Electrophysiologic characteristics of glial cells

Connexin 43-Mediated Astroglial Metabolic Networks Contribute to the Regulation of the Sleep-Wake Cycle

Astrocytes produce and supply metabolic substrates to neurons through gap junction-mediated astroglial networks. However, the role of astroglial metabolic networks in behavior is unclear. Here, we demonstrate that perturbation of astroglial networks impairs the sleep-wake cycle. Using a conditional Cre-Lox system in mice, we show that knockout of the gap junction subunit connexin 43 in astrocytes throughout the brain causes excessive sleepiness and fragmented wakefulness during the nocturnal active phase. This astrocyte-specific genetic manipulation silenced the wake-promoting orexin neurons located in the lateral hypothalamic area (LHA) by impairing glucose and lactate trafficking through astrocytic networks. This global wakefulness instability was mimicked with viral delivery of Cre recombinase to astrocytes in the LHA and rescued by in vivo injections of lactate. Our findings propose a novel regulatory mechanism critical for maintaining normal daily cycle of wakefulness and involving astrocyte-neuron metabolic interactions.

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.

The problem of astrocyte identity

Astrocytes were the original neuroglia of Ramón y Cajal but after 100 years there is no satisfactory definition of what should comprise this class of cells. This essay takes a historical and philosophical approach to the question of astrocytic identity. The classic approach of identification by morphology and location are too limited to determine new members of the astrocyte population. I also critically evaluate the use of protein markers measured by immunoreactivity, as well as the newer technique of marking living cells by using promoters for these same proteins to drive reporter genes. These two latter approaches have yielded an expanded population of astrocytes with diverse functions, but also mark cells that traditionally would not be defined as astrocytes. Thus we need a combination of measures to define an astrocyte but it is not clear what this combination should be. The molecular approach, especially promoter driven fluorescent reporter genes, does have the advantage of pre marking living astrocytes for electrophysiological or imaging recordings. However, lack of sufficient understanding of the behavior of the inserted constructs has led to unclear results. This approach will no doubt be perfected with time but at present an acceptable, practical definition of what constitutes the class of astrocytes remains elusive.

Localization and regulation of cyclo-oxygenase-1 and -2 and neuronal nitric oxide synthase in mouse spinal cord

Prostaglandins are important mediators in spinal nociceptive processing. They are produced by cyclo-oxygenase isoforms, cyclo-oxygenase-1 and -2, which are both constitutively expressed in the central nervous system. The present immunohistochemical study details localization and regulation of cyclo-oxygenase-1 and -2 and neuronal nitric oxide synthase in lumbar spinal cord before and after induction of a painful paw inflammation in mice. Cyclo-oxygenase-1 immunoreactivity was found in glial cells of the dorsal and ventral horns, but not in neurons. In unstimulated mice, cyclo-oxygenase-2 immunoreactivity was found in motoneurons of the ventral horns and in lamina X, but not in dorsal horn neurons. After induction of a paw inflammation with zymosan, cyclo-oxygenase-2 immunoreactivity increased dramatically in dorsal horn neurons of laminae I-VI and X, paralleled by a significant increase in prostaglandin E(2) release from lumbar spinal cord. Cyclo-oxygenase-2 was co-localized with neuronal nitric oxide synthase immunoreactivity in several neurons in superficial laminae of the dorsal horns and in the area surrounding the central canal. Nitric oxide synthase was distributed in the cytoplasm and extended to processes of some neurons. In contrast, electron microscopy revealed that cyclo-oxygenase-2 immunoreactivity was restricted to the nuclear membrane and rough endoplasmic reticulum. It is shown in the present study that both cyclo-oxygenase isoforms are constitutively expressed in the spinal cord, cyclo-oxygenase-1 in glial cells of the dorsal and ventral horns and cyclo-oxygenase-2 in motoneurons. After induction of a hindpaw inflammation, several dorsal horn neurons express cyclo-oxygenase-2. Some of them are also positive for neuronal nitric oxide synthase, which is also induced following peripheral inflammation. Intracellularly, cyclo-oxygenase-2 is bound to the membranes of the nucleus and endoplasmic reticulum, whereas neuronal nitric oxide synthase is found in the cytoplasm.

Dynamic control of presynaptic Ca(2+) inflow by fast-inactivating K(+) channels in hippocampal mossy fiber boutons

Analysis of presynaptic determinants of synaptic strength has been difficult at cortical synapses, mainly due to the lack of direct access to presynaptic elements. Here we report patch-clamp recordings from mossy fiber boutons (MFBs) in rat hippocampal slices. The presynaptic action potential is very short during low-frequency stimulation but is prolonged up to 3-fold during high-frequency stimulation. Voltage-gated K(+) channels in MFBs inactivate rapidly but recover from inactivation very slowly, suggesting that cumulative K(+) channel inactivation mediates activity-dependent spike broadening. Prolongation of the presynaptic voltage waveform leads to an increase in the number of Ca(2+) ions entering the terminal per action potential and to a consecutive potentiation of evoked excitatory postsynaptic currents at MFB-CA3 pyramidal cell synapses. Thus, inactivation of presynaptic K(+) channels contributes to the control of efficacy of a glutamatergic synapse in the cortex.

Upregulation of metabotropic glutamate receptor subtype mGluR3 and mGluR5 in reactive astrocytes in a rat model of mesial temporal lobe