Selective modulation of morphine anfinocicepfion, but not development of tolerance, by 8 receptor agonists

Co-administration of 8 opioid agonists at doses which do not produce measurable antinociception were demonstrafed 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 8 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 It agonists are modulated by 8 agonists, the development of antinociceptive tolerance is not.