Anatomical projections of the dorsomedial hypothalamus to the periaqueductal grey and their role in thermoregulation: a cautionary note.

The DMH is known to regulate brown adipose tissue (BAT) thermogenesis via projections to sympathetic premotor neurons in the raphe pallidus, but there is evidence that the periaqueductal gray (PAG) is also an important relay in the descending pathways regulating thermogenesis. The anatomical projections from the DMH to the PAG subdivisions and their function are largely elusive, and may differ per anterior-posterior level from bregma. We here aimed to investigate the anatomical projections from the DMH to the PAG along the entire anterior-posterior axis of the PAG, and to study the role of these projections in thermogenesis in Wistar rats. Anterograde channel rhodopsin viral tracing showed that the DMH projects especially to the dorsal and lateral PAG. Retrograde rabies viral tracing confirmed this, but also indicated that the PAG receives a diffuse input from the DMH and adjacent hypothalamic subregions. We aimed to study the role of the identified DMH to PAG projections in thermogenesis in conscious rats by specifically activating them using a combination of canine adenovirus-2 (CAV2Cre) and Cre-dependent designer receptor exclusively activated by designer drugs (DREADD) technology. Chemogenetic activation of DMH to PAG projections increased BAT temperature and core body temperature, but we cannot exclude the possibility that at least some thermogenic effects were mediated by adjacent hypothalamic subregions due to difficulties in specifically targeting the DMH and distinct subdivisions of the PAG because of diffuse virus expression. To conclude, our study shows the complexity of the anatomical and functional connection between the hypothalamus and the PAG, and some technical challenges in studying their connection.

Transcription factors in the development of medial hypothalamic structures.

The hypothalamus has historically been subdivided into nuclei, agglomerations of cell bodies that are visually distinct in histological sections. Regulatory functions of metabolism have been assigned to the various hypothalamic nuclei principally by analysis of animals with lesions of individual nuclei but also via various means of stimulation, such as cooling or heating probes. Biochemical and molecular specificity of these studies became possible with the identification and synthesis of neurotransmitters as well as the means to manipulate the expression of endogenous neurotransmitters and their receptors by genetic means . The arcuate nucleus (ARC) is likely to be the primary site for neurons that sense circulating fuels and energy reserves (POMC/CART neurons, NPY/AGRP neurons), whereas the paraventricular nucleus (PVN) receives input from the ARC and harbors many of the releasing factors (CRF, TRH, vasopressin, and oxytocin) that control pituitary hormone release. The ventromedial nucleus (VMN) receives input from the ARC and plays a critical role in energy balance in parallel with the ARC. The VMN and PVN also send descending projections to the autonomic nervous system and other pathways that control ingestive behavior and metabolism. Developmental analyses have revealed that the neurons that comprise the hypothalamic nuclei arise by differentiation and migration from stem cells within the ventricular zone. Based on recent work, it is becoming clear that coordination between numerous transcription factors that determine specification, survival, and migration is necessary for the formation of the hypothalamus, with each nucleus being determined by its own unique set of factors. In this minireview, we will provide a selective view of the roles that transcription factors play in the developing hypothalamus.

Connectivity pattern suggests that incerto-hypothalamic area belongs to the medial hypothalamic system.

The incerto-hypothalamic area (IHy) is a poorly defined diencephalic region located at the junction of the medial hypothalamus and zona incerta (ZI). This region is characterized by the presence of the A13 dopaminergic group and also cells expressing melanin-concentrating hormone (MCH) and cocaine- and amphetamine-regulated transcript (CART). The dopaminergic neurons appear to influence luteinizing hormone secretion, but the role of the MCH/CART-expressing cells is unclear. Even though IHy presents a singular neurochemistry, it has long been assumed that it is also part of the zona incerta. By injecting biotinylated dextran amine into the IHy and ZI of adult male Wistar rats, we analyzed the efferent projections from the IHy in comparison to the ZI. We have found that ZI projects mainly to laterally located brain stem structures, whereas the main efferents from the IHy are the reuniens thalamic nucleus, precommissural nucleus, posterior hypothalamic area and dorsolateral periaqueductal gray matter. The IHy projection pattern is quite similar to that of the anterior hypothalamic area and our hodological results suggest that IHy belongs to the medial hypothalamic system and might be part of the defensive behavior system. The IHy could be an integrative area associated with the regulation of neuroendocrine functions related to motivated behaviors, which are mediated by the medial hypothalamus.

