dihydromorphine and naloxazone

In addition to morphine-selective mu 2 and enkephalin-preferring delta sites, recent evidence supports the presence within the central nervous system of a common site with very high affinity for both enkephalins and opiates termed the mu 1 site. This concept of a common, very high affinity site for multiple neurotransmitters is a unique concept in neuropharmacology, differing from classical transmitter systems which possess multiple receptor classes for a single transmitter. This review will address both the biochemical and pharmacological evidence supporting the existence of this site.

Topics: Aging; Analgesia; Animals; Autoradiography; Binding, Competitive; Brain; Dihydromorphine; Endorphins; Enkephalin, Leucine; Enkephalin, Leucine-2-Alanine; Enkephalins; Ethylmaleimide; Morphine; Naloxone; Neurotransmitter Agents; Phylogeny; Receptors, Opioid; Receptors, Opioid, mu; Respiration; Substance-Related Disorders; Tissue Distribution

Receptor binding experiments with masking of mu- and delta-receptors in the presence of an excess of unlabelled dihydromorphine and [D-Ala2, D-Leu5]enkephalin have been carried out. They indicate that 12 to 17% of original high affinity binding of [3H]Mr 2034 [-)(1R,5R,9R,2"S)-5,9-dimethyl-2-tetrahydrofurfuryl-2'-hydroxy-6, 7-benzomorphan hydrochloride), an opioid kappa-agonist, could then be detected as kappa-receptor sites in brain membranes both from untreated rats and from rats pretreated with naloxazone, too. Because of the irreversible blockade of the high affinity binding sites by naloxazone (naloxone hydrazone) treatment, the masking effects of mu- and delta-selective ligands seem to be mediated by the low affinity binding sites of these opioid receptor types. As could be shown before with [3H]Mr 2034, another kappa-agonist, [3H]bremazocine does not seem to be affected in its binding properties by naloxazone treatment of the rat in vivo. Displacement studies with several opioid agonists and antagonists, [3H]Mr 2034 and [3H]ethylketocyclazocine as radioligands in brain membranes from naloxazone treated rats and untreated controls provided further support for the evidence of two different kappa-receptors.

Topics: Animals; Benzomorphans; Binding, Competitive; Cyclazocine; Dihydromorphine; Enkephalin, Leucine; Enkephalin, Leucine-2-Alanine; Ethylketocyclazocine; In Vitro Techniques; Kinetics; Male; Morphinans; Naloxone; Rats; Receptors, Opioid; Receptors, Opioid, kappa

Further investigations into the molecular actions of the 14-hydroxydihydromorphinone hydrazones (naloxazone, oxymorphazone, and naltrexazone) have suggested that their irreversible actions can be explained by the formation of their azines. These azines, naloxonazine, naltrexonazine, and oxymorphonazine, irreversibly block opiate binding in vitro 20- to 40-fold more potently than their corresponding hydrozones, naloxazone, naltrexazone, and oxymorphazone. The blockade of binding by naloxonazine shows the same selectivity for high affinity, or mu1, sites as naloxazone.

Topics: Animals; Brain; Cell Membrane; Dihydromorphine; Drug Stability; Hydromorphone; Kinetics; Naloxone; Naltrexone; Oxymorphone; Receptors, Opioid; Structure-Activity Relationship

Detailed displacements of [3H]dihydromorphine by ketocyclazocine and SKF 10,047, [3H]ethylketocyclazocine by SKF 10,047, and [3H]SKF 10,047 by ketocyclazocine are all multiphasic, suggesting multiple binding sites. After treating brain tissue in vitro with naloxazone, all displacements lose the initial inhibition of 3H-ligand binding by low concentrations of unlabeled drugs. Together with Scatchard analysis of saturation experiments, these studies suggest a common site which binds mu-, kappa, and sigma-opiates and enkephalins equally well and with highest affinity (KD less than 1 nM). The ability of unlabeled drugs to displace the low affinity binding of [3H]dihydromorphine (KD = 3 nM), [3H]ethylketocyclazocine (KD = 4 nM), [3H]SKF 10,047 (KD = 6 nM), and D-Ala2-D-Leu5-[3H]enkephalin (KD = 5 nM) remaining after treating tissue with naloxazone demonstrates unique pharmacological profiles for each. These results suggest the existence of distinct binding sites for kappa- and sigma-opiates which differ from those sites which selectively bind morphine (mu) and enkephalin (delta).

Topics: Animals; Binding Sites; Brain; Culture Techniques; Cyclazocine; Dihydromorphine; Enkephalins; Ethylketocyclazocine; Ligands; Male; Naloxone; Narcotics; Phenazocine; Rats; Rats, Inbred Strains; Receptors, Opioid; Tritium

The receptor binding of the kappa agonist [3H]ethylketocyclazocine to brain homogenates in vitro and ketocyclazocine (kappa) analgesia in vivo has been investigated and compared to morphine, a mu agonist. Saturation analysis of [3H]ethylketocyclazocine binding in both mice and rats yielded biphasic Scatchard plots similar to those of opiate mu agonists, antagonists, enkephalins, and endorphins. Treatment of brain membranes with monovalent and divalent cation, chelating agents, protein-modifying reagents, and enzymes affected [3H]ethylketocyclazocine binding in a manner similar to that of [3H]morphine. Naloxazone, a long-acting antagonist that selectively abolished high-affinity [3H-DAla2,Met5]enkephalinamide binding in vivo, also selectively blocked high-affinity [3H]ethylketocyclazocine binding. Evaluation of analgesia with writhing and tail-flick assays in animals whose high-affinity binding sites were blocked by naloxazone demonstrated a 6- to 7-fold increase in median effective dose (ED50) values of ketocyclazocine. This decrease in analgesic potency was comparable to morphine's decreased potency in similarly treated mice. These biochemical and pharmacological results suggest that the analgesic properties of both kappa and mu agonists may be mediated through the same subpopulation of receptors, the high-affinity binding sites.

Topics: Analgesics; Animals; Brain; Cyclazocine; Dihydromorphine; Dose-Response Relationship, Drug; Enkephalin, Methionine; Enkephalins; Ethylketocyclazocine; In Vitro Techniques; Mice; Morphine; Motor Activity; Naloxone; Rats; Receptors, Opioid; Receptors, Opioid, kappa