Diurnal Properties of Tonic and Synaptic GABA Receptor-mediated Currents in Suprachiasmatic Nucleus Neurons

Synaptic and extrasynaptic GABA receptor (GABAR)-mediated neurotransmission is a critical component of the suprachiasmatic nucleus (SCN) neuronal network. However, the properties of the GABA tonic current () and its origin remain unexplored. Spontaneous GABA postsynaptic currents (sGPSCs) and were recorded from SCN neurons with the whole cell voltage-clamp technique at different times of the day. GABAR antagonists (bicuculline, gabazine, and picrotoxin) inhibited sGPSC and induced an outward shift of the holding current, which defined the amplitude. The sGPSC frequency, synaptic charge transfer, and amplitude all demonstrated significant diurnal rhythms, with peaks in the middle of the day [zeitgeber time (ZT)7-8] and nadirs at night (ZT19-20). The amplitude increased proportionally with the sGPSC frequency and synaptic charge transfer during the day and required action potential-mediated GABA release, which was confirmed by TTX application. The activation of presynaptic GABA receptors by baclofen did not significantly alter the of neurons with low-frequency sGPSC. The equilibrium potential (Eq) for was similar to the Eq for chloride and GABA receptor-activated currents. showed outward rectification at membrane potentials over the range of -70 to -10 mV and then was linear at voltages greater than -10 mV. GABAR containing α4-, α5-, and δ-subunits were expressed in SCN, and their contribution to was confirmed by application of the GABAR agonist 4,5,6,7-tetrahydroisoxazolo[5,4-]pyridin-3-ol (THIP) and the GABAR inverse agonist 11,12,13,13-tetrahydro-7-methoxy-9-oxo-9-imidazo[1,5-]pyrrolo[2,1-][1,4]benzodiazepine-1-carboxylic acid ethyl ester (L655,708). Thus, the was mediated by extrasynaptic GABARs activated predominantly by GABA diffusing out of GABAergic synapses. A tonic current () mediated by GABA receptors (GABARs) containing α4-, α5- and δ-subunits was observed in the suprachiasmatic nucleus. The amplitude strongly depended on the action potential-mediated synaptic release of GABA. The equilibrium potential for corresponds to that for GABA currents. The frequency of GABA postsynaptic currents and I amplitude increased during the day, with peak in the middle of the day, and then gradually declined with a nadir at night, thus showing a diurnal rhythm.