Recent statistics indicate that one in five people worldwide suffer from moderate to severe chronic pain and that one in three are unable or less able to maintain an independent lifestyle due to their pain.
The trauma of battle, attack by a predator, and even everyday stress often reduce the perception of pain. This phenomenon, called stress-induced analgesia (SIA), was first recognized by researchers after World War II. Little is known about the brain mechanisms responsible for SIA (i.e. the pain relief seen in response to stress). Research into the brain function underlying SIA may help identify novel clinical treatments for severe pain, by targeting these brain pathways.
Researchers at Oregon Health & Science University studied the role of the amygdala in SIA, utilizing an animal model to help identify mechanisms of pain relief. The amygdala or “almond” is a tiny cluster of neurons deep within the limbic or emotional centers of the forebrain. The amygdala is involved in fear and stress responses, and also has strong connections to the brain's principal pain modulation pathway, located in the brainstem.
The results of this study, A Candidate Neural Circuit for Stress-Induced Analgesia, were presented by Nathan R. Selden, MD, PhD, FACS, during the 75th Annual Meeting of the American Association of Neurological Surgeons in Washington, D.C. Co-authors are Justin Ortiz, MD, Liesl Close, BA, and Mary M. Heinricher, PhD.
Researchers tested the hypothesis that stress-induced release of noradrenaline into the central nucleus of the amygdala (CeA) mediates SIA, by microinjecting precise doses of the a2-adrenergic agonist, clonidine into lightly anesthetized adult male Taconic rats, according to an Institutional Animal Care and Use Committee (IACUC)-approved protocol. Clonidine mimics the brain's stress neurotransmitter, norepinephrine. Following injection, the rats were monitored to analyze the effect of the injection on thermal pain perception. Tail-flick withdrawal latencies (TFL) to noxious heat were used to evaluate pain perception. The following results were noted: