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Biomedical subjects

G Pelletier

Publications and source records attributed to G Pelletier.

At least 379 records · Page 21Linked to original sources
PubMed1989

Localization and characterization of prolactin-like immunoreactivity in the pituitary of the frog Rana ridibunda.

Distribution and quantification of PRL in the pituitary gland of the frog Rana ridibunda were investigated using a high-affinity antiserum raised against bullfrog prolactin (PRL). The immunoreactive PRL-producing cells were distributed throughout the pars distalis, the highest density of cells being observed in the rostral region of the adenohypophysis facing the neurointermediate lobe. The dorsal region of the pars distalis contained only a few scattered PRL-immunoreactive cells. At the electron microscopic level, PRL-containing cells were visualized using the immunogold procedure. PRL-immunoreactive material was exclusively stored in secretory granules (size ranging from 200 to 700 nm in diameter). Neither the rough endoplasmic reticulum nor the Golgi apparatus were immunolabeled. Using a radioimmunoassay method we have compared the displacement curves obtained with bullfrog PRL and acetic extracts from Rana ridibunda pituitary. The two binding curves were not completely parallel, suggesting the existence of slight variations of the amino acid sequences of PRL in the two species. The concentration of PRL in the green frog adenohypophysis appeared somewhat higher (35.3 +/- 8.8 micrograms/mg protein) than that in the bullfrog pituitary. These results validate the use of an antiserum to bullfrog PRL to investigate the regulation of PRL secretion in Rana ridibunda.

Animals↗
PubMed1989

Ontogeny of prolactin mRNA in the rat pituitary gland as evaluated by in situ hybridization.

We have studied the ontogeny of prolactin (PRL) messenger RNA (mRNA) in male and female rats. Quantitative in situ hybridization was performed on sections of fixed pituitaries using a 32S-labeled PRL cDNA probe. With this technique, hybridization signal was first detected on day 19 of gestation. The PRL mRNA levels were very low in foetuses and newborn animals. Higher PRL mRNA levels were found in 5-day-old animals. Thereafter, mRNA concentrations regularly increased to reach a plateau at 60 and 90 days of age in males and females, respectively. Sexual dimorphism was first observed in 20-day-old animals, the PRL mRNA levels being higher in the female than in the male. This difference in PRL mRNA became more marked after puberty such that in 90-day-old animals the amounts of PRL mRNA in females were 2.7 times those observed in males. Since sexual dimorphism in PRL mRNA levels occurs well before sexual dimorphism in PRL secretion, which takes place first during puberty, it is suggested that during sexual maturation PRL secretion is regulated translationally as well as transcriptionally.

Animals↗
PubMed1989

Castration and testosterone effects on endogenous and somatocrinin-induced growth hormone release in intact and castrated male pigs.

Growth hormone (GH) release is influenced mainly by two hypothalamic factors, growth hormone-releasing factor (GRF) and somatostatin and is modulated by other hormones such as gonadal steroids. The objective of this study was to determine if castration (CA) and exogenous testosterone (TE) affect endogenous and GRF-induced GH release. Purebred Yorkshire male pigs (n = 32) were assigned to one of the following treatments: T1:CA; T2:CA +/- TE; T3: intact (IN); T4: IN +/- TE, in a 2 x 2 factorial design. Piglets were castrated at 3 days of age. Testosterone propionate (1.0 mg/kg) in sesame oil (2 ml) or sesame oil alone was injected sc SID during a 10-day period before each sampling day at 9, 15 and 21 weeks of age. Jugular blood samples were collected for a 6-hr period preceding and following iv injection of hGRF (1-29)NH2 (10 micrograms/kg). These procedures were repeated at 9, 15 and 21 weeks of age. The overall mean GH levels and the area under the GH peaks before and after GRF stimulation were lower (P less than .05) in castrated animals than in intact animals. Testosterone treatment increased (P less than .05) circulating TE levels and increased the amplitude of the endogenous GH peaks but did not affect (P greater than .05) the GRF-induced GH release. Increasing age produced a marked reduction of the amplitude of the GH peaks, the area under the GH peaks, the baseline mean and the overall mean GH levels during the 6-hr period preceding GRF injection. The present data support the hypothesis that castration in pigs reduces circulating and GRF-induced GH release. Exogenous testosterone for 10 days did not stimulate endogenous or GRF-induced GH release with the exception of the amplitude of the endogenous GH peaks.

