μ Opiate receptors are selectively labelled by [3H]carfentanil in human and rat brain

Milt Titeler, Robert A. Lyon, Michael J. Kuhar, James F. Frost, Robert F. Dannals, Sigrun Leonhardt, Amy Bullock, Laura T. Rydelek, Donald L. Price, Robert G. Struble

Research output: Contribution to journalArticlepeer-review

Abstract

[11C]Carfentanil is a potent opioid agonist currently in use as a specific PET (position emission tomography) scan radioligand for brain μ opioid receptors. In order to investigate the receptor interactions of carfentanil in detail [3H]carfentanil was used as a radioligand for labelling receptors in rat and human brain tissue homogenates. [3H]Carfentanil was found to bind saturably and with high affinity (KD = 0.08 ± 0.01 nM) to membranes prepared from human cortical (Bmax = 42 ± 3 fmol/mg) and thalamic (Bmax = 84 ± 3 fmol/mg) tissues and rat cortex (Bmax = 82 ± 4 fmol/mg) and deincephalon (Bmax = 105 ± 5 fmol/mg). Association (1.23 ± 0.19 × 1010Mol-1 × min-1 and dissociation rate (0.19 ± 0.03 min-1) constants were determined in human cortical tissues; results from studies in rat cortical, and rat diencephalon tissue homogenates produced similar kinetic rate constants. Competition studies with a variety of drugs indicated that [3H]carfentanil interacts primarily with μ opioid receptors in the four tissues studied; the affinities of a series of non-radioactive opioid ligands were essentially identical in the four tissues (correlation coefficients = 0.88-0.93). Naloxone, morphine, DAGO ([D-Ala2-MePhe4-Gly-ol5]enkephalin), DADL (D-Ala2-D- Leu5]enkaphalin) and EKC (ethylketazocine) potently displaced specific [3H]carfentanil binding with nM potency while the κ agonist U-69593, the σ agonists (+)-SKF 10047, (+)-3-PPP ((3-hydroxyphenyl)-N-propylpiperidine) and haloperidol and PCP (phencyclidine) were less potent displacing agents. The higher affinities of DAGO and morphine versus DADL for the [3H]carfentanil binding site indicates that δ opioid receptors are not being labelled. These data indicate that [3H]carfentanil is a high affinity, specific μ opioid receptor radioligand that may be of use in vitro for studying μ opioid receptors and supports the PET scanning data indicating the μ opioid receptor specificity of [11C]carfentanil. The molecular kinetic rate constants determined in vitro will be of assistance in using modelling procedures to interpret PET scanning information.

Original languageEnglish (US)
Pages (from-to)221-228
Number of pages8
JournalEuropean Journal of Pharmacology
Volume167
Issue number2
DOIs
StatePublished - Aug 22 1989

Keywords

  • PET scanning
  • [H]Carfentanil
  • μ Opioid receptors

ASJC Scopus subject areas

  • Pharmacology

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