Fig. 8.
D2 and D4 but not D1 receptor blockade attenuates VTA-mediated inhibition of firing evoked by single-pulse stimulation of the FF. Bars represent the percentage change in firing probability in response to single pulses to the FF after burst stimulation of the VTA before drug administration (gray bar) and 10–20 min after drug administration (black bars). These values are expressed as a percentage change relative to the evoked-firing probability observed when the FF was stimulated alone (open bar; for comparative purposes). Thus, a score of 0% indicates that no spike firing is elicited by FF stimulation after VTA stimulation, whereas FF stimulation alone is always 100%. Only the D2 antagonist eticlopride, the D2/D4 antagonist haloperidol, and the D4 antagonist CP 293,019 were effective in attenuating the VTA-induced inhibition, whereas the D1 antagonist SCH23390 and the NMDA antagonist CPP were without effect. Significance at *p < 0.05, **p < 0.01 versus pre-drug condition.