Synaptic networks must dynamically adapt to environmental cues, changes in muscle size or synapse number during development, and variable levels of stimulation over time. To achieve this feat, multiple layers of communication are necessary between pre- and post-synaptic sites as well as between heterosynaptic cells.

The Kaplan lab previously demonstrated that mutants of the muscle microRNA mir-1 display diminished ACh release, manifested as a decrease in the frequency of endogenous neurotransmitter release as well as a decrease in the stimulus-evoked current at neuromuscular junctions (NMJs) in C. elegans. This data suggested that postsynaptic miR-1 controls a retrograde signal that modulates presynaptic function. We provide evidence that the worm orthologues of the synaptic adhesion molecules Neuroligin (NLG-1) and Neurexin (NRX-1) are required for the conductance of this retrograde signaling pathway. Despite wild type baseline transmission at cholinergic synapses, mutations in nlg-1 and nrx-1 completely abolish the retrograde signal induced in mir-1 mutants. Respectively, this suppression is rescued by the expression of NLG-1 in cholinergic neurons, and by expression of NRX-1 in the muscle. We confirmed that NLG-1 is endogenously expressed in cholinergic neurons and localizes to presynaptic moieties in those cells, and that muscle-expressed NRX-1 localizes in a pattern that is consistent with postsynaptic apposition to the nerve cord, in contrast to prior observations of these molecules in mammals. Collectively, these data indicate that NLG-1 and NRX-1 in worms may be acting as trans-synaptic binding partners to mediate presynaptic release via a retrograde signal at the NMJ.

In addition to coordinating pre- and post-synaptic function, processing of neural information in the broader view of a network is thought to occur by the integration of excitatory and inhibitory synaptic inputs. To study this phenomenon, we exploited the C. elegans body wall musculature, which is innervated by both excitatory inputs (via acetylcholine) and inhibitory inputs (via GABA). We analyzed the functional connectivity of neighboring cholinergic and GABAergic neurons, and their postsynaptic muscle cells, using a variety of fluorescently labeled proteins as markers to detect in vivo responses to perturbations in synaptic transmission. This work aimed to clarify the extent to which excitatory and inhibitory inputs interact with each other, ultimately to better understand how the nervous system is able to maintain functionality despite fluctuating levels of excitation and inhibition.