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The isolation and identification of rat prolactin (Kwa, 1965; Kwa, van der Bent & Prop, 1967) made it possible to develop a radioimmunoassay method for the estimation of prolactin in plasma and in pituitary tissue (Kwa & Verhofstad, 1967). Results are now given for plasma levels of prolactin, expressed as ng./ml. in rats of the highly inbred strains U and R, Amsterdam and the F 1 hybrids (R x U) during their oestrous cycle, using the rat prolactin prepared by the above mentioned method as standard (15·6 i.u./mg.).
Daily vaginal smears were taken from the age of 10 to 12 weeks from rats maintained under controlled illumination (14 hr. light, 10 hr. darkness). Five rats of each genetic constitution were chosen; all had regular 5-day cycles. Blood samples were collected daily between 14.00 and 15.00 hr. from the occular venous plexus. The samples were used in the radioimmunoassays at the
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After the isolation from oestrogen-induced prolactin-producing tumours in mice of a protein (mouse pituitary tumour protein = MPTP), a radioimmunoassay has been developed based on 125I-labelled MPTP (labelled by the Chloramine-T method of Greenwood, Hunter & Glover, 1963) and a rabbit antiserum to MPTP suitable for the estimation of prolactin in pituitary tissue homogenates and mouse plasma (H. G. Kwa, F. Verhofstad & E. M. van der Bent, unpublished observations). Parallel inhibition curves obtained by (1) unlabelled MPTP, (2) mouse anterior lobe tissue and (3) plasma from pseudopregnant mice suggested that MPTP was prolactin. Results are now given for plasma levels of prolactin, expressed in mμg. of the MPTP standard, in female mice during the oestrous cycle and in μg. at various time intervals after implantation of isologous pituitary tissue under the renal capsule.
Blood samples were collected under light ether anaesthesia from the occular venous plexus, and the plasma
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In a study undertaken to try to correlate plasma levels, estimated by radioimmunoassay, and morphological signs of hyperactivity of the prolactin cells, three groups of spayed female rats bearing pituitary isografts were studied. These received (a) no oestrogen, (b) a low dose of 2 mg oestrone/l drinking water (0·5 mg/l being the dose that is sufficient to give vaginal cornification in spayed female rats), or (c) a high dose of 50 μg oestrone in oil/day (sufficient to induce pituitary tumours in rats). Varying degrees of prolactin secretory activity of both isografts and pituitary glands in situ were found, as shown by means of electron microscopy by the development of the endoplasmic reticulum, the Golgi region, and the number of secretory granules. The morphological signs of secretory activity correlated with the dosage of oestrogen, but plasma levels of prolactin in the rats without oestrogen treatment did not differ significantly from those
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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
