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Bicuculline Increases and Muscimol Reduces the Phase-Delaying Effects of Light and VIP/PHI/GRP in the Suprachiasmatic Region

Laboratory of Neuroendocrinology and Behavior, Departments of Biology and Psychology, Georgia State University, Atlanta, GA 30303

Kim L. Huhman

Laboratory of Neuroendocrinology and Behavior, Departments of Biology and Psychology, Georgia State University, Atlanta, GA 30303

Tokunbo O. Babagbemi

Laboratory of Neuroendocrinology and Behavior, Departments of Biology and Psychology, Georgia State University, Atlanta, GA 30303

H. Elliott Albers

Laboratory of Neuroendocrinology and Behavior, Departments of Biology and Psychology, Georgia State University, Atlanta, GA 30303

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 (PHI), and gastrin-releasing peptide (GRP) to phase delay hamster activity rhythms. Microinjection of the GABA_A agonist, muscimol, significantly (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 muscimol or bicuculline into the suprachiasmatic nucleus (SCN) produced little or no effect on circadian phase when no light pulses were provided. Coadministration of muscimol with VIP/PHI/GRP significantly (p < .01) reduced the phase-delaying effect of VIP/PHI/GRP, whereas administration of bicuculline with VIP/PHI/ GRP significantly (p <.05) increased 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.

Key Words: circadian • GABA • GABAA • hamster • light • phase-shift • suprachiasmatic nucleus • vasoactive intestinal peptide • peptide histidine isoleucine • gastrin-releasing peptide

Journal of Biological Rhythms, Vol. 11, No. 2, 137-144 (1996)
DOI: 10.1177/074873049601100206


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