Roles of mu, delta and kappa opioid receptors in spinal and supraspinal mediation of gastrointestinal transit effects and hot-plate analgesia in the mouse.

The opioid receptors involved in the mediation of thermal analgesia (55 degrees C hot-plate) and inhibition of gastrointestinal transit at the spinal and supraspinal levels were studied in unanesthetized mice. Five receptor-selective compounds were evaluated for effectiveness in eliciting analgesia and inhibiting transit after both i.c.v. and intrathecal administration; these included the proposed mu agonist, [D-Ala2, N-methyl-Phe4, Gly5-ol]enkephalin (DAGO), the proposed delta agonists, [D-Pen2, L-Pen5]enkephalin (DPLPE), [D-Pen2, D-Pen5]enkephalin (DPDPE) (conformationally constrained delta selective enkephalin analogs) and [D-Thr2, Thr6, Leu5]enkephalin (DTTLE), and the proposed kappa agonist, trans-3,4-dichloro-N-methyl-N-[2-(1-pyrolidinyl)-cyclohexyl]- benzeneacetamide methanesulfonate (U-50,488H), as well as the nonselective mu-acting agonist, morphine. All compounds were found to produce analgesia after i.c.v. administration; the rank order of potency by the i.c.v. route was DAGO greater than DTTLE greater than morphine greater than DPLPE greater than DPDPE greater than U-50,488H. The analgesic effectiveness of most of these agonists given i.c.v. was evident for up to 40 min, with only DTTLE and U-50,488H having briefer time courses. Similarly, all the compounds produced analgesic responses after intrathecal administration, with the rank order of potency by this route being DTTLE greater than morphine greater than DAGO greater than DPLPE greater than DPDPE greater than U-50,488H, and all compounds (except U-50,488H) had durations of action of up to 20 to 40 min. These agonists also inhibited gastrointestinal transit after intrathecal administration, with a rank order of potency of DAGO greater than DTTLE greater than DPLPE greater than morphine greater than DPDPE greater than U-50,488H.(ABSTRACT TRUNCATED AT 250 WORDS)