Direct projections from the central amygdaloid nucleus to the hypothalamic paraventricular nucleus: possible role in stress-induced adrenocorticotropin release

The amygdala, particularly the central amygdaloid nucleus, is important for the expression of adrenocorticotropin and corticosterone responses during stress. The aim of the present study was to determine if the central amygdaloid nucleus directly innervated the hypothalamic paraventricular nucleus. To accomplish this aim, the Phaseolus vulgaris leucoagglutinin lectin anterograde tracing method was used. Injections of the tracer into the medial central amygdaloid nucleus resulted in axonal and terminal labeling within the medial and lateral parvocellular parts of the caudal paraventricular nucleus. A dense patch of labeling was observed within the lateral wing of the lateral part of the parvocellular paraventricular nucleus. Only a few labeled axons were observed within the paraventricular nucleus of animals that had lectin injections localized to the lateral part of the central nucleus. Tracer injections localized to the medial amygdaloid nucleus resulted in axonal and terminal labeling primarily within the anterior parvocellular and periventricular regions of the paraventricular hypothalamic nucleus. Sparse to moderate axonal and terminal labeling was observed within the magnocellular parts of the paraventricular nucleus in animals that had injections of tracer into either the medial central nucleus or the medial nucleus. No labeling was observed within the paraventricular nucleus of animals that had injections of lectin within other amygdaloid nuclei or adjacent regions of the striatum. The results demonstrated a topographically organized projection from the amygdala to the hypothalamic paraventricular nucleus. The central nucleus mainly innervates the caudal lateral and medial parvocellular paraventricular nucleus. The medial nucleus innervates the rostral parvocellular parts of the paraventricular nucleus. These pathways could form the anatomical substrates of amygdaloid modulation of neuroendocrine responses to stressors.     

The hypothalamic paraventricular nucleus has a pivotal role in regulation of prolactin release in lactating rats

The affect of paraventricular nucleus (PVN) lesions on PRL secretory response to suckling was studied in adult female rats. Basal levels of PRL were similar in the control and lesioned groups. Substantial decreases in PRL levels occurred after separation of pups from their mothers in the control as well as lesioned animals. When mothers and pups were reunited, the circulating PRL concentrations of the control groups rose immediately from basal values of 50-100 micrograms/liter to reach peaks of 450-550 micrograms/liter. PVN lesions significantly decreased the suckling-induced rise of PRL levels. Furthermore, PVN lesions abolished the high amplitude, episodic pattern of PRL release in continuously lactating rats. These findings are consistent with the view that PVN neurons produce PRL releasing factor(s), which is (are) required for normal secretory patterns of PRL in lactating rats.     

Effect of stress and adrenalectomy on urocortin II mRNA expression in the hypothalamic paraventricular nucleus of the rat

Urocortin II (Ucn II) is a novel corticotropin-releasing hormone (CRH)-related peptide discovered as a selective agonist for type-2 CRH receptor. In the rat or mouse brain, Ucn II mRNA shows weak expression mainly in the hypothalamic paraventricular nucleus (PVN) and the locus coeruleus (LC). Understanding the regulation of Ucn II mRNA expression under varying conditions provides new insights into central stress response. We examined expression of Ucn II mRNA in the PVN and LC following immobilization stress, water deprivation, and adrenalectomy. Rats subjected to immobilization stress exhibited a dramatic induction of Ucn II mRNA expression in the parvocellular part of the PVN at the end of 2 h of immobilization. In contrast, water deprivation for 3 days induced Ucn II mRNA expression mainly in the magnocellular part of the PVN. Although water-deprived rats showed a marked decrease in their food intake, pair-fed rats failed to alter PVN Ucn II mRNA expression, suggesting that osmotic stimuli per se, but not reduced food consumption during water deprivation, caused Ucn II mRNA induction in the magnocellular part of the PVN. Adrenalectomized rats failed to show an increase in Ucn II mRNA in the PVN when compared to sham-operated rats. Double-label in situ hybridization revealed colocalization of Ucn II mRNA in approximately 45% of the CRH mRNA-expressing cells in the parvocellular part of the PVN following immobilization, or colocalization in most of the vasopressin mRNA-expressing cells in the magnocellular part of the PVN following water deprivation. In the LC, no induction of Ucn II mRNA was observed in any of the three experimental conditions, indicating that the regulation of Ucn II mRNA expression was site-specific. The results show a stressor-specific regulation of Ucn II mRNA expression in the PVN and raise the possibility that Ucn II mRNA plays a modulatory role in stress-induced alteration of anterior and posterior pituitary function, depending on the type of stress.     

