GABAERGIC MODULATION OF OPTIC NERVE-EVOKED FIELD POTENTIALS IN THE RAT SUPRACHIASMATIC NUCLEUS

The suprachiasmatic nuclei (SCN) at the base of the hypothalamus are k nown 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 neurot ransmitters in the SCN, and has been implicated in the modulation of p hotic responses of the SCN circadian pacemaker. This study sought to e xamine the effect of GABAergic compounds on optic nerve-evoked SCN fie ld 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) in hibited SCN field potentials by a similar amount, (22%). Conversely, t he 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 ti mes indicate that there were no qualitative day-night differences in G ABAergic activity on SCN field potentials. This study concludes that, in general, GABA(A) activity tends to increase, and GABA(B) activity t ends to decrease the response of SCN neurons to optic nerve stimulatio n.