Effect of estrogen on the lordosis-inhibiting action of ketanserin and SB 206553.

The effects of the 5-HT(2A/2C) receptor antagonist, ketanserin, and the 5-HT(2C) receptor antagonist, SB 206553, on lordosis behavior were investigated in ovariectomized rats hormonally primed with estradiol benzoate (EB) (0.5 or 25 microg) and progesterone (500 microg). Both ketanserin and SB 206553 inhibited lordosis behavior after infusion into the ventromedial nucleus of the hypothalamus (VMN), but ketanserin was slightly more effective than the 5-HT(2C) receptor antagonist. Either drug was more effective in rats primed with 0.5 microg EB than in rats hormonally primed with 25 microg EB. These findings support the suggestion that estrogen may enhance functioning of the 5-HT(2) receptor family and thereby protect against the 5-HT(2) receptor antagonists. These data are consistent with prior suggestions that estrogen modulates functioning of 5-HT(2) receptors within the VMN and that 5-HT(2) receptors play a facilitatory role in the modulation of female rat lordosis behavior.

Differential modulation of feline defensive rage behavior in the medial hypothalamus by 5-HT1A and 5-HT2 receptors.

Previous studies have established that the expression of defensive rage behavior in the cat is mediated over reciprocal pathways that link the medial hypothalamus and the dorsolateral quadrant of the midbrain periaqueductal gray matter (PAG). The present study was designed to determine the roles played by 5-HT(1A) and 5-HT(2C) receptors in the medial hypothalamus on the expression of defensive rage behavior elicited from electrical stimulation of the PAG. Monopolar stimulating electrodes were placed in the midbrain PAG from which defensive rage behavior could be elicited by electrical stimulation. During the course of this study, defensive rage was determined by measuring the latency of the "hissing" component of this behavior. Cannula-electrodes were implanted into sites within the medial hypothalamus from which defensive rage behavior could also be elicited by electrical stimulation in order that serotonergic compounds could be microinjected into behaviorally identifiable regions of the hypothalamus at a later time. Microinjections of the 5-HT(1A) receptor agonist 8-OHDPAT (0.1, 1.0 and 3.0 nmol) into the medial hypothalamus suppressed PAG-elicited hissing in a dose-dependent manner. Administration of the 5-HT(1A) antagonist p-MPPI (3.0 nmol) blocked the suppressive effects of 8-OHDPAT upon hissing. The suppressive effects of 8-OHDPAT were specific to defensive rage behavior because this drug (3 nmol) facilitated quiet biting attack. Microinjections of the 5-HT(2C) receptor agonist (+/-)-DOI hydrochloride into the medial hypothalamus (0.5, 1.0, and 3.0 nmol) facilitated the occurrence of PAG-elicited hissing in a dose-dependent manner. In turn, these facilitating effects were blocked by pretreatment with the selective 5-HT(2) antagonist, LY-53,857, which was microinjected into the same medial hypothalamic site. The findings of this study provide evidence that activation of 5-HT(1A) and 5-HT(2) receptors within the medial hypothalamus exert differential modulatory effects upon defensive rage behavior elicited from the midbrain PAG of the cat.

Biphasic role of dopamine on female sexual behaviour via D2 receptors in the mediobasal hypothalamus.

