Postnatal emergence of mature release properties in terminals of rat fast- and slow-twitch muscles

Motor nerve terminals in adult mammalian slow-twitch muscles have lower levels of spontaneous and evoked neurotransmitter release than terminals in fast-twitch muscles. These reflect adaptive differences, allowing terminals in slow (postural) muscles to sustain release during the prolonged firing trains experienced in vivo. Here we ask whether these differences in terminal release properties in Sprague-Dawley rat extensor digitorum longus (EDL, fast) and soleus (slow) muscles reflect their early cytodifferentiation in the embryo or whether they might be adaptations to their distinct mature activity patterns, which emerge around two weeks postnatally. We find that the mature pattern of differences in release arise through co-ordinated increases in presynaptically dependent release properties (quantal content, spontaneous release frequency and evoked potential amplitude), beginning at three weeks, which are particularly substantial in EDL. In contrast, other synaptic properties are either consistently greater in the same muscle throughout development (evoked potential kinetics, muscle fibre diameter) or display no systematic muscle type-dependent differences (terminal area, input resistance, spontaneous release amplitude). Thus, the emergence of adaptive differences in terminal release properties correlates with the differentiation of locomotor activity patterns in postnatal rat hindlimb muscles.