Effects of GBR 12909 on locomotor activity and dopamine turnover in mice: comparison with apomorphine

The effects of GBR 12909 1-[2-[bis(4-fluorophenyl)methoxy]-ethyl]-4- [3-phenylpropyl]piperazine, a very potent and selective dopamine uptake inhibitor, and apomorphine, a dopamine receptor agonist, alone and in combination were investigated on locomotor activity and dopamine turnover in discrete brain regions of mice. The levels of dopamine and its metabolites were examined 40 min after the administration of GBR 12909 and/or apomorphine, when the effects of the drugs on locomotor activity were approximately at a peak. GBR 12909 (10 mg/kg i.p.) reversed a low dose of apomorphine (0.05 mg/kg s.c.)-induced suppression in locomotor activity and significantly increased this activity. Despite the dramatic change in the behavior, GBR 12909 did not influence the decrease in 3,4-dihydroxyphenylacetic acid (DOPAC)/dopamine ratio (which is one of the indications of transmitter turnover) induced by a low dose of apomorphine in the nucleus accumbens and striatum. In contrast, GBR 12909 did not enhance the high-dose apomorphine (2 mg/kg s.c.)-induced hyperlocomotion, and did not modify the larger decrease in dopamine turnover produced by the high dose of apomorphine in the frontal cortex, nucleus accumbens and striatum. This suggests that postsynaptic dopamine receptors may reach maximum stimulation at a high dose of apomorphine. These results indicate that a behavioral change induced via stimulation of postsynaptic dopamine receptors does not necessarily lead to an alteration in dopamine turnover.