Differential sensitivity to Zolpidem of IPSPs activated by morphologically identified CA1 interneurons in slices of rat hippocampus

Hippocampal pyramidal cells express several alpha-subunits, which determine the affinity of GABA(A) (gamma-aminobutyric acid) receptors for benzodiazepine site ligands. This study asked whether inhibitory postsynaptic potentials (IPSPs) elicited by specific interneuronal subclasses were differentially sensitive to the alpha 1-preferring agonist Zolpidem, i.e. whether different receptors mediate different inhibitory connections. Paired intracellular recordings in which the presynaptic cell was an interneuron and the postsynaptic cell a CA1 pyramid were performed in slices of adult rat hippocampus. Resultant IPSPs were challenged with Zolpidem, cells filled with biocytin and identified morphologically. IPSPs elicited by fast spiking (FS) basket cells (n = 9) were enhanced more than IPSPs elicited by regular spiking (RS) basket cells (n = 10). At FS basket cell synapses the efficacy of Zolpidem was equivalent to that of Diazepam, while RS basket cell IPSPs are enhanced 50% less by Zolpidem than by Diazepam. Thus, while alpha 1 subunits may dominate at synapses supplied by FS basket cells, RS basket cell synapses also involve alpha 2/3 subunits. Two bistratified cell IPSPs tested with Zolpidem did not increase in amplitude, despite powerful enhancements of bistratified cell IPSPs by Diazepam, consistent with previous indications that these synapses utilize alpha 5-containing receptors. Enhancements of basket cell IPSPs by Zolpidem and Diazepam were bi- or triphasic with steep amplitude increases separated by plateaux, occurring 10-15, 25-30 and 45-55 min after adding the drug to the bath. The entire enhancement was, however, blocked by the antagonist Flumazenil (n = 7). Flumazenil, either alone (n = 3), or after Zolpidem, reduced IPSP amplitude to similar to 90% of control, suggesting that alpha 4-containing receptors were not involved.