Saturable binding of morphine to rat brain-stem slices and the effect of chronic morphine treatment.

The binding of morphine to brainstem slices was studied with nanomolar concentrations of (3H)-morphine. Rat brainstem slices were incubated at 35 degrees C and immediately homogenized. Bound morphine was estimated using Millipore filters to separate unbound morphine. Binding of labelled morphine was markedly inhibited by either non-labelled morphine or naloxone, indicating that a substantial portion of the binding is saturable. The saturable morphine binding to slices was significantly greater than that to brainstem tissue homogenized prior to incubation with morphine, whereas non-saturable binding was the same in both preparations. Saturable binding of morphine to brainstem slices was lower in rats rendered tolerant to morphine (using pellet implantation) than in non-tolerant controls. It appears that chronic morphine treatment decreases the affinity of the opiate receptor for morphine. It is concluded that the slice is the preferable preparation for the in vitro estimation of opiate receptor binding and that chronic morphine treatment alters the binding of morphine to brainstem.