Dopamine has been implicated in the control of sexual behaviour, but its role seems quite complex and controversial. The aim of the present experiments was to investigate the effects of dopamine (DA) acting on D2 receptors in the mediobasal hypothalamus (MBH) on sexual behaviour in female sheep. To achieve this, the D2 agonist, quinpirole, was administered bilaterally via microdialysis probes into the MBH of ovariectomized ewes either before or after oestradiol (E2) administration. Quinpirole (100 ng/ml) infused for 6 h just before E2 hastened the onset of oestrus behaviour and the luteinizing hormone surge, whereas the same treatment given 6-12 h or 18-21 h after E2 decreased the intensity of sexual receptivity without affecting LH or prolactin secretion. We then tested the hypothesis that E2 stimulates the onset of oestrus partly by decreasing DA activation of D2 receptors. In this case the D2 antagonists pimozide or spiperone (100 ng/ml) were infused into the MBH via microdialysis probes for 11 h in the absence of E2 administration. A significant number of ewes showed induction of receptivity with both antagonists, although its intensity was significantly lower than that induced by E2. These treatments generally did not significantly alter extracellular concentrations of monoamines or aminoacids although quinpirole modulated the ability of sexual interactions to increase noradrenaline release. These experiments show that DA acts via D2 receptors in the MBH to control female sexual behaviour in a biphasic manner: the onset of sexual motivation and receptivity requiring an initial increase in activation followed by a decrease. This dual action could explain some of the controversies concerning DA action on sexual behaviour.

The medial hypothalamic defensive system: hodological organization and functional implications.

The hypothalamus is a relatively small division of the vertebrate forebrain that plays especially important roles in neural mechanisms assuring homeostasis, defense, and reproduction. Previous studies from our laboratory have suggested a distinct circuit in the medial hypothalamic zone as critically involved in the organization of innate defensive behavior. Thus, after exposure to a natural predator known to elicit innate defensive responses, increased Fos levels in the medial zone of the hypothalamus have been found restricted to the anterior hypothalamic nucleus, dorsomedial part of the ventromedial nucleus, and dorsal premammillary nucleus (PMd). Previous anatomical studies have shown that these Fos-responsive cell groups in the medial hypothalamus are interconnected in a distinct neural system, in which the PMd appears to be a critical element for the expression of defensive responses elicited by the presence of a predator. The purpose of this review is to provide an overview of what is currently known about the functional and hodological organization of this hypothalamic circuit subserving defensive responses.

Neuroendocrine regulation of sexually dimorphic brain structure and associated sexual behavior in male rats is genetically controlled.

Steroid hormones, particularly 17beta-estradiol (E2), regulate the development and expression of neural structures and sexual behavior. Recently, we demonstrated that E2-regulated responses are controlled by quantitative trait loci. In this study, we quantified 1) volume of the sexually dimorphic nucleus (SDN) of the preoptic area (POA); 2) medial basal hypothalamic (MBH)-POA aromatase and 5alpha-reductase enzyme activities during prenatal development and in adults; 3) serum LH, testosterone, FSH, E2, prolactin (PRL), and corticosterone levels; 4) reproductive organ (i.e., testis and ventral prostate) weights; and 5) male mating behavior in Noble (NB/Cr) and Wistar-Furth (WF/NCr) rat strains to determine the genetic influence on the measured parameters. Maximal phenotypic divergence in male SDN-POA volumes was seen between NB/Cr versus WF/NCr and BDIX/Cr rats (among nine rat strains initially examined), with the average SDN-POA volume of NB/Cr male rats being significantly greater ( approximately 30%) than that of either WF/NCr or BDIX/Cr males. Subsequent experiments investigated WF/NCr versus NB/Cr male rats in further detail. Significantly higher MBH-POA aromatase activity was seen in adult WF/NCr versus NB/Cr males, while MBH-POA 5alpha-reductase rates were not significantly different (within or between sex) for the two rat strains assayed. Serum LH levels were significantly higher (by greater than sixfold) in WF/NCr versus NB/Cr males, whereas testis organ:body weight and ventral prostate:body weight ratios in WF/NCr versus NB/Cr males were significantly smaller (by approximately 6-fold for testis and approximately 1.5-fold for prostate values). Serum FSH levels were significantly higher (by twofold) in WF/NCr versus NB/Cr males. However, serum testosterone levels were not significantly different, whereas E2 levels were approximately twofold higher (but not significantly different) in WF/NCr versus NB/Cr animals. No significant differences were found in basal (i.e., nonstress) serum PRL or corticosterone levels between the WF/NCr and NB/Cr males. In male copulatory tests, NB/Cr males exhibited significantly more aggressive sexual behavior (e.g., in mounting, intromission, and ejaculation parameters) compared with WF/NCr males. Taken together, these findings indicate that WF/NCr males are, in general, low responders, whereas NB/Cr males are high responders to hormonal signals. The obtained data suggest that the correlative, phenotypic variation in SDN-POA volume (i.e., structure) and reproductive hormone patterns and mating behavior (i.e., function) of WF/NCr versus NB/Cr males is regulated by potentially E2-mediated mechanisms that are genetically controlled.

