Ultrastructural localization of galanin immunoreactivity in the rat median eminence

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Galanin enhancement of gonadotropin-releasing hormone-stimulated luteinizing hormone secretion in female rats is estrogen dependent

The hypothalamic peptide GnRH is the primary neuroendocrine signal regulating pituitary LH in females. The neuropeptide galanin is cosecreted with GnRH from hypothalamic neurons, and in vitro studies have demonstrated that galanin can act at the level of the pituitary to directly stimulate LH secretion and also augment GnRH-stimulated LH secretion. Several lines of evidence have suggested that the hypophysiotropic effects of galanin are important for the generation of preovulatory LH surges. To determine whether the pituitary actions of galanin are enhanced by the preovulatory steroidal milieu, LH responses to galanin administration (with or without GnRH) were examined in: 1) ovariectomized (OVX); 2) OVX, estrogen (E)-primed; and 3) OVX, E- and progesterone-treated female rats. Results from the study indicate that galanin enhances GnRH-stimulated LH secretion only in the presence of E (in OVX, E-primed, or E- and progesterone-treated rats). Galanin alone does not directly stimulate LH secretion under any of the steroid conditions examined. In the absence of gonadal steroids (OVX rats), galanin inhibits GnRH-stimulated LH secretion. These findings suggest that the primary pituitary effect of galanin is to modulate GnRH-stimulated LH secretion, and that the potentiating effects of galanin occur only in the presence of E.

Testosterone-dependent effects of galanin on pituitary luteinizing hormone secretion in male rats

Galanin is a 29-amino-acid peptide that colocalizes with GnRH in hypothalamic neurons. High concentrations of galanin are present in portal vessel blood of both male and female rats, and galanin receptors are present on gonadotropes in both sexes. Results from studies of female rats indicate that galanin acts at the level of the pituitary to directly stimulate LH secretion and also to enhance GnRH-stimulated LH secretion. The effects of galanin on pituitary LH secretion in male rats are relatively uncharacterized; thus, the present in vivo study was conducted 1). to examine the ability of galanin to affect basal or GnRH-stimulated LH secretion in male rats and 2). to determine whether the effects of galanin on LH secretion in male rats are testosterone-dependent. All three doses of galanin used (1, 5, and 10 micro g/pulse) significantly enhanced GnRH-stimulated LH secretion in intact male rats. Only the highest dose of galanin directly stimulated LH secretion (without GnRH coadministration) in intact males. Galanin did not directly stimulate LH secretion or enhance GnRH-stimulated LH secretion in castrated male rats. In fact, the highest dose of galanin inhibited GnRH-stimulated LH secretion in castrated males. Upon testosterone replacement, the ability of galanin to directly stimulate LH secretion and to enhance GnRH-stimulated LH secretion was restored in castrated males. These results suggest a role for galanin in the regulation of LH release in male rats and demonstrate that testosterone upregulates the ability of the pituitary to respond to the stimulatory effects of galanin.

Galanin: analysis of its coexpression in gonadotropin-releasing hormone and growth hormone-releasing hormone neurons

Galanin is coexpressed in a subset of gonadotropin-releasing hormone (GnRH) and growth hormone-releasing hormone (GHRH) neurons in the brain and has an important role in the neuroendocrine regulation of gonadotropin and growth hormone secretion. Our overall goal has been to understand the functional significance of galanin as a cotransmitter with GnRH and GHRH in the regulation of these important physiologic processes. To this end, we studied the regulation of galanin's expression in GnRH and GHRH neurons under a variety of physiologic and experimental conditions. Using double-label in situ hybridization and computerized image analysis, we observed that in GnRH neurons, galanin's expression is increased over the course of development in both sexes. Galanin achieves a higher basal expression in GnRH neurons in females, and it is sexually differentiated in the adult as a result of the differential exposure to testosterone during the neonatal critical period. Galanin is induced in GnRH neurons coincident with and subsequent to the proestrous luteinizing hormone surge (reflecting the combined action of estradiol and progesterone) acting indirectly on GnRH neurons through a synaptic relay. Galanin's expression in GnRH neurons is inhibited during lactation, when the neuroendocrine reproductive axis is relatively quiescent. In GHRH neurons, the expression of galanin is also induced over the course of development in both sexes. Galanin's expression in GHRH neurons in the adult is sexually differentiated, but in this case, its expression is higher in males than females, reflecting the stimulatory effect of testosterone on galanin in the male. Galanin's expression in GHRH neurons is induced by growth hormone (GH), whereas the absence of GH leads to a reduction of galanin mRNA in these same cells. On the basis of these observations, we conclude that galanin is an important target for regulation by many hormones, and we postulate that as a cotransmitter, galanin acts presynaptically to modulate the secretion of GnRH and GHRH, possibly by altering their pulsatile release patterns, which in turn influences the release of the gonadotropins and GH from the pituitary.

