Comparison of peptidic and nonpeptidic δ-opioid agonists on guanosine 5′-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding in brain slices from Sprague-Dawley rats

Previous studies have demonstrated that peptidic and nonpeptidic δ-opioid receptor agonists have different effects depending on the measure. For example, nonpeptidic δ-opioid agonists, but not peptidic agonists, produce convulsions in rats, and in vitro studies suggested that peptidic and nonpeptidic δ-opioid agonists might have differential mechanisms of receptor down-regulation. The present study evaluated potential differences between peptidic and nonpeptidic δ-opioid agonists in their ability to activate G proteins using guanosine 5′-O-(3-[ ³⁵S]thio)triphosphate ([³⁵S]GTPγS) autoradiography experiments in rat brain slices. The peptidic agonist [D-Pen²,D- Pen⁵]-enkephalin and the nonpeptidic agonist (+)BW373U86 [(+)-4-[α(R)-α-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl] -(3-hydroxyphenyl)methyl]-N,N-diethylbenzamide] demonstrated concentration-dependent increases in [³⁵S]GTPγS binding that were attenuated by the δ-opioid antagonist naltrindole. (+)BW373U86 was more potent and efficacious than the peptidic agonist, and this difference remained consistent across brain regions where significant stimulation was observed. In addition, multiple δ-opioid compounds were evaluated for their agonist activity in this assay. These data suggested that differences between peptidic and nonpeptidic δ-opioid agonists in behavioral studies were most likely caused by differences in agonist efficacy. Finally, these data also revealed that [³⁵S]GTPγS autoradiography could be used to compare efficacy differences among agonists across various brain regions in rat brain slices.