Development of the noradrenergic, serotonergic, and dopaminergic innervation of neocortex

This chapter addresses numerous developmental questions raised by the presence of extensive innervation of neocortex by at least three distinct sets of extrathalamic afferents. It reviews the available developmental data. These data are much less comprehensive than the data dealing with adult innervation patterns in three basic ways. First, in most developmental studies, the details of the developing patterns are difficult to discern, as the time points are not closely spaced or comprehensive. Second, while numerous neocortical regions have been studied in detail in adult organisms, a much smaller number have been studied in detail during development. Third, nearly all studies of the development of monoaminergic innervation patterns in neocortex have been performed in rodents or carnivores. Thus, the data available for primates are limited to a few time points and a very few neocortical areas. The conceptual issues raised by the available data are discussed. There are two major issues: (1) how monoamine axons, arising at a considerable distance from the neocortex, find their way into the cortical mantle, distribute themselves in the manner appropriate to each transmitter system, and find their appropriate classes of target neurons, and (2) what developmental influence(s) these fibers and their transmitters exert on the development of intrinsic cortical circuitry or the distribution of other cortical afferents, particularly with reference to the issue of plasticity.