Differential expression of GAP-43 mRNA in adult central cholinergic neuronal populations

Abstract

The present study addresses a question of differential expression for a ‘plasticity’ gene within neurons identified by neurotransmitter type. A method combining immunohistochemical localization of choline acetyltransferase (ChAT) with in situ hybridization histochemistry (ISHH) in the same brain sections was used to quantitate the levels of mRNA for the growth-associated protein GAP-43 (neuromodulin) in rat central cholinergic neuronal populations. We found that many cholinergic neurons in the adult rat brain express levels of GAP-43 mRNA comparable to other brain regions noted for their expression of this plasticity gene. GAP-43 expression in cholinergic cell groups appeared to be highly heterogeneous; this was often true even for cholinergic neurons within the same brain region. A dorsal-ventral gradient in GAP-43 mRNA levels was evident in the rostral basal forebrain cholinergic groups; i.e., medial septum, nucleus basalis magnocellularis and the vertical limb of the diagonal band expressed intermediate levels, while the horizontal limb of the diagonal band and the substantia innominata expressed higher levels of GAP-43 mRNA. Cholinergic interneurons of the striatum were grouped in several populations according to mRNA levels, from very low to very high expression. Similarly, GAP-43 mRNA levels in the cholinergic neurons of the nucleus basalis magnocellularis/ substantia innominata were quite variable. The expression of GAP-43 mRNA in brainstem cholinergic groups (laterodorsal tegmental and pedunculopontine nuclei) was in nearly uniform populations of somewhat lower levels. While the expression of GAP-43 mRNA in cholinergic neurons was heterogeneous, virtually every ChAT-positive neuron sampled contained GAP-43 mRNA at levels significantly over background. The highly heterogeneous expression of neuromodulin/GAP-43 in central cholinergic neurons may relate to the differential survival of these cells in brain disease or injury, or may reflect on differential involvement of GAP-43 in regulation of neurotransmitter release.