Diurnal properties of tonic and synaptic GABA_A receptor-mediated currents in suprachiasmatic nucleus neurons

Abstract

Synaptic and extrasynaptic GABA_A receptor (GABA_AR)-mediated neurotransmission is a critical component of the suprachiasmatic nucleus (SCN) neuronal network. However, the properties of the GABA_A tonic current (<i>I</i>_tonic) and its origin remain unexplored. Spontaneous GABA_A postsynaptic currents (sGPSCs) and <i>I</i>_tonic were recorded from SCN neurons with the whole cell voltage-clamp technique at different times of the day. GABA_AR antagonists (bicuculline, gabazine, and picrotoxin) inhibited sGPSC and induced an outward shift of the holding current, which defined the <i>I</i>_tonic amplitude. The sGPSC frequency, synaptic charge transfer, and <i>I</i>_tonic 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 <i>I</i>_tonic 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_B receptors by baclofen did not significantly alter the <i>I</i>_tonic of neurons with low-frequency sGPSC. The equilibrium potential (Eq) for <i>I</i>_tonic was similar to the Eq for chloride and GABA_A receptor-activated currents. <i>I</i>_tonic showed outward rectification at membrane potentials over the range of -70 to -10 mV and then was linear at voltages greater than -10 mV. GABA_AR containing α4-, α5-, and δ-subunits were expressed in SCN, and their contribution to <i>I</i>_tonic was confirmed by application of the GABA_AR agonist 4,5,6,7-tetrahydroisoxazolo[5,4-<i>c</i>]pyridin-3-ol (THIP) and the GABA_AR inverse agonist 11,12,13,13<i>a</i>-tetrahydro-7-methoxy-9-oxo-9<i>H</i>-imidazo[1,5-<i>a</i>]pyrrolo[2,1-<i>c</i>][1,4]benzodiazepine-1-carboxylic acid ethyl ester (L655,708). Thus, the <i>I</i>_tonic was mediated by extrasynaptic GABA_ARs activated predominantly by GABA diffusing out of GABAergic synapses.<b>NEW & NOTEWORTHY</b> A tonic current (<i>I</i>_tonic) mediated by GABA_A receptors (GABA_ARs) containing α4-, α5- and δ-subunits was observed in the suprachiasmatic nucleus. The <i>I</i>_tonic amplitude strongly depended on the action potential-mediated synaptic release of GABA. The equilibrium potential for <i>I</i>_tonic corresponds to that for GABA_A currents. The frequency of GABA_A postsynaptic currents and I_tonic 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.