A variety of
neurotransmitters and other chemical substances are released into the extracellular space in the brain in response to acute ischemic stress, and the
biological actions of these substances are exclusively mediated by receptor-linked second messenger systems. One of the well-known second messenger systems is
adenylate cyclase, which catalyzes the generation of
cyclic AMP, triggering the activation of
cyclic AMP-dependent protein kinase (PKA). PKA controls a number of cellular functions by phosphorylating many substrates, including an important
DNA-binding
transcription factor,
cyclic AMP response element binding protein (CREB). CREB has recently been shown to play an important role in many physiological and pathological conditions, including synaptic plasticity and neuroprotection against various insults, and to constitute a convergence point for many signaling cascades. The autoradiographic method developed in our laboratory enables us to simultaneously quantify alterations of the second messenger system and local cerebral blood flow (lCBF).
Adenylate cyclase is diffusely activated in the initial phase of acute
ischemia (< or = 30 min), and its activity gradually decreases in the late phase of
ischemia (2-6 h). The areas of reduced
adenylate cyclase activity strictly coincide with
infarct areas, which later become visible. The binding activity of PKA to
cyclic AMP, which reflects the functional integrity of the
enzyme, is rapidly suppressed during the initial phase of
ischemia in the ischemic core, especially in vulnerable regions, such as the CA1 of the hippocampus, and it continues to decline. By contrast, PKA binding activity remains enhanced in the peri-
ischemia area. These changes occur in a clearly lCBF-dependent manner. CREB phosphorylation at a
serine residue, Ser(133), which suggests the activation of CREB-mediated transcription of genes containing a CRE motif in the nuclei, remains enhanced in the peri-
ischemia area, which is spared of
infarct damage. On the other hand, CREB phosphorylation at Ser133 rapidly diminishes in the ischemic core before the histological damage becomes manifest. The Ca2+ influx during membrane depolarization contributes to CREB phosphorylation in the initial phase of post-ischemic recirculation, while PKA activation and other signaling elements seem to be responsible in the later phase. These findings suggest that derangement of
cyclic AMP-related intracellular signal transduction closely parallels ischemic neuronal damage and that persistent enhancement of this signaling pathway is important for neuronal survival in acute
cerebral ischemia.