ABSTRACT

Prolactin secretion throughout the oestrous cycle in the rat remains at a low level and fairly constant, with the exception of the surge at pro-oestrus. The present study was designed to characterize possible changes in pulsatile patterns of prolactin during the oestrous cycle of the adult female rat. Mean values of prolactin increased from dioestrus-2 to pro-oestrus and then decreased to the values found at dioestrus-1. The number of peaks remained fairly constant in any phase of the oestrous cycle. The absolute amplitude of the peaks increased numerically but was not statistically significant from dioestrus-2 to pro-oestrus then decreasing until dioestrus-1. No changes in the relative amplitude or duration of the peaks throughout the oestrous cycle were detected. The results indicated that there is a similar pulsatile pattern of prolactin at any stage of the oestrous cycle, when samples were obtained during the morning.

Journal of Endocrinology (1993) 137, 43–47

Abstract

Much is known about the fact that thyrotrophin-releasing hormone (TRH) and vasoactive intestinal peptide (VIP) stimulate prolactin secretion but areas of uncertainty remain. This work was undertaken to describe the effects of TRH and VIP on the pulsatile secretion pattern of prolactin, in adult sham-operated and pituitary-grafted hyperprolactinaemic female rats. Two pulses of TRH (1 μg/rat) or one pulse of VIP (20 μg/rat) were given 60 or 120 min after the period of blood sampling. Pituitary grafting increased the mean values of prolactin, absolute amplitude and duration of the peaks and decreased their frequency, compared with control animals. In sham-operated rats, TRH elevated prolactin levels by increasing the absolute and relative amplitudes and duration of the pulses, along with a decrease in their frequency. No priming effects of TRH were observed in this study. Hyperprolactinaemia blunted TRH effects on the pulsatile secretion pattern of prolactin. In sham-operated rats, VIP administration increased the absolute and relative amplitudes of the prolactin peaks. None of the other parameters studied were changed. In pituitary-grafted animals, VIP administration increased the absolute and relative amplitudes of the prolactin peaks but to a lesser extent compared with controls. These data suggest that TRH and VIP affect prolactin pulsatility differentially. The effects of TRH and VIP were blunted to some extent by exposure to previously elevated circulating prolactin levels.

Journal of Endocrinology (1994) 142, 581–586

Male and female Wistar rats were made hyperprolactinaemic by grafting two pituitary glands of litter-mate donors under the kidney capsule at 30 days of age. Other animals were sham-operated at the same age to serve as controls. Plasma levels of prolactin, LH and FSH were measured by double-antibody radioimmunoassay. Basal preoperative prolactin levels of ∼ 10 ng/ml increased after the transplantation in both male and female rats, reaching values of ∼ 180 ng/ml. Levels of LH were significantly reduced in these hyperprolactinaemic rats, whereas an increase in FSH values was seen. After administration of LH releasing hormone (LH-RH) a reduced LH response was seen but there was no response of FSH to LH-RH or even a decrease in FSH values. Prolactin levels were also reduced by LH-RH injection. Although an increase in prolactin levels was observed in control animals after a challenge with oestradiol benzoate, reduced increments were seen in experimental animals. The positive feedback effect of oestradiol benzoate on LH in females was reduced in pituitary-grafted rats but a potentiation of the FSH positive feedback could be clearly detected. This study suggests a dissociation of LH and FSH regulation in hyperprolactinaemia.

ABSTRACT

Nine-month-old female rats bearing an ectopic pituitary gland (from a litter-mate) under the right kidney capsule since day 30 of life and their sham-operated controls, were treated with a dopamine agonist (lysuride) or antagonist (metoclopramide). Plasma prolactin and LH levels were measured by double-antibody radioimmunoassays. Vaginal smears were taken before and during the treatment periods. Eight months after the operation, a significant (P < 0·01) increase in basal prolactin levels together with a significant (P < 0·05) reduction in LH values and permanent dioestrus occurred in the grafted animals when compared with controls. Lysuride treatment resulted in a marked reduction in plasma prolactin levels both in control and grafted rats over the whole 12 days of treatment, together with a partial restoration of plasma LH levels on day 1. From day 7 onwards a depression in LH values was again observed. Oestrous cycles were partially restored at the beginning of the treatment, but after 7 days dioestrus returned. Metoclopramide administration induced a significant (P< 0·001) increase in basal prolactin levels in both grafted and control rats. Basal plasma LH values were unaffected in controls when compared with vehicle-treated animals. An increase could be seen in hyperprolactinaemic rats after 7 or 12 days of treatment however. The LH response to the administration of LH releasing hormone (LHRH) was greater in the experimental and control metoclopramide-treated rats when compared with vehicle-treated rats. Vaginal smears were not altered in the control animals but there was a significant increase in the number of oestrous smears in grafted animals given the dopamine antagonist partially restoring the cycle pattern. After LHRH administration plasma prolactin levels decreased in vehicle-treated grafted and control animals, whereas only a tendency to lower values or no modification in basal levels was observed with lysuride or metoclopramide treatments. All these data suggest that increased plasma prolactin levels cannot modify LH secretion directly. This influence may be exerted, however, through increased hypothalamic and in-situ pituitary dopamine detected in hyperprolactinaemic animals.

