Organization of vasotocin-immunoreactive cells and fibers in the canary brain

The distribution of vasotocin (VT)-immunoreactive neuronal perikarya and fibers in the canary (Serinus canaria) was investigated with immunohistological techniques. The results suggest that most VT-stained cell bodies are located in three diencephalic regions. First, a large number of densely packed neurons are found in the paraventricular nucleus (PVN) and the anterior preoptic nucleus. Neurons here vary widely in size and shape. Small-size rounded neurons and large-size multipolar neurons appear to concentrate in separate subdivisions. Second, a series of loosely organized cell groups of medium- to large-size cells occurs in the lateral parts of the hypothalamus. These aggregates of neurons apparently correspond to subdivisions of the supraoptic nucleus (SON). Third, diffusely distributed, lightly stained cells are found dorsal to the paraventricular nucleus in the dorsal diencephalon. A number of cells of this group seem to be located in the basal septal area and bed nucleus of the stria terminalis. Immunoreactive fibers and varicosities concentrate in brain regions that are associated with neuroendocrine, autonomic, and limbic functions. Axons from the PVN and SON form compact bundles of the hypothalamohypophysial tract in the lateral hypothalamus and then funnel into the internal zone of the medium eminence (ME). Furthermore, a heavy innervation seems to be present in the palisadal, external zone of the ME. A substantial number of fibers appear to leave the PVN toward extrahypothalamic areas. Most extrahypothalamic VT fibers innervate telencephalic and brainstem regions that are thought to be involved in mediation of limbic and autonomic functions. These areas include the lateral and medial septum, the lateral habenula, the substantia grisea centralis, the area ventralis (Tsai), the locus coeruleus, raphe nuclei, the nucleus tractus solitarii, and lateral medulla. In addition, fibers with immunoreactivity for VT innervate structures such as the optic tectum and the nucleus ovoidalis that have been implicated in sensory processing of visual and auditory information. Finally, VT fibers and varicosities occur in centers including the nucleus robustus archistriatalis and nucleus intercollicularis that have been implicated in vocal control.     

Corticotropin-releasing hormone-synthesizing neurons of the human hypothalamus receive neuropeptide Y-immunoreactive innervation from neurons residing primarily outside the infundibular nucleus

Immunohistochemical single- and double-labeling studies were performed on the hypothalami of postmortem human brains to elucidate the distribution of corticotropin-releasing hormone (CRH)-immunoreactive (IR) neuronal elements and their interaction with the neuropeptide Y (NPY)-ergic neuronal system. The great majority of CRH-IR perikarya were found in the paraventricular nucleus (PVN), whereas a considerable number of CRH-IR neurons were also observed in the periventricular and infundibular nuclei. The dorsomedial nucleus and the perifornical region contained only scattered CRH-IR neurons. Dense CRH-IR fiber networks were found throughout the hypothalamus. However, the medial preoptic, the dorsolateral part of the supraoptic, the suprachiasmatic, the ventromedial, and the different mammillary nuclei showed a relative paucity of fibers. The terminal fields of NPY-IR axons overlapped the distribution of CRH-IR neurons in the hypothalamus. NPY-IR axon varicosities were juxtaposed to both dendrites and perikarya of the majority of CRH-IR neurons residing in the paraventricular, periventricular, and infundibular nuclei. These neurons were frequently contacted by multiple NPY axons that either formed baskets around their perikarya or completely ensheathed the emanating CRH dendrites. Because NPY and agouti-related protein (AGRP) are co-contained in neurons of the human infundibular nucleus, we used AGRP as a marker of NPY fibers originating exclusively from the infundibular nucleus. Only a small proportion of CRH neurons in the PVN was contacted by AGRP-IR axon varicosities, suggesting that NPY-IR innervation of CRH neurons in the PVN derive mainly from regions outside the infundibular nucleus. The present morphological findings support the view that NPY regulates the CRH system of the human hypothalamus and therefore at least some of the effects of NPY on metabolic, autonomic, and endocrine functions may be mediated through CRH.     

