The effect of prolyl oligopeptidase inhibition on extracellular acetylcholine and dopamine levels in the rat striatum.

Prolyl oligopeptidase (PREP, EC 3.4.21.26) inhibitors have potential as cognition enhancers, but the mechanism of action behind the cognitive effects remains unclear. Since acetylcholine (ACh) and dopamine (DA) are known to be associated with the regulation of cognitive processes, we investigated the effects of two PREP inhibitors on the extracellular levels of ACh and DA in the rat striatum using in vivo microdialysis. KYP-2047 and JTP-4819 were administered either as a single systemic dose (50 μmol/kg∼17 mg/kg i.p.) or directly into the striatum by retrodialysis via the microdialysis probe (12.5, 37.5 or 125 μM at 1.5 μl/min for 60 min). PREP inhibitors had no significant effect on striatal DA levels after systemic administration. JTP-4819 significantly decreased ACh levels both after systemic (by ∼25%) and intrastriatal (by ∼30-50%) administration. KYP-2047 decreased ACh levels only after intrastriatal administration by retrodialysis (by ∼40-50%) when higher drug levels were reached, indicating that higher brain drug levels are needed to modulate ACh levels than to inhibit PREP. This result does not support the earlier hypothesis that the positive cognitive effects of PREP inhibitors in rodents would be mediated through the cholinergic system. In vitro specificity studies did not reveal any obvious off-targets that could explain the observed effect of KYP-2047 and JTP-4819 on ACh levels, instead confirming the concept that these compounds have a high selectivity towards PREP.