A comparison of the receptor constants of morphine and ethylketocyclazocine for analgesia and inhibition of gastrointestinal transit in the rat

The efficacies and dissociation constants of proposed mu and kappa receptor agonists (morphine and ethylketocyclazocine, respectively) were compared using the method of partial irreversible blockade (with buprenorphine) and Stephenson's theory of drug action. While there was good agreement between the dissociation constant (KA) of morphine in analgesia (3.3 x 10(-5) M) and in inhibition of gastrointestinal transit (1.1 x 10(-5) M), the KA of ethylketocyclazocine differed by an order of magnitude in these endpoints (3.2 x 10(-6) M and 6.7 x 10(-5) M, respectively). The efficacies of morphine were found to be similar for the two effects studied (4.23 and 5.26), while those for ethylketocyclazocine differed markedly (2.06 and 10.39). The fraction of receptors remaining unblocked after buprenorphine was consistent for the test but not for the agonist, indicating a different distribution of receptors for the two endpoints. Our results strongly suggest that morphine induces analgesia, and slows transit in the small intestine, through the same type of receptor. The same conclusion cannot be drawn for ethylketocyclazocine.     

Differential antagonism of mu agonists by U50,488H in the rat

Agonist, and antagonist effects of the proposed kappa opioid agonist, U50,488H (U50) have been studied in an experimental model of seizure activity (flurothyl-induced seizure threshold) (ST) and in the central modulation of spontaneous, volume-induced micturition contractions (bladder motility) (BM) in rats. Intracerebroventricular (i.c.v.) administration of U50 (at the doses tested) did not produce any agonist effect in either ST or in BM. In contrast, i.c.v. administration of [D-Ala2, NMPhe4, Gly-ol]enkephalin (DAGO) or etorphine, agonists with activity at mu opioid receptors, produced an elevation of ST and inhibition of BM. The elevation in ST produced by etorphine (0.004 nmol) was prevented by prior treatment with U50. In contrast, the approximately equieffective elevation in ST resulting from DAGO was not affected by U50 pretreatment. Similarly, pretreatment of rats with U50 antagonized the approximately equieffective BM effects of etorphine, but not those of DAGO. As both DAGO and etorphine are thought to exert their effects via the opiate mu receptor, the results may be consistent with the view that subpopulations of mu receptors exist within the central nervous system; these sites may be differentially associated with the kappa receptor.     

A comparison of the central gastrointestinal antitransit effects of morphine and bombesin in the mouse

The gastrointestinal motor effects of centrally-given morphine and bombesin were compared in mice. Both compounds produced a dose-related decrease in the propulsion of a marker along the gut when given by the intracerebroventricular (i.c.v.) or intrathecal (i.th.) routes. Co-administration of the same compound by both routes was found to produce a marked increase in potency for morphine, but only a slight increase in potency for bombesin. Isobolographic analysis of the gut effects of these compounds revealed a multiplicative brain-spinal cord interaction for morphine, but an additive interaction for bombesin. These results are consistent with the interpretation that morphine can act at either the level of the brain or the spinal cord, activating independent pathways which ultimately converge to alter gut propulsion. In contrast, spinal bombesin requires communication to supraspinal sites in order for its gut effects to occur, suggesting activation of a common outflow pathway from the central nervous system.     

Antinociceptive effects of [D-Ala2]deltorphin II, a highly selective delta agonist in vivo

The present study has characterized the antinociceptive actions of [D-Ala2]deltorphin II following intracerebroventricular (i.c.v.) administration in the mouse tail-flick test. [D-Ala2]deltorphin II produced dose- and time-related antinociception, with maximal effects at +10 min and significant antinociception which lasted for 40-60 min. [D-Ala2]deltorphin II was 13-fold more potent than i.c.v. [D-Pen2, D-Pen5]enkephalin (DPDPE), a second highly selective delta agonist, and approximately equipotent with i.c.v. morphine in producing antinociception. The antinociceptive effects of i.c.v. [D-Ala2]deltorphin II and DPDPE, but not those of morphine, were antagonized by the selective delta antagonist, ICI 174,864. In contrast, pretreatment with the non-equilibrium mu antagonist, beta-funaltrexamine blocked morphine antinociception, but failed to antagonize [D-Ala2]deltorphin II and DPDPE antinociception. These data indicate that [D-Ala2]deltorphin II produced its antinociceptive effects at a supraspinal delta receptor. [D-Ala2]deltorphin II appears to be the most appropriate delta opioid agonist currently available for studies in vivo and support the involvement of delta receptors in supraspinal antinociception.     

Preclinical Assessment of the Analgesic Pharmacology of NKTR-181 in Rodents

Cellular and Molecular Neurobiology - Pharmacological evaluation of the mu-opioid receptor (MOR) agonist properties of NKTR-181 in rodent models. Graded noxious stimulus intensities were used in...     

