Positive feedback by autoexcitatory neuropeptides in neuroendocrine bag cells of Aplysia

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Abstract
Neurohormones are often secreted in large amounts from neuroendocrine cells during episodes of synchronous, repetitive spike activity. We report evidence that this pattern of activity in the neuroendocrine bag cells of Aplysia involves positive feedback by autoexcitatory transmitters. Intracellular stimulation of individual bag cells caused slow depolarizing afterpotentials and synchronous afterdischarges in the entire population of bag cells. Application of the bathing medium collected during bag cell activity mimicked these responses. Application of alpha-, beta-, or gamma-bag cell peptides (BCPs), 3 structurally related neuropeptides released from bag cells, also mimicked these responses. These autoexcitatory BCPs fulfill most of the strict criteria necessary for classification as neurotransmitters in this system. This is the first biological activity reported for beta- and gamma-BCPs and brings to 4 the number of bag cell neuropeptides derived from the egg-laying hormone/BCP precursor that are putative cotransmitters. Positive feedback by autoexcitatory transmission may provide a general mechanism for the generation of episodic activity in neuroendocrine systems.