Guidance cues have a well-characterized role in axon and dendrite guidance ( Chisholm et al., 2016 Dickson, 2002 Keleman and Dickson, 2001 Tessier-Lavigne and Goodman, 1996 Zlatic et al., 2009), but their role in regulating the subcellular position of synapses has yet to be tested. An alternative mechanism relies on axon guidance cues to restrict pre-synaptic access to one of several acceptable postsynaptic targets (the ‘guidance cue’ model). elegans whereby local clustering of cell surface molecules on a postsynaptic neuron dictates synapse position ( Ango et al., 2004 Colón-Ramos et al., 2007 Klassen and Shen, 2007 Mizumoto and Shen, 2013). This model is supported by evidence in mouse and C. The first model relies on molecular differences between two subcellular domains to restrict synapse formation to one domain (the ‘labeled arbor’ model). Two distinct developmental models could explain subcellular synaptic specificity. dbd forms synapses specifically with the medial dendritic domain, and does not synapse with the lateral dendritic domain or the output domain. ![]() ( C) Electron microscopy reconstruction of dbd neurons (magenta) and A08a neurons (green) morphologies in one abdominal (A) segment (A1 left and A1 right) of the Drosophila ventral nerve cord (posterior view). ( B) Schematic of fly A08a neuron (green) with a dbd neuron (magenta) forming synapses onto the medial dendrite and an A02l neuron (orange) forming synapses onto the lateral dendrite. ( A) Schematic of mouse neocortical pyramidal neuron (green) with a martinotti neuron (magenta) forming synapses onto the distal dendrite and the bitufted neuron (orange) forming synapses onto the proximal dendrite. Although the precise subcellular positioning of synapses is important for proper circuit function, the mechanisms necessary to achieve such specificity are just starting to be explored ( Telley et al., 2016). The precise targeting of inhibitory neurons to distinct subcellular domains of their target neurons has profound effects on neural processing and circuit function by gating action potential initiation, providing a substrate for plasticity, altering mEPSP amplitude, and modulating dendritic integration ( Bloss et al., 2016 Hao et al., 2009 Miles et al., 1996 Pouille et al., 2013 Tobin et al., 2017). In mammals, cortical pyramidal neurons receive input from martinotti neurons on their distal dendrites and basket neurons on their proximal dendrites ( Huang et al., 2007) ( Figure 1A). In Drosophila, the giant fiber descending neuron targets a specific dendritic domain of the tergotrochanteral motor neuron in a fast jump escape circuit ( Godenschwege et al., 2002 Godenschwege and Murphey, 2009). In addition to specificity at a cellular level, neural circuits also exhibit synaptic specificity at the subcellular level (reviewed in Yogev and Shen, 2014). ![]() From the Drosophila larva with 10,000 neurons to the human with 80 billion neurons, all neurons are faced with the challenge of identifying the correct subset of synaptic partners among many potential target neurons. Nervous system function is determined by the precise connectivity of neurons.
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