Transient expression of acetylcholinesterase in the posterior ventral cochlear nucleus of rat brain.

In this report we partially characterize a pathway projecting to the posterior ventral cochlear nucleus (PVCN) of the rat brain that transiently expresses a high level of acetylcholinesterase (AChE). The AChE-positive axons form a network that envelops a discrete region of the PVCN that includes the octopus cell region and some cells rostral to it. AChE is first detectable by postnatal day 3 (P3), peaks in expression at about P7-10, and is barely detectable in our preparations by P15. We previously reported that neurons in the octopus cell region express high levels of alpha7 nAChR mRNA and alpha-bungarotoxin binding during the same time period. In light microscopic immunocytochemical studies using antibodies to the vesicular acetylcholine transporter (VAChT), we could not identify immunopositive boutons in the developing regions of the PVCN that express high levels of AChE-positive fibers despite distinct punctate labeling in other brain regions. Systematic electron microscopic examination of AChE histochemical staining throughout the PVCN revealed intense labeling of axons, but synaptic sites were devoid of reaction product. The source of the AChE-positive fibers is not known, but the fibers are not auditory nerve axons and probably not collaterals of the olivocochlear bundle.