Ontogenesis of opiate receptors in regions of the ovine brain.

We have studied the ontogenesis of opiate receptors in homogenates prepared from different regions of the ovine brain using 3H-naloxone and 3H-D-Ala2, Met5-enkephalinamide. At the concentration employed (0.5 nM) these 3H-opiates labeled both high (KD less than 0.5 nM) and low (KD greater than 1.3 nM) affinity binding sites. When midgestation fetus (68-77 day), late gestation fetus (83-134 day), and neonate (147-173 day) brain regions were compared the majority of regions showed a steady increase in 3H-opiate binding with development. However, marked variation was noted in postnatal development when neonate and adult (greater than 730 days) brains were compared. In some brain regions binding plateaued or increased slightly (thalamus, hippocampus, temporal cortex, and occipital cortex), while opiate receptor binding decreased significantly from neonate to adult in other regions (hypothalamus, striatum, and midbrain). In the pons/medulla, binding was highest in fetal tissues and declined during postnatal life. A comparison of 3H-naloxone binding in adult ovine and rat brains indicated similar regional distribution and concentrations of opiate receptors between these species, although the ovine midbrain and hippocampus contained only 25-30% of the binding sites present in comparable rat brain regions.