Opiate binding to brain slices and ontogenesis of hypothalamic [3H]naloxone binding sites

We have developed a radioligand binding assay based on the use of hypothalamic slices and have examined the ontogenesis of [³H]naloxone binding sites in male and female rats. [³H]NAL binding is reversible, saturable, stereospecific, of high affinity, readily displaceable by morphine and is sensitive to phenoxybenzamine. These characteristics suggest that [³H]NAL readily binds to opiate receptors in brain slices. With this assay we have demonstrated that: (a) there is an age-related increase in opiate binding sites in rat hypothalamus, (b) there are sex differences in the binding affinity of the sites and (c) the values of B_max are approximately 2–5-fold higher than the levels previously reported from assays with brain homogenates.