C. elegans moves away from ultraviolet and blue light, and this avoidance is dependent on LITE-1, a putative light sensor, as well as the uncloned
lite-2 and
lite-3 (Edwards..Miller 2008). We found that worms also stop pharyngeal pumping in response to light. For example, worms immediately stop pumping when exposed to violet light (the "acute response") and maintain reduced pumping after light is removed (the "sustained response").
Mutations in
lite-1,
lite-2 or
lite-3 have little effect on the acute response but lead to a completely defective sustained response, such that pumping quickly recovers to pre-light levels after light is removed. After testing neurotransmission mutants we found that
eat-4, a vesicular glutamate transporter, is required for the acute but not the sustained response to light.
unc-13, a regulator of neurotransmitter release, is required for both responses. Since
eat-4;
lite-1 mutants lack both the acute and sustained responses,
eat-4 and
lite-1 likely act in parallel, indicating that an additional light sensor upstream of
eat-4 might be present in the worm.
The C. elegans nervous system consists of 2 anatomical networks connected by gap junctions between the main network's RIP neurons and the pharyngeal network's I1 neurons. Consistent with the hypothesis that the main network is involved in the pumping response to light, laser ablation of both I1s yielded worms that lacked the acute but not the sustained light response, similar to
eat-4 mutants. To investigate whether
eat-4 acts downstream of the I1s, we ablated individual classes of
eat-4-expressing pharyngeal neurons (M3s, NSMs, and I5). We found that these ablations had no effect on the worm's light response, indicating that
eat-4 likely acts outside the pharynx to control the acute response. We also tested available mutants of other glutamate transporters and glutamate receptors, but all responded normally to light.
We plan to identify the sites-of-action of
lite-1 and
eat-4 via cell ablation and cell-specific rescue experiments. Additionally, we plan to image calcium to measure the physiological sequence in which these neurons are activated. We also plan to do a mutagenesis screen for additional mutants defective in the pumping response to light to seek the light sensor upstream of
eat-4, the glutamate receptor downstream of
eat-4, and the signaling molecules downstream of
lite-1. We hope that such analysis will elucidate important principles about neural communication that are relevant more generally across species.