Writhing
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                                                An intraperitoneal injection of acetic acid or phenylquinone induces a transient state of somatic (visceral) pain. The ability of putative analgesic compounds to block the physiological response is measured.
                                                AB
                                                Morphine and the kappa opioid agonist, U50488, dose-dependently decrease the number of writhes induced by phenylquinone (A) and acetic acid (B).