Quantitative autoradiography of the development of mu opiate binding sites in rat brain

The regional developmental appearance of mu binding sites in rat brain was examined by quantitative autoradiography of 3H-dihydromorphine binding in rats 2, 14, 21, and 28 days old. Labeling with 3H-dihydromorphine was heterogeneous in adult rat brains, as previously reported by other laboratories. Levels of 3H-dihydromorphine binding ranged from approximately 250 nCi/g tissue in the interpeduncular nucleus and 100 nCi/g tissue in the habenula to 40 nCi/g tissue in the hypothalamus and periaqueductal gray. Some areas, particularly white matter regions, had no detectable specific binding. The density of 3H-dihydromorphine binding increased in all regions between 2 and 28 days of age. The increases in 3H-dihydromorphine binding in various regions of rat brain developed at different rates. Maximal densities were seen by 14 days of age in most regions examined, including the caudate, hippocampus, amygdala, and hypothalamus. Binding in the medial thalamus and quadrigeminal plate, however, did not reach maximal levels until 21 days. Although quantitative autoradiography offers major advantages in the examination of the regional distribution of opiate binding sites, variability both between sections from the same brain and between sections from different brains demonstrate some of the difficulties associated with this type of experimental approach.