Morphine and nigrostriatal function in the rat and mouse: the role of nigral and striatal opiate receptors.

Using local injections of drugs and hemisection experiments, a comparison was made of the actions of morphine on the dopaminergic nigrostriatal system of the rat and mouse. In the rat, morphine appeared to act exclusively at presynaptic opiate receptors on dopaminergic nerve endings in the striatum. Activation of these receptors resulted in enhanced dopamine synthesis but with no associated increase in dopamine release. In the mouse, morphine acted at the level of the substantia nigra to enhance both striatal dopamine synthesis and release. The exact localization of these receptors in the substantia nigra remains to be determined.