Gain of Function in Fhm - 1 Cav 2 . 1 Knock - in Mice Is Related to the Shape of 1 the Action Potential

22 23 Familial hemiplegic migraine type-1 (FHM1) is caused by missense mutations in the CACNA1A 24 gene that encodes the α1A pore-forming subunit of CaV2.1 Ca channels. We used knock-in (KI) 25 transgenic mice harbouring the pathogenic FHM-1 mutation R192Q to study neurotransmission at the 26 calyx of Held synapse and cortical layer 2/3 pyramidal cells (PCs). Using whole cell patch clamp 27 recordings in brainstem slices we confirmed that KI CaV2.1 Ca channels activated at more 28 hyperpolarizing potentials. However, calyceal presynaptic calcium currents (IpCa) evoked by 29 presynaptic action potentials (APs) were similar in amplitude, kinetic parameters and neurotransmitter 30 release. 31 CaV2.1 Ca channels in cortical layer 2/3 PCs from KI mice also showed a negative shift in their 32 activation voltage. PCs had APs with longer durations and smaller amplitudes than the calyx of Held. 33 AP evoked Ca currents (ICa) from PCs were larger in KI compared to WT mice. In contrast, when ICa 34 were evoked in PCs by calyx of Held AP waveforms, we observed no amplitude differences between 35 WT and KI mice. In the same way, Ca currents evoked at the presynaptic terminals (IpCa) of the calyx 36 of Held by the AP waveforms of the PCs had larger amplitudes in R192Q KI mice that in WT. These 37 results suggest that longer time courses of pyramidal APs were a key factor for the expression of a 38 synaptic gain of function in the KI mice. In addition, our results indicate that consequences of FHM1 39 mutations might vary according to the shape of APs in charge of triggering synaptic transmission 40 (neurons in the calyx of Held vs. excitatory/inhibitory neurons in the cortex), adding to the complexity 41 of the pathophysiology of migraine. 42 43 Key-words: knock-in mice, CaV2.1 channels, migraine mutation, calyx of Held, cortical pyramidal 44 cells. 45 46 47