Acetylcholinesterase as an early marker of the differentiation of dorsal thalamus in embryonic rats.

The enzyme acetylcholinesterase (AChE) is transiently expressed in rats by neurons of the principal sensory thalamic nuclei, although these neurons do not use acetylcholine as a neurotransmitter. Reports that AChE expression begins at late embryonic stages led to the proposal that AChE may function in the establishment of connections, but not in earlier events. However, we find AChE reactivity in rat dorsal thalamus 5 days earlier than previously described. Cells that form the ventrobasal complex (VB), the dorsal lateral geniculate nucleus (dLG) and the medial geniculate nucleus, express AChE as they migrate and aggregate into definitive nuclei. AChE-positive cells are occasionally observed in the dorsal thalamic neuroepithelium, but are more common in others regions of the diencephalic neuroepithelium. AChE reactivity delineates VB and dLG earlier than Nissl-stained cytoarchitecture. These findings indicate that AChE is an early marker of neuronal differentiation. Certain properties of AChE, together with its early detection, are consistent with a proposed role in the migration of principal sensory neurons and their organization into discrete nuclei.