Cocaine-induced mu opioid receptor occupancy within the striatum is mediated by dopamine D2 receptors

Abstract

Previous studies by our laboratory have demonstrated that the mu opioid receptor antagonist, CTAP, blocks the rewarding effects of cocaine when it is injected directly into the nucleus accumbens or ventral tegmental area (VTA). This finding suggests that cocaine is causing the release of endogenous opioid peptides which activate mu opioid receptors within the nucleus accumbens and VTA. The purpose of the present study was to characterize the dose-response and time-course of mu receptor occupancy following systemic cocaine administration and to determine if release of endogenous opioids by cocaine is mediated by activation of D1 or D2 dopamine receptors. Quantitative in vitro receptor autoradiography was used to measure the regional displacement of (3)H-DAMGO binding following cocaine administration. Adult male Sprague-Dawley rats were given intraperitoneal (i.p.) injections of cocaine and their brains were removed at various times and prepared for mu opioid receptor quantitation. To determine the role of dopamine D1 and D2 receptors in the effect of cocaine on mu receptor occupancy, rats were injected with the selective D1 or D2 receptor antagonists SCH23390 or eticlopride prior to cocaine. For all studies, (3)H-DAMGO binding to mu opioid receptors was measured in the nucleus accumbens, caudate putamen, frontal cortex, olfactory tubercle and VTA. Results demonstrate that cocaine administration caused a time- and dose-dependent reduction in (3)H-DAMGO binding within the nucleus accumbens core and shell. The reduction in mu receptor binding was attenuated by pretreatment with eticlopride. These results suggest that cocaine, acting via D2 dopamine receptors, can cause the release of an endogenous opioid peptide that binds to mu opioid receptors within the nucleus accumbens.

Figure 1. Coronal Rat Brain Sections Illustrating the Regions of Quantitation of Mu Opioid Receptor Binding

Receptor densities from both the left and right hemisphere for each region of interest were obtained. The distance of each section from bregma is given in millimeters (Paxinos and Watson, 1986). NAcc=nucleus accumbens; VTA=ventral tegmental area.

Figure 2. Total and Non-Specific ³H-DAMGO Binding to Representative Brain Sections

Brains were obtained at various times following injection of 10 mg/kg cocaine: time=0 min (2A), time=5 min (2B), time=10 min (2C), time=20 min (2D), time=30 min (2E), and non-specific binding for T=0 min (2F).

Figure 3. Cocaine Time-Course Study

Animals received a single injection of saline (1 ml/kg i.p.) or cocaine (10 mg/kg i.p.) and were euthanized either 0, 5, 10, 20 or 30 minutes later. ³H-DAMGO binding was quantified in the nucleus accumbens and plotted as a % of binding in the saline-injected rat at T=0. One-way ANOVA with Bonferroni post-hoc analyses revealed a significant reduction in ³H-DAMGO binding in the nucleus accumbens (NAcc) core and shell at 10 and 20 minutes following cocaine injection (*=P<0.05, **=P<0.01 vs 0 min). Data are presented as mean (±SD). (N=4 animals/group)

Figure 4. Cocaine Dose-Response Study

Animals received a single injection of saline (1 ml/kg i.p.) or cocaine (2.5, 10 or 20 mg/kg i.p.) and were euthanized ten minutes later. ³H-DAMGO binding was quantified in the nucleus accumbens and plotted as a % of binding in saline controls. One-way ANOVA with Bonferroni post-hoc analyses revealed a significant reduction in ³H-DAMGO binding in the nucleus accumbens core and shell for animals injected with 2.5 and 10 mg/kg cocaine (**=P<0.01 vs 0 mg/kg). Data are presented as mean (±SD). (N=4 animals/group)

Figure 5. Dopamine D1 and D2 Receptor Antagonist Study

Animals were injected with saline (1 ml/kg i.p.), the D1 selective antagonist SCH23390 (0.5 mg/kg i.p.) or the D2 selective antagonist eticlopride (1 mg/kg i.p.). Thirty minutes later, animals in one of the two saline-injected groups received saline (1 ml/kg i.p.) and all other animals received cocaine (10 mg/kg i.p.). Ten minutes later, brains were obtained. ³H-DAMGO binding was quantified in the nucleus accumbens and plotted as a % of that in saline controls. One-way ANOVA with Bonferroni post-hoc analyses revealed a significant reduction in ³H-DAMGO binding in the NAcc core and shell in animals pretreated with saline prior to 10 mg/kg cocaine when compared to saline-saline animals. This reduction was attenuated in animals pretreated with eticlopride, but not SCH23390. Data are presented as mean (±SD). (N=4 animals/group) (*=P<0.05, **=P<0.01 vs saline + saline).