An MRI-compatible and quantifiable mechanical stimulator for allodynia in a rat model of neuropathic pain

We developed an MRI-compatible and quantifiable mechanical stimulator for rats. Functional MRI (fMRI) was used to investigate brain activations resulting from mechanical stimulation in normal rats and in a rat model of neuropathic pain. In the previous MRI-compatible mechanical stimulator, stimulation intensity was not adjustable. In this study, the strength of mechanical stimulation was controlled by von Frey filaments, which were used for mechanical nociception assessment. It provides us to investigate correlations between behavioral sensitivities in von Frey tests and BOLD signal changes during mechanical stimulation. In order to transmit mechanical force to a stimulation site under the strong magnetic field of a 7-T MRI system, a tube-rod structure consisting of nonmagnetic materials was used. The mechanical stimulation evoked a change in blood oxygenation level dependent (BOLD) signals in normal rats. Changes in brain activation were investigated at around- and supra-threshold conditions of mechanical nociception using the filaments for 15 g and 60 g forces. The mechanical stimulation from the 60-g-force filament, which was over the mechanical nociceptive threshold, induced strong brain activation in the areas related to nociceptive pain perception. This result was consistent with that associated with strong electrical stimulation. Mechanical stimulation in the neuropathic pain model evoked brain activity even at around-threshold conditions of mechanical nociception. Higher brain activity in the neuropathic pain model compared with normal rats was considered to be associated with allodynia.