In the present study,we produced null-mutant mice of neuropsin, an extracel
lular matrix serine protease, to examine the neural functions of this prote
in particularly in the hippocampus. Golgi-Cox impregnation and NissI-staini
ng revealed morphological change of cell soma in the mutant mice compared t
o wild-type mice. However, Golgi-Cox impregnation revealed no apparent chan
ge in the dendritic arborization and spine density. Quantitative electronmi
croscopic analysis revealed that number of asymmetrical synapses were signi
ficantly decreased in the stratum radiatum, the major terminal field of Sch
affer-collaterals, whereas free boutons still holding synaptic vesicles but
with no synaptic specialization were increased in number in the same micro
scopic fields. An increased number of parvalbumin-immunoreactive cells (kno
wn as fast spiking cells) in mutant was also observed. These results strong
ly suggest that neuropsin is involved in connectivity of a group of CA1 syn
apses and consequently in the hippocampal networking.