A method for site selectivity analysis applied to opiate receptors

Methodological aspects on drug receptor binding analysis

Although drug receptors occur in relatively low concentrations, they can be visualized by the use of appropriate radioindicators. In most cases the procedure is rapid and can reach a high degree of accuracy. Specificity of the interaction is studied by competition analysis. The necessity of using several radioindicators to define a receptor population is emphasized. It may be possible to define isorecptors and drugs with selectivity for one isoreceptor.

Fluorescent probes for opioid receptors

Naloxone, naltrexone and oxymorphone were labeled in position 6 with fluorescein by coupling their hydrazone analogs with fluorescein isothiocyanate. Naloxone was also coupled in a similar way with rhodamine-B. The compounds thus obtained were: "6-FN", [1-(N)-fluoresceinyl naloxone thiosemicarbazone]; "6-FNX", [1-(N)-fluoresceinyl naltrexone thiosemicarbazone]; "6-FO", [1-(N)-fluoresceinyl oxymorphone thiosemicarbazone]; "6-RN", [1-(N)-tetramethylrhodaminyl-B naloxone thiosemicarbazone]. These compounds were tested for opioid receptor binding in rat brain synaptosomal plasma membranes and for biological activity on the guinea pig ileum. All compounds retained activity, showing some changes in affinity and binding profile compared to their parent compounds. "6-FN", for example, showed decreased (approximately 10x) affinity compared to naloxone in opiate displacement analysis but retained potency in displacement of opioid peptides. Antagonist activity of "6-FN" in the guinea pig ileum bioassay was about ten-fold less than that of naloxone.

Comparison of the effects of beta-endorphin and morphine on exploratory and socio-sexual behaviour in the male rat

The socio-sexual and exploratory behaviour profiles were studied in male rats after intracerebral administration of beta-endorphin and morphine. Two social test situations were used, one with a sexually receptive female and one with a castrated male as the stimulus subject. Time (10-60 min)-response and dose (0.25-5.0 millimicron)-response relationships were investigated. A dose of 5 millimicron of beta-endorphin or morphine decreased exploratory activity and socio-sexual approaches. At lower doses (0.5-1.0 millimicron) beta-endorphin produced a more selective inhibitory effect on copulatory behaviour than morphine, which still caused a general depression of both social and sexual behaviours. In contrast to morphine, beta-endorphin increased amicable social contacts at a dose (1 millimicron) which decrease sexual responses. The dissimilarities in morphine and beta-endorphin effects suggest involvement of different brain areas or opiate receptors. The particular behaviour profiles observed after treatment with beta-endorphin indicate the possibility that beta-endorphin influences mechanisms implicated in perception or interpretation of environmental stimuli.

d-Propoxyphene acts differently from morphine on opioid receptor-effector mechanisms

The actions of d-propoxyphene and morphine on opioid receptor mechanisms were compared. In assay measuring receptor binding selectivity in vitro, with dihydromorphine, naloxone and an enkephalin analogue as radioactive ligands, d-propoxyphene differed from morphine in having a higher relative affinity for sites occupied by the peptide. Naloxone was more potent in antagonizing morphine- than d-propoxyphene-induced antinociception in the mouse hot-plate test. In morphine-tolerant mice showing three-fold lower morphine sensitivity the antinociceptive efficacy of d-propoxyphene was unchanged. The results indicate differences in receptor-effector mechanisms between d-propoxyphene and morphine.

Radioreceptor assay of methionine-enkephalin-like substance in human cerebrospinal fluid

A sensitive and simple radioreceptor assay system for measuring methionine(met)-enkephalin-like substance in human cerebrospinal fluid (CSF) was developed using a particulate fraction of rat brain as a receptor preparation and [3H]dihydromorphine as a radiolabeled ligand in the presence of 1 mM ethylenediamine tetracetate (EDTA) and 2 mM magnesium acetate. Metenkephalin-like substance was purified from CSF by the combination of Sephadex G-10 and SP-Sephadex (H+) column chromatographies to be free of sodium and large molecular weight substance such as beta-endorphin. The assays were carried out on samples obtained from normal subjects and patients with the disease of the brain or pituitary by lumbar or ventricular puncture. Mean level in samples obtained from normal subjects by lumbar puncture was 2.6 +/- 1.0 pmoles/ml.

Distribution of opiate-like substances in rat tissues

Rat tissues were tested for their ability to inhibit the binding of [3H]dihydromorphine or [3H]naloxone to membrane-bound opiate receptors. By this criterion, morphine-like substances were found in lung, heart, liver, and kidney as well as in brain. The relative activity of the extracts, based on initial tissue weight, differed with the radioactive lignand employed. With dihydromorphine, the order was as above. With naloxone, lung was most active, followed by heart, brain, liver, and kidney. The ability of all tissue extracts to inhibit opiate binding was reduced by 100 mM NaC1 and slightly reduced by 1 mM MnC1(2). Gel filtration using Sephadex G-25 indicated that the inhibitory substances were heterogeneous in molecular weight. Only with brain and kidney extracts was there significant activity at the elution volume where enkephalins would be expected. Fractionation using Amberlite XAD-2, a resin which selectively absorbs hydrophobic materials, again indicated that the major protion of activity in all tissue extracts was due to substances other than enkephalins.

Opiate receptors for behavioral analgesia resemble those related to the depression of spinal nociceptive neurons

With naloxone as antagonist, a dose-ratio analysis of the depression by morphine of nociceptive neurons in the spinal cord reveals that this opiate depression of single unit activity has the same pharmacological properties as observed with morphine analgesia. This suggests that the opiate receptor, mediating the observed cellular depression, and those mediating analgesia are presumably the same.

A dose ratio comparison of the interaction between morphine and cyclazocine with naloxone in rhesus monkeys on the shock titration task

Dose-response curves for morphine (0.5-20 mg/kg) were obtained in 4 rhesus monkeys performing the discrete trials shock titration task. Naloxone (5-500 microgram/kg) uniformly produced a dose dependent, parallel shift of the morphine dose-response curves to right. Plots for the 4 animals of the Log (dose ratio--1) versus --Log (naloxone) revealed that the line had a slope of -0.96. The value of the pA2 was 7.2. Similar experiments were carried out in two rhesus monkeys using cyclazocine. Naloxone (50-1000 microgram) similarly produced a rightward dose dependent, parallel shift of the cyclazocine dose response curves. Unlike the results obtained with morphine, the pA2 for naloxone obtained with cyclazocine was 6.3 and the slope of the dose ratio line was -1.6.