6-cyano-7-nitroquinoxaline-2-3-dione and Acidosis

During the initial minutes of cerebral ischemia, lactic acid accumulates and acidifies brain pH to 6.0-6.7. Glutamate is also released during ischemia that activates glutamate receptors and induces excitotoxicity. While glutamate excitotoxicity is well established to induce ischemic injury, a role of lactic acidosis in ischemic brain damage is poorly understood. This study analyzes acidosis neurotoxicity in hippocampal slice cultures in the presence or absence of lactate. At pH 6.7, neuronal loss was similar whether or not lactate was present. At pH 6.4, neuronal loss was significantly greater in the presence of lactate suggesting that lactate potentiates the acidosis toxicity. At pH 6.4 in the presence of lactate, NMDA or non-NMDA receptor antagonists reduced neuronal loss, while in the absence of lactate, NMDA or non-NMDA receptor antagonists had little effect. [3H]-Glutamate uptake was inhibited by acidic pH, and the amount of inhibition was significantly greater in the presence of lactate. These findings suggest that lactate plays a role in acidosis neurotoxicity by inducing excitotoxicity. Lactic acidosis and excitotoxicity have been previously thought to be independent events during ischemia. This study suggests that during ischemia, lactic acidosis contributes to excitotoxic neuronal loss.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Acidosis; Analysis of Variance; Animals; Animals, Newborn; Cell Death; Dizocilpine Maleate; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Glucose; Glutamic Acid; Hippocampus; Hydrogen-Ion Concentration; In Vitro Techniques; Intracellular Space; Lactic Acid; Neurotoxicity Syndromes; Propidium; Rats; Rats, Sprague-Dawley; Time Factors; Tritium

Neurons from many brainstem nuclei involved in respiratory control increase their firing rate in response to acidosis in vitro, suggesting that they are central chemoreceptors. This property has been considered to be either unique to neurons involved in respiratory control, or at least very unusual for non-respiratory neurons. However, recordings of intrinsic pH responses of neurons have not been made from enough non-respiratory regions of the CNS to be certain this assumption is true. Here, we have quantified changes in firing rate of neurons cultured from the hippocampus (n=43), neocortex (n=33), and cerebellum (n=29) in response to changes in CO(2) between 3% and 9% (pH approximately 7.6-7.2) after blockade of glutamatergic and GABAergic transmission. The responses of neurons from these three regions were similar, with a subset of neurons (12% of the total 105) inhibited by acidosis, decreasing their firing rate to a mean of 70% of control in response to a decrease in pH of 0.2. Some neurons (5% of total) were stimulated by acidosis, with an increase in firing rate to a mean of 175% of control in response to a decrease in pH of 0.2. We previously quantified chemosensitivity of neurons from the medullary raphe using the same methods [W. Wang, J.H. Pizzonia, G.B. Richerson, Chemosensitivity of rat medullary raphe neurones in primary tissue culture, J. Physiol., 511 (1998) 433-450]. Compared to these non-respiratory neurons, more raphe neurons were stimulated by acidosis (22%), and the average response was greater (to 300% of control) in response to the same stimulus. Thus, over a physiologically relevant pH range, stimulation by acidosis occurs in a significant percentage of neurons not involved in respiratory chemoreception. However, the degree of chemosensitivity of these neurons was less than medullary raphe neurons under the same conditions. Chemosensitivity is not an all-or-none neuronal property, and the degree of chemosensitivity may be relevant to the role neurons play in sensing pH in vivo.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Acidosis; Action Potentials; Animals; Animals, Newborn; Cells, Cultured; Cerebellum; Chemoreceptor Cells; Fibroblast Growth Factor 2; Fibroblast Growth Factor 5; Fibroblast Growth Factors; Hippocampus; Hydrogen-Ion Concentration; Neocortex; Neurons; Raphe Nuclei; Rats; Respiratory Center