Connectomics has focused primarily on the mapping of synaptic links in thebrain; yet it is well established that extrasynaptic volume transmission,especially via monoamines and neuropeptides, is also critical to brainfunction. Here we present a draft monoamine connectome, along with a partialneuropeptide connectome, for the nematode C. elegans, based on new andpublished expression data for biosynthetic genes and receptors. Thus, theneuronal connectome can be represented as a multiplex network, with synaptic,gap junction, and neuromodulatory layers representing alternative modes ofinterneuronal interaction and with distinct network structures. In particular,the monoamine network exhibits novel topological properties, with a highlydisassortative star-like structure and a rich-club of interconnectedbroadcasting hubs. Despite the low degree of overlap between layers, we findhighly significant modes of interaction, pinpointing network locations andmultilink motifs where aminergic and neuropeptide signalling modulate synapticactivity. The multilayer connectome of C. elegans represents a clear exemplarof a biological multiplex network and provides a prototype for understandinghow extrasynaptic signalling can be integrated into the wired circuitry inlarger brains.
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