Animals↗
PubMed1989

Plasma lactose after weaning and its relationship with lactose content of milk, post-weaning plasma oestradiol-17 beta and weaning to mating interval in sows.

Twenty first-liter sows were used to investigate the relationship between plasma lactose concentrations following weaning and milk production, total milk content of lactose, post-weaning plasma oestradiol-17beta (E2) concentration and weaning to mating interval. Milk production was estimated from 6 out of 8 hourly successive "weighing-suckling-weighing" of piglets on day 21 of a 28-day lactation. Sows were cannulated in the jugular vein and sampled at 8-h intervals from 2 h to 66 h after weaning. Plasma lactose concentrations (means +/- SE) after weaning increased from 52 +/- 4 microM at the beginning of sampling to a peak value of 183 +/- 23 microM 40 h later and then decreased to 91 +/- 11 microM 66 h after weaning (P less than 0.001). Milk production on day 21 (7.5 +/- 0.3 kg) and the corresponding milk content of lactose (400.5 +/- 0.2 g) were not related (P greater than 0.10) to area under the curve, timing, amplitude and spreading of the lactose peak in plasma. Similarly, none of the characteristics of the lactose peak were related (P greater than 0.10) to weaning to mating interval. There was a linear increase (P less than 0.0001) in mean plasma E2 concentrations (means +/- SE) from 5.6 +/- 0.3 pg/ml, 2 h after weaning, to 12.9 +/- 1.2 pg/ml at the end of the sampling period. On a within-sow basis, there was a correlation (r = 0.28; P less than 0.01) between post-weaning plasma lactose and E2 concentrations. Results showed the existence of the post-weaning peak of plasma lactose in sows.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Neuropeptide Y (NPY) innervation of the ovine pineal gland.

A dense network of neuropeptide Y (NPY)-like immunoreactive (NPY-LI) fibres was revealed in the ovine pineal gland at the light microscope level. The dorsal and peripheral regions of the gland contained the most dense concentration of NPY-LI fibres with relatively few fibres in the mid-region and almost none in the pineal stalk. The effect of NPY in conjunction with isoproterenol (ISO) on cyclic AMP (cAMP) accumulation and noradrenaline (NA) on melatonin synthesis was investigated using in vitro techniques. NPY had no effect on the stimulation of cAMP or melatonin synthesis by the adrenergic agonists.

Animals↗
PubMed1989

Hypercalcaemia associated with chronic viral hepatitis.

A patient with long lasting non-parathyroid hormone mediated hypercalcaemia occurring within the context of hepatitis B virus chronic hepatitis is reported. Hepatocellular carcinoma and bone malignancy were carefully excluded. The biological pattern associated hypercalcaemia with normal phosphataemia, low nephrogenic cAMP level and high level of tubular reabsorption of phosphate. The usual causes of hypercalcaemia were ruled out. Hypercalcaemia may represent a rare biological feature of some advanced liver disease. The underlying mechanisms remain to be elucidated.

Calcium↗
PubMed1989

Influence of the central nucleus of the amygdala on the content of corticotropin-releasing factor in the median eminence.

The central nucleus of the amygdala possesses numerous neurons containing corticotropin-releasing factor (CRF). This study demonstrates a striking decrease of the CRF-like immunoreactivity in the median eminence at both 1 and 2 weeks after bilateral lesions of the amygdaloid central nucleus. Lesion of the amygdaloid central nucleus did not alter the neurophysin-like immunoreactivity in the internal zone of the median eminence, indicating the integrity of the efferent neurophysin-containing fibers of the supraoptic and paraventricular hypothalamic nuclei. However, there was a concomitant decrease of neurophysin and CRF-like immunoreactivity in the external zone of the median eminence. These results substantiate the hypothesis that the amygdaloid central nucleus can influence the content of CRF-like material in the median eminence via a multisynaptic pathway involving the synthesis of CRF at the level of the paraventricular nucleus of the hypothalamus. The exact mechanism by which lesion of the amygdaloid central nucleus influences the CRF content in the median eminence remains to be determined.

Amygdala↗
PubMed1989

Effect of corticotropin-releasing factor on the release and synthesis of prolactin.

