The concept of nociception in invertebrates is complex. While it is recognized that many different invertebrates, including mollusks, nematodes, insects and crustaceans, exhibit nociception (response to a noxious stimulus), the idea that invertebrates might “feel” pain is still open to interpretation.1 At the heart of this issue is the fact that it is difficult to determine whether stimulus avoidance behavior in invertebrates is more than a reflex, and whether discomfort registers in the emotional sense that we associate pain with in higher vertebrates.
The presence of opioid receptors in the nervous system of invertebrates has been confirmed in nematodes, mollusks and some insects.3 Administration of opioids, such as morphine, increases the latency of response to a stimulus, such as an electric shock or heat, in many species of invertebrate, whereas opioid antagonists, such as naloxone, seem to abolish this effect.2 However, the data available are not consistent across invertebrate taxa. Information on arachnid nociception is particularly sparse. Our preliminary experiments on Chilean rose tarantulas (Grammostola rosea), using a noxious thermal stimulus, suggest that tarantulas consistently remove the affected limb from the stimulus, and that opioids such as morphine and butorphanol alter this behavior. These findings will be discussed as well as invertebrate nociception and implications for captive management and research.
1. Gunkel, C., and G.A. Lewbart. 2008. Anesthesia and analgesia of invertebrates, In: Fish, R. F., P.J. Danneman,
2. M. Brown, and A. Karas (eds.). Anesthesia and Analgesia in Laboratory Animals, 2nd ed. Elsevier, London. 535–545.
3. Kavaliers, M. Evolutionary and comparative aspects of nociception. 1988. Brain Res. Bull. 21:923–931.
4. Tobin, D.M. and C.L. Bargmann. 2004. Invertebrate nociception: behaviors, neurons and molecules. J. Neurobiol. 61:161–174.