Neuromodulation in Tourette syndrome: dopamine and beyond.

Almost since the beginning of research on Tourette syndrome (TS), tics have been linked to a dysfunction of the dopamine (DA) system. At first, this assumption was mainly based on clinical findings of DA antagonists being the most effective drug in treating tics, but in recent years nuclear imaging has enabled a much deeper understanding of DA neurotransmission in TS. Based on the findings of various PET and SPECT studies the first part of the review discusses four hypotheses on DA dysfunctions in TS: (i) DA hyperinnervation, (ii) supersensitive DA receptors, (iii) pre-synaptic DA abnormality and (iv) DA tonic-phasic dysfunction. According to the latter hypothesis, reduced levels of tonic DA in the extracellular space lead to higher concentrations of DA in the axon terminal and an increase of stimulus-dependent DA release. The second part of the review addresses the modulating role of DA in some major clinical features of TS, like the exacerbation with stress or infection and the association with deficient sensorimotor gating.