Neural Signaling Dynamics of Conditioning in C. elegans

Retrograde signaling from downstream effectors (i.e., motor neurons) can modulate plasticity. Much research has focused on the learned association of closely timed sensory stimuli. By comparison, there is less research probing the potential influence of how or if activation at downstream neuromuscular junctions (NMJ) could modulate associative conditioning. Using channelrhodopsin activation of body wall muscle and different motor neuron subsets (cholinergic motor neurons that drive contraction and GABAergic motor neurons that drive relaxation of muscle) in the Caenorhabditis elegans (C. elegans) model system, we examined if concurrent excitation in these downstream circuits influences associative conditioning. Conditioning consisted of pairing two distinct sensory stimuli, mechanosensory (vibration) and blue light (~480nm). Each stimulus drives a locomotor response on its own and we have shown that pairing delivery of these two stimuli alters the subsequent locomotor response to vibration. Animals that expressed channelrhodopsin in the body wall muscle (pmyo-3::ChR2), excitatory motor neurons (punc-17::ChR2) or the inhibitory motor neurons (punc-47::ChR2) received associative vibration-light conditioning. Thus, the blue light stimulus simultaneously functioned as both associating sensory stimulus and activator of channelrhodopsin, when the necessary cofactor was present, all-trans-retinol (ATR+). Results showed wild type C. elegans typically pause for a longer duration following associative vibration-light conditioning. Following vibration-light conditioning, pmyo-3::ChR2 exhibited a complete disruption of learning. While trained ATR+ punc-17::ChR2 and punc-47::ChR2 animals showed partially disrupted conditioned locomotor behavior, as compared to controls. Together, this data suggests that co-activation of the downstream body wall muscle and motorneurons interferes with upstream associative conditioning.