Key points• So far, increased excitability and calcium handling problems have been discussed as causes for motoneuron death in amyotrophic lateral sclerosis (ALS) mainly on the basis of studies in juvenile presymptomatic mice.• We developed a brainstem preparation to analyse excitability and calcium handling during disease progression up to disease endstage of motoneurons in an ALS mouse model. • Increased excitability of motoneurons is not seen at disease endstage, challenging this factor as a direct cause for motoneuron death in ALS.• We show that calcium handling is remodelled during disease progression from mitochondrial uptake to mitochondrial uptake failure and increased plasma membrane extrusion, providing a compensatory mechanism that fails at disease endstage and might lead to a toxic calcium overload of the cells. • Supporting this newly described compensatory endeavour of the motoneurons might be a promising therapeutic strategy.Abstract Amyotrophic lateral sclerosis is a progressive neurodegenerative disease that targets some somatic motoneuron populations, while others, e.g. those of the oculomotor system, are spared. The pathophysiological basis of this pattern of differential vulnerability, which is preserved in a transgenic mouse model of amyotrophic lateral sclerosis (SOD1 G93A ), and the mechanism of neurodegeneration in general are unknown. Hyperexcitability and calcium dysregulation have been proposed by others on the basis of data from juvenile mice that are, however, asymptomatic. No studies have been done with symptomatic mice following disease progression to the disease endstage. Here, we developed a new brainstem slice preparation for whole-cell patch-clamp recordings and single cell fura-2 calcium imaging to study motoneurons in adult wild-type and SOD1 G93A mice up to disease endstage. We analysed disease-stage-dependent electrophysiological properties and intracellular Ca 2+ handling of vulnerable hypoglossal motoneurons in comparison to resistant oculomotor neurons. Thereby, we identified a transient hyperexcitability in presymptomatic but not in endstage vulnerable motoneurons. Additionally, we revealed a remodelling of intracellular Ca 2+ clearance within vulnerable but not resistant motoneurons at disease endstage characterised by a reduction of uniporter-dependent mitochondrial Ca 2+ uptake and enhanced Ca 2+ extrusion across the plasma membrane. Our study challenged the notion that hyperexcitability is a direct cause

Burst firing in medial substantia nigra (mSN) dopamine (DA) neurons has been selectively linked to novelty-induced exploration behavior in mice. Burst firing in mSN DA neurons, in contrast to lateral SN DA neurons, requires functional ATP-sensitive potassium (K-ATP) channels both in vitro and in vivo. However, the precise role of K-ATP channels in promoting burst firing is unknown. We show experimentally that L-type calcium channel activity in mSN DA neurons enhances open probability of K-ATP channels. We then generate a mathematical model to study the role of Ca dynamics driving K-ATP channel function in mSN DA neurons during bursting. In our model, Ca influx leads to local accumulation of ADP due to Ca-ATPase activity, which in turn activates K-ATP channels. If K-ATP channel activation reaches levels sufficient to terminate spiking, rhythmic bursting occurs. The model explains the experimental observation that, in vitro, coapplication of NMDA and a selective K-ATP channel opener, NN414, is required to elicit bursting as follows. Simulated NMDA receptor activation increases the firing rate and the rate of Ca influx, which increases the activation of K-ATP. The model suggests that additional sources of hyperpolarization, such as GABAergic synaptic input, are recruited in vivo for burst termination or rebound burst discharge. The model predicts that NN414 increases the sensitivity of the K-ATP channel to ADP, promoting burst firing in vitro, and that that high levels of Ca buffering, as might be expected in the calbindin-positive SN DA neuron subpopulation, promote rhythmic bursting pattern, consistent with experimental observations in vivo. NEW & NOTEWORTHY Recently identified distinct subpopulations of midbrain dopamine neurons exhibit differences in their two primary activity patterns in vivo: tonic (single spike) firing and phasic bursting. This study elucidates the biophysical basis of bursts specific to dopamine neurons in the medial substantia nigra, enabled by ATP-sensitive K channels and necessary for novelty-induced exploration. A better understanding of how dopaminergic signaling differs between subpopulations may lead to therapeutic strategies selectively targeted to specific subpopulations.