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The Journal of Neuroscience, October 15, 2002, 22(20):9063-9069

Neurohormone Secretion Persists after Post-Afterdischarge Membrane Depolarization and Cytosolic Calcium Elevation in Peptidergic Neurons in Intact Nervous Tissue

Stephan Michel and Nancy L. Wayne

Department of Physiology, David Geffin School of Medicine at University of California at Los Angeles, Los Angeles, California 90095

The purpose of this work was to test the hypothesis that an electrical afterdischarge (AD) causes prolonged elevation in cytosolic calcium levels that is associated with prolonged secretion of egg-laying hormone (ELH) from peptidergic neurons in intact nervous tissue of Aplysia. Using a combination of radioimmunoassay measurement of ELH secretion, electrophysiological measurement of membrane potential, and optical imaging of the concentration of intracellular free calcium ions ([Ca²⁺]_i), we verified that there was persistent secretion of ELH after the end of the AD; this was accompanied by prolonged post-AD membrane depolarization and prolonged post-AD elevation in [Ca²⁺]_i. Extracellular treatment with the calcium chelator EGTA had no effect on the pattern or magnitude of ELH secretion or on the post-AD membrane potential (V_m) and post-AD Ca²⁺ signal, ruling out a role for extracellular calcium in the post-AD elevation of [Ca²⁺]_i. Both V_m and [Ca²⁺]_i returned to baseline well before ELH secretion, such that neither prolonged membrane depolarization nor prolonged Ca²⁺ signaling can fully account for the extent of the persistent secretion of ELH. These findings suggest a unique relationship between membrane excitability, Ca²⁺ signaling, and prolonged neuropeptide secretion.

Key words: action potential; Aplysia; bag cell neurons; calcium imaging; calcium signaling; egg-laying hormone; exocytosis; membrane potential; neuroendocrine; neurosecretion

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Copyright © 2002 Society for Neuroscience  0270-6474/02/22209063-07$05.00/0

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