Testing the Benefits of Rat Tickling on a Rodent Model of Persistent Inflammatory Pain

Chronic pain is a debilitating health problem that affects billions of people worldwide, requiring new, safe, and effective treatments. Current approaches to addressing this issue include nocifensive (pain-related) behaviour testing in animals and investigations of specific molecular determinants to observe how they influence pain behaviours in rodent models. Pain is a biopsychosocial phenomenon, even in rodents, and emotional/stressful states and basal experiences can modulate pain outputs, which can be a major confound in nocifensive behaviour testing. This project explored the effects of rat tickling (playful handling by the experimenter that mimics natural rough-and-tumble play) on the lab’s well-established persistent inflammatory pain model using von Frey filament (VFF) testing and recordings of 22- (alarm calls) and 50-kHz (indicative of positive affect) ultrasonic vocalizations (USVs). Adult male Sprague Dawley rats underwent tickle training or gentle handling followed by evoked somatosensory VFF testing. Baseline testing was followed by a subcutaneous left hind paw footpad injection of 0.3 mL of the pro-inflammatory substance Complete Freund’s Adjuvant (CFA) or vehicle. The animals then underwent VFF testing 24-, 48-, and 72-hours post-injection to verify that pain hypersensitivity was induced. Both tickled and gentle handled rats produced similar VFF testing results, but tickled rats produced more 50-kHz USVs than gentle handled rats. These results suggest both conditions are effective acclimation strategies, but that tickling seems to increase the production of 50-kHz USVs, suggesting improved welfare of the animals. We conclude that animal welfare is an important factor to consider when conducting VFF nocifensive behaviour testing.