Stress and colchicine do not induce the release of galanin from the external zone of the median eminence

The aim of the present study was to verify the hypothesis that stress exposure modifies the content and release of galanin in the hypothalamic paraventricular nucleus and the median eminence. Colchicine and immobilization served as stress stimuli, and the changes in galanin immunoreactivity were compared with those in corticotropin-releasing hormone and vasopressin. In control animals, a limited number of galanin perikarya were identified in the paraventricular nucleus. The high dose (75 microg) of colchicine enhanced galanin in both parvicellular and magnocellular subdivisions, as analysed 72 h later. In the median eminence, galanin accumulated only in the external zone. High-dose colchicine did not affect galanin, while corticotropin-releasing hormone and vasopressin were depleted from the median eminence. Immobilization (120 min) neither alone nor in combination with colchicine influenced galanin immunoreactivity in the external zone. The low dose of colchicine induced an unexpected accumulation of galanin in the internal zone of the median eminence, which was further increased by subsequent immobilization. In the external zone, low-dose colchicine induced a complete disappearance of vasopressin, substantial depletion of corticotropin-releasing hormone and no changes in galanin immunoreactivity. The present studies demonstrate that galanin in the external zone of the median eminence is not influenced by colchicine or by immobilization stress.

Ependymal development, proliferation, and functions: a review

A survey of the literature shows that proliferation of ependyma occurs largely during the embryonic and early postnatal periods of development in most species. Differentiation of these cells proceeds along particular regional and temporal gradients as does the expression of various cytoskeletal (vimentin, cytokeratins, glial fibrillary acidic protein) and secretory proteins (S-100). Turnover declines significantly postnatally, and only low levels of residual activity persist into adulthood under normal conditions. Although the reported response of ependyma to injury is somewhat equivocal, only limited regenerative capacity appears to exist and to varying degrees in different regions of the neuraxis. Proliferation has been most often observed in response to spinal cord injury. Indeed, the ependyma plays a significant role in the initiation and maintenance of the regenerative processes in the spinal cord of inframammalian vertebrates. In the human, however, ependyma appears never to regenerate at any age nor re-express cytoskeletal proteins characteristic of immature cells. The functions of ependyma including tanycytes, a specialized form of ependymal cell that persists into adulthood within circumscribed regions of the nervous system, are still largely speculative. Fetal unlike mature ependyma is believed to be secretory and is believed to play a role in neurogenesis, neuronal differentiation/axonal guidance, transport, and support. In the adult brain, mature ependyma is not merely an inert lining but may regulate the transport of ions, small molecules, and water between the cerebrospinal fluid and neuropil and serve an important barrier function that protects neural tissue from potentially harmful substances by mechanisms that are still incompletely understood.

Ultrastructural localization of galanin and galanin receptors in the guinea pig median eminence

The aim of this work was to compare the localization of galanin and galanin receptors in the guinea pig median eminence at the light and electron microscopic level. Concerning galanin the highest labeling was shown in the external part of the median eminence. At the ultrastructural level, galanin immunoreactivity was observed only in nerve terminals containing granular vesicles of approximately 120 nm in diameter. Light microscopic autoradiographs of semithin sections exhibited a moderate labeling in the external part of the median eminence. Galanin receptors were labeled in vitro on semithin sections (2 microm) using the highly specific radioligand [125I]galanin. Ultrastructural data showed that most of galanin binding sites overlaid membrane appositions between nerve terminals and also between nerve terminal and tanycyte. By considering the percentages in the distribution of the binding it appeared that galanin receptors were located on some nerve ending membranes. Our observations were not really in favor of a presence of receptors in tanycytes. The presence of galanin nerve endings in the external part suggests that like in the rat the peptide may have a direct hypophysiotrophic role. In contrast, the occurrence of numerous binding sites gives additional arguments in favor of a local action (paracrine and/or autocrine) of galanin occurring via galanin receptors located essentially on the pericapillary nerve terminals in the guinea pig median eminence.

Growth hormone inhibits the hypophysectomy-induced expression of galanin in hypothalamic neurons of the toad (Bufo arenarum hensel)

The expression of the neuropeptide galanin was analyzed by immunohistochemistry in magnocellular and preoptic hypothalamic neurons of toads following hypophysectomy (HPX) and pars distalectomy (PDX). There was a marked increase in the galanin-like immunoreactive expression in magnocellular hypothalamic cells 3 days after HPX, followed by a decrease to normal levels after 7 days. No changes in the expression of galanin were detected after PDX in these neurons when compared to controls. Moreover, 7 days after HPX or PDX the number of cells expressing galanin was significantly increased in the preoptic area, where numerous intraependymal cells were intensely immunoreactive. The hypophysis grafts into the hind limb in HPX or PDX animals prevented increased galanin-like immunoreactivity in preoptic cells but not in magnocellular neurons. Similarly, PDX toads given growth hormone showed no GAL-LI in the intraependymal preoptic cells. These results suggest the presence of a region regulation of galanin expression in the preoptic area by hypophyseal hormones, in particular growth hormone.

