Immunohistochemical evaluation of cholinergic neurons in the rat superior olivary complex

The cholinergic system in the rat superior olivary complex (SOC) was evaluated by immunohistochemistry for choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) and histochemistry for acetylcholinesterase (AChE). ChAT-positive somata were found mostly in the lateral superior olive (LSO) and ventral nucleus of the trapezoid body (VNTB). In the LSO, there were both rostral-caudal and medial-lateral gradients in concentration of ChAT-positive somata; the highest concentration was in the middle of the rostral-caudal extent and the most medial part. The estimated total number of ChAT-positive neurons in the LSO was similar to previous estimates of the total number of lateral olivocochlear neurons. Two groups of ChAT-positive somata were found in the VNTB: a dorsolateral group of larger, multipolar, and more darkly labeled neurons and a ventromedial group of smaller, oval, and more lightly labeled neurons, which was about 5 times as numerous. There was a caudal-to-rostral increase in number of neurons in each group. VAChT immunoreactivity, predominantly localized in puncta, was seen in LSO, VNTB, and LNTB, and, to a lesser extent, in other parts of the SOC. VAChT-positive somata were also found in the VNTB and medial LSO. This distribution pattern of VAChT was generally similar to that of ChAT. AChE labeling had a similar appearance to ChAT labeling in the VNTB but differed in the LSO, where AChE labeling was lighter and associated more with neuropil than with somata.