Hypothalamic thyrotropin-releasing hormone (TRH)-containing neurons involved in the hypothalamic-hypophysial-thyroid axis. Light microscopic immunohistochemistry

The localization of neurons containing immunoreactive thyrotropin-releasing hormone (TRH) was examined in the hypothalamus of intact, propythiouracil (PTU)-treated, and colchicine-treated adult rats. In intact animals, immunoreactive TRH neurons were occasionally found in the paraventricular and dorsomedial nuclei and in the anterior and lateral hypothalamic areas. In PTU-treated animals, the cellular appearance of the hypothalamus with the exception of the paraventricular nucleus was almost similar to that of intact animals. In the paraventricular nucleus, only the cells localized in the periventricular and medial parvocellular subdivisions significantly increased in number and became hypertrophic in comparison with intact animals. The distribution of immunoreactive fibers in the hypothalamus was almost equal among the 3 animal groups with the exception of that in the median eminence, in which the fibers were most densely concentrated in intact animals, and most sparse in PTU-treated rats. The fibers projecting into the median eminence were distinguished into the periventricular and lateral pathways, which are derived from the neurons in the periventricular and medial parvocellular subdivisions of the paraventricular nucleus, respectively. Thus, among immunoreactive TRH neurons in the hypothalamus, only those in the periventricular and medial parvocellular subdivisions of the paraventricular nucleus may be involved in the hypothalamic-hypophysial-thyroid axis.     

alpha-Melanocyte-stimulating hormone is contained in nerve terminals innervating thyrotropin-releasing hormone-synthesizing neurons in the hypothalamic paraventricular nucleus and prevents fasting-induced suppression of prothyrotropin-releasing hormone gene expression.

The hypothalamic arcuate nucleus has an essential role in mediating the homeostatic responses of the thyroid axis to fasting by altering the sensitivity of prothyrotropin-releasing hormone (pro-TRH) gene expression in the paraventricular nucleus (PVN) to feedback regulation by thyroid hormone. Because agouti-related protein (AGRP), a leptin-regulated, arcuate nucleus-derived peptide with alpha-MSH antagonist activity, is contained in axon terminals that terminate on TRH neurons in the PVN, we raised the possibility that alpha-MSH may also participate in the mechanism by which leptin influences pro-TRH gene expression. By double-labeling immunocytochemistry, alpha-MSH-IR axon varicosities were juxtaposed to approximately 70% of pro-TRH neurons in the anterior and periventricular parvocellular subdivisions of the PVN and to 34% of pro-TRH neurons in the medial parvocellular subdivision, establishing synaptic contacts both on the cell soma and dendrites. All pro-TRH neurons receiving contacts by alpha-MSH-containing fibers also were innervated by axons containing AGRP. The intracerebroventricular infusion of 300 ng of alpha-MSH every 6 hr for 3 d prevented fasting-induced suppression of pro-TRH in the PVN but had no effect on AGRP mRNA in the arcuate nucleus. alpha-MSH also increased circulating levels of free thyroxine (T4) 2.5-fold over the levels in fasted controls, but free T4 did not reach the levels in fed controls. These data suggest that alpha-MSH has an important role in the activation of pro-TRH gene expression in hypophysiotropic neurons via either a mono- and/or multisynaptic pathway to the PVN, but factors in addition to alpha-MSH also contribute to the mechanism by which leptin administration restores thyroid hormone levels to normal in fasted animals.     

Immunoreactive leu-enkephalin in the monkey hypothalamus including observations on its ultrastructural localization in the paraventricular nucleus

