Long-term changes of striatal D-2 receptors in rats chronically exposed to morphine under aversive life conditions

Chronic morphine treatment has been shown to cause the development of hyperreactivity of the dopamine system detected as the increased behavioral and biochemical responses to the action of specific dopamine agonists. Furthermore, inverted changes in animal behavioral reactivity to the stimulation of presynaptic, proposed D-2 receptors by apomorphine in a low dose was found in our previous study when morphine was chronically used in animals under conditions of restraint. To estimate the nature and proposed receptor mechanisms of changes found in behavioral reactivity due to chronic morphine administration in aversive life conditions at the level of highly sensitive D-2 receptors, the density and affinity of [3H] spiroperidol binding sites was studied in these animals two weeks after the last opiate administration. Increased density and affinity of D-2 receptors probably indicating their hypersensitivity was found in animals chronically exposed to two-hour restraint stress, while a significant decrease in density accompanied by increase in affinity of these receptors was typical to rats chronically exposed to morphine under conditions of restraint. The data are discussed in aspects of quantitative and qualitative changes in D-2 receptors, and their proposed mechanisms and functional significance in the mediation of modified organism's functional state due to chronic opiate administration in different environmental conditions.