Local and Non‐Local Sensitization Following Repetitive von Frey Filament Testing in Unoperated Sprague‐Dawley and Lewis Rats

ABSTRACT Plantar and dorsal von Frey (VF) filament assays are widely used to assess mechanical nociception in rodents following peripheral nerve injury, spinal cord injury, and analgesic interventions. However, the effects of repeated testing on withdrawal thresholds and potential sensitization have not been well defined. In the current study, we performed various plantar and dorsal VF assays on unoperated male and female Lewis or Sprague‐Dawley rats with minimally or highly repetitive VF testing schedules across 1–15 days with various rest intervals between days. Plantar VF assays were performed at the tibial‐innervated midplantar region eight times per day at hourly intervals (Group #1). Dorsal VF filaments were applied to sural‐ and saphenous‐innervated regions under four different schedules: (Group #2) eight times at hourly intervals per day at bilateral sural regions; (Group #3) once per day at bilateral sural and saphenous regions; (Group #4) twice per day at bilateral sural and saphenous regions; and (Group #5) eight times per day at hourly intervals at the left sural region and twice per day at all other regions to detect non‐local effects. Minimally repetitive dorsal VF tests (Groups #3 and 4) did not produce sensitization. In contrast, highly repetitive plantar (Group #1) and dorsal VF tests (Groups #2 and #5) all produced significant local sensitization that intensified across consecutive days of testing. Group #5 also showed non‐local sensitization at minimally tested regions. Recovery from sensitization occurred after rest intervals following highly repetitive dorsal VF tests (Groups #2 and 5), but not plantar VF tests (Groups #1). Animal restlessness due to repeated handling and testing did not affect results. No significant sex (Groups #1, 2, and 5) or strain differences (Groups #2 and 5) were found within or across the groups for average VF withdrawal thresholds or sensitization. In summary, highly repetitive VF tests can produce sensitization and should be treated as a methodological variable.