Effects of beta-casomorphin on dentate hippocampal field potentials in freely moving rats

Intracerebroventricular administration of 166 nmoles of the exogenic opioid beta-casomorphin (5) produced a potent and reversible depression of the compound action potential evoked in dentate granule cells by stimulation of the medial perforant path, whereas the extracellularly recorded excitatory postsynaptic potential is left unchanged. This in vivo effect of beta-casomorphin was obviously different from those observed previously in the CA 1 region in hippocampal slice experiments. The results suggest that more than one opioid mechanism determines the granule cell excitability. Some of the possible mechanisms involved in the effects of beta-casomorphins in the hippocampus are briefly discussed.