Synthesis and biological activities of position one and three transposed analogs of the opioid peptide YKFA

Tyr-c[D-Lys-Phe-Ala], YKFA, is a potent opioid peptide analog with subnanomolar IC₅₀s toward mu and delta receptors. Transposing Phe and Tyr, a modification found to promote mu antagonist activity in opioid/somatostatin hybrids, gave surprisingly high mu agonist activities for several related analogs, considering the lack of a 1-position hydroxyl function.     

Vascular non-protein thiols: prooxidants or antioxidants in atherogenesis?

Abstract-cell-mediated lipoprotein oxidation may be due to generation of non-protein thiols (NP-SH) from cystine with formation of oxidizing species. However, NP-SH, especially GSH, may also exert antioxidant effects in vitro and in vivo. To further investigate whether vascular NP-SH could be prooxidants or antioxidants in atherosclerosis, we have correlated the aortic content of NP-SH with that of lipoperoxides in 10 rabbits fed on a fat-enriched atherogenic diet for 9 weeks. As compared to 7 control rabbits, aortic NP-SH and lipoperoxides were significantly increased in the fat-fed animals. The levels of NP-SH were strongly and inversely correlated with those of lipid peroxidation in the atherosclerotic aorta (r(s) -0.92, P < 0.0001 for thiobarbituric acid reactive substances, and r(s) -0.80, P < 0.01 for fluorescent damage products of lipid peroxidation). A similar trend was evident also in the control rabbits (r(s) -0.60 for both indices of lipid peroxidation). Thus, the present data suggest that vascular NP-SH exert significant antioxidant-antilipoperoxidative effects in vivo especially in fat diet-related atherogenic conditions.     

Novel ligands for the opioid receptors: synthesis and structure-activity relationships among 5'-aryl and 5'-heteroaryl 17-cyclopropylmethyl-4,5 alpha-epoxypyrido[2',3':6,7]morphinans

A series of pyridomorphinans possessing an aryl (10a-s) or heteroaryl (11a-h) substituent at the 5'-position of the pyridine ring of 17-cyclopropylmethyl-4,5 alpha-epoxypyrido[2',3':6,7]morphinan was synthesized and evaluated for binding and functional activity at the opioid delta, mu, and kappa receptors. All of these pyridomorphinans bound with higher affinity at the delta site than at mu or kappa sites. The binding data on isomeric compounds revealed that there exists greater bulk tolerance for substituents placed at the o-position of the phenyl ring than at m- or p-positions. Among the ligands examined, the 2-chlorophenyl (10l), 2-nitrophenyl (10n), 2-pyridyl (11a), and 4-quinolinyl (11g) compounds bound to the delta receptor with subnanomolar affinity. Compound 10c with the p-tolyl substituent displayed the highest mu/delta selectivity (ratio=42) whereas compound 10l with the 2-chlorophenyl substituent displayed the highest kappa/delta selectivity (ratio=23). At 10 microM concentration, the in vitro functional activity determined using [(35)S]GTP-gamma-S binding assays showed that all of the compounds were antagonists devoid of any significant agonist activity at the delta, mu, and kappa receptors. Antagonist potency determinations of three selected ligands revealed that the p-tolyl compound 10c is a potent delta selective antagonist. In the [(35)S]GTP-gamma-S assays this compound had a functional antagonist K(i) value of 0.2, 4.52, and 7.62 nM at the delta, mu, and kappa receptors, respectively. In the smooth muscle assays 10c displayed delta antagonist potency with a K(e) value of 0.88 nM. As an antagonist, it was 70-fold more potent at the delta receptors in the MVD than at the mu receptors in the GPI. The in vitro delta antagonist profile of this pyridomorphinan 10c resembles that of the widely used delta selective antagonist ligand naltrindole.     

Inhibition of pain responses by activation of CB(2) cannabinoid receptors

Cannabinoid receptor agonists diminish responses to painful stimuli. Extensive evidence demonstrates that CB(1) cannabinoid receptor activation inhibits pain responses. Recently, the synthesis of CB(2) cannabinoid receptor-selective agonists has allowed testing whether CB(2) receptor activation inhibits pain. CB(2) receptor activation is sufficient to inhibit acute nociception, inflammatory hyperalgesia, and the allodynia and hyperalgesia produced in a neuropathic pain model. Studies using site-specific administration of agonist and antagonist have suggested that CB(2) receptor agonists inhibit pain responses by acting at peripheral sites. CB(2) receptor activation also inhibits edema and plasma extravasation produced by inflammation. CB(2) receptor-selective agonists do not produce central nervous system (CNS) effects typical of cannabinoids retaining agonist activity at the CB(1) receptor. Peripheral antinociception without CNS effects is consistent with the peripheral distribution of CB(2) receptors. CB(2) receptor agonists may have promise for the treatment of pain and inflammation without CNS side effects.