Triplicated bursting neuron R15 in an Aplysia abdominal ganglion: non-symmetrical coupling, common synaptic inputs and response to dopamine.

An abdominal ganglion of the mollusc Aplysia californica was found to contain 3 neurons in the place normally occupied by a single R15 cell. The 3 neurons exhibited properties characteristic of R15 neurons including spontaneous bursts. The bursts appeared asynchronously in spite of electrotonic coupling between them. The coupling function approximated a low pass filter with a cut-off frequency between 0.02 and 0.05 Hz in accordance with a measured coupling time-constant of 5--10 sec. Coupling measured in the cell body was found to be stronger for hyperpolarizing currents than for depolarizing currents injected into any of the 3 cells. This 'symmetrical rectification' can be explained by a rectifying axonal membrane interposed between the site of coupling and the site of recording. All 3 cells were found to have dopamine receptors and to receive common synaptic inputs. Since the coupling efficiency was found to vary depending on the direction of current flow, depolarizing synaptic inputs and spike burst generation remain autonomous.