Unilateral microinjection of neurotensin in the ventral tegmental area of the rat (2.5 micrograms/0.5 microliter) produced behavioural excitation illustrated by contralateral circling. Given orally, SR 48692, a selective and potent non-peptide neurotensin receptor antagonist, significantly reduced these rotations with a triphasic dose-effect relationship. Inhibition occurred at 0.12 mg/kg; further increases in dose up to 2.5 mg/kg produced no significant antagonism, then at doses > or = 5 mg/kg, a second phase of antagonism was observed. Bilateral injection of neurotensin (0.5 microgram each side) into the nucleus accumbens antagonized the increase in locomotor activity following intraperitoneal injection of amphetamine. Given orally, SR 48692 reduced dose-dependently (0.1-1 mg/kg) these intra-accumbens neurotensin effects. Using high pressure liquid chromatography with electrochemical detection, we showed that microgram amounts of neurotensin injected into the ventral tegmental area increased dihydroxyphenylacetate/dopamine ratios in the nucleus accumbens. Using in vivo voltammetry techniques, we found that the injection of nanogram and picogram amounts of neurotensin in the ventral tegmental area stimulated dopamine efflux in the nucleus accumbens. None of these biochemical changes were affected by SR 48692 (0.1-10 mg/kg). These results indicate complex interactions between neurotensin and the mesolimbic dopamine system. More particularly, the differential ability of SR 48692 to affect neurotensin-evoked behavioural versus biochemical changes supports the concept of neurotensin receptor heterogeneity.