Immunohistochemical studies of intrahypothalamic somatostatin-containing neurons in rat

Intrahypothalamic somatostatin-containing neurons were investigated immunohistochemically. In intact rats, immunoreactive cell bodies appeared in the rostral periventricular area, and immunoreactive beaded fibers were observed to terminate in the median eminence and to form delicate networks surrounding immunonegative cell bodies within the medial preoptic, suprachiasmatic, arcuate, ventromedial and premammillary nuclei. Intraventricular colchicine infusion resulted in the appearance of immunoreactive cell bodies in the arcuate, ventromedial and suprachiasmatic nuclei, and an increase in the number of cell bodies seen in the periventricular area. Complete deafferentation of the medial-basal hypothalamus excluding the rostral periventricular area caused the immunoreactive structures in the median eminence to disappear and enhanced the staining of periventricular cell bodies. In the arcuate and ventromedial nuclei, the immunoreactive fiber networks were left intact and the immunoreactive cell bodies were occasionally recognized. Horizontal knife cut between the arcuate nuclei and median eminence did not alter immunoreactivity in either region. Neonatal administration of MSG caused only the disappearance of arcuate nuclei. The results indicate that two kinds of somatostatin neuronal systems exist in rat hypothalamus: one is involved in the production of hormonal somatostatin and the other serves for the regulation of neuronal activities in restricted hypothalamic nuclei.     

Biochemical mapping of somatostatinergic systems in rat brain: Effects of periventricular hypothalamic and medial basal amygdaloid lesions on somatostatin-like immunoreactivity in discrete brain nuclei

Immunohistochemical studies have demonstrated the presence of somatostatin (growth hormone release-inhibiting factor)-positive cell bodies in the periventricular nucleus of the hypothalamus and in the medial-basal amygdala. In order to map biochemically the projections of these cell groups, electrolytic lesions were made in these structures and somatostatin was measured by radioimmunoassay in microdissected brain nuclei. Bilateral destruction of the periventricular nucleus significantly decreased somatostatin-like immunoreactivity (SLI) in the median eminence, and in the rostral periventricular, medial preoptic and arcuate nuclei. Bilateral lesions placed in the medial-basal amygdala significantly decreased SLI in the median eminence and suprachiasmatic nucleus. Similar depletions were observed following lesions of the stria terminalis. These results suggest that both the periventricular and amygdaloid somatostatin systems may participate in the regulation of growth hormone secretion via their projections to the median eminence and other hypothalamic nuclei.     

Efferents from medial basal forebrain and hypothalamus in the rat. II. An autoradiographic study of the anterior hypothalamus.

Using tritiated amino acid autoradiography, the efferent projections of the anterior hypothalamic area (AHA) were studied in albino rats. Axons from AHA neurons were not confined to local projections in the hypothalamus. Ascending AHA axons ran through the preoptic region, joined the diagonal band and distributed in the lateral septum. Descending AHA efferents within the hypothalamus coursed in a bundle ventromedial to the fornix. Projections were observed to the dorsomedial, ventromedial, arcuate and dorsal premammillary nuclei, and to the median eminence. Sweeping dorsomedially in the posterior hypothalamus, some AHA axons distributed in the central grey. AHA axons staying ventral projected to the supramammillary region, ventral tegmental area, raphe nuclei and midbrain reticular formation. Other AHA efferents distributed to the periventricular thalamus, to the medial amygdala via the stria terminalis or supraoptic commissure, and to the lateral habenula through the stria medullaris. For comparison with the AHA, efferent projections from the paraventricular nucleus (PVN) and from the ventromedial nucleus and adjacent basal hypothalamus (VMR) were studied. Projections from PVN neurons were not restricted to the median eminence and neurohypophysis. PVN efferents also distributed to many of the same regions as did those of the AHA but had somewhat different fiber trajectories and longer descending projections. VMR efferents were more widespread than those of the AHA, with projections extending into the lateral zona incerta and pontine reticular formation. Projections from the AHA were distinct from those of the medial preoptic area (mPOA). For example, while AHA axons descended in a bundle ventromedial to the fornix, mPOA axons ran in the medial forebrain bundle. Such anatomical differences may underlie experimentally demonstrated functional differences between the mPOA and AHA, for instance, in mediation of male and female sex behaviors.     

Efferents from medial basal forebrain and hypothalamus in the rat. I. An autoradiographic study of the medial preoptic area.

