Opioid receptor modulation of feeding-evoked dopamine release in the rat nucleus accumbens.

Feeding is associated with increases in the activity of the mesolimbic dopamine (DA) system which originates in the ventral tegmental area (VTA) and projects heavily to the nucleus accumbens. The present study used in vivo brain microdialysis to assess the contribution of opioid receptors in feeding-evoked DA release in the nucleus accumbens. Feeding in 18 h food-deprived rats increased DA release by about 50% above baseline. Systemic injection of the opioid receptor antagonist naltrexone (1 mg/kg, s.c.) blocked the effect of feeding on DA release and reduced the amount of food consumed. Unilateral application of naltrexone (100 microM) in the VTA via a microdialysis probe failed to affect the DA response to feeding, the amount of food consumed, or the latency to eat. In contrast, intra-VTA naltrexone significantly reduced the effect of systemic heroin (0.5 mg/kg, s.c.) on accumbal DA release. These results indicate that: (1) opioid receptor activation is a component of the neural substrates of deprivation-induced feeding: (2) opioid receptors in the VTA do not contribute significantly to feeding-associated increases in DA release in the nucleus accumbens; and (3) heroin-induced increases in accumbal DA release are mediated, at least in part, by opioid receptors in the VTA.