I. INTRODUCTION

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The functional cerebral changes that have been detected in the brains of hypertensive animals occur predominantly in the hypothalamus and medulla. The justification for limiting a review to certain changes that take place in the hypothalamus (and certain areas immediately anterior) stems from the conclusion that hypertension appears to be due to an increase in medullary pressor activity due to a suppression from above of medullary inhibitory activity (94, 95). Furthermore, there is the observation by Julius (288) based on observations made on patients with essential hypertension that "... it appears that hypertension is not a disease of blood pressure regulation. The pressure is set at a high level but around that setting it is regulated in a normal fashion."

The central nervous system's medullary control of the arterial pressure stems from a tonic excitatory center, situated in a nucleus in a rostro-ventral position with spinal excitatory fibers to the spinal intermediolateral nucleus which controls sympathetic ganglia and the adrenal medulla. The medulla's excitatory center is under the influence of the hypothalamus, the midbrain, a medullary inhibitory center slightly more caudal than the excitatory center, and the nucleus tractus solitarius (94, 503).

Functional alterations in particular hypothalamic nuclei either raise or lower the blood pressure by altering sympathetic nervous activity. The nuclei are closely interconnected and also communicate with many other areas in the central nervous system both rostrally and caudally. There are efferent spinal neurons (93, 503, 695) that project to the midbrain and medulla and to spinal sympathetic preganglionic neurons while afferent stimuli come from the midbrain and medulla and from chemo- and pressure-sensitive neurons in the heart, the aorta, the carotids (21, 141, 316, 503) and from the viscera, particularly the kidneys (583). After a brief account of the anatomical layout of the hypothalamic nuclei, their multiple interconnections and the effect of the antero third ventricle ventral (AV₃V) lesion on the blood pressure the review describes the widespread changes in neurotransmitter and neuromodulator functions that occur in the hypothalamus of hypertensive rats. The review ends with a discussion on the possible origin of these hypothalamic changes which, via the sympathetic nervous system, appear to play a predominant role in most forms of hypertension.

	II. DISPOSITION OF CERTAIN HYPOTHALAMIC NUCLEI INVOLVED IN HYPERTENSION

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The areas in the hypothalamus that are most concerned with the blood pressure lie on either side of the whole length of the third ventricle: a distance of ~4.25 mm in the rat. In the midline anteriorly, forming the principal part of the anterior wall of the third ventricle (the lamina terminalis or end wall), there is the median preoptic nucleus that arches upward and posteriorly over the ventricle (360). Near the dorsal end of the median preoptic nucleus there is a vascular area, the subfornical organ (SFO), and at the ventral end of the nucleus, there is another vascular body, the organum vascularis lamina terminalis (OVLT), which is attached to the underlying optic chiasm by a short strand of connective tissue. These are the structures that form the lamina terminalis of the third ventricle. Midway between the SFO and the OVLT there is the anterior commissure that crosses the posterior surface of the median optic nucleus horizontally. Both the SFO and OVLT contain neurons with secreting granules and vacuoles and capillaries that are fenestrated. This arrangement permits these neurons to monitor blood levels of substances such as peptides that may not be able to penetrate the blood-brain barrier in other parts of the central nervous system. The SFO projects to most regions of the more caudal paraventricular nucleus and to the supraoptic nucleus (18), which lies immediately lateral to and adjoining the optic chiasma. The supraoptic nucleus is regulated, in part, by projections from the suprachiasmatic nucleus, which lies next to the midline on the upper surface of the optic chiasma (124, 615). Anterior to the structures of the lamina terminalis there is a relatively large area, the septum, which stretches from the corpus callosum above to the optic chiasma below.

Immediately surrounding the most anterior projection of the floor of the third ventricle, the preoptic recess, there is the periventricular preoptic hypothalamic nucleus with ventrally the anterior part of the medial optic nucleus in the lateral portion of which is embedded the anterolateral preoptic nucleus. More posteriorly and dorsally there is the most anterior part of the paraventricular nucleus. The ventrally placed medial preoptic nucleus extends posteriorly as far as the most anterior edge of the anterior hypothalamic nucleus which appears lateral and ventral to the more dorsally placed paraventricular nucleus. There are inputs into the preoptic and medial preoptic areas from the anterior hypothalamic area, the lateral hypothalamus, and the ventral and dorsal medial nuclei. The medial preoptic nucleus also receives afferents from various brain stem nuclei including the locus coerulus and the median raphe magna (198) and projects to the periaqueductal gray neurons, which themselves project to the medulla (506).

As the paraventricular nucleus extends posteriorly, it enlarges laterally, whereas ventrally it becomes the periventricular nucleus. In the rat the paraventricular nucleus, which is an integrating center for many physiological functions, occupies less than one-third of a millimeter on either side of the third ventricle. It is subdivided into a parvocellular area near the midline, a more lateral magnocellular area, and at least eight subdivisions (595). Out of a total of ~21,000 neurons (both sides), 19% are in the magnocellular area (312). Some neurons in the parvocellular part of the paraventricular nucleus connect directly with the midbrain and others have descending projections which make direct contact with the rostro-ventrolateral medulla, the nucleus tractus solitarius (116, 594), and spinal sympathetic preganglionic neurons (371, 544). Some of the spinally projecting neurons receive information on extracranial vascular pressures and chemical changes (368). Some fibers from the periventricular area of the paraventricular nucleus project to the median eminence and the arcuate nucleus (622).

Into the parvocellular division of the paraventricular nucleus there are inputs from the anterior hypothalamic area, the lateral hypothalamic, and from the ventromedial and dorsomedial nuclei; there are also a few projections from the preoptic area. The suprachiasmatic nucleus also projects to the periventricular and dorsal parts of the parvocellular area of the paraventricular nucleus. The magnocellular division of the paraventricular nucleus only appears to have projections from the dorsomedial and preoptic nuclei. The SFO projects to most regions of the paraventricular nucleus.

More posteriorly, lateral to the posterior continuation of the anterior hypothalamic nucleus, there is the lateral hypothalamic area and below it the tuber cinereum. And as the area of the tuber cinereum diminishes caudally, the median eminence forms the floor of the ventricle. At this level the ventromedial and arcuate nuclei are evident while dorsal to the dorsomedial nucleus there is the dorsal hypothalamic area. The arcuate nucleus projects to the supraoptic nucleus and the SFO (515). The dorsal, ventromedial, and paraventricular nuclei interconnect and lie medial to the lateral hypothalamus, which mainly contains fibers coming from these nuclei, and there are connections between the dorsomedial nucleus and the circumventricular organs. The dorsomedial nucleus (32) contains direct and indirect connections with sympathetic and parasympathetic systems and is influenced by peripheral afferents via the nucleus solitarus, the parabrachial nucleus, and the sympathetic intermediolateral columns.

