Progressive circuit hyperexcitability in mouse neocortical slice cultures with increasing duration of activity silencing

Abstract

Forebrain neurons deprived of activity become hyperactive when activity is restored. Rebound activity has been linked to spontaneous seizures in vivo following prolonged activity blockade. Here we measured the time course of rebound activity and the contributing circuit mechanisms using calcium imaging, synaptic staining and whole cell patch clamp in organotypic slice cultures of mouse neocortex. Calcium imaging revealed hypersynchronous activity increasing in intensity with longer periods of deprivation. While activity partially recovered three days after slices were released from five days of deprivation, they were less able to recover after ten days of deprivation. However, even after the longer period of deprivation, activity patterns eventually returned to baseline levels. The degree of deprivation-induced rebound was age-dependent, with the greatest effects occurring when silencing began in the second week. Pharmacological blockade of NMDA receptors indicated that hypersynchronous rebound activity did not require Hebbian plasticity evoked. In single neuron recordings, input resistance roughly doubled with a concomitant increase in intrinsic excitability. Synaptic imaging of pre- and postsynaptic proteins revealed dramatic reductions in the number of presumptive synapses with a larger effect on inhibitory than seen in excitatory synapses. Putative excitatory synapses colocalizing PSD-95 and Bassoon declined by 39% and 56% following five and ten days of deprivation, but presumptive inhibitory synapses colocalizing gephyrin and VGAT declined by 55% and 73% respectively. The results suggest that with prolonged deprivation, a progressive reduction in synapse number is accompanied by a shift in the balance between excitation and inhibition and increased cellular excitability.