Résumé :
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Congenital myasthenic syndromes are characterized by muscle weakness during exercice. The mutations affects key events of the synaptic transmission but the consequence on the muscle contraction and the pathophysiology are poorly understood. Indeed, the utilization of AChE inhibitors to rescue the muscle function is quite unpredictable. The complete inhibition of AChE leads to a complete arrest of the tetanic contractions whereas the muscle contracts well in chronic absence of AChE. To revisit the consequences of the synaptic transmission on the muscle contraction, we have constructed a new set-up to record the strength of the diaphragm during the time after electrical stimulations ex-vivo. The sensitivity and the dynamic of our new set-up allow the quantification of the muscle strength over the time triggered by non-tetanic stimulations. To challenge the synaptic transmission without alteration of contractile properties of the muscle and to analyze the synaptic events, we used only train of 4 stimulations at different frequencies. When we applied the four stimulations at 2Hz each 30 sec, the amplitude and the shape of the 4 contractions are identical during hours at 32_C. To ensure the fast inhibition of AChE, we used 3?M neostigmine, an AChE inhibitor. The acute inhibition of AChE changes dramatically the amplitude of the 4 contractions within minutes after application of neostigmine. After 20 min, the maximal strength of the muscle was reduced by half. In contrast in absence of AChE, the amplitude of twitch contraction is higher than in wt, and the amplitude fading of the 4th contraction versus the 1rst is around 0.7, a signature of a muscle weakness. It is in general believed that the reduction of the contraction is due to the desensitization of the nAChR, but it is not clear how the reduction of the size of the synaptic area in absence of AChE prevents this desensitization. Alternatively, the fast dynamic of the change during the 10 min after inhibition of AChE may suggest a rapid adaptation of the synaptic transmission. To evaluate whether the reduction the density of nAChR may prevent the reduction of the twitch contraction after AChE inhibition, we blocked part of nAChR with _-bungarotoxin. After application of neostigmine, the muscle is affected with the temporary variations of the contraction, but not the reduction after 20min. We will present complementary experiments to explain at which level the synaptic transmission is changed.
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