Vasopressin-containing neurons of the hypothalamic parvocellular paraventricular nucleus of the jerboa: plasticity related to immobilization stress

The corticotropin-releasing hormone (CRH) neurons of the hypothalamic parvocellular paraventricular nucleus (PVN) have a high potential for phenotypical plasticity, allowing them to rapidly modify their neuroendocrine output, depending upon the type of stressors. Indeed, these neurons coexpress other neuropeptides, such as cholecystokinin (CCK), vasopressin (VP), and neurotensin, subserving an eventual complementary function to CRH in the regulation of the pituitary. Unlike in rats, our previous data showed that in jerboas, CCK is not coexpressed within CRH neurons in control as well as stressed animals. The present study explored an eventual VP participation in the phenotypic plasticity of CRH neurons in the jerboa. We analyzed the VP expression within the PVN by immunocytochemistry in male jerboas submitted to acute stress. Our results showed that, contrary to CRH and CCK, no significant change concerned the number of VP-immunoreactive neurons following a 30-min immobilization. The VP/CRH coexpression within PVN and median eminence was investigated by double immunocytochemistry. In control as well as stressed animals, the CRH-immunopositive neurons coexpressed VP within cell bodies and terminals. No significant difference in the number of VP/CRH double-labeled cells was found between both groups. However, such coexpression was quantitatively more important into the posterior PVN as compared with the anterior PVN. This suggests an eventual autocrine/paracrine or endocrine role for jerboa parvocellular VP which is not correlated with acute immobilization stress. VP-immunoreactive neurons also coexpressed CCK within PVN and median eminence of control and stressed jerboas. Such coexpression was more important into the anterior PVN as compared with the posterior PVN. These results showed the occurrence of at least two VP neuronal populations within the jerboa PVN. In addition, the VP expression did not depend upon acute immobilization stress. These data highlight differences in the neuroendocrine regulatory mechanisms of the stress response involving CRH/CCK or VP. They also underline that adaptative physiological mechanisms to stress might vary from one mammal species to another.     

Differential effects of refeeding on melanocortin-responsive neurons in the hypothalamic paraventricular nucleus

To explore the effect of refeeding on recovery of TRH gene expression in the hypothalamic paraventricular nucleus (PVN) and its correlation with the feeding-related neuropeptides in the arcuate nucleus (ARC), c-fos immunoreactivity (IR) in the PVN and ARC 2 h after refeeding and hypothalamic TRH, neuropeptide Y (NPY) and agouti-related protein (AGRP) mRNA levels 4, 12, and 24 h after refeeding were studied in Sprague-Dawley rats subjected to prolonged fasting. Despite rapid reactivation of proopiomelanocortin neurons by refeeding as demonstrated by c-fos IR in ARC alpha-MSH-IR neurons and ventral parvocellular subdivision PVN neurons, c-fos IR was present in only 9.7 +/- 1.1% hypophysiotropic TRH neurons. Serum TSH levels remained suppressed 4 and 12 h after the start of refeeding, returning to fed levels after 24 h. Fasting reduced TRH mRNA compared with fed animals, and similar to TSH, remained suppressed at 4 and 12 h after refeeding, returning toward normal at 24 h. AGRP and NPY gene expression in the ARC were markedly elevated in fasting rats, AGRP mRNA returning to baseline levels 12 h after refeeding and NPY mRNA remaining persistently elevated even at 24 h. These data raise the possibility that refeeding-induced activation of melanocortin signaling exerts differential actions on its target neurons in the PVN, an early action directed at neurons that may be involved in satiety, and a later action on hypophysiotropic TRH neurons involved in energy expenditure, potentially mediated by sustained elevations in AGRP and NPY. This response may be an important homeostatic mechanism to allow replenishment of depleted energy stores associated with fasting.     

