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 conte xts. When the animals were tested outside of their home cages surround ed 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 v entral tegmental area facilitated food-related behaviors, decreasing t he 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, wherea s the effects on feeding were negligible. These effects intra-VTA DAMG O increased were greatly enhanced in rats that underwent repeated trea tments with amphetamine. On the other hand, when food-related behavior s were studied in a home-cage, where access to the food supply was ach ieved by entry into a tunnel, latency to feed and total food-intake we re not enhanced in tests made during either the dark or the light phas e (Experiment 3 and 4). This was true whether powdered standard lab ch ow or a highly palatable food was available. It appears that when a nu mber of alternative incentive stimuli are available, increases in dopa mine transmission such as that induced by intra-VTA DAMGO may ultimate ly have the effect of interfering with behavior normally directed prim arily to one of these stimuli, by enhancing the salience of others. Th ese effects bears some resemblance to the effects of tail-pinch and el ectrical brain stimulation of the medial forebrain bundle-lateral hypo thalamic area on the responses to natural incentive stimuli.