Cell Calcium 42 (2007) 59–69 Role of mitochondria in kainate-induced fast Ca 2+ transients in cultured spinal motor neurons Julian Grosskreutz a,c,d,∗ , Kirsten Haastert b,d , Maarten Dewil f , Philip Van Damme f , Geert Callewaert e , Wim Robberecht f , Reinhard Dengler c,d , Ludo Van Den Bosch f a Academic Neurology Unit, University of Sheffield, Medical School, United Kingdom b Department of Neuroanatomy, Medical School Hannover, Carl-Neuberg-Strasse 1, 30625 Hannover, Germany c Department of Neurology, Medical School Hannover, Germany d Center for Systems Neuroscience (ZSN), Hannover, Germany e Laboratory of Physiology, University of Leuven, Belgium f Laboratory of Neurobiology, University of Leuven, Campus Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium Received 5 January 2006; received in revised form 9 October 2006; accepted 7 November 2006 Available online 22 January 2007 Abstract 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 Ca 2+ homeostasis with near physiological AMPA receptor activation. Using fast solution exchange on patch-clamped cultured neurons, kainate (KA) was applied for 2s. This induced a transient increase in the cytosolic Ca 2+ concentration ([Ca 2+ ] c ) for seconds. Inhibition of the mitochondrial uniporter by RU-360 abolished the decay of the Ca 2+ transient and caused immediate [Ca 2+ ] c overload. Repetitive short KA stimulation caused a slowing of the decay of the Ca 2+ transient and a gradual increase in peak and baseline [Ca 2+ ] 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 Ca 2+ transients. We conclude that motor neurons have an insufficient mitochondrial capacity to buffer large Ca 2+ 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. © 2006 Elsevier Ltd. All rights reserved. Keywords: Amyotrophic lateral sclerosis; AMPA receptors; Excitotoxicity; Calcium-induced calcium release; Mitochondrial uniporter; Calcium imaging; Patch-clamp 1. Introduction Amyotrophic lateral sclerosis (ALS) is a fatal neurodegen- erative disease characterized by the death of motor neurons in the motor cortex, brain stem and spinal cord. This selective motor neuron degeneration results in death of the patient after a mean disease duration of 3 years. The only drug proven to ∗ Corresponding author at: Academic Neurology Unit, University of Sheffield, Medical School, E-Floor, Beech Hill Road, Sheffield S10 2RX, United Kingdom. Tel.: +44 114 226 1311; fax: +44 114 226 1201. E-mail address: j.grosskreutz@sheffield.ac.uk (J. Grosskreutz). slow ALS in patients is riluzole which mediates at least part of its neuroprotective effect via a presynaptic reduction of glutamatergic stimulation of motor neurons [1]. About 10% of ALS cases are familial and 20 % of these are caused by mutations in the superoxide dismutase 1 (SOD1) gene. Despite intensive research, the exact nature of the gain of function for which mutations in SOD1 are responsible is not yet clarified (for a review see Ref. [2]) but AMPA receptor-mediated excitotoxicity seems to be involved. First, mutant SOD1 can damage the glial glutamate transporter responsible for the removal of glutamate from the synap- tic cleft [3]. Second, AMPA receptor antagonists prolonged 0143-4160/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ceca.2006.11.010