An in vitro receptor binding and in vivo microdialysis study was performed to further investigate the modulation of dopamine (DA) D2 receptors by neurotensin (NT) peptides. Saturation experiments with the D2 agonist [3H]NPA (N-propylnorapomorphine) showed that 10 nM of NT, 10 nM of neuromedin N (NN) and 1 nM of the C-terminal NT-(8-13) fragment significantly increased the KD values by 125%, 181%, and 194%, respectively without significantly affecting the Bmax value of the [3H]NPA binding sites in coronal sections of rat ventral forebrain mainly containing the nucleus accumbens (Acb) and the olfactory tubercle. In line with the previous findings that NT can increase GABA release in the Acb and that NT receptors are not found on DA terminals in this brain region, the present in vivo microdialysis study demonstrated that local perfusion of NT (1 nM) counteracted the D2 agonist pergolide (2 mu M) induced inhibition of GABA, but not of DA release in the rat Acb. This result indicates that NT counteracts the D2 agonist induced inhibition of GABA release in the rat Acb, via an antagonistic postsynaptic NT/D2 receptor interaction as also suggested by the inhibitory regulation of D2 receptor affinity in the Acb by the NT peptides demonstrated in the present receptor binding experiments. Thus, the neuroleptic and potential antipsychotic profile of the NT peptides may involve an antagonistic NT/D2 receptor regulation in the ventral striatum.