We have found that two endogenous neuropeptides in Aplysia, the small cardioactive peptides SCPA and SCPB, facilitate synaptic transmission from siphon mechano-sensory neurons and enhance the defensive withdrawal reflex that these sensory neurons mediate. Single-channel recording revealed that these peptides close a specific K+ channel, the S channel, which is sensitive to cAMP. Moreover, the peptides increase cAMP levels in these sensory neurons. This reduction in K+ current slows the repolarization of the action potential in these cells, which increases transmitter release. In these actions, the SCPs resemble both noxious sensitizing stimuli, which enhance the reflex, and serotonin. Bioassay of HPLC fractions of abdominal ganglion extracts and immunocytochemistry indicate that both the SCPs and serotonin are present in the ganglion and are found in processes close to the siphon sensory neurons, suggesting that these transmitters may be involved in behavioral sensitization. Recent evidence suggests that one group of identified facilitatory interneurons, the L29 cells, does not appear to contain either the SCPs or serotonin but may use yet another facilitatory transmitter. Thus, it appears that several transmitters can converge to produce presynaptic facilitation in the sensory neurons of the defensive withdrawal reflex. All of the transmitters studied here, the SCPs and serotonin, act via an identical molecular cascade: cAMP-dependent closure of the S-K+ channel, broadening of the presynaptic action potential, and facilitation of transmitter release.

Abrams, T. W., Castellucci, V. F., Camardo, J. S., Kandel, E. R., & Lloyd, P. E. (1984). Two endogenous neuropeptides modulate the gill and siphon withdrawal reflex in Aplysia by presynaptic facilitation involving cAMP-dependent closure of a serotonin-sensitive potassium channel. Proceedings of the National Academy of Sciences, 81(24), 7956–7960.