Biomedical subjects

H Vaudry

Publications and source records attributed to H Vaudry.

At least 379 records · Page 21Linked to original sources

PubMed1991

Dopamine regulates the electrical activity of frog melanotrophs through a G protein-mediated mechanism.

Recently we have demonstrated that dopamine inhibits action potentials in cultured frog melanotrophs through D2 receptor-mediated activation of hyperpolarizing potassium current and reduction of calcium and sodium currents. Herein, the respective roles of G proteins, guanosine-5'-triphosphate and adenosine-3':5'-cyclic-monophosphate in dopamine-induced electrical responses were investigated using the whole-cell patch-clamp technique. Pretreatment of melanotrophs with pertussis toxin (1 microgram/ml) abolished the hyperpolarization and arrest of action potentials evoked by dopamine (1 microM) in 77% of the cells studied. Addition of guanosine-5'-O-(2-thiodiphosphate) (500 microM) to the intracellular solution did not alter the effects of a first exposure to dopamine, but completely blocked the response of cultured melanotrophs to subsequent pulses of dopamine. In cells which were dialysed with guanosine-5'-O-(3-thiotriphosphate) (100 microM) dopamine caused a sustained hyperpolarization and an irreversible inhibition of spikes. Voltage-clamp recordings with electrodes containing guanosine-5'-O-(3-thiotriphosphate), showed that the increase of potassium current and decrease of calcium and sodium currents caused by dopamine were irreversible. These effects were not modified when the pipette contained, in addition to guanosine-5'-O-(3-thiotriphosphate), a high concentration of adenosine-3':5'-cyclic-monophosphate (100 microM) together with the inhibitor of phosphodiesterases 3-isobutyl-1-methylxanthine (100 microM). It is concluded that, in cultured frog melanotrophs, a pertussis toxin-sensitive G protein is implicated in the coupling of dopamine D2 receptors to activation of potassium channels and inhibition of calcium and sodium channels. Our results also indicate that the G protein-mediated signal transduction does not involve the adenylate cyclase system.

Action Potentials↗

PubMed1991

Release of pro-thyrotropin-releasing hormone connecting peptides PS4 and PS5 from perifused rat hypothalamic slices.

Thyrotropin-releasing hormone prohormone contains five copies of the thyrotropin-releasing hormone progenitor sequence Gln-His-Pro-Gly, each flanked by pairs of basic amino acids and separated by intervening sequences (connecting peptides). Using a perifusion system for rat hypothalamic slices, we have studied the ionic mechanisms underlying the release of two connecting peptides originating from the thyrotropin-releasing hormone precursor: prepro-thyrotropin-releasing hormone-(160-169) (Ps4) and prepro-thyrotropin-releasing hormone-(178-199) (Ps5). Quantification of these two peptides in the effluent fluid was performed using sensitive and highly specific radioimmunoassay procedures. Reverse phase high performance liquid chromatography analysis of the effluent perifusate showed that released peptides co-eluted with synthetic Ps4 and Ps5. The secretion of Ps4 and Ps5 was stimulated by depolarizing agents such as (i) high potassium concentrations, (ii) ouabain, an Na+/K(+)-ATPase inhibitor, and (iii) veratridine, a stimulator of voltage-operated Na+ channels. The response to potassium (70 mM) was not affected by the specific Na+ channel blocker tetrodotoxin. The K+ channel blocker tetraethylammonium did not modify K(+)-evoked release of Ps4 and Ps5. These data suggest that voltage-operated Na+ channels are not involved in the stimulatory effect of high K+ on the release of Ps4 and Ps5. The lack of effect of picrotoxin, a Cl- channel blocker, on the secretion of the connecting peptides indicates that chloride ions play a minor role in the release process. In contrast, deprivation of Ca2+ in the perifusion medium suppressed K(+)-evoked release of the two peptides, indicating that voltage-operated Ca2+ channels are implicated in the release process. Taken together, the present results show that non-thyrotropin-releasing hormone peptides originating from the thyrotropin-releasing hormone precursor are secreted by mediobasal hypothalamic fragments. The release of these peptides is stimulated by depolarization through a calcium-dependent process. These data indicate that Ps4 and Ps5 may be released at the level of the median eminence into the portal circulation, suggesting that these peptides may play a role in the control of anterior pituitary cells.

