In the present study we characterized the cholecystokinin receptor regulation of (i) the dopamine D2 agonist binding sites in striatal sections including the nucleus accumbens and (ii) GABA and dopamine release in the central part of the rat nucleus accumbens, by combining the in vitro filter wipe-off and the in vivo microdialysis techniques. In the binding study we demonstrate that sulphated cholecystokinin octapeptide (1 nM) increased (219 +/- 30%) the KD value of the D2 agonist [3H]N-propylnorapomorphine binding sites in sections from the striatum including the accumbens. This effect was counteracted by the cholecystokinin-B antagonist PD134308 (50 nM). In a parallel study using microdialysis in the central nucleus accumbens, we found that local perfusion with sulphated cholecystokinin octapeptide (1 microM) induced an increase in GABA (135 +/- 7%) and dopamine (146 +/- 8%) release which was unaffected by the cholecystokinin-A antagonist L-364,718 (10 nM). In contrast, when the cholecystokinin-B antagonist PD134308 (10 nM) was co-perfused with the peptide it prevented the increase in dopamine and decreased GABA release (-24 +/- 2%). This reduction was counteracted by the addition to the perfusate medium of the cholecystokinin-A antagonist or the cholinergic muscarinic M2 receptor antagonist AF-DX 116 (0.1 microM). Taken together, these data demonstrate that the facilitation by sulphated cholecystokinin octapeptide of GABA and dopamine release in the central accumbens probably reflects an inhibitory effect of the peptide on both pre- and postsynaptic D2 receptors, mediated via cholecystokinin-B receptor activation. In addition, for the first time we provide evidence for a differential cholecystokinin-A and -B receptor-mediated regulation of GABA transmission in the central accumbens, where the cholecystokinin-B receptor exerts a dominant excitatory influence while the cholecystokinin-A receptor mediates an inhibition of GABA release via a local muscarinic M2 receptor.