The spinal pharmacology of acutely and chronically administered opioids.

Systematic studies on the pharmacology of the antinociceptive activity of spinally administered agents have emphasized the action in animal models of mu and delta opioid receptors. Importantly, aside from receptor selectivity, the opioid agonists differ in the property of efficacy. Agents with high efficacy (large receptor reserves) show smaller rightward shifts in their dose-response curves in the face of a given degree of opioid tolerance or when the stimulus intensity is elevated. With regard to loss of drug effect with long-term exposure (tolerance), multiple mechanisms may be considered, including changes in stimulus intensity, change in afferent processing, or changes in receptor number/coupling. Basic studies are providing insights into these several mechanisms.