Optogenetic Analysis of a Nociceptor Neuron and Network Reveals Ion Channels Acting Downstream of Primary Sensors

Buchmann Institute for Molecular Life Sciences, Goethe-University Frankfurt, Max von Laue Str. 15, 60438 Frankfurt, Germany.
Current biology: CB (Impact Factor: 9.57). 04/2012; 22(9):743-52. DOI: 10.1016/j.cub.2012.02.066
Source: PubMed


Nociception generally evokes rapid withdrawal behavior in order to protect the tissue from harmful insults. Most nociceptive neurons responding to mechanical insults display highly branched dendrites, an anatomy shared by Caenorhabditis elegans FLP and PVD neurons, which mediate harsh touch responses. Although several primary molecular nociceptive sensors have been characterized, less is known about modulation and amplification of noxious signals within nociceptor neurons. First, we analyzed the FLP/PVD network by optogenetics and studied integration of signals from these cells in downstream interneurons. Second, we investigated which genes modulate PVD function, based on prior single-neuron mRNA profiling of PVD.
Selectively photoactivating PVD, FLP, and downstream interneurons via Channelrhodopsin-2 (ChR2) enabled the functional dissection of this nociceptive network, without interfering signals by other mechanoreceptors. Forward or reverse escape behaviors were determined by PVD and FLP, via integration by command interneurons. To identify mediators of PVD function, acting downstream of primary nocisensor molecules, we knocked down PVD-specific transcripts by RNAi and quantified light-evoked PVD-dependent behavior. Cell-specific disruption of synaptobrevin or voltage-gated Ca(2+) channels (VGCCs) showed that PVD signals chemically to command interneurons. Knocking down the DEG/ENaC channel ASIC-1 and the TRPM channel GTL-1 indicated that ASIC-1 may extend PVD's dynamic range and that GTL-1 may amplify its signals. These channels act cell autonomously in PVD, downstream of primary mechanosensory molecules.
Our work implicates TRPM channels in modifying excitability of and DEG/ENaCs in potentiating signal output from a mechano-nociceptor neuron. ASIC-1 and GTL-1 homologs, if functionally conserved, may denote valid targets for novel analgesics.

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    • "C. elegans is particularly useful in the study of nociception as it exhibits a clear and reproducible withdrawal behavior, involving a reversal and change in direction away from the noxious stimulus (Wittenburg and Baumeister, 1999; Tobin and Bargmann, 2004). Recently, optogenetic activation of C. elegans nociceptors has been used to characterize molecules associated with nociception (Husson et al., 2012). Channelrhodopsin 2 was expressed under the control of the F49H12.4 "
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