Target-dependent modulation of presynaptic structure and function at identified neuromuscular synapses

Motoneurons of Helisoma trivolvis can be divided into two categories of synaptogenic strategies: selective and promiscuous. B19, a neuron with a selective strategy, requires 24 hours of appropriate muscle contact to upregulate neurotransmitter secretion in culture. Additionally, contact with these muscle fibers induces neuritic branching in B19. Neurons B5 and B110 possess neurosecretory function prior to muscle contact, as indicated by transmitter reporter assays. Due to these differences in developmental regimens, the underlying mechanisms of synapse formation were investigated to determine their structural and functional underpinnings.; Transcriptional and protein synthesis inhibition in B19 prior to muscle contact disrupted target-dependent induction of acetylcholine release. Studies of the temporal relationship between the onset of target contact and the suppression of translation indicated that new macromolecular expression was required for both initial “tagging”, and subsequent development, of functional presynaptic sites. Such protein synthesis-dependent mechanisms were not required by B5, indicating that promiscuous synaptogenesis involves a fundamentally different developmental program than that of more constrained neurons.; Early stages of synaptic induction in B19 are governed by cAMP-dependent mechanisms. Pharmacological elevation of PKA activity induced secretory upregulation and branching in the absence of target contact. However, blocking PKA activity prior to the establishment of appropriate muscle contacts did not block target-induced branching. Therefore, muscle-induced alterations in presynaptic secretory function, but not neuritic architecture, involved elevations in PKA activity. These results suggest that alterations in B19 growth cone behavior and synaptogenesis involve multiple signaling pathways during neuromuscular regeneration.; During regeneration, adult neuromuscular connectivity patterns are reestablished between neurons B19 and B110. Both neurons share the same muscle targets, but differ in synaptic strengths. These connections and differences are recapitulated in vitro. Synaptic efficacy, particularly in B110, is influenced by target-dependent, homeostatic mechanisms that impact on presynaptic quantal content. Such homeostatic controls provide an integrative mechanism for the polysynaptogenesis ongoing at Helisoma muscle fibers during motor system reconstruction. Taken together, these findings demonstrate the contrasting cellular mechanisms that govern promiscuous, and more selective, strategies of neuronal synaptogenesis.