High-frequency Trains Of Action Potentials Research Articles

The Role of CaV2.1 Channel Facilitation in Synaptic Facilitation

SUMMARYActivation of CaV2.1 voltage-gated calcium channels is facilitated by preceding calcium entry. Such self-modulatory facilitation is thought to contribute to synaptic facilitation. Using knockin mice with mutated CaV2.1 channels that do not facilitate (Ca IM-AA mice), we surprisingly found that, under conditions of physiological calcium and near-physiological temperatures, synaptic facilitation at hippocampal CA3 to CA1 synapses was not attenuated in Ca IM-AA mice and facilitation was paradoxically more prominent at two cerebellar synapses. Enhanced facilitation at these synapses is consistent with a decrease in initial calcium entry, suggested by an action-potential-evoked CaV2.1 current reduction in Purkinje cells from Ca IM-AA mice. In wild-type mice, CaV2.1 facilitation during high-frequency action potential trains was very small. Thus, for the synapses studied, facilitation of calcium entry through CaV2.1 channels makes surprisingly little contribution to synaptic facilitation under physiological conditions. Instead, CaV2.1 facilitation offsets CaV2.1 inactivation to produce remarkably stable calcium influx during high-frequency activation.

Molecular Determinants of Photocurrent Kinetics of the Red Light Activable Channelrhodopsin Chrimson

Channelrhodopsin (ChR) are light gated ion channels which expressed in neurons enable the temporal and spacial precise regulation of membrane voltage and action potential firing by light [1]. Recently an extended screen of 61 potential ChR genes for photocurrents in human kidney cells discovered a far-red activable ChR from the freshwater algae Chlamydomonas noctigama CnChR1 also named Chrimson [2]. Chrimson shows peak photocurrents at 590 nm more than 50 nm further red shifted than previous ChRs and is therefor best suited for optogenetic experiments requiring deep tissue penetration or dual color activation with blue and red light. Photocurrent kinetics are key properties of ChRs determining their potential optogenetic application. Whereas fast photocurrent kinetics enable the temporally precise triggering of high frequency action potential trains, slow photocurrent kinetics increase the operational range of ChRs to lower light intensities and enable the prolonged manipulation of membrane voltage by single low intensity light pulses. In the well-characterized CrChR2 from Chlamydomonas reinhardtii the residue D156 has been identified as a key determinant of photocurrent kinetics possibly participating in the reprotonation of the retinal Schiff base [3]. In Chrimson this aspartate is not conserved. Interestingly photocurrent kinetics are still fast indicating different kinetic determinants in Chrimson than in CrChR2. By sight-directed mutagenesis we investigated amino acid substitution in Chrimson compared to CrChR2 in the inner gate, central gate and in the retinal binding pocket and functionally characterized kinetic properties of these mutants in whole cell patch clamp measurements. Whereas specific substitutions in all three regions of the protein were affecting channel kinetics, especially the retinal binding pocket proved essential for fast photocurrent kinetics.

Loss of sensory input increases the intrinsic excitability of layer 5 pyramidal neurons in rat barrel cortex.

Development of the cortical map is experience dependent, with different critical periods in different cortical layers. Previous work in rodent barrel cortex indicates that sensory deprivation leads to changes in synaptic transmission and plasticity in layer 2/3 and 4. Here, we studied the impact of sensory deprivation on the intrinsic properties of layer 5 pyramidal neurons located in rat barrel cortex using simultaneous somatic and dendritic recording. Sensory deprivation was achieved by clipping all the whiskers on one side of the snout. Loss of sensory input did not change somatic active and resting membrane properties, and did not influence dendritic action potential (AP) backpropagation. In contrast, sensory deprivation led to an increase in the percentage of layer 5 pyramidal neurons showing burst firing. This was associated with a reduction in the threshold for generation of dendritic calcium spikes during high-frequency AP trains. Cell-attached recordings were used to assess changes in the properties and expression of dendritic HCN channels. These experiments indicated that sensory deprivation caused a decrease in HCN channel density in distal regions of the apical dendrite. To assess the contribution of HCN down-regulation on the observed increase in dendritic excitability following sensory deprivation, we investigated the impact of blocking HCN channels. Block of HCN channels removed differences in dendritic calcium electrogenesis between control and deprived neurons. In conclusion, these observations indicate that sensory loss leads to increased dendritic excitability of cortical layer 5 pyramidal neurons. Furthermore, they suggest that increased dendritic calcium electrogenesis following sensory deprivation is mediated in part via down-regulation of dendritic HCN channels.