Neuropeptide-Y-immunoreactive innervation of thyrotropin-releasing hormone-synthesizing neurons in the rat hypothalamic paraventricular nucleus

The association of neuropeptide-Y (NPY)-immunoreactive (IR) axon terminals with TRH-synthesizing neurons in the rat hypothalamic paraventricular nucleus (PVN) has been studied. Immunocytochemical single and double labeling studies were performed at both light and electron microscopic levels using antiserum to NPY and, as a marker of TRH-containing neurons, antisera recognizing the N-terminal flanking peptides of the TRH prohormone, prepro-TRH-(25-50) and prepro-TRH-(53-74). At the light microscopic level, a diffuse group of TRH-IR cell bodies were observed in the anterior parvocellular subdivision of the PVN and became more numerous and densely clustered in the medial and periventricular parvocellular subdivisions. NPY-IR fibers were observed to innervate all subdivisions of the PVN, but were particularly dense in the anterior, medial, and periventricular parvocellular subdivisions of the nucleus, where they appeared to contact TRH-synthesizing perikarya and neuronal processes. At the ultrastructural level, numerous NPY-IR axon terminals containing labeled vesicles were either tightly juxtaposed to TRH-producing neurons or seen to establish both symmetric and asymmetric synaptic contacts with TRH-containing cell bodies and dendrites. Some NPY-IR axon terminals also established synaptic contacts with unlabeled PVN perikarya and processes or were found in close apposition to blood vessels. These data provide a morphological basis to suggest NPY-mediated neuroendocrine regulation over the biosynthesis and/or secretion of TRH in the PVN. Reports of the colocalization of NPY and catecholamines in the same axon terminals raises the possibility of a potential interaction between NPY and catecholamines to influence TRH neurons in the PVN. Morphological evidence for synaptic interactions between NPY-IR axon terminals and non-TRH-containing neurons in the PVN further suggests that this peptide may influence other neuroendocrine systems.     

Glutamatergic inputs in the hypothalamic paraventricular nucleus maintain sympathetic vasomotor tone in hypertension

The paraventricular nucleus (PVN) of the hypothalamus is critical to the regulation of sympathetic output. The PVN hyperactivity is known to cause increased sympathetic nerve activity in spontaneously hypertensive rats (SHRs). The purpose of this study was to determine whether glutamatergic input to the PVN contributes to heightened sympathetic outflow in hypertension. Lumbar sympathetic nerve activity, mean arterial blood pressure, and heart rate were recorded from anesthetized SHRs and Wistar-Kyoto (WKY) rats. Bilateral microinjection of an N-methyl-D-aspartate receptor antagonist, 2-amino-5-phosphonopentanoic acid, or a non-N-methyl-D-aspartate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, into the PVN dose-dependently decreased lumbar sympathetic nerve activity, mean arterial blood pressure, and heart rate in SHRs but not in WKY rats. Bilateral microinjection of kynurenic acid into the PVN also significantly decreased lumbar sympathetic nerve activity, mean arterial blood pressure, and heart rate in SHRs but not in WKY rats. Furthermore, microinjection of gabazine, a specific GABA(A) receptor antagonist, into the PVN increased lumbar sympathetic nerve activity, mean arterial blood pressure, and heart rate in both SHRs and WKY rats. Notably, this response was significantly attenuated in SHRs compared with that in WKY rats. In addition, kynurenic acid abolished the sympathoexcitatory and pressor responses to microinjection of gabazine into the PVN in both SHRs and WKY rats. Thus, this study provides new functional evidence that resting sympathetic vasomotor tone is maintained by tonic glutamatergic input in the PVN in SHRs. Removal of GABAergic inhibition results in augmented glutamatergic input in the PVN, which probably constitutes an important source of excitatory drive to the brain stem vasomotor neurons in hypertension.     

Noradrenergic and dopaminergic activity in the hypothalamic paraventricular nucleus after naloxone-induced morphine withdrawal

