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

Localization and role of galanin in the thyroid gland of Podarcis sicula lizard (reptilia, lacertide)

Galanin (GAL) is a 29-amino acid residue neuropeptide, which was initially isolated from porcine intestine extracts and since then, widely found in a variety of vertebrate organs, in correlation with multiple neuro-hormonal actions exerted and so receiving a constantly growing attention. Moreover, although the studies undertaken so far suggest a local intrathyroidal peptidergic regulatory action, the exact role of GAL on thyroid gland remains to be established. The aim of this study was to determine in the lizard, Podarcis sicula, (1) the presence of GAL immunoreactivity in the thyroid gland and (2) the short- and long-term effects of in vivo GAL administration by intraperitoneal injection on thyroid gland physiology. First of all, the presence of GAL in the thyroid gland of P. sicula was demonstrated by immunohistochemical technique (avidin-biotin-peroxidase complex--ABC method). Second, the role of GAL in the control of thyroid gland activity was studied in vivo using light microscopy (LM) technique coupled to a specific radioimmunoassay for thyroid-stimulating hormone (TSH) and thyroid hormones (T(4) and T(3)). Prolonged GAL administration [(0.4 mg/100 g body wt)/day] increased T(4) and T(3) release, but decreased the plasma concentration of TSH. In addition, using LM clear signs of stimulation of the thyroid gland were observed. These findings suggest that systemic administration of GAL was able to stimulate the thyroid gland of the lizard both at morphological and physiological level.

Distribution of galanin-like immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri)

Galanin is a 29-amino acid peptide widely distributed in the central nervous system of vertebrates. The organization of galaninergic systems is well known in teleosts, the most advanced actinopterygians, but no data are available on primitive bony fish. To extend the evolutionary analysis of galaninergic systems we studied the distribution of galanin-like immunoreactive (GAL-ir) cells and fibers in the sturgeon brain, since chondrosteans are among the most primitive extant actinopterygians. Double-immunolabeling experiments were performed to compare the distribution of galanin with that of neurophysin, tyrosine hydroxylase, and serotonin. Numerous GAL-ir cells of cerebrospinal fluid-contacting (CSF-C) type were found in the ventral telencephalon, preoptic area, and in the tuberal and caudal hypothalamus. The distribution of GAL-ir elements in the sturgeon brain shows many similarities to that observed in other vertebrates, but also important differences, such as the abundance of GAL-ir CSF-C cells, which appear to be a primitive characteristic. GAL-ir neurons observed in the sturgeon telencephalic hemispheres perhaps represent the basic organization of common ancestors of bony fishes and tetrapods. In the preoptic-hypophyseal system, GAL-ir cells appeared to be related not only with neurophysin-expressing neurons (in the tuberal hypothalamus) but also with serotoninergic and catecholamines-synthesizing neurons (in preoptic and tuberal nuclei). Numerous GAL-ir fibers were observed in the median eminence and neural lobe of the hypophysis, indicating that galanin may play a role in the modulation of hypophyseal secretion. GAL-ir neurons were absent from the sturgeon brainstem, suggesting that their presence in other vertebrates could represent an evolutionary recent acquisition.

Evidence for the c-ret protooncogene product (c-Ret) expression in the spinal tanycytes of adult rat

Expression of the protein product of c-ret (c-Ret) in the spinal cord of the adult rat was examined immunohistochemically at both electron and light microscopic levels. In the cervical, thoracic and lumbar segments of the spinal cord, a large number of c-Ret immunoreactive cells were found in both ependymal and subependymal layers of the central canal. These cells were ovoid or triangular in shape and had a well developed single cytoprocess which protruded into the central canal. None of the neuropeptides and neuronal markers examined, including substance P, CGRP, galanin, neuropeptide Y, tyrosine hydroxylase, methionine-enkephalin, choline acetyltransferase and glially fibrilally acidic protein, was present in these c-Ret immunoreactive cells in the spinal cord. Ultrastructurally, a desmosome-like structure was found between the apical part of the cytoprocess and the ependymal cell. These morphological observations indicated that c-Ret positive cells are spinal tanycytes. The present results suggest that spinal tanycytes in the rat express a trophic factor receptor and may respond to GDNF in the cerebrospinal fluid.

