Iontophoresis of enkephalins potentiates evoked field potentials in the in vitro rat hippocampus.

Effects of iontophoresis of an active opioid peptide, D-Ala2-Met-enkephalinamide (2dA), on evoked field potentials were studied in the hippocampus in vitro preparation. Population spike responses which correspond to the summatory firing of the pyramidal neurons were recorded with glass microelectrodes from the CA1 cell layer. It is found that 2dA greatly enhanced the amplitude of population spike. The excitatory effect was opiate-specific, because it was antagonized by iontophoretically applied naloxone, a potent narcotic antagonist. On the contrary, an inactive enkephalin analog, N-acetyl-(D-Ala2, Lys3)-Met-enkephalinamide, was without effect. It is concluded that opioid peptides exert a specific excitatory action on the pyramidal neurons in the in vitro hippocampus.