Serotonergic inhibition of afterdischarge in peptidergic bag cells.

Abstract

AbstractThe neuroendocrine bag cells of Aplysia generate a long‐lasting afterdischarge causing the release of the peptide egg‐laying hormone (ELH) into the hemolymph (Stuart and Strumwasser, 1980). This afterdischarge activity can be elicited in vitro by electrical stimulation of a pleurovisceral connective (PVC) nerve. It has been shown previously that bag cell afterdischarge and egg‐laying behavior can also be induced by a purified neuroactive peptide isolated from the atrial gland in the reproductive tract (Heller et al., 1980). The authors find that the afterdischarge can be inhibited in a dose‐dependent manner by the extracellular application of low concentrations of serotonin (5 × 10−7 M). This inhibitory action of serotonin is blocked by the serotonin antagonist D‐butaclamol. Serotonin has been found to prevent egg laying (and associated behaviors) induced by injection of this atrial gland peptide in vivo but not that induced by injection of ELH. Afterdischarge activity in bag cells consists of a short initial phase during which the spike frequency is high and a longer, secondary phase in which the spike frequency is much lower (Kupfermann and Kandel, 1970). Intracellular recordings during serotonin inhibition of afterdischarge indicate significant decreases in spike amplitude (15%) and duration (18%) compared with controls. In addition, the latency and threshold of directly elicited spikes, in the absence of afterdischarge, were increased considerably as a result of serotonin. Arguments are presented from these data that serotonin may be preventing the conductance decrease associated with the long‐lasting second phase of afterdischarge and that this effect of serotonin can be overcome by application of the potassium channel blocker tetraethylammonium (TEA). Although a serotonergic input to the bag cell neurons has not yet been described, the pleurovisceral connective nerves, known to contain bag cell process, have the highest concentration of serotonin receptors of any nervous tissue in Aplysia (Drummond, Bucher, and Levitan, 1980). It remains to be determined whether these represent serotonergic synapses on bag cell processes. The authors speculate that serotonin, considered a transmitter mediating sensitization (arousal) in other systems in Aplysia, may act to suppress an ongoing peptide hormone release, which would have triggered a behavior program in the freely behaving animal, thus allowing some degree of behavioral plasticity as a result of changed conditions in the external environment.