The posterior hypothalamic nucleus lies immediately posterior to the dorsal hypothalamic area at a level, where ventrally, the infundibular stalk emerges. The posterior hypothalamic nucleus extends posteriorly for a relatively short distance level with the most rostral edge of the ventrally placed mammillary nuclei. It receives a large number of afferents from other parts of the hypothalamus particularly from the anterior hypothalamic and the ventromedial nuclei and a few from the supraoptic, suprachiasmatic, and paraventricular nuclei. In addition, there are a few from the lateral and dorsal hypothalamic areas and the mammillary nuclei (701). It also receives many afferents from the brain stem including the substantia nigra, the periaqueductal gray matter, the rostral raphe nuclei, and the locus coerulus (525). Neurons from the posterior hypothalamus have descending projections to several autonomic regulating regions in the midbrain and medulla and to sympathetic preganglionic neurons in the mediolateral cell columns of the spinal cord (529, 651). The function of the posterior hypothalamus is very dependent on the activity of the locus coeruleus which lies more caudally.

A ventral lesion placed into the anterior part of the third ventricle (AV₃V) has been used to study the hypothalamic control of the blood pressure in the rat (60). The lesion, which partially destroys several areas to a varying extent, is produced by skewering, from the front, the anterior ventral corner of the third ventricle in the midline. The most ventral portion of the anterior end wall of the ventricle (the lamina terminalis) between the anterior commissure above and the optic chiasm below is destroyed with the loss of the OVLT. The damage extends posteriorly and horizontally, on either side and close to the midline of the slitlike ventricle, as a narrow tunnel of destruction into the walls of the anterior and ventral parts of the ventricle. The periventricular hypothalamic nucleus and the median preoptic nucleus are particularly involved, and the lesion may then extend posteriorly into the anterior hypothalamus and paraventricular area. According to Brody and Johnson (60), "the medial preoptic nucleus, the anterior hypothalamic nucleus and the paraventricular nucleus were largely intact, in most cases, and usually sustained little or no apparent damage beyond their medial borders."

	III. FUNCTIONAL DISTURBANCES IN THE HYPOTHALAMUS OF HYPERTENSIVE ANIMALS

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In 1969 Yamori and Okamoto (687) sectioned the brains of the spontaneously hypertensive rat (SHR) in vivo in various ways and came to the conclusion that the rise in arterial pressure is due to an increase in hypothalamic "tonic influence." It has since been established that the rise in sympathetic nervous activity in hypertension is associated with multiple functional abnormalities in certain sites in the hypothalamus, brain stem, and medulla (87). The individual contribution and site of action within the hypothalamus of most of these abnormalities in the SHR and other forms of hypertension are described below.

	IV. CATECHOLAMINES

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In catecholaminergic neurons, L-tyrosine is converted to L-dopa (dihydroxy-phenylalanine) by tyrosine hydroxylase, a rate-limiting step. Tyrosine hydroxylase activation requires tetrahydroperidine cofactor, oxygen, and ferrous iron. Dopa decarboxylase (L-aromatic amino acid decarboxylase) converts L-dopa to dopamine (dihydroxyphenylalanine). Dopamine is taken up in storage vesicles and deaminated to 3,4-dihydroxyphenylacetic acid. In the vesicles dopamine is converted to norepinephrine by dopamine -hydroxylase, the essential cofactors of which are ascorbic acid and cuprous ion (Cu²⁺). Norepinephrine is methylated by phenylethanolamine-N-methyltransferase (PNMT) to form epinephrine.

Norepinephrine has a negative feedback action by inhibiting the conversion of L-tyrosine to L-dopa. Following release from storage vesicles into the synaptic cleft, most of the norepinephrine (~80%) reenters the membrane back into the cytoplasmic pool, and then by active transport against a high concentration gradient, it is returned into storage granules. The two main divisions of catecholamine receptors  and  have been further subdivided. Molecular cloning has characterized at least three postjunctional ₁-receptors and four prejunctional ₂-receptors. Three -receptors have been identified (497).

With the use of a fluorescent histochemical technique (275) to localize the presence of catecholamines, which correlates quantitatively with their content (320) in neurons and fibers, the distribution of catecholamines is mainly centered on the paraventricular, periventricular, and dorsomedial nuclei. Most of the catecholaminergic fibers ascend from the medulla and travel to the paraventricular nucleus (444). The medial preoptic, anterior hypothalamic, and posterior hypothalamic nuclei demonstrate scattered fluorescence, while the ventromedial nucleus appears almost clear of activity. In neurons from the parvocellular area of the paraventricular nucleus, which can be either neuroendocrine or preautonomic, norepinephrine regulation occurs for the most part either as a -adrenoreceptor-mediated inhibition or via ₁-receptor-mediated activation of intrahypothalamic glutaminergic circuits (127). In the magnocellular portion of the paraventricular nucleus, norepinephrine has an inhibitory effect on vasopressin-secreting neurons and an excitatory effect on oxytocin-secreting neurons (280).

The earliest observations were in vitro studies of catecholamine turnover, vesicular storage capacity, release, uptake, catabolism, density of innervation and of varicosities, enzyme function, and receptor binding capacity on tissue sections and synaptosomes. The results tended to be incongruous (256). The position was clarified by the use of in vivo injection and microperfusion techniques (460). Microperfusion of catecholamines or ₂-agonists into the anterior hypothalamic area lowers the blood pressure (47, 587) and when injected into the paraventricular nucleus raises the blood pressure (237). Conversely, injections of 6-hydroxydopamine (6-OHD) into the anterior hypothalamic nucleus raises the blood pressure (28). 6-OHD causes degeneration of catecholaminergic nerve endings, particularly noradrenergic and adrenergic (227); hypothalamic dopamine neural group however are resistant (700). The pressor effect of catecholamines injected into the paraventricular nucleus is due in part to the release of arginine vasopressin (AVP). The injection of a catecholamine, phenylephrine, into the posterior hypothalamus also raises the arterial pressure (420), and the injection of interleukin-1 into the lateral ventricles decreases norepinephrine secretion from the posterior hypothalamus and lowers the blood pressure (85).

