Optical Postsynaptic Measurement of Vesicle Release Rates for Hippocampal Synapses Undergoing Asynchronous Release during Train Stimulation

Abstract

Developing hippocampal neurons in microisland culture were found to undergo rapid depression of excitatory synaptic activity caused by consumption of their readily releasable pool (RRP) of vesicles in response to 20 Hz trains of stimulation. Associated with depression was a switch to an asynchronous release mode that maintained transmission at a high steady-state rate equivalent to ϳ2.1 RRPs per second. We have applied postsynaptic Ca 2ϩ imaging to directly monitor these asynchronous release events to estimate both the steady rate of transmitter release and the number of quanta within the RRP at individual hippocampal synapses. Based on the frequency of asynchronous release measured at individual synapses postsynaptically using Ca 2ϩ imaging (5-17 sec after train stimulation) and with knowledge of the time course by which asynchronous release rates decay, we estimate that individual hippocampal synapses exhibit (in response to train stimulation) peak release rates of up to 21 quanta per second from an RRP that contains, on average, 10 quanta. Use-dependent block of evoked synaptic activity by MK-801 [(ϩ)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine maleate] confirmed that synapses undergoing asynchronous release are not significantly different from the general population with regard to their composition of NMDA receptor and/or release probability. Given that high-frequency trains deplete the synapse of readily releasable quanta (and that these release rates can only be maintained for a few seconds), these high rates of asynchronous release likely reflect refilling of vesicles from a reserve pool and not necessarily the continuous action of a relatively slow clathrin-and endosomedependent process.

Calcium Signaling Pathways Mediating Synaptic Potentiation Triggered by Amyotrophic Lateral Sclerosis IgG in Motor Nerve Terminals

“…3B). Interestingly, the increment in asynchronous neurotransmission did not reduce synchronous release at any frequency of stimulation studied (QC at 20 and 40 Hz), as could be expected from previous studies (David and Barrett, 2003;Otsu and Murphy, 2004).…”

“…3D). Studies at neuromuscular and hippocampal synapses have shown that an increase in the asynchronous release reduces the synchronous release, probably because of the fact that vesicles involved in synchronous and asynchronous release derive from the same source (David and Barrett, 2003;Otsu and Murphy, 2004) (also see Sara et al, 2005).…”

“…Thus, during and after a train of action potentials (30 s at 20 or 40 Hz), ALS-IgG promoted asynchronous and delayed release. The ALSIgG effect on asynchronous and spontaneous release, but not on synchronous release, could be because the ALS-IgG affects mechanisms associated with those modes of transmitter release (i.e., intracellular calcium stores) (Narita et al, 2000;David and Barrett, 2003;Otsu and Murphy, 2004) that involve a different type of synaptic vesicles pool (Sara et al, 2005) and distinct release machinery (Hua et al, 1998). Furthermore, at mouse NMJ, a cross talk between synchronous and asynchronous release is apparent only when asynchronous release reaches ϳ1000 quanta/s (David and Barrett, 2003), a higher rate than that induced by ALS-IgG (ϳ1000 quanta/min).…”

Properties of Synaptic Vesicle Pools in Mature Central Nerve Terminals

“…This notion was corroborated by the continuous release in those experiments of FM dye that had been preloaded into both the RRP and reserve vesicles. Indeed, high frequency electrical stimulation of hippocampal cells (a method normally used to trigger the RRP) leads to gradual mobilization of reserve vesicles (57).…”

Increased asynchronous release and aberrant calcium channel activation in amyloid precursor protein deficient neuromuscular synapses

“…The asynchronous release competes with synchronous release for the recovered quanta since they are derived from the same pool of readily releasable vesicles (Elmqvist and Quastel, 1965;Hagler and Goda, 2001;Lu and Trussell, 2000). As such, elevated asynchronous release can reduce future synchronous release (Otsu and Murphy, 2004). To test whether enhanced asynchronous release might be involved in the greater depression seen in APP −/− mice, we first measured asynchronous release by quantifying the frequency of minilike responses during a 10 min train at 30 Hz ( Figure 4).…”