Whole brain and regional [125I]-α-bungarotoxin binding in developing rat

Abstract These experiments were conducted in order to determine if the total number of binding sites for [ 125 I]-α-bungarotoxin ([ 125 I]-α-BGT) in rat brain increases and then decreases during postnatal development as predicted by comparison with skeletal muscle, and, if so, to determine at approximately what age the peak in binding occurs in the brain as a whole. A further purpose was to investigate the time-course of development of the [ 125 I]-α-BGT binding sites in several brain regions. Specific binding for [ 125 I]-α-BGT was studied using the pellets from a 20 min, 14,000 × g centrifugation of rat brain homogenates from 4 or 5 postnatal ages. At least three binding assays were done per region and per age, on cerebral cortex, cerebellum, caudate-putamen, posterior hippocampus, pons-medulla and whole brain. In most regions, the [ 125 I]-α-BGT specific binding is measurable, but is low at day one, peaks at about 12–20 days and declines by adulthood. With a few exceptions, these data hold true whether binding is expressed as specific binding per mg protein, specific binding per gram wet tissue, or total specific binding per brain region. The absolute number of specifically bound [ 125 I]-α-BGT molecules is undistorted by simultaneous or non-linear growth of cells uninvolved with α-BGT binding and, thus, is the measurement most useful in determining developmental changes. Whole brain has the same age-related pattern as in the majority of the brain regions, i.e., compared to 19–20 days, the adult brain actually has fewer total binding sites.