Capsaicin-evoked brain activation and central sensitization in anaesthetised rats: A functional magnetic resonance imaging study

Functional magnetic resonance imaging (fMRI) of blood oxygen level dependent (BOLD) haemodynamic responses was used to study the effects of the noxious substance capsaicin on whole brain activation in isofluorane anaesthetised rats. Rats (n = 8) received intradermal injection of capsaicin (30 μg/5 μl), or topical cream (0.1%) capsaicin and BOLD responses were acquired for up to 120 min. Effects of capsaicin versus placebo cream treatment on the BOLD response to a 15 g mechanical stimulus applied adjacent to the site of cream application were also studied. Both injection and cream application of capsaicin activated brain areas involved in pain processing, including the thalamus and periaqueductal grey (PAG) (p < 0.05, corrected for multiple comparisons). Capsaicin also produced increases in BOLD signal intensity in other regions that contribute to pain processing, such as the parabrachial nucleus and superior colliculus. Mechanical stimulation in capsaicin-treated rats, but not placebo-treated rats, induced a significant decrease in BOLD signal intensity in the PAG (p < 0.001). These data demonstrate that the noxious substance capsaicin produces brain activation in the midbrain regions and reveals the importance of the PAG in central sensitization.