Irreversible binding of [³H]β-funaltremaxine to brain slices of morphine-tolerant and -dependent mice

The specific, irreversible binding of [³H]β-funaltrexamine (β-FNA) to μ opioid receptors in slices from the corpus striatum and midbrain region containing the ventral tegmentum area (VTA) was accomplished by incubation with [³H]β-FNA followed by a washing procedure with 1 μM unlabeled naltrexone in the washing medium. Regional distribution studies of the specific, irreversible binding of [³H]β-FNA revealed that slices of striatum and midbrain were richest in μ opioid receptors. Slices of cerebral cortex and medulla-pons had much lower amounts and the cerebellum had virtually no μ opioid receptors. Examination of the kinetics of the specific, irreversible binding of [³H]β-FNA revealed that although the maximum binding of β-FNA to striatal and midbrain slices of control and morphine tolerant/dependent mice did not differ, the rate at which β-FNA associated with the receptor was increased in the slices from morphine tolerant/dependent animals. The increase in association probably occurred at the initial recognition step before alkylation takes place and is attributable to an increase in affinity of μ opioid receptors for β-FNA in both the striatal and midbrain regions of morphine tolerant/dependent mice. This finding supports and expands earlier conclusions that the affinity of μ opioid receptors for opioid antagonists is increased during the development of opiate tolerance and dependence in mice.