Development of a Stimulator for the Characterization of Mechanical-Evoked Pain-Related Supra-Spinal Processing Using BOLD-fMRI in Rodents

Objective: Investigation of pain-related brain processing in animal models is often performed with unspecific stimuli that are not representative of clinically relevant pain phenomena such as punctate hyperalgesia or pressure pain. In order to explore cerebral processing of mechanically evoked pain with functional Magnetic Resonance Imaging (fMRI), a MRI-compatible spatially- and strength-specific mechanical stimulator incorporating either von-Frey filaments or an air-puff system was developed. With this device, mechanical stimuli can be applied to various aspects on the rat hind paw (HP). Methods: The mechanical stimulator consists of an electro-pneumatic unit connected to the part delivering the mechanical stimulation (MS) via a non-magnetic spring for punctate MS (using a von-Frey filament) or via a Lure-lock unit for pressure pain (air-puffs). The strengths of stimuli were calibrated against the delivered air pressure and weight exerted on a pressure pad or in vivo. BOLD fMRI was performed in a 9.4T MRI scanner during calibrated MS at increasing air pressure. Results: We observed a linear relationship between air pressure and force. These calibrations provided quantitative, adjustable, precise and reproducible sub- and supra-threshold MS. Changes in brain activation were investigated up to a force of 154 g using von-Frey filaments, while the maximum force was 30.9 g for air-puffs. Stimulation demonstrated a significant difference between the two types of MS. Unilateral suprathreshold MS induced strong bilateral brain activation in the areas related to pain processing for both types of MS. However, the patterns of brain activity in subcortical areas evoked by von-Frey MS were different from that evoked by air-puffs. Conclusion: The precise delivery of calibrated and reproducible punctate and static MS allows for the specific assessment of clinically relevant supra-spinal correlates of pain-related processes using BOLD fMRI. Significance: A novel device for clinically relevant MS in BOLD fMRI pain studies in rats, providing results that may be directly translated to human pain research.