Action Potentials↗

PubMed1991

Transient expression of somatostatin receptors in the rat visual system during development.

The ontogeny of somatostatin receptors in the rat visual system was studied by auto-radiography, using [125I-Tyr0,DTrp8]S14 as a radioligand. The binding sites showed high affinity for somatostatin and somatostatin analogues, and were regulated by GTP as early as day 16 of fetal life (E16), indicating that they represent functional somatostatin receptors. The density of somatostatin receptors was quantified by computerized image-analysis of film autoradiograms, and by grain counting on emulsion-coated slides. During fetal life, somatostatin receptors were observed in the retina, optic nerve, optic chiasma, optic tract, and lateral geniculate nucleus. The highest densities of somatostatin receptors were measured from E16 to E18 in the retina and primary optic pathways. During the first postnatal days, the density of somatostatin receptors decreased dramatically in the retina. In both the optic pathways and dorsal lateral geniculate nucleus, somatostatin receptors gradually disappeared, and the levels of somatostatin receptors were almost undetectable at postnatal day 21 (P21). Conversely, the density of somatostatin receptors remained stable in the ventral lateral geniculate nucleus during the early postnatal life (P0-P7). The timing of expression and the localization of somatostatin receptors in the developing visual system suggest that the immature ganglion cells are responsible for the expression of these evanescent somatostatin receptors. After eye opening, the distribution patterns of somatostatin receptors in the retina and the lateral geniculate nucleus were similar to those observed in adults. In particular, from P14 onwards, somatostatin receptors were concentrated in the inner plexiform layer and, to a lesser extent, in the ganglion cell and photoreceptor layers. In the ventral lateral geniculate nucleus, a heterogeneous distribution of somatostatin receptors was noted, the highest densities being found in the intergeniculate leaflet and the medial zone limiting the parvo-magnocellular interface. The distribution of somatostatin receptors in the retina and the ventral lateral geniculate nucleus after the second postnatal week, together with the presence of somatostatin-like immunoreactive elements in these structures, provide support for the involvement of somatostatin as a neurotransmitter or neuromodulator in the visual system of the adult rat. Conversely, the transient expression of somatostatin receptors observed before maturation and complete organization of the optic pathways suggests that somatostatin plays a trophic role during development of the visual system.

Aging↗

PubMed1991

Central-type benzodiazepines inhibit release of alpha-melanocyte-stimulating hormone from the rat hypothalamus.

In a previous work, we have shown that GABA inhibits the release of alpha-melanocyte-stimulating hormone (alpha-melanotropin) from hypothalamic neurons through activation of GABAA receptors [Delbende et al. (1989) Brain Res. 497, 86-93]. Since GABA-gated channel activity can be allosterically modulated by a variety of compounds including benzodiazepines, we have investigated the effect of benzodiazepines in the control of alpha-melanotropin release by the rat basal hypothalamus. This study was conducted in vitro using perifused rat hypothalamic slices and the amount of alpha-melanotropin release was monitored with a sensitive and highly specific radioimmunoassay. Infusion of clonazepam (50 microM), a selective agonist for central-type benzodiazepine binding sites, induced an inhibition of KCl (50 mM)-evoked alpha-melanotropin release. The inhibitory effect of clonazepam was rapid and reversible. Administration of Ro 15-1788 (100 microM), a specific antagonist for central-type benzodiazepine receptors or SR 95531, a GABAA receptor antagonist, completely reversed the inhibitory effect of clonazepam. In addition, Ro 15-1788 and SR 95531 both enhanced the amplitude of the response observed during prolonged KCl infusion on alpha-melanotropin neurons, suggesting the existence of a tonic inhibitory effect of endogenous GABA and/or benzodiazepines in the release of alpha-melanotropin by hypothalamic neurons. To investigate further the effect of benzodiazepines in the regulation of alpha-melanotropin neurons, rats were treated in vivo with clonazepam (5 mg/kg) or the non-selective benzodiazepine receptor agonist diazepam (3 mg/kg). Both compounds caused a significant increase in the content of alpha-melanotropin and beta-endorphin in the rat hypothalamus within 3 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Ranakinin: a novel NK1 tachykinin receptor agonist isolated with neurokinin B from the brain of the frog Rana ridibunda.

