Hyperprolactinemia exerts a negative effect on the beta-endorphin content of the rat neurointermediate pituitary lobe

Daily changes of GABA and taurine concentrations in various hypothalamic areas are affected by chronic hyperprolactinemia

This study was designed to characterize, in anterior, mediobasal, and posterior hypothalamic and median eminence, the 24h changes of gamma aminobutyric acid (GABA) and taurine (TAU) contents in adult male rats and to analyze whether chronic hyperprolactinemia may affect these patterns. Rats were turned hyperprolactinemic by a pituitary graft. Plasma prolactin (PRL) levels increased after pituitary grafting at all time points examined. A disruption of the circadian rhythm was observed in pituitary-grafted rats, whereas GABA and TAU content followed daily rhythms in all areas studied in controls. In the mediobasal hypothalamus, two peaks for each amino acid were found at midnight and midday. In the anterior hypothalamus, GABA and TAU showed only one peak of concentration at midnight. In the posterior hypothalamus, the values of both GABA and TAU were higher during the light as compared to the dark phase of the photoperiod. In the median eminence GABA content peaked at 20:00h, the time when TAU exhibited the lowest values. Hyperprolactinemia abolished the 24h changes of GABA in the mediobasal hypothalamus and reduced its content as compared to controls. Hyperprolactinemia advanced the diurnal peak of TAU to 12:00h in the mediobasal hypothalamus and did not modify the 24:00h peak. In the anterior hypothalamus, hyperprolactinemia increased GABA and TAU contents during the light phase while it decreased them during the dark phase of the photoperiod. In the posterior hypothalamus hyperprolactinemia did not modify GABA or TAU patterns as compared to controls. In the median eminence hyperprolactinemia increased the 20:00h peak of GABA and shift advanced the decrease in TAU content at 20:00h and its maximum at 24:00h as compared to controls. These data show that GABA and TAU content exhibit specific daily patterns in each hypothalamic region studied. PRL differentially affects the daily pattern of these amino acids in each hypothalamic region analyzed.

Modulation of ornithine decarboxylase activity by prolactin in seminal vesicles of the rat

The stimulatory effect of prolactin on sexual accessory glands is well established, though the mechanism of trophic action remains poorly understood. We have therefore assessed the effect of high levels of prolactin on ornithine decarboxylase activity and polyamine content in seminal vesicles (SV) of adult rats. Hyperprolactinaemia was induced by implantation of tissue fragments of a prolactin-secreting tumour 7315a, and the rats killed at 35 days post-inoculation. Serum levels of prolactin were increased significantly (p < 0.001) in tumour-bearing rats. Levels of testosterone in serum were reduced markedly, whereas LH levels remained unchanged. SV weight in the experimental group was not significantly different from that in the corresponding control group. A clear increase in ornithine decarboxylase activity of SV was observed (p < 0.001) in the hyperprolactinaemic rats. However, there was no change in the polyamine content of SV in tumour-bearing rats, compared to the control group. These results indicate that ornithine decarboxylase activity in SV of adult rats is regulated mainly by prolactin. This experimental model may be useful for identification of some of the events involved in the trophic action of prolactin.

Insulin binding and action on adipocytes from female rats with experimentally induced chronic hyperprolactinemia

We studied insulin binding and action in adipocytes from female rats with chronic hyperprolactinemia induced by grafting an anterior pituitary gland under the right kidney capsule. Normal basal insulin plasma levels were detected. An increase in insulin binding due to an increased number of receptors was observed (grafted: 193,000 +/- 13,000 (6) receptors/cell vs. controls: 136,000 +/- 17,000 (6) receptors/cell, P less than 0.05). No changes in receptor affinity were detected (ED50 grafted: 2.3 X 10(-9) M and ED50 controls: 1.6 X 10(-9) M). The antilipolytic activity of insulin was significantly decreased in adipocytes from rats with hyperprolactinemia, indicating an insulin-resistant state in these animals. These findings suggest that the chronic hyperprolactinemic state can modify receptor and post-receptor insulin events in rat parametrial adipose tissue.

Studies on the mechanism of corticotrophin-mediated desensitization of corticosterone secretion by rat adrenocortical cells

Long-term exposure of the adrenocortical cells in vivo or in vitro to high concentrations of ACTH results in a diminution of the responsiveness of these cells to a subsequent stimulation of corticosterone release by ACTH. Conflicting studies have been published on the mechanism of this 'desensitization' phenomenon. Dispersed adrenocortical cells prepared from the hypertrophic/hyperplastic adrenal glands of rats bearing the ACTH/PRL-secreting rat pituitary tumour 7315a showed an increased basal release of corticosterone, but had lost their ability to respond further to ACTH. However, corticosterone release in response to dibutyryl cyclic AMP (dbcAMP), cholera toxin and forskolin remained intact. Pretreatment of normal rats for 3, 9 and 21 days with 50 micrograms/rat/day of a long-acting ACTH depot preparation induced a dose-dependent increase in basal corticosterone release by the adrenocortical cells prepared from these animals and a dose-dependent decrease in the sensitivity to ACTH. However, the responsiveness of the adrenocortical cells prepared from the adrenal glands of control and ACTH pretreatment rats to dbcAMP, cholera toxin and forskolin was similar. In addition, pretreatment with ACTH in vivo did not affect the sensitivity of the adrenocortical cells in vitro to calmodulin inhibition by haloperidol and 11 beta-hydroxylase inhibition by etomidate. It is concluded that long-term exposure of the adrenal gland to high concentrations of ACTH in vitro results in an excessive activation of corticosterone release by the adrenocortical cells in vitro, which is accompanied by a loss of sensitivity to ACTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of branched-chain amino acids on beta-endorphin in the pituitary of the rat

Despite evidence that the branched-chain amino acids (BCAAs) influence brain metabolism and neurotransmitter synthesis, there is little information on the neuroendocrine effects of the BCAAs. We now report that administration of a mixture of the BCAAs 3 times daily for 12 days to Sprague-Dawley rats decreased the concentration of beta-endorphin immunoreactivity (BEI) in the neurointermediate lobe of the pituitary. In BCAA-treated rats, BEI was 21 +/- 4 compared to control levels of 57 +/- 12 (ng/microgram of protein). The BCAAs had no effect on the level of BEI in the adenohypophysis. These findings suggest that changes in availability of leucine, isoleucine, and valine mediate specific neuroendocrine responses to metabolic perturbations in the rat.