The distribution of immunoreactive leu-enkephalin neurons and fibers in the monkey hypothalamus, including ultrastructural localization in the paraventricular nucleus (PVN), was examined with the peroxidase-antiperoxidase immunocytochemical method. Immunoreactive leu-enkephalin cell bodies and fibers were present in the PVN, the region of the dorsal nucleus and nucleus of the anterior commissure, the dorsomedial nucleus, ventromedial nucleus, and lateral hypothalamus. Within the PVN labeled cells were found mostly in the medial parvocellular region, and a smaller proportion including some large cells was present in the lateral, and dorsolateral zones. Immunoreactive neurons contained numerous large granular vesicles (LGV) which ranged from 63 to 235 nm in size, suggesting that at least some enkephalin-containing neurons belong to the population of neurosecretory cells. Positive neurons were postsynaptic to four types of unlabeled axon terminals. Leu-enkephalin-containing fibers (some of which were myelinated) and boutons contained small clear vesicles and numerous LGV. Axon terminals made synaptic contacts with the cell bodies, primary and distal dendrites of unlabeled neurons. The findings show that enkephalin-containing neurons in the PVN integrate a variety of neuronal inputs and provide morphological evidence for the inhibiting influence of enkephalins on the firing rate of PVN neurons. It may be speculated that the effects of opioids on the release of vasopressin and other substances possibly originating from PVN neurons may be regulated in part within the nucleus by locally synapsing axons belonging to enkephalin-containing neurons.     

GABA-ergic innervation of thyrotropin-releasing hormone-synthesizing neurons in the hypothalamic paraventricular nucleus of the rat

To determine whether GABA-ergic axons are anatomically situated to directly influence TRH neurons in the PVN, double-labeling light- and electronmicroscopic immunocytochemistry was performed using antisera against glutamic acid decarboxylase (GAD) and prothyrotropin-releasing hormone (proTRH). In the anterior, periventricular and medial parvocellular subdivisions of the PVN, GAD-immunoreactive (IR) axon varicosities were closely apposed to all proTRH containing cell bodies and proximal dendrites. Ultrastucturally, GAD-IR nerve terminals established symmetric type synapses with both perikarya and dendrites of proTRH-IR neurons, indicating the inhibitory nature of the contacts. Since a subpopulation of neuropeptide Y (NPY) neurons in the hypothalamic arcuate nucleus co-synthesize GABA, and NPY-containing neurons of arcuate nucleus origin densely innervate TRH neurons in the PVN, we performed triple labeling immunocytochemistry to elucidate the origin of the GAD-IR innervation of hypophysiotropic TRH neurons. While axons co-containing GAD and NPY were observed throughout the PVN, only approximately 10% of GAD-IR terminals in contact with TRH neurons were found to contain NPY-immunoreactivity. We conclude that GABA-ergic neurons are in position to act directly on hypophysiotropic TRH neurons and while this innervation arises partly from neurons in the arcuate nucleus that co-synthesize NPY, the majority of the GABA-ergic input arises from other neuronal groups.     

An immunohistochemical study of the organization of catecholaminergic cells and terminal fields in the paraventricular and supraoptic nuclei of the hypothalamus

The distribution of catecholaminergic fibers and cell bodies in the paraventricular and supraoptic nuclei of the hypothalamus was investigated with immunohistochemical methods in the adult albino rat. Sections through the nuclei were stained with antisera to the catecholamine synthesizing enzymes tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT). The results suggest that adrenergic (PNMT-stained) fibers innervate the entire parvocellular division of the paraventricular nucleus, although the highest density of fibers was found in the medial part of the division. Only widely scattered adrenergic fibers are found in the magnocellular division of the nucleus and in the supraoptic nucleus. Noradrenergic fibers appear to innervate the periventricular zone of the paraventricular nucleus and those parts of the paraventricular and supraoptic nuclei that contain predominantly vasopressinergic neurons in both the normal and in the homozygous Brattleboro rat. Significant numbers--somewhat more than 500--of dopaminergic (TH-stained) neurons are found in the paraventricular nucleus; the cells are distributed throughout the nucleus but are concentrated in the medial and periventricular parts of the parvocellular division. Double-labeling experiments with the retrogradely transported tracer true blue indicate that between 4% and 8% of the dopaminergic neurons in the paraventricular nucleus project to the region of the dorsal vagal complex and/or thoracic levels of the spinal cord. It is concluded that adrenergic inputs to the paraventricular nucleus may influence cells that project to the median eminence and to preganglionic autonomic cell groups in the medulla and spinal cord. Noradrenergic inputs to the supraoptic and paraventricular nuclei may influence primarily vasopressinergic cells that project to the posterior lobe of the pituitary, as well as cells in the periventricular part of the paraventricular nucleus that project to the median eminence.     

