Different opioid systems may participate in post-electro-convulsive shock (ECS) analgesia and catalepsy

Abstract

Administration of electroconvulsive shock (ECS) to rats results in post-ictal analgesia and catalepsy both of which can be partially reversed by the opiate antagonists, naloxone and naltrexone. Tolerance to both phenomena develops following daily ECS administration for 10 days. However, qualitatively different patterns of tolerance development of analgesia and catalepsy are seen. Naloxone treatment prior to ECS provides partial protection against the development of tolerance to ECS-induced catalepsy but does not present the tolerance to post-ECS analgesia. In contrast, the long-lasting opiate antagonist, naltrexone, blocked the development of tolerance to ECS analgesia. Furthermore, the same animals that showed tolerance to the analgesic effects of repeated ECS failed to show analgesia following the administration of 10 mg/kg of morphine while naltrexone, but not naloxone, treatment prior to ECS blocked the development of cross-tolerance to morphine analgesia. A dose-response investigation of morphine's action (5, 10, 15 and 20 mg/kg) in additional animals receiving 10 daily administrations of ECS reveals that a greater tolerance to morphine's motor inhibitory effect than to its analgesic effect results from repeated ECS administration. Finally, animals receiving daily injections of either a low (10 mg/kg) or a high (100 mg/kg) dose of morphine for 10 days showed markedly attenuated post-ECS analgesia and catalepsy. However, whereas similar effects of opiate antagonists and the chronic administration of both doses of morphine were observed with post-ECS catalepsy, analgesia was least affected by naloxone (50% of control) and most affected by the chronic high dose of morphine (14% of control). Furthermore, a similar degree of tolerance to post-ECS analgesia was seen following either repeated ECS in drug-naive animals or the chronic administration of the high dose of morphine. Thus, the partial naloxone blockade of ECS analgesia and the more substantial attenuation of post-ECS analgesia seen in morphine-tolerant animals provide different estimates of opioid involvement in these phenomena. It is proposed that these results may demonstrate the involvement of different opioid systems in analgesia and catalepsy and it is suggested that more than one opioid system may also be involved in post-ECS analgesia.