CELLULAR MECHANISMS GOVERNING SYNAPSE FORMATION - LESSONS FROM IDENTIFIED NEURONS IN CULTURE

The accessibility of embryonic and adult neurons within invertebrate n ervous systems has made them excellent subjects for neurobiological st udy. The ability to readily identify individual neurons, together with their great capacity for regeneration, has been especially beneficial to investigations of synapse formation and the specificity of neurona l connectivity. Many invertebrate neurons survive for long periods fol lowing isolation into primary cell culture. In addition, they readily extend new neuritic arbors and form electrical and chemical connection s at sites of contact. Thus, cell culture approaches have allowed neur oscientists greater access to, and resolution of, events underlying ne urite out-growth and synaptogenesis. Studies of identified neuromuscul ar synapses of Helisoma have determined a number of signaling mechanis ms involved in transsynaptic communication at sites of neuron-target c ontact. At these sites, both anterograde and retrograde signals regula te the transformation of growth cones into functional presynaptic term inals. We have found that specific muscle targets induce both global a nd local changes in neurotransmitter secretion and intracellular calci um handling. Here we review recent studies of cultured Helisoma synaps es and discuss the mechanisms thought to govern chemical synapse forma tion in these identified neurons and those of other invertebrate speci es.