Previous research has shown an increase in hypothalamo-pituitary-adrenal axis activity following naloxone administration to morphine-dependent rats. In the present study, we investigated the adaptive changes in the noradrenaline (NA) and dopamine (DA) systems in the hypothalamic paraventricular nucleus (PVN) during morphine dependence and withdrawal. Additionally, we examined the possible change in 3',5'-cyclic adenosine monophosphate (cAMP) levels in that nucleus under the same conditions. Rats were made dependent on morphine by morphine or placebo (na?ve) pellet implantation for 7 days. On day 8, rat groups received an acute injection of saline or naloxone (1 mg/kg subcutaneously) and were decapitated 30 min later. NA and DA content as well as their metabolite production in the PVN were estimated by HPLC/ED. Both plasma corticosterone levels and cAMP concentration in the PVN were measured by RIA. Naloxone administration to morphine-dependent rats (withdrawal) induced a pronounced increase in the production of both the NA metabolite MHPG and the DA metabolite DOPAC and an enhanced NA and DA turnover. Furthermore, an increase in corticosterone secretion was observed in parallel to the changes in catecholamine turnover. However, no alterations in cAMP levels were seen during morphine withdrawal. These results raise the possibility that catecholaminergic afferents to the PVN could play a significant role in the alterations of PVN functions and consequently in the pituitary-adrenal response during morphine abstinence syndrome. These data provide further support for the idea of adaptive changes in catecholaminergic neurons projecting to the PVN during chronic morphine exposure.     

Involvement of brainstem catecholaminergic inputs to the hypothalamic paraventricular nucleus in estrogen receptor alpha expression in this nucleus during different stress conditions in female rats

In the present study, we determined the involvement of brainstem catecholaminergic inputs to the paraventricular nucleus (PVN) on estrogen receptor alpha (ERalpha) expression in this nucleus during conditions of 48-h fasting, 2-deoxy-d-glucose (2DG)-induced acute glucoprivation and 1-h immobilization, in ovariectomized rats. Our approach was to examine the effect of lesioning catecholaminergic inputs to the PVN using DSAP [saporin-conjugated anti-DBH (dopamine-beta-hydroxylase)]. Bilateral injection of DSAP into the PVN, 2 wk before stress, prevented fasting-, glucoprivation-, and immobilization-induced increase in ERalpha-immunopositive cells in the PVN. The DBH-immunoreactive (ir) terminals in the PVN were severely depleted by DSAP injection in all experimental groups. Among the brainstem noradreneregic cell groups examined, DBH-ir cell bodies were significantly reduced in the A2 region of all experimental groups treated with DSAP compared with the saporin- and vehicle-injected controls. PVN DSAP injection caused a small, but not significant, decrease in A1 DBH-ir cell bodies in fasted and immobilized rats, and a significant, but slight, reduction in A1 DBH-ir cell bodies of iv 2DG- injected rats compared with PVN vehicle-injected or PVN saporin-injected controls. The A6 DBH-ir cell bodies in all experimental groups treated with DSAP, saporin, or vehicle did not show any significant difference. These results suggest that the brainstem catecholaminergic inputs to the PVN, especially from the A2 cell group, may play a major role in mediating the induction of ERalpha expression in the PVN by metabolic stressors such as fasting, acute glucoprivation, and less specific stressors, such as immobilization, in female rats.     

Stimulation of central and systemic oxytocin release by histamine in the paraventricular hypothalamic nucleus: evidence for an interaction with norepinephrine

Central histaminergic neurons have been implicated in the control of oxytocin (OT) secretion in various physiological conditions, including parturition and lactation. The present studies investigated whether histamine also influences the central intranuclear release of OT, which is known to be important in the activation of OT neurons, and the possible interaction of histamine with norepinephrine in systemic and central OT release. Microdialysis probes were placed immediately adjacent to the hypothalamic paraventricular nucleus (PVN) and used for administration of artificial cerebrospinal fluid (ACSF) vehicle, ACSF containing histamine, ACSF containing histamine in combination with a specific H1 or H2 histamine receptor antagonist, or ACSF containing histamine and the alpha-adrenergic antagonist phentolamine. Dialysates and plasma were collected, and OT concentrations were determined using RIA. Dialysis of the PVN with ACSF containing histamine significantly increased the release of OT systemically and centrally within the PVN. Furthermore, the increases in OT concentration in dialysates and plasma were prevented by simultaneous administration of chlorpheniramine (an H1 receptor antagonist) or ranitidine (an H2 receptor antagonist) as well as by the adrenergic antagonist phentolamine. These data demonstrate that histamine acts within the PVN to increase both systemic and intranuclear release of OT. Furthermore, the increased OT release induced by histamine is dependent upon stimulation of both H1 and H2 histaminergic receptors and subsequent activation of alpha-noradrenergic receptors. These findings suggest that histamine induces systemic and intranuclear OT release by stimulating the release of norepinephrine.     

