Novel quantal analysis method reveals conservation of average charge injected by excitatory synapses across cortical neurons of different sizes

In chemical synapses of the central nervous system (CNS), information is transmitted via the presynaptic release of a vesicle (or ‘quantum’) of neurotransmitter, which elicits a postsynaptic electrical response with an amplitude termed the ‘quantal size’. This key determinant of neural computation is hard to measure reliably due to its small size and the multiple sources of noise within neurons and electrophysiological recordings. Measuring amplitudes of miniature postsynaptic currents (mPSCs) or potentials (mPSPs) at the cell soma is generally thought to offer a technically straightforward way to estimate quantal sizes, as each of these miniature responses (or ‘minis’) is generally thought to be elicited by the spontaneous release of a single neurotransmitter vesicle. However, a somatically recorded mini is typically massively attenuated compared with at its input site, and a significant fraction are indistinguishable from (or cancelled out by) background noise fluctuations. Here, as part of patch clamp data analysis software called ‘minis’, we describe a novel quantal analysis method that estimates the ‘electrical size’ of the synapse by combining somatic recordings of background physiological noise with and without minis with simulations. With the help of a genetic algorithm, simulations are successfully used to infer the combined amplitude and rise time distribution of minis that would otherwise be inaccessible due to low signal-to-noise ratios. The estimated distributions reveal a striking inverse dependence of mean minis’ amplitudes on cell’s total capacitance (proportional to cell size, more exactly, extracellular membrane surface area) that firmly supports the conservation of the average ‘electrical size’ (in terms of injected charge) of excitatory cortical synapses in rats, across neocortical pyramidal neurons of very different sizes.