Peptide opioid antagonist separates peripheral and central opioid antitransit effects

The purpose of these investigations was to determine 1) whether peripherally located mu, delta and kappa opioid receptors can inhibit the rate of gastrointestinal transit and, if so, 2) do peripheral opioid receptors mediate the constipation caused by systemic morphine? and 3) whether constipation can be separated from analgesia on the basis of different sites of action. We studied the effects of peripherally administered (s.c.) mu, delta and kappa opioid receptor selective agonists on the rate of gastrointestinal transit in mice. We used peptidergic agonists with high peripheral selectivity (limited ability to cross the blood-brain barrier) including [MePhe³,D-Pro⁴]morphiceptin (PL017) (mu), [D-Pen²,D-Pen⁵]enkephalin (DPDPE) (delta) and Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg (dynorphin 1-9) (kappa). As peripheral selectivity is dose-related, we included the hot-plate test as an index of that dose at which each compound lost its peripheral selectivity and entered the central nervous system. When given s.c., [MePhe³,D-Pro⁴]morphiceptin inhibited transit (IC₅₀ = 0.37 mg/kg s.c.) at doses much lower than those needed to produce analgesia (A₅₀ = 30 mg/kg s.c.), indicating that peripheral mu receptors can inhibit transit independently of central mu receptors. The independence of peripheral mu antitransit receptors from central receptors was demonstrated further as the lack of antagonism of s.c. [MePhe³,D-Pro⁴]morphiceptin antitransit effects by simultaneous i.c.v. administration of the mu receptor antagonist D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH₂ (CTP) (1 μg i.c.v.). [D-Pen²,D-Pen⁵]enkephalin and Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg did not inhibit transit when given s.c., even at very high doses, indicating that peripheral delta and kappa receptors do not mediate transit. Morphine given s.c. inhibited transit (IC₅₀ = 1 mg/kg s.c.) at doses similar to those which also caused analgesia (A₅₀ = 4 mg/kg s.c.). Effects of coadministration of CTP (1 μg i.c.v. or 1 mg/kg s.c.) with s.c. morphine varied with the dose of morphine: at subanalgesic doses of morphine (≤ 1 mg/kg s.c.), its antitransit effects were blocked totally by s.c. CTP, but not i.c.v. CTP, whereas at analgesic doses (> 1 mg/kg s.c.), it was blocked partially, but nearly equally, by both i.c.v. and s.c. CTP. These results suggest that at subanalgesic doses, systemic morphine acts at peripheral mu receptors (not delta or kappa) to inhibit transit, and at analgesic doses, it acts through both central and peripheral opioid receptors. It therefore seems unlikely that the antitransit effects of morphine can be separated from analgesia on the basis of different (peripheral vs. central) sites of action.