Corticotropin-releasing factor (CRF) has been characterized on the basis of its intrinsic activity to release corticotropin from cultured rat anterior pituitary cells. Injected in intact rats, CRF increases adrenocorticotropic hormone (ACTH) release. Endogenous CRF-like immunoreactivity was detected in the cytoplasm and nucleus of corticotrophs. Using an antirat CRF serum, a similar location of CRF-like immunoreactivity was observed in lactotrophs: cytoplasmic matrix, secretory granules, nucleus and, to a lesser degree, the plasma membrane level were stained. One injection of CRF increased the plasma ACTH concentration 4-fold after 15 min, while plasma prolactin (PRL) increased 2.7-fold 5 min after injection. In vitro, incubation of female pituitary cells with rat CRF (10(-10)-10(-8) M) had no significant effect on PRL secretion. In contrast, after 4 days of in vitro pretreatment with 17 beta-estradiol (10(-9) M), rat CRF stimulated PRL secretion by 42%. In situ hybridization of whole pituitary slices showed that rat CRF injection significantly increased the labeling of corticotrophs using an ACTH-cDNA probe, but had no significant effect on the labeling of lactotrophs using a PRL riboprobe. These results indicate that CRF is a factor which can modulate PRL release but not the synthesis of PRL.

Adrenocorticotropic Hormone↗
PubMed1989

Thyroid hormone regulation of neurons staining for a pro-TRH-derived cryptic peptide sequence in the rat hypothalamic paraventricular nucleus.

The influence of thyroid hormones on neurons staining for a pro-TRH-derived cryptic peptide sequence in the rat hypothalamic paraventricular nucleus was assessed by using quantitative immunocytochemistry. The antiserum used for immunocytochemical studies recognized a cryptic fragment of pro-TRH located in the Phe 178-Ser 186 region of the precursor. Hypothyroid female rats obtained following 1 month administration of 6-n-propyl-2-thiouracil (PTU) as well as euthyroid animals were treated with a low (3 micrograms/kg/day) or high (67 micrograms/kg/day) dose of L-thyroxine (T4) for 4-10 days before sacrifice. All the animals received an intraventricular injection of colchicine 2 days prior to perfusion with 4% paraformaldehyde. PTU treatment induced an increase (45% over control) in the number of immunostained neurons. The administration of a low dose of T4 to hypothyroid animals slightly increased the number of staining for a pro-TRH-derived cryptic sequence neurons, while being uneffective in euthyroid rats. In hypothyroid animals, a 4-day treatment with the high dose of T4 did not produce any changes whereas the 7-day treatment reduced the number of positive cells to 42% of control. In euthyroid rats treated with the high dose of T4 for 4 and 7 days, the number of stained neurons was reduced to 60 and 37% of control, respectively. These results indicate that thyroid hormones can modify the amounts of staining for a pro-TRH-derived cryptic sequence in hypothalamic paraventricular neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1989

Electron microscopic autoradiographic localization of prolactin mRNA in rat pituitary.

Recent immunoelectron microscopic studies have shown that immunoreactive prolactin (PRL) in rat pituitary can be detected not only in typical PRL cells, characterized by large secretory granules, but also in another type of cell, which contains small secretory granules. To determine whether or not these two cell types are involved in PRL biosynthesis, we developed a procedure to investigate PRL gene expression by using in situ hybridization at the ultrastructural level. Rat pituitary was fixed and vibratome sections were incubated with a PRL [35S]-cDNA probe and subsequently flat-embedded in Araldite. Semi-thin and ultra-thin sections were processed for autoradiography. The results indicate that only the two PRL cell types were labeled. When immunolabeling for PRL was applied to ultra-thin sections, only immunopositive cells were seen to contain silver grains. In these cells the silver grains were associated with the rough endoplasmic reticulum and nucleus. When a growth hormone (GH) [35S]-cDNA probe was used as a control, only GH-secreting cells were labeled. This study confirms that the two PRL cell types are involved in biosynthesis of PRL. Moreover, this simple in situ hybridization technique provides a new approach to accurately localize mRNA in complex tissue and to investigate the subcellular distribution of mRNA under differing experimental conditions.

Animals↗
PubMed1989

Stimulation of androgen-dependent gene expression by the adrenal precursors dehydroepiandrosterone and androstenedione in the rat ventral prostate.

