Orm Caenorhabditis elegans. The nervous system of C. elegans is invariant in terms of neuronal position, quantity and morphology of neurons and as a result genetic screens have identiWed quite a few genes which might be involved in specifying neuronal fate and these that underlie a selection of physiological processes (Hobert 2005; Schafer 2005; Barr and Garcia 2006; Goodman 2006). Several diVerent varieties of sensory neurons have been described in C. elegans making use of physiological strategies too as genetic tools. The ASH pair of neurons have ciliated sensory endings inside the worm’s anterior finish (or “nose”, the amphid neurons) and laser removal of those neurons signiWcantly lowers the avoidance response to stimulation of the worm’s anterior, a so-called “nose touch” withdrawal, whereas animals lacking all other amphid neurons except for ASH display regular avoidance behavior (Kaplan and Horvitz 1993). Two other neurons,FLP and OLQ, also play a minor role within this avoidance behavior. There is certainly also strong proof that the ASH neuron is involved in avoidance behavior to very osmotic option, octanol and acid (Troemel et al. 1995; Sambongi et al. 2000; Hilliard et al. 2002) and it has been suggested that the ASH neuron acts like polymodal nociceptors in mammals (reviewed in Tobin and Bargmann 2004). The function of your ASH neuron is just not special to C. elegans, as current analysis of avoidance behavior in 5 other species of nematode worm has shown that the part of ASH is largely conserved (Srinivasan et al. 2008). Exceptions integrated the added requirement of ADL neurons for complete higher osmotic solution avoidance behavior in Pristionchus paciWcus and Degarelix In Vivo diVerences in basal stimulus sensitivity believed to become resulting from adaptation of species to their respective niches (Srinivasan et al. 2008). A thermal avoidance behavior has also been observed in C. elegans where upon exposure to 3 a reXex escape response is evoked (Wittenburg and Baumeister 1999). Though it is actually recognized that neurons controlling thermotaxis aren’t involved in the avoidance response, the nociceptive neurons that detect noxious heat in C. elegans are nonetheless unknown. Interestingly, capsaicin was noticed to sensitize the heat response, but evoked no acute behavior. In conclusion, it seems that C. elegans as well as other nematodes possess neurons, which speciWcally react to noxious stimuli, the ASH neuron being the very best characterized so far. Arthropoda The final invertebrate that may be discussed in detail will be the arthropod Drosophila melanogaster which, like C. elegans, is an organism that lends itself to genetic evaluation. D. melanogaster undergo a 4-day larval stage and touching larvae with a probe causes them to pause and move away from the stimulus. Even so, a heated probe (2 ) evokes a corkscrew-like rolling behavior, evoked in as tiny as 0.4 s (Tracey et al. 2003). Strong mechanical stimulation evokes a similar behavior, indicating that this may be a nociceptive response to damaging stimuli. The sensory neurons essential for this response will be the class IV multidendritic neurons that terminate inside the periphery in the larvae, attached to epidermal cells (Hwang et al. 2007). Proof that they function as nociceptors came from experiments exactly where channelrhodopsin-2 was expressed in diVerent multidendritic neuron classes and behavior observed upon photoactivation. Only activation in class IV neurons triggered nocifensive rolling, whereas activation in classes II and III neurons evoked an accordion-like behavior indicative of a role in pr.