BICUCULLINE INCREASES AND MUSCIMOL REDUCES THE PHASE-DELAYING EFFECTSOF LIGHT AND VIP PHI/GRP IN THE SUPRACHIASMATIC REGION/

The present study investigated the effects of gamma-amino butyric acid (GABA)(A)-active drugs on the ability of light or coadministration of vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PH I), and gastrin-releasing peptide (GRP) to phase delay hamster activit y rhythms. Microinjection of the GABA(A) agonist, muscimol, significan tly (p < .01) reduced the phase-delaying effect of Light administered at circadian time (CT) 13.5. By contrast, microinjection of the GABA(A ) antagonist, bicuculline, significantly (p < .01) increased the phase -delaying effect of Light administered at CT 13.5. Microinjection of m uscimol or bicuculline into the suprachiasmatic nucleus (SCN) produced little or no effect on circadian phase when no light pulses were prov ided. Coadministration of muscimol with VIP/PHI/GRP significantly (p < .01) reduced the phase-delaying effect of VIP/PHI/GRP, whereas admini stration of bicuculline with VIP/PHI/GRP significantly (p < .05) incre ased the phase-delaying effect of these peptides. These data indicate that changes in GABA(A) activity within the SCN can modulate the phase -delaying effects of light and VIP/PHI/GRP during the early portion of subjective night.