Antagonism by physostigmine of the "running fit" caused by levorphanol, a morphine congener, in mice

Effects of diabetes on spontaneous locomotor activity in mice

Spontaneous locomotor activity in diabetic mice was significantly greater than that in non-diabetic mice. Haloperidol and SCH23390, a selective dopamine D1-receptor antagonist, significantly reduced spontaneous locomotor activity in diabetic mice, but not in non-diabetic mice. Spontaneous locomotor activity in diabetic mice was also reduced by pretreatment with naltrindole, a selective delta-opioid receptor antagonist, and 7-benzylidenenaltrexone, a selective delta1-opioid receptor antagonist. The rate of dopamine turnover in the limbic forebrain in diabetic mice was significantly higher than that in non-diabetic mice. These findings suggest that the enhanced spontaneous locomotor activity in diabetic mice may result from increased dopamine neurotransmission, which might be due to an increase in dopamine release in mesolimbic dopamine systems. The increased dopamine neurotransmission in diabetic mice may also be due to the up-regulation of delta-opioid receptor-mediated functions.

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.

Inhibitory and stimulatory effects of morphine on locomotor activity in mice: biochemical and behavioral studies

A possible interaction between the opposite effect (inhibition and stimulation) of morphine on locomotor activity in mice and monoaminergic systems in the striatum was studied. Ten minutes after systemic administration, morphine at 1.25 mg/kg decreased locomotor activity and the levels of 3-methoxytyramine (3-MT), whereas at 20 mg/kg locomotor activity and 3-MT levels increased. At the same time, no change in the other monoamine metabolite (DOPAC, HVA, MHPG, and 5-HIAA) levels was observed. Sixty minutes after administration, morphine at 1.25 mg/kg did not induce any change in locomotor activity or in all the monoamine metabolite levels measured. On the other hand, morphine at 20 mg/kg maintained an initial increase in locomotor activity and increased not only 3-MT levels, but also other metabolite (DOPAC, HVA, MHPG, and 5-HIAA) levels. These results suggest that, at low dosages, the inhibitory effect of morphine on locomotor activity in mice may be related to a decrease of the presynaptic dopamine release in the striatum and that the stimulatory effect of morphine, at high dosages, may be related to an increase of the presynaptic dopamine release in the striatum.

Morphine enhances high-affinity choline uptake in mouse striatum

The effects of morphine (2 mg/kg-60 mg/kg) on cholinergic neuronal activity were examined by the method of high-affinity, Na+-dependent [3H]choline uptake into synaptosomes isolated from mouse corpus striatum. Acute administration of analgesic doses of morphine (10 mg/kg, 20 mg/kg) significantly stimulated choline uptake into synaptosomes in a naloxone-reversible manner. When synaptosomes were directly exposed to pharmacologically effective concentrations of morphine (0.1 microM-10.0 microM) in vitro however, choline transport was not significantly different from control transport, suggesting that morphine (10 mg/kg, 20 mg/kg) does not stimulate choline uptake by a direct effect on the cholinergic nerve terminal. The possibility that acute morphine administration indirectly enhances striatal cholinergic neuronal activity by inhibiting dopaminergic function was supported pharmacologically since the dopaminergic agonists, apomorphine (10 mg/kg) or amantadine (50 mg/kg), reversed the stimulatory effect of morphine on choline uptake. High-affinity choline transport into synaptosomes was not significantly different from control uptake in response to a sub-analgesic dose of morphine (2 mg/kg) or in response to 60 mg/kg, a dose that elicited hypermotility. These data suggest that analgesic doses of morphine may indirectly enhance cholinergic neuronal activity in the mouse corpus striatum.

Biphasic effects of a potent enkephalin analogue (D-Met2,Pro5)-enkephalinamide and morphine on locomotor activity in mice

The effects of morphine and a potent enkephalin analogue on spontaneous locomotion have been compared in mice. In doses of 3-10 mg/kg SC both compounds induced a brief reduction of motor activity followed by a period of behavioral hyperactivity. Similar receptorial mechanisms are suggested in mediation of their motor effects.

Effect of humoral modulators of morphine-induced increase in locomotor activity of mice

The effect of humoral modulators on the morphine-induced increase in locomotor activity of mice was studied. The subcutaneous administration of 10 mg/kg of morphine-HC1 produced a marked increase in locomotor activity in mice. The morphine-induced hyperactivity was potentiated by scopolamine and attenuated by physostigmine. In contrast, both methscopolamine and neostigmine, which do not penetrate the blood-brain barrier, had no effect on the hyperactivity produced by morphine. Pretreatment of mice with alpha-methyltyrosine (20 mg/kg i.p., one hour), an inhibitor of tyrosine hydroxylase, significantly decreased the activity-increasing effects of morphine. On the other hand, pretreatment with p-chlorophenylalamine (3 X 320 mg/kg i.p., 24 hr), a serotonin depletor, caused no significant change in the hyperactivity. The study suggests that the activity-increasing effects of morphine are mediated by the release of catecholamines from adrenergic neurons in the brain. And the results are consistent with the hypothesis that morphine acts by retarding the release of acetylcholine at some central cholinergic synapses. It is also suggested from collected evidence that the activity-increasing effects of morphine in mice are mediated by mechanisms different from those which mediate the activity-increasing effects of morphine in rats.

Disturbed patterns of behaviour in morphine tolerant and abstinent rats

1. Eating, drinking and spontaneous motor activity were studied in rats receiving large daily doses of morphine. These forms of behaviour were largely suppressed when the rats were made abstinent and were restored when morphine was given again.2. Compensation for depressions of behaviour during abstinence did not seem sufficient to account for all the stimulant effects of morphine in tolerant rats. Morphine also had slight stimulant actions in non-tolerant rats.3. In tolerant rats, the repeated pairing of the effects of morphine with the re-emergence of behaviour such as eating and drinking may intensify the rewarding value of the drug.