Further Observations on the Serotonergic Cerebral Neurones of Helisoma (Mollusca, Gastropoda): The Case for Homology with the Metacerebral Giant Cells

Abstract

A bilaterally symmetrical pair of serotonergic neurones in the cerebral ganglia of the snail Helisoma trivolvis has major features in common with the serotonergic cerebral cells of other gastropods, including those of Helix and Aplysia. The cells were found to receive a high level of inhibitory synaptic activity which is important in determining their firing level. In the cells of isolated cerebral ganglia, tonic inhibitory synaptic potentials, uniform in frequency and amplitude, occur continually at a rate of about 2/8. In the cells of cerebral ganglia left intact with the periphery and buccal ganglia, phasic inhibitory activity occurs in addition to the tonic activity. Phasic inhibitory input could be observed at regular intervals, correlated with feeding motor activity in the buccal ganglia. Other properties of these cells include: a linear relationship between membrane potential and current injected into the cell; communication between bilateral cells in about 50% of the preparations studied; and sodium-based somatic action potentials. Some of these properties of Helisoma SCCs are different from those of Helix and Aplysia. We propose that the Helisoma SCCs are homologous to those in other gastropods and that the differences observed are the result of evolutionary adaptation of the cells to different functional roles.