Changes in the kinetics of calcium signals in response to high frequency stimulation in the cultured hippocampal neurons

AbstractAnalysis of the presynaptic action potential’s (APsyn) role in synaptic facilitation in hippocampal pyramidal neurons has been difficult due to size limitations of axons. We overcame these size barriers by combining high resolution optical recordings of membrane potential, exocytosis and Ca2+ in cultured hippocampal neurons. These recordings revealed a critical and selective role for Kv1 channel inactivation in synaptic facilitation of excitatory hippocampal neurons. Presynaptic Kv1 channel inactivation was mediated by the Kvβ1 subunit, and had a surprisingly rapid onset that was readily apparent even in brief physiological stimulation paradigms including paired-pulse stimulation. Genetic depletion of Kvβ1 blocked all broadening of the APsyn during high frequency stimulation and eliminated synaptic facilitation without altering the initial probability of vesicle release. Thus using all quantitative optical measurements of presynaptic physiology, we reveal a critical role for presynaptic Kv channels in synaptic facilitation at small presynaptic terminals of the hippocampal neurons upstream of exocytic machinery.SignificanceNerve terminals generally engage in two opposite and essential forms of synaptic plasticity (facilitation or depression) during high frequency stimulation that play critical roles in learning and memory. Measurements of the electrical impulses (action potentials) underlying these two forms of plasticity has been difficult in small nerve terminals due to their size. In this study we deployed a combination of optical measurements of vesicle fusion and membrane voltage to overcome this previous size barrier. Here, we found a unique molecular composition of Kv1 channel β-subunits that causes broadening of the presynaptic action essential to synaptic facilitation. Disruption of the Kvβ1 inactivation mechanism switches excitatory nerve terminals into a depressive state, without any disruption to initial probability of vesicle fusion.

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Q-SNARE Syntaxin 7 confers actin-dependent rapidly replenishing synaptic vesicles upon high activity

10.1101/2020.08.13.249672 ◽

2020 ◽

Author(s):

Yasunori Mori ◽

Koichiro Takenaka ◽

Yugo Fukazawa ◽

Shigeo Takamori

Keyword(s):

High Frequency ◽

Transmitter Release ◽

Synaptic Vesicles ◽

Hippocampal Neurons ◽

Actin Polymerization ◽

Repetitive Stimulation ◽

Snare Proteins ◽

Endosomal Membrane ◽

Kinetics Of ◽

Cultured Hippocampal Neurons

AbstractReplenishment of readily releasable synaptic vesicles (SVs) with vesicles in the recycling pool is important for sustained transmitter release during repetitive stimulation. Kinetics of replenishment and available pool size define synaptic performance. However, whether all SVs in the recycling pool are recruited for release with equal probability is unknown. Here, using comprehensive optical imaging for various presynaptic endosomal SNARE proteins in cultured hippocampal neurons, we demonstrate that part of the recycling pool bearing the endosomal Q–SNARE Syntaxin 7 (Stx7) is preferentially mobilized for release during high–frequency repetitive stimulation. Recruitment of the SV pool marked with the Stx7–reporter requires high intra–terminal Ca2+ concentrations and actin polymerization. Furthermore, disruption of Stx7 function by overexpressing the N–terminal domain selectively abolished this pool. Thus, our data indicate that endosomal membrane fusion involving Stx7 is essential for adaptation of synapses to respond high-frequency repetitive stimulation.

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