THE INTRINSIC OPTICAL SIGNAL EVOKED BY CHIASM STIMULATION IN THE RAT SUPRACHIASMATIC NUCLEI EXHIBITS GABAERGIC DAY-NIGHT VARIATION

Infrared light transmittance imaging was used in rat hypothalamic slic es to record an intrinsic optical signal (IOS) of the cell ensemble in the suprachiasmatic nuclei (SCN), the locus of the endogenous circadi an clock. Upon optic chiasm stimulation, a transient IOS was observed in an area conforming to the known retinohypothalamic tract innervatio n in the ventral SCN. An increase in extracellular Mg2+ concentration to 10 mM reduced the IOS, suggesting that the elicited IOS is dependen t on synaptic transmission. D-2-Amino-5-phosphonopentanoic acid and mu scimol suppressed the elicited IOS, indicating that NMDA and GABAA rec eptor-mediated mechanisms were involved in cell ensemble activity refl ected in the IOS. The extracellularly recorded spiking of SCN neurons located outside the IOS area remained largely unaffected by the affere nt stimulus. Neurons located within the IOS area responded with a depr essed electrical discharge, manifesting an inverse relationship betwee n single-unit discharge and the optical measure. The influence of the endogenous circadian rhythm on the elicited IOS was assessed by carryi ng out daytime-dependent concentration-response experiments. NMDA and non-NMDA receptor specific compounds did not exhibit significant day-n ight differences, whereas GABA-specific ligands showed a significant d ay-night variation in activity. The competitive GABAA receptor antagon ist bicuculline enhanced the IOS exclusively in the daytime SCN. 5 alp ha-Pregnane-3 alpha,21-diol-20-one (allotetrahydrodeoxy-corticosterone ), a neuroactive steroid that potentiates GABAergic inhibition, suppre ssed the IOS in the night-time SCN more than in the daytime SCN. This suggests that in the rat the level of extracellular GABA is higher in the night-time SCN compared to the daytime SCN.