Androgens play a major role in the development, growth, and function of accessory sexual organs, especially the prostate. However, the testis is not the sole source of circulating androgens in man, since the adrenal gland secretes dehydroepiandrosterone (DHEA), DHEA sulfate, and androstenedione (delta 4-dione) in large quantities. The aim of the present study was to investigate the effect of plasma concentrations of DHEA and delta 4-dione similar to those found in adult man on sensitive and specific markers of androgen action in the rat ventral prostate. In addition to ventral prostate weight, we have measured the steady state levels of the mRNAs encoding the C1 component of rat prostatic binding protein (PBP-C1) and spermine-binding protein (SBP) using 35S-labeled cDNA probes for in situ hybridization. One week after castration, ventral prostate weight fell 84%, while prostatic 5 alpha-dihydrotestosterone (DHT) and androgen-dependent mRNAs were undetectable. When administered via Silastic implants to castrated adult rats for 1 week, plasma concentrations of 1.37 +/- 0.06 ng/ml DHEA or 0.43 +/- 0.08 ng/ml delta 4-dione independently caused increases in ventral prostate weight to 33% and 65% of normal values, respectively. The same plasma levels of DHEA and delta 4-dione resulted in high intraprostatic levels of DHT to 1.19 +/- 0.34 and 3.66 +/- 0.89 ng/g tissue, respectively. Furthermore, DHEA caused an increase in the steady state levels of PBP-C1 and SBP mRNAs to 50% and 57% of the normal state, respectively, while delta 4-dione caused increases corresponding to 80% and 119% of control values, respectively. Castrated adult rats receiving testosterone at a concentration of 1.66 +/- 0.37 ng/ml plasma maintained normal ventral prostate weight and gene expression levels. The present results demonstrate that circulating levels of the adrenal steroids DHEA and delta 4-dione comparable to those found in man cause an important stimulation of androgen-dependent gene expression in the rat, probably after their conversion to DHT in the prostatic tissue itself.

Adrenal Glands↗
PubMed1989

Thyroid and glucocorticoid hormone regulation of rat pituitary growth hormone messenger ribonucleic acid as revealed by in situ hybridization.

The effect of thyroid and glucocorticoid hormones on mRNA have been largely studied using tumor cell lines. To investigate the role of these hormones in vivo, we have used in situ hybridization techniques to study GH mRNA regulation in the rat pituitary. The amount of mRNA encoding for GH was dramatically reduced after 1 month of treatment with 6-N-propyl-2-tiouracil. However, daily injections of L-T4 for 10 days restored GH mRNA to control levels. Similarly, adrenalectomy decreased GH mRNA levels, and subsequent daily injections of dexamethasone for 1 and 5 days restored the levels of GH mRNA to those before adrenalectomy. Moreover, 6-N-propyl-2-tiouracil treatment in combination with adrenalectomy dramatically reduced pituitary levels of GH mRNA, which were increased by administration of both thyroid hormones and glucocorticoids. These results show that the effects of thyroid and glucocorticoid hormones on GH synthesis are largely mediated by the hypothalamus. Our results obtained in implanted pituitaries under the kidney capsula demonstrated that these two hormones also exert a smaller effect of GH mRNA directly at the pituitary level.

Adrenalectomy↗
PubMed1989

Effects of active immunization against somatostatin on serum growth hormone concentration in growing pigs: influence of fasting and repetitive somatocrinin injections.

Three experiments were conducted with growing pigs actively immunized against a protein-conjugated somatostatin (SRIF) in Freund's adjuvant. In the first experiment, blood from 24-week-old pigs (seven immunized and eight control) was sampled at 20-min intervals for 6 h to evaluate basal GH concentrations. The animals were then injected iv with porcine GH-releasing factor (GRF)-(1-29)NH2 (10 micrograms/kg). Before GRF stimulation, immunized animals had higher (P less than 0.05) baseline mean GH levels (2.6 vs. 1.4 ng/ml) and area under the GH curve (AUC; 1632 vs. 779 ng/min.ml); they also had higher AUC after GRF administration (4268 vs. 1972 ng/min.ml). In a second experiment eight immunized and eight control pigs were injected iv four times at 90-min intervals with porcine GRF (10 micrograms/kg). Control pigs responding to the first injection did not respond to the second and third, and those responding to the second did not respond to the first, third, and fourth, indicating a decreased responsiveness that was longer than 3 h post-GRF response in control pigs. SRIF-immunized pigs had a more consistent GH response to the GRF injections. Overall, a reduced response was observed after the second and the fourth injections in immunized pigs, although five and six of eight animals had a GH peak response higher than 10 ng/ml during these periods. In a third experiment, effects of fasting, GRF, and SRIF immunization were studied. Immunization and fasting had their own positive effects on serum GH levels. Immunization increased baseline mean GH levels (5.0 vs. 2.2 ng/ml) and total AUC before (2318 vs. 1073 ng/min.ml) and after (1886 vs. 910 ng/min.ml) iv GRF stimulation (10 micrograms/kg) compared to controls. Fasting increased the mean baseline GH level (4.5 vs. 2.6 ng/ml), and it increased AUC before exogenous GRF stimulation (2009 vs. 1392 ng/min.ml). In conclusion, SRIF in pigs seems to be a potent GH-governing factor, since, when inhibited, baseline mean GH levels increase, and a consistent response to GRF is observed. Fasting could increase GH concentrations by different ways: decreasing SRIF release and increasing GRF release or modifying the sensitivity of the somatotrophs to both factors.