Enkephalin-immunoreactive neurons in the parvicellular subdivisions of the paraventricular nucleus project to the external zone of the median eminence.

The enkephalin-immunoreactive neurons that project to the external zone of the median eminence were identified on thin paraffin and thick vibratome sections using a combination of retrograde labeling with peripherally administered Fluoro-Gold and immunocytochemistry. The vast majority of the enkephalin-immunoreactive neurons that project to the external zone of the median eminence (ME) reside in the paraventricular nucleus (PVN) of the hypothalamus. Within the PVN, the majority of these hypophysiotropic neurons are located in the medial parvicellular subdivision, while a smaller number can be detected in the anterior and the periventricular subdivisions. Although many enkephalin-immunoreactive neurons are present in other hypophysiotropic areas of the hypothalamus, such as the medial preoptic area, the anterior periventricular area, and the arcuate nucleus, only a few of these can be retrogradely labeled from the ME. These results provide morphological evidence for the key role of paraventricular enkephalin-immunoreactive neurons in the regulation of neuroendocrine functions.     

Enhanced binocular interaction in the visual cortex of normal kittens subjected to intracortical norepinephrine perfusion

Immunoreactive fibers and varicosities in the hypothalamic paraventricular nucleus (PVN) were examined by light- and electronmicroscopy, following treatment of brain slices with specific antibodies to adrenocorticotropin (ACTH), beta-endorphin (beta-End) and alpha-melanotropin (alpha-MSH) peptides. In an attempt to provide a more precise, quantitative definition of the densities of immunoreactive elements, sections were analyzed by computer based image-analysis techniques. Fibers and varicosities immunostained with the 3 different antibodies displayed an identical distribution pattern throughout the nucleus suggesting that they are parts of the same, arcuate pro-opiomelanocortin (POMC) neuron system. Although immunoreactive varicosities were found all over the PVN, it was possible to identify a characteristic, density distribution pattern. At the ultrastructural level, immunoreactive presynaptic nerve terminals were observed forming symmetrical synaptic contacts with unlabeled dendrites. The majority of immunoreactive elements were found in the dorsal parvo- and caudal magnocellular subdivisions which give rise to long projections to the lower brainstem. Moderate density of POMC neural elements was observed in the anterior and medial (ventral portion) parvocellular subdivisions which project to the external zone of the median eminence. Only a few, widely scattered immunostained varicosities are found in the medial and lateral magnocellular subdivisions which project to the neurohypophysis. A combined lesion and immunocytochemical approach has shown that the bulk of the afferent neuronal input from arcuate POMC cells enters the PVN from a ventral direction.     

Noradrenergic innervation of vasopressin-and oxytocin-containing neurons in the hypothalamic paraventricular nucleus of the macaque monkey: Quantitative analysis using double-label immunohistochemistry and confocal laser microscopy

