Synaptic stimulation of Aplysia peptidergic neurons can activate hormone secretion in the absence of an afterdischarge

For over 20 years, the bag cell neurons of the marine mollusk Aplysia have been used to investigate second-messenger pathways that mediate effects of synaptic stimulation on ion currents and membrane excitability, presumably leading to exocytotic release of the neuropeptide egg-laying hormone (ELH). It is widely cited that a train of action potentials, called an afterdischarge, is necessary for activating cellular events leading to ELH secretion. Using a combination of electrophysiology, optical imaging of calcium signaling, and radioimmunoassay of ELH secretion, we show that an afterdischarge is not required for ELH secretion. Electrical stimulation that failed to produce afterdischarges but did lead to prolonged membrane depolarization and a rise in intracellular calcium concentration was sufficient to stimulate significant ELH release.