An extract of the whole brain of the frog Rana ridibunda contained high concentrations of substance P-like immunoreactivity, measured with an antiserum directed against the COOH-terminal region of mammalian substance P and neurokinin B-like immunoreactivity, measured with an antiserum directed against the NH2-terminus of neurokinin B. The primary structure of the substance P-related peptide (ranakinin) was established as: Lys-Pro-Asn-Pro-Glu-Arg-Phe-Tyr-Gly-Leu-Met-NH2. Mammalian substance P was not present in the extract. The primary structure of the neurokinin B-related peptide was established as: Asp-Met-His-Asp-Phe-Phe-Val-Gly-Leu-Met-NH2. This amino acid sequence is the same as that of mammalian neurokinin B. Ranakinin was equipotent with substance P and [Sar9,Met(O2)11]substance P in inhibiting the binding of 125I-Bolton-Hunter-[Sar9,Met(O2)11]substance P, a selective radioligand for the NK1 receptor, to binding sites in rat submandibular gland membranes (IC50 1.6 +/- 0.3 nM; n = 5). It is concluded that ranakinin is a preferred agonist for the mammalian NK1 tachykinin receptor subtype.

Amino Acid Sequence↗

PubMed1991

Neuropeptide-Y in the trout brain and pituitary: localization, characterization, and action on gonadotropin release.

Using a specific antiserum raised against synthetic neuropeptide-Y (NPY), the distribution of NPY-like immunoreactivity in the brain and pituitary of the trout Oncorhynchus mykiss has been examined with the indirect immunofluorescence and peroxidase-antiperoxidase methods. The highest density of NPY-immunoreactive elements was found in the basal telencephalon and hypothalamus. In particular, NPY-immunoreactive neurons were located in the nucleus entopeduncularis and the preoptic nucleus. NPY-immunoreactive fibers were observed throughout the trout brain. The preoptic nucleus, the suprachiasmatic nucleus, and the nucleus entopeduncularis were densely innervated. In addition, NPY-positive fibers were detected in the nucleus lateralis tuberis and in the distal and intermediate lobes of the pituitary. The NPY-like peptide of the trout brain was characterized by combining HPLC analysis and radioimmunological detection. Serial dilutions of trout hypothalamus and pituitary extracts produced displacement curves that were parallel to the standard curve. HPLC analysis resolved a major peak which was slightly less hydrophobic than porcine NPY. The possible effect of NPY, either alone or in combination with a GnRH antagonist, on gonadotropin (GtH) release from trout pituitaries was investigated using a perifusion system technique. Graded concentrations of synthetic NPY induced a dose-dependent stimulation of GtH release. The stimulatory activities of NPY and various short chain analogs on GtH release were compared: the order of potency was NPY greater than NPY-(2-36) greater than NPY-(16-36) greater than NPY-(25-36). This result suggests that the biological determinant of NPY is located in the C-terminal part of the molecule. Administration of a short pulse of NPY or GnRH (10(-7) M each) induced a marked stimulation of GtH release. Prolonged infusion of the GnRH antagonist D-Phe2-6,Pro3-GnRH induced a significant reduction of GnRH-evoked GtH secretion. In addition, the GnRH antagonist blocked NPY-induced GtH release. The widespread distribution of NPY in the trout brain suggests the involvement of this neuropeptide in a variety of physiological functions. The present data support the view that NPY, released by nerve terminals in the distal lobe of the pituitary, may act presynaptically on GnRH fibers to modulate GtH release.

Animals↗

PubMed1991

Dopamine inhibits inositol phosphate production, arachidonic acid formation, and corticosteroid release by frog adrenal gland through a pertussis toxin-sensitive G-protein.