Distribution of catecholamine-synthesizing enzymes and some neuropeptides in the median eminence-arcuate nucleus complex (MEARC) of the immature female pig

The presence of the catecholamine-synthesizing enzymes tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (D beta H) and some neuropeptides, including neuropeptide Y (NPY), Leu5-enkephalin (LENK), vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP), substance P (SP), galanin (GAL) and somatostatin (SOM) was investigated in nerve fibres and perikarya of the median eminence-arcuate nucleus complex (MEARC) of the sexually immature female pigs by means of the immunohistochemical avidin-biotin complex method. Although immunoreactivities to all the studied substances were found in nerve fibres of the porcine MEARC, there were differences in the distribution and density of particular subsets of nerve fibres within the complex. While loose D beta H-immunoreactive (D beta H-IR) and dense TH-, NPY- and VIP-IR nerve meshworks occurred predominantly in the internal layer of the MEARC, nerve fibres immunoreactive to TH, CGRP, SOM, SP and LENK were more numerous in the external than in the internal layer of the median eminence (ME). Numerous TH-, D beta H-, NPY-, VIP-, SP- and CGRP-IR perivascular nerve fibres were also observed within both layers of the median eminence. There were also differences in the distribution of a particular subset of neurons within the porcine MEARC: NPY-, VIP-, GAL-, SP- and TH-IR (but not D beta H-IR) perikarya were found in the arcuate nucleus, while in the median eminence only subpopulations of NPY-, VIP and GAL-IR neurons were observed.

The cellular localization of vasoactive intestinal peptide (VIP) in the mouse median eminence by immuno-electron microscopy

The aim of this study was to examine, by use of pre-embedding immunocytochemistry, the ultrastructural localization of vasoactive intestinal peptide (VIP) immunoreactivity in the mouse median eminence. VIP immunoreactivity was observed in axonal profiles. The VIP-immunoreactive axonal profiles were in close proximity to non-immunoreactive axonal profiles that contained dense granular vesicles and clear vesicles and also to processes of tanycytes. VIP-immunoreactive terminals were observed in the proximity of the perivascular space and in the neuropil. Our results suggest that VIP-immunoreactive axon terminals may possibly interact with other non-immunoreactive axon terminals containing peptide and/or other transmitters at the level of the median eminence or may be released to the portal vasculature thereby to effect anterior pituitary cells.

Galanin in the hypothalamus of fed and fasted lean and obese Zucker rats

Galanin (GAL), a 29 aminoacid peptide, is widely distributed in the central nervous system and especially in the hypothalamus. It strongly stimulates food intake when it is injected in the paraventricular nucleus (PVN) of normal rats. The obese Zucker rat with a well-established hyperphagia is characterized by a general dysregulation of some important neuropeptides involved in the regulation of feeding behavior e.g. neurotensin, NPY or CCK and the aim of this study was to measure GAL in different microdissected brain areas in lean (Fa/Fa) and obese (fa/fa) male Zucker rats. As feeding status may modulate the central peptide concentrations, it was measured in ad libitum fed rats and in 48-h fasted rats of both genotypes. GAL was measured by a specific radioimmunoassay in the arcuate nuclei (ARC) and parvocellular (PVNp) and magnocellular (PVNm) parts of the PVN as well as in the median eminence (ME), median preoptic area (MPOA), supraoptic (SON) and dorsomedian (DMN) nuclei. Two-way analysis of variance revealed a very significant effect of genotype in the PVNp (P < 0.001), SON (P < 0.001) and in the ME (P < 0.02). No significant variations at all were noted in the ARC, PVNm, MPOA and DMN. GAL concentrations were more than doubled in the PVNp and SON of ad lib obese rats when compared to the ad lib lean rats (P < 0.005). On the other hand, in the ME where GAL concentration was about 4-fold greater than in the other areas, there was a 20 to 30% decrease in GAL concentrations in the obese rat (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Proteoglycan synthesis in normotensive and spontaneously hypertensive rat arteries in vitro

Proteoglycans (PGs) were analyzed and compared in the media of the thoracic aorta, abdominal aorta, left carotid artery and superior mesenteric artery of age-matched Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Two ages were examined; 10 week old, during the development of hypertension and 28 week old, when hypertension is well established in the SHR. Large chondroitin sulfate PG, large heparan sulfate PG and biglycan (PGI) and decorin (PGII) small PGs were identified. Biglycan was the predominant small PG found in all arteries. Newly synthesized PGs were labelled in vitro with 35SO4 for quantitation. The synthesis of large and small PGs was similar in the media of the thoracic aorta, abdominal aorta, left carotid artery, and superior mesenteric artery. The large to small ratio value, a measure of the artery PG composition, was also similar among the four arteries but was highest in the mesenteric artery. In both WKY and SHR arteries there was significantly decreased PG synthesis in the 28-week old compared to 10-week old animals. This was especially true for large PG. Hypertensive changes in PG synthesis were seen mainly in the carotid artery. In this artery, synthesis of both large and small PG was increased in the SHR, at both ages. The ratio of large to small PG was not significantly different between SHR and WKY arteries. We conclude that 28-week old WKY and SHR rat arteries synthesize less large and small PG than 10-week old arteries. The most prominent change seen in hypertensive rats is an increase in PG synthesis in the carotid artery.