GCaMP6f-expressing chromaffin cells in murine adrenal slices exhibit dynamic spontaneous calcium responses that do not require nerve input

Chromaffin cells of the adrenal medulla exhibit spontaneous action potentials and intracellular Ca2+ responses that, when dissociated in vitro, result in catecholamine secretion. However, spontaneous activity in these cells is nearly abolished when splanchnic nerve input is blocked in vivo. To address this discrepancy, we examined spontaneous Ca2+ responses in chromaffin cells using an intermediate preparation: adrenal slices from transgenic mice expressing the genetically encoded Ca2+ indicator GCaMP6f selectively in these cells. Within this preparation, the three-dimensional structure and extracellular environment surrounding chromaffin cells are retained, as is input from distal splanchnic nerve terminals, which spontaneously release the neurotransmitter acetylcholine. Chromaffin cells within these slices displayed spontaneous Ca2+ responses with frequencies and amplitudes that varied greatly within and between individual cells. However, population averages remained stable, providing a tool to measure the cellular and molecular mechanisms underlying these responses. While overall average frequencies and amplitudes of spontaneous Ca2+ responses depended on the influx of extracellular Ca2+ through voltage-gated Ca2+ channels, they did not require Ca2+ release from intracellular stores, splanchnic nerve input, or activation of voltage-gated sodium channels. Together, these results suggest that spontaneous activity in chromaffin cells in adrenal slices is generated autonomously, similar to that in dissociated chromaffin cells. By contrast, spontaneous activity in chromaffin cells in intact animals likely represents a distinct form that depends on basal input from the intact splanchnic nerve. This study provides a foundation to further explore the diverse mechanisms mediating chromaffin cell activation.