Cyclobenzaprine, brand name Flexeril, is a muscle relaxant medication used to relieve skeletal muscle spasms and associated pain in acute musculoskeletal conditions. It is the most well-studied drug for this application, and it also has been used off-label for fibromyalgia treatment. A new bedtime formulation of cyclobenzaprine is under development for the management of fibromyalgia syndrome. The mechanism of action for cyclobenzaprine is unclear. Studies from the 1980s in rats indicated cyclobenzaprine activates the locus ceruleus in the brain stem, leading to an increased release of norepinephrine in the ventral horn of the spinal cord, and the subsequent inhibitory action of norepinephrine on alpha motor neurons. Cyclobenzaprine has been considered structurally related to the first-generation tricyclic antidepressants. Such tricyclics, including amitriptyline, act to inhibit the uptake of norepinephrine, resulting in increased transynaptic norepinephrine concentration.
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