Regional differences in glial cell modulation of synaptic transmission

Mitochondrial Function in Astrocytes Is Essential for Normal Emergence from Anesthesia in Mice

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

In mice, restriction of loss of the mitochondrial complex I gene Ndufs4 to glutamatergic neurons confers a profound hypersensitivity to volatile anesthetics.Astrocytes are crucial to glutamatergic synapse functioning during excitatory transmission.

WHAT THIS ARTICLE TELLS US THAT IS NEW

In a tamoxifen-activated astrocyte-specific Ndufs4(KO) mouse, the induction EC50s for tail clamp in both isoflurane and halothane were similar between the control and astrocyte-specific Ndufs4(KO) mice at 3 weeks after 4-hydroxy tamoxifen injection. However, the emergent concentrations in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were half that of the controls.Similarly, the induction EC50s for loss of righting reflex were similar between the control and astrocyte-specific Ndufs4(KO) mice; concentrations for regain of righting reflex in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were much less than the control.Thus, mitochondrial complex I function within astrocytes is essential for normal emergence from anesthesia.

BACKGROUND

In mice, restriction of loss of the mitochondrial complex I gene Ndufs4 to glutamatergic neurons confers a profound hypersensitivity to volatile anesthetics similar to that seen with global genetic knockout of Ndufs4. Astrocytes are crucial to glutamatergic synapse functioning during excitatory transmission. Therefore, the authors examined the role of astrocytes in the anesthetic hypersensitivity of Ndufs4(KO).

METHODS

A tamoxifen-activated astrocyte-specific Ndufs4(KO) mouse was constructed. The specificity of the astrocyte-specific inducible model was confirmed by using the green fluorescent protein reporter line Ai6. Approximately 120 astrocyte-specific knockout and control mice were used for the experiments. Mice were anesthetized with varying concentrations of isoflurane or halothane; loss of righting reflex and response to a tail clamp were determined and quantified as the induction and emergence EC50s. Because norepinephrine has been implicated in emergence from anesthesia and astrocytes respond to norepinephrine to release gliotransmitters, the authors measured norepinephrine levels in the brains of control and knockout Ndufs4 animals.

RESULTS

The induction EC50s for tail clamp in both isoflurane and halothane were similar between the control and astrocyte-specific Ndufs4(KO) mice at 3 weeks after 4-hydroxy tamoxifen injection (induction concentration, EC50(ind)-isoflurane: control = 1.27 ± 0.12, astrocyte-specific knockout = 1.21 ± 0.18, P = 0.495; halothane: control = 1.28 ± 0.05, astrocyte-specific knockout = 1.20 ± 0.05, P = 0.017). However, the emergent concentrations in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were less than the controls for tail clamp; (emergence concentration, EC50(em)-isoflurane: control = 1.18 ± 0.10, astrocyte-specific knockout = 0.67 ± 0.11, P < 0.0001; halothane: control = 1.08 ± 0.09, astrocyte-specific knockout = 0.59 ± 0.12, P < 0.0001). The induction EC50s for loss of righting reflex were also similar between the control and astrocyte-specific Ndufs4(KO) mice (EC50(ind)-isoflurane: control = 1.02 ± 0.10, astrocyte-specific knockout = 0.97 ± 0.06, P = 0.264; halothane: control = 1.03 ± 0.05, astrocyte-specific knockout = 0.99 ± 0.08, P = 0.207). The emergent concentrations for loss of righting reflex in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were less than the control (EC50(em)-isoflurane: control = 1.0 ± 0.07, astrocyte-specific knockout = 0.62 ± 0.12, P < 0.0001; halothane: control = 1.0 ± 0.04, astrocyte-specific KO = 0.64 ± 0.09, P < 0.0001); N ≥ 6 for control and astrocyte-specific Ndufs4(KO) mice. For all tests, similar results were seen at 7 weeks after 4-hydroxy tamoxifen injection. The total norepinephrine content of the brain in global or astrocyte-specific Ndufs4(KO) mice was unchanged compared to control mice.

CONCLUSIONS

The only phenotype of the astrocyte-specific Ndufs4(KO) mouse was a specific impairment in emergence from volatile anesthetic-induced general anesthesia. The authors conclude that normal mitochondrial function within astrocytes is essential for emergence from anesthesia.

