In-vitro secretion of prolactin and growth hormone in the presence of melatonin by pituitary gland from mink kept under long or short days

Autumnal timing of photoperiodic manipulation critical via melatonin to winter pelage development in mink

Quantitive collection of night-time urine every 2nd week was used to elucidate the temporal relationship between changes in melatonin production, weight gain, and priming of the winter pelage under natural light conditions and under a long photoperiod (18L: 6D) imposed at different times in four groups of female mink during autumn. In mink maintained outdoors under the natural photoperiod (group 1), melatonin excretion was 1·80 (s.d. 0·80) ng per night (no. = 3) at the beginning of September, thereafter it began to decrease slowly until November when it was only 0·62 (s.d. 0·32) ng per night. These mink had a normal autumn moult during the first half of October and the winter pelage was mature at the end of November. When mink maintained outdoors were transferred to 18L: 6D on 7 September (group 2), excretion of melatonin decreased promptly, autumn moult was disrupted and winter fur priming delayed. Long photoperiod after mid October (group 3), did not interfere with pelage maturation despite the decrease in melatonin production. Mink implanted with melatonin on 14 July (group 4) showed very high melatonin excretion in early September. In these animals the winter pelage growth was unaffected by the long-day conditions; the pelage matured in mid October. Body weight increased in mink of all groups during autumn. This increase levelled off or a weight loss (group 2) was seen along with the final hair maturation. Activation of hair follicles occurred during a 4 to 6 week period. According to these results, melatonin is the photoperiodic signal to autumnal weight increase and autumn moult but seems not to be necessary for later pelage growth and maturation.

Mink growth hormone structural-functional relationships: effects of renaturing and storage conditions

Fourier-transform infrared spectroscopy, in vitro bioassay and enzyme-linked immunoassay were used to study the structural-functional relationships of recombinant mink growth hormone (mGH), refolded and stored under different conditions. Porcine GH (pGH) was synthesized and used as an example. These two hormones, when refolded and stored the same way, had the same secondary structures, biological and immunological efficacy, and biological potency. Only the immunological potency differed, mGH being significantly less potent than pGH. Renaturation pH and storing frozen or at 4 degrees C in 5% glycerol did not affect either the secondary structure or the activity. However, freeze-drying raised the content of buried alpha-helices and lowered that of solvated alpha-helices and of unordered structures. These conformational changes were associated with a reduction of immunological and biological potency of mGH and of immunological potency of pGH. These findings provide original information on the secondary structure of mGH, and show that conformational changes induced by lyophilization adversely affect its activity.

Immunohistochemical study of somatotrophs in pituitary pars distalis of male viscacha (Lagostomus maximus maximus) in relation to the gonadal activity

Somatotrophs were identified and quantified in pituitary pars distalis of male viscachas (Lagostomus maximus maximus) during the annual reproductive cycle, after the administration of melatonin, after castration and in different growth stages by immunohistochemistry and morphometric analysis. In adult male viscachas, the somatotrophs were distributed throughout the pars distalis during the reproductive cycle. They were oval, pyramidal or round in shape with a large round nucleus. The percentage immunopositive area, the major cellular diameter and the number of cells decreased during the gonadal regression period in relation to the values found in the reproductive period. The administration of melatonin did not provoke any variations of the morphometric parameters studied. On the contrary, a significant decrease in the percentage immunopositive area, in the major cellular diameter and in the number of somatotrophs in castrated viscachas was observed. The study of different growth stages showed that these morphometric parameters increased from immature to adult animals in the reproductive period. The results obtained suggested that the variations of the morphometric parameters of somatotrophs are more related to the gonadal development and activity than to a direct effect of melatonin.

The pars tuberalis: the missing link in the photoperiodic regulation of prolactin secretion?

The endocrine function of the pars tuberalis of the pituitary gland has been an enigma for many years. Recent work suggests that one of its primary functions in seasonal mammals is to mediate photoperiodically regulated changes in prolactin secretion via an unidentified factor called tuberalin.

Effects of melatonin in vivo upon luteinizing hormone and prolactin releases induced by opiate receptor antagonists in adult male rats

The effects of melatonin on LH and PRL releases induced by treatment with naloxone, naloxone methyliodide and nalmefene were studied in adult male rats. Subcutaneous melatonin injection (1.4 mg/Kg) had no effect on LH secretion, but caused an inhibition effect (84%) on LH release induced by naloxone (2.4 mg/Kg). Melatonin too totally inhibited LH secretion induced by naloxone methyliodide (2.8 mg/Kg) and nalmefene (2 mg/Kg) when it was simultaneously administered with each opioid receptor antagonist. Melatonin alone had no significant effect on serum PRL levels, but decreased by 25.5% the inhibitory effect potency of nalmefene on PRL secretion after simultaneous injections. The inhibitory effect potency of naloxone on PRL release increased (16%) when it was administered with melatonin. Simultaneous injection of melatonin with naloxone methyliodide inhibited PRL release (78%) while naloxone methyliodide alone did not modify this secretion. The results obtained with a quaternary opioid antagonist indicate that the opioid receptor type which mediates LH and PRL responses is located respectively outside and inside the blood-brain barrier. Our findings show that opiate antagonists and their quaternary ammonium salts affect secretion of LH and PRL through different mechanisms susceptible to the influence of melatonin.