Lateral evaginations from the third ventricle into the rat mediobasal hypothalamus: an amplification of the ventricular route.

In this work we report the existence of several evaginations extending out of the third ventricle within the mediobasal hypothalamus of the rat. In coronal sections, these evaginations appear as very narrow canaliculi integrating a canalicular system, which increases the contact surface between the ventricular lining and the nervous tissue. Consequently these evaginations enlarge the ventricular route for the transport of active principles present in the cerebrospinal fluid, such as (neuro)hormones and neurotransmitters. The mediobasal hypothalamus includes the arcuate nucleus and the median eminence (both involved in neuroendocrine mechanisms and in the regulation of pituitary function). A possible implication of our finding is that the neuroactive substance-containing ventricular cerebrospinal fluid may reach the intercellular spaces of the surrounding neuropil of the arcuate nucleus. According to literature these substances cross the ependyma of the lateral wall of the infundibular recess of the third ventricle. We suggest that such substances might also pass through the ependymal lining of the canalicular system, which displays the same ultrastructural characteristics as the rest of the ependyma of the lateral wall of the third ventricle. Therefore, the arcuate neurons may be influenced not only by synaptic inputs (afferent fibers) but also by non-synaptic diffusion neurotransmission (by means of neuroactive substances present in the cerebrospinal fluid).

Ibotenate lesion of the medial hypothalamus alters the salt intake and pressor responses to activation of the median preoptic nucleus in rats.

We investigated the influence of ibotenic acid lesions of the medial hypothalamus (MH) on salt appetite and arterial blood pressure responses induced by angiotensinergic and adrenergic stimulation of the median preoptic nucleus (MnPO) of rats. Previous injection of the adrenergic agonists norepinephrine, clonidine, phenylephrine, and isoproterenol into the MnPO of sham MH-lesioned rats caused no change in the sodium intake induced by ANG II. ANG II injected into the MnPO of MH-lesioned rats increased sodium intake compared with sham-lesioned rats. Previous injection of clonidine and isoproterenol increased, whereas phenylephrine abolished the salt intake induced by ANG II into the MnPO of MH-lesioned rats. Previous injection of norepinephrine and clonidine into the MnPO of sham MH-lesioned rats caused no change in the mean arterial pressure (MAP) induced by ANG II. Under the same conditions, previous injection of phenylephrine increased, whereas isoproterenol reversed the increase in MAP induced by angiotensin II (ANG II). ANG II injected into the MnPO of MH-lesioned rats induce a decrease in MAP compared with sham-lesioned rats. Previous injection of phenylephrine or norepinephrine into the MnPO of MH-lesioned rats induced a negative MAP, whereas pretreatment with clonidine or isoproterenol increased the MAP produced by ANG II injected into the MnPO of sham- or MH-lesioned rats. These data show that ibotenic acid lesion of the MH increases the sodium intake and pressor responses induced by the concomitant angiotensinergic, alpha 2 and beta adrenergic activation of the MnPO, whereas alpha 1 activation may have opposite effects. MH involvement in excitatory and inhibitory mechanisms related to sodium intake and MAP control is suggested.

Efferent projections from the ovarian steroid receptor-containing area of the ventrolateral hypothalamus in female guinea pigs.

