The acetylcholine system in the brain of cyclostomes with special references to the telencephalon.

The distribution of cholinergic neurons is studied in the brains of cyclostomes (Lampetra fluviatilis and Myxine glutinosa) representing an early stage of vertebrate evolution. Histochemical localization of acetylcholinesterase (AChE) is complemented by quantitative determinations of acetylcholine (ACh), cholinacetyltransferase (ChAT) and AChE. The determinations are carried out in homogenates of whole brains, the brain regions telencephalon, diencephalon, mesencephalon and medulla oblongata. The telencephalon is further subdivided and studied in more detail.The transmitter distribution found in cyclostomes is compared to data from other vertebrates. Observations on the ACh-distribution in the telencephalon are discussed with the problems of identifying telencephalic homologies.