Penicillin induced hyperexcitability in the in vitro hippocampal slice can be unrelated to impairment of somatic inhibition.

The effects induced by penicillin (PEN) upon the synaptic responses of CA1 hippocampal pyramidal cells (HPCs) were studied in the 'in vitro' slice. Low concentrations of PEN (0.17-0.34 mM) evoked an increase in amplitude and duration of the orthodromic excitatory post-synaptic potential induced by stratum radiatum, while stimuli which were subthreshold in control conditions became effective in eliciting action potentials. These changes were not paralleled by any decrease of the recurrent inhibitory post-synaptic potential due to inhibitory interneurons located at or near the soma. However, the latter decreased and then disappeared as PEN concentrations were brought to levels higher than 0.68 mM. Since low concentrations of PEN increase CA1 HPCs responsiveness without decreasing somatic inhibition, it is concluded that this action is probably due to a reduced dendritic inhibitory mechanism.