Previous reports on the rat and monkey hypothalamus have revealed a dense noradrenergic innervation within the hypothalamic paraventricular nucleus as assessed by dopamine-β-hydroxylase immunohistochemistry. These single-label analyses were unable to delineate the cellular structures which receive this catecholaminergic innervation. Double-label preparations in the rat hypothalamic paraventricular nucleus have demonstrated synaptic interactions between noradrenergic varicosities and magnocellular neurons. However, the density and distribution of varicosities contacting chemically identified magnocellular neurons have not been assessed at the light or electron microscopic level. In this report, single-label immunohistochemistry was used to assess the morphology and distribution of vasopressin - and oxytocin-immunoreactive neurons within the macaque hypothalamic paraventricular nucleus. In addition, double-label immunohistochemistry was combined with confocal laser scanning microscopy to quantify the number of dopamine-β-hydroxylase-immunoreactive varicosities in apposition to magnocellular neurons expressing vasopressin or oxytocin immunoreactivity. The morphology of chemically identified neurons was also compared to magnocellular neurons in the monkey hypothalamic paraventricular nucleus which were filled with Lucifer Yellow in order to assess the somatodendritic labeling of the immunohistochemical preparation. Qualitative assessment of immunohistochemically identified magnocellular cells indicated that vasopressin- and oxytocin-containing neurons are observed throughout the rostrocaudal extent of the monkey hypothalamic paraventricular nucleus, demarcating this structure from the surrounding anterior hypothalamus. The distribution of the two nonapeptides is complementary, with vasopressin-immunoreactive neurons having a greater somal volume and located in a more medial aspect of the mid and caudal hypothalamic paraventricular nucleus relative to oxytocin-immunoreactive perikarya. For the double-label preparations, a series of confocal optical sections was assessed through the total somal volume of vasopressin- and oxytocin-immunoreactive neurons along with the corresponding dopamine-β-hydroxylase-immunoreactive varicosities in the same volume of tissue, generating a varicosity-to-neuron ratio which was further characterized morphologically to assess afferent input to the soma and proximal dendrites. Quantitative analysis revealed that vasopressin-immunoreactive neurons received approximately two thirds of their dopamine-β-hydroxylase-immunoreactive varicosities in apposition to the proximal dendrites and one third in apposition to the somata. Furthermore, vasopressin-immunoreactive neurons received a greater innervation density than oxytocin-immunoreactive neurons, which did not have a differential distribution of varicosities on the proximal dendrites and somata. The distribution of dopamine-β-hydroxylase-immunoreactive afferents on magnocellular neurons in the hypothalamic paraventricular nucleus may reflect a physiological role of this circuit in terms of preferential release of vasopressin from magnocellular neurons upon noradrenergic stimulation.     

Projections from the lateral geniculate nucleus to the hypothalamus of the Mongolian gerbil (Meriones unguiculatus): an anterograde and retrograde tracing study.

The lateral geniculate nucleus of the thalamus sends efferents to the hypothalamic suprachiasmatic nucleus, which is involved in generation and entrainment of several circadian rhythms. It seems reasonable to believe that the lateral geniculate conveys visual information about the length of the photoperiod to the circadian oscillator. In order to study in more detail the topographical relationship between the lateral geniculate and the suprachiasmatic nucleus, anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde tracing with wheatgerm agglutinin coupled to horseradish peroxidase (WGA-HRP) were performed in the gerbil. After iontophoretic injections of PHA-L in the lateral geniculate, a large number of PHA-L-immunoreactive fibers and nerve terminals were observed in the ventrolateral part of the suprachiasmatic nucleus. Nerve fibers were also present in the ventromedial and dorsolateral portions, particularly in the caudal half of the nucleus. PHA-L-immunoreactive nerve fibers continued outside the borders of the suprachiasmatic nucleus to the adjacent anterior hypothalamic, the periventricular, and the subparaventricular areas. A moderate number of fibers entered the lateral hypothalamic area and the tuber cinerum via the optic tract and chiasm. Moreover, the paraventricular nucleus, the supraoptic nucleus, the medial preoptic area, the lateral preoptic area, and the supramammillary nucleus contained a few labeled fibers. In all parts of the hypothalamus receiving an input from the lateral geniculate, fine beaded immunoreactive fibers with varicosities and nerve terminals were observed, some of which were found in close apposition to hypothalamic neurons. Only after labeling of neurons in the intergeniculate leaflet of the lateral geniculate nucleus, fibers were found in the hypothalamus. This topographical organization of the geniculohypothalamic pathway was supported by retrograde tracing after injections of WGA-HRP in the suprachiasmatic area. In these experiments, retrograde labeled neurons were observed in the intergeniculate leaflet and, in agreement with the anterograde studies, most of labeling was observed in the ipsilateral side. These results confirm that the suprachiasmatic nucleus receives a substantial input from the intergeniculate leaflet of the lateral geniculate. Moreover, the present data demonstrate that the suprachiasmatic nucleus is not the only nucleus that receives a direct visual input. Thus other hypothalamic areas might be influenced by a direct rhythmic neuronal input as well.     