Efferent projections from the medial and periventricular preoptic area, bed nucleus of the stria terminalis and nuclei of the diagonal band were traced using tritiated amino acid autoradiography in albino rats. Medial and periventricular preoptic area efferents were not restricted to short-axon projections. Ascending projections from the medial preoptic area (mPOA) were traced through the diagonal band into the septum. Descending mPOA axons coursed in the medial parts of the medial forebrain bundle. Projections to most hypothalamic nuclei, including the arcuate nucleus and median eminence, were observed. In the midbrain, mPOA efferents were distributed in the central grey, raphe nuclei, ventral tegmental area and reticular formation. Projections from the mPOA were also observed to the amygdala through the stria terminalis, to the lateral habenula through the stria medullaris, and to the periventricular thalamus. Axons of the most medial and periventricular preoptic area (pvPOA) neurons had a distribution similar to more lateral mPOA neurons but their longest-axoned projections were weaker. The pvPOA did not send axons through the stria medullaris but did project more heavily than the more lateral mPOA to the arcuate nucleus and median eminence. Projections from the bed nucleus of the stria terminalis (nST) were in most respects similar to those from the medial preoptic area, with the major addition of a projection to the accessory olfactory bulb. The nuclei of the diagonal band of Broca (nDBB) gave a different pattern of projections than mPOA or nST, projecting, for instance, to the medial septum and hippocampus. Descending nDBB efferents ran in the ventral portion of the medial forebrain bundle. Among hypothalamic cell groups, only the medial mammillary nuclei received nDBB projections. nDBB efferents also distributed in the medial and lateral habenular nuclei and the mediodorsal thalamic nucleus.     

A subset of progesterone target neurons have axonal projections to the midbrain

Progestin-concentrating neurons in the preoptic area and hypothalamus that project to the midbrain in the female rat were identified using the combined steroid hormone autoradiography-retrograde axonal tracing technique. Progesterone target neurons were most abundant in the periventricular preoptic area and the medial preoptic nucleus, and in the ventromedial and arcuate nuclei of the hypothalamus. In the medial preoptic area as a whole, about 14% of the progestin-concentrating cells were afferent to the midbrain. More specifically, 23% of medial preoptic nucleus progesterone target neurons communicated directly with midbrain cell groups, whereas a much smaller percentage (2%) of periventricular preoptic target neurons projected to the midbrain. In the medial basal hypothalamus as a whole, 11% of the progestin-concentrating cells detected sent axons to the midbrain. This proportion was slightly higher in the ventromedial nucleus (15%), and much lower in the arcuate nucleus (3%). In the dorsal and lateral hypothalamic areas, close to 30% of the progesterone target neurons sent axons to the midbrain, although the total number and density of target cells in the two latter areas was low. These data support the idea that transduction by forebrain target neurons of the progesterone signal into altered synaptic transmission in the midbrain is one route through which progesterone can influence a variety of behaviors.     

Efferents of the medial preoptic area in the guinea pig: An autoradiographic study

Medial preoptic axons were traced into the diagonal band of Broca and septum, particularly lateral septum. Other labeled fibers could be followed dorsally from medial preoptic area injections adjacent to the stria medullaris, and in the periventricular fiber system and the stria terminalis and its bed nucleus. The anterior and medial amygdaloid nuclei were labeled by fibers via the stria terminalis and others arching over the optic tract and through the substantia innominata. The lateral habenula was labeled. Labeled periventricular fibers reached the periventricular nucleus of the thalamus. Descending efferents were traced principally below the fornix and in the adjacent lateral hypothalamus to label the anterior hypothalamus, the tuberal nuclei, and median eminence. Axons of the medial preoptic area joined the medial part of the medial forebrain bundle and distributed to the reticular formation and the central gray of the midbrain and pons. A small amount of contralateral connections were described.     

The hypothalamic ‘tuberoinfundibular’ system of the rat as demonstrated by horseradish peroxidase (HRP) microiontophoresis

Microiontophoresis of horseradish peroxidase (20%) into the median eminence of the rat has allowed visualization of perikarya and axon projections of the tuberoinfundubular system after retrograde transport. Cells projecting to the median eminence were found in the periventricular regions of the hypothalamus and were particularly pronounced in dorsal portions of the rostral arcuate nucleus, the medial division of the paraventricular nucleus, and within the anterior periventricular nucleus. Labeling of perikarya within the ventromedial nucleus was rarely found. No labeling by HRP was found within cells of the dorsomedial, anterior, suprachiasmatic, preoptic, lateral hypothalamic nuclei or within the septal and amygdaloid nuclei. Axons from identifiable cells were located within the periventricular neuropil and contained within the baso-lateral portions of the hypothalamic-hypophysial tract.     