Noradrenergic afferents facilitate the activity of tuberoinfundibular neurons of the hypothalamic paraventricular nucleus

The role of ascending noradrenergic projections of medullary origin in regulating the activity of tuberoinfundibular neurons of the hypothalamic paraventricular nucleus (PVN) was examined in pentobarbital-anesthetized male Sprague-Dawley rats. Discrete electrical stimulation of either the A1 or the A2 noradrenaline cell group areas of the caudal medulla enhanced the probability of firing in a substantial proportion of antidromically identified tuberoinfundibular PVN cells tested. Notably, no inhibitory effects were observed. Destruction of the PVN noradrenergic terminal plexus by local application of the neurotoxin 6-hydroxydopamine 1 day prior to electrophysiological experiments abolished the effects of both A1 and A2 stimulation. These findings indicate that noradrenergic afferents can exert a facilitatory influence on the activity of a population of tuberoinfundibular PVN neurons, thus supporting earlier suggestions that central noradrenergic structures can enhance the release of certain anterior pituitary hormones.     

Neuronal cell bodies in the hypothalamic paraventricular nucleus mediate stress-induced renin and corticosterone secretion

The present studies were undertaken to determine the involvement of neurons in the hypothalamic paraventricular nucleus (PVN) in stress-induced renin secretion. The stressor was a 10-min conditioned emotional response (CER) paradigm. Bilateral electrolytic lesions in the PVN prevented the stress-induced increase in plasma renin activity (PRA), and plasma renin concentration (PRC). Stress-induced corticosterone secretion was also blocked, supporting the histological verification and suggesting that the lesion included corticosterone-releasing factor neurons in the PVN. Stress-induced renin secretion appears to be restricted to the PVN, as electrolytic lesions in the nucleus reuniens, dorsal and caudal to the PVN, did not prevent the stress-induced increase in either PRA or PRC. The next step was to determine whether cell bodies in the PVN or fibers of passage through the PVN mediate the stress-induced increase of these hormones. For this purpose, bilateral stereotaxic injections of the cell-selective neurotoxin ibotenic acid (10 micrograms/microliter; 0.3 microliters per side) were performed 14 days prior to the stress procedure. Histological evaluation of the tissue revealed cell death and lysis in the PVN. Ibotenic acid injection into the PVN prevented the effect of stress on PRA, PRC and corticosterone levels. None of the lesions prevented the stress-induced rise in plasma prolactin concentration. These results suggest that neurons in the PVN play an important role in mediating stress-induced increases in renin and corticosterone but not prolactin secretion.     

Role of cholecystokinin in corticotropin release: coexistence with vasopressin and corticotropin-releasing factor in cells of the rat hypothalamic paraventricular nucleus.

Cholecystokinin-8 (CCK8)-containing cell bodies in the parvocellular region of the rat paraventricular nucleus (PVN) contain vasopressin and corticotropin-releasing factor (CRF). The CCK8 and vasopressin in these cells can readily be visualized in adrenalectomized, but not in shamoperated animals. Furthermore, CCK8 levels as measured by RIA change in the PVN and in the median eminence in response to adrenalectomy. CCK8 has a stimulatory effect on corticotropin (ACTH) release from primary cultures of the anterior pituitary. This stimulation is additive with that produced by vasopressin; CCK8 plus vasopressin have an effect as great as CRF in stimulating ACTH release. Our results suggest that CCK8 may participate in the regulation of ACTH release under certain physiological conditions.     

11beta-hydroxysteroid dehydrogenase type 2 activity in hypothalamic paraventricular nucleus modulates sympathetic excitation

