Shaping of IPSCs by endogenous calcineurin activity

Synaptic inhibition, mediated by GABA(A) receptors, regulates neuronal firing, influences coincidence detection (Konig et el., 1996), and can synchronize the output of neural circuits (Cobb et al., 1995). Although GABA(A) receptors can be modulated by phosphorylation, few studies have directly addressed the role of such modulation at synapses, where the nonequilibrium conditions of receptor activation are quite different from those often used to study GABA(A) receptors in vitro. Here we promoted endogenous phosphorylation by inhibiting specific phosphatases in rat hippocampal neurons and compared the effects on IPSCs with GABA(A) channel responses in outside-out patches. Brief and saturating GABA pulses (5 msec; 10 mM) activated patch currents resembling the IPSC. Inhibition of calcineurin (protein phosphatase 2B), but not phosphatases 1 or 2A, produced a similar shortening of iPSC and patch responses, as did nonspecific inhibition of dephosphorylation using ATPγS or high concentrations of intracellular phosphate. Calcineurin inhibition increased the microscopic ligand unbinding rate, which was measured using the competitive antagonist 2- (3-carboxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide, suggesting that the iPSC shortening was partly caused by destabilization of the ligand binding site. Calcineurin inhibition also increased the rate and extent of macroscopic receptor desensitization. These results show that endogenous regulation by kinases and calcineurin can produce substantial changes in the iPSC duration by altering the unbinding and gating kinetics of the GABA(A) receptor. Dynamic regulation of synaptic inhibition may thus allow for the tuning of circuit behavior at the level of individual inhibitory synapses.