Pridopidine selectively occupies sigma-1 rather than dopamine D2 receptors at behaviorally active doses

RATIONALE: Dopamine stabilizers have stimulatory actions under low dopamine tone and inhibitory actions under high dopamine tone without eliciting catalepsy. These compounds are dopamine D(2) receptor (D(2)R) antagonists or weak partial agonists and may have pro-mnemonic and neuroprotective effects. The mechanism underlying their stimulatory and neuroprotective actions is unknown but could involve sigma-1R binding. OBJECTIVES: The present study examined sigma-1R and D(2)R occupancy by the dopamine stabilizer pridopidine (ACR16) at behaviorally relevant doses in living rats. METHODS: Rats were administered 3 or 15 mg/kg pridopidine, or saline, before injection of the radiotracer (11)C-SA4503 (sigma-1R) or (11)C-raclopride (D(2)R). Some animals received 60 mg/kg pridopidine and were only scanned with (11)C-raclopride. Cerebral (11)C-SA4503 binding was quantified using metabolite-corrected plasma input data and distribution volume (V(T)) calculated by Logan graphical analysis. (11)C-raclopride binding was quantified using striatum-to-cerebellum ratios and binding potentials calculated with a simplified reference tissue model. RESULTS: Cunningham-Lassen plots indicated sigma-1R occupancies of 57 ± 2 and 85 ± 2 % after pretreatment of animals with 3 and 15 mg/kg pridopidine. A significant (44–66 %) reduction of (11)C-raclopride binding was only observed at 60 mg/kg pridopidine. CONCLUSIONS: At doses shown to elicit neurochemical and behavioral effects, pridopidine occupied a large fraction of sigma-1Rs and a negligible fraction of D(2)Rs. Significant D(2)R occupancy was only observed at a dose 20-fold higher than was required for sigma-1R occupancy. The characteristics of dopamine stabilizers may result from the combination of high sigma-1R and low D(2)R affinity.