Acid-sensing Ion Channels 1a (ASIC1a) inhibition of neuromuscular synaptic transmission, their role in skeletal muscle fatigue

LINO, NOELIA; URBANO, FRANCISCO; GONZÁLEZ-INCHAUSPE, CARLOTA; GONZÁLEZ, LAURA; LÓPEZ, MARÍA EUGENIA; WUNSCH, AMANDA; WEMMIE, JOHN; UCHITEL, OSVALDO

Washington DC

Congreso; 41th Annual Meeting of Neuroscience 2011; 2011

Society for Neuroscience

CNS changes in extracellular pH have considerable influence on the physiology of neurons. At the neuromuscular junction (NMJ), H+ are co-release with acethylcholine and are pump into the synaptic cleft after prolonged stimulation (Michaelson & Angel, 1980; Füldner & Stadler, 1982; Tabares et al., 2007). Recently, acid-sensing ion channels (ASICs) has been identified and proposed to regulate neuromuscular synaptic transmission in response to acidosis. Thus, our first goal was to elucidate the role of ASIC1a channels at the NMJs from levator auris longus muscles. We found that ASIC1a immunofluorescence co-localized with the presynaptic marker synaptophysin. Genetically disrupting ASIC1a (ASIC1a-/- mice) or inhibiting them with psalmotoxin increased the frequency of spontaneous miniature endplate potentials (MEPPs), while lowering extracellular pH to 6.0 reduced the frequency of MEPPs. Also, endplate potential facilitation during trains of repetitive stimulation (75 Hz, 5 sec) was significantly increased in the absence of ASIC1a. Furthermore, activation of ASIC1a inhibited FM2-10-labeled vesicle turnover, evidencing faster FM2-10 time courses of distaining for ASIC1a-/- NMJs compared to wildtypes, affecting both fast and slow releasable pools.Considering our evidence and the fact that acidosis may have a protective role against muscle fatigue, we then investigated the influence of ASIC1a over skeletal muscle physiology. In order to evaluate muscle strength, the animals were hanged from a wire grid with their front paws, while their tails were pulled backwards. ASIC1a-/- mice exhibited more strength than wildtype mice (i.e.: grip strength test, 0,33±0,03 kg (n=11) and 0,27±0,01 kg (n=9), respectively; Student´s t-test, p<0.05). Conversely, a significant decrease was observed in muscle endurance -quantified by measuring the latency to fall after mice were hanged from a wire- for ASIC1a-/- compared to wildtype mice (i.e.: wire hang test, 2,18±0,36 min (n=20) and 17,35±3,27 min (n=12), respectively; Student?s t-test, p<0.001). Furthermore, we recorded in vitro muscle contraction at different stimulation frequencies from ASIC1a-/- and wildtype mice.Together our results suggest that ASIC1a are present at the NMJ of adult mice, where they may exert a negative modulation of synaptic transmission in response to changes in the extracellular pH. In addition, marked differences observed in muscle strength between wildtype and ASIC1a-/- mice support the idea that ASIC1a could have an important role preventing skeletal muscle fatigue.