The efferent connections of the suprachiasmatic nucleus of the hypothalamus

The efferent connections of the suprachiasmatic nucleus of the hypothalamus have been studied in the rat by the injection of ³H-proline into the nucleus and the surrounding regions of the rostral hypothalamus, and by the injection of the enzyme marker, horseradish peroxidase, into the region of the ventromedial hypothalamic nucleus. After an injection of ³H-proline confined to the ventral portion of the suprachiasmatic nucleus, transported label can be followed, in the autoradiographs, dorsally and caudally in the periventricular area as far as the caudal end of the ventromedial nucleus, into the triangular area between this nucleus and the arcuate nucleus, and along the ventral aspect of the tuberal region, just lateral to the ventromedial nucleus. A small number of silver grains are also seen over the internal lamina of the median eminence. No label can be followed rostrally or immediately lateral to the nucleus. Comparable injections into adjoining regions of the hypothalamus (especially the anterior hypothalamic area, the medial preoptic area, and the retrochiasmatic region) show transported label over the same regions, but with a somewhat different pattern of grain distribution; in addition, the anterior hypothalamic area shows an extensive projection through the medial forebrain bundle to the mammillary and supramammillary nuclei, the midbrain tegmentum, and certain of the midline thalamic nuclei. Although it is difficult in our autoradiographs to distinguish between the course of the efferent fibers from the suprachiasmatic nucleus and the zones in which they terminate, our evidence favors a termination among the cells of the periventricular area, and upon dendrites of the cells in the ventromedial, dorsomedial and arcuate nuclei, which extend beyond the limits of the nuclei into the periventricular area and to the area beneath the ventromedial nucleus.     

Distribution of neuropeptide Y-like immunoreactivity in the hypothalamus of the adult golden hamster

The distribution of neuropeptide Y (NPY)-like immunoreactivity within the hypothalamus of the adult golden hamster was investigated with conventional immunohistochemical techniques. Neuropeptide Y immunoreactive cell bodies were found in greatest numbers in the arcuate nucleus while a few stained perikarya were seen in the internal and subependymal zones of the median eminence. Isolated perikarya were observed in the anterior commissure and supracommissural portion of the interstitial nucleus of the stria terminalis. Immunoreactive axons were located throughout the hypothalamus with the highest concentrations in the subependymal and internal zones of the median eminence, the interstitial nucleus of the stria terminalis, the medial preoptic area, and in the following nuclei: periventricular, suprachiasmatic, paraventricular, perifornical, median preoptic, and arcuate. Moderate to dense plexuses of immunoreactive fibers were observed in the anterior, lateral, and posterior hypothalamic areas and in the infundibular stalk. The supraoptic nucleus and lateral preoptic area displayed a small number of labeled axons whereas the ventromedial nucleus contained only a few fibers. NPY immunoreactive fibers were present in the optic tract and in the dorsomedial aspect of the optic chiasm. Labeled fibers penetrated the ependymal lining of the third ventricle throughout the ventral aspect of the periventricular zone. Additional fibers were observed in the pia lining the ventral aspect of the hypothalamus. This systematic analysis of hypothalamic NPY immunoreactivity in the adult golden hamster suggests that a portion of the labeled fibers display a distribution that is similar to previously described noradrenergic fibers in the hypothalamus.     

Coexpression of dynorphin and neurokinin B immunoreactivity in the rat hypothalamus: Morphologic evidence of interrelated function within the arcuate nucleus

