Chemical neuroanatomy of the cholinergic neurons in the cephalopod octopus and the gastropod limax

Acetylcholine is present in the nervous system of all animals and was the first molecule to be identified as a neurotransmitter. In vertebrates, the understanding of the cholinergic function was dramatically improved by the development of a reliable visualization method using immunohistochemistry for choline acetyltransferase, the acetylcholine-synthesizing enzyme; eventually providing a clear description of the cholinergic neurons and their organization in the so-called "cholinergic pathway". Later, visualization of the cholinergic system proved to be invaluably relevant to linking the cholinergic system with central function and behavior.Gastropods and cephalopods are two classes of mollusks that are most frequently studied by neurobiologists. Electrophysiological, pharmacological and biochemical studies have provided evidence indicating that these mollusks use acetylcholine as a neurotransmitter, and that cholinergic neurotransmission may play a role in various behavioral, cognitive and physiological functions in these animals. However, no reliable method which provides a complete description of the cholinergic neurons and their pathway in mollusks has yet been established, thereby impeding the study of the cholinergic function in such animals. For many years, only a small number of cholinergic neurons were identified in a few gastropod species, and in cephalopods, cholinergic neurons could not be identified at all.Recently, the cholinergic neurons and fibers in the nervous system of the octopus and the slug were identified by immunohistochemical methods using an antiserum specific to the rat common type of choline acetyltransferase. The present chapter is intended to review the current knowledge about the molluskan cholinergic mechanisms and the recent progress made in the identification of cholinergic neurons and circuitry, which provides the neuroanatomical basis of a certain type of cholinergic system in mollusks. © 2012 by Nova Science Publishers, Inc. All rights reserved.