Evidence that opioid receptors in the substantia nigra pars reticulata are relevant in regulating the function of striatal efferent pathways.

A tonic activity in the electromyogram (EMG) was induced in conscious rats by injections of morphine either systemically or into the caudate nucleus. This activity was antagonized by injections of naloxone into the substantia nigra pars reticulata. These findings suggest that opioid receptors in this brain area are relevant in regulating the function of striatal efferent pathways.