Animals↗
PubMed1989

Inhibitory effect of androgen on estrogen-induced prolactin messenger ribonucleic acid accumulation in the male rat anterior pituitary gland.

Estrogens are known to exert specific stimulatory effects on basal and dopamine-inhibited PRL secretion and synthesis as well as on PRL gene expression. However, dihydrotestosterone (DHT) and progesterone (P), although inactive alone, can reverse the effect of 17 beta-estradiol (E2) on PRL release both in vivo and in vitro. Using castrated male rats, we have studied the effect of E2 (0.25 micrograms), P (2 mg), or DHT (100 micrograms) administered twice daily for 14 days alone or in combination on pituitary PRL mRNA levels measured by quantitative in situ hybridization. Treatment with E2 increased the accumulation of PRL mRNA by about 2.6-fold. Administration of P or DHT alone failed to modify PRL mRNA concentrations. However, DHT could prevent by 80% the stimulatory effect of E2 on PRL mRNA levels. Similar results were obtained by dot blot hybridization assay. The effects of sex steroids on PRL mRNA were closely paralleled to pituitary PRL content measured by RIA. The present data demonstrate that the effect of sex steroids on immunodetectable PRL result from a modulation of PRL mRNA accumulation. The sexual dimorphism observed in pituitary PRL content results from a 3.5-fold greater accumulation of PRL mRNA in intact females than in male rats. These results also clearly show that quantitative in situ hybridization is a powerful tool in the investigation of the regulation of gene expression in addition to providing valuable information on the localization of specific mRNA.

Androgens↗
PubMed1989

Identification of vasotocin-like immunoreactivity in chromaffin cells of the frog adrenal gland: effect of vasotocin on corticosteroid secretion.

The presence of neurohypophyseal nonapeptides in the adrenal gland of nonmammalian vertebrates and the possible action of these regulatory peptides on corticosteroid secretion have never been investigated. We have applied the indirect immunofluorescence technique to examine whether vasotocin (AVT) and/or mesotocin (MT) are located in frog adrenal (interrenal) tissue. Using antisera against AVT and tyrosine hydroxylase, we found that all chromaffin cells contain an AVT-like peptide. Labeling of consecutive sections with phenylethanolamine-N-methyltransferase or AVT antibodies showed that both noradrenaline- and adrenaline-storing cells contain AVT-like immunoreactivity. In contrast no labeling of frog adrenal slices was observed using a MT antiserum. At the ultrastructural level, the immunogold technique revealed that the AVT-immunoreactive peptide is sequestered in chromaffin granules with varying electron densities. Filtration of frog adrenal tissue extracts on Sep-Pak C-18 cartridges showed that the elution profile of the AVT-like peptide was similar to that of synthetic AVT. The apparent concentration of AVT in the adrenal was 2.7 ng/g tissue. Since chromaffin cells represent approximately one third of all interrenal cells, the actual concentration of AVT in chromaffin tissue was about 8 ng/g tissue. The role of AVT in the regulation of frog adrenal steroidogenesis was studied in vitro using perifused frog interrenal slices. Graded doses of AVT (10(-10)-10(-7) M) induced a dose-dependent stimulation of both corticosterone and aldosterone secretion. The other neurohypophyseal peptides (vasopressin, oxytocin, and MT) were also able to enhance corticosteroid secretion, but AVT was by far the most potent stimulator of steroidogenesis. Prolonged administration (4 h) of AVT induced a rapid increase in corticosterone and aldosterone output, followed by a gradual decline of corticosteroid secretion. These results show that an AVT-like peptide is stored in chromaffin granules of frog adrenal gland. Our data also indicate that synthetic AVT is a potent stimulator of corticosteroid secretion by frog interrenal cells. Since in amphibians adrenocortical and chromaffin cells are intimately intermingled, these results suggest that AVT produced by chromaffin cells may regulate corticosteroid release locally, through a cell to cell mode of communication.