Aldosterone stimulates the sympathetic nervous system by binding to a select population of brain mineralocorticoid receptors (MR). These MR have an equal affinity for corticosterone that is present in substantially higher concentrations, but are held in reserve for aldosterone by activity of the enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD-2), which converts corticosterone to an inactive metabolite. Thus, colocalization of MR and 11beta-HSD-2 activity may help identify brain regions that mediate the effects of aldosterone. The present studies tested the hypothesis that 11beta-HSD-2 activity regulates MR-mediated responses in the paraventricular nucleus (PVN) of the hypothalamus, a forebrain region implicated in sympathetic regulation. Real-time-polymerase chain reaction revealed the presence of 11beta-HSD-2 mRNA in PVN. In anesthetized adult male Sprague-Dawley rats, microinjection of the 11beta-HSD-2 inhibitor carbenoxolone (CBX) into PVN increased mean arterial pressure, heart rate, and renal sympathetic nerve activity. Intracerebroventricular injections of CBX excited PVN neurons and increased mean arterial pressure, heart rate, and renal sympathetic nerve activity. The ability of CBX to increase sympathetic activity by inhibiting 11beta-HSD-2, thereby permitting corticosterone to activate MR, was confirmed by the following: Intracerebroventricular glycyrrhizic acid, another 11beta-HSD-2 inhibitor, mimicked the sympathoexcitatory effects of CBX; the sympathoexcitatory effects of CBX were blocked by spironolactone, a MR antagonist. Neither CBX nor glycyrrhizic acid elicited a response in adrenalectomized rats. These findings suggest that MR in PVN contribute to sympathetic regulation and may be activated by aldosterone or corticosterone (or cortisol in humans) depending on the state of 11beta-HSD-2 activity.     

L-dopa stimulates release of hypothalamic growth hormone-releasing hormone in humans

A sensitive RIA for human GH-releasing hormone-(1-44)-NH2 [hGHRH-(1-44)-NH2] was developed which allows its measurement in human plasma extracts. The assay did not detect hGHRH-(1-37)-OH or hGHRH-(1-40)-OH. A method to extract hGHRH from plasma was developed using silicic acid and acid-acetone, by which recovery of synthetic hGHRH-(1-44)-NH2 from plasma averaged 74.3%. Serial dilutions of plasma extracts gave an inhibition curve parallel with that of synthetic hGHRH-(1-44)-NH2 in the RIA system. On Sephadex G-50 columns, hGHRH-like immunoreactivity (hGHRH-LI) in plasma extracts eluted as a single peak corresponding to hGHRH-(1-44)-NH2. This hGHRH-LI peak, when subjected to reverse phase HPLC, emerged at the position where hGHRH-(1-44)-NH2 was eluted. hGHRH-LI was detectable in the peripherally circulating plasma of all subjects tested. The mean basal level of plasma hGHRH-LI in normal subjects was 9.4 +/- 0.7 (+/- SE) pg/ml (n = 22; range, 2.8-18.1 pg/ml), comparable to the basal plasma hGHRH-LI concentration in patients with hypothalamic lesions (11.3 +/- 1.1 pg/ml; n = 7). Oral administration of L-dopa (0.5 g) caused a significant increase in both plasma hGHRH-LI and GH levels in normal subjects, and the plasma hGHRH-LI peak slightly preceded or coincided with that of plasma GH in individual subjects. There was also a significant correlation between plasma hGHRH-LI and the GH rises after L-dopa administration when their net increments were compared. All of the patients with hypothalamic lesions had significant increases in plasma GH after hGHRH-(1-44)-NH2 injection (1 microgram/kg BW, iv), indicating the presence of functioning somatotrophs in their pituitaries. When L-dopa was orally administered to these patients, neither plasma hGHRH-LI nor GH concentration changed throughout a 120-min observation period. These findings suggest that 1) hGHRH, immunologically and chromatographically indistinguishable from synthetic hGHRH-(1-44)-NH2, is detectable in peripheral plasma in humans; 2) L-dopa stimulates the release of hypothalamic hGHRH, alterations of which are reflected in changes in peripheral levels; and 3) the source of circulating hGHRH is not restricted to the hypothalamus, since hGHRH-LI is present in the peripheral plasma of patients with hypothalamic lesions in amounts similar to those found in normal subjects.     

Inhibition of the hypothalamic paraventricular nucleus in spontaneously hypertensive rats dramatically reduces sympathetic vasomotor tone

