Developmental change of cholinergic activity in the forebrain of the zebra finch during song learning.

Song learning in the male zebra finch closely correlates with the development of song-control nuclei, the high vocal center (HVc), the robust nucleus of the archistriatum (RA) and the lateral magnocellular nucleus of the anterior neostriatum (IMAN). We previously found that acetylcholine (ACh) content increases transiently in the RA during the sensitive period of song learning. In the present experiment, development of cholinergic neurons by immunohistochemical methods using monoclonal choline acetyltransferase (ChAT) antibodies, and enzymic activity of ChAT and acetylcholinesterase (AChE) were examined in the RA during the sensitive period of song learning. The developmental change of ChAT activity in the RA was closely parallel to that of the ACh concentration. On the other hand, the density of ChAT-immunoreactive fibers changed little in the RA. This indicates that the transient increase of ACh content during the sensitive period is due to ChAT activity, but not to the increasing density of the cholinergic fibers. The ACh in the RA might be related to plastic changes in the synapse of RA neurons.