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 μM, 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 μM; n = 7), and (±)3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 10 μM, n = 6), respectively. Reversal potentials for the evoked IPSCs and EPSCs were consistent with mediation via GABA_A 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 GABA_A 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.