Adrenal Cortex↗
PubMed1989

Regulation of pro-gonadotropin-releasing hormone gene expression by sex steroids in the brain of male and female rats.

The fine modulation of gonadotropin gene expression and secretion is well recognized to be regulated by sex steroids through their direct action both at the anterior pituitary level and on the pulsatile pattern of GnRH secretion at the hypothalamic level. Since the influence of sex steroids on hypothalamic GnRH mRNA levels remains to be elucidated, quantitative in situ hybridization was used to study the effect of sex steroids on cellular levels of pro-GnRH mRNA in adult rats of both sexes. The effects of 14-day gonadectomy as well as administration of 17 beta-estradiol (E2, 0.25 micrograms) or dihydrotestosterone (DHT, 100 micrograms) twice a day during 14 days to gonadectomized animals were evaluated. In addition, the effect of progesterone (P, 2 mg, twice daily) alone or in the presence of E2 was also studied in ovariectomized animals. Hybridization was performed using a 35S-labeled cDNA probe encoding rat pro-GnRH and the corresponding mRNA levels were assessed by counting the number of silver grains overlying labeled neurons. In male rats, castration induced a highly significant 65% increase (compared to intact rats) in the mean number of grains per neuron. Administration of E2 or DHT to castrated animals completely prevented the post castration rise in pro-GnRH mRNA levels. In female animals, the effect of ovariectomy was less striking than in the male, a 25% increase (P less than 0.001) being observed. Treatment with E2 or DHT also completely prevented the increase in pro-GnRH mRNA levels induced by ovariectomy. Moreover, treatment with P in ovariectomized animals markedly potentiated the inhibitory effect of E2 on pro-GnRH mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Effect of serotonin on alpha-melanocyte-stimulating hormone secretion from perifused frog neurointermediate lobe: evidence for the presence of serotonin-containing cells in the frog pars intermedia.

We have examined the presence of 5-hydroxytryptamine (serotonin; 5-HT) in the intermediate lobe of the frog pituitary and investigated the effect of exogenous 5-HT on alpha-melanocyte-stimulating hormone (alpha-MSH) release from the perifused neurointermediate lobe (NIL). Using a specific antiserum against 5-HT, the indirect immunofluorescence technique revealed the presence of 5-HT-like immunoreactivity (5-HT-LI) in discrete cells, generally gathered in small clusters among parenchymal cells, and in numerous neurites surrounding melanotrophic cells. At the electron microscopic level, using a silver-gold intensification procedure, 5-HT-LI was localized in dense-core secretory vesicles within specific pituitary cells which appear to be different from pituitary melanotrophs. Dense accumulation of gold particles was also observed in nerve fibres running between parenchymal cells. A combination of high-performance liquid chromatography analysis and electrochemical detection showed the presence of both 5-HT and its metabolite 5-hydroxyindol acetic acid (5-HIAA) in frog NIL extracts (534 +/- 40 and 1245 +/- 65 (S.E.M.) pg/mg wet tissue respectively). Administration of graded doses of 5-HT (from 1 to 30 mumol/l) to perifused frog NIL induced a dose-dependent inhibition of alpha-MSH release. Repeated pulses of 5-HT (10 mumol/l each) induced a reproducible inhibition of alpha-MSH without any desensitization phenomena. The inhibitory effect of 5-HT was partially blocked by the serotonergic antagonists methysergide and ICS-205-930 (10 mumol/l each). Concomitant administration of methysergide and ICS-205-930 (10 mumol/l each) totally abolished 5-HT-evoked inhibition of alpha-MSH. Fenfluramine, a releaser of 5-HT, induced a slight but significant reduction of alpha-MSH secretion. While 5-HT caused a marked inhibition of alpha-MSH release from intact NIL, 5-HT was devoid of effect on acutely dispersed pars intermedia cells suggesting that 5-HT does not exert a direct action on pituitary melanotrophs. We have examined the effect of specific dopaminergic, GABAergic and alpha-adrenergic antagonists on 5-HT-induced alpha-MSH inhibition. We observed that sulpiride and SR 95531 (10 mumol/l each) did not affect the response of NIL to 5-HT while yohimbine (10 mumol/l) suppressed the inhibitory action of 5-HT. Taken together, our results indicate that discrete cells of the frog pars intermedia contain the neurotransmitter 5-HT which may act locally to inhibit alpha-MSH release.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