Bioluminescence imaging of mitochondrial Ca2+dynamics in soma and neurites of individual adult mouse sympathetic neurons

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Abstract
Changes in the cytosolic Ca(2+) concentration ([Ca(2+)](c)) are essential for triggering neurotransmitter release from presynaptic nerve terminals. Calcium-induced Ca(2+) release (CICR) from the endoplasmic reticulum (ER) may amplify the [Ca(2+)](c) signals and facilitate neurotransmitter release in sympathetic neurons. In adrenal chromaffin cells, functional triads are formed by voltage-operated Ca(2+) channels (VOCCs), CICR sites and mitochondria. In fact, mitochondria take up most of the Ca(2+) load entering the cells and are essential for shaping [Ca(2+)](c) signals and exocytosis. Here we have investigated the existence of such functional triads in sympathetic neurons. The mitochondrial Ca(2+) concentration ([Ca(2+)](m)) in soma and neurites of individual mouse superior cervical ganglion (SCG) neurons was monitored by bioluminescence imaging of targeted aequorins. In soma, Ca(2+) entry through VOCCs evoked rapid, near millimolar [Ca(2+)](m) increases in a subpopulation of mitochondria containing about 40% of the aequorin. Caffeine evoked a similar [Ca(2+)](m) increase in a mitochondrial pool containing about 30% of the aequorin and overlapping with the VOCC-sensitive pool. These observations suggest the existence of functional triads similar to the ones described in chromaffin cells. In neurites, mitochondria were able to buffer [Ca(2+)](c) increases resulting from activation of VOCCs but not those mediated by caffeine-induced Ca(2+) release from the ER. The weaker Ca(2+) buffering by mitochondria in neurites could contribute to facilitate Ca(2+)-induced exocytosis at the presynaptic sites.