Selective modulation of morphine antinociception, but not development of tolerance, by delta receptor agonists

Co-administration of delta opioid agonists at doses which do not produce measurable antinociception were demonstrated to produce an increase in the antinociceptive potency of morphine in the mouse tail-flick test. In contrast, co-administration of equi-antinociceptive combinations of a delta agonist plus morphine for three days resulted in the development of less tolerance to morphine antinociceptive actions. The data indicate that while acute antinociceptive effects of opioid mu agonists are modulated by delta agonists, the development of antinociceptive tolerance is not.