Wavelength dependence of acute, light-evoked recruitment of brainstem pain-modulating systems in female rats

Chronic pain states are often associated with sensory hypersensitivities, including acute sensitivity to light. White light exposure has been shown to activate a significant subset of pain-facilitating neurons and suppress firing of pain-inhibiting neurons in the rostral ventromedial medulla (RVM) leading to behavioral hyperalgesia. However, some recent evidence suggests that different wavelengths of light may have distinct effects on pain sensitivity. We therefore sought to understand how specific wavelengths of light influence the activity of RVM pain-modulating neurons. We recorded the activity of identified, white light-responsive ON- and OFF-cells in the RVM of lightly anesthetized female rats, and tested their responses to the blue, green, and red components of a white light stimulus that was sufficient to alter their activity. We found that the majority of RVM OFF- and ON-cells responded during exposure to blue and green, but not red, light, with suppression of antinociceptive OFF-cell firing and activation of pronociceptive ON-cells. This pronociceptive influence of short- and middle-wavelength light was manifested as a significant thermal hyperalgesia during exposure to blue and green light. Our results demonstrate that the pain-modulating cells of the RVM can be engaged by light of specific wavelengths, and that this results in significant behavioral hyperalgesia.