Electric shock (ES) produces an opiate-like analgesia and catalepsy in rats, which may be mediated by endogenous opioid systems. The present study was designed to investigate the alterations of endogenous opioid receptors by an intermittent ES application. In a case of opiate agonists, [3H]dihydromorphine and [3H]d-Ala-Met-enkephalinamide, the application of ES produced a decrease of [3H]ligands' binding capacity. On the contrary, the binding capacity of the opiate antagonist [3H]naloxone was increased by the ES application. These results suggest that endogenous putative opioid agonist- and antagonist-type receptors are differently regulated in vivo.
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