The ventrolateral hypothalamus (VLH) in female guinea pigs includes a subset of neurons which contain estrogen and progestin receptors, and which are implicated in the regulation of female sexual behavior by steroid hormones. However, little is known about where these neurons project, and consequently which other brain areas are involved in sexual behavior in female guinea pigs. The anterograde tracer Phaseolus vulgaris-Leucoagglutinin was used to label efferents from the ovarian steroid receptor-containing part of the VLH. To identify the correct placement of the tracer specifically within the group of neurons containing estrogen receptors, medial hypothalamic sections were also immunostained for estrogen receptors. Forebrain areas receiving dense projections from the ventrolateral hypothalamus included the bed nucleus of the stria terminalis, medial preoptic area, anterior hypothalamic area, anterior ventromedial hypothalamus, and caudal ventrolateral hypothalamus. The midbrain central gray was also heavily labeled. Moderate innervation was observed in the forebrain in the basolateral amygdala, medial preoptic nucleus, lateroanterior hypothalamic nucleus, dorsal hypothalamic areas, posterior hypothalamus, zona incerta, and in the midbrain interspersed among the central and lateral tegmental tracts. The major efferent pathways from the VLH appeared to travel rostrally through the mediobasal hypothalamus and preoptic area, and caudally via the medial thalamic nuclei and periventricular fiber system. These findings are similar to those of previous studies tracing the efferents from the ventromedial nucleus in rats and from the lateral hypothalamus in guinea pigs. Many of these areas that receive input from the steroid receptor rich area within the VLH are likely to be involved in the regulation of female sexual behavior.

Ipsilateral alterations in tryptophan hydroxylase activity in rat brain after hypothalamic 5,7-di-hydroxytryptamine lesion.

The in vivo relationship between the amounts of tryptophan hydroxylase (TPH) protein and its intrinsic synthetic activity, measured by quantifying the amounts of alpha-[3H]methyl-5-hydroxytryptamine (alpha-[3H]M5-HT), is reported in cell body and terminal areas of intact and disturbed serotonergic neurons following a unilateral 5,7-dihydroxytryptamine (5,7-DHT) lesion of the dorsolateral hypothalamus. Five days after the lesion, the relationships between TPH and its synthetic product 5-HT were evaluated on adjacent brain sections in serotonergic cells bodies of the dorsal raphe nucleus (DRN) and nerve fibres of the medial forebrain bundle (MFB). On the side contralateral to the lesion, TPH and alpha-[3H]M5-HT levels in the intact hemi-DRN exhibited a caudo-rostral distribution and were positively and significantly correlated (p < or = 0.001); the calculated TPH-specific activity was 0.76 nCi of alpha-[3H]M5-HT formed per U TPH. In the MFB, quantitative measurements of TPH and alpha-[3H]M5-HT showed no correlation between enzyme and product and no specific activity for TPH could be determined. On the side ipsilateral to the lesion, the density of TPH-immunoreactive fibers was drastically decreased in the dorsolateral hypothalamus where a significant reduction in TPH content (45.5% of control side, P < 0.001) was found. In the overall ipsilateral hemi-DRN, TPH and alpha-[3H]M5-HT levels, their correlation as well as TPH-specific activity were unaltered by the lesion but a significant increase in alpha-[3H]M5-HT and TPH contents was observed in the lateral wings of the DRN. The lesion also induced a significant increase in alpha-[3H]M5-HT and TPH levels (136% and 93.8%, P < 0.001, respectively) in the ipsilateral MFB, which resulted in a positive and significant correlation between these two markers and yielded a TPH-specific activity of 1.0 nCi of alpha-[3H]M5-HT formed per U TPH. TPH topological area was also significantly increased in the lateral aspect of the ipsilateral MFB 5 days post lesion. These results show that 5-HT synthesis in the intact DRN is proportional to and dependent on TPH activity while in the MFB, 5-HT accumulation appears unrelated to TPH content which is most likely in an inactive enzymatic form. Moreover, the data show that a local disruption of serotonergic terminals in the dorsolateral hypothalamus does not affect 5-HT synthesis in the overall ipsilateral DRN neurons but results in local activation of TPH within the serotonergic projection neurons and the ipsilateral MFB, as evidenced by active de novo synthesis of 5-HT. Altogether the results point to circumscribed activation of compensatory mechanisms in 5-HT synthesis after selective destruction of serotonergic terminals.

Medial amygdaloid suppression of predatory attack behavior in the cat: I Role of a substance P pathway from the medial amygdala to the medial hypothalamus.

