PMC

Acetic acid-induced writhing in CB1 KO and WT mice. Top, writhing in response to intraperitoneal injection of 0.56% acetic acid. Ordinate: mean number of writhes. Asterisk denotes statistical significance compared with WT (p<0.05). Bottom, effects of morphine on writhing expressed as percent inhibition of writhing. Abscissa: dose of morphine in milligrams per kilogram. Ordinate: percent inhibition of writhing. Bottom inset, effects of morphine on writhing (raw data)

(A) Experimental design depicting the within-subject procedure. In daily sessions, mice were given a 0.9% dose of acetic acid in one physical chamber, Context A, and vehicle injections in a separate physical chamber, Context B. On the final day, mice received an injection of 0.9% acetic acid solution in each context. Writhing behaviors were assessed on days 1 and 4 for the first 30 min following injection. Hindpaw sensitivity was measured 45 min following injection. (B) von Frey withdrawal thresholds of male mice (n = 8) on days 1 and 4 of the paradigm. Acetic acid injection on day 1 (0.9%) induced hindpaw mechanical hypersensitivity (‘hyperalgesic descending control of nociception [DCN]’). Hindpaw mechanical sensitivity differed between the contexts following 0.9% acetic acid injection on day 4, however; males exhibited contextually mediated analgesia in the acid-paired chamber relative to the vehicle-paired chamber on day 4. (C) von Frey withdrawal thresholds of female mice (n = 8) on days 1 and 4 of the paradigm. Acetic acid injection on day 1 (0.9%) induced hindpaw mechanical hypersensitivity (‘hyperalgesic DCN’). Hindpaw mechanical sensitivity differed between the contexts following 0.9% acetic acid injection on day 4, however; females exhibited contextually mediated analgesia in the acid-paired chamber relative to the vehicle-paired chamber on day 4. (D) von Frey withdrawal thresholds of male and female mice replotted to highlight results in the acetic acid paired chamber. Male mice exhibited similar levels of hindpaw mechanical sensitivity in the acid-paired chamber on days 1 and 4. Female mice exhibited less hindpaw sensitivity following 0.9% acetic acid injection on day 4 than they did following 0.9% acetic acid injection on day 1 suggesting the development of conditioned analgesia. (E) Writhing behavior of male mice on days 1 and 4 of the paradigm. On day 1, injection of 0.9% acid induced more writhing than injection of vehicle. Writhing frequency of male mice was similar in both contexts on day 4. (F) Writhing behavior of female mice on days 1 and 4 of the paradigm. On day 1, injection of 0.9% acid induced more writhing than injection of vehicle. Writhing frequency of female mice was lower in the acid-paired context on day 4 than in the vehicle-paired context. (G) Writhing behavior of male and female mice replotted to highlight results in acetic acid paired chamber. Both sexes exhibited less writhing on day 4 relative to day 1.

(A) C57BL/6 mice were treated with anti-Ly6G antibodies or isotype controls for 24 hours and tested in the acetic acid-induced writhing model (n = 5). Reduced numbers of neutrophils was confirmed in the blood (B) and the peritoneal cavity (C) after the writhing model (n = 5). *P<0.05 and ***P<0.001.