GABA AND GLUTAMATE MEDIATE RAPID NEUROTRANSMISSION FROM SUPRACHIASMATIC NUCLEUS TO HYPOTHALAMIC PARAVENTRICULAR NUCLEUS IN RAT

1. Intracellular sharp electrode and whole-cell patch-clamp recording from characterized paraventricular nucleus (PVN) neurones in rat hypot halamic slices were used to study the synaptic mechanisms and associat ed neurotransmitters that mediate their response to suprachiasmatic nu cleus (SCN) stimulation. 2. Electrical stimulation restricted to SCN e voked short-latency inhibitory postsynaptic potentials (IPSPs) or comb inations of IPSPs and excitatory postsynaptic potentials (EPSPs) in al l (n = 59) PVN neurones tested. Type I neurones (n = 18) were magnocel lular and a majority (13/18) demonstrated monosynaptic IPSPs that reve rsed polarity at the chloride equilibrium potential and were sensitive to bicuculline. 3. Type II (n = 10) and III parvocellular (n = 13), a nd unclassifiable neurones (n = 18) displayed combinations of IPSPs an d EPSPs following similar stimuli applied to SCN. IPSP blockade with b icuculline uncovered SCN-evoked monosynaptic dual-component EPSPs that mere sensitive to N-methyl-D-aspartate (NMDA) and non-NMDA receptor a ntagonists. In addition, chemical microstimulation within SCN was asso ciated with transient increases in spontaneous EPSPs recorded from the se PVN neurones. 4. These data imply that the amino acids GABA and glu tamate are important mediators of fast monosynaptic transmission from SCN to defined neurones in PVN, and are candidates for conveying circa dian rhythmicity to PVN regulation of neuroendocrine and autonomic pro cesses.