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Many studies have shown the existence of functional interactions between central neurotransmitter systems and the hypothalamo-pituitary adrenal axis. Mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) are regulated by multiple factors including glucocorticoids themselves. Neurotransmitters such as serotonin (5-hydroxytryptamine: 5-HT) can regulate brain corticosteroid receptors in a complex way. The present study examined the short-term (48 h) effects of parachlorophenylalanine (PCPA), a drug which specifically inhibits 5-HT synthesis, on corticosteroid receptor levels and on the expression of their respective messenger ribonucleic acids (mRNA) in the rat hippocampus, hypothalamus and brain stem. The study was performed in bilaterally adrenalectomized animals, in order to avoid potential drug-induced changes in plasma corticosterone levels, which could secondarily regulate MR and GR. Short-term inhibition of 5-HT synthesis by PCPA significantly increased the number of hippocampal MR-binding sites. PCPA treatment did not alter the number of GR-binding sites in the hippocampus, hypothalamus and brain stem. We observed no change in the affinities of GR and MR sites in all the structures studied. In PCPA-treated rats, restoration of control 5-HT levels by injection of its immediate precursor, 5-hydroxytryptophan (5-HTP) brings the number of hippocampal MR-binding sites back to control levels. It can therefore be concluded that the increase in number of MR-binding sites induced by acute PCPA treatment is dependent on the decrease in 5-HT levels. The increase in hippocampal MR binding sites was correlated with an induction of their messengers, suggesting that 5-HT modulates the synthesis of MR protein. Although PCPA did not modify the number of hippocampal GR-binding sites, a decrease in hippocampal GR mRNA expression was observed. This study shows that 5-HT inhibits hippocampal mineralocorticoid receptor synthesis and that this effect is not mediated by changes in corticosterone hormone secretion, and illustrates the existence of complex mechanisms for corticosteroid receptor regulation in the hippocampus.

Ovariectomized female rats were implanted with estradiol containing silastic implants to induce constant circulating levels of the steroid, and sacrificed every 2 h in order to determine neuroendocrine rhythms. Under these conditions, we observed very marked circadian fluctuations in the hypothalamic concentrations of corticotropin-releasing factor (CRF), luteinizing hormone-releasing hormone (LHRH) and thyrotropin-releasing hormone (TRH), and in plasma levels of adrenocorticotropin (ACTH), corticosterone, luteinizing hormone (LH) and prolactin; the amplitude of the prolactin cycle was in particular much higher than in non-chronically estrogenized animals. The daily variation in CRF, ACTH and corticosterone showed significant rank correlations. Changes in hypothalamic content of LHRH and TRH were biphasic; the increase observed during the light period was abruptly interrupted by a depletion episode, coincident with the period of maximal LH and prolactin secretion, respectively. The initial phase of ACTH, LH and prolactin increments occurred between 11.00 and 15.00 h, and was relatively well synchronized. The steepest rise in ACTH and prolactin occurred at the same time, and preceded that of LH by a constant lag of about 2 h. After that initial period, secretion kinetics of the three hormones followed an independent pattern. The data suggest that increased secretion of several hormones results from activation of neural mechanisms occurring within a limited period of the 24-hour cycle.