Modification of morphine-induced locomotor activity by pertussis toxin: biochemical and behavioral studies in mice

The effect of pertussis toxin (PTX) on the locomotor-enhancing action of systemic and intracerebroventricular (i.c.v.) morphine was investigated in mice. Mice were i.c.v. injected with either PTX (0.25 and 0.5 micrograms) or saline as a control. The s.c. (5-20 mg/kg) and i.c.v. (7-30 nmol) administration of morphine produced a dose-related locomotor-enhancing action in control mice. The peak effect of morphine (30 nmol, i.c.v.)-induced hyperlocomotion was observed 90 min after the morphine injection. At the same time, morphine significantly increased dopamine (DA) metabolism in the limbic forebrain (nucleus accumbens and olfactory tubercle). Similarly, the selective mu-opioid receptor agonist [D-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAGO, 4 nmol, i.c.v.) also significantly increased locomotor activity and DA metabolism in the limbic forebrain. Both morphine- and DAGO-induced hyperlocomotion and elevation of DA turnover were antagonized by pretreatment with the mu antagonist beta-funaltrexamine (beta-FNA). These results suggest that the locomotor-enhancing action of morphine results from the activation of central mu-opioid receptors, and that the activation of the mesolimbic DA system may be involved in the expression of morphine-induced hyperlocomotion in mice. Furthermore, pretreatment with PTX (0.5 micrograms, i.c.v., 6 days prior to the testing) significantly reduced hyperlocomotion and elevation of DA turnover in the limbic forebrain which had been induced by administrations of morphine (30 nmol, i.c.v.) and DAGO (4 nmol, i.c.v.). These findings suggest that the central PTX-sensitive GTP-binding protein (G-protein) mechanism may play an important role in opioids-induced locomotor-enhancing action. Furthermore, the activation of mesolimbic DA transmission by mu-opioid agonists may also be mediated by a PTX-sensitive G-protein mechanism in mice.