Astrocytes contribute to regulation of extracellular calcium and potassium in the rat cerebral cortex during spreading depression

Processes and components participating in the generation of intrinsic optical signal changes in vitro

Imaging of intrinsic optical signals has become an important tool in the neurosciences. To better understand processes underlying changes in intrinsic optical signals, we studied electrical stimulation at varying strengths in hippocampal slices of adult Wistar rats. Following serial stimulation we observed an increase in light transmittance in all tested slices. During antidromic stimulation at minimum stimulation strength the increase in light transmittance was 75 +/- 8% (P < 0.05), and during orthodromic minimum stimulation 19.6 +/- 5.6% (P < 0.001) in the stratum pyramidale of the CA1-region. During orthodromic stimulation no significant difference between submaximum, maximum and supramaximum stimulation was found, indicating saturation. In contrast, submaximum antidromic stimulation yielded 56.2 +/- 12% (P < 0.05) of maximum stimulation strength, indicating recruitment. In a further set of experiments serial stimulation was carried out under glial blockade with fluoroacetate (FAC) or blockage of mitochondrial function. Amplitude and slope of the intrinsic optical signal significantly decreased in the presence of FAC (amplitude: 36 +/- 6%, P < 0.01; slope: 37 +/- 11% as compared with baseline conditions, P < 0.05). This suggests a glial participation in signal generation. Rotenone, an inhibitor of mitochondrial complex I, yielded decreased amplitudes of the intrinsic optical signal (27 +/- 7% after 40 min, P < 0.01). Our data indicate that the intrinsic optical signal change reflects type and strength of neuronal activation and point to glia and mitochondria as important participants in signal generation.

Astrocytic contributions to bioenergetics of cerebral ischemia

Astrocytes are multifunctional cells that interact with neurons and other astrocytes in signaling and metabolic functions, and their resistance to pathophysiological conditions can help restrict loss of tissue after an ischemic event provided adequate nutrients are supplied to support their requirements. Astrocytes have substantial oxidative capacity and mechanisms to upregulate glycolytic capability when respiration is impaired. An astrocytic enzyme that synthesizes a powerful activator of glycolysis is not present in neurons, endowing astrocytes with the ability to sustain ATP production under restrictive conditions. The monocarboxylic acid transporter (MCT) isoforms predominating in astrocytes are optimized to facilitate very large increases in lactate flux as lactate concentration increases within (1-3 mM) and above (>3 mM) the normal range. In sharp contrast, the major neuronal MCT serves as a barrier to increased transmembrane transport as lactate rises above 1 mM, restricting both entry and efflux. Lactate can serve as fuel during recovery from ischemia but direct evidence that lactate is oxidized by neurons (vs. astrocytes) to maintain synaptic function is lacking. Astrocytes have critical roles in regulation of ionic homeostasis and control of extracellular glutamate levels, and spreading depression associated with ischemia places high demands on energy supplies in astrocytes and contributes to metabolic exhaustion and demise. Disruption of Ca2+ homeostasis, generation of oxygen free radicals and nitric oxide, and mitochondrial depolarization contribute to astrocyte death during and after a metabolic insult. Novel pharmaceutical agents targeted to astrocytes and hyperoxic therapy that restores penumbral oxygen level during energy failure might improve postischemic outcome.

Lasting accelerative effects of 1 Hz and 20 Hz electrical stimulation on cortical spreading depression: relevance for clinical applications of brain stimulation

Clinical applications of brain stimulation have been increasing during the last decade; however, the mechanisms of action remain unknown. One proposed mechanism of action is that repetitive stimulation modulates cortical excitability. Herein, we explore the question of whether repetitive electric stimulation increases cortical excitability as indexed by the cortical spreading depression. Twenty-four Wistar rats were divided into three groups according to the treatment: sham, 1-Hz and 20-Hz stimulation. Stimulation was applied to the left frontal cortex through a pair of epidurally implanted silver-wire electrodes. The cortical spreading depression-features were analysed at three time points (one day before, one day after and 2 weeks after treatment) in both the stimulated and unstimulated hemisphere. A 3 x 2 x 3 factorial anova with repeated measures showed significant differences in the main effect of time (P < 0.0001), hemisphere (P = 0.0002) and stimulation group (P = 0.008). The interaction between time vs. hemisphere vs. stimulation group was also significant (P < 0.0001). Posthoc analysis demonstrated that 1-Hz and 20-Hz repetitive electrical stimulation significantly increased the velocity of cortical spreading depression in the stimulated hemisphere. Furthermore, 20-Hz stimulation showed a greater effect on cortical spreading depression compared to 1-Hz stimulation. The results show that 1-Hz and 20-Hz repetitive electrical stimulation results in an increase in cortical spreading depression velocity that is associated with the frequency and the hemisphere of stimulation. Furthermore, the effects are found to be long lasting. We believe that these findings have strong relevance to support the clinical application of therapies involving electrical stimulation for diseases of reduced cortical excitability.

Inhibition of aconitase in astrocytes increases the sensitivity to chemical convulsants

Although there is evidence that astrocytes support neuronal function, the contribution of astrocytes to seizure onset and termination is not known. To determine whether there are changes in seizure susceptibility or neuronal damage when the ability of astrocytes to generate ATP is reduced, 0.5 nmol of fluorocitrate (FC) was injected into the right ventricle. Injection of FC alone did not produce electrographic or behavioral seizures and did not stress or injure neurons or astrocytes, as measured with silver stain and immunohistochemistry for HSP32 or HSP72. However, in animals pretreated with FC, administration of kainic acid, at a dose that does not initiate seizures in control animals (7 mg/kg), caused wet dog shakes and neuronal damage in the hilus. Wet dog shakes did not cause any neuronal damage in control animals. If the dose of FC was increased to 0.75 nmol, then subsequent administration of the same dose of kainic acid (7 mg/kg) caused stage 3-5 seizures. Injection of FC also reduced the dose of pilocarpine needed to produce seizures. Given simultaneously with FC, isocitrate, which bypasses the biochemical inhibition of aconitase, blocked the effects of FC in both kainic acid and pilocarpine treated animals. The data demonstrate that inhibition of aconitase in astrocytes lowers the doses of both kainic acid and pilocarpine that will cause behavioral seizures and may increase neuronal vulnerability to seizures.