Physiological Activity Depresses Synaptic Function through an Effect on Vesicle Priming

Abstract

Neurons engage compensatory, homeostatic synaptic changes to maintain their overall firing rate. We examined the induction and expression of a persistent presynaptic adaptation. We explored the effect of mild extracellular potassium elevation to increase hippocampal pyramidal neuron spiking over a physiological range. With several days of mild depolarization, glutamate release adapted, as revealed by an increased mismatch between the number of active, FM1-43-positive, glutamatergic synapses and the total number of synapses defined by vesicular glutamate transporter-1 antibody staining. Surprisingly, the adaptation of glutamate terminals was all-or-none; recycling vesicle pool size at remaining active synapses was not significantly altered by the adaptation. Tetrodotoxin (TTX), but not postsynaptic receptor blockade, reversed depolarization-induced adaptation, and TTX added to normal incubation medium increased the number of active synapses, suggesting that normal spiking activity sustains a steady-state percentage of inactive terminals. Chronic mild depolarization depressed EPSCs and decreased the size of the readily releasable pool of vesicles (RRP). Several hours of 10 Hz electrical stimulation also depressed the RRP size, confirming that spiking alone induces adaptation and that strong stimulation induces more rapid presynaptic adaptation. Despite the importance of RRP alteration to the adaptation, ultrastructural experiments revealed no changes in docked or total synaptic vesicle numbers. Furthermore, ␣-latrotoxin induced vesicle release at adapted synapses, consistent with the idea that adaptation resulted from changes in vesicle priming. These results show that glutamatergic neurotransmission persistently adapts to changes in electrical activity over a wide physiological range.

A Specific Role for Ca2+-Dependent Adenylyl Cyclases in Recovery from Adaptive Presynaptic Silencing

“…For experiments in which electrical activity was blocked, 500 nM tetrodotoxin (TTX) plus 25 M D-APV and 1 M NBQX were added immediately at 4 -5 DIV. These experiments were then conducted at 11-14 DIV (6 -10 d of treatment), as described previously (Moulder et al, 2006). For agents dissolved in DMSO, final DMSO concentration was Յ0.1%.…”

“…We have described a form of presynaptic adaptation by which glutamatergic presynaptic terminals respond to changes in activity with a binary adjustment in the release competence of their synaptic vesicles (Moulder et al, 2004(Moulder et al, , 2006. Presynaptic silencing operates at normal levels of activity.…”

“…Presynaptic silencing operates at normal levels of activity. In hippocampal cultures, ϳ25% of terminals are silent basally, and block of electrical activity decreases this percentage (Moulder et al, 2006). Furthermore, modest depolarization and associated spiking decrease the percentage of active synapses (Moulder et al, 2006).…”

“…In hippocampal cultures, ϳ25% of terminals are silent basally, and block of electrical activity decreases this percentage (Moulder et al, 2006). Furthermore, modest depolarization and associated spiking decrease the percentage of active synapses (Moulder et al, 2006). The mechanism of this adaptation to a wide range of activity levels appears to involve primarily a change in vesicle priming (Moulder et al, 2006).…”

“…Furthermore, modest depolarization and associated spiking decrease the percentage of active synapses (Moulder et al, 2006). The mechanism of this adaptation to a wide range of activity levels appears to involve primarily a change in vesicle priming (Moulder et al, 2006). Such adaptive mechanisms may exist in part to limit the likelihood that glutamate signaling will run away unchecked, thus avoiding possible excitotoxicity.…”