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 slices to record an intrinsic optical signal (IOS) of the cell ensemble in the suprachiasmatic nuclei (SCN), the locus of the endogenous circadian clock. Upon optic chiasm stimulation, a transient IOS was observed in an area conforming to the known retinohypothalamic tract innervation in the ventral SCN. An increase in extracellular Mg2+ concentration to 10 mM reduced th IOS, suggesting that the elicited IOS is dependent on synaptic transmission. D-2-amino-5-phosphonopentanoic acid and muscimol suppressed the elicited IOS, indicating that NMDA and GABAA receptor-mediated mechanisms were involved in cell ensemble activity reflected in the IOS. The extracellularly recorded spiking of SCN neurons located outside the IOS area remained largely unaffected by the afferent stimulus. Neurons located within the IOS area responded with a depressed electrical discharge, manifesting an inverse relationship between single-unit discharge and the optical measure. The influence of the endogenous circadian rhythm on the elicited IOS was assessed by carrying out daytime-dependent concentration-response experiments. NMDA and non-NMDA receptor specific compounds did not exhibit significant day-night differences, whereas GABA-specific ligands showed a significant day-night variation in activity. The competitive GABAA receptor antagonist bicuculline enhanced the IOS exclusively in the daytime SCN. 5alpha-Pregnane-3alpha,21-diol-20-one (allotetrahydrodeoxycorticosterone), a neuroactive steroid that potentiates GABAergic inhibition, suppressed 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 night-time SCN compared to the daytime SCN.