Presynaptic mitochondria in functionally different motor neurons exhibit similar affinities for Ca2+ but exert little influence as Ca2+ buffers at nerve firing rates in situ.

Mitochondria accumulate within nerve terminals and support synaptic function, most notably through ATP production. They can also sequester Ca(2+) during nerve stimulation, but it is unknown whether this limits presynaptic Ca(2+) levels at physiological nerve firing rates. Similarly, it is unclear whether mitochondrial Ca(2+) sequestration differs between functionally different nerve terminals. We addressed these questions using a combination of synthetic and genetically encoded Ca(2+) indicators to examine cytosolic and mitochondrial Ca(2+) levels in presynaptic terminals of tonic (MN13-Ib) and phasic (MNSNb/d-Is) motor neurons in Drosophila, which, as we determined, fire during fictive locomotion at approximately 42 Hz and approximately 8 Hz, respectively. Mitochondrial Ca(2+) sequestration starts in both terminals at approximately 250 nM, exhibits a similar Ca(2+)-uptake affinity (approximately 410 nM), and does not require Ca(2+) release from the endoplasmic reticulum. Nonetheless, mitochondrial Ca(2+) uptake in type Is terminals is more responsive to low-frequency nerve stimulation and this is due to higher cytosolic Ca(2+) levels. Since type Ib terminals have a higher mitochondrial density than Is terminals, it seemed possible that greater mitochondrial Ca(2+) sequestration may be responsible for the lower cytosolic Ca(2+) levels in Ib terminals. However, genetic and pharmacological manipulations of mitochondrial Ca(2+) uptake did not significantly alter nerve-stimulated elevations in cytosolic Ca(2+) levels in either terminal type within physiologically relevant rates of stimulation. Our findings indicate that presynaptic mitochondria have a similar affinity for Ca(2+) in functionally different nerve terminals, but do not limit cytosolic Ca(2+) levels within the range of motor neuron firing rates in situ.