Seasonal changes in the normal variability in release properties of motor nerve terminals in Rana pipiens.

During a 2 1/2 year period, we studied 298 identified endplates from 40 sartorius muscles, correlating their morphology with their synaptic release properties. There is a remarkably large range in evoked and spontaneous quantal release levels when junctions are studied in a low-Ca2+ Ringer. This diversity in release efficacy persists when release is normalized to identified nerve terminal length. We also found that release was significantly larger in the winter (Dec-Feb) than in the spring and summer (Mar-Aug), suggesting seasonal regulation of release properties.