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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 -melanocyte-stimulating hormone (-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 µmol/l) to perifused frog NIL induced a dose-dependent inhibition of -MSH release. Repeated pulses of 5-HT (10 µmol/l each) induced a reproducible inhibition of -MSH without any desensitization phenomena. The inhibitory effect of 5-HT was partially blocked by the serotonergic antagonists methysergide and ICS-205-930 (10 µmol/l each). Concomitant administration of methysergide and ICS-205-930 (10 µmol/l each) totally abolished 5-HT-evoked inhibition of -MSH. Fenfluramine, a releaser of 5-HT, induced a slight but significant reduction of -MSH secretion. While 5-HT caused a marked inhibition of -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 -adrenergic antagonists on 5-HT-induced -MSH inhibition. We observed that sulpiride and SR 95531 (10 µmol/l each) did not affect the response of NIL to 5-HT while yohimbine (10 µmol/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 -MSH release. Our data also suggest that the inhibitory effect of 5-HT is mediated via presynaptic stimulation of catecholamine (possibly norepinephrine) release from adrenergic nerve endings terminating in the intermediate lobe of the frog pituitary.

Journal of Endocrinology (1989) 122, 135–146

This article has been cited by other articles:

				M. Garnier, M. Lamacz, L. Galas, S. Lenglet, M.-C. Tonon, and H. Vaudry Pharmacological and Functional Characterization of Muscarinic Receptors in the Frog Pars Intermedia Endocrinology, August 1, 1998; 139(8): 3525 - 3533. [Abstract] [Full Text] [PDF]