ALTERED FUNCTION OF CAV2.1 CALCIUM CHANNELS LEADS TO CHANGES IN SYNAPTIC TRANSMISSION IN BRAIN STEM SYNAPSES FROM GENETICALLY MODIFIED CACNA1A MICE

CARLOTA GONZÁLEZ INCHAUSPE; MARIANO DI GUILMI; ARN M.J.M VAN DEN MAAGDENBERG; RUNE R. FRANTS; MICHEL D. FERRARI; IAN D. FORSYTHE; OSVALDO D. UCHITEL

Givat Ram Campus of the Hebrew University of Jerusalem. Israel

Otro; School on "The Presynaptic Terminal: Assembly and Function"; 2007

Institute for Advanced Studies, Givat Ram Campus of the Hebrew University of Jerusalem

Genetic analyses have revealed an important association of the gene encoding the P/Q-type voltage dependent Ca2+ channel a1A subunit with hereditary neurological disorders. The generation of a1A-null mutant mice (KO) (Hee-Sup Shin et al. 1999, PNAS 96: 15245) and a knockin mice with the R192Q familial hemiplegic migraine (FHM1) mutation (van den Maagdenberg et al., 2004, Neuron 41: 701) allows a critical examination of features of neurotransmission dependent on Ca2+ influx. We use auditory brainstem slices containing the Medial Nucleus of the Trapezoid Body (MNTB), whose principal neurons receive a giant synapse (calyx of Held). This synapse is glutamatergic and forms part of the binaural auditory pathway responsible for sound localization. Whole cell patch clamp technique is used to measure presynaptic Ca2+ currents (IpCa) at the calyx of Held nerve terminal and neurotransmitter induced excitatory postsynaptic currents (EPSCs) at the MNTB neurons.       We investigate for both mutant mice, the electrophysiological properties of calcium currents and the alterations in the efficacy of synaptic activity compared to control WT mice. Some particular aspects under study are IpCa  amplitude and kinetic (current-voltage relationship, activation, deactivation and inactivation properties), the relation between calcium currents and EPSCs, short term plasticity phenomena under high frequency synaptic transmission, activity dependent facilitation of IpCa  and EPSCs and presynaptic modulation of transmitter release through G protein-coupled receptors. The current–voltage (I–V) relationship of presynaptic Ca2+ currents revealed that Cav2.1 Ca2+ channels activate at more negative potentials in R192Q KI and at more positive potentials in the KO mice, suggesting an increase and decrease of action potential induced Ca2+ influx, respectively.       No differences were observed in the amplitude of postsynaptic currents at the calyx of the KI animals, however striking differences were found in the recovery time after short term depression (STD). Repetitive firings cause STD, due to rapid depletion of synaptic vesicles, which recovers after many seconds of rest during which replenishment of the readily releasable pool of synaptic vesicles occurs. Recovery from STD induced by 10 and 100 Hz stimulation of the MNTB afferent axons is significantly faster in KI than in the WT animals. On the contrary, at the calyx of P/Q KO animals where P/Q-type Ca2+ channels are replaced by N-type channels, the recovery process is slowed down.       These results are in line with the hypothesis of a gain of function of the KI mutation with an increase in Ca2+ influx during the action potential and with the loss of function observed at the KO synapses due to the lack of P/Q channels.       It has been proposed that both release and replenishment of synaptic vesicles are highly dependent on the level of Ca2+ influx. Such influx may provide an important gain control mechanism to adjust synaptic strength. Therefore, alterations in the degree or kinetics of these phenomena could result in important changes in network activity leading to ataxia epilepsy and migraine.