title: No evidence of morphine analgesia to noxious shock in the shore crab, Carcinus maenas.
abstract: 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.
authors.affiliation: School of Biological Sciences, Queen's University, Belfast, MBC, 97 Lisburn Road, Belfast, Northern Ireland BT9 7BL, United Kingdom.
authors.forename: Stuart
authors.initials: S
authors.lastname: Barr
authors.forename: Robert W
authors.initials: RW
authors.lastname: Elwood
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D001143:Arousal / Q000187:drug effects; D001362:Avoidance Learning / Q000187:drug effects; D003386:Brachyura / Q000187:drug effects*; D004597:Electroshock; D004924:Escape Reaction / Q000187:drug effects*; D006801:Humans; D007267:Injections; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009043:Motor Activity / Q000187:drug effects; D017288:Pain Threshold / Q000187:drug effects*; D011930:Reaction Time / Q000187:drug effects
chemical_list: D000701:Analgesics, Opioid; D009020:Morphine