The locus coerulus caudal to the hypothalamus in the floor of the fourth ventricle strongly influences the function of the posterior hypothalamus. The injection of the neuro-stimulating amino acid L-glutamate into the locus coerulus increases the norepinephrine content of the posterior hypothalamus and raises the blood pressure. These effects are attenuated by a prior injection of the catecholaminergic neurotoxin 6-hydroxytryptamine (6-OHD) (305) into the posterior hypothalamus.

The extensive functional interconnection between catecholamines and most of the other neurotransmitters and neuromodulators that are involved in the control of the blood pressure are described below.

There are three in vivo microperfusion-dialysis studies in the whole rat which show that in the SHR there is an increased norepinephrine release from the paraventricular nucleus (488, 673) and from the posterior lateral hypothalamus (440). In one study, the increased release at 7-10 wk exceeded the raised levels at 12-14 wk (488). In another in vivo study at 9 wk, however, the release of norepinephrine (and epinephrine) from the posterior hypothalamus of the SHR was lower than from the Wistar-Kyoto rat (WKY), although the release of dopamine was raised (631). The precision of the in vivo microperfusion experiments and the focal nature of the changes in these experiments is evident by the finding that in the areas immediately surrounding the paraventricular nucleus and the dorsomedial area norepinephrine release is not raised (673).

Another microperfusion study has been carried out in which the perfusate levels of 4-hydroxy-3 methoxyphenylglycol (MOPEG), the major metabolite of norepinephrine, has been used as an index of norepinephrine release. It was found that the release of MOPEG from the anterior hypothalamic nucleus of the 9- to 10-wk-old SHR is substantially greater than from the WKY (99). The release of MOPEG from the posterior hypothalamus, however, was no different from that of the WKY.

The first study that demonstrated the importance of cerebral catecholamines in hypertension was the effect of the central administration of 6-OHD (229). In SHR, in which the blood pressure had risen to 160-170 mmHg and would subsequently plateau at 190-210 mmHg at 12-14 wk, two intracerebroventricular injections of 6-OHD at 7 wk caused a sustained fall in arterial pressure to ~140 mmHg until the 12th week when the pressure then gradually rose to the same levels as in control SHR (162, 229). If, however, 6-OHD was administered to the SHR at 12 wk, it only produced a transient and moderate fall in pressure lasting <1 wk, a response similar to that of a normal animal (229).

An increase in norepinephrine release from the paraventricular and posterior hypothalamic nuclei has also been measured, with microperfusion, in the hypertension, which results from a constricting ligature around the aorta just above the kidneys (598) and from 5/6 removal of renal mass (86), but it does not occur in the DOCA plus salt (489). The injection, however, of 6-OHD intracerebroventricularly 7-10 days before the administration of DOCA and salt or the application of a clip on a renal artery prevents the blood pressure from rising (180, 208).

In vitro studies of hypothalamic slices from 8- to 10-wk-old SHR have also shown there is an increased secretion of norepinephrine from the paraventricular nucleus (408).

The usual rise in arterial pressure in the SHR occurs on a "normal" intake of sodium. A rise in sodium intake from 1 to 8% in the 9- to 10-wk old SHR, however, causes a further rise in arterial pressure and a diminution of the preexisting compensatory depressor increase in MOPEG (and therefore, probably, norepinephrine) release from the anterior hypothalamus (99, 435, 677). It is probable that this reduction is due, in part, to the simultaneous increase in the local concentration of the neuroinhibitor atrial natriuretic peptide (691).

The venous drainage of the human brain is asymmetrical. Usually the right internal jugular vein drains the saggital sinus as its main tributary, and the cerebral cortex is its predominant field of drainage. The left internal jugular is typically a continuation of the straight sinus and drains predominantly the subcortical area. Ferrier et al. (175) report that in patients with essential hypertension norepinephrine release from the subcortical region is significantly greater and that this increase correlates with the increase in total body spillover of norepinephrine (i.e., overall sympathetic nervous activity) and with renal norepinephrine spillover. The subcortical region includes the medulla, pons, and the hypothalamus, and most norepinephrine is released from the locus coeruleus in the floor of the fourth ventricle. These findings indicate that in essential hypertension, as in hypertension in animals, there is a central disturbance of catecholaminergic metabolism in the inferior part of the brain, which is related to the increase in sympathetic nervous activity, but the contribution of the hypothalamus is not distinguishable.

	V. CHOLINERGIC MECHANISMS

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Acetylcholine is synthesized from acetyl coenzyme A and choline by the action of choline acetyltransferase. It is packaged into synaptic vesicles at high concentrations by an active transport process and is released from the vesicles by exocytosis in response to calcium entry. Once released, acetylcholine is rapidly broken down by acetylcholinesterase that is present in high concentrations wherever acetylcholine acts as a neurotransmitter. Destruction is rapid; ~90% of the released acetylcholine may be hydrolyzed before it gets across the synaptic cleft onto its receptor on the postsynaptic membrane (497). The acetic acid produced by the hydrolysis is rapidly removed by various biochemical pathways while the choline is actively transported back into the nerve terminal where it can be recycled to synthesize acetylcholine (296).

All five subdivisions of muscarinic receptors (M1-5) and the nicotine receptor are found in the brain; they are diffusely present in the hypothalamus.

The intracerebroventricular injection of carbachol or neostigmine induce strong fos-like immunoreactivity in the paraventricular, posterior, ventral premamillary, and the supra, median, and medial preoptic area. Only moderate activity is present in the SFO and the OVLT (57, 353, 517). With the use of Koelle's (314) thiocholine method for the localization of acetylcholinesterase, the highest concentrations are in the dorsomedial and paraventricular nuclei with high concentrations in the median eminence, premamillary, and posterior hypothalamic nuclei (275). A monoclonal antiserum against choline-acetyltransferase, however, reveals a rather different pattern. In the preoptic area, immunostaining is found particularly in the medial and median preoptic areas, whereas in the hypothalamus there are numerous immunoreactive cells in the lateral hypothalamic area, the dorsal premamillary, ventromedial hypothalamic nuclei, and the area surrounding the fornix. Substantial numbers of immunoreactive cells are also seen in the dorsomedial, posterior, and ventral premamillary nuclei. There are also many such cells between the periventricular zone and the anterior hypothalamic nucleus and between the paraventricular zone and the anterior hypothalamus (498).

