Opiates And Opioid Peptides

The Opiate drugs Morphine and Heroin, administered posttraining, impair retention in laboratory animals. The memory impairment is not state-dependent: Administration of opiates prior to retention testing does not decrease the impairment. Opiate-receptor ANTAGONISTS, including NALOXONE and NALTREXONE, enhance memory and block the memory impairment produced by opiates. Endogenous opioid peptides (brain peptides that mimic the effect of morphine, heroin, and other opiates) also affect memory. The opioid beta-endorphin is released in the brain when animals are exposed to novel training situations. Memory impairment is induced by posttraining injections of beta-ENDORPHIN as well as by injections into several brain regions, including the amygdaloid complex and medial septum. Opiate antagonists administered into these brain regions enhance memory. Unlike the effects of opiate drugs, the memory impairment induced by beta-endorphin may be due, at least in part, to the induction of state-dependency: Under some conditions beta-endorphin administered (or endogenously released) prior to memory testing may lessen the memory impairment induced by a posttraining injection of the peptide.

Despite the widespread and long-standing use of opiate drugs by humans, there have been no systematic studies on the effect of morphine, heroin, or other opiates on human memory. Chronic opiate users do show memory deficits, but these may result from general deterioration rather than from any specific effect of the opiates. Acute administration of opiates (as in preanesthetic medication, for example) may induce a temporary amnesia. The failure of patients to remember experiences immediately prior to surgery may be due, at least in part, to an amnestic effect of the opiates used for ANALGESIA (PAIN suppression). The effect of opiate antagonists has been explored clinically in the treatment of dementias, but with limited success.

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