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

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Abstract
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 α-melanocyte-stimulating hormone (α-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 α-MSH release. Repeated pulses of 5-HT (10 μmol/l each) induced a reproducible inhibition of α-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 α-MSH. Fenfluramine, a releaser of 5-HT, induced a slight but significant reduction of α-MSH secretion. While 5-HT caused a marked inhibition of α-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 α-adrenergic antagonists on 5-HT-induced α-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 α-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

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