Irreversible binding of [3H]beta-funaltrexamine to brain slices of morphine-tolerant and -dependent mice.

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