Distribution of neurotransmitter secretion in growing axons

Neurotransmitter secretion from the nerve terminal is mediated by the fusio n of synaptic vesicles with the plasma membrane. It is generally believed t hat neurotransmitter release in mature synapses is localized to the presyna ptic nerve terminals. To probe the topology of neurotransmitter secretion a long developing axons in culture, we recorded membrane currents from myocyt es manipulated into contact with axons. At the early stages of growth, exoc ytotic events were detected along the axon as well as at the growth cone. A t the later stages of growth, neurotransmitter secretion adopted the form o f a smooth proximodistal gradient, with the highest level of activity at th e growth cone region. Our results reveal the existence of a previously undetected early stage of axonal growth and suggest developmental regulation in the pattern of neurot ransmitter secretion along the growing nerve processes. (C) 1999 IBRO. Publ ished by Elsevier Science Ltd.