RELIABILITY OF HIGH-FREQUENCY NEUROTRANSMISSION IN REDUCED EXTRACELLULAR Ca2+

Extracellular Ca²⁺ concentration in the brain is estimated at ~1.2 mM. However, reports indicate this value may change during sleep and in response to trauma, ischemia and seizure. Here, we examine how such changes in [Ca²⁺] alter the reliability of neurotransmission as a function of stimulation frequency. Brain slices from C57 mice containing MNTB neurons were used for whole-cell current clamp recordings. Presynaptic action potentials were generated by stimulating the afferent fibers, and the number of postsynaptic AP failures was determined. In 0.8 mM [Ca²⁺], AP failure rate was 35±5% at 100 Hz. Surprisingly, increasing stimulation to 500 Hz decreased the failure rate to 15±5% (N=34, p<10^-4). Increasing [Ca²⁺] to 1.2 mM or 2.0 mM resulted in a decrease in the number of failures at 100 Hz. Unlike 0.8 mM [Ca²⁺], in these Ca²⁺ concentrations raising stimulation frequency to 500 Hz caused an increase in failures (1.2 mM Fail_100Hz=19±7% vs Fail_500Hz=36±11%, n=10, p=0.006; 2 mM Fail_100Hz=12±7.4% vs Fail_500Hz=50±16%, N=6, p=0.008). We quantified synaptic strength as EPSP′, the maximum value of dVm/dt during the response to stimulation. The amplitude of the first EPSP′ in each train was 92±8 V/s in 0.8 mM Ca²⁺, and increased superlinearly with increasing [Ca²⁺] (1.2 mM: 255±22 V/s, 2.0 mM: 410±28 V/s). Steady-state EPSP′ also increased with [Ca²⁺], but this increase was modest: (0.8 mM: 102±7 V/s, 1.2 mM: 127±11 V/s, 2.0 mM: 149±11 V/s). In 0.8 mM [Ca²⁺], we observed facilitation of EPSP′ at 500 Hz (1.6±0.06 fold) but not at 100 Hz. Raising [Ca²⁺] to 1.2 mM significantly reduced facilitation (1.2±0.03-fold, p<10^-4 compared to 0.8 mM [Ca²⁺]). In 2.0 mM [Ca²⁺] we observed depression of the EPSP′ rather than facilitation. In conclusion, in 2.0 mM Ca²⁺, a relatively low failure rate at 100 Hz increases at 500 Hz due to synaptic depression. In contrast, synaptic facilitation in 0.8 mM Ca²⁺ contributes significantly to the increase in reliability of neurotransmission at 500 Hz compared to 100 Hz. Surprisingly, in these conditions the synapse may thus transmit in band-pass mode.

This work was supported by CAPES, CAPES-PrINT, FAPEMIG (APQ-00842-23), and NIH-5R01DC012938