Persistent Defect in Transmitter Release and Synapsin Phosphorylation in Cerebral Cortex After Transient Moderate Ischemic Injury

Abstract

Background and Purpose-Synaptic transmission is highly vulnerable to metabolic perturbations. However, the long-term consequences of transient metabolic perturbations on synapses are not clear. We studied the long-lasting changes in synaptic transmission and phosphorylation of presynaptic proteins in penumbral cortical neurons after transient moderate ischemia. Methods-Rats were subjected to 1 hour of middle cerebral artery occlusion. After reperfusion, electric activity of neurons in the peri-infarct region was recorded intracellularly and extracellularly in situ. Phosphorylation of synapsin-I and tyrosine residues was studied by immunohistochemistry. Results-Neurons in the penumbra displayed no postsynaptic potentials 1 to 3 hours after recirculation. However, these cells were able to generate action potentials and were responsive to glutamate, suggesting that postsynaptic excitability was preserved but the synaptic transmission was blocked because of a presynaptic defect. The synaptic transmission was still depressed 24 hours after recirculation in neurons in the peri-infarct area that survived ischemia. The amount of immunoreactive synapsin-I, synaptophysin, and synaptotagmin was not appreciably changed for 72 hours after reperfusion. However, phosphorylation of synapsin-l was significantly decreased, whereas phosphotyrosine immunoreactivity was increased, suggesting a selective defect in synapsin-I phosphorylation. Conclusions-These data demonstrate that synaptic transmission may be permanently impaired after transient moderate brain injury. Since postsynaptic excitability is preserved, the transmission failure is likely to be caused by presynaptic mechanisms, one of which may be impaired phosphorylation of presynaptic proteins.

Ischemic Cerebral Damage

“…8,35,36 Thereafter, changes in synaptic responses after variable periods of hypoxia or ischemia have been shown frequently in vitro 37,38,39,40 and in vivo, 41,42,43 and several causes of presynaptic and postsynaptic ischemic failure have been postulated.…”

“…They found decreased phosphosynapsin immunoreactivity, suggesting a permanent ischemia-induced phosphorylation defect. 42 Evidence of failure of presynaptic calcium channels as a cause of presynaptic hypoxic failure dates from the 1980s and 1990s and has been demonstrated by voltage clamp experiments [52][53][54] and measurements of extracellular in intracellular calcium concentrations in vitro. [55][56][57] Elevated intracellular Ca 2ϩ levels result from hypoxia-induced inflow from outside and release from internal stores.…”

Zinc-Dependent Multi-Conductance Channel Activity in Mitochondria Isolated from Ischemic Brain

“…During ischemia, mitochondria sense oxygen levels and decrease ATP production (Sugawara et al, 1999;Wang et al, 2003). The effects of energy depletion on neurons (Howard et al, 1998;Tian and Baker, 2000;Fleidervish et al, 2001;Bolay et al, 2002) include neuronal membrane depolarization, synaptic glutamate release, a rise in cytosolic Ca 2ϩ , and swelling of cells (Choi, 1994;Sattler and Tymianski, 2000;Nishizawa, 2001;Zukin et al, 2004). Just after reperfusion, ATP levels and membrane polarization are restored .…”

Spatial Memory Impairment Without Apoptosis Induced by the Combination of Beta-Amyloid Oligomers and Cerebral Ischemia Is Related to Decreased Acetylcholine Release in Rats

“…High K + -evoked release may reflect stored neurotransmitter or the efficiency of synapse vesicle recycling in presynapses. Ischemia induces presynaptic defects because phosphorylation of synapsin-I, a synaptic vesicle protein, is decreased (32). Moreover, Aβ oligomers localize to cell processes (25).…”