Locomotor activity of rats after injection of various opiods into the nucleus accumbens and the septum mediale

Abstract

The possible role of the nucleus accumbens (ACB) and, in some experiments, the septum mediale (SM) in mediating alterations in locomotor activity, produced by various opioids, was evaluated in the rat, the drug being injected either into the left central part of the ACB or into the left SM. Morphine, predominantly acting at the mu-type receptors, given in larger doses (13 or 40 nmol into the ACB) produced depression of locomotor activity and catalepsy, whereas 2.5 nmol were ineffective. Co-administration of naloxone into the ACB suppressed the effects of morphine.d-ala²,d-leu⁵-enkephalin (DADL), a preferential delta-type receptor agonist, produced a biphasic effect on locomotor activity, namely an inhibition of it and a catalepsy, followed by a locomotor activation. This effect was observed after 4 or 13 nmol; 1 nmol produced a delayed locomotor activation without any previous inhibition, whereas 0.4 or 0.1 nmol were ineffective. Equimolar doses of naloxone, when coadministered with DADL, only partially antagonized these effects of DADL. In contrast, co-administration of a small dose of DADL and an excess dose of naloxone into the ACB produced an immediate increase in locomotor activity. Injections of a predominant kappa type receptor agonist, MR 2033-Cl, were ineffective. Injection of DADL, either alone or combined with naloxone into the SM (1 nmol) produced an immediate stimulation of locomotor activity. The results suggest that locomotor depression and catalepsy may be mediated by opioid receptors of the mutype in the ACB, whereas locomotor stimulation is due to an action on delta type receptors, an effect which may be masked by simultaneous stimulation of mu-type receptors under certain conditions. Kappa-type receptors apparently are not involved in any of these effects. In addition, a diffusion of DADL to the adjacent SM may contribute to locomotor stimulation.