Morphine suppression of substantia nigra zona reticulata neurons in the rat: implicated role for a novel striatonigral feedback mechanism.

In lightly anesthetized rats, systemic administration of morphine exerted a naloxone-reversible suppression of spontaneously discharging neurons located primarily in the substantia nigra zona reticulata, either at rest or after their activation by microinjection of dopamine into the caudate nucleus. Inactivation of these neurons within the striatonigral feedback pathway is suggested to complement a direct action on the nigrostriatal dopaminergic cells by the opiate, resulting in the morphine suppression of caudate neuronal activities.