The analgesic and respiratory depressant actions of metorphamide in mice and rabbits

Antagonistic effects of naloxone and naloxonazine on sufentanil-induced antinociception and respiratory depression in rats

Several binding studies in rodent brain homogenates have revealed two distinct micro-opiate binding sites based on differences in binding affinity of several opiate peptides and opiate alkaloids. Naloxonazine (NLZ), which preferentially binds to the high affinity micro(1) sites, is often used to discriminate between pharmacological effects mediated by micro(1) and micro(2) binding sites. The present series of experiments were undertaken to compare the opioid antagonistic properties of naloxonazine and naloxone (NLX) (a non-selective micro(1)-antagonist) on intravenous (i.v.) and intrathecal (i.t.) sufentanil (SUF)-induced antinociception and respiratory depression. The opioid antagonists were given either intravenously at 5 min after SUF, or subcutaneously (s.c.) 24 h prior to the opioid. Intravenous NLX and NLZ reduced the i.v. and i. t. SUF-induced antinociception, hypercapnia and hypoxia when given directly after the opioid. There were no major differences in activity between both antagonists. Pretreatment with 30 mg/kg NLX did not reverse the i.v. or i.t. SUF-induced antinociception and respiratory depression. Subcutaneous pretreatment with doses up to 30 mg/kg NLX only partially antagonized the i.v. SUF-induced antinociception, while a complete reversal was present of the opioid-induced hypercapnia and hypoxia. With regard to i.t. SUF, doses up to 30 mg/kg NLZ were unable to reduce the antinociception. The respiratory depression was partially affected; with 30 mg/kg NLZ, the i.t. SUF-induced hypercapnia returned to baseline levels, whereas the SUF-induced hypoxia was only minimally affected. These results challenge the classical view of the selectivity of NLZ for the high affinity micro(1) binding sites. They further fail to conform an exclusive role for micro(2) receptor sites in the respiratory depression and spinal analgesia induced by a strong lipophilic opioid such as SUF in rats.

Differential roles of opioid receptors in respiration, respiratory disease, and opiate-induced respiratory depression

In summary, these findings indicate the importance of designing future experiments that delineate between opioid and nonopioid forms of respiratory disease and dysfunction, and the need to identify means of diagnosing them in order to achieve successful recovery. Apparently there is great diversity between animal species in terms of contributions of endogenous opioids to tonic control of ventilation, and future work should strive to identify which species is most appropriate as a model of human ventilatory control and disease. Certain opioid receptor types appear to be linked to independent respiratory functions. For instance, mu receptors in the brain stem produce strong inhibitory actions on respiratory parameters, including RR, VT, VE, and CO2 sensitivity. These effects have been observed in vivo and by electrophysiologic recordings in vitro. Delta receptors may also exert some inhibitory effect on respiration, especially in the NTS. In the CNS, the ventral surfaces of the medulla and pons, especially the NTS and NA, seem to be important sites for opioid-induced inhibition of respiration, whereas the spinal cord probably is not involved in opioid-mediated ventilatory depression. Kappa receptors appear to be devoid of respiratory depressant activity, whereas sigma receptors may stimulate some ventilatory parameters. Morphine and similar pure mu agonists, such as fentanyl and oxymorphine, probably produce their analgesic and respiratory depressant effects through stimulation of mu receptors. Mixed agonists/antagonists that have mu antagonist (or partial agonist) activity plus kappa agonist and/or sigma agonist activity show a ceiling effect for respiratory depression. Future tests need to determine which opioid receptor may be responsible for the ceiling effect. In addition, the effects of mu, delta, kappa, and sigma selective agonists on hypoxic drive should also be determined, as a drug that stimulates hypoxic sensitivity in the face of hypercapnic depression may produce less overall respiratory depression due to counteractive effects. In the future, clinically optimal opiates should have more specificity of action than those available now. This may be achieved by creating drugs selective for single receptors or by creating drugs with desirable combinations of receptor selectivities. The combinations of mixed agonists/antagonists with pure mu agonists currently in use today are promising, as they provide analgesia with reduced respiratory depression. In the early days of opiate research and development, combination drug regimens were thoroughly tested to determine the "ideal ratios" that would retain analgesic properties but not the other undesirable effects such as respiratory depression (196).(ABSTRACT TRUNCATED AT 400 WORDS)

BAM-18: analgesia, hyperalgesia and locomotor effects

BAM-18, a proenkephalin A-derived opioid peptide, is widely distributed throughout rat CNS and displays high affinity for both mu and kappa opioid receptors. In the present study, BAM-18 was tested in two analgesia paradigms, tail-flick and hot-plate. Injections were centrally administered through a chronically implanted unilateral cannula in the lateral ventricle. In the tail-flick, low doses of BAM-18 (5 micrograms) produced a hyperalgesia while high doses of BAM-18 (50 micrograms) produced an analgesic response. Naloxone (10 mg/kg, s.c.) reversed the BAM-18-induced analgesia and unmasked a persistent hyperalgesia. Morphine-induced (1 microgram) analgesia was completely reversed by 5 micrograms BAM-18. In the hot-plate test, high doses of BAM-18 produced analgesia, with no hyperalgesia observed at any dose. Naloxone reversed the BAM-18-induced analgesia. The locomotor effects of BAM-18 did not differ from those of morphine except in effective dose (50 micrograms vs. 5 micrograms, respectively). Opioid and non-opioid effects of BAM-18 are discussed and compared with other endogenous peptides.

Distribution pattern of metorphamide compared with other opioid peptides from proenkephalin and prodynorphin in the bovine brain

Metorphamide is a [Met]-enkephalin-containing opioid octapeptide with a C-terminal alpha-amide group. It is derived from proenkephalin and is, so far, the only endogenous opioid peptide with a particularly high affinity for mu opioid (morphine) receptors, a somewhat lesser affinity for kappa opioid receptors, and a relatively low affinity for delta opioid receptors. The concentrations of metorphamide in the bovine caudate nucleus, the hypothalamus, the spinal cord, and the neurointermediate pituitary were determined by radioimmunoassay and chromatography separation procedures. Metorphamide concentrations were compared with the concentrations of eight other opioid peptides from proenkephalin and prodynorphin in identical extracts. The other opioid peptides were [Met]-enkephalyl-Arg6-Phe7 and [Met]-enkephalyl-Arg6-Gly7-Leu8 from proenkephalin; alpha-neoendorphin, beta-neoendorphin, dynorphin A(1-8), dynorphin A(1-17), and dynorphin B from prodynorphin; and [Leu]-enkephalin, which can be derived from either precursor. All opioid peptides were present in all four bovine neural tissues investigated. Metorphamide concentrations were lower than the concentrations of the other proenkephalin-derived opioid peptides. They were, however, similar to the concentrations of the prodynorphin-derived opioid peptides in the same tissues. Marked differences in the relative ratios of the opioids derived from prodynorphin across brain regions were observed, a finding suggesting differential posttranslational processing. Differences in the ratios of the proenkephalin-derived opioids across brain regions were less pronounced. The results from this study together with previous findings on metorphamide's mu opioid receptor binding and bioactivities suggest that the amounts of metorphamide in the bovine brain are sufficient to make this peptide a candidate for a physiologically significant endogenous mu opioid receptor ligand.