Experimental evidence indicates that the hypothalamic paraventricular nucleus modulates sympathetic vasomotor tone and blood pressure and that this modulation is altered in some cardiovascular diseases. This study tested the hypothesis that this nucleus exerts a more significant tonic excitatory modulation of basal sympathetic vasomotor activity in spontaneously hypertensive rats. In anesthetized, artificially-ventilated rats, bilateral microinjections of the GABA(A) receptor agonist, muscimol (1 to 1.5 nmoles per side), into the paraventricular nucleus produced a depressor and sympathoinhibitory response that did not recover. When compared with normotensive rats, this response was more marked in spontaneously hypertensive rats, where lumbar sympathetic nerve discharge was reduced by 75 +/- 3% and mean arterial pressure fell from 119 +/- 7 mm Hg to 58 +/- 3 mm Hg. Blockade of excitatory and inhibitory amino acid receptors in the rostral ventrolateral medulla significantly attenuated this response. Microinjections of small volumes (<20 nL) of GABA were used to localize precisely the responsive region of the paraventricular nucleus. Unilateral injections of GABA into the dorsomedial cap of the paraventricular nucleus induced a brisk depressor (decrease of 42 +/- 4 mm Hg), sympathoinhibitory (decrease by 72 +/- 2%), and bradycardic (decrease of 77 +/- 16 bpm) response. The mechanisms underlying the sympathoinhibition after inactivation of the paraventricular nucleus are not elucidated, but evidence discussed suggests the involvement of a supracollicular sympathoinhibitory pathway. The results presented demonstrate that the paraventricular nucleus exerts a powerful, tonic effect on the control of sympathetic vasomotor tone under basal conditions in anesthetized rats and that this is enhanced in spontaneously hypertensive rats.     

Triiodothyronine exerts direct cell-specific regulation of thyrotropin-releasing hormone gene expression in the hypothalamic paraventricular nucleus

Thyroid hormone administered systemically exerts negative feedback control of biosynthesis of the TRH pro-hormone in the hypothalamic paraventricular nucleus (PVN), the origin of neurons that regulate anterior pituitary TSH secretion, but not in any other group of TRH-synthesizing neurons in the brain. To determine whether this response is mediated by direct effects on PVN neurons, we studied the effect of unilateral stereotaxic implants of L-T3 into the anterior hypothalamus on the concentration of pro-TRH mRNA and pro-TRH in the PVN of hypothyroid rats. Because hypothalamic-pituitary-thyroid function is also regulated by central catecholamines, we also determined the effect of unilateral ablation of ascending catecholaminergic fibers to one side of the PVN by stereotaxic injection of 6-hydroxydopamine or transection of ascending catecholaminergic pathways. T3-implanted hypothyroid animals showed a marked reduction in pro-TRH mRNA and immunoreactive pro-TRH in medial parvocellular neurons of the PVN on the same side as the implant, but not in contralateral PVN neurons or TRH-synthesizing neurons in other hypothalamic regions. In contrast, hypothyroid animals implanted with pellets of hormonally inactive 3,5-diiodo-L-thyronine showed intense symmetric hybridization and immunoreaction product in both wings of the PVN. Despite marked unilateral reduction in the catecholamine innervation to the PVN, no reduction in pro-TRH mRNA or immunoreactive pro-TRH was observed in the PVN on the affected side compared to that on the unaffected side. These studies demonstrate that negative feedback regulation of thyroid hormone occurs directly on TRH neurons and is restricted only to those in the PVN tuberoinfundibular system.     

Agouti-related protein containing nerve terminals innervate thyrotropin-releasing hormone neurons in the hypothalamic paraventricular nucleus.

Gene expression for agouti-related protein (AGRP), an endogenous antagonist of melanocortin receptors, has been localized to the hypothalamic arcuate nucleus, where it colocalizes with neuropeptide Y (NPY). Having reported that the NPY innervation of hypophysiotropic TRH neurons in the hypothalamic paraventricular nucleus (PVN) originates primarily from NPY-producing neurons in the arcuate nucleus, here we examined the possibility that TRH neurons in the PVN are similarly innervated by AGRP nerve terminals. Using immunohistochemistry, AGRP-containing cell bodies were found almost exclusively in the arcuate nucleus, but their projections were distributed widely in the hypothalamus, most conspicuously in the paraventricular (PVN), arcuate and dorsomedial nuclei, and the posterior hypothalamic area. Ablation of the arcuate nucleus by the neonatal administration of monosodium glutamate obliterated nearly all AGRP-immunoreactivity in the hypothalamus. In the PVN, double-labeling light and electron microscopic immunohistochemistry revealed that TRH neurons receive dense innervation by AGRP nerve terminals, with the frequent occurrence of axosomatic and axodendritic synapses (mainly of the symmetrical type). These findings provide morphological basis to hypothesize a role for AGRP in the arcuato-paraventricular pathway, in the down-regulation of the hypothalamic-pituitary-thyroid axis, which occurs as an adaptive response to starvation.