γ-Aminobutyrate, α-carboxy-2-nitrobenzyl ester selectively blocks inhibitory synaptic transmission in rat dentate gyrus

γ-Aminobutyrate, α-carboxy-2-nitrobenzyl ester (cGABA) is a stable photoactivatable probe used to study γ-aminobutyrate (GABA) receptors. GABA is released from this compound when it is exposed to ultraviolet light, but little is known about the electrophysiological effects of the compound itself. Whole cell patch clamp recordings on rat hippocampal slices demonstrated that cGABA blocked polysynaptic inhibitory postsynaptic currents (IPSCs) evoked in dentate granule cells by antidromic stimulation of the mossy fibers. It also reduced monosynaptically evoked IPSCs with an IC 50 of 28 μM. In contrast, cGABA had no effect on excitatory postsynaptic currents (EPSCs) evoked by perforant path stimulation. The effect of cGABA was not mediated by depression of GABA release through activation of presynaptic GABA B receptors. cGABA inhibited muscimol-evoked currents by only 15% at a concentration of 40 μM. At this same concentration, it reduced the mean frequency of miniature inhibitory postsynaptic potentials by 71%, their mean peak amplitude by 44%, their mean decay time constant by 26% and the mean charge transfer per event by 52%. These effects may be explained by a phenothiazine-like modification of GABA A receptor kinetics and/or a selective block of somatic GABA synapses.