The atypical excitation by opiates and opioid peptides of hippocampal pyramidal cells can be antagonized by iontophoresis of naloxone, the γ-aminobutyric acid antagonist bicuculline, or magnesium ion. The recurrent inhibition of these cells evoked by transcallosal stimulation of the contralateral hippocampus is blocked by enkephalin but only shortened by acetylcholine. The results suggest that the opioids excite pyramidal neurons indirectly by inhibition of neighboring inhibitory interneurons (probably containing γ-aminobutyric acid). This mechanism may be pertinent to the electrographic signs of addictive drugs.
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