The medial amygdala is known to powerfully suppress predatory attack behavior in the cat, but the mechanisms underlying such modulation remain unknown. The present study tested the hypothesis that medial amygdaloid suppression of predatory attack is mediated, in part, by a pathway from the medial amygdala to the medial hypothalamus which utilizes substance P as a neurotransmitter. Stimulating electrodes were implanted into the medial amygdala and cannula electrodes were implanted into both the medial and lateral hypothalamus. Predatory attack behavior was elicited by electrical stimulation of the lateral hypothalamus. In the first phase of the study, paired trials compared attack latencies of single stimulation of the lateral hypothalamus with those following dual stimulation of the lateral hypothalamus and medial amygdala. Attack latencies were significantly elevated following dual stimulation of the medial amygdala and lateral hypothalamus. In the second phase of the study, dose and time dependent decreases in response suppression were noted following the infusion of the substance P (NK1) receptor antagonist, CP96.345 (in doses of 0.05, 0.5 and 2.5 nmol) into the medial hypothalamus. In third phase of the study, the effects of microinjections of the substance P receptor agonist, [Sar9.Met(O2)11]-substance P (in doses of 0.5, 1.0 and 2.0 nmol), directly into the medial hypothalamus upon lateral hypothalamically elicited predatory attack behavior were determined. Microinfusion of this drug elevated attack response latencies in a dose- and time-dependent manner. In addition, pretreatment with CP96,345 into the medial hypothalamus blocked the suppressive effects of subsequent delivery of [Sar9,Met(O2)11]-substance P into the same medial hypothalamic site. Other parts of the study demonstrated the presence of: (1) high densities of substance P receptors in the ventromedial hypothalamus, and (2) neurons that are positively labeled for substance P that project from the medial amygdala to the ventromedial hypothalamus as demonstrated by retrograde labeling with Fluoro-Gold. These findings provide support for the hypothesis that medial amygdaloid suppression of lateral hypothalamically elicited predatory attack behavior includes a substance P pathway from the medial amygdala to the medial hypothalamus. The findings further suggest that stimulation of the medial amygdala activates substance P receptors in the medial hypothalamus, thus triggering an inhibitory mechanism from the medial to the lateral hypothalamus, resulting in suppression of predatory attack behavior.

Comparative morphology of the pituitary gland in Australian flying foxes (Megachiroptera: genus Pteropus).

BACKGROUND: Investigations of reproductive endocrinology of flying foxes (genus Pteropus) have been hampered by inadequate information on the normal morphology of the megachiropteran pituitary. METHODS: The novel technique of graphical three-dimensional (3-D) reconstruction, supported by more traditional anatomical techniques, have now been used to examine the shapes of, the interrelations between, the lobes of the pituitary of the little red flying fox, Pteropus scapulatus. Statistical analysis of data from three species tested whether there were changes in pituitary size with annual cycles in function, particularly with key stages of reproduction. RESULTS: In the three species of Australian flying foxes examined, the hypophyseal cleft is closed; the pars intermedia extends over the rostral, ventral, and lateral surfaces of the neural lobe. The pars distalis is broad rostrally and extends over two-thirds of the lateral and ventral pars intermedia. The hypophyseal recess is broad at the median eminence, then narrows and extends through the infundibulum to, but not into, the neural lobe. In adult animals the pituitary weight was 10.0 +/- 0.3 mg (mean +/- s.e.) in P. scapulatus, 14.7 +/- 0.5 mg in Pteropus poliocephalus (greyheaded flying foxes), and 18.7 +/- 1.5 mg in Pteropus alecto (black flying foxes). Pituitary weight was not significantly affected by reproductive stage. CONCLUSIONS: Thus histologically, the adenohypophysis and neurohypophysis are similar to those of other mammals. Comparative differences in pituitary size reflected differences in species body size rather than cyclical reproduction.

Sexual differences in vasopressin receptor binding within the ventrolateral hypothalamus in golden hamsters.