Considerable evidence suggests that dynorphin and neurokinin B (NKB) neurons in the hypothalamic arcuate nucleus participate in the sex-steroid regulation of reproduction. In the present study, we used dual-label immunofluorescence to explore the distribution of prodynorphin and proNKB immunoreactivity in the rat hypothalamus. Additionally, we investigated whether arcuate prodynorphin-ir (immunoreactive) neurons expressed the neurokinin 3 receptor (NK3R) or nuclear estrogen receptor-alpha (ERalpha). We found that the majority of prodynorphin-ir neurons in the rat arcuate nucleus expressed proNKB, whereas nearly all (99%) of the proNKB neurons were immunoreactive for prodynorphin. The arcuate nucleus was the only site in the hypothalamus where neuronal somata coexpressing prodynorphin and proNKB-immunoreactivity were identified. A dense plexus of double-labeled prodynorphin/proNKB-ir fibers was found within the arcuate nucleus extending to the median eminence and throughout the periventricular zone of the hypothalamus. Prodynorphin/proNKB fibers were also identified in the paraventricular nucleus, anterior hypothalamic area, medial preoptic area, median preoptic nucleus, anteroventral periventricular nucleus, and bed nucleus of the stria terminalis in a distribution consistent with previously described arcuate nucleus projections. Interestingly, the majority of prodynorphin-ir neurons in the arcuate nucleus expressed NK3R, and nearly 100% of the prodynorphin-ir neurons contained nuclear ERalpha. Our results suggest that there is a close functional relationship between dynorphin and NKB peptides within the arcuate nucleus of the rat, which may include an autofeedback loop mediated through NK3R. The diverse hypothalamic projections of fibers expressing both prodynorphin and proNKB provide evidence that these neurons may participate in a variety of homeostatic and neuroendocrine processes.     

Immunohistochemical localization of avian pancreatic polypeptide-like immunoreactivity in the rat hypothalamus

The distribution of avian pancreatic polypeptide-like (APP) immunoreactivity within the rat hypothalamus was investigated with the indirect immunoperoxidase method. APP immunoreactive perikarya are found in largest numbers in the retrochiasmatic area, the arcuate nucleus, and the supracommissural portion of the interstitial nucleus of the stria terminalis. Small clusters of immunoreactive neurons are also consistently observed in the ventral aspect of the medial preoptic area and lateral hypothalamic area, immediately dorsolateral to the optic chiasm and tracts. These neurons are apparent in all animals but are more intensely strained and occur in larger numbers following colchicine pretreatment. Other immunoreactive neurons are visible only in colchine-treated rats and are scattered throughout the anterior and lateral hypothalamic areas and the supramammillary nucleus. Immunoreactive axons and terminal fields present an extensive and highly characteristic distribution throughout the hypothalamus, which in many instances exhibits differential distribution within specific subfields of hypothalamic nuclei and areas. The heaviest concentrations of APP immunoreactive axons are present in the periventricular nucleus throughout the rostrocaudal extent of the hypothalamus, the ventrolateral portion of the suprachiasmatic nucleus, the retrochiasmatic area, the parvocellular paraventricular nucleus, the ventral supraoptic nucleus, the perifornical nucleus, the ventral dorsomedial nucleus, and the arcuate nucleus. Moderate plexuses of immunoreactive fibers are also present in the medial preoptic area, the anterior and lateral hypothalamic areas, the nucleus circularis, the median eminence, and the ventral premammillary area. Other areas, such as the ventromedial nucleus, contain virtually no immunoreactive axons but are encapsulated by a dense plexus of immunoreactive terminals. The distribution of a major component of APP immunoreactive fibers exhibits a marked similarity to that of previously described norepinephrine-containing hypothalamic afferents. Other groups of APP immunoreactive perikarya and fibers appear to represent components of intrinsic diencephalic systems.     

The distribution of growth-hormone-releasing factor (GRF) immunoreactivity in the central nervous system of the rat: an immunohistochemical study using antisera directed against rat hypothalamic GRF

