Impairment of cerebral cAMP-mediated signal transduction system and of spatial memory function after microsphere embolism in rats.

The transcription factor cAMP-responsive element binding protein (CREB) has been implicated in synaptic plasticity and memory. The purpose of the present study was to characterize alterations in the cAMP/protein kinase A (PKA)/CREB system after sustained cerebral ischemia. Sustained cerebral ischemia was induced by injection of 900 microspheres (48 microm in diameter) into the right (ipsilateral) hemisphere of rats. Alterations in the CREB, PKA, and cAMP levels in the cerebral cortex and hippocampus were examined up to 7 days after microsphere embolism. Immunoblotting analysis showed a decrease in the immunoreactivity of phosphorylated CREB (pCREB) in the ipsilateral hemisphere on the third day after microsphere embolism, whereas that of the total CREB was not altered. An electrophoretic gel mobility shift assay showed a decrease in the cAMP response element (CRE)-DNA binding activity of CREB in the ischemic region on the third day after the microsphere embolism. Cytosolic PKA C beta in the ipsilateral hemisphere was selectively decreased on the first day after the microsphere embolism, whereas the levels of another catalytic subunit, C alpha, and a regulatory subunit, RII alpha, were not altered. Immunoreactivity of the PKA catalytic subunit C alpha in the nucleus of the ipsilateral hemisphere was decreased on the third day after the embolism. The decreases in the pCREB, CRE-DNA binding activity, and PKA C alpha and C beta levels lasted at least up to 7 days after the operation. A decrease in the cAMP content was also seen in the ipsilateral hemisphere throughout the experiment. Furthermore, microsphere embolized rats showed prolongation of the escape latency in the water maze task determined on the seventh to ninth day after the operation. Our results suggest that sustained cerebral ischemia may impair the phosphorylation and CRE-DNA binding activity of CREB and that these effects may be one of the possible causes for learning and memory dysfunction.