Apoptosis and necrosis occur in separate neuronal populations in hippocampus and cerebellum after ischemia and are associated with differential alterations in metabotropic glutamate receptor signaling pathways

It was evaluated whether postischemic neurodegeneration is apoptosis and oc curs with alterations in phosphoinositide-linked metabotropic glutamate rec eptors (mGluRs) and their associated signaling pathways. A dog model of tra nsient global incomplete cerebral ischemia was used. The CA1 pyramidal cell s and cerebellar Purkinje cells underwent progressive delayed degeneration. By in situ end-labeling of DNA, death of CA1 and Purkinje cells was greate r at 7 days than 1 day after ischemia, whereas death of granule neurons in dentate gyrus and cerebellar cortex was greater at 1 than at 7 days. Ultras tructurally, degenerating CAI pyramidal neurons and cerebellar Purkinje cel ls were necrotic; in contrast, degenerating granule neurons were apoptotic. In agarose gels of regional DNA extracts, random DNA fragmentation coexist ed with internucleosomal fragmentation. By immunoblotting of regional homog enates, mGluR1 alpha, mGluR5, phospholipase C beta (PLC beta), and G alpha( q/11) protein levels in hippocampus at 1 and 7 days after ischemia were sim ilar to control levels, but in cerebellar cortex, mGluR1 alpha. and mGluR5 were decreased but PLC beta was increased. By immunocytochemistry, mGluR an d PLC beta immunoreactivity dissipated in CA1 and cerebellar Purkinje cell/ molecular layers, whereas immunoreactivities for these proteins were enhanc ed in granule neurons. It was concluded that neuronal death after global is chemia exists as two distinct, temporally overlapping forms in hippocampus and cerebellum: necrosis of pyramidal neurons and Purkinje cells and apopto sis of granule neurons. Neuronal necrosis is associated with a loss of phos phoinositide-linked mGluR transduction proteins, whereas neuronal apoptosis occurs with increased mGluR signaling.