Immunohistochemical methods have been used to chart the distribution of rat hypothalamic growth-hormone-releasing factor (rhGRF) immunoreactivity in the brains of normal and colchicine-treated adult albino rats. The results suggest the existence of at least two distinct rhGRF-containing systems: one responsible for delivery of the peptide to portal vessels in the median eminence, and one whose relationship, if any, to hypophysiotropic function is less direct. A dense plexus of rhGRF-stained fibers was found throughout the external lamina of the median eminence that is the route by which the peptide is delivered to the anterior pituitary. This projection appears to arise primarily from a group of rhGRF-immunoreactive neurons centered in the arcuate nucleus. Some 1,000-1,500 rhGRF-positive neurons were counted on each side of the brain in rats pretreated with colchicine. Colocalization studies, using a sequential double staining technique, indicated that a subset of rhGRF-immunoreactive neurons in the arcuate region contain neurotensin immunoreactivity. No evidence was obtained for colocalization of rhGRF with either of two pro-opiomelanocortin-derived peptides (alpha-melanocyte-stimulating hormone, adrenocorticotropic hormone (1-24)) in individual neurons in the arcuate nucleus. Much smaller groups of neurons were localized in the parvicellular division of the paraventricular nucleus of the hypothalamus and in the dorsomedial nucleus, and it is unclear whether they contribute to the plexus of rhGRF-stained fibers in the median eminence. The only other region in the rat brain in which rhGRF-stained cells were found reliably was in the area that roughly encapsulates the caudal aspect of the ventromedial nucleus of the hypothalamus. Because cells in this region are not known to project to the median eminence, they may be assumed to contribute to the extrahypophysiotropic rhGRF-stained projections outlined below. From the level of the arcuate and ventromedial nuclei, rhGRF-immunoreactive fibers could be traced along the base of the brain and through the periventricular system to discrete terminal fields limited almost exclusively to the hypothalamus and adjoining parts of the basal telencephalon. All parts of the periventricular region of the hypothalamus receive an input, including the preoptic and anterior parts in which somatostatin-containing neurons that project to the median eminence are clustered. Other prominent terminal fields were localized in discrete parts of the dorsomedial, paraventricular, suprachiasmatic, and premammillary nuclei, and in the medial preoptic and lateral hypothalamic areas.(ABSTRACT TRUNCATED AT 400 WORDS)     

Distribution of pituitary adenylate cyclase activating polypeptide (PACAP) immunoreactivity in the hypothalamus and extended amygdala of the rat

Pituitary adenylate cyclase activating polypeptide (PACAP) is found in two forms of 27 and 38 amino acids (PACAP-27 and PACAP-38 respectively) in the mammalian central nervous system. Using antibodies to these two forms of PACAP, we examined the distribution of PACAP immunoreactivity in the rat hypothalamus and a number of extrahypothalamic areas. The patterns of immunostaining for PACAP-27 and PACAP-38 were similar: prominent terminal labelling was present in the retrochiasmatic area, median eminence, and posterior periventricular nucleus of the hypothalamus as well as the bed nucleus of the stria terminalis and amygdaloid complex. After colchicine treatment, immunopositive cell bodies were found in the preoptic region of the periventricular zone of the hypothalamus, the suprachiasmatic and paraventricular hypothalamic nuclei, neural structures adjacent to the median eminence (including the retrochiasmatic area, arcuate nucleus, ventromedial hypothalamus, and tuber cinereum), and the lateral mammillary and supramammillary nuclei. In all these areas, immunolabelling appeared specific since it was abolished by preabsorption of primary antisera with the appropriate PACAP peptide. However, the number of immunopositive cells in the suprachiasmatic nucleus was also reduced by preabsorption of PACAP-27/38 antisera with vasoactive intestinal polypeptide, suggesting that a subpopulation of cells in the suprachiasmatic nucleus express a peptide which has significant sequence homology with both PACAP-27/38 and vasoactive intestinal polypeptide. The distribution of PACAP immunoreactivity throughout the hypothalamus, bed nucleus of the stria terminalis, and amygdala suggests the involvement of PACAP in a number of processes including limbic, autonomic, and neuroendocrine functions as well as regulation of the circadian pacemaker. © 1996 Wiley-Liss, Inc.     

Monosodium glutamate lesions in rat hypothalamus studied by immunohistochemistry for gonadotropin releasing hormone, neurotensin, tyrosine hydroxylase, and glutamic acid decarboxylase and by autoradiography for [3H] estradiol

Adult male and female rats treated neonatally with monosodium glutamate (MSG) exhibit lesions in the arcuate nucleus of the hypothalamus. Immunohistochemical analysis of the distribution of tyrosine hydroxylase (TH), glutamic acid decarboxylase (GAD), neurotensin (NT) and gonadotropin-releasing hormone (GnRH) reveals substantial destructions of tuberoinfundibular dopamine and NT systems accompanied by a marked reduction of immunoreactivity in the median eminence. GAD immunoreactivity in the arcuate nucleus and median eminence is greatly reduced, while GnRH containing structures in the mediobasal hypothalamus are not noticeably affected. Evaluation of autoradiograms after intravenously administered [3H] estradiol in the ventral hypothalamus indicate an almost complete loss of target neurons in the arcuate nucleus but not in the nearby ventromedial nucleus. The results suggest that: (a) NT- and dopamine-containing neurons of the arcuate nucleus project to the median eminence via tuberoinfundibular NT and dopaminergic pathways; (b) GABA in the median eminence originates to a major extent from neurons of the arcuate nucleus through a tuberoinfundibular GABAergic system; (c) GnRH is produced in the rat outside the arcuate nucleus; (d) the MSG-induced lesion in the basal tuberal region abolishes or strongly diminishes estradiol target neurons in the arcuate nucleus.     

