It is now firmly established that the central nervous system has the capability of synthesizing bioactive steroids, and the distribution of most steroidogenic enzymes has now been determined in the brain of various groups of vertebrates. However, the neuronal mechanisms controlling the activity of neurosteroid-producing cells in the brain has received little attention. In the present study, we have investigated the effect of CRH, a neuropeptide that plays a crucial role in the stress response, on neurosteroid biosynthesis in the brain of the frog Pelophylax ridibundus. Double labeling of brain sections showed that steroid-synthesizing neurons are frequently innervated by CRH-containing fibers. The diencephalic regions where most steroidogenic cell bodies are located are also enriched with CRH receptor (CRH-R)-like immunoreactivity. Exposure of frog hypothalamic explants to graded concentrations of CRH produced a dose-dependent increase in the formation of various neurosteroids including 17-hydroxypregnenolone (17OH-Δ5P), dehydoepiandrosterone (DHEA), corticosterone (B), tetrahydrodeoxycorticosterone (THDOC), progesterone (P) and tetrahydroprogesterone (THP). The stimulatory effect of CRH on neurosteroid biosynthesis was mimicked by urocortin, sauvagin and urotensin-I, and by the CRH agonist stressin-1. CRH-induced neurosteroid production was markedly attenuated by CRH-R1 and CRH-R2 receptor antagonists, suggesting that the effect of CRH on neurosteroidogenesis is mediated through both receptors. To examine the possible involvement of neurosteroids in the stress response, frogs were exposed to shaking stress before incubating hypothalamic explants with tritiated pregnenolone. Hypothalamic tissue from stressed animals displayed enhanced production of neurosteroids. The stress-induced stimulation of neurosteroid formation was significantly reduced by icv pretreatment with CRH-R1 or CRH-R2 receptor antagonists. Altogether, these observations indicate that CRH does not only stimulate the corticotropic axis but also exerts a direct stimulatory effect on the biosynthesis of neurosteroids. The behavioral effects of CRH may thus be mediated, at least in part, through increased production of neurosteroids.
Supported: FEDER, INSERM, INSERM-JSPS, INSERM-FRSQ, STAR, Haute-Normandie