We have previously shown that dopamine-evoked inhibition of corticosteroid production from adrenocortical cells is mediated through a decrease in prostaglandin biosynthesis. Since the catecholamine did not alter the stimulatory effect of arachidonic acid, it was proposed that dopamine may inhibit the formation of arachidonate from glycerophospholipids. To test this hypothesis, the effect of dopamine on phosphoinositol lipid metabolism was investigated in frog interrenal (adrenal) tissue. In [3H]myo-inositol-prelabeled frog interrenal slices, a short pulse of dopamine (50 microM) induced a biphasic effect on inositol phosphate production: a transient (1-min) increase, followed by a sustained inhibition. Concurrently, dopamine induced a transient reduction followed by a sustained increase in polyphosphoinositides. A 10-min pulse of the D2 dopamine receptor agonist apomorphine (50 microM) elicited a significant inhibition of basal levels of inositol phosphates (tris-, bis-, and mono-), and an increase in plasma membrane phosphoinositol lipid contents. The inhibitory effect of dopamine on inositol phosphate formation and corticosteroid release was abolished by a 24-h incubation of interrenal slices with pertussis toxin. In [3H]arachidonic acid-prelabeled interrenal slices, dopamine also decreased diacylglycerol (DG) and arachidonic acid (AA) concentrations. A delay of 1 min was observed between inhibition of DG and arachidonate, suggesting that AA is probably generated from DG. We conclude that in the adrenal cortex, activation of dopamine D2 receptors is coupled to a phosphoinositide-specific phospholipase-C mediated via a pertussis toxin-sensitive G-protein. Taken together, our data indicate that inhibition of inositol phosphate and AA formation is one of the mechanisms by which dopamine controls corticosteroid production by adrenocortical cells.

Adrenal Cortex Hormones↗

PubMed1991

Distribution and characterization of endogenous benzodiazepine receptor ligand (endozepine)-like peptides in the rat gastrointestinal tract.

Endozepine is the generic name for a family of peptides that are capable of displacing benzodiazepines and the 3-carboxylate ester of beta-carboline from their specific binding sites on synaptosomal membranes. The 104-amino acid polypeptide diazepam-binding inhibitor (DBI) and the octadecaneuropeptide (ODN) generated by tryptic digestion of DBI are two members of the endozepine family. In the present study we have used RIA, HPLC, in situ hybridization, and immunohistochemical techniques to identify and localize endozepine-like molecules in the rat gastrointestinal tract. Significant amounts of endozepine-like immunoreactivity (LI) were detected throughout the gut; the highest concentrations were found in the duodenum and antrum. HPLC analysis revealed that the immunoreactive material eluted as a major peak with a higher retention time than that of synthetic ODN. The distribution of the immunoreactive peptide(s) was studied using the peroxidase-antiperoxidase technique at the light microscope level. Endozepine-LI was localized only in the epithelial cell layer of the intestine in both goblet cells and enterocytes. In the stomach, endozepine-LI appeared to be restricted to deep layer of the epithelial cells. The diffuse neuroendocrine cells (amine precursor uptake and decarboxylation system) as well as myenteric and neuronal cells were devoid of immunoreactivity. A good correlation was observed between RIA and immunocytochemical data, in that the esophagus, which contained very low concentrations of endozepine-LI, also exhibited weak immunostaining of secretory cells. In situ hybridization using a 35S-labeled cRNA probe showed that endozepine mRNA was located in the mucosa. Taken together, these results show that in the rat, epithelial cells synthesize endozepine-LI material. Since epithelial cells also contain a high density of peripheral-type benzodiazepine-binding sites, our data indicate that endozepines may play a role in water, electrolyte, and/or mucus regulation in the rat gastrointestinal tract. The occurrence of high levels of endozepine-LI in the rat stomach also suggests that endozepines can be involved in the regulation of gastric acid secretion through modulation of local gamma-aminobutyric acid-ergic neurotransmission.

Animals↗

PubMed1991

Primary structure of frog pituitary adenylate cyclase-activating polypeptide (PACAP) and effects of ovine PACAP on frog pituitary.