The noradrenergic innervation of vasopressin neurons in the paraventricular nucleus of the hypothalamus: An ultrastructural study using radioautography and immunocytochemistry

Immunocytochemical and radioautographic procedures were combined at the ultrastructural level to study the noradrenergic synaptic input to vasopressin neurons in selected portions of the paraventricular nucleus of the hypothalamus (PVN) of the rat. Radioactive norepinephrine (NE) was infused into the lateral ventricle or applied topically to the region of the PVN. After appropriate survival times, brain tissues were processed for ultrastructural immunocytochemical demonstration of vasopressin using a monoclonal antibody. [3H]NE varicosities were detected by electron microscopic radioautography. In the periventricular zone of the PVN, radioactive varicosities were numerous accounting for 20-30% of all nerve terminals in this zones. These NE terminals primarily innervated dendritic processes of non-vasopressinergic neurons. Although an occasional axosomatic synapse was observed, input to vasopressin positive neurons was exclusively to their dendrites. In the lateral magnocellular sub-nucleus of the PVN (designed pvl2), noradrenergic terminals were fewer in number accounting for only 1-2% of the total. These terminals were found predominately but not exclusively making axodendritic synapses onto non-vasopressin processes. In both regions, many of the radiolabeled terminals had well-defined membrane appositions with their post-synaptic partners which included a synaptic cleft and post-synaptic density of varying thickness. In both the periventricular zone and the lateral magnocellular regions, noradrenergic varicosities were seen in close proximity to numerous blood vessels.     

Efferent connections from the lateral hypothalamic region and the lateral preoptic area to the hypothalamic paraventricular nucleus of the rat

The lateral preoptic and lateral hypothalamic regions contain the majority of the cell groups embedded in the fibre trajectories of the medial forebrain bundle on its course through the hypothalamus. Recent studies have extended considerably the parcellation of the lateral hypothalamic region, and, therefore, the need to emphasize new insights into the anatomical organisation of projections from the neurons of the lateral hypothalamic region. In the present study we describe the anatomical organisation of efferent projections from the lateral preoptic and lateral hypothalamic regions to the hypothalamic paraventricular nucleus (PVN) on the basis of retrograde- and anterograde-tracing techniques. Iontophoretic injections of the retrograde tracer, cholera toxin subunit B, into the PVN revealed that most hypothalamic nuclei project to the PVN. Within the lateral hypothalamic region, retrogradely labelled cells were concentrated in the intermediate hypothalamic area, the lateral hypothalamic area, and the perifornical nucleus, whereas fewer retrogradely labelled cells were found in the lateral preoptic area. To determine the distribution of terminating fibres in subnuclei of the heterogeneous PVN, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were delivered into distinct areas of the lateral hypothalamic region. Neurons of the intermediate hypothalamic area projected mainly to the PVN subnuclei, which contained parvicellular neuroendocrine cells. In contrast, neurons of the rostral and tuberal parts of the lateral hypothalamic area and the perifornical nucleus projected to the PVN subnuclei, which contained parvicellular neurons that send descending projections to preganglionic cell groups in the medulla and spinal cord. The perifornical nucleus was the only area within the lateral hypothalamic region that consistently innervated magnocellular perikarya of the PVN. Finally, all areas of the lateral hypothalamic region contributed substantially to fibres terminating in the perinuclear shell of the PVN. These results demonstrate that anatomically distinct areas of the lateral hypothalamic region have distinct projections to subnuclei of the PVN and further substantiate the view that the lateral hypothalamic region as well as the PVN constitute anatomically and functionally heterogeneous structures. © 1994 Wiley-Liss, Inc.     

Importance of endogenous nitric oxide synthase in the rat hypothalamus and amygdala in mediating the response to capsaicin

