Ca2+-Induced Ca2+Release inAplysiaBag Cell Neurons Requires Interaction Between Mitochondrial and Endoplasmic Reticulum Stores

Abstract

Intracellular Ca2+is influenced by both Ca2+influx and release. We examined intracellular Ca2+following action potential firing in the bag cell neurons of Aplysia californica. Following brief synaptic input, these neuroendocrine cells undergo an afterdischarge, resulting in elevated Ca2+and the secretion of neuropeptides to initiate reproduction. Cultured bag cell neurons were injected with the Ca2+indicator, fura-PE3, and subjected to simultaneous imaging and electrophysiology. Delivery of a 5-Hz, 1-min train of action potentials (mimicking the fast phase of the afterdischarge) produced a Ca2+rise that markedly outlasted the initial influx, consistent with Ca2+-induced Ca2+release (CICR). This response was attenuated by about half with ryanodine or depletion of the endoplasmic reticulum (ER) by cyclopiazonic acid. However, depletion of the mitochondria, with carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone, essentially eliminated CICR. Dual depletion of the ER and mitochondria did not reduce CICR further than depletion of the mitochondria alone. Moreover, tetraphenylphosphonium, a blocker of mitochondrial Ca2+release, largely prevented CICR. The Ca2+elevation during and subsequent to a stimulus mimicking the full afterdischarge was prominent and enhanced by protein kinase C activation. Traditionally, the ER is seen as the primary Ca2+source for CICR. However, bag cell neuron CICR represents a departure from this view in that it relies on store interaction, where Ca2+released from the mitochondria may in turn liberate Ca2+from the ER. This unique form of CICR may be used by both bag cell neurons, and other neurons, to initiate secretion, activate channels, or induce gene expression.