Disinhibitory effect of phencyclidine in the hippocampus in vitro: PCP receptors implicated

The effects of phencyclidine (PCP) and two dioxolane stereoisomers, dexoxadrol and levoxadrol, on hippocampal inhibition were compared. Field potentials were recorded in the CA1 pyramidal cell layer in the rat hippocampal slices in vitro. Recurrent inhibition of the population spikes evoked orthodromically by stimulation of the Schaffer collaterals was induced by antidromic conditioning stimulation at appropriate time intervals before the orthodromic stimulation. The drugs were applied by micropressure ejection in concentrations which did not affect the unconditioned population spike. After PCP or dexoxadrol administration, the orthodromically evoked population spike was much less reduced by the antidromic conditioning stimulation than before, suggesting that the recurrent inhibition was diminished. Levoxadrol had only negligible effect. Since dexoxadrol has many PCP-like pharmacological properties but levoxadrol does not, we concluded that PCP attenuates hippocampal recurrent inhibition by activating the PCP receptors. It is suggested that this action results in depression of excitatory synaptic transmission from axon collaterals to the inhibitory interneuron with possible involvement of the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptor.