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Restless legs syndrome

Revisiting the dopamine hypothesis from the spinal cord perspective

From the Department of Physiology (S.C., S.H.) and Department of Neurology (D.R.), Emory University School of Medicine, Atlanta, GA is currently with the Department of Biomedical Engineering, Emory University School of Medicine and Georgia Institute of Technology.

Address correspondence and reprint requests to Dr. Shawn Hochman, Emory University School of Medicine, Department of Physiology, 615 Michael St., Atlanta, GA 30322; e-mail: shochm2{at}emory.edu

Restless legs syndrome (RLS) involves abnormal limb sensations that diminish with motor activity, worsen at rest, have a circadian peak in expression in the evening and at night, and can severely disrupt sleep. Primary treatment is directed at CNS dopaminergic systems, particularly activation of D₂-like (D₂, D₃, and D₄) receptors. Although RLS affects 2% to 15% of the general population, the neural circuitry contributing to RLS remains speculative, and there is currently no accepted animal model to enable detailed mechanistic analyses. Traditional views suggest that RLS arises from supraspinal sources which favor facilitation of the flexor reflex and emergence of the RLS phenotype. The authors forward the hypothesis that RLS reflects a dysfunction of the little-studied dorsoposterior hypothalamic dopaminergic A11 cell group. They assert that, as the sole source of spinal dopamine, reduced drive in this system can lead to spinal network changes wholly consistent with RLS. The authors summarize their recent investigations on spinal cord dopamine dysfunction that rely on lesions centered on A11, and on studies in D₃ receptor knockout (D₃KO) mice. Excessive locomotor behavior is evident in both sets of animals, and D₃KO mice exhibit facilitation rather than the expected depression of spinal reflexes in the presence of dopamine as well as a reversal in their circadian expression of the rate-limiting enzyme for dopamine synthesis, tyrosine hydroxylase. Taken together, these findings are consistent with an involvement of spinal dopamine dysfunction in the etiology of RLS, and they argue that the D₃KO mouse might serve as a relevant animal model to study the underlying mechanisms of RLS.

Disclosure: The authors report no conflicts of interest.

Received October 12, 2005. Accepted in final form March 16, 2006.

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