Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease.

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

The role of protein kinase C in the regulation of serotonin-2A receptor expression.

We have investigated in C6 glioma cells the involvement of protein kinase C (PKC) in the regulation of serotonin-(2A) receptor (5-HT(2A) receptor) expression by agonist treatment. Comparison of the time-courses of agonist-induced downregulation of receptor number and mRNA indicate that a decrease in the number of 5-HT(2A) receptor binding sites in response to serotonin (5-HT) treatment is preceded by a decrease in 5-HT(2A) receptor mRNA. This decrease in 5-HT(2A) receptor mRNA as a result of agonist exposure was not due to a change in the stability or half-life of the transcript. Pretreatment of cells with the PKC inhibitor bisindolylmaleimide blocked the decrease in 5-HT(2A) receptor mRNA levels, and attenuated the down-regulation of 5-HT(2A) receptor binding sites induced by treatment with 5-HT. Experiments performed with the PKC inhibitors calphostin C and Gö 6976 confirmed that PKC was involved in the regulation of 5-HT(2A) receptor mRNA by agonist and implicate the conventional subgroup of PKC isoforms. Western blot analysis, using isoform-specific anti-PKC antibodies showed that under our culture conditions C6 glioma cells express the conventional isoforms PKC alpha, PKC gamma, as well as the novel isoforms PKC delta, PKC epsilon, and the atypical isoforms PKC lambda and PKC iota. Upon treatment with 5-HT for 10 min levels of the conventional isoforms PKC alpha and PKC gamma increased in the nuclear fraction. Taken together, our results implicate PKC alpha and/or PKC gamma in the regulation of 5-HT(2A) mRNA receptor and binding sites in response to agonist treatment.

Regulation of 5-HT(2A) receptor mRNA levels and binding sites in rat frontal cortex by the agonist DOI and the antagonist mianserin.

In the present study we have characterized the time course of effect of administration of the serotonin(2) (5-HT(2)) receptor antagonist mianserin, or the 5-HT(2) receptor agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), on 5-HT(2A) receptor binding sites and mRNA levels in rat frontal cortex. Radioligand binding and ribonuclease protection assays were performed with separate hemispheres of frontal cortex from each animal to examine concomitant changes in 5-HT(2A) receptor sites and mRNA levels. The decrease in cortical 5-HT(2A) receptor sites in response to chronic DOI administration was not accompanied by changes in 5-HT(2A) receptor mRNA. A single injection of DOI produced a transient decrease in 5-HT(2A) receptor mRNA levels detected 1 h post-injection. The density of 5-HT(2A) receptor sites, however, was not significantly reduced following a single injection of DOI. The down-regulation of cortical 5-HT(2A) receptor sites in response to a single injection of mianserin was accompanied by reductions in 5-HT(2A) receptor mRNA levels. Following 4 days of mianserin administration, however, we did not observe a change in 5-HT(2A) receptor mRNA levels, although 5-HT(2A) receptor density was decreased. Thus, changes in receptor mRNA may initially contribute to the down-regulation of 5-HT(2A) receptors in response to acute mianserin administration. Sustained changes in 5-HT(2A) receptor mRNA, however, appear not to be involved in maintaining the down-regulation of 5-HT(2A) receptor number with chronic mianserin administration. Mechanisms other than the regulation of receptor mRNA levels appear to underlie the down-regulation of 5-HT(2A) receptor sites in response to chronic administration of the agonist DOI.