Interaction between highly selective mu and delta opioids in vivo at the rat spinal cord

Possible interactions between mu- and delta-receptors in the rat spinal cord were studied using the radiant-heat-induced tail flick response and the highly selective mu- or delta-ligands: [NMePhe3,D-Pro4] morphiceptin(PL-17) and cyclic[D-Pen2,D-Pen5]enkephalin(DPDPE). Intrathecal infusions of PL-17 (0.5 microgram/hr) for 5 days caused 61- and 6-fold shifts to the right of the dose-response curves for PL-17 and DPDPE, respectively. Since PL-17 and DPDPE are highly selective agonists for mu- and delta-receptors, the partial cross-tolerance to the delta-agonist induced by PL-17 is probably not resultant from a cross-reactivity between these ligands. These data suggest that there may be an interaction between mu- and delta-receptors in the rat spinal cord.