Highlights
- •Different wavelengths of light elicit different biological responses.
- •Rats exposed to red light develop diffuse nociception, an injury-free pain model.
- •Nociception was reversed by medications commonly used clinically for chronic pain.
Abstract
We report the development and characterization of a novel, injury-free rat model in
which nociceptive sensitization after red light is observed in multiple body areas
reminiscent of widespread pain in functional pain syndromes. Rats were exposed to
red light-emitting diodes (RLED) (LEDs, 660 nm) at an intensity of 50 Lux for 8 hours
daily for 5 days resulting in time- and dose-dependent thermal hyperalgesia and mechanical
allodynia in both male and female rats. Females showed an earlier onset of mechanical
allodynia than males. The pronociceptive effects of RLED were mediated through the
visual system. RLED-induced thermal hyperalgesia and mechanical allodynia were reversed
with medications commonly used for widespread pain, including gabapentin, tricyclic
antidepressants, serotonin/norepinephrine reuptake inhibitors, and nonsteroidal anti-inflammatory
drugs. Acetaminophen failed to reverse the RLED induced hypersensitivity. The hyperalgesic
effects of RLED were blocked when bicuculline, a gamma-aminobutyric acid-A receptor
antagonist, was administered into the rostral ventromedial medulla, suggesting a role
for increased descending facilitation in the pain pathway. Key experiments were subjected
to a replication study with randomization, investigator blinding, inclusion of all
data, and high levels of statistical rigor. RLED-induced thermal hyperalgesia and
mechanical allodynia without injury offers a novel injury-free rodent model useful
for the study of functional pain syndromes with widespread pain. RLED exposure also
emphasizes the different biological effects of different colors of light exposure.
Perspective
This study demonstrates the effect of light exposure on nociceptive thresholds. These
biological effects of red LED add evidence to the emerging understanding of the biological
effects of light of different colors in animals and humans. Understanding the underlying
biology of red light-induced widespread pain may offer insights into functional pain
states.
Key words
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Article info
Publication history
Published online: May 02, 2019
Accepted:
April 15,
2019
Received in revised form:
February 23,
2019
Received:
November 8,
2018
Footnotes
This work was supported, in part, by the National Center for Complementary and Alternative Health (R01AT009716,) (M.I. and R.K.), by the National Institute for Neurological Disorders and Stroke (1R01NS098772 to RK), a Career Development Award from the NINDS (A.P.), and a Children's Tumor Foundation NF1 Synodos grant (to R.K.), and start-up seed funds (to M.I. and R.K.). A.M. was partially supported by a Young Investigator Award from the Children's Tumor Foundation. The University of Arizona partially funded this work through the Start-up seed funds for both Mohab Ibrahim and Amol Patwardhan. The authors have no conflicts of interest to declare.
Supplementary data accompanying this article are available online at www.jpain.org and www.sciencedirect.com.
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© 2019 by the American Pain Society
