How the nervous system adjusts to sensory loss

The brain has a remarkable capacity to respond to sensory loss by re-tuning remaining functioning senses. This form of neural plasticity is termed cross-modal plasticity. For example, blind individuals compensate for the loss of vision through increased hearing performance. At the neuronal level, cross-modal plasticity consists of neural circuit reorganization in both the impaired and intact sensory modalities. However, due to the complexity of the system, not much is known about the molecular and cellular processes that underlie these changes. Previous work in our lab examined cross-modal plasticity in C. elegans by comparing olfactory performance between normal worms and worms deprived of body touch sensation due to mutations in their touch receptor, necessary for gentle touch sensation. These mutants exhibited an enhanced sense of smell stemming from a tightened odor-locomotion coupling. To gain deeper understanding of the essential molecular and cellular foundations of the observed cross-modal plasticity, we sought to reveal the extent to which the touch receptor neurons (TRNs) themselves may assume new function in worms that cannot sense touch, and how their presence or absence is reflected in odor-locomotion coupling. We compared locomotion patterns worms with dysfunctional touch receptors and worms that lack the TRNs altogether. We expected the latter to show increased odor-locomotion coupling. Surprisingly, odor-locomotion coupling in worms lacking TRNs was similar in strength to WT. These results indicate that in worms lacking touch sensation, the TRNs remain functional, and probably necessary for cross-modal plasticity modulations. Indeed, the TRNs of touch-deficient worms may signal that touch information is unavailable. We have further, shown that such signaling could be carried by neuropeptides secreted by the TRNs. These findings reveal the importance of sensory neurons even when they are unable to sense and sheds light on different modes of downstream signaling by these neurons.