Mesoaccumbens dopamine-opiate interactions in the control over behaviour by a conditioned reinforcer

These experiments examined the role of dopamine-opiate interactions in the ventral tegmental area (VTA) and nucleus accumbens in the mediation of reinforcement-related behaviour. It has been shown previously that opiates induce a dopamine-dependent increase in locomotor activity in rats when infused into the VTA, and a dopamine-independent hyperactivity when infused into the nucleus accumbens. The present study investigated the generality and significance of these two findings, by examining dopamine-opiate interactions in the control over behaviour exerted by a conditioned reinforcer (CR), an arbitrary stimulus which gains control by association with primary reinforcement. Rats were trained to associate a light/noise stimulus with sucrose reinforcement, and the efficacy of the CR in controlling behaviour was assessed by measuring its ability to support a new lever pressing response. Responding on one lever (CR lever) produced the CR, responding on the other lever had no programmed consequences. In experiment 1, intra-accumbens infusions of d-amphetamine (10 micrograms), the D1 dopamine receptor agonist SKF-38393 (0.1 microgram), the D2 dopamine receptor agonist LY-171555 (quinpirole; 0.1 microgram) or the opiate receptor agonist [D-Ala2]-methionine enkephalinamide (DALA; 1 microgram) selectively increased responding on the CR lever. Infusion with DALA intra-VTA had no effect. However, pretreatment with DALA intra-VTA (10 x 1 microgram/day) subsequently reduced the selectivity of the response to infusions intra-accumbens with d-amphetamine or SKF-38393, and blocked the response to LY-171555 or DALA. Pretreatment also shifted to the right the dose-response function for DALA intra-accumbens. In experiment 2, intra-accumbens infusions of d-amphetamine, SKF-38393, LY-171555 or DALA again increased responding on the CR lever only. Pretreatment with intra-accumbens d-amphetamine (5 x 1 microgram/day) reduced the selectivity of the response subsequently to d-amphetamine, and blocked the response to SKF-38393, LY-171555 or DALA. In experiment 3, intra-accumbens infusions of the mu-opiate receptor agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (0.003-0.1 microgram), or the delta-opiate receptor agonist [D-Pen2,5]-enkephalin (0.03-1 microgram) enhanced selectively responding on the CR lever. Thus, the dopamine-dependent locomotor-stimulant properties of intra-VTA infusions of opiates are associated with impaired conditioned reinforcer efficacy. Finally, repeated stimulation of the mesoaccumbens dopamine pathway may compromise the dopamine-independence of the opiate system within the nucleus accumbens.