We recently reported that cholecystokinin (CCK) excited nucleus accumbens (NAc) cells and depressed excitatory synaptic transmission indirectly through gamma-aminobutyric acid (GABA), acting on presynaptic GABAB receptors (Kombian et al. [2004] J. Physiol. 555:71-84). The present study tested the hypothesis that CCK modulates inhibitory synaptic transmission in the NAc. Using in vitro forebrain slices containing the NAc and whole-cell patch recording, we examined the effects of CCK on evoked inhibitory postsynaptic currents (IPSCs) recorded at a holding potential of -80 mV throughout CCK-8S caused a reversible inward current accompanied by a concentration-dependent decrease in evoked IPSC amplitude. Maximum IPSC depression was approximately 25% at 10 microM, with an estimated EC50 of 0.1 microM. At 1 microM, CCK-8S induced an inward current of 28.3 +/- 4.8 pA (n=6) accompanied by an IPSC depression of -18.8% +/- 1.6% (n=6). This CCK-induced IPSC depression was blocked by pretreatment with proglumide (100 microM; -3.7% +/- 6.9%; n=4) and by LY225910 (100 nM), a selective CCKB receptor antagonist (4.4% +/- 2.6%; n=4). It was not blocked by SCH23390 (10 microM; -23.5% +/- 1.3%; P < 0.05; n=7) or sulpiride (10 microM; -21.8% +/- 5.1%; P <0.05; n=4), dopamine receptor antagonists. By contrast, it was blocked by CGP55845 (1 microM; -0.4% +/- 3.4%; n=5) a potent GABAB receptor antagonist, and by forskolin (50 microM; 9.9% +/- 5.2%; n=4), an adenylyl cyclase activator, and H-89 (1 microM; 6.9% +/- 3.9%; n=4), a protein kinase A (PKA) inhibitor. These results indicate that CCK acts on CCKB receptors to increase extracellular levels of GABA, which then acts on GABAB receptors to decrease IPSC amplitude.