Relationship of Neuronal Nitric Oxide Synthase Immunoreactivity to GnRH Neurons in the Ovariectomized and Intact Female Rat

The present study has used a rat neuronal nitric oxide synthase (nNOS) antibody to examine the relationship of nNOS immunoreactivity to GnRH neurons in the ovariectomized and intact diestrous and proestrous rat. A striking band of nNOS-immunoreactive cells was identified in the rostral preoptic area which began in the median preoptic nucleus and organum vasculosum of the lamina terminalis and formed an inverted Y-type distribution above the rostral third ventricle at the level of the anteroventral periventricular nucleus. Another band of nNOS-immunoreactivity was found extending through the internal zone of the median eminence into the arcuate nucleus. Although nNOS immunoreactivity was not detected within GnRH neuronal cell bodies in any of the experimental groups, GnRH perikarya located in the rostral preoptic area, but not elsewhere, were found to be surrounded by nNOS-containing cells. In the median eminence, nNOS and GnRH immunoreactivities were distributed separately in the internal and external zones, respectively.
These results provide evidence that, regardless of their pattern of activity, GnRH neurons in the female rat do not express nNOS. Instead, a close anatomical relationship between nNOS-immunoreactive cells and GnRH perikarya and fibers has been identified within specific sub-regions of the rostral preoptic area and in the median eminence. Such findings are compatible with a role for NO at both sites in regulating the release of GnRH throughout the estrous cycle.     

Distribution of somatostatin in the rat brain: Telencephalon and diencephalon

The distribution of somatostatin (SRIF) was examined using the unlabeled antibody enzyme method of immunocytochemistry on thick 30-50 microns Vibratome sections. The greatest population of SRIF-neurons was observed along the ventricular wall in the preoptic area and anterior hypothalamus. Dense accumulations of fibers were observed in the suprachiasmatic, ventromedial and arcuate nuclei, the internal and external zone of the median eminence, the organum vasculosum of the lamina terminalis and the subfornical organ. Extrahypothalamic sites of SRIF-containing neurons and fibers were also observed throughout the telencephalon. The widespread distribution of SRIF is consistent with radioimmunoassay data and suggests a diverse physiological role for somatostatin.     

Monosodium glutamate lesions in rat hypothalamus studied by immunohistochemistry for gonadotropin releasing hormone, neurotensin, tyrosine hydroxylase, and glutamic acid decar☐ylase and by autoradiography for [H]estradiol

Adult male and female rats treated neonatally with monosodium glutamate (MSG) exhibit lesions in the arcuate nucleus of the hypothalamus. Immunohistochemical analysis of the distribution of tyrosine hydroxylase (TH), glutamic acid decar☐ylase (GAD), neurotensin (NT) and gonadotropin-releasing hormone (GnRH) reveals substantial destructions of tuberoinfundibular dopamine and NT systems accompanied by a marked reduction of immunoreactivity in the median eminence. GAD immunoreactivity in the arcuate nucleus and median eminence is greatly reduced, while GnRH containing structures in the mediobasal hypothalamus are not noticeably affected. Evaluation of autoradiograms after intravenously administered [³H]estradiol in the ventral hypothalamus indicate an almost complete loss of target neurons in the arcuate nucleus but not in the nearby ventromedial nucleus. The results suggest that: (a) NT- and dopamine-containing neurons of the arcuate nucleus project to the median eminence via tuberoinfundibular NT and dopaminergic pathways; (b) GABA in the median eminence originates to a major extent from neurons of the arcuate nucleus through a tuberoinfundibular GABAergic system; (c) GnRH is produced in the rat outside the arcuate nucleus; (d) the MSG-induced lesion in the basal tuberal region abolishes or strongly diminishes estradiol target neurons in the arcuate nucleus.     

