Induction of Inhibitory Synaptic Plasticity Enhances Tonic Current by Increasing the Content of α5-Subunit Containing GABA Receptors in Hippocampal Pyramidal Neurons.

It is known that besides synaptic inhibition, there is a persistent component of inhibitory drive mediated by tonic currents which is believed to mediate majority of the total inhibitory charge in hippocampal neurons. Tonic currents, depending on cell types, can be mediated by a variety of GABA receptor (GABAR) subtypes but in pyramidal neurons, α5-subunit containing receptors were found to be predominant. Importantly, α5-GABARs were implicated in both inhibitory and excitatory synaptic plasticity as well as in a variety of cognitive tasks. In the present study, we asked whether the protocol that evokes NMDAR-dependent GABAergic inhibitory long-term potentiation (iLTP) also induces the plasticity of tonic inhibition in hippocampal pyramidal neurons. Our whole-cell patch-clamp recordings revealed that the induction of this type of iLTP is associated with a marked increase in tonic current. By using the specific inverse agonist of α5-containing GABARs (L-655,709) we provide evidence that this plastic change in tonic current is correlated with an increased proportion of this type of GABARs. On the contrary, the iLTP induction did not affect the tonic current potentiated by THIP, indicating that the pool of δ subunit-containing GABARs receptors remains unaffected. We conclude that the α5-GABARs-dependent plasticity of tonic inhibition is a novel dimension of the neuroplasticity of the inhibitory drive in the hippocampal principal neurons. Overall, α5-containing GABARs emerge as key players in a variety of plasticity mechanisms operating over a large span of time and spatial scales.