Increased EPSCs but decreased presynaptic Ca2+ influx in S218L Cav2.1 knock-in migraine mice.

MN DI GUILMI ; C GONZÁLEZ INCHAUSPE; F J URBANO; ID FORSYTHE; A VAN DEN MAAGDENBERG ; JGG BORST; OD UCHITEL

Washington DC

Congreso; Annual Meeting of the Society for Neuroscience; 2011

Society for Neuroscience

Genes encoding the P/Q-type voltage dependent Ca2+ channel CaV2.1 subunits have been linked to hereditary neurological disorders. The generation of CaV2.1 knock-in (KI) transgenic mice with the S218L familial hemiplegic migraine (FHM1) mutation (Tottene et. al. 2005) have allowed to examine physiological features of neurotransmission dependent on Ca2+ influx. We used KI S218L mice to study presynaptic Ca2+ currents (IpCa) and excitatory postsynaptic currents (EPSCs) at the calyx of Held. During whole-cell patch-clamp recordings, a shift of the peak of the I-V curve to more negative potentials was observed (Vm for peak IpCa current density; WT: -10 mV, n = 10; KI: -20 mV, n = 5). Steady-state activation curves were also significantly shifted between WT and KI mice, activating at more negative potentials in the transgenic model (-23.0 ± 1.7 mV, n = 7 for WT and -36.7±6.5 mV, n = 5 for KI). The time course of activation of IpCa was faster in the KI than in WT mouse in the range between -30 and + 20 mV. On the other hand, presynaptic calcium currents (IpCa) evoked by action potential (AP) waveforms had less amplitude in KI than WT (WT: 1.6±0.2 nA, n=9 and KI:0.9±02 nA, n=5). Additionally, Ca2+ current facilitation after 100 Hz train of APs was significant reduced in KI compared to WT mice (last amplitude/first amplitude*100: 20±3%, n=8 for WT and 7±2%, n=5 for KI). Both EPSC amplitudes (KI: 8.5±1.2 nA, n=? and WT: 5.5±0.5 nA, n=?) and miniature EPSC frequencies (KI: 8.6±2.5 Hz and WT: 1.6±0.3 Hz) were significant higher in KI than in WT. Although the release probability was the same for both genotypes, the size of the releasable pool was higher for KI. The synaptic activity in the transgenic model showed less rate of short term depression with a faster recovery after either 10 or 100 Hz frequency trains.  Our results suggest that the calcium channel activation shift might be increasing calcium influx at resting membrane potential. Such calcium concentration increment would lead to a decrement in action potential-induced calcium influx due to channel inactivation. Moreover, an acceleration of calcium dependent processes on the exocitosis machinery might explain the observed decrement in EPSC amplitude regardless of the observed reduction in IpCa.