In the following studies, the presence of a sexual difference in arginine-vasopressin (AVP) receptor binding was tested within the ventrolateral hypothalamus (VLH), an area rich in gonadal steroid receptors. The density of AVP receptor binding was estimated by in vitro quantitative autoradiography within the entire rostro-caudal extent of the VLH. The density of AVP binding was higher in males than in females at all levels of this area. Furthermore, dependency on testosterone treatment was also compared between gonadectomized males and females. While gonadectomy resulted in a near total disappearance of binding in both males and females, testosterone treatment resulted in equally high levels of binding in both sexes. Indeed, a high density of AVP receptor binding was observed at all levels of the VLH in both testosterone-treated males and females. These results show that adult female golden hamsters are equally capable as males of expressing high levels of AVP receptor binding in the VLH in response to high levels of testosterone. Together, our results suggest that, while AVP receptor binding within the VLH is sexually different in gonadally-intact animals, these differences are not related to differential responsiveness to testosterone, but rather to a differential production and availability of the hormone.

Localization and developmental pattern of vasoactive intestinal polypeptide binding sites in the human hypothalamus.

Using a quantitative in vitro autoradiographic approach, vasoactive intestinal polypeptide (VIP) binding site densities were compared in the post-mortem hypothalamus of human neonate/infant and adult. The densities were similar during development in most of the hypothalamic nuclei and areas examined underlying the stability of 125I-VIP binding sites in the post-mortem hypothalamus of young and adult individuals. However, the ventral part of the medial preoptic area, the medial, lateral, and supramammillary nuclei were characterized by an increase of 125I-VIP binding with age. In young and adult individuals, the highest densities of hypothalamic 125I-VIP binding sites were detected in the supraoptic and infundibular nuclei; the ependyma; the organum vasculosum of the lamina terminalis; the horizontal limb of the diagonal band of Broca; the ventral part of the medial preoptic area (in adult); the suprachiasmatic, paraventricular, and periventricular nuclei; and the medial and lateral mammillary nuclei in adult. Moderate densities were found in the vertical limb of the diagonal band of Broca, the bed nucleus of the stria terminalis, the ventral part of the medial preoptic area in neonate/infant, the medial and lateral mammillary nuclei in neonate/infant, the supramammillary nucleus in adult, the dorsal hypothalamic area, and the ventromedial nucleus. Low to moderate binding site densities were observed in the other hypothalamic regions of young or adult individuals. The nonspecific binding ranged from 15% of the total binding in the anterior hypothalamus to 20% in the mediobasal and posterior hypothalamic levels. Taken together, these results provide evidence for a large distribution of VIP binding sites in neonate/infant and adult human hypothalamus suggesting the implication of VIP in the development of this brain structure and the maintenance of its various functions.

In vitro GnRH release from the isolated medial basal hypothalamus of the male guinea pig: evidence for the existence of two pools of releasable GnRH.

We studied in vitro GnRH output from the isolated medial basal hypothalamus (MBH) of the adult intact male guinea pig. Both basal release and the response to veratridine (V), which exerts a depolarizing action by increasing Na+ permeability, were assessed using a static incubation system. Spontaneous GnRH release from single MBHs was pulsatile, with mean pulse frequency ('Ultra' program; 3 CV threshold) of 0.86 +/- 0.17 pulse/h (range: 0.17-1.75) for 7.5 min fractions collected over a 240 to 360 min observation period (n = 10). Exposure to 20 microM V for a 15-60 min period resulted in increased GnRH output characterized by a consistently biphasic pattern of release, with an immediate response, and a secondary increase usually observed with a delay of 30 to 60 min relative to the introduction of V. This characteristic pattern of response was seen for tested V concentrations ranging between 0.1 and 50 microM. The Na+ channel blocker tetrodotoxin (TTX, 3 microM), the calcium chelator EGTA (20 mM) and the calcium channel blocker verapamil (1 mM) significantly suppressed basal GnRH release and effectively prevented the GnRH response to V when added simultaneously with V. The secondary response to V could not be blocked selectively by tetrodotoxin introduced following V or from the last 7.5 min of v exposure onwards. In contrast, this secondary response could be selectively suppressed with either EGTA or verapamil. We conclude that the secondary component of the GnRH response to V is the result of delayed, calcium dependent secretory events triggered by the initial depolarizing action of V. These findings indicate the presence of two distinct pools of releasable GnRH in the MBH of the adult male guinea pig. The relevance of this finding to the pulsatile mode of GnRH release remains to be established. When, for purposes of comparison, isolated MBHs from 50-day-old male rats were studied, a clearly different, monophasic pattern of response to V was observed. Whether this observation is related to the known differences in distribution of hypothalamic GnRH between the rat and the guinea pig, still needs to be elucidated.

