Motor controls of opaline secretion in Aplysia californica

1. Using combined morphological and electrophysiological techniques, we have identified motor neurons in the right pleural ganglion of Aplysia californica that contribute to the release of opaline from the opaline gland. 2. Three pleural ganglion neurons were found to meet the requirements for identification as opaline gland motor neurons by a) sending processes in nerve P5, which innervates the gland; b) producing contractions of the gland in the absence of central synaptic activity; and c) producing excitatory junctional potentials (EJPs) in cells making up the opaline gland itself. The neurons can be reliably located and have been designated PLR1, PLR2, and PLR3. 3. When gland contraction is measured by the change in luminal pressure, the gland response is a graded function of low-frequency spike activity in the motor neurons. 4. Presumptive EJPs recorded from opaline gland cells are reversibly decreased in size by high extracellular Mg2+ and reversibly increased in size by raising the concentration of extracellular Ca2+. These results suggest that the presumptive EJPs are chemically mediated. The presumptive EJPs show facilitation and posttetanic potentiation. 5. The identified opaline motor neurons may constitute a significant portion of the motor input to the opaline gland via nerve P5 since hyperpolarization of the cells prevents the opaline gland response elicited by right connective stimulation in vitro. 6. We have compared the properties of the opaline motor neurons with the previously identified properties of the ink motor neurons (6--9, 19). Like the ink motor neurons, the opaline motor neurons have high resting potentials, are electrically coupled, and have no spontaneous spike activity. They also receive a slow and long-lasting evoked depolarizing synaptic input, which appears to be mediated by a decreased conductance mechanism. The firing pattern of the opaline motor neurons produced by synaptic input shows the same delayed bursting pattern previously described for the ink motor neurons. 7. The biophysical properties and synaptic input to the ink motor neurons have been shown to affect the features of inking behavior (4, 6--9, 19). The opaline motor neurons share some of these biophysical characteristics and mediate a defensive behavior similar to ink release. Further comparisons of these behaviors and their underlying neural circuits may provide a better understanding of the extent to which cellular biophysical properties and patterns of synaptic input influence the features of the behaviors that individual neurons mediate.