Effects of estrous cyclicity on the expression of the galanin receptor Gal-R1 in the rat preoptic area: a comparison with the male

Variations in the number of galanin receptor (Gal-R1)-expressing cells and levels of Gal-R1 messenger RNA (mRNA) were determined in the preoptic area in intact female rats throughout the phases of the estrous cycle and compared with those in the male. Female and male Wistar rats were fixed by perfusion with 4% paraformaldehyde. Cryostat sections were hybridized with a 35S-labeled antisense Gal-R1 riboprobe. The number of Gal-R1 mRNA-expressing cells was lower in the rostral preoptic area than in the medial preoptic area. During the estrous cycle, the highest number of Gal-R1 mRNA-expressing cells in the rostral preoptic region was detected at 0800 h on proestrus, whereas in the medial preoptic area, the maximum number was observed at 1800 h on estrus. Gal-R1 mRNA levels in individual cells were low during diestrus and increased at estrus in both areas. In the male, the number of mRNA-expressing cells and the hybridization signal were significantly lower than those in females during estrus. The results demonstrate that Gal-R1 gene expression in the preoptic area varies during the estrous cycle and is low in males. Short term treatment of ovariectomized rats with estradiol plus progesterone caused significantly decreased preoptic Gal-R1 mRNA levels compared with those after treatment with estrogen only. These observations suggest that in the preoptic area, expression of Gal-R1 is influenced by progesterone. The variation in Gal-R1 expression is likely to influence the extent to which galanin can influence the preoptic cells implicated in the control of neighboring GnRH cells.

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.

Semi-quantitative ultrastructural analysis of the localization and neuropeptide content of gonadotropin releasing hormone nerve terminals in the median eminence throughout the estrous cycle of the rat

The ultrastructural appearance of gonadotropin releasing hormone-immunoreactive elements was studied in the external zone of the median eminence of adult female Wistar rats. On the one hand, the purpose of the study was to determine the distribution of gonadotropin releasing hormone terminals towards the parenchymatous basal lamina at the level of hypothalamo-hypophyseal portal vessels, throughout the estrous cycle. On the other hand, we have semi-quantified the gonadotropin releasing hormone content in nerve terminals or preterminals during this physiological condition. A morphometric study was coupled to a colloidal 15 mn gold postembedding immunocytochemistry procedure. Animals were killed at 09.00 on diestrus II, 0.900, 10.00, 13.00, 17.00 and 18.00 on proestrus and 09.00 on estrus (n = 4-8 rats/group). A preliminary light microscopic study was carried out to identify an antero-posterior part of median eminence strongly immunostained by anti-gonadotropin releasing hormone antibodies but which was, in addition, easily spotted. This last condition was necessary to make a good comparison between each animal. Contacts between gonadotropin releasing hormone nerve terminals and the basal lamina were observed only the day of proestrus. Such contacts, however, were rare and in the great majority of cases, gonadotropin releasing hormone terminals are separated from basal lamina by tanycytic end feet. The morphometric analysis showed no significant variation in average distance between gonadotropin releasing hormone terminals and capillaries throughout the estrous cycle. Consequently, it did not appear that a large neuroglial plasticity exists during the estrous cycle. However, the observation of contacts only on proestrus together with some ultrastructural images evoke the possibility of a slight plasticity. The semi-quantitative results show that the content of gonadotropin releasing hormone in the nerve endings presented two peaks on proestrus: one at 09.00 (23 +/- 5 particles/micrograms2, P < 0.03) before the onset of luteinizing hormone surge, and the second at 18.00 (16 +/- 2 particles/micrograms2, P < 0.01) concomitantly with the luteinizing hormone surge, when compared to baseline values on proestrus 10.00 (8 +/- particles/micrograms2).