Neural connections of the suprachiasmatic nucleus and medial hypothalamus of the Syrian hamster (Mesocricetus auratus).

The neural connections of brain areas involved in the control of circadian rhythms were investigated in the Syrian hamster by observing the anterograde and retrograde neural labelling resulting from injections of HRP-WGA into the suprachiasmatic nucleus (SCN) and subparaventricular zone (SPZ). The neural connections of these 2 brain areas in the strongly photoperiodic hamster are very similar to those in the more weakly photoperiodic rat. Two differences were observed: (1) in the rat, but not in the hamster, the SCN projects to the ventral geniculate nucleus and intergeniculate leaflet of the thalamus; and (2) reciprocal contralateral neural connections appear to be more numerous in the hamster than in the rat SCN. These differences may relate to the differences in photoperiodic sensitivity of the 2 species.

Injecting 5-HT into the PVN does not prevent feeding induced by injecting 8-OH-DPAT into the raphe.

The selective 5-hydroxytryptamine1A (5-HT1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) activates raphe somatodendritic autoreceptors, leading to an inhibition of 5-HT neuronal activity and reduced synthesis and release of 5-HT in forebrain terminal areas. One behavioural consequence of this is increased feeding in satiated rats. Because injections of 5-HT agonists into the medial hypothalamus suppress feeding, it has been proposed that 8-OH-DPAT-induced feeding may involve a reduction of 5-HT release within this area. This hypothesis was tested by examining the ability of 5-HT injected into the medial hypothalamus to reverse the feeding-stimulant action of 8-OH-DPAT following injection into the dorsal raphe or median raphe. Two groups of rats, maintained with free access to food at all times, were used. Each was prepared with two cannulae, one aimed at the paraventricular nucleus (PVN) of the medial hypothalamus and the other at either the dorsal raphe nucleus or median raphe nucleus. Food intake over the next hour was increased following dorsal raphe or median raphe injections of 8-OH-DPAT (1 and 0.5 microgram, respectively). These effects were not blocked by injections of 7.5 or 15 micrograms 5-HT into the PVN. However, 15 micrograms 5-HT did attenuate the feeding-stimulant action of 10 micrograms norepinephrine injected into the PVN. These results do not support the hypothesis that a reduction in 5-HT release within the medial hypothalamus is responsible for the feeding-stimulant action of 8-OH-DPAT.

Short-day effects of melatonin on luteinizing hormone secretion in the ewe: evidence for central sites of action in the mediobasal hypothalamus.

Experiments were designed to localize the central sites of action of melatonin involved in the control of seasonal reproduction. Ewes were exposed to long days and received microimplants of melatonin in the preoptic area (n = 9), anterior hypothalamus (n = 4), dorsolateral hypothalamus (n = 4), or mediobasal hypothalamus (n = 12). The effects of implants were determined by comparison with control ewes (untreated or sham-operated, n = 10) and with ewes treated with an s.c. implant of melatonin (n = 8) or ewes subjected to short days (n = 8). All ewes were ovariectomized and treated s.c. with a silastic capsule of estradiol. Melatonin released in the preoptic area as well as in the anterior and lateral hypothalamus did not cause any difference as compared with the controls (no stimulation of LH secretion and no inhibition of prolactin secretion). In contrast, melatonin implanted in the mediobasal hypothalamus caused an increase in LH secretion in 7 of the 12 ewes on Day 53.0 +/- 4.2 after implantation (mean +/- SEM). Their response was not different compared with that of ewes treated s.c. with melatonin or exposed to short days either in terms of timing (Day 56.3 +/- 6.2 and 59.5 +/- 3.1, respectively, for controls) or of amplitude of the LH response. Similarly, melatonin caused only a reduction of prolactin secretion in the mediobasal, s.c., and short-day groups. It is concluded that the mediobasal hypothalamus or the surrounding tissues could be the sites of action of melatonin involved in the control of seasonal reproduction.