Pituitary adenylate cyclase-activating polypeptide (PACAP), a peptide of the glucagon-secretin-vasoactive intestinal polypeptide superfamily, was isolated in pure form from the brain of the European green frog, Rana ridibunda. The primary structure of the peptide indicates that evolutionary pressure to conserve the complete amino acid sequence has been very strong. Frog PACAP comprises 38 amino acid residues and contains only 1 substitution (isoleucine for valine at position 35) compared with human/ovine/rat PACAP. In the presence of the phosphodiesterase inhibitor isobutylmethylxanthine, synthetic ovine PACAP-(1-38) produced a dose-dependent increase in the concentration of cAMP in isolated frog anterior pituitary fragments (ED50 = 2.1 +/- 0.6 x 10(-7) M; mean +/- SE; n = 6). Maximum stimulation (an approximately 8-fold increase in concentration over basal values) was produced by 10(-6) M peptide. The truncated form of PACAP [PACAP-(1-27)] also produced a dose-dependent increase in cAMP in frog anterior pituitary fragments, and the potency of the peptide (ED50 = 5.9 +/- 0.6 x 10(-8) M) was comparable to that of PACAP-(1-38). The data suggest, therefore, that the function as well as the structure of PACAP have been conserved during the evolution of amphibia to mammals.

1-Methyl-3-isobutylxanthine↗

PubMed1991

Self-inhibition of steroid secretion by amphibian adrenocortical cells is not mediated through glucocorticoid receptors.

To investigate a possible direct action of glucocorticoids on adrenal steroidogenesis, the effect of corticosterone on the conversion of pregnenolone into various metabolites by frog adrenal tissue was examined. Frog interrenal slices were incubated with [3H]pregnenolone (1 mCi/ml) and the various labelled metabolites analysed by reverse-phase high-performance liquid chromatography. With the methanol gradient used, five identified steroids were resolved: progesterone, 11-deoxycorticosterone, corticosterone, 18-hydroxycorticosterone and aldosterone. Corticosterone (10 micrograms/ml) induced a 45-80% decrease in all steroids synthesized from [3H]pregnenolone. In contrast, the glucocorticoid agonist dexamethasone did not reduce the rate of conversion of pregnenolone into its metabolites. In addition, the inhibitory effect of corticosterone was not reversed by the specific glucocorticoid antagonist RU 43044. These results show that corticosterone exerts a direct inhibitory effect on adrenal steroid secretion. In addition, our data indicate that the ultra-short regulation induced by corticosterone is not mediated through glucocorticoid receptors.

Adrenal Cortex↗

PubMed1991

[The intermediate lobe of the pituitary, model of neuroendocrine communication].

The intermediate lobe of the pituitary is composed of a homogeneous population of endocrine cells, the melanotrophs, which secrete several bioactive peptides including alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. In contrast to most endocrine glands which are richly vascularized, the intermediate lobe of the pituitary contains very few blood vessels; in some species, the pars intermedia is virtually totally avascular. In contrast, pituitary melanotrophs are richly supplied by nerve fibers originating from the hypothalamus. The pars intermedia thus appears as a pure model of neuroendocrine communication, i.e. it is an archetype of the mode of transducing interface between the central nervous system and endocrine effectors. In mammalian species, different types of nerve terminals containing dopamine, norepinephrine, gamma-aminobutyric acid (GABA) and serotonin have been identified. In lower vertebrates, particularly in fish and amphibians, the pars intermedia is also innervated by peptidergic fibers which are though to take part in regulation of the secretory activity of the melanotroph. In these animals, the pars intermedia is regarded as a major center of neuroendocrine integration and an exceptional model to investigate the process of communication between the brain and the endocrine glands. The purpose of the present review is to summarize our current knowledge on the synthesis, processing and release of peptide hormones from pars intermedia cells and to survey the multiple regulatory mechanisms which are involved in the control of the activity of pituitary melanotrophs. Proopiomelanocortin, a multifunctional precursor. Pituitary melanotrophs synthetise a major precursor protein called proopiomelanocortin (POMC) which generates through proteolytic cleavage several biologically active peptides including adrenocorticotropic hormone (ACTH), endorphins and MSHs. In lower vertebrates, alpha-MSH is generally considered as the major hormone secreted by melanotrophs, in that it is involved in the process of skin colour adaptation. The post-translational processing of POMC, which yields to the mature hormones released by melanotrophs, includes a number of steps: glycosylation, phosphorylation, tissue-specific proteolytic cleavage, amidation and acetylation. Some of these posttranslational modifications can be regulated by neuroendocrine factors. For instance, in frogs, it has been shown that dopamine inhibits acetylation of alpha-MSH and thus reduces the secretion of the biologically active form of the peptide. The intermediate lobe of the pituitary: a model of neuroendocrine integration. In most vertebrate species, the intermediate lobe of the pituitary is innervated by catecholamine-containing fibers. In particular, the presence of dopaminergic nerve fibers has been observed in the pars intermedia of mammals and poikilotherms.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗

