Neuroactive steroids alter the circadian system of the Syrian hamster in aphase-dependent manner

Several steroid compounds affect neuronal function, primarily by modulating the GABA(A) receptor complex. A circadian variation in the brain concentra tion of neurosteroids has been reported in rats and humans. We have previou sly reported that natural occurring or synthetic neuroactive steroids such as androsterone and alphaxalone also have a rhythmic effect on behavior (an esthetic and anticonvulsant activity) and GABAergic activity. In the presen t work, we have tested the ability of neuroactive steroids to phase shift c ircadian rhythms in hamsters. The GABA(A) negative modulator dehydroepiandr osterone sulphate (DHEAS) elicited phase advances when administered at CT 6 , while the positive modulator androsterone lacked any effect at this time. A complete phase response curve for DHEAS revealed a nonphotic-like effect . DHEAS also blocked the circadian effects of light, while androsterone ind uced photic-like responses. There is also evidence that neurosteroids may b e present and even synthesized in the SCN. Collectively, the results so far indicate that some neuroactive steroids might modulate the activity of the circadian clock.