Ventral tegmental area opioid mechanisms and modulation of ingestive behavior

In this paper we report on the effects of intra-VTA infusion of opioid agonists on rat ingestive behavior in a variety of experimental contexts. When the animals were tested outside of their home cages surrounded only by food-pellets (Experiment 1), the injection of the mu-opioid agonist DAMGO, but not the kappa-opioid agonist U-50,488H, into the ventral tegmental area facilitated food-related behaviors, decreasing the latency to feed and increasing the number of interactions with food. When, as in Experiment 2, gnawable objects and a drinking tube were also available, intra-VTA DAMGO gnawing and drinking behaviors, whereas the effects on feeding were negligible. These effects intra-VTA DAMGO increased were greatly enhanced in rats that underwent repeated treatments with amphetamine. On the other hand, when food-related behaviors were studied in a home-cage, where access to the food supply was achieved by entry into a tunnel, latency to feed and total food-intake were not enhanced in tests made during either the dark or the light phase (Experiment 3 and 4). This was true whether powdered standard lab chow or a highly palatable food was available. It appears that when a number of alternative incentive stimuli are available, increases in dopamine transmission such as that induced by intra-VTA DAMGO may ultimately have the effect of interfering with behavior normally directed primarily to one of these stimuli, by enhancing the salience of others. These effects bears some resemblance to the effects of tail-pinch and electrical brain stimulation of the medial forebrain bundle-lateral hypothalamic area on the responses to natural incentive stimuli.