The suprachiasmatic nuclei (SCN) at the base of the hypothalamus are known to be the site of the endogenous circadian pacemaker in mammals. The SCN are innervated by the retinohypothalamic tract, which conveys photic information to the SCN. GABA is one of the most abundant neurotransmitters in the SCN, and has been implicated in the modulation of photic responses of the SCN circadian pacemaker. This study sought to examine the effect of GABAergic compounds on optic nerve-evoked SCN field potentials recorded in rat horizontal hypothalamic slices. The GABA(A) agonist muscimol (10 mu M) potentiated SCN field potentials by 23%, while application of the GABA(A) antagonist bicuculline (10 mu M) inhibited SCN field potentials by a similar amount, (22%). Conversely, the GABA(B) agonist baclofen (1.0 mu M) inhibited SCN field potentials by 48%, while the GABA(B) antagonist phaclofen (0.5 mM) augmented SCN field potentials by 62%. Recordings performed at both day and night times indicate that there were no qualitative day-night differences in GABAergic activity on SCN field potentials. This study concludes that, in general, GABA(A) activity tends to increase, and GABA(B) activity tends to decrease the response of SCN neurons to optic nerve stimulation.
