Mesolimbic dopamine (DA) neurons in the ventral tegmental area (VTA) that project to the nucleus accumbens (NAc) are part of a well-defined pathway involved in reward processing (Nestler, 2005). GABAergic interneurons in the VTA maintain a tonic inhibition over dopaminergic neurons. Binding of β-endorphin or morphine to MOPRs on these interneurons will decrease their activity, resulting in disinhibition of the DA neurons and elevations of DA in the NAc (Johnson and North, 1992). This dopamine influx has been associated with reward and reinforcement and is believed to contribute to the development of drug dependence (Di Chiara and Imperato, 1988; Wise and Bozarth, 1985).
