N no locomotive response to mechanical stimulation. However, a longitudinal incision, via the animal’s body, produced behind the brain and continued posteriorly along the midline for most from the animal’s length (hence creating a L-shaped cut via the worm’s physique), didn’t prevent locomotory escape behavior, suggesting the presence of a diVuse sensory neural network (Koopowitz 1973). Presumably pin prick represents a noxious stimulus and therefore the evoked behavior may very well be considered asJ Comp Physiol A (2009) 195:1089Mollusca Nociceptors and nociceptive behavior have also been investigated in quite a few species of Mollusca. For example, the land snail, Cepaea nemoralis, responds to placement on a hotplate (0 ) with stereotypical lifting from the anterior portion on the extended foot. The expression of opioid receptors and endogenous ligands for these receptors is thought of basic in determining whether or not or not nociception can occur (Sneddon 2004). Thus, it is fascinating to note that opiate agonists enhanced withdrawal latency, which might be blocked by the opiate receptor antagonist, naloxone (Kavaliers et al. 1983). The use of 1 and two opioid receptor agonists also improved response latency (Thomas et al. 1997) and immunohistochemical staining indicates the presence of endogenous -receptor agonists (Sakharov et al. 1993). The hot-plate test is often a common model for measuring nociception in rodents and opiates normally enhance the withdrawal latency though strain diVerences in basal withdrawal latency and also the magnitude in the eVect of morphine do occur (Mogil et al. 1996). Therefore, the action of opioid receptor agonistsantagonists upon withdrawal latency supports the hypothesis that the foot lifting response in C. nemoralis is indeed a nocifensive behavior. Probably the most intensively studied Mollusca could be the gastropod, Aplysia californica. The Wrst potentially nociceptive sensory neurons within a. californica, innervating the siphon and mantle, had been identiWed within the left E (LE) cluster in the abdominal ganglion (Castellucci et al. 1970). Initial studies indicated that these have been low threshold mechanoreceptors (Byrne et al. 1974), but this was later shown to become on account of sensitization induced by tightly pinning out the siphon (Illich and Walters 1997). Within a “free siphon” model low-level tactile stimuli that evoked siphon withdrawal failed to activate LE cells. Even so, upon reaching activation threshold LE-cell activity improved with stimulus strength and maximal activity occurred when crushing tearing stimuli, causing physique wall damage, had been applied. They are characteristics of nociceptors, cells tuned to detect noxious stimuli. A second group of sensory neurons will be the ventrocaudal (VC) cells from the pleural ganglia. Tactile pressure for the organism’s posterior generates graded responses in these cells, which adapt slowly to maintained stimulation. Despite the fact that responsive to weak stimuli, 2-Palmitoylglycerol Cannabinoid Receptor VC-cells respond most vigorously to pinching in the posterior, which simultaneously evokes “tail” withdrawal, suggestive that pinching is noxious and, as a result, that VC-cells are acting as nociceptors. Certainly electrical activation of a VC sensory neuron induced motor neuron activation and withdrawal on the “tail”, or additional properly the posterior, supporting this theory (Walters et al. 1983). This potential of VC-cells to respond to weak stimulation and most vigorously toT 20mV 200ms Ptouch 7gPN7g 21gFig. three Intracellular recordings from T-, P- and N-cells.