Aquaporin-4 water channels and synaptic plasticity in the hippocampus

Aquaporin-4 (AQP4) is the major water channel expressed in the central nervous system (CNS) and is primarily expressed in glial cells. Many studies have shown that AQP4 regulates the response of the CNS to insults or injury, but far less is known about the potential for AQP4 to influence synaptic plasticity or behavior. Recent studies have examined long-term potentiation (LTP), long-term depression (LTD), and behavior in AQP4 knockout (KO) and wild-type mice to gain more insight into its potential role. The results showed a selective effect of AQP4 deletion on LTP of the Schaffer collateral pathway in hippocampus using an LTP induction protocol that simulates pyramidal cell firing during theta oscillations (theta-burst stimulation; TBS). However, LTP produced by a different induction protocol was unaffected. There was also a defect in LTD after low frequency stimulation (LFS) in AQP4 KO mice. Interestingly, some slices from AQP4 KO mice exhibited LTD after TBS instead of LTP, or LTP following LFS instead of LTD. These data suggest that AQP4 and astrocytes influence the polarity of long-term synaptic plasticity (potentiation or depression). These potentially powerful roles expand the influence of AQP4 and astrocytes beyond the original suggestions related to regulation of extracellular potassium and water balance. Remarkably, AQP4 KO mice did not show deficits in basal transmission, suggesting specificity for long-term synaptic plasticity. The mechanism appears to be related to neurotrophins and specifically brain-derived neurotrophic factor (BDNF) because pharmacological blockade of neurotrophin trk receptors or scavenging ligands such as BDNF restored plasticity. The in vitro studies predicted effects in vivo of AQP4 deletion because AQP4 KO mice performed worse using a task that requires memory for the location of objects (object placement). However, performance on other hippocampal-dependent tasks was spared. The results suggest an unanticipated and selective role of AQP4 in synaptic plasticity and spatial memory, and underscore the growing appreciation of the role of glial cells in functions typically attributed to neurons. Implications for epilepsy are discussed because of the previous evidence that AQP4 influences seizures, and the role of synaptic plasticity in epileptogenesis.

Amino acid transport system A is involved in inflammatory nociception in rats

Previous studies have indicated that central sensitization is a state of increased excitability of nociceptive neurons in the spinal dorsal horn following peripheral tissue injury and/or inflammation and astrocytes play an important role in the central sensitization. The current study investigated the role of amino acid transport system A in central sensitization and hyperalgesia induced by intraplantar injection of formalin in rats. Formalin (5%, 50μl) injected subcutaneously into the unilateral hindpaw pad induced typical biphase nociceptive behaviors, including licking/biting and flinching of the injected paw and an increase of glial fibrillary acid protein (GFAP, an activated astrocyte marker) expression in spinal dorsal horn, and these effects could be attenuated by intrathecal injection of the competitive inhibitor of amino acid system A transporter, methylaminoisobutyric acid (MeAIB, 0.1, 0.3, 0.5, and 0.7mmol), in a dose-dependent manner. Intrathecal injection of vehicle (PBS) had no effect on the formalin-induced nociceptive behaviors and increase of the GFAP. These findings suggest that amino acid transport system A is involved in inflammation-induced nociception, and inhibition of this transporter system results in inhibition of the central sensitization and hyperalgesia.

Glutaminase-deficient mice display hippocampal hypoactivity, insensitivity to pro-psychotic drugs and potentiated latent inhibition: relevance to schizophrenia

Dysregulated glutamatergic neurotransmission has been strongly implicated in the pathophysiology of schizophrenia (SCZ). Recently, presynaptic modulation of glutamate transmission has been shown to have therapeutic promise. We asked whether genetic knockdown of glutaminase (gene GLS1) to reduce glutamatergic transmission presynaptically by slowing the recycling of glutamine to glutamate, would produce a phenotype relevant to SCZ and its treatment. GLS1 heterozygous (GLS1 het) mice showed about a 50% global reduction in glutaminase activity, and a modest reduction in glutamate levels in brain regions relevant to SCZ pathophysiology, but displayed neither general behavioral abnormalities nor SCZ-associated phenotypes. Functional imaging, measuring regional cerebral blood volume, showed hippocampal hypometabolism mainly in the CA1 subregion and subiculum, the inverse of recent clinical imaging findings in prodromal and SCZ patients. GLS1 het mice were less sensitive to the behavioral stimulating effects of amphetamine, showed a reduction in amphetamine-induced striatal dopamine release and in ketamine-induced frontal cortical activation, suggesting that GLS1 het mice are resistant to the effects of these pro-psychotic challenges. Moreover, GLS1 het mice showed clozapine-like potentiation of latent inhibition, suggesting that reduction in glutaminase has antipsychotic-like properties. These observations provide further support for the pivotal role of altered glutamatergic synaptic transmission in the pathophysiology of SCZ, and suggest that presynaptic modulation of the glutamine-glutamate pathway through glutaminase inhibition may provide a new direction for the pharmacotherapy of SCZ.