Melatonin modifies prolactin release induced by opiate antagonists in male rats

The effect of melatonin on the prolactin (PRL) release induced by treatment with naloxone, naloxone methyliodide, naltrexone and nalmefene were studied in adult male rats. Subcutaneous melatonin injection (1.4 mg/Kg) had no significant effect on serum PRL levels, but decreased by 29% and 26% respectively the inhibitory effect potency of naltrexone (2.5 mg/Kg) and nalmefene (2 mg/Kg) on PRL secretion after simultaneous injections. The inhibitory effect potency of naloxone on PRL release increased (16%) when it was administered with melatonin. Simultaneous injection of melatonin with naloxone methyliodide (2.8 mg/Kg) inhibited PRL release (77.5%) while naloxone methyliodide alone did not modify this secretion. The results obtained with a quaternary opioid antagonist indicate that the opioid receptor type which mediates PRL response is located inside the blood-brain barrier. Our findings show that opiate antagonists and their quaternary ammonium salts affect secretion of PRL through mechanism susceptible to the influence of melatonin.

Effect of melatonin on pituitary secretion of prolactin in vitro during delayed implantation and the periimplantation period in the spotted skunk

The pars tuberalis of the western spotted skunk is the only region of the brain known to bind 2-[125I]iodomelatonin, thus suggesting that this region of the pituitary might be essential in mediating the effects of melatonin in this species. Female skunks were killed during short- and long-day photoperiods. Hemisections of the adenohypophysis, with and without the pars tuberalis were cultured in the presence and absence of 10 nM melatonin to determine whether the pars tuberalis mediates the previously observed suppressive effects of melatonin on the seasonal increase in plasma prolactin levels in this species. Pituitaries obtained during long-day photoperiods secreted significantly more prolactin than those obtained during the short-day photoperiod. Melatonin had no effect on pituitary response to gonadotropin releasing hormone and failed to inhibit prolactin secretion during the 48 h culture period. However, melatonin significantly reduced the inhibitory effects of dopamine on prolactin secretion. The latter response was observed in both photoperiods and in the presence and absence of the pars tuberalis. These results suggest that the pars tuberalis does not mediate the inhibitory effect of melatonin on prolactin secretion in the absence of the hypothalamus.

Embryonic diapause in vertebrates

Embryonic diapause occurs in many species of vertebrates, but the physiological mechanisms which control this fascinating process are exceedingly different in the diverse groups which employ this reproductive strategy. In nonmammalian species and some bats, reduction in rate of embryonic development is temperature dependent, but this is not the case in most mammals. Development becomes arrested at the blastocyst stage of embryogenesis in mammals which exhibit delayed implantation, whereas postimplantation development is continuous but retarded in species exhibiting delayed development. The hormonal control of diapause is remarkably different in the various species. Pituitary secretion of prolactin prevents implantation in the tammar wallaby but hastens renewed development and implantation in the mink and spotted skunk. Ovariectomy results in the eventual death of blastocysts in mustelids but induces renewed development and implantation in the armadillo. Luteal function, as evidenced by elevated progesterone secretion, is essentially constant in the roe deer and armadillo, whereas the luteal cells fail to complete their differentiation and secrete low levels of progesterone in carnivores. Progesterone will induce implantation in the tammar wallaby, but estrogen is required to induce renewed development and implantation in rodents. Neither progesterone and/or estrogens appear to be capable of stimulating implantation in carnivores. The uterus plays an important role in maintaining the embryos in a viable state throughout the period of diapause. In many species the uterus undergoes histological changes and secretes increased amounts of protein, yet we still do not understand the role, if any, these proteins play in initiating renewed embryonic development. Thus the phenomenon of embryonic diapause still holds many mysteries for scientists to solve.

Retention and metabolic fate of [3H]-melatonin in the spotted skunk

Two experiments were designed to determine which tissues accumulate [3H]-melatonin and the metabolic fate of this hormone in the spotted skunk. Tritiated melatonin was injected into the jugular vein of 10 anesthetized skunks 1-3 h before the onset of darkness and allowed to circulate for 22 min before the vasculature was flushed with saline to clear radioactivity from the blood. Selected tissues were removed from five skunks and oxidized in a Packard Biological Oxidizer which yielded 95 +/- 5% recovery of radioactivity. Relatively high amounts of radioactivity were found in the pineal (367 +/- 304 dpm/mg tissue), ovary (69 +/- 38 dpm/mg), pituitary (89 +/- 56 dpm/mg), liver (107 +/- 29 dpm/mg), and kidney (63 +/- 15 dpm/mg). Relatively small amounts of [3H] were found in different brain regions (approximately 6-7 dpm/mg). The uterus, pancreas, and temporalis muscle also accumulated radioactivity (approximately 13 dpm/mg). The lung retained the least amount of radioactivity (4 +/- 1.3 dpm/mg). In the second experiment, hypothalami, pituitaries, and ovaries were removed from the remaining five females. Radioactivity from these tissues was extracted and subjected to thin-layer chromatography. Melatonin accounted for approximately 70% of the radioactivity recovered while 6-hydroxymelatonin and unidentified more polar compounds made up the majority of the melatonin metabolites. These data indicate that tissues other than the hypothalamus are able to accumulate [3H]-melatonin.