PubMed1990

Characterization of the cDNA encoding proopiomelanocortin in the frog Rana ridibunda.

In the amphibian pars intermedia, secretion of proopiomelanocortin (POMC)-derived peptides is controlled by multiple factors including classical neurotransmitters and neuropeptides. To pursue questions concerning the regulation of POMC gene expression in Rana ridibunda, we have isolated and characterized a full-length cDNA for frog POMC. A cDNA clone isolated from a frog pituitary library contains an open-reading frame of 780-bp that predicts a 260 amino acid POMC protein. The structure of frog POMC demonstrates considerable amino acid sequence similarity with POMC from other species. In particular, the sequence of alpha-melanotropin (alpha-MSH) is identical in frog and all mammalian species studied so far, while adrenocorticotropin (ACTH) and beta-endorphin exhibit 79% and 84% homology with their human counterpart. Frog POMC contains only one potential asparagine-linked N-glycosylation signal (Asn-Ser-Thr) within the gamma-MSH domain. The alpha-MSH sequence is C-terminally flanked by the Gly-Lys-Lys amidation signal while the joining peptide is not amidate.

Amino Acid Sequence↗

PubMed1990

Effect of acetylcholine on the electrical and secretory activities of frog pituitary melanotrophs.

The activity of melanotroph cells of the amphibian pars intermedia is regulated by multiple factors including classical neurotransmitters and neuropeptides. In this study, we have examined the possible involvement of acetylcholine (ACh) in the regulation of electrical and secretory activities of frog pituitary melanotrophs. Electrophysiological recordings were conducted on cultured cells by using the patch-clamp technique in the whole-cell configuration. In parallel, alpha-MSH release from acutely dispersed pars intermedia cells was studied by means of the perifusion technique. In all cells tested in the current-clamp mode, superfusion with ACh (10(-6) M) gave rise to a depolarization associated with an enhanced frequency of action potentials. Administration of ACh (10(-6) M) to perifused cells also induced stimulation of alpha-MSH release. These results indicate that the neurotransmitter ACh exerts a direct stimulatory effect on pituitary melanotrophs. The action of ACh on electrical and secretory activities was mimicked by muscarine (10(-5) M), while ACh-induced alpha-MSH secretion was completely abolished by the muscarinic antagonist atropine (10(-6) M). The depolarizing effect of muscarine was suppressed by the specific M1 muscarinic antagonist pirenzepine (10(-5) M), indicating the existence of a M1 subtype muscarinic receptor in frog pars intermedia cells. In addition, using a monoclonal antibody against calf muscarinic receptors, we have visualized, by the immunofluorescence technique, the presence of muscarinic receptor-like immunoreactivity in cultured intermediate lobe cells. Electrophysiological recordings showed that nicotine (10(-5) M) induces membrane depolarization associated with an increase of the frequency of action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗

PubMed1990

Expression of somatostatin receptors is impaired in the cerebellum of developing Brattleboro rats.

