SR2-NEURONS IN CULTURE( AND QUANTAL EVENTS AT EXCITATORY SYNAPSES BETWEEN MOUSE HIPPOCAMPAL)

1. Whole-cell recording from pairs of adjacent mouse hippocampal neuro ns in culture was used to study the quantal properties of action poten tial-evoked excitatory synaptic transmission and to demonstrate the us e of Sr2+ in quantifying those properties. 2. In the presence of extra cellular Sr2+, excitatoray postsynaptic currents (EPSCs) were followed by an after-discharge of miniature excitatory postsynaptic currents ( mEPSCs) lasting 1-2 s and generated by evoked asynchronous release of presynaptic quanta of transmitter. Like the EPSC of which it is though t to be an extension, the after-discharge was modulated by procedures expected to modulate Sr2+ influx into the nerve terminal. The number o f mEPSCs in the after-discharge was decreased by increasing extracellu lar [Mg2+], and increased by increasing extracellular [Sr2+] Or increa sing the number of action potentials used to evoke the after-discharge . 3. EPSCs recorded in media containing either 1 mM Ca2+ or 6 mM Sr2were of similar amplitude. Adding Sr2+ to low-Ca2+ media increased EPS C amplitude, while adding Sr2+ to high-Ca2+ media lowered EPSC amplitu de. These results suggest that extracellular Sr2+ is less effective th an Ca2+ in supporting quantal release. 4. The levels of extracellular Ca2+, Mg2+ and Sr2+ were adjusted so that most after-discharge mEPSCs were discrete and comparable in numbers to the quantal events that con tributed to the corresponding evoked EPSCs. In a series of twenty-five pairs of neurons, the mean amplitude of mEPSCs recorded at -80 mV was 35 +/- 10 pA and the mean coefficient of variation was 0.50 +/- 0.10 (range, 0.26-0.62). The mEPSC amplitude histogram was positively skewe d. 5. In ten pairs of neurons, the mean and variance of EPSCs and mEPS Cs and quantal content were determined from samples of more than 100 e voked events (in superfusion solutions containing (mM): 0.5 Ca2+, 2 Sr 2+ and 10 Mg2+) and mean quantal content was determined from the ratio of amplitudes of the mean EPSC and mEPSC. A binomial quantal analysis produced values of 2-12 for N-app (apparent number of independent syn apses) and 0.25-0.75 for p(app) (apparent probability of releasing a q uantum at one of those synapses). These parameters predicted the numbe r of observed failures. The observed coefficient of variation for quan tal content predicted the observed coefficient of variation of the EPS C amplitude when the coefficient of variability of quantal amplitude o f after-discharge mEPSCs was taken into account. 6. In six pairs of ne urons, where more than 250 evoked events were recorded, the observed a mplitude histogram for EPSCs could be approximated by a predicted ampl itude distribution generated from the estimated binomial parameters an d an empirical function describing the amplitude distribution of after -discharge mEPSCs. 7. The observation that parameters derived from mEP SCs that contribute to the Sr2+-generated after-discharge can predict the shape of the EPSC amplitude distribution and a quantal content con sistent with the observed failure rate and EPSC amplitude variance, su ggests that this subset of mEPSCs has the same properties as the quant al events released around the time of the peak of the corresponding EP SCs. The use of Sr2+ to evoke after-discharges of mEPSCs should allow unambiguous determination of the extent to which modification of synap tic strength is pre- or postsynaptic.