Quinpirole elicits differential in vivo changes in the pre- and postsynaptic distributions of dopamine D(2) receptors in mouse striatum: relation to cannabinoid-1 (CB(1)) receptor targeting

RATIONALE: The nucleus accumbens (Acb) shell and caudate-putamen nucleus (CPu) are respectively implicated in the motivational and motor effects of dopamine, which are mediated in part through dopamine D(2)-like receptors (D(2)Rs) and modulated by activation of the cannabinoid-1 receptor (CB(1)R). The dopamine D(2/D3) receptor agonist, quinpirole elicits internalization of D(2)Rs in isolated cells; however, dendritic and axonal targeting of D(2)Rs may be highly influenced by circuit-dependent changes in vivo and potentially influenced by endogenous CB(1)R activation. OBJECTIVE: We sought to determine whether quinpirole alters the surface/cytoplasmic partitioning of D(2)Rs in striatal neurons in vivo. METHODS: To address this question, we examined the electron microscopic immunolabeling of D(2) and CB(1) receptors in the Acb shell and CPu of male mice at 1 h following a single subcutaneous injection of quinpirole (0.5 mg/kg) or saline, a time point when quinpirole reduced locomotor activity. RESULTS: Many neuronal profiles throughout the striatum of both treatment groups expressed the D(2)R and/or CB(1)R. As compared with saline, quinpirole-injected mice showed a significant region-specific decrease in the plasmalemmal and increase in the cytoplasmic density of D(2)R-immunogold particles in postsynaptic dendrites without CB(1)R-immunolabeling in the Acb shell. However, quinpirole produced a significant increase in the plasmalemmal density of D(2)R immunogold in CB(1)R negative axons in both the Acb shell and CPu. CONCLUSIONS: Our results provide in vivo evidence for agonist-induced D(2)R trafficking that is inversely related to CB(1)R distribution in postsynaptic neurons of Acb shell and in presynaptic axons in this region and in the CPu.