GABAergic regulation of light-induced c-Fos immunoreactivity within the suprachiasmatic nucleus

Analysis of the photic induction of c-Fos immunoreactivity (-ir) within the suprachiasmatic nucleus (SCN) has proven to be a powerful tool with which to study the neurochemical mechanisms involved in phase shifting the circad ian clock. Some systemically administered GABAergic drugs inhibit light-ind uced phase shifts and c-Fos-ir, whereas others inhibit light-induced phase shifts without affecting c-Fos-ir. More recently, we have found that inject ion of GABAergic drugs directly into the SCN region can have dramatically d ifferent effects on light-induced phase shifts than following their systemi c administration. The present study investigated the effects of GABA(A) and GABA(B) agonists and antagonists injected into the SCN region on c-Fos-ir within the SCN. Microinjection of either a GABAA agonist, muscimol, or a GA BA(B) agonist, baclofen, into the SCN region significantly reduced light-in duced c-Fos-ir within the SCN when administered before light exposure at ci rcadian time (CT) 13.5 or CT 19. In contrast, microinjection of a GABA(A) a ntagonist, bicuculline, but not a GABA(B) antagonist, CGP-35348, into the S CN region increased light-induced c-Fos-ir within the SCN when administered before light exposure at CT 13.5 or CT 19. These data indicate that GABAer gic agonists and antagonists injected directly into the SCN region alter li ght-induced Fos-ir in a manner similar to their effects on light-induced ph ase shifts. Comparison of these data with previous studies examining the ef fects of systemically administered GABAergic drugs suggests that GABAB-acti ve drugs have similar effects whether given systemically or within the SCN, but that GABAA-active drugs have more complex effects on c-fos induction a nd have multiple sites of action. (C) 1999 Wiley-Liss, Inc.