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Behav Processes. 2011 Mar;86(3):340-4. doi: 10.1016/j.beproc.2011.02.002. Epub 2011 Feb 13.

No evidence of morphine analgesia to noxious shock in the shore crab, Carcinus maenas.

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School of Biological Sciences, Queen's University, Belfast, MBC, 97 Lisburn Road, Belfast, Northern Ireland BT9 7BL, United Kingdom.


A number of criteria have been suggested for testing if pain occurs in animals, and these include an analgesic effect of opiates (Bateson, 1991). Morphine reduces responses to noxious stimuli in crustaceans but also reduces responsiveness in a non-pain context. Here we use a paradigm in which shore crabs receive a shock in a preferred dark shelter but not if they remain in an unpreferred light area. Analgesia should thus enhance movement to the preferred dark area because they should not experience 'pain'. However, morphine inhibits rather than enhances this movement even when no shock is given. Morphine produces a general effect of non-responsiveness rather than a specific analgesic effect and this could also explain previous studies claiming analgesia. However, we question the utility of this criterion of pain and suggest instead that behavioural criteria be employed.

[Indexed for MEDLINE]

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