Shifting motivational states: The effects of nucleus accumbens dopamine and opioid receptor activation on a modified effort-based choice task

The nucleus accumbens (NAc) is critical for regulating the appetitive and consummatory phases of motivated behavior. These experiments examined the effects of dopamine and opioid receptor manipulations within the NAc during an effort-based choice task that allowed for simultaneous assessment of both phases of motivation. Male Sprague-Dawley rats received bilateral guide cannulas targeting the NAc core and were tested in 1-hr sessions with free access to rat chow and the choice to work for sugar pellets on a progressive ratio 2 (PR2) reinforcement schedule. Individual groups of rats were tested following stimulation or blockade of NAc D1-like or D2-like receptors, stimulation of mu-, 8-, or kappa-opioid receptors, or antagonism of opioid receptors. Behavior was examined under ad libitum conditions and following 23-h food restriction. NAc blockade of the D1-like receptors or stimulation of the D2 receptor reduced break point for earning sugar pellets; D2 receptor stimulation also modestly lowered chow intake. NAc mu-opioid receptor stimulation increased intake of the freely-available chow while simultaneously reducing break point for the sugar pellets. In non-restricted conditions, 8-opioid receptor stimulation increased both food intake and breakpoint. There were no effects of stimulating NAc D1 or kappa receptors, nor did blocking D2 or opioid receptors affect task behavior. These data support prior literature linking dopamine to appetitive motivational processes, and suggest that mu and 8-opioid receptors affect food-directed motivation differentially. Specifically, mu-opioid receptors shifted behavior towards consumption, and 8-opioid receptor enhanced both sugar-seeking and consumption of the pabulum chow when animals were not food restricted.

Automated summary

The nucleus accumbens plays a critical role in regulating motivated behavior, with dopamine and opioid receptors within it having varying effects on appetitive and consummatory phases of motivation. Stimulation or blockade of different receptor types showed distinct impacts on food intake and sugar-seeking behavior, suggesting differential roles of mu- and 8-opioid receptors in food-directed motivation.