Evidence for the behavioral supersensitivity of dopamine D2 receptors without receptor up-regulation in morphine-abstinent rats

The effect of morphine tolerance-dependence and abstinence on the characteristics of dopamine D₂ receptors in brain regions and spinal cord was determined in the rat. Male Sprague-Dawley rats were implanted s.c. under light ether anesthesia with 6 morphine pellets for a 7-day period, each containing 75 mg of morphine free base. Rats implanted with placebo served as controls. This procedure resulted in the development of tolerance to morphine as evidenced by decreased analgesic response to a challenge dose of morphine. Similarly, the development of physical dependence was evidenced by a decreased in body weight and colonic temperature after morphine pellet removal (withdrawal). The binding characteristics (B_max andK_d values) of [³H]spiroperidol to dopamine D₂ receptors were determined in the tissues of morphine-tolerant and morphine-abstinent rats. In the tolerant rats, the pellets were left intact at the time of sacrificing, whereas, in the abstinent rats the pellets were removed 18 h prior to sacrificing. The binding of [³H]spiroperidol was determined in membranes prepared from brain regions (hypothalamus, hippocampus, cortex, pons and medulla, midbrain, corpus striatum and amygdala) and spinal cord of rats from various treatment groups. [³H]Spiroperidol bound to brain regions and spinal cord at a single high affinity site. TheB_max or theK_d values in brain regions and spinal cord of morphine-tolerant and -abstinent rats did not differ from their respective placebo controls. The behavioral responses to a selective dopamine D₂ receptor agonist, 2-bromo-α-ergocryptine were also determined in the morphine-abstinent rats. In morphine-abstinent rats, increased behavioral activity, such as total distance travelled, number of movements, and the number of stereotypic movements was seen as compared to placebo controls. The dose of 2-bromo-α-ergocryptine which by itself had no effect on any type of behavioral activity in placebo-treated rats, increased the total distance travelled, horizontal activity, number of movements, and movement time in morphine-abstinent rats. Although in morphine-tolerant or morphine-abstinent rats, the characteristics of [³H]spiroperidol binding to dopamine D₂ receptors in brain regions and spinal cord were unchanged, the supersensitivity was observed to behavioral responses of 2-bromo-α-ergocryptine, a selective dopamine D₂ receptor agonist. These results provide an evidence for behavioral responses of 2-bromo-α-ergocryptine, a selective up-regulation in morphine abstinent rats. Previously, we have show that dopamine D₁ receptors are unaffected in morphine tolerant rats but are modified in morphine-abstinent rats. Thus, in the morphine abstinent process a significant difference was noted in the biochemical characteristics of dopamine D₁ and D₂ receptors.