Kainic acid-induced seizures enhance dentate gyrus inhibition by downregulation of GABA(B) receptors.

Seizures cause a persistent enhancement in dentate synaptic inhibition concurrent with, and possibly compensatory for, seizure-induced hippocampal hyperexcitability. To study this phenomenon, we evoked status epilepticus in rats with systemic kainic acid (KA), and 2 weeks later assessed granule cell inhibition with paired-pulse stimulation of the perforant path (PP) in vitro. Controls demonstrated three components of paired-pulse inhibition: early inhibition (10-30 msec), intermediate facilitation (30-120 msec), and late inhibition (120 msec to 120 sec). After seizures, inhibition in all components was enhanced significantly. The GABA(A) antagonist bicuculline blocked only early enhanced inhibition, demonstrating that both GABA(A) and GABA(B) postsynaptic receptors contribute to seizure-induced enhanced inhibition. In controls, the GABA(B) antagonist CGP 35348 increased both GABA(A) and GABA(B) responses in granule cells, suggesting that CGP 35348 acts presynaptically, blocking receptors that suppress GABA release. In contrast, slices from KA-treated rats were markedly less sensitive to CGP 35348. To test the hypothesis that GABA(B) receptors regulating GABA release are downregulated after seizures, we measured paired-pulse suppression of recurrent IPSPs, or disinhibition, using mossy fiber stimuli. Early disinhibition (< 200 msec) was reduced after seizures, whereas late disinhibition remained intact. CGP 35348 blocked the early component of disinhibition in controls and, to a lesser extent, reduced disinhibition in KA slices. However, paired monosynaptic IPSPs recorded intracellularly showed no difference in disinhibition between groups. Our findings indicate that seizure-induced enhancement in dentate inhibition is caused, at least in part, by reduced GABA(B) function in the polysynaptic recurrent inhibitory circuit, resulting in reduced disinhibition and heightened GABA release.