Is morphine-induced akinesia related to inhibition of reflex activation of flexor alpha-motoneurones? Role of the nucleus accumbens.

Unilateral injections of morphine (5-15 micrograms) into the nucleus accumbens of non-anesthetized rats produces a decrease of locomotor activity and a catalepsy. In a similar dose-response relationship, injections of morphine into this area inhibited the reflex activation of alpha-motoneurones by mild tetanic stimulation of the ipsilateral peroneal nerve (flexor alpha-motoneurones) in halothane-anesthetized rats. All these effects were antagonized by systemic administration of naloxone. Our results suggest that a reduction in reflex activation of flexor alpha-motoneurones by proprioceptive stimuli seems to be relevant for the development and/or mediation of akinesia and catalepsy, and that the nucleus accumbens plays a key role in these effects produced by morphine.