B.  Cholinergic Interactions With Catecholamines, Nitric Oxide, and AVP

1.  Catecholamines

The integrity of central catecholaminergic neurons appears to be critical for expression of the cholinergic response (56, 459). The rise in arterial pressure and increase in sympathetic nervous activity induced by an increase in central cholinergic activity is linked to activation of central catecholaminergic mechanisms. For instance, muscarinic receptor agonists and acetylcholinesterase inhibitors increase the activity of tyrosine hydroxylase (349) and the synthesis and release of brain catecholamines (306). Microperfusion and dialysis of the paraventricular nucleus of the normal rat with nicotine stimulates norepinephrine release (548), and superfusion of the posterior hypothalamus of the cat with nicotinic drugs or acetylcholine elicits a rise in arterial pressure that is abolished by hypothalamic superfusion with -adrenoreceptor blocking agents (39). There is also some evidence that clonidine exerts an inhibitory effect on central cholinergic neurons involved in the regulation of the arterial pressure (68). Some neurons contain both norepinephrine and acetylcholinesterase, and the prolonged administration of muscarinic agonists oxotenosine and pilocarpine increase markedly the number of ₂-adrenoreceptors throughout the rat brain (251). The increase in blood pressure observed after the central administration of carbachol is inhibited by the prior injection into the lateral ventricle of 6-OHD (208) and by the administration of bethanidine and guanethidine. -Adrenoreceptors may also be involved in that the effects of the central administration of carbachol can be blocked by the central administration of propranolol. Finally, the inhibitory effect of catecholaminergic agonists on acetylcholine release from cholinergic terminals provides evidence of a feedback system between catecholaminergic and cholinergic systems (56, 151).

In rat neuronal cultures, carbachol causes a time- and concentration-dependent increase in cGMP levels, which is antagonized by atropine. This response is depressed by nitric oxide (NO) synthase inhibitors, which suggests that, as elsewhere, muscarinic receptor stimulation increases NO production in the cerebral cortex (91).

The injection of acetylcholine into the supraoptic nucleus of an anesthetized dog during a water diuresis has no effect on the blood pressure but inhibits the urine flow from denervated kidneys (471), which suggests there has been an increase in plasma AVP. Intracerebroventricular administration of carbachol into conscious rats causes an increase in AVP secretion and blood pressure that is blocked by the muscarinic blocker atropine but not by a nicotinic blocker (271).

In the normal rat, injection of choline intracerebroventricularly or of carbachol or neostigmine into either the posterior area of the paraventricular nucleus, the dorsal portion of the anterior hypothalamic nucleus, the posterior hypothalamic nucleus and the ventromedial nuclei increases the arterial pressure and renal sympathetic nerve activity (12, 56, 59, 67, 385, 475). Injections of carbachol into the dorsomedial nuclei, however, evoke a fall in blood pressure (53, 242) while neostigmine either causes an increase in pressure or has no effect (53). Injections into the lateral hypothalamic nucleus only evoke insignificant responses (59). Because of the relative absence of M₁ muscarinic receptors in the posterior hypothalamic nuclei (200) and the observation that the hypertensive effect of carbachol can be partially blocked by the injection into the nucleus of the M₁-M₂-M₃ receptor antagonist 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP), it has been considered that the hypertensive effect of carbachol on the posterior hypothalamus is due principally to stimulation of M₂-M₃ receptors (52, 678). The injection of carbachol into the third or lateral ventricle of unanesthetized rats also releases AVP, which contributes to the early rise in arterial pressure (250, 331, 676). Intravenously administered physostigmine raises the blood pressure by its effect on the rostral ventrolateral medulla (483).

The intravenous injection of atropine in the conscious SHR evokes an age-dependent fall in blood pressure, the younger rats (11 wk) being significantly less sensitive than the older rats (88) (15-20 wk). It has no significant effect in the WKY. The intravenous injection of methyl atropine, however, which does not readily enter the brain, induces a transient rise in blood pressure. This suggests that the depressor effect of intravenous atropine in the SHR is due to a central action that suppresses increased cholinergic activity.

High-affinity choline uptake (330, 684) in freshly prepared crude synaptosomes from the hypothalamus of the SHR, although greater than that of the Wistar-Lewis rat, is not different from the WKY (625). And choline acetyltransferase (ChAT) activity as measured in sections obtained by micropuncture is reduced in the paraventricular, dorsomedial, and posterior hypothalamic nuclei, a change which is more pronounced with age (242). In contrast, the hypotensive effect of hemicholinium (HC-3), ethyl-choline aziridium, and 4-DAMP intracerebroventricularly and into the posterior hypothalamus and paraventricular nucleus indicate that there is an increase in hypothalamic cholinergic activity particularly in the posterior hypothalamus.

HC-3 is a synthetic substance which, at low concentrations, selectively blocks sodium high-affinity neuronal uptake of choline and reduces acetylcholine synthesis (330, 684). At high concentrations it can block first nicotinic and then muscarinic receptors. Single acute injections of HC-3 into the lateral ventricle of young and adult WKY have no effect on the blood pressure (54). In the 12-wk-old SHR, such injections reduce the blood pressure, an effect which becomes more pronounced with age until 18-20 wk (201). Five- to eight-week lateral ventricle injections of HC-3 cause no significant reduction in blood pressure. Single central injections of HC-3 also reduce the blood pressure in the DOCA salt, aortic constriction, and Grollman, renal hypertensive rats. The magnitude of the depressor response in the rat with aortic constriction increases with age (201).

Chronic lateral ventricle infusions of HC-3 for 21 days in the 5-wk-old SHR decreases hypothalamic acetylcholine levels and suppresses the development of hypertension during the 21 days (647). In contrast, a similar infusion at 18 wk only reduces the blood pressure for the first 8 days, after which the blood pressure returns toward control values during which time hypothalamic acetylcholine levels also return to normal despite the continuing HC-3 injection.

Microinjection of HC-3 into the lateral septal area, the paraventricular, and the posterior hypothalamic nuclei of the adult SHR reduce the blood pressure (58, 537). About 60% of the acute depressor effect of HC-3 injected into the posterior hypothalamic nucleus can be abolished by an injection of choline into the nucleus (58). When ethylcholine aziridium, a cholinergic neurotoxin, is injected into the posterior hypothalamus of 1- and 3-mo-old SHR and WKY, there is a substantial fall in arterial pressure, although not to normal in the SHR (158). When injected into the anterior hypothalamic nucleus of the 4-wk-old SHR, it does not prevent the onset or severity of the hypertension (158).

4-DAMP mustard, an M₁, M₂, and M₃ receptor antagonist which decreases the density of muscarinic receptors in the hypothalamus by ~60%, has no significant effect on receptors in the brain stem (55). Following a bilateral injection of 4-DAMP mustard bilaterally into the posterior hypothalamic nuclei of the SHR, there is a fall in blood pressure (although not to normal) that takes several days to return to the original level (55). These findings suggest that the hypertensive effect of the increase in cholinergic activity in the hypothalamus including the posterior hypothalamus is independent of changes in cholinergic activity in the brain stem and medulla.

