Modification of Morphine Analgesia by Altering Central Catecholaminergic and 5-Hydroxytryptaminergic Activity

Abstract

The roles of central catecholamines and 5-hydroxytryptamine in morphine analgesia were investigated. Drugs that are relatively selective in modifying central monoaminergic activity were used. Nociceptive response was tested by the modified hot plate method. L-Dopa and apomorphine antagonized while chlorpromazine, haloperidol, alpha-methyl-para-tyrosine, and clonidine enhanced morphine analgesia. Para-chlorophenylalanine blocked but L-tryptophan potentiated this effect of morphine. All these changes were significant. The results of the present study indicated that increased brain catecholaminergic or decreased 5-hydroxytryptaminergic activity inhibited the analgesic effect of morphine. On the contrary, decreased central catecholaminergic or increased 5-hydroxytryptaminergic activity enhanced morphine analgesia.