Electrophysiological analysis of suprachiasmatic nucleus projections to the ventrolateral preoptic area in the rat.

The circadian pacemaker housed in the suprachiasmatic nucleus (SCN) synchronizes daily sleep-wake cycles, presumably by modulating the sleep-wake regulatory system, including ventrolateral preoptic area (VLPO) neurons. We used whole-cell patch-clamp recording to study the projections from the SCN to the VLPO in horizontal slices of rat hypothalamus. Single-pulse stimulation of the SCN region elicited postsynaptic currents (PSCs) in 20 of 66 neurons (30%) recorded within the VLPO region as verified by intracellular biocytin labelling. At a holding potential of -60 mV, the evoked PSCs had an amplitude of 17.6 +/- 3.2 pA (SEM) and a latency of 6.3 +/- 0.5 ms (n = 10). There was a trend for simple excitatory postsynaptic currents (EPSCs) to be evoked in the VLPO cluster, simple inhibitory postsynaptic currents (IPSCs) in the extended VLPO, and a combination of EPSCs and IPSCs in both regions. IPSCs were blocked reversibly by bicuculline (10 microm, n = 11). In both the presence and absence of bicuculline, EPSCs had fast and slow components that were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 microm; n = 7), and (+/-)3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 10 microm, n = 6), respectively. Reversal potentials for the evoked IPSCs and EPSCs were consistent with mediation via GABAA and ionotropic glutamate receptors, respectively. These results suggest that the SCN region provides both inhibitory and excitatory inputs to single VLPO neurons, which are mediated, respectively, by GABAA receptors and by both non-NMDA and NMDA glutamate receptors. These projections may play important roles in conveying circadian input to systems in the preoptic area that regulate sleep and waking.