A microiontophoretic study of the actions of mu-, delta- and kappa-opiate receptor agonists in the rat brain.

The actions of mu-, delta- and kappa-opiate receptor agonists have been compared on the activity of single neurones in the brain stem, caudate nucleus and hippocampus of the rat, using the technique of microiontophoresis. In the brain stem and caudate nucleus the predominant effect of all the opiate agonists tested was depression of neuronal activity which was antagonized by naloxone. The selectivity of naloxone as an opiate receptor antagonist was indicated by its lack of effect on gamma-aminobutyric acid (GABA)-induced responses. In the hippocampus both mu- and delta-agonists mainly caused an increase in neuronal firing rates, though some neurones were depressed. In contrast, all the kappa-agonists, including the proposed endogenous ligand for the kappa-receptor, dynorphin, caused depression of neuronal activity. All of these effects were antagonized by naloxone. There was a clear distinction in the areas within the hippocampus in which the mu- and delta-agonists produced different effects. Neurones in the pyramidal cell layer were always excited by these drugs, whereas neurones in the granule cell layer of the dentate gyrus were always depressed by the same drug.