Early postnatal changes in presynaptic potassium sensitivity

Amplitude histograms of miniature endplate potentials (MEPPs) and the overall frequency of skew-MEPPs and bell-MEPPs were examined in 5 and 15 mM potassium solutions at postnatal day (PD) 3, PD 10 and PD 27 neuromuscular junctions. Temporal non-uniformities in spontaneous release produced clusters of bell-MEPPs at PD 0-PD 3 junctions. PD 3 nerve terminals that preferentially released skew-MEPPs (5 mM potassium) were significantly (P less than 0.01) less sensitive to elevations in potassium than more mature (PD 10) junctions that preferentially released bell-MEPPs. Increases in the potassium concentration at PD 3 junctions increased the frequency of bell-MEPPs and altered the MEPP amplitude distribution profile by significantly (P less than 0.01) reducing the percentage of skew-MEPPs. Although the potassium sensitivity of PD 10 and PD 27 preparations were as expected for adult preparations, there was an increase in overall MEPP frequency in 5 mM potassium between PD 10 and PD 27. These results suggest that early postnatal increases in the number of presynaptic calcium channels establish adult levels of depolarization sensitivity and promote the preferential spontaneous release of bell-MEPPs. Since these changes occur during an early period of synapse elimination, they may play a critical role in synapse stabilization.