Presynaptic GABA_B receptors regulate retinohypothalamic tract synaptic transmission by inhibiting voltage-gated Ca²⁺ channels

Presynaptic GABA_B receptor activation inhibits glutamate release from retinohypothalamic tract (RHT) terminals in the suprachiasmatic nucleus (SCN). Voltage-clamp whole cell recordings from rat SCN neurons and optical recordings of Ca²⁺-sensitive fluorescent probes within RHT terminals were used to examine GABA_B-receptor modulation of RHT transmission. Baclofen inhibited evoked excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner equally during the day and night. Blockers of N-, P/Q-, T-, and R-type voltage-dependent Ca²⁺ channels, but not L-type, reduced the EPSC amplitude by 66, 36, 32, and 18% of control, respectively. Joint application of multiple Ca²⁺ channel blockers inhibited the EPSCs less than that predicted, consistent with a model in which multiple Ca²⁺ channels overlap in the regulation of transmitter release. Presynaptic inhibition of EPSCs by baclofen was occluded by ω-conotoxin GVIA (≤72%), mibefradil (≤52%), and ω-agatoxin TK (≤15%), but not by SNX-482 or nimodipine. Baclofen reduced both evoked presynaptic Ca²⁺ influx and resting Ca²⁺ concentration in RHT terminals. Tertiapin did not alter the evoked EPSC and baclofen-induced inhibition, indicating that baclofen does not inhibit glutamate release by activation of Kir3 channels. Neither Ba²⁺ nor high extracellular K⁺ modified the baclofen-induced inhibition. 4-Aminopyridine (4-AP) significantly increased the EPSC amplitude and the charge transfer, and dramatically reduced the baclofen effect. These data indicate that baclofen inhibits glutamate release from RHT terminals by blocking N-, T-, and P/Q-type Ca²⁺ channels, and possibly by activation of 4-AP-sensitive K ⁺ channels, but not by inhibition of R- and L-type Ca²⁺ channels or by Kir3 channel activation.