Morphine-induced analgesia in rats withdrawn from concurrent nimodipine and morphine treatment.

The effect of repeated administration of dihydropyridine calcium channel antagonist, nimodipine, given concurrently with morphine on the development of tolerance to the antinociceptive actions of morphine in rats was studied. In acute experiments nimodipine (1 mg/kg i.p.) enhanced the antinociceptive effect of morphine (2.5 mg/kg s.c.) in the hot-plate and tail immersion tests. Daily administration of morphine either for 10 days (increasing the daily dose from 20 to 35 mg/kg) or for 24 days (increasing the daily dose from 20 to 70 mg/kg) induced tolerance to the antinociceptive effect of a challenge dose of morphine (10 mg/kg) administered 24 h after the withdrawal from chronic morphine. Concurrent administration of nimodipine (1 mg/kg per day) with morphine for 10 or 24 days augmented the reduction of the antinociceptive effect of morphine. Neither acute nor repeated administration of nimodipine with morphine altered the concentrations of morphine or its metabolite morphine 6-glucuronide in the brain tissue or in the plasma. The observed further reduction in the nociceptive response in morphine tolerant animals pre-treated with nimodipine is, most probably, due to the adaptive changes in the central dihydropyridine calcium channels induced by the withdrawal from repeated nimodipine treatment.

Conditioned place preference induced by a combination of L-dopa and a COMT inhibitor, entacapone, in rats.

The interaction of dopamine (DA) precursor L-dopa and catechol-O-methyltransferase (COMT) inhibitor, entacapone, was examined in rats using conditioned place preference (CPP) paradigm to assess reinforcement, and by measuring DA metabolism in the striatum and the limbic forebrain. Neither L-dopa (100 mg/kg i.p.) nor entacapone (30 mg/kg i.p.) alone induced CPP, but in combination they induced significant CPP. Entacapone alone had no effect on limbic or striatal DA concentrations, while it reduced the concentrations of the COMT products 3-methoxytyramine (3-MT), a metabolite reflecting DA release, and homovanillic acid (HVA) in both brain areas. L-dopa elevated limbic but not striatal 3-MT. L-dopa also slightly elevated limbic DA but had no effect on striatal DA concentration. L-Dopa-induced increase of 3-MT was attenuated by entacapone. Our results show for the first time that L-dopa is able to produce CPP in intact animals. This effect may be related to the findings that L-dopa increases synaptic DA concentrations in the limbic areas, and entacapone may enhance this elevation as it prevents the synaptic metabolism of DA.

Involvement of opioid mu 1 receptors in morphine-induced conditioned place preference in rats.

The main purpose of this study was to evaluate the role of mu 1-opioid receptors in morphine reward. Therefore, we studied the ability of a mu 1-selective antagonist, naloxonazine [15 mg/kg intraperitoneally (IP)], to antagonize the conditioned place preference (CPP) induced by morphine [3 mg/kg subcutaneously (SC)]. In addition, effects of naloxonazine on morphine-induced catalepsy (15 mg/kg), analgesia (3 mg/kg), and hyperthermia (3 mg/kg) were studied. For comparison, the effects of a nonselective opioid receptor antagonist, naltrexone (2.5 mg/kg SC), and a selective delta-opioid receptor antagonist, naltrindole (2 mg/kg IP), on CPP induced by morphine were investigated. Morphine-induced CPP was clearly antagonized by pretreatment with naloxonazine and naltrexone (12 h and 20 min prior to morphine, respectively) but not by naltrindole (15 min before morphine). Naloxonazine also antagonized morphine-induced catalepsy and analgesia but not morphine-induced hyperthermia. Naltrindole did not modify morphine-induced catalepsy. These results suggest an active role for mu 1-opioid receptors in morphine reward, whereas morphine-induced hyperthermia does not appear to be mediated by mu 1-opioid receptors. Furthermore, delta-opioid receptors seem to be without significance in morphine-induced reward.

Behavioural and neurochemical sensitization of morphine-withdrawn rats to quinpirole.

The sensitivity of dopamine D2-like receptors in morphine-withdrawn rats was studied using the selective agonist quinpirole. Morphine was administered twice daily increasing the daily dose from 20 to 50 mg/kg during 7 days. Twenty-four hours after the last morphine administration the rats were given quinpirole (0.01-1 mg/kg) and their behavior was assessed. Withdrawal from repeated morphine treatment enhanced yawning behavior and penile erections induced by small doses (0.01-0.1 mg/kg) as well as the intensity of stereotypy induced by a large dose (1.0 mg/kg) of quinpirole. In the morphine-withdrawn rats the dose of 1 mg/kg of quinpirole caused less yawning than in the control rats, whereas the number of erections induced by this dose was enhanced as compared with the control animals. In the control rats, the striatal and limbic concentrations of dopamine metabolites, 3,4-dihydroxphenylacetic acid (DOPAC), and homovanillic acid (HVA), were not clearly affected by the smallest dose of quinpirole. However, the small dose of quinpirole (0.01 mg/kg) significantly reduced the levels of DOPAC and HVA in the striatum and limbic forebrain of the rats withdrawn from morphine either for 24 or 48 h. These findings indicate that withdrawal from repeated morphine treatment enhances the sensitivity of dopamine D2-like receptors.