Differential development of cholinergic nerve terminal markers in rat brain regions: implications for nerve terminal density, impulse activity and specific gene expression

During critical developmental periods, cholinergic activity plays a key role in programming the development of target cells. In the current study, ontogeny of cholinergic terminals and their activity were constrasted in 4 brain regions of the fetal and neonatal rat using choline acetyltransferase activity, which is unresponsive to changes in impulse flow, and [ 3H]hemicholinium-3 binding, which labels the high-affinity choline transporter that upregulates in response to increased neuronal stimulation. In all 4 regions (cerebral cortex, midbrain + brainstem, striatum, hippocampus) choline acetyltransferase activity increased markedly from late gestation through young adulthood, but generally did so in parallel with the expansion of total membrane protein, reflective of axonal outgrowth and synaptic proliferation. In contrast, [ 3H]hemicholinium-3 binding was extremely high in late gestation and immediately after birth, declined in the first postnatal week and then rose again into young adulthood. The ontogenetic changes reflected alterations primarily in the number of binding sites ( B max) and not in binding affinity. Only the latter phas of development of [ 3H]hemicholinium-3 binding corresponded to the ontogenetic changes in choline acetyltransferase activity; in the hippocampus, there were disparities even in young adulthood, where [ 3H]hemicholinium-3 binding showed a spike of activity centered around the 5th to 6th postnatal week, whereas choline acetyltransferase did not. Correction of binding for membrane protein development did not eliminate any of the major differences in developmental patterns between the two markers. These results suggest that development of the choline transporter binding site is regulated independently of the outgrowth of the bulk of cholinergic nerve terminal. By implication, either cholinergic nerve impulse activity is extremely high in fetal and early neonatal stages, despite the relative sparsity of terminals, or the choline transporter labeled by [ 3H]hemicholinium-3 is being transiently overexpressed in cells that do not possess the site in the mature nervous system.