Divergent effects of painful nerve injury on mitochondrial Ca²⁺ buffering in axotomized and adjacent sensory neurons

Mitochondria critically regulate cytoplasmic Ca²⁺ concentration ([Ca²⁺]_c), but the effects of sensory neuron injury have not been examined. Using FCCP (1 μM) to eliminate mitochondrial Ca²⁺ uptake combined with oligomycin (10 μM) to prevent ATP depletion, we first identified features of depolarization-induced neuronal [Ca²⁺]_c transients that are sensitive to blockade of mitochondrial Ca²⁺ buffering in order to assess mitochondrial contributions to [Ca²⁺]_c regulation. This established the loss of a shoulder during the recovery of the depolarization (K⁺)-induced transient, increased transient peak and area, and elevated shoulder level as evidence of diminished mitochondrial Ca²⁺ buffering. We then examined transients in Control neurons and neurons from the 4th lumbar (L4) and 5th lumbar (L5) dorsal root ganglia after L5 spinal nerve ligation (SNL). The SNL L4 neurons showed decreased transient peak and area compared to control neurons, while the SNL L5 neurons showed increased shoulder level. Additionally, SNL L4 neurons developed shoulders following transients with lower peaks than Control neurons. Application of FCCP plus oligomycin elevated resting [Ca²⁺]_c in SNL L4 neurons more than in Control neurons. Whereas application of FCCP plus oligomycin 2 s after neuronal depolarization initiated mitochondrial Ca²⁺ release in most Control and SNL L4 neurons, this usually failed to release mitochondrial Ca²⁺ from SNL L5 neurons. For comparable cytoplasmic Ca²⁺ loads, the releasable mitochondrial Ca²⁺ in SNL L5 neurons was less than Control while it was increased in SNL L4 neurons. These findings show diminished mitochondrial Ca²⁺ buffering in axotomized SNL L5 neurons but enhanced Ca²⁺ buffering by neurons in adjacent SNL L4 neurons.