Calcium channels of the P/Q subtype mediate transmitter release at the neuromuscular junction and at many central synapses, such as the calyx of Held. Transgenic mice in which ␣ 1A channels are ablated provide a powerful tool with which to test compensatory mechanisms at the synapse and to explore mechanisms of presynaptic regulation associated with expression of P/Q channels. Using the calyx of Held preparation from the knock-out (KO) mice, we show here that N-type channels functionally compensate for the absence of P/Q subunits at the calyx and evoke giant synaptic currents [approximately two-thirds of the magnitude of wild-type (WT) responses]. However, although evoked paired-pulse facilitation is prominent in WT, this facilitation is greatly diminished in the KO. In addition, direct recording of presynaptic calcium currents revealed that the major functional difference was the absence of calcium-dependent facilitation at the calyx in the P/Q KO animals. We conclude that one physiological function of P/Q channels is to provide additional facilitatory drive, so contributing to maintenance of transmission as vesicles are depleted during high throughput synaptic transmission.

Specific missense mutations in the CACNA1A gene, which encodes a subunit of voltage-gated Ca V 2.1 channels, are associated with familial hemiplegic migraine type 1 (FHM1), a rare monogenic subtype of common migraine with aura. We used transgenic knock-in (KI) mice harboring the human pathogenic FHM1 mutation S218L to study presynaptic Ca 2ϩ currents, EPSCs, and in vivo activity at the calyx of Held synapse. Whole-cell patch-clamp recordings of presynaptic terminals from S218L KI mice showed a strong shift of the calcium current I-V curve to more negative potentials, leading to an increase in basal [Ca 2ϩ ] i , increased levels of spontaneous transmitter release, faster recovery from synaptic depression, and enhanced synaptic strength despite smaller action-potential-elicited Ca 2ϩ currents. The gain-of-function of transmitter release of the S218L mutant was reproduced in vivo, including evidence for an increased release probability, demonstrating its relevance for glutamatergic transmission. This synaptic phenotype may explain the misbalance between excitation and inhibition in neuronal circuits resulting in a persistent hyperexcitability state and other migraine-relevant mechanisms such as an increased susceptibility to cortical spreading depression.

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González Inchauspe C, Urbano FJ, Di Guilmi MN, Forsythe ID, Ferrari MD, van den Maagdenberg AMJM, Uchitel OD. Gain of function in FHM-1 Ca v 2.1 knock-in mice is related to the shape of the action potential. J Neurophysiol 104: 291-299, 2010. First published May 19, 2010 doi:10.1152/jn.00034.2010. Familial hemiplegic migraine type-1 FHM-1 is caused by missense mutations in the CACNA1A gene that encodes the ␣ 1A pore-forming subunit of Ca V 2.1 Ca 2ϩ channels. We used knock-in (KI) transgenic mice harboring the pathogenic FHM-1 mutation R192Q to study neurotransmission at the calyx of Held synapse and cortical layer 2/3 pyramidal cells (PCs). Using whole cell patch-clamp recordings in brain stem slices, we confirmed that KI Ca V 2.1 Ca 2ϩ channels activated at more hyperpolarizing potentials. However, calyceal presynaptic calcium currents (I pCa ) evoked by presynaptic action potentials (APs) were similar in amplitude, kinetic parameters, and neurotransmitter release. Ca V 2.1 Ca 2ϩ channels in cortical layer 2/3 PCs from KI mice also showed a negative shift in their activation voltage. PCs had APs with longer durations and smaller amplitudes than the calyx of Held. AP-evoked Ca 2ϩ currents (I Ca ) from PCs were larger in KI compared with wild-type (WT) mice. In contrast, when I Ca was evoked in PCs by calyx of Held AP waveforms, we observed no amplitude differences between WT and KI mice. In the same way, Ca 2ϩ currents evoked at the presynaptic terminals (I pCa )of the calyx of Held by the AP waveforms of the PCs had larger amplitudes in R192Q KI mice that in WT. These results suggest that longer time courses of pyramidal APs were a key factor for the expression of a synaptic gain of function in the KI mice. In addition, our results indicate that consequences of FHM-1 mutations might vary according to the shape of APs in charge of triggering synaptic transmission (neurons in the calyx of Held vs. excitatory/inhibitory neurons in the cortex), adding to the complexity of the pathophysiology of migraine.