Differential development of cholinergic-like neurons in the superior olive: a light microscopic study

To better understand the development of cholinergic-like neurons within the superior olivary complex, we investigated the onset and distribution of tw o well-known markers of cholinergic-like neurons in hamsters: choline acety ltransferase (ChAT) and acetylchlolinesterase (AChE). From embryonic day (E ) 14 through postnatal day (P) 0, olivary cells immunopositive for ChAT wer e restricted to the rostral periolivary (RPO) area. Between PO and P3, ChAT -positive cells are found in progressively more caudal and ventral perioliv ary locations. Although rostral and ventral periolivary cells exhibited an early onset of ChAT expression, stable numbers were not reached until P4. I n contrast, ChAT expression within the lateral superior olive (LSO) is not visible until after PO and higher numbers of ChAT-positive cells are obtain ed by P5. The AChE expression lags several days but follows roughly the sam e pattern of onset as for ChAT. Additionally in rostral and ventral peri ol ivary regions as well as in the LSO, there were fewer AChE-labeled cells th an ChAT-labeled cells. The observed temporal relationships in cholinergic-l ike expression within olivary cells suggest that different cholinergic-like populations may be defined on the basis of the onset of neurotransmitter-r elated enzymes: RPO cells are first, cells in ventral periolivary regions a re second, and cells associated with the LSO are last. The differences obse rved in the onset of ChAT and AChE expression may reflect differences in th e timing of target innervation as well as differences in synaptogenesis.