u-50488 and 7-benzylidenenaltrexone

The existence of heterodimeric opioid receptors has introduced greater complexity to the in vivo characterization of pharmacological selectivity of agonists by antagonists. Because of the possibility of cooperativity between receptors organized as heterodimers, it is conceivable that selective antagonists may antagonize an agonist bound to a neighboring, allosterically coupled receptor. As a consequence, the in vivo selectivity of an opioid antagonist may depend on the organizational state of receptors that mediate analgesia. In this regard, phenotypic delta- and kappa-opioid receptors have been proposed to arise from different organizational states that include oligomeric delta-kappa heterodimers and homomeric delta and kappa receptors. In view of the evidence for analgesia mediated by delta-kappa heterodimers in the spinal cord, but not the brain, we have investigated the selectivity of pharmacologically selective delta and kappa antagonists in mice by both i.t. and i.c.v. routes of administration to evaluate changes in selectivity. Using pharmacologically selective delta (benzylidenenaltrexone, naltrindole, and naltriben) and kappa (norbinaltorphimine) antagonists versus delta ([D-Pen(2),D-Pen(5)]-enkephalin and deltorphin II) and kappa [3,4-dichloro-N-methyl-N-[(1R,2R)-2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide (U50488) and bremazocine] agonists, the delta-1/delta-2 selectivity ratios were found to be dependent on the route of administration (i.t. versus i.c.v.). The data from different routes of administration suggest that differences in molecular recognition between spinal delta-kappa heterodimers and supraspinal homomeric delta and kappa receptors may contribute to the divergent selectivity ratios of selective antagonists. In view of the observed tissue-dependent selectivity, we suggest that multiple opioid antagonists be employed routinely in establishing agonist selectivity in vivo.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Animals; Benzylidene Compounds; Enkephalin, D-Penicillamine (2,5)-; Injections, Intraventricular; Injections, Spinal; Ligands; Male; Mice; Mice, Inbred ICR; Naltrexone; Narcotic Antagonists; Receptors, Opioid, delta; Receptors, Opioid, kappa

Severe, intermittent hypoxia (hypoxic conditioning) induces an acute adaptation such that survival time during a subsequent hypoxic challenge is increased. The opioid antagonist, naloxone, and the delta-selective antagonists, naltrindole and 7-benzylide-nenaltrexone (BNTX), block this adaptation. The current study continued the pharmacological characterization of this acute adaptation to hypoxia by using selective opioid agonists. [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (1 mg/kg s.c.), U50488H [trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeacetamide methane sulfonate]; 30 mg/kg s.c.] and [D-Pen2,D-Pen5]-enkephalin (DPDPE; 100 mg/kg s.c.) further augmented the hypoxic conditioning induced increase in survival time. DPDPE (56.1 mg/kg of peptide i.v.) increased survival time of naive mice independently of hypoxic conditioning and decreased body temperature. The DPDPE-induced increase in survival time was blocked by the delta-1-selective antagonist, BNTX (0.6 mg/kg s.c.), but not by the delta-2-selective antagonist, naltrindole (10 mg/kg s.c.). However, the DPDPE-induced decrease in body temperature was not blocked by either BNTX or naltrindole. These results supported our hypothesis that the mechanism of acute hypoxic adaptation involves an endogenous delta-1 opioid pathway and demonstrated that activation of a delta-1 receptor mimicked acute hypoxic adaptation induced by intermittent hypoxia.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Adaptation, Physiological; Animals; Benzylidene Compounds; Body Temperature; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Enkephalins; Hypoxia; Male; Mice; Naltrexone; Pyrrolidines; Receptors, Opioid, delta

The antinociception induced by beta-endorphin given intracerebroventricularly (i.c.v.) has been previously demonstrated to be mediated by the release of Met-enkephalin and subsequent stimulation of delta receptors in the spinal cord for antinociception. The present study was designed to determine what type of opioid receptor, delta 1 or delta 2, in the spinal cord is involved in i.c.v. beta-endorphin-induced antinociception. Antinociception was assessed by the tail-flick test in male ICR mice. NTB (0.2-20 nmol) and NTI (0.22-2.2 nmol), selective delta 2 receptor antagonists, given intrathecally (i.t.) dose-dependently attenuated i.c.v. beta-endorphin-induced inhibition of the tail-flick response. On the other hand, BNTX (0.02-2.2 nmol), a selective delta 1 receptor antagonist, given i.t., did not block i.c.v. beta-endorphin-induced antinociception. The tail-flick inhibition induced by DAMGO, a mu receptor agonist, or U50,488H, a kappa receptor agonist, was not blocked by i.t. BNTX, NTB or NTI. It is concluded that delta 2 but not delta 1 receptors in the spinal cord are involved in i.c.v. beta-endorphin-induced antinociception.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Animals; Benzylidene Compounds; beta-Endorphin; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Injections, Intraventricular; Injections, Spinal; Male; Mice; Mice, Inbred ICR; Naltrexone; Narcotic Antagonists; Nociceptors; Pain Measurement; Pyrrolidines; Receptors, Opioid, delta; Spinal Cord