Rapid Critical Period Induction by Tonic Inhibition in Visual Cortex

Abstract

Mice lacking a synaptic isoform of glutamic acid decarboxylase (GAD65) do not exhibit ocular dominance plasticity unless an appropriate level of GABAergic transmission is restored by direct infusion of benzodiazepines into the brain. To better understand how intracortical inhibition triggers experience-dependent changes, we dissected the precise timing requirement for GABA function in the monocular deprivation (MD) paradigm. Diazepam (DZ) or vehicle solution was infused daily before and/or during 4 d of MD in GAD65 knock-out mice. Extracellular singleunit recordings from the binocular zone of visual cortex were performed at the end of deprivation. We found that a minimum treatment of 2 d near the beginning ofMDwas sufficient to fully activate plasticity but did not need to overlap the deprivation per se. Extended delay afterDZinfusion eventually led to loss of plasticity accompanied by improved intrinsic inhibitory circuit function. Two day DZtreatment just after eye opening similarly closed the critical period prematurely in wild-type mice. Raising wild-type mice in complete darkness from birth delayed the peak sensitivity toMDas in other mammals. Interestingly, 2 d DZ infusion in the dark also closed the critical period, whereas equally brief light exposure during dark-rearing had no such effect. Thus, enhanced tonic signaling through GABAA receptors rapidly creates a milieu for plasticity within neocortex capable of triggering a critical period for ocular dominance independent of visual experience itself.