GCaMP6f-expressing chromaffin cells in murine adrenal slices exhibit dynamic spontaneous calcium responses that do...

Chromaffin cells of the adrenal medulla exhibit spontaneous action potentials and intracellular Ca²⁺ responses that result in catecholamine secretion when dissociated in vitro. 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 in 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 is 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 intact splanchnic nerve. This study provides a foundation to further explore the diverse mechanisms mediating chromaffin cell activation.