Somatostatin (SRIF) receptors are expressed in the external granule cell layer of the rat cerebellum during early postnatal life. The aim of the present study was to investigate the distribution and biochemical characteristics of SRIF binding sites in the cerebellum of homozygous (vasopressin deficient) Brattleboro rats, which exhibit a selective impairment of their granule cell layer. This study has been conducted in 13-day-old rats by means of membrane-binding assay and autoradiography using [125I-Tyr0,DTrp8]S14 as a radioligand. In the cerebellum of homozygous Brattleboro rats, Scatchard plot analysis revealed the existence of a single class of SRIF receptors with similar Kd values as in Long-Evans or heterozygous Brattleboro rats (180-200 pM). Conversely, a marked reduction of the concentration of SRIF binding sites was observed in Brattleboro rats as compared to heterozygous or Long-Evans rats. In homozygous Brattleboro rats, autoradiographic studies revealed that the concentration of SRIF receptors was reduced in all lobules of the cerebellum as compared to Long-Evans. In addition, the magnitude of the decrease of receptor concentration was greater than the loss of granule cells observed in the homozygous Brattleboro rat. These results indicate that the expression of SRIF receptors by immature granule cells of the cerebellum is markedly reduced in Brattleboro rats. Whether the impairment of SRIF receptors in diabetes insipidus rats can directly be ascribed to vasopressin deficiency remains to be determined.

Animals↗

PubMed1990

Localization and characterization of neuropeptide Y in the brain of Microcebus murinus (Primate, Lemurian).

The distribution of neuropeptide Y (NPY) in the brain of the lemur Microcebus murinus was determined by immunocytochemistry with the aid of a highly specific antiserum against synthetic porcine NPY. When compared with previous immunohistochemical data obtained in primates and other mammalian species, the localization of NPY-immunoreactive (IR) structures in the Microcebus murinus brain revealed particular features. (1) Numerous NPY-IR perikarya and a dense network of IR nerve terminals were found in the supraoptic and suprachiasmatic nuclei, respectively. The occurrence of NPY-IR perikarya in the supraoptic nucleus, also reported in the squirrel monkey, seems to be specific to primates. In the squirrel monkey, the suprachiasmatic nucleus exhibits only a moderate innervation, whereas in humans it appears totally devoid of NPY-IR fibers. (2) IR perikarya and axon processes were observed in many upper brainstem areas, in particular in the interpeduncular, raphe pontine, dorsal tegmental, parabrachial, and dorsal raphe nuclei, in the locus coeruleus, the nucleus of the solitary tract, and the reticular formation; in this latter area, the occurrence of two categories of NPY-IR neurons was demonstrated on the basis of their morphology and localization, suggesting that they may play distinct roles. (3) NPY-IR nerve processes could be traced over a long distance. (4) For the first time, numerous NPY-IR terminals were observed close to the lumen of the various cerebral ventricles. The immunoreactive NPY-like peptide was characterized by combining high performance liquid chromatography (HPLC) analysis and radioimmunoassay quantification. The dilution curves obtained with synthetic porcine NPY and serial dilutions of occipital cortex, paraventricular and supraoptic hypothalamus, posterior hypothalamus, medulla oblongata, or preoptic area extracts were parallel. The highest amounts of NPY were measured in the hypothalamus and telencephalon. HPLC analysis resolved a single peak of NPY-like immunoreactivity that exhibited the same retention time as synthetic porcine NPY. The distribution of NPY in the lemurian brain is discussed with respect to phylogeny and putative functions.

Animals↗

PubMed1990

Electrophysiological evidence for the existence of GABAA receptors in cultured frog melanotrophs.

The neurotransmitter GABA exerts a biphasic effect on alpha-melanocyte-stimulating hormone (alpha-MSH) secretion from pars intermedia cells: GABA induces a rapid and transient stimulation followed by a sustained inhibition of alpha-MSH release. In the present study, we have investigated the effect of GABA on the electrophysiological properties of frog melanotrophs in primary culture using the patch-clamp technique in the whole cell configuration. In all cells tested, GABA stimulated an inward current and induced depolarization. A transient period of intense firing was consistently observed at the onset of GABA administration. During the depolarization phase, the membrane potential reached a plateau corresponding to the Cl- equilibrium potential. When repeated hyperpolarizing pulses were applied, an increase of membrane conductance was observed throughout the response evoked by GABA. The effect of GABA was abolished by the chloride channel blocker picrotoxin, and by antagonists of GABAA receptors (bicuculline and SR 95531). The depolarizing action of GABA was mimicked by muscimol, an agonist of GABAA receptors. Taken together, our results indicate that the rapid and transient stimulation of alpha-MSH release induced by GABA can be accounted for by activation of a chloride conductance which causes membrane depolarization. These data support the notion that the transient stimulation of alpha-MSH secretion induced by GABA can be accounted for by membrane depolarization which provokes activation of voltage-operated calcium channels. Since no evidence was found for GABA-induced hyperpolarization, the intracellular mechanisms leading to the strong inhibitory effect of GABA on alpha-MSH secretion remain to be elucidated.

