Preferential Gs protein coupling of the galanin Gal1 receptor in the μ-opioid-Gal1 receptor heterotetramer

Recent studies have proposed that heteromers of μ-opioid receptors (MORs) and galanin Gal1 receptors (Gal1Rs) localized in the mesencephalon mediate the dopaminergic effects of opioids. The present study reports converging evidence, using a peptide-interfering approach combined with biophysical and biochemical techniques, including total internal reflection fluorescence microscopy, for a predominant homodimeric structure of MOR and Gal1R when expressed individually, and for their preference to form functional heterotetramers when co-expressed. Results show that a heteromerization-dependent change in the Gal1R homodimeric interface leads to a switch in G-protein coupling from inhibitory Gi to stimulatory Gs proteins. The MOR-Gal1R heterotetramer, which is thus bound to Gs via the Gal1R homodimer and Gi via the MOR homodimer, provides the framework for a canonical Gs-Gi antagonist interaction at the adenylyl cyclase level. These novel results shed light on the intense debate about the oligomeric quaternary structure of G protein-coupled receptors, their predilection for heteromer formation, and the resulting functional significance

Cocaine and several σ receptor ligands inhibit dopamine uptake in rat caudate-putamen

Cocaine and several σ receptor ligands inhibit dopamine uptake via a common site. This is evidenced by a concentration-dependent inhibition of dopamine uptake and displacement of the binding of [ 3H]WIN 35,428 (also called CFT), a cocaine analog with high affinity for the dopamine transporter. Since several σ receptor ligands have been shown to block the stimulant effects of cocaine, this site may serve as a target for future drug development to treat cocaine abuse

The cocaine-like behavioral effects of meperidine are mediated by activity at the dopamine transporter

Meperidine has atypical opioid receptor agonist effects and shares some structural features with the phenyltropane (WIN) analogs of cocaine. In combination with 0.1 mg/kg naltrexone, meperidine produced cocaine-like discriminative stimulus effects in monkeys, whereas morphine was inactive. Both cocaine and meperidine inhibited [ 3H]dopamine uptake in chopped rat caudate putamen with comparable potencies; meperidine differed from cocaine in that its effects could be characterized as having predominantly a single high-affinity component. Morphine was not active in inhibiting [ 3H]dopamine uptake, indicating that the effect of meperidine was not via a classic μ-opioid receptor agonist action. Further, meperidine but not morphine displaced [ 3H]WIN 35,428 (2β-carbomethoxy-3β-(4-fluorophenyl)tropane) binding. These data suggest that the actions of meperidine that are atypical of opioids are due to activity at the dopamine transporter. In addition, meperidine appears to interact predominantly with the high-affinity component of the dopamine transporter, and this high-affinity component may be the site of importance for the production of cocaine's behavioral effects