Electrophysiology and pharmacology of projections from the suprachiasmaticnucleus to the ventromedial preoptic area in rat

Extracellular and whore-cell patch-clamp recordings were made from neurons in the ventromedial preoptic area in rat horizontal brain slices. Responses to single-pulse electrical stimulation of the ipsilateral suprachiasmatic nucleus were characterized using peristimulus time histograms or postsynapt ic current recordings, and bath application of neurotransmitter receptor an tagonists. Extracellular recordings showed that suprachiasmatic nucleus sti mulation (50-150 mu A) elicited a short-latency suppression in 35 of 64 neu rons (55%), with the majority (29/35, 83%) showing a biphasic response cons isting of a short-latency suppression followed by a long-duration activatio n. In addition, 14 cells (22%) showed activation only, while 15 (23%) were unresponsive. The GABA(A) receptor antagonist bicuculline (5-10 mu M) rever sibly blocked suppressions evoked by suprachiasmatic nucleus stimulation (2 0/20 cells). In the majority of these neurons (13/20), bicuculline also unm asked an activation in response to stimulation. Activations elicited by sup rachiasmatic nucleus stimulation, either in the presence or absence of bicu culline, were blocked by the non-N-methyl-D-aspartate and N-methyl-D-aspart ate glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and (+/ -)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10/10 cells). 6,7- Dinitroquinoxaline-2,3-dione (10 mu M) selectively and reversibly blocked t he initial, short-duration (<50 ms) activation, but had no effect on the lo nger-duration activation. In contrast, (+/-)-3-(2-carboxypiperazin-4-yl)-pr opyl-1-phosphonic acid (10 mu M) appeared to inhibit the lone-duration acti vation selectively without affecting the initial rapid activation. Combined applications of the two ionotropic glutamate receptor antagonists blocked stimulation-induced activations completely. All the pharmacological effects were concentration dependent. Whole-cell patch-clamp recordings showed tha t suprachiasmatic nucleus stimulation elicited inhibitory postsynaptic curr ents or a combination of inhibitory and excitatory postsynaptic currents in 25 of 33 neurons tested. The inhibitory postsynaptic currents had short on set latencies (4.9 +/- 0.3 ms) and a reversal potential of -56.0 +/- 3.8 mV (n = 5), and were reversibly blocked by bicuculline (5-10 mu M, 4/4 cells) . In the presence of bicuculline (5-10 mu M), excitatory postsynaptic curre nts had short onset latencies (4.7 +/- 0.5 ms), and had a fast and a slow c omponent. (+/-) 3-(2-Carboxypiperazin-4-yl)-propyl-1-phosphonic acid blocke d the slow, but not the fast, component, whereas 6,7-dinitroquinoxaline-2,3 -dione blocked the fast, but not the slow, component (n = 7). These results suggest that the projection from the suprachiasmatic nucleus conveys both inhibitory (GABA) and excitatory (glutamate) inputs to the ven tromedial preoptic area. GABA(A) receptor and both non-N-methyl-D-aspartate and N-methyl-D-aspartate glutamate receptors mediate these influences. The se inputs may be responsible for conveying information related to circadian phase from the suprachiasmatic nucleus to regions of the preoptic area kno wn to be involved in regulation of sleep/waking and other physiological fun ctions. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserv ed.