Levodopa effects on [ (11)C]raclopride binding in the resting human brain

Area of Science:

• Neuroscience
• Pharmacology
• Radiochemistry

Background:

• Synaptic dopamine (DA) release can displace [11C]raclopride (RAC*) from dopamine D2-like receptors.
• Presynaptic dopaminergic abnormalities are implicated in Tourette Syndrome (TS).
• Exogenous levodopa may offer a method to assess presynaptic dopaminergic function.

Purpose of the Study:

• To determine if levodopa induces measurable synaptic DA release in healthy individuals at rest.
• To gather pilot data on levodopa's effects in individuals with TS.

Main Methods:

• A double-blind, crossover study involving 10 carbidopa-pretreated, neuroleptic-naïve adults (5 TS, 5 controls).
• Positron emission tomography (PET) with RAC* was used to measure synaptic DA release.
• Measurements were taken four times per participant, before and during levodopa or placebo infusion on separate days.

Main Results:

• DA release decreased between scans on both levodopa and placebo days, possibly due to habituation.
• Levodopa did not significantly reduce striatal RAC* binding in either group.
• A significant difference in levodopa's effect on DA release was observed in the right midbrain, with controls showing displacement and TS subjects showing increased RAC* binding.

Conclusions:

• Habituation to study procedures may explain decreased DA release during scans.
• Low baseline firing of mesostriatal DA neurons might explain the lack of significant levodopa effect in the striatum.
• The observed midbrain DA release difference between groups warrants further investigation, despite the small sample size.