Monosynaptic Pathway Between the Arcuate Nucleus Expressing Glial Type II Iodothyronine 5'-Deiodinase mRNA and the Median Eminence-Projective TRH Cells of the Rat Paraventricular Nucleus

Recent evidence suggests that the thyroid regulation of thyrotropin-releasing hormone (TRH)-containing neurons in the paraventricular nucleus of the hypothalamus involves the activation of other hypothalamic neural circuits. For example, the arcuate nucleus and not the paraventricular nucleus contains the highest enzyme activity of 5′-deiodinase type II, an enzyme that is pivotal for the local synthesis of T3. This experiment was undertaken to demonstrate whether a monosynaptic pathway exists between the arcuate nucleus and those TRH cells of the paraventricular nucleus that are neuroendocrine, i.e. project to the external layer of the median eminence. A specific cRNA probe derived from the coding region of deiodinase type II was used for the in situ hybridization histochemistry which was combined with immunocytochemistry for a specific marker of glial cells, glial fibrillary acidic protein (GFAP). The hybridization signals were present within the hypothalamus in the arcuate nucleus–median eminence region and in the periventricular area. The periventricular labeling was localized to the ependymal layer of the third ventricle and no hybridization product was detected in the paraventricular nucleus and other hypothalamic nuclei adjacent to the third ventricle. Within the median eminence, numerous cells containing the hybridization product were located in the internal layer adjacent to the floor of the third ventricle and in the external layer adjacent to the surface of the brain. In the dorso- and ventromedial regions of the arcuate nucleus, deiodinase type II mRNA-containing cells were also detected. Numerous type II deiodinase mRNA-containing cells in the median eminence and arcuate nucleus were also found to be immunopositive for GFAP. The abundance of arcuate cells expressing the hybridization product was lower than those in the periventricular region or in the median eminence. The anterograde tracer, Phaseolus vulgaris leucoagglutinin, was injected into the medial parts of the arcuate nucleus where the in situ hybridization experiment detected deiodinase type II mRNA. Simultaneously with the anterograde tracing, the retrograde tracer, Fluoro-Gold, was injected into either the median eminence or the general circulation. Light and electron microscopic double and triple immunolabeling experiments on vibratome sections of colchicine-pretreated animals revealed that arcuate fibers innervate TRH cells within the parvicellular region of the paraventricular nucleus. Populations of these TRH cells receiving afferents from the arcuate nucleus were also retrogradely labelled from either the median eminence or the general circulation indicating their direct role in the regulation of thyrotropin secretion from the anterior pituitary. The majority of arcuate nucleus efferents on TRH cells were found to establish symmetrical synaptic connections. The present results provided direct evidence of a monosynaptic pathway between the hypothalamic site of local thyroid hormone production, the arcuate nucleus, and neuroendocrine TRH cells in the paraventricular nucleus. This signalling modality may play an important role in thyroid feedback on TRH cells. Since the arcuate nucleus is involved in the regulation of central mechanisms controlling diverse homeostatic functions, including reproduction and feeding, the pathway described in this study may also carry integrated signals related to reproduction and ingestion to TRH-producing cells.     

Altered CNS neuroanatomical organization of spontaneously hypertensive (SHR) rats

Compared to Wistar-Kyoto (WKY) normotensive control rats, spontaneously hypertensive (SHR) rats have significantly reduced brain weights(−10.6%) and brain volumes (−11.8%). Computerized morphometric analysis of soma cross-sections areas of single neurons in 12 selected hypothalamic regions revealed significant differences between SHR and WKY animals. Neurons from the periventricular, medial and lateral preoptic nuclei and ventromedial hypothalamus show significantly increased soma cross-sectional areas in SHR animals when compared to normotensive controls. Cells located in the two circumventricular organs, organ vasculosum lamina terminalis (OVLT) and subfornical organ (SFO), also showed significantly greater cross-sectional areas in the SHR. In contrast, neurons in the paraventricular and arcuate nuclei and dorsomedial hypothalamus were significantly smaller spontaneously hypertensive rats when compared to normotensive controls. Only neurons in supraoptic nucleus, lateral and anterior hypothalamus have equivalent cross-sectional areas in WKY and SHR animals. Differences also exist in the number of cells in certain nuclei in SHR animals. Cell densities in periventricular preoptic nucleus, paraventricular nucleus, arcuate nucleus, ventromedial and anterior hypothalamus, organ vasculosum lamina terminalis and subfornical organ were reduced in SHR animals compared to WKY controls. Because of decreased brain weight and volume along with observed morphometric differences in individual neuronal soma size and cell densities, it is suggested that the SHR brain differs significantly from normotensive control rats. The differences may underlie some of the abnormalities in cardiovascular and endocrine regulation associated with neurogenic hypertension.