The increase in cholinergic activity in the SHR which most of these observations suggest is mediated at least in part through enhanced density of muscarinic receptors. Muscarinic receptor sites in the SHR are increased as early as 1 wk of age and continue to increase until 11 wk (244). mRNA encoding of five muscarinic receptors in the whole hypothalamus has revealed that there is 40-50% increase in excitatory M₁ subtype and a decrease in the inhibitory M₄ subtype during the establishment of the hypertension (662). The changes are more pronounced at 12 wk of age. The levels of the other subtypes M₂, M₃, and M₅ in the SHR were not significantly different from the WKY (662). A change in muscarinic receptor density does not occur in renal hypertension (681), but the density is increased in the Dahl salt-sensitive rat (154).

	VI. ANGIOTENSIN

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All the components of the renin-angiotensin system, renin, angiotensinogen, angiotensin-(1---7), angiotensin converting enzyme, angiotensin II, and angiotensin II receptor subtypes have been identified in brain neurons and glial cells (182, 189, 363). The angiotensinogen gene is exclusively expressed in astrocytes (363). Angiotensin II modulates voltage-activated ion channels of neurons which leads to depolarization and an increase in intraneural calcium (170, 689).

There are two distinct subtypes of angiotensin II receptors, AT₁ and AT₂. Most of the known actions of angiotensin II are mediated by AT₁ receptors, which belong to the G protein-coupled receptor superfamily.

Ligand binding, immunochemistry, and electrophysiological techniques agree that in the hypothalamus angiotensin has its major impact on the blood pressure by its effect on the structures at the anterior part of the third ventricle, particularly those that form part of the lamina terminalis, i.e., the SFO, the median optic nucleus, and the OVLT (360, 430, 517). "The OVLT `sees' both CSF and blood angiotensin, whereas the SFO only detects blood-borne angiotensin" (465). The angiotensin pressor area appears to be mainly below the level of the anterior commissure where the OVLT is the most prominent structure, and it includes the margins of the preoptic and anterior hypothalamic nuclei (238). These areas interconnect and also have important connections to and from the paraventricular and supraoptic nucleus and their AVP- and oxytocin-secreting cells, and to the medulla and spinal cord. The posterior magnocellular area of the paraventricular nucleus and the ventromedial/median eminence region of the hypothalamus are closely involved in the pressor effect of intracerebroventricular administered angiotensin II (279, 283).

Intracerebroventricular angiotensin II increases turnover of norepinephrine in certain hypothalamic nuclei, particularly the most rostrally placed (589) and the paraventricular nucleus (361, 465, 579). The turnover of dopamine is not affected.

In cultured neurons from the rat brain, angiotensin II not only causes a significant increase of angiotensin II receptors and in the concentration of neuronal and media norepinephrine, it also stimulates norepinephrine reuptake, thus mitigating the rise of norepinephrine in the extraneuronal environment (494). There is also evidence of a negative feedback loop in that catecholamines inhibit angiotensin II release and reduce the number of angiotensin II receptors (494).

An intracerebroventricular injection of angiotensin II (0.7-5.7 fmol) causes an increased release of AVP (465), an effect which is diminished by the injection of a GABA agonist (51).

The intracerebroventricular administration of angiotensin II (539, 589) and the microinjection of doses of angiotensin II as low as 50 fmol (464) into the OVLT/median optic nucleus area cause a rise in blood pressure associated with an increase in sympathetic nervous activity (167, 539, 619, 661). The pressor effect of the central administration of angiotensin II is mediated by activation of AT₁ receptors. There is some evidence that the pressor effect of an acute intracerebroventricular injection of angiotensin II is due in part to the release of AVP (249, 593). In the rat, intracerebroventricular transfection of the human angiotensin-converting enzyme gene increases blood pressure and heart rate and is associated with increased hypothalamic production of AVP, the transgene is widely expressed in periventricular cells, the cortex, hypothalamic nuclei, and the brain stem (417).

The pressor effect of intracerebroventricular angiotensin II is exaggerated in 12-wk-old normal rats fed a high-sodium diet from the age of 2-3 wk. The exaggeration of the pressor effect is associated with acute retention of sodium, is abolished by renal denervation, and does not occur in a 10-wk-old normal rat given a high-sodium diet for 2 wk (437).

In the SHR, there are more than twice as many cells and fibers in the supraoptic and paraventricular nuclei which stain for angiotensin II-like immunoreactivity (665), and the turnover (188) of angiotensin II and its concentration (468) are greater in the hypothalamus of the adult SHR than in the WKY. Angiotensinogen mRNA in the preoptic area of the SHR is increased, a rise which is apparent at 4 wk of age and becomes more pronounced with age (555). In the two-kidney, two-clip (2K2C) renal hypertensive rat, the angiotensinogen concentration in the hypothalamus is increased and the rise in pressure occurs despite central depletion of catecholamines with 6-OHD (23). In the 2K2C renal hypertensive Wistar rat, Basso et al. (23) found that the angiotensinogen concentration in the hypothalamus, 6 wk after placing clips, was also increased (and that the rise in arterial pressure occurred despite central depletion of catecholamines with 6-OHD), whereas Lou et al. (367) in the two-kidney, one-clip (2K1C) hypertensive Sprague-Dawley rat reported a decrease in hypothalamic mRNA at 19 days, but not at 40 days.

The pressor response to the injection of angiotensin II into the preoptic area in the SHR is two to three times greater than in the WKY (388). Both the 4- and 14-wk-old SHR have significantly greater concentrations of angiotensin II binding sites in the median preoptic nucleus, the SFO and the paraventricular nucleus than the WKY (224). In the DOCA plus salt hypertensive rat angiotensin II binding in the median preoptic nucleus, the SFO, and the paraventricular nucleus is increased (562).

