Effect of a Novel Prolyl Endopeptidase Inhibitor, JTP‐4819, on Prolyl Endopeptidase Activity and Substance P‐ and Arginine‐Vasopressin‐Like Immunoreactivity in the Brains of Aged Rats

: The effects of a novel prolyl endopeptidase (PEP) inhibitor, (S)‐2‐[[(S)‐2‐(hydroxyacetyl)‐1‐pyrrolidinyl]carbonyl]‐N‐(phenylmethyl)‐1‐pyrrolidinecarboxamide (JTP‐4819), on the PEP activity in the brain and on the contents of substance P (SP)‐ and arginine‐vasopressin (AVP)‐like immunoreactivity (LI) in the cerebral cortex and hippocampus of young and aged rats were investigated using enzyme immunoassay. JTP‐4819 exhibited a concentration‐dependent in vitro inhibitory action on PEP activity in the brains of both young and aged rats, with IC50 values of ∼0.7 and 0.8 nM, respectively. A single dose of JTP‐4819 (3 mg/kg, p.o.) increased the SPLI content in the cerebral cortex but not the hippocampus of aged rats (23–24 months old). In addition, repeated administration of JTP‐4819 (1 mg/kg, p.o., for 21 days) increased the SPLI content in the cerebral cortex and restored the SPLI content in the hippocampus, which had decreased with aging. In contrast, single (1 mg/kg, p.o.) and repeated (1 mg/kg, p.o., for 21 days) administration of JTP‐4819 only tended to increase the AVPLI content of the hippocampus and cerebral cortex in aged rats, respectively. These results indicate that JTP‐4819 increases the cerebral and hippocampal SPLI content in aged rats by inhibiting the action of PEP.