Gamma-aminobutyric acid (GABA), and its biosynthetic enzyme, glutamic decarboxylase, are widely distributed in the suprachiasmatic nucleus (SCN). In the present study, we examined the role of the GABAA receptor on in vitro SCN responses to photic-like signals. We found that 100 μM GABAA receptor antagonist bicuculline partially blocked field potentials evoked by optic nerve stimulation. NMDA- and SP-induced phase shifts of SCN neuronal activity rhythms, were blocked with 10 μM bicuculline. Application of 100 μM bicuculline alone induced phase advance of SCN neuronal activity rhythm. These results show that NMDA- and SP-induced phase shifts are blocked by bicuculline and suggest GABA has an important role as neurotransmitter in the neuronal network regulating phase shifts of the circadian clock.
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