Hypothalamic neuronal cultures have been established from 1-day-old WKY and SHR strain rats (370, 493, 702). They contain paraventricular and supraoptic nuclei, anterior, lateral and posterior, and dorsal and ventromedial nuclei, and the cultures consist of 85-90% neuronal cells with 10-15% astroglial cells. Neuronal cultures from the SHR hypothalamus express two to four times higher levels of functional AT₁ receptors with parallel increases in mRNAs for both AT_1A and AT_1B receptor subtypes (233, 493, 495, 582). And in such cultures angiotensin II stimulates tyrosine hydroxylase (TH) activity, TH mRNA, norepinephrine uptake, c-fos mRNA, and norepinephrine uptake significantly more when the neurons originate from SHR rather than WKY (370, 702). Stimulation of TH mRNA is mediated by AT₁ receptor subtypes, which results in an increase in its transcription and involves activation of phospholipase C and protein kinase C (702). In vivo confirmation of the stimulating effect of angiotensin II on TH activity and TH mRNA has been obtained in adult WKY and SHR when the level of stimulation in the SHR has been found to be significantly greater than in the WKY (702). Overall, these observations are consistent with the increase in AT₁ receptor gene expression in the SHR (493). The greater level of angiotensin II receptors found in the SHR hypothalamus does not include AT₂ receptor subtype (590). The level of angiotensinogen mRNA in the preoptic area of the SHR is higher than in the WKY at 7 and 16 wk, although at 4 wk the difference is insignificant (555).

Intervention in the hyperactivity of the hypothalamic angiotensin system lowers the blood pressure in the SHR (30). The intracerebroventricular or anterior hypothalamic injection of an angiotensin-converting enzyme inhibitor or angiotensin receptor antagonist in the SHR (31, 269, 469, 692), Dahl salt-sensitive rat (614), DOCA plus salt (273) and 2K1C renal hypertensive (165) rat lowers the blood pressure but is without effect on the blood pressure of normal rats. In the SHR, the depressor effect is greater in an animal on a high salt intake (686). In the Dahl salt-sensitive rat, an intracerebroventricular infusion of an angiotensin receptor blocker also prevents the usual increase in sympathetic nervous activity (265). Injection of an angiotensin receptor antagonist into the posterior hypothalamic nucleus of the SHR produces no significant effect on the blood pressure (692), which supports the conclusion that the hypothalamic localization of the increase in angiotensin hyperactivity is in the anterior hypothalamus.

More recently, antisense oligodeoxynucleotides, targeted to angiotensinogen mRNA and AT₁ receptor mRNA, injected into the lateral ventricle have significantly reduced the blood pressure of SHR (294, 467, 470, 667). Antisense oligodeoxynucleotides targeted to angiotensinogen do not lower the blood pressure of normotensive rats (466).

The bilateral injection of antisense oligonucleotide targeted solely to angiotensinogen into the paraventricular nuclei of the SHR does not reduce the blood pressure (294), suggesting that the increase in angiotensin in the paraventricular nucleus does not contribute to the sustained rise in blood pressure. There is some fragmentary evidence, however, that the effect of the excess angiotensin in the paraventricular nucleus on the blood pressure is to exaggerate baroreflex responses. In normal dogs in which the sinoaortic nerves and the vagi have been sectioned, the introduction of angiotensin into the paraventricular nucleus increases the renal sympathetic nerve activity response to electrical stimulation of the left cardiac sympathetic nerve (660).

	VII. NATRIURETIC PEPTIDES

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Four so-called natriuretic peptides have been identified, atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), and urodilatin. ANP is natriuretic, is present in plasma and in the hypothalamus, and is most abundant in the auricle. BNP is natriuretic, is present in plasma, and is most abundant in the ventricle of the heart (427). There is no BNP mRNA in the hypothalamus. CNP, which is neither natriuretic nor present in plasma, is found predominantly in the brain, particularly in the hypothalamus (243) and vascular endothelial cells (586). Urodilatin, which is natriuretic, is only present in the urine. It is produced in the kidney (540)

There are three main natriuretic peptide receptors (NPRs). NPR-A and NPR-B are particulate guanyl cyclases with an extracellular binding site, a region which spans the membrane and an intracellular tail that catalyzes the conversion of GTP to cGMP. NPR-C, which lacks an intracellular catalytic tail, does not appear to be involved in the generation of cGMP. It is thought to be a clearance receptor for all forms of natriuretic peptides, which controls plasma natriuretic peptide concentration. If the effect of natriuretic peptides on the brain is related to the production of cGMP, it will depend on astrocytes, since these are the only brain cells that generate cGMP upon exposure to natriuretic peptides. CNP also reduces astrocyte glutamate reabsorption (138), which should stimulate neuronal activity. ANP and CNP are equally potent in their ability to generate cGMP in rat brain (206). In cell cultures, cGMP is rapidly exported out of astrocytes and acts extracellularly to inhibit sodium/hydrogen exchange and reduce intracellular pH (623). Hypothalamic ANP, like other hypothalamic neurotransmitter and neuromodulator peptides, is released in a Ca²⁺-dependent manner (554).

Stimulation of the AV₃V region induces a rapid rise in plasma ANP (20), whereas lesions of the AV₃V region are followed by a marked fall in plasma ANP and a significant suppression of the normal rise that occurs with volume expansion (10). Because these changes are accompanied by little or no change in atrial ANP, it suggests that brain ANP can alter plasma ANP (20).

The distribution of ANP immunoreactive cells is particularly dense in the anterior part of the hypothalamus (399, 530, 581), including much of the area included in the AV₃V area, the OVLT, the SFO, the periventricular nucleus, the medial preoptic nucleus, the preoptic suprachiasmatic nucleus, and the anterior hypothalamic nucleus. The density of ANP-containing cells is less in the paraventricular nucleus, the dorso- and ventromedial nucleus extending to the arcuate nucleus and the medial mamillary nucleus. The axons of ANP-containing cells project to the median eminence and neural lobe (10).

ANP mRNA is widely distributed in the preoptic area, the paraventricular and arcuate nucleus, and throughout the lateral inferior and posterior areas of the hypothalamus, including the mamillary nucleus. The distribution of CNP mRNA is less widely distributed, overlapping that of ANP mRNA in the preoptic and arcuate nuclei (243). In contrast to the distribution of ANP immunoreactive cells, there is a surprising paucity of cells containing ANP or CNP mRNA in the anterior hypothalamic area (see below). The paraventricular nucleus, which contains a relatively low density of ANP mRNA cells, contains, however, much ANP immunoreactivity (243). The significance of this discrepancy between the distribution of ANP mRNA and immunoreactivity, particularly in the paraventricular nucleus, is not understood. Herman et al. (243) have suggested that, as ANP antibodies cross react with other species possessing cysteine rings near the carboxy terminus, it is possible that the protein product detected by immunocytochemistry is derived from some other molecule than ANP. In addition to the natriuretic peptides formed in the brain, those present in the blood and cerebrospinal fluid (CSF) can affect neurons in organs that lack a blood-brain barrier, such as the SFO and OVLT.

