Enzymatic synthesis of acetylcholine by a serotonin-containing neurone from Helix

Abstract

ACETYLCHOLINE (ACh) and serotonin are strong neurotransmitter candidates in the snail species Helix aspersa and Helix pomatia^1,2. Experiments¹ have indicated that the enzymes responsible for their biosynthesis, choline acetyltransferase (ChAc, EC 2.3.1.6.) and L-aromatic amino acid decarboxylase (AAD, EC 4.1.1.26.), were specifically localised to separate populations of neurones in H. aspersa, and similarly for H. pomatia (M.R.H., unpublished). Out of 150 neurones, 36 were found to contain ChAc only (detection limit 2 pmol h⁻¹) and 20 were found to contain AAD only (detection limit 8 pmol h⁻¹). This clear biochemical differentiation between presumably ‘cholinergic’ neurones and presumably ‘serotonergic’ neurones has been previously noted in other invertebrates^3,4. It was therefore surprising when, in two laboratories (St Andrews and Kjeller), we independently found that the two homologous giant metacerebral neurones, which contain serotonin⁵ and are thought to use it as a neurotransmitter⁶, could also enzymatically synthesise ACh. The coincidence of these two enzymatic activities in the giant serotonin cell (GSC) is a unique and therefore intriguing situation among characterised invertebrate nerve cells.