Naloxone opioid receptor binding

Opioid receptor binding Naloxone (Simantow and Snyder,... 

Naloxone blocks receptor binding of endogenous opioids, called endorphins or enkephalins, produced by the pituitary, as well as blocking the effects of administered narcotics, such as morphine. These blocking drugs are used to awaken patients operated on under opiate anesthesia. Persons in coma from ingesting illegal narcotics, such as morphine or heroin, wake up within seconds after administration of naloxone. 

The opioid antagonists naloxone and naltrexone bind to aU three opioid receptors, p, K, and 8. These compounds are antagonists due to their inability to elicit downstream effects of these receptors once bound (Sarton et al. 2008 Yaksh and Rudy 1977). Interestingly, both antagonists have a high binding affinity for MORs. Naloxone is used to reverse the effects of an acute opioid overdose because of its rapid onset of action. Naltrexone elicits similar actions, but has a longer onset and duration of action and hence, is used for the maintenance of treatment for opioid addicts. 

Our results suggest that the numerically predominant 153 nM 3H-(+)SKF-10,047 site represents the PCP opioid receptor. The much less abundant haloperidol - sensitive 3.6 nM 3H-SKF-10,047 binding site that we have described appears to represent the same entity as a class of sites reported by several other groups utilizing diverse strategies, including use of H-SKF-10,047 in the presence of etorphine (Su 1982) 3H-ethylketoSycl azocine (EKC) in the presence of excess naloxone (Tam 1983) 3H-(+)SKF-10,047 or (+)3H-... 

Naloxone is a pure opioid antagonist that binds competitively to opioid receptors but does not produce an analgesic response. It is used to reverse the toxic effects of agonist and agonist-antagonist opioids. 

Accordingly, library 2 in contrast to library 1 must contain (at least) one ligand that, in concentrations of 1 pM and 10 nM, is capable of reducing the specific binding of morphine to the //-opioid receptor. Considering the composition of the two libraries, the conclusion that the component with an affinity for the //-opioid receptor must be naloxone is naturally trivial, but this issue could also be addressed by further examining the relevant binding samples (Fig. 7.12). 

To this end, the pellets remaining from the competitive MS binding assay were, after several washing steps, resuspended in binding buffer and incubated with a great excess of competitor (50 pM (+)-methadone) to liberate the unknown bound ligand (as well as the bound marker). Then the supernatants obtained by centrifugation were analyzed by LC-ESl-MS/MS. In addition to morphine as the marker, naloxone was identified as the hit that had been searched for. Thereby, the relative concentrations of marker (2.93 nM) and hit (2.30 nM) pointed to the fact that the hit had a similar affinity to the //-opioid receptor as the marker [65]. 

This strategy was realized in a competitive MS binding assay examining the affinity of naloxone for //-opioid receptors (with morphine as marker under the conditions described above for library screening). The respective experiments led... 

Fig. 7.13 Representative binding curve obtained by nonlinear regression from a competitive MS binding assay for //-opioid receptors, in which naloxone competes with morphine as marker. The points describe nonbound morphine quantified by LC-ESl-MS/MS at concentrations of nonbound naloxone determined simultaneously by LC-ESl-MS/MS.

In October 2002, the FDA approved two new medications for treating opiate addiction, both developed by Reckitt Benckiser Pharmaceuticals. The new drugs, Subutex (buprenorphine hydrochloride) and Suboxone tablets (buprenorphine hydrochloride and naloxone hydrochloride) contain buprenorphine, a partial opioid agonist. Like methadone, buprenorphine binds to the brain s opioid receptors, but produces significantly reduced pleasurable effects than heroin. 

Opioid antagonists bind to the opioid receptor with high affinity and have low efficacy. The pure antagonists block the effects of opioids at all opioid receptors. However, as previously discussed, the dose required for naloxone blockade of the jx-receptor versus the k-opioid receptor is several times as much. All opioid antagonists will precipitate withdrawal in opioid-dependent patients. 

Sequence Determination of the Brain Peptide Leucine Enkephalin A group of peptides that influence nerve transmission in certain parts of the brain has been isolated from normal brain tissue. These peptides are known as opioids, because they bind to specific receptors that also bind opiate drugs, such as morphine and naloxone. Opioids thus mimic some of the properties of opiates. Some researchers consider these peptides to be the brain s own pain killers. Using the information below, determine the amino acid sequence of the opioid leucine enkephalin. Explain how your structure is consistent with each piece of information. 

Opioid receptors. Direct binding of highly radioactive opiates has permitted localization of specific opiate receptors of several types.863-866 The three major types (p, 8, k) are all 7-helix receptors coupled to adenylate cyclase, K+ and Ca2+ channels, and the MAP kinase cascade.866 The p receptors bind morphine most tightly.867 8673 These receptors are found in various cortical and subcortical regions of the brain. Most narcotics are polycyclic in nature and share the grouping indicated in Fig. 30-30. However, the flexible molecule methadone binds to the same receptors.868 Among antagonists that block the euphoric effects of opiates the most effective is naloxone (Fig. 30-30). 

An examination of the structure-activity relationship (SAR) of a series of enkephalin analogues and morphine derivatives in competing with the binding of radioisotope labeled DADLE and FK 33-824 or opiates revealed that DADLE binds to an opioid receptor selective to Leu-enkephalin and its analogues. In contrast, FK 33-824, morphine, or naloxone binds to opioid receptors with selectivity in favor of morphine and derivatives simi-... 

An altered pattern of cortical [3H]U 69593 binding to the kappa receptor has been reported in schizophrenia (Royston et al., 1991). However, by in situ hybridization, transcripts for the K-receptor, as well as its endogenous ligand prodynorphin, were reported unchanged in dorsolateral PFC and ACC in schizophrenia (Peckys and Hurd, 2001). This is consistent with a previous study in which no differences in [3H]etorphine and [3H]naloxone binding to opioid receptors were reported in striatum in schizophrenia (Owen et al., 1985). Additional postmortem studies are warranted to further clarify the role of the endogenous opioid system in schizophrenia. 

Owen F, Bourne RC, Poulter M, Crow TJ, Paterson SJ, et al. 1985. Tritiated etorphine and naloxone binding to opioid receptors in caudate nucleus in schizophrenia. Br J Psychiatry 146 507-509. 

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