Role of mitochondria in kainate-induced fast Ca²⁺ transients in cultured spinal motor neurons

Motor neuron death in amyotrophic lateral sclerosis (ALS) has been linked to selective vulnerability towards AMPA receptor-mediated excitotoxicity. We investigated intracellular mechanisms leading to impairment of motor neuron Ca²⁺ homeostasis with near physiological AMPA receptor activation. Using fast solution exchange on patch-clamped cultured neurons, kainate (KA) was applied for 2 s. This induced a transient increase in the cytosolic Ca²⁺ concentration ([Ca²⁺]_c) for seconds. Inhibition of the mitochondrial uniporter by RU-360 abolished the decay of the Ca²⁺ transient and caused immediate [Ca²⁺]_c overload. Repetitive short KA stimulation caused a slowing of the decay of the Ca²⁺ transient and a gradual increase in peak and baseline [Ca²⁺]_c in motor neurons, but not in other neurons, indicating saturation of the mitochondrial buffer. Furthermore, mitochondrial density was lower in motor neurons and, in a network of neurons with physiological synaptic AMPA receptor input, RU-360 acutely induced an increase in Ca²⁺ transients. We conclude that motor neurons have an insufficient mitochondrial capacity to buffer large Ca²⁺ elevations which is partly due to a reduced mitochondrial density per volume compared to non-motor neurons. This may exert deleterious effects in motor neuron disease where mitochondrial function is thought to be compromised.