Shaping of IPSCs by Endogenous Calcineurin Activity

Abstract

Synaptic inhibition, mediated by GABA A receptors, regulates neuronal firing, influences coincidence detection (Kö nig et al., 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 ATP␥S 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.

Interaction of Calcineurin and Type-A GABA Receptor γ2Subunits Produces Long-Term Depression at CA1 Inhibitory Synapses

“…The final illustration of the cellular and molecular mechanisms underlying bidirectional regulation of GABA A receptors mediated by kinases and phosphatases in synaptic plasticity can teach us much about what kinds of molecules are needed to build arrays of the GABA A receptor regulations. CaN is reported to modulate the channel kinetics of the GABA A receptors in cultured neurons (Jones and Westbrook, 1997). However, we found that there were no changes of the unitary GABA A R-IPSC rise times or their decay time constants after activation of CaN.…”

γ-Aminobutyric acid type A receptors modulate cAMP-mediated long-term potentiation and long-term depression at monosynaptic CA3–CA1 synapses

“…This depression recovered within 1 h after Sp-cAMPS was washed out with normal ACSF (data not shown). This result drew our attention to involvement of inhibitory synaptic transmission in Sp-cAMPS-induced LTP and to the modulation of GABA A receptors by phosphorylation (21,22). We therefore examined the effect of Sp-cAMPS on synaptic transmission in the presence of picrotoxin, a GABA A receptor antagonist.…”

“…Other evidence also suggests that activation of protein phosphatases favors the induction of LTD, whereas activation of protein kinases such as PKA, CaMK (calcium calmodulin kinase)-II, or CaMKIV promotes LTP (6,40). Furthermore, modulations of GABA A receptors by PKA and protein phosphatases can eliminate and enhance the inhibitory function of GABA A receptors, respectively (21,22). It is therefore possible that PKA-activated phosphorylation of both GABA A and AMPA receptors can subsequently inhibit activation of protein phosphatases, ensuring that the direction of synaptic plasticity is primarily toward potentiation.…”

The γ-aminobutyric acid type A (GABA A ) receptor-associated protein (GABARAP) promotes GABA A receptor clustering and modulates the channel kinetics

“…The size and duration of excitatory and inhibitory postsynaptic potentials are largely determined by transmitter release and postsynaptic channel kinetics (15,(22)(23)(24). Modulation of kinetics is an important mechanism to modulate synaptic transmission (25)(26)(27)(28).…”