Animals↗

PubMed1990

Characterization of alpha-melanocyte-stimulating hormone (alpha-MSH)-like peptides in discrete regions of the rat brain. In vitro release of alpha-MSH from perifused hypothalamus and amygdala.

The neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) is synthesized by discrete populations of hypothalamic neurons which project in different brain regions including the cerebral cortex, hippocampus and amygdala nuclei. The purpose of the present study was to identify the alpha-MSH-immunoreactive species contained in these different structures and to compare the ionic mechanisms underlaying alpha-MSH release at the proximal and distal levels, i.e. within the hypothalamus and amygdala nuclei, respectively. The molecular forms of alpha-MSH-related peptides stored in discrete areas of the brain were characterized by combining high-performance liquid chromatography (HPLC) separation and radioimmunoassay detection. In mediobasal and dorsolateral hypothalamic extracts, HPLC analysis confirmed the existence of a major immunoreactive peak which co-eluted with the synthetic des-N alpha-acetyl alpha-MSH standard. In contrast, 3 distinct forms of immunoreactive alpha-MSH, which exhibited the same retention times as synthetic des-, mono- and di-acetyl alpha-MSH, were resolved in amygdala nuclei, hippocampus, cortex and medulla oblongata extracts. The proportions of acetylated alpha-MSH (authentic alpha-MSH plus diacetyl alpha-MSH) contained in these extrahypothalamic structures were, respectively, 78, 80, 60 and 92% of the total alpha-MSH immunoreactivity. In order to compare the ionic mechanisms underlaying alpha-MSH release from hypothalamic and extrahypothalamic tissues, we have investigated in vitro the secretion of alpha-MSH by perifused slices of hypothalamus and amygdala nuclei. High potassium concentrations induced a marked increase of alpha-MSH release from both tissue preparations. However, a higher concentration of KCl was required to obtain maximal stimulation of amygdala nuclei (90 mM) than hypothalamic tissue (50 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylation↗

PubMed1990

Effect of platelet-activating factor on hypothalamic and hypophyseal pro-opiomelanocortin-related peptides and hypothalamo-pituitary-adrenal axis in the rat.

To examine the effect of platelet-activating factor (PAF-acether) on pro-opiomelanocortin (POMC)-related peptides and on the hypothalamo-pituitary-adrenal axis, we administered PAF-acether and BN 52021, a selective PAF-acether antagonist, to freely moving rats. Minipumps loaded with either PAF-acether (30 micrograms/kg) or the vehicle alone were connected to the jugular vein for 7 days and positioned under the back skin of rats. A group of animals treated with PAF-acether also received 15 mg/kg of BN 52021 orally twice a day. In vivo treatment with PAF-acether alone or in association with BN 52021 did not affect the hypothalamic concentrations of corticotropin-releasing factor (CRF), alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. Using a perifusion system for rat hypothalamic slices, we did not observe any effect of PAF-acether on spontaneous or potassium-induced release of alpha-MSH in vitro. In addition, treatment of rats with PAF-acether alone or in association with BN 52021 did not modify the alpha-MSH or beta-endorphin concentration in the neurointermediate lobe of the pituitary. In contrast, in vivo administration of PAF-acether caused a significant reduction of ACTH concentration in the anterior lobe of the pituitary and a marked decrease in the corticosterone level in plasma and adrenal glands. The inhibitory effect of PAF-acether was reversed by concomitant administration of BN 52021. The ineffectiveness of PAF-acether to modulate in vitro ACTH release from perifused anterior pituitary fragments ruled out a direct effect of PAF-acether on corticotrophs. These findings support the view that PAF-acether exerts a specific inhibitory effect on the hypothalamo-pituitary-adrenal axis.

Adrenocorticotropic Hormone↗