Characterization of spontaneous inhibitory postsynaptic currents in cultured rat retinal amacrine cells

Spontaneous postsynaptic current is a reflection of spontaneous neurotransmitter release that plays multiple roles in a variety of neurobiological activities. In the present study, we recorded spontaneous inhibitory postsynaptic currents (sIPSCs) by patch-clamp techniques in cultured rat retinal GABAergic amacrine cells (ACs), which provide inhibitory inputs to both bipolar and ganglion cells in the inner retina, and examined if and how Ca2+ was involved in the induction of spontaneous GABA release from the terminals of these cells. sIPSCs were completely blocked by application of either 10 μM bicuculline or 10 μM gabazine, and the reversal potential of sIPSCs was close to ECl−, indicating that these events were exclusively mediated by GABAA receptors. Increase of external Ca2+ concentrations from 2 to 5 mM significantly enhanced the frequency, but did not change the amplitude of sIPSCs. In contrast, perfusion of Ca2+-free external solution greatly reduced the events of sIPSCs and decreased the amplitude of sIPSCs. Consistently, the non-selective voltage-gated calcium channel blocker CdCl2 (200 μM) considerably suppressed both the frequency and the amplitude of sIPSCs. Furthermore, the ryanodine receptor (RyR) antagonist dantrolene (10 μM) failed to affect sIPSCs, while the inositol 1,4,5-trisphosphate (IP3) receptor antagonists 2-aminoethyl diphenylborinate (2-APB, 20 μM) and xestospongin C (XeC, 1 μM) significantly decreased the frequency of sIPSCs. In the presence of SKF96365 (10 μM), a non-specific transient receptor potential channel (TRP) blocker, 2-APB persisted to show its effect on sIPSCs. These results suggest that spontaneous GABA release from the terminals of GABAergic ACs is Ca2+-dependent, and both extracellular calcium influx through presynaptic calcium channels and Ca2+ release through activation of the IP3-sensitive pathway, but not the ryanodine-sensitive one, from intracellular stores are responsible for the generation of sIPSCs under our experimental conditions.