Effects of diazepines and barbiturates on hippocampal recurrent inhibition

The effects of two diazepines (diazepam and Ro 11-7800) and 3 barbiturates (thiamylal, pentobarbitol and phenobarbital) on GABA-mediated recurrent inhibition were assessed on single hippocampal pyramidal cells and on population spikes using extracellular recording techniques. Recurrent inhibition was evoked in spontaneously active CA1 pyramidal cells by stimulation of the fimbria or the alveus with single shocks. Microiontophoretic application of Ro 11-7800 or systemic application of diazepines or barbiturates resulted in an increase of the duration of the inhibition and in a concomitant depression of the spontaneous firing in most neurones tested. When the firing rates were kept constant artificially, using excitant amino acids, a prolongation of the recurrent inhibition was observed with barbiturates but not with diazepines. The duration of the inhibition, which was assessed from CA1 population spikes elicited by double shocks to the fimbria, was prolonged following systemic application of diazepines or barbiturates. It is concluded that both diazepines and barbiturates are able to potentiate GABAergic recurrent inhibition in the hippocampus. The demonstration of this effect appears to depend critically on certain experimental conditions.