Activation of dopamine D sub(4) receptors within the anterior cingulate cortex enhances the erroneous expectation of reward on a rat slot machine task

Using a rodent slot machine task (rSMT), we have previously shown that rats, like humans, are susceptible to the reinforcing effects of winning signals presented within a compound stimulus array, even when the pattern generated predicts a negative rather than a positive outcome such as during a "near-miss". The dopamine D sub(4) receptor critically mediates the erroneous reward expectancy generated on such trials. D sub(4) receptors are particularly enriched within frontal and limbic areas activated during slot machine play, such as the anterior cingulate cortex (ACC). We therefore selectively inactivated the ACC to confirm involvement of this region in rSMT performance, and subsequently examined the specific contribution of local D sub(4) receptors. ACC inactivations generally impaired animals' ability to optimally differentiate winning from losing outcomes. Local administration of the D sub(4) agonist PD168077 had a qualitatively similar effect, but increased reward expectancy was only evident on archetypal "near-miss" trials i.e. when the first two of three stimuli in the array were concordant with a rewarding outcome, and only the last stimulus critically signalled a non-win. These data indicate that the ACC is critically involved in parsing the appropriate response when competing stimulus-outcome associations are activated, and that signalling via D sub(4) receptors may play a particularly important role in gating the temporal and spatial summation of salient events. Such findings provide novel insights into the mechanism underlying the erroneous expectations of reward generated when playing slot machines, and suggest a mechanism by which D sub(4) receptor antagonists may be effective in treating gambling disorder.