There is a certain overlap, but the principle receptor for ANP is NPR-A and for CNP is NPR-B (96, 668). The clearance receptor NPR-C has much less stringent structural requirements for ligand binding than either NPR-A or -B. It is present in such rostral areas as the olfactory bulb, the medial frontal cortex, the cingulate gyrus, and the lateral septal nucleus; there are no NPR-C receptors in AV₃V structures such as the median, preoptic, supraoptic and paraventricular nuclei and SFO or in the rest of the hypothalamus. NPR-A is distributed in the same area as the NPR-C receptors and in the SFO, the median preoptic, supraoptic, the paraventricular nucleus and certain areas caudal to the hypothalamus (63). No NPR-B sites have been detected in the rat brain (63), nor has any high-affinity guanylyl cyclase-linked receptor that is specific for BNP (668).

In the anesthetized rat, microinjections into the paraventricular nucleus of ANP in doses as low as 1.3 fmol have an inhibitory effect on the electrical activity of single neurons within the nucleus (580). In hypothalamic slice preparations in vitro, ANP has a direct inhibitory effect on neurons in the AV₃V area and the parvocellular part of the paraventricular nucleus but no effect on the electrical activity of magnocellular neurons in the paraventricular and supraoptic nucleus (431, 672). However, ANP applied to the rat SFO in vitro, in concentrations as low as 10¹⁰ M, induces a brief period of excitation (76). In vivo the injection of 100 ng ANP into the cerebral ventricles increases afferent renal sympathetic nerve activity (512).

In vitro, ANPs inhibit the release of norepinephrine from hypothalamic neurons (183), and injections of ANP (2-5 µg) into the lateral ventricles of the normal rat decrease the amount of norepinephrine, dopamine, and their metabolites in the hypothalamus (419). Microperfusion of ANP into the anterior hypothalamus of conscious WKY, however, elicits an unexplained small rise in the concentration of norepinephrine metabolite MOPEG, whereas microperfusion of ANP into the anterior hypothalamus of the SHR induces a substantial fall in the concentration of MOPEG (451). Conversely, microinjections of norepinephrine into AV₃V region stimulates the release of ANP by stimulating cholinergic neurons to stimulate ANP neurons (8). Microinjection of carbachol into the AV₃V region increases the content of ANP in the hypothalamus and raises plasma ANP (20).

An infusion of either ANP or BNP into the third ventricle inhibits vasopressin secretion, and the effect is greater with BNP (685). Conversely, AVP stimulates ANP gene expression and secretion from hypothalamic neurons (348). There is also evidence that ANP inhibits the release of ACTH, prolactin, and growth hormone (397).

The central administration of ANP blocks some of the actions of centrally administered angiotensin II such as its effect on water intake, salt intake, and AVP release, although the rise in blood pressure is not always affected (9, 89, 414, 418, 601). The topical application of ANP on the SFO selectively depresses the excitatory action of angiotensin II but has no effect on the excitatory action of angiotensin II on the supraoptic nucleus (685).

In normal rats aldosterone, but not dexamethasone, increases ANP in the supraoptic, paraventricular, perifornical, and lateral hypothalamic nuclei. A high salt intake has a variable effect on hypothalamic content of ANP, decreasing it in one study (611) and increasing it in two others (282, 601). Volume loading raises and depletion lowers the concentration of ANP in the OVLT, paraventricular and medial preoptic nucleus, and the SFO (443).

The effect on the blood pressure of natriuretic peptide injected into the lateral ventricles of normal animals is not consistent. One group found that rat ANP [atriopeptin III and ANP-(5---28)] raised the blood pressure of conscious rats, whereas human ANP-(1---25) did not (691). This same group, however, found that microperfusion of the anterior hypothalamus of the conscious rat did not raise the blood pressure (451). Others (340, 512) using similar amounts of rat ANP injected into the lateral ventricles of the anesthetized and the conscious rat and the conscious sheep also obtained no significant change in blood pressure, but microinjection of 2-4 pmol ANF into the suprachiasmatic nucleus raised the blood pressure of the anesthetized rat (560).

The evidence in the SHR is also inconsistent. In contrast to the normal rat, microperfusion of ANP into the hypothalamus of the conscious SHR raises the blood pressure (451). Most investigators have found the content of immunoreactive ANP in the hypothalamus of the SHR to be raised at 4 wk, before the onset of hypertension, and at 8, 12, and 18 wk, when it is established (131, 272, 282, 317, 520). And microinjection of 0.055-0.55 µg of a blocking antibody to ANP into the anterior hypothalamus of the SHR lowers the blood pressure and decreases the local concentration of the major norepinephrine metabolite 3-methoxy-4-hydroxyphenylglycol (691). The ANP blocking antibody does not lower the blood pressure when injected into the posterior hypothalamus of the SHR or into the anterior hypothalamus of the WKY. Similarly, some investigators have found that the content of immunoreactive ANP is raised in the anterior part of the hypothalamus in the Dahl salt-sensitive rat (197, 585), the salt-loaded reduced renal mass rat (195), and the DOCA salt rat (196). The distribution of the increased content of ANP in the DOCA salt rat, however, was found to be different in that it is entirely focused on the OVLT and the SFO (196). In the reduced renal mass rat, the rise in ANP concentration in the hypothalamus was demonstrated to be independent of the blood pressure in that it occurred in rats in which the pressure was controlled with the angiotensin-converting enzyme inhibitor quinapril, which does not cross the blood-brain barrier in SHR (195, 520). However, it was reversed by lisinopril, which does cross the barrier, suggesting that the hypothalamic rise in ANP is in part due to local angiotensin II-related mechanisms.

In contrast, however, Bahner et al. (17) found a decrease in ANP in the SFO, the perifornical and periventricular hypothalamic nuclei and the medial preoptic nuclei, and the paraventricular and striae terminalis of the 4- and 12-wk-old SHR. In the median eminence, an increase in ANF occurred at 4 wk. The same authors reported that in both the Dahl salt-sensitive and salt-resistant rats a high salt intake induced a decrease in the content of ANP in the OVLT, the SFO, and the supraoptic nuclei and that in the salt-sensitive rat there was, in addition, a significant increase in ANP in the paraventricular nucleus (197).

There is also some disagreement on the content of ANP mRNA in the SHR. Komatsu et al. (317) found an increased content in the 17-wk-old SHR mainly localized to the anterior hypothalamic area, and the AV₃V area, including segments of the periventricular nucleus, the paraventricular nucleus, the suprachiasmatic nucleus, and the rostro-dorsal part of the ventromedial nucleus (317). But Chen et al. (103) found no difference in atrial peptide mRNA in anterior, posterior, or ventral hypothalamus between 10-wk-old SHR and WKY fed 1 or 8% salt diet for the preceding 3 wk (103).

