The effect of intradermal NGF injection on temporal changes in cutaneous mechanical and thermal sensitivity in pigs

Background and aims: We investigated effects of intradermal NGF injection on cutaneous mechanical and thermal sensitivity in pigs. Methods: Eight female pigs (50±5kg; 15 weeks of age) were habituated to the procedures before testing. Treatment pigs (n=4) received 25 x 20μl intradermal injections into a 5x5cm area tail root of either 10μg NGF (in PBS; 0.1% BSA) or PBS vehicle (control). Two mechanical (von Frey filaments (vF), pressure application measurement device (PAM)) and one thermal assay (CO2 laser) were used. Behavioural responses to stimulation at a 2x2cm test area on the dorsal tail base were measured during test sessions 1 day before and 3, 7, 14 and 21 days after NGF or vehicle, while the pigs were kept in a test cage and accompanied by a pen mate for social support. Within the test session each assay involved three stimulations per pig. Data were analysed by GLM repeated measures ANOVA.Results: NGF injection induced increased (p<0.05) punctate mechanical sensitivity 3-7 days post injection, compared with controls. At 14 and 21 days no significant effects on noxious sensitivity were detected.Conclusions: In this pilot study we showed signs of mechanical hyperalgesia on day 3 and 7, but the measurements of NGF-induced cutaneous sensitivity appeared to differ depending on the mode of stimulation used. Further research is needed to clarify whether these findings, apparently differing from the temporal profiles found in humans, are species differences or can be explained by methodological differences.