Postnatal development of cholinergic neurons in the rat: I. Forebrain

The postnatal development of cholinergic projection and local-circuit neurons in the rat forebrain was examined by use of choline acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) histochemistry. Although regional nuances were apparent, a general trend emerged in which cholinergic projection neurons in the basal nuclear complex (i.e., medial septal nucleus, vertical and horizontal diagonal band nuclei, magnocellular preoptic field, substantia innominata, nucleus basalis, and nucleus of the ansa lenticularis) demonstrated ChAT-like immunoreactivity earlier in postnatal development than intrinsically organized cholinergic cells in the caudate-putamen nucleus and nucleus accumbens, although this disparity was less apparent for local circuit neurons in the olfactory tubercle and Islands of Calleja complex. Ontologic gradients of enzyme expression also existed in some regions. A lateral to medial progression of ChAT and AChE appearance was observed as a function of increasing postnatal age in the nucleus accumbens and rostral caudate-putamen nucleus. By comparison, a rostrocaudal gradient of expression of ChAT-like immunoreactivity was apparent within the basal nuclear complex. Moderate to intense ChAT positivity, for example, appeared first in the medial septal nucleus. Furthermore, compared to more caudal regions, a greater proportion of AChE-positive neurons in rostral aspects of the basal forebrain expressed ChAT immunoreactivity on postnatal day 1, a difference that was no longer present by postnatal day 5. Cholinergic neurons in all forebrain regions also underwent an initial stage of progressive soma and proximal-dendrite hypertrophy, which peaked during the third postnatal week, followed by a period of cell-body and dendritic shrinkage that persisted into the fifth postnatal week when adult configurations were reached. These soma and dendritic size increases and decreases were not correlated with the magnitude of postnatal ChAT expression, which increased progressively until adult levels were attained approximately by the third to fifth weeks after birth. Expression of AChE in putative cholinergic neurons appeared to precede that of ChAT, especially in the caudate-putamen complex. Staining intensity of AChE also incremented earlier than that of ChAT.