Synaptic and non-synaptic release of GABA occurs in the suprachiasmatic nucleus. GABAA-PHASIC and GABAA-TONIC receptors are active in the suprachiasmatic nucleus. GABA has inhibitory and excitatory actions on suprachiasmatic neurons. GABAA and GABAB receptors modulate the phase of the circadian pacemaker. GABA can both inhibit and mediate the effects of light on the circadian pacemaker.