Hypothalamic ANP receptors in the SHR are quantitatively and qualitatively different from ANP receptors in the WKY. There is a reduced number of receptors, particularly in the SFO, the paraventricular nucleus, and the choroid plexus (62, 221, 521, 522). In addition, the affinity of the ANP receptors in SHR is reduced, but they have an increased sensitivity to ANP (62) in that they generate larger quantities of cGMP (221). In contrast, in DOCA plus salt and Dahl salt-sensitive forms of hypertension there is an increase in ANP binding sites in the SFO (585, 669).

	VIII. VASOPRESSIN

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Circulating AVP is produced in the supraoptic nucleus and magnocellular neurons in the paraventricular nucleus. It is transported on its carrier neurophysin II to the posterior pituitary gland from where it is released into the circulation. Release is dependent on both osmotic and nonosmotic pathways (547, 650). Osmolality activates neurons throughout the anterior hypothalamus including the magnocellular neurons, the SFO, the OVLT and the median preoptic nucleus. Projections from the SFO, the OVLT, and the median preoptic nucleus activate both excitatory and inhibitory interneurons that project to the supraoptic and paraventricular nucleus. Nonosmotic stimuli such as vomiting and baroreceptor changes affect the medulla which, via the nucleus tractus solitarius and the area postrema, influence the supraoptic and paraventricular nucleus. In addition to hormonal AVP, there are AVP-containing neural pathways that stretch from the parvocellular area of the paraventricular nucleus and from the suprachiasmatic nucleus to the brain stem and to the spinal cord (114, 115, 390, 595) where the fibers make direct synaptic contact with sympathetic preganglionic neurons.

AVP receptors V1a, V1b, and V2 are G protein coupled. V1a and V1b receptors act through phosphatidylinositol hydrolysis, whereas V2 receptors are coupled to adenylate cyclase (402). There are V1 receptors in the suprachiasmatic (150) nucleus, the SFO, the OVLT, the arcuate nucleus, and astrocytes (291). Oxytocin receptors are found predominantly in the ventromedial nucleus (150), the SFO, and the OVLT (291). There is no evidence to suggest that there are any V2 receptors in the brain.

The intracerebroventricular administration of AVP to a normal rat reduces the content of norepinephrine in the anterior hypothalamus, the median eminence, and the arcuate nucleus (612). Conversely, the pressor effect of the central administration of an ₁-adrenergic agonist to a normal rat does not occur in a rat with congenital diabetes insipidus, suggesting that the pressor effect of central catecholamines in a normal rat is contingent on the intraneural presence of AVP (247).

In vitro, acetylcholine is a potent stimulus for AVP release from hypothalamo-neurohypophysial explants in organ culture from normal rats (574). In vivo, the central administration of acetylcholine or carbachol in the intact rat stimulates muscarinic receptors, possibly in the paraventricular nucleus, to increase AVP secretion (271). These findings support the evidence that AVP neurons are cholinergically innervated (561).

Intracerebroventricular injection of ANP reduces the concentration of plasma AVP (591) and prevents the hypertensive effect of an intracerebroventricular injection of AVP (584) (see below) in both the WKY and SHR.

The pressor effect of centrally administered angiotensin II may be in part due to the pressor effect of AVP (51).

Microinjection of V1 agonists into the lateral ventricle or third ventricle of conscious or anesthetized rats causes a brisk rise in blood pressure with marked increases in efferent splanchnic and renal nerve activity, whereas V2 agonists and oxytocin have no effect on the blood pressure (179, 510, 576, 584, 640, 705, 706). The hypertensive effect of the central administration of a V1 agonist also occurs in the anesthetized Brattleboro congenital diabetes insipidus rat, indicating that the rise in arterial pressure is a central effect and not due to the release of endogenous AVP into the circulation (473).

In contrast, microinjection of 5 pmol AVP into the SFO of the anesthetized rat lowers the blood pressure (576). This central depressor effect of AVP in the anesthetized rat appears to act by enhancing baroreflex-induced bradycardia and reducing cardiac output (401). A central depressor effect of AVP was first suspected when it was found that, although the rise in plasma AVP is the same whether AVP is administered via the inferior vena cava or the vertebral arteries, the rise in arterial pressure is less with the arterial infusion (356).

Overall the evidence suggests that in hypertension there is an increase in AVP release from the hypothalamus. Plasma AVP is raised in the SHR (123, 400), the Dahl salt-sensitive rat (389), essential hypertension (436), and in the DOCA plus salt rat (409). Urinary excretion of AVP is also raised in the SHR (123) and essential hypertension (311). In the SHR, the rise in plasma AVP is accompanied by a fall in the content of AVP (and oxytocin) in the hypothalamus (140, 186, 239, 337, 406, 407, 409, 573). In the 4-wk-old SHR AVP mRNA levels in the paraventricular and supraoptic nuclei are two- to threefold higher than in the WKY, and at 10 wk, the level is still 30-40% higher. At 24 wk the content of radioimmunoassayable AVP (and oxytocin) in the paraventricular nucleus is reduced (406). Oxytocin mRNA levels, however, in the SHR paraventricular and supraoptic nucleus at 4 wk are not different from the WKY, and at 10 wk they are ~10% lower than in WKY (646).

AVP release has also been measured in vitro from either a triangular slice of tissue from the basal hypothalamus which includes the supraoptic nucleus with its AVP axons and their extension to the median eminence and their termination in the attached neural lobe, or a punched out region (441) from the paraventricular and supraoptic nuclei (29, 408). At 4 wk, basal release of AVP and oxytocin is less in the SHR than in the WKY (29), but from 5 to 8 wk, although there is now no difference in basal release, which remains low, stimulated release of AVP with either acetylcholine (569) or potassium (140) is significantly greater in the SHR than in the WKY explants. At 18 wk this difference is no longer detectable. Hyperresponsiveness does not occur with an osmotic stimulus (572). There is one study (570) that has provided in vivo evidence that in young SHR the AVP system is hyperresponsive to at least one physiological stimulus. It was found that the rise in plasma AVP in response to a decrease in plasma volume is greater in the SHR (570). There is a similar increase in hypothalamic release of AVP in vivo in the DOCA plus salt rat which appears independent of the blood pressure and the salt intake (104).

It is not clear whether these changes in central AVP contribute to the rise in pressure. The development of stroke-prone SHR (SHRSP) homozygous for hypothalamic diabetes insipidus has yielded an animal in which AVP is not detectable in the plasma or the