J. Endocr. (1984) 100, 141–148

ABSTRACT

To study the role of testosterone on the regulation of the hypothalamic-pituitary-testicular axis, young intact male Wistar rats were given acute (24 h) or chronic (5 days) subcutaneous treatments of 500 μg testosterone propionate (TP) or vehicle alone. Plasma LH, prolactin and testosterone levels were measured both basally and after administration of LH-releasing hormone (LHRH) or human chorionic gonadotrophin (hCG) by means of specific radioimmunoassay systems using materials supplied by the NIADDK. After acute treatment with TP there was an increase in basal plasma testosterone concentrations and no modification in the hCG response when compared with vehicle-treated animals. No difference could be detected in basal plasma testosterone levels after the chronic treatment, but a significant reduction in the hCG response was observed. Both acute and chronic treatments with TP resulted in a significant decrease of basal plasma LH levels. A reduced LH response to LHRH in acutely treated rats and no response in the chronically treated rats was detected. Plasma prolactin levels showed an increase after both acute and chronic treatments. To evaluate the possible role of the increased plasma prolactin levels on the above modifications during TP treatment, another group of animals was treated with TP and bromocriptine (dopamine agonist) simultaneously to avoid the increase in plasma prolactin levels. In this situation, neither basal plasma LH levels nor the response to LHRH were altered when compared to vehicle-treated rats; a normal testosterone response to hCG stimulation was observed in spite of the high basal plasma testosterone levels. All these observations suggest that increased prolactin levels may exert a modulatory role on the negative feedback effect of testosterone both at the testicular and central levels.

J. Endocr. (1985) 105, 423–427

ABSTRACT

It was recently reported that anterior pituitary tissue transplanted to an ectopic site contains measurable amounts of dopamine and noradrenaline. To examine the possibility of local catecholaminergic control of prolactin secretion from ectopic pituitaries, pituitary grafted and sham-operated female rats were submitted to several pharmacological treatments modifying catecholamine synthesis. Administration of a single dose of α-methyl-p-tyrosine (α-MPT) significantly reduced dopamine content in the graft, while noradrenaline content was not modified. Similar changes in the contents of dopamine and noradrenaline after α-MPT administration were observed in the hypothalamus and in the in-situ pituitary in both grafted and sham-operated rats. Plasma concentrations of prolactin were increased in both grafted and sham-operated rats after administration of α-MPT. A single injection of l-3,4-dihydroxyphenylalanine (l-DOPA) increased dopamine content in the ectopic pituitary gland without altering the noradrenaline content, and produced similar effects in the hypothalamus and in-situ pituitary of grafted and control rats. Plasma prolactin concentrations were decreased by l-DOPA in both pituitary grafted and control rats. Administration of dl-treo-dihydroxyphenylserine (DOPS) increased noradrenaline content in the ectopic pituitary and reduced plasma prolactin concentrations in pituitary grafted rats. In contrast, injection of DOPS to control rats increased both hypothalamic noradrenaline content and plasma prolactin concentrations. These results suggest that dopamine and noradrenaline present in the ectopic pituitary tissue have a role in mediating prolactin release from pituitary transplants.

J. Endocr. (1987) 113, 45–49

Abstract

This work was designed to investigate the effects of cyclosporine on prolactin secretion by an ectopically grafted heterologous pituitary gland, and on the hypothalamic content of norepinephrine, dopamine and serotonin. The administration of cyclosporine prevented the augmentation in plasma prolactin levels which occurred following an ectopic graft of a litter-mate pituitary gland. In contrast, in sham-operated rats, cyclosporine increased prolactin levels on day 8 of treatment. Both pituitary grafting and cyclosporine treatment in sham-operated rats decreased hypothalamic norepinephrine content. In grafted rats, cyclosporine returned hypothalamic norepinephrine to normal. Hypothalamic serotonin content decreased 8 days after pituitary grafting but increased to the values of control animals after cyclosporine administration. Cyclosporine treatment for 2 and 8 days increased serotonin content in sham-operated animals. As expected, the hypothalamic dihydroxyphenylacetic acid/dopamine index increased after pituitary grafting and administration of cyclosporine for 8 days resulted in a further increase. Cyclosporine administration for 2 days, however, decreased this index to the values observed in control animals while drug treatment of control rats for 8 days decreased the dihydroxyphenylacetic acid/dopamine index. In vitro release of prolactin from the ectopic gland was markedly decreased in animals treated with cyclosporine for 2 days and this effect was less evident in 8-day treated rats. These data suggest that the impairment of the local immune reaction after pituitary grafting, by cyclosporine administration for 2 days, prevents the augmentation of plasma prolactin levels and normalizes the hypothalamic catecholamine parameters, thus suggesting that the effects of cyclosporine at the hypothalamo-pituitary axis are exerted through changes in prolactin secretion by the graft, although chronic administration of cyclosporine may also have a direct effect on the hypothalamus.

Journal of Endocrinology (1995) 144, 159–164