Dysfunction of the Dentate Basket Cell Circuit in a Rat Model of Temporal Lobe Epilepsy

Temporal lobe epilepsy is common and difficult to treat. Reduced inhibition of dentate granule cells may contribute. Basket cells are important inhibitors of granule cells. Excitatory synaptic input to basket cells and unitary IPSCs (uIPSCs) from basket cells to granule cells were evaluated in hippocampal slices from a rat model of temporal lobe epilepsy. Basket cells were identified by electrophysiological and morphological criteria. Excitatory synaptic drive to basket cells, measured by mean charge transfer and frequency of miniature EPSCs, was significantly reduced after pilocarpine-induced status epilepticus and remained low in epileptic rats, despite mossy fiber sprouting. Paired recordings revealed higher failure rates and a trend toward lower amplitude uIPSCs at basket cell-to-granule cell synapses in epileptic rats. Higher failure rates were not attributable to excessive presynaptic inhibition of GABA release by activation of muscarinic acetylcholine or GABA(B) receptors. High-frequency trains of action potentials in basket cells generated uIPSCs in granule cells to evaluate readily releasable pool (RRP) size and resupply rate of recycling vesicles. Recycling rate was similar in control and epileptic rats. However, quantal size at basket cell-to-granule cell synapses was larger and RRP size smaller in epileptic rats. Therefore, in epileptic animals, basket cells receive less excitatory synaptic drive, their pools of readily releasable vesicles are smaller, and transmission failure at basket cell-to-granule cell synapses is increased. These findings suggest dysfunction of the dentate basket cell circuit could contribute to hyperexcitability and seizures.

Synaptic Vesicles: Turning Reluctance Into Action

Vesicle availability partly determines the efficacy of synaptic communication in the CNS. The authors recently found that some hippocampal glutamate vesicles exhibit reluctance to exocytose during short, high-frequency action potential trains. These same vesicles can be “coaxed” into exocytosis by increased Ca2+entry, by direct depolarization of synaptic terminals, or by challenge with hypertonic sucrose, a tool used to cause fusion of the population of release-ready synaptic vesicles. Interestingly, the authors did not find evidence of reluctance at hippocampal GABA synapses, suggesting that vesicle reluctance might be a negative feedback mechanism to prevent runaway excitation. It is also possible that synapses exhibit reluctance to retain a dormant population of readily accessible vesicles, ready to respond to triggers such as enhanced Ca2+ influx or neuromodulators. Recent work from the calyx of Held synapse suggests that reluctance might arise from inactivation of Ca2+ channels. The authors review this work, along with several other potential mechanisms of reluctance.

A computational model of synaptic transmission at the calyx of Held

The calyx of Held is a giant glutamatergic synapse in the mammalian auditory pathway designed to ensure faithful transmission of high frequency action potential trains. Pre- and postsynaptic recordings from this synapse reveal several forms of facilitation and depression. A computational model of synaptic transmission has been developed to investigate the mechanisms underlying modulation at the calyx of Held. The model includes paired-pulse facilitation and depression due to vesicle depletion and postsynaptic AMPA receptor desensitization. Accurately matching the experimental time course and magnitude of the depression requires both slow and fast replenishment of vesicles at the presynaptic release sites. The model demonstrates that the temporal accuracy of postsynaptic spike generation depends on the amplitude of the EPSC and so accuracy decreases as the EPSCs depress.