Imaging brain activation: simple pictures of complex biology

Elucidation of biochemical, physiological, and cellular contributions to metabolic images of brain is important for interpretation of images of brain activation and disease. Discordant brain images obtained with [(14)C]deoxyglucose and [1- or 6-(14)C]glucose were previously ascribed to increased glycolysis and rapid [(14)C]lactate release from tissue, but direct proof of [(14)C]lactate release from activated brain structures is lacking. Analysis of factors contributing to images of focal metabolic activity evoked by monotonic acoustic stimulation of conscious rats reveals that labeled metabolites of [1- or 6-(14)C]glucose are quickly released from activated cells as a result of decarboxylation reactions, spreading via gap junctions, and efflux via lactate transporters. Label release from activated tissue accounts for most of the additional [(14)C]glucose consumed during activation compared to rest. Metabolism of [3,4-(14)C]glucose generates about four times more [(14)C]lactate compared to (14)CO(2) in extracellular fluid, suggesting that most lactate is not locally oxidized. In brain slices, direct assays of lactate uptake from extracellular fluid demonstrate that astrocytes have faster influx and higher transport capacity than neurons. Also, lactate transfer from a single astrocyte to other gap junction-coupled astrocytes exceeds astrocyte-to-neuron lactate shuttling. Astrocytes and neurons have excess capacities for glycolysis, and oxidative metabolism in both cell types rises during sensory stimulation. The energetics of brain activation is quite complex, and the proportion of glucose consumed by astrocytes and neurons, lactate generation by either cell type, and the contributions of both cell types to brain images during brain activation are likely to vary with the stimulus paradigm and activated pathways.

Metabolic coupling between glia and neurons is necessary for maintaining respiratory activity in transverse medullary slices of neonatal mouse

The respiratory rhythm is generated and regulated by a neuronal network within the lower brainstem. While the neuronal mechanisms of rhythm generation have been extensively investigated, the contribution of glial cells remains to be determined. Here we report the effect of specific blockade of the glial Krebs cycle and glutamine synthetase on the neuronal activity of the respiratory network. Application of 5 mM fluoroacetate, which selectively blocks the glial Krebs cycle, suppressed rhythmic respiratory burst activity. Substitution of either the Krebs cycle substrate isocitrate (3 mM) or glutamine (3 mM) restored rhythmic network activity. Blockade of glutamine synthetase by methionine sulfoximine (0.5 mM) suppressed rhythmic burst activity as well. Resubstitution of glutamine (3 mM) was able to restore rhythmic activity in the presence of methionine sulfoximine. This data demonstrates that the glutamate-glutamine cycle in astrocytes and their supply of glutamine to neuronal glutamatergic terminals is essential for the rhythm generation in the respiratory centre.

Metabotropic glutamate receptors and blockade of glial Krebs cycle depress glycinergic synaptic currents of mouse hypoglossal motoneurons

Metabotropic glutamate receptors are known to depress synaptic transmission by inhibiting transmitter release from presynaptic nerve terminals. This study reports the effects of presynaptic metabotropic glutamate receptor activation on inhibitory synaptic transmission in hypoglossal motoneurons in brainstem slice preparations of neonatal mice. Whole-cell patch-clamp recordings were performed on hypoglossal motoneurons of 2-6-day-old mice. Monosynaptic glycinergic currents were elicited by electrical stimulation of the nucleus of Roller. Application of the specific metabotropic glutamate receptor agonists (+/-)-1-aminocyclopentane-trans-1,3,dicarboxylic acid (t-ACPD), (2S, 2'R,3'R)-2-(2',3'-dicarboxylcyclopropyl)-glycine (DCG-IV) or L-2-amino-4-phosphonobutyric acid (L-AP4) depressed stimulus-evoked glycinergic inhibitory postsynaptic currents (IPSCs) by an average of 39.5, 59.4 and 39.2%, respectively. In the presence of t-ACPD, glycinergic miniature IPSCs were reduced in frequency but not in amplitude, which is indicative of a presynaptic mechanism. A similar reduction of IPSC amplitude was observed in the presence of elevated extracellular glutamate or during application of D, L-threo-hydroxyaspartate (THA), a blocker of glutamate transport, respectively. The data suggest that uptake of glutamate, which is predominately carried out by glial cells, can prevent spill-over of glutamate and activation of metabotropic glutamate receptors. A reduction of IPSCs was also observed following application of monofluoroacetic acid, a substance acting specifically on glial cells. Our results suggest that glial regulation of extracellular glutamate uptake can prevent spill-over of glutamate, and glutamatergic depression of glycinergic inhibition in hypoglossal motoneurons.

The role of glial cells in synaptic function

Glial cells represent the most abundant cell population in the central nervous system and for years they have been thought to provide just structural and trophic support to neurons. Recently, several studies were performed, leading to the identification of an active interaction between glia and neurons. This paper focuses on the role played by glial cells at the level of the synapse, reviewing recent data defining how glia is determinant in synaptogenesis, in the modulation of fully working synaptic contacts and in synaptic plasticity.