Although capsaicin has been shown to activate certain neuronal groups in the hypothalamus and amygdala, the neurotransmitters involved and the exact mechanism of action are not clearly understood at present. The aim of this study was to examine the hypothesis that the effect of capsaicin in the rat hypothalamus and amygdala primarily involves direct activation of the endogenous nitric oxide synthase (NOS) neurons responsible for the synthesis of nitric oxide (NO). Subcutaneous capsaicin injection in male rats, compared with vehicle, caused a significant increase in Fos expression in the paraventricular nucleus (PVN), supraoptic nucleus (SON), and medial and cortical amygdala. The expression of nicotinamide adenine dinucleotide phosphate diaphorase, a histochemical marker for NOS, was also increased in these brain areas in addition to the periventricular and lateral hypothalamic area and central amygdaloid nucleus. Also, capsaicin significantly increased the expression of neuronal NOS messenger RNA and protein in the PVN, SON, and medial amygdala as demonstrated by in situ hybridization and immunohistochemistry, respectively. A higher proportion of the NOS neurons in the PVN, periventricular region, SON and amygdala showed Fos expression in response to capsaicin than vehicle injection. There was little, if any, Fos activation in the NOS-positive neurons in the lateral hypothalamic area. The capsaicin-induced activation of the hypothalamic PVN and SON neurons and the medial amygdaloid nucleus was attenuated in the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) -pretreated animals in comparison with the inactive enantiomer D-NAME. These observations indicate that activation of the endogenous NOS system and production of NO constitute a major pathway through which capsaicin exerts its effect within the hypothalamus and amygdala.     

Medullary synaptic inputs to Thyrotropin-Releasing Hormone (TRH)-containing neurons in the hypothalamus: an ultrastructural study combining WGA-HRP anterograde tracing with TRH immunocytochemistry

Ascending projections from the A1/C1 cell group and from the A2 cell group in the medulla oblongata was studied in the light microscope by anterograde tracing ofPhaseolus vulgaris leucoagglutinin and in the electron microscope by anterograde tracing of wheat germ agglutinin-coupled horseradish peroxidase (WGA-HRP) combined with thyrotropin-releasing hormone (TRH) immunocytochemistry in the hypothalamic paraventricular nucleus (PVN). WGA-HRP-labeled axon terminals originating from neurons in the A1/C1 or the A2 cell group were found to make synaptic contacts with TRH-containing cell bodies and dendrites in the medial parvocellular part of the PVN, usually forming axo-dendritic synapses. Of all the afferent synapses on TRH neurons in the PVN, 9.8–20.9% of the presynaptic axon terminals were WGA-HRP-positive. This indicates that each brain stem catecholaminergic cell group that contribute to innervation of the PVN is in a position to modulate the activity of TRH neurons.     

Glucagon like peptide-1 (7-36) amide (GLP-1) nerve terminals densely innervate corticotropin-releasing hormone neurons in the hypothalamic paraventricular nucleus

Glucagon like peptide-1 (7-36) amide (GLP-1), a potent regulator of glucose homeostasis, is also produced in the central nervous system and has been implicated in the control of hypothalamic-pituitary function and food intake. GLP-1 immunoreactive (IR) fibers and terminals are widely distributed in the septum, hypothalamus, thalamus and brainstem, likely originating from GLP-1-IR neuronal cell bodies from the nucleus of the solitary tract of the medulla oblongata. Central administration of GLP-1 increases plasma corticosterone levels and elicits c-fos expression in corticotropin releasing hormone (CRH) neurons of the hypothalamic paraventricular nucleus (PVN). To identify the endogenous neurocircuitry that may underlie this response, the present study determined whether there is an innervation of PVN CRH neurons by GLP-1-containing nerve terminals. GLP-1-IR fibers and nerve terminals were found in the parvocellular parts of the PVN, with highest concentrations in the anterior and medial parvocellular subdivisions. The magnocellular divisions of the PVN also showed moderate numbers of GLP-1-IR nerve fibers. Double immunolabelling revealed numerous GLP-1-IR nerve fibers in close apposition to approximately 65% of detectable CRH neurons in the medial parvocellular subdivision of the rat PVN. At the ultrastructural level, GLP-1-IR terminals were observed to establish synapses on both perikarya and dendrites of CRH neurons. These findings support the hypothesis that the GLP-1-induced activation of CRH neurons and the associated pituitary-adrenocortical activation may be accomplished by GLP-1's direct action on hypophysiotropic CRH neurons. Since central CRH is also thought to be an anorexigenic factor and GLP-1 neurons contain leptin receptors, activation of CRH neurons in the PVN by GLP-1 may contribute to the complex neuroendocrine and metabolic actions by the adipostatic hormone, leptin.