Identified cholinergic neurons R2 and LPl1 control mucus release in Aplysia

1. R2 and LPl1 are homologous giant cholinergic neurons in the nervous system of Aplysia with overlapping and almost symmetrical axonal trees extending over most of the body wall. In spite of much experimental study, the behavioral role of the cells has remained unknown. 2. After intrasomatic injection of R2 and LPl1 with horseradish peroxidase (HRP), the giant cell axons were traced to the periphery and found to contact subepidermal glands in the body wall exclusively. 3. The axons penetrated the glandular basal lamina, indenting the gland cell cytoplasm, and expanded into varicosities containing putative cholinergic transmissive sites. 4. Histochemical characterization of the contents of the glands showed that they contain mucus, suggesting that the giant cells control mucus release from the body wall. 5. Stimulation of R2 or LPl1 resulted in glandular discharge, as measured both by an increase in the appearance of protein and of mucus on the body wall. 6. R2 and LPl1 control mucus release from the body wall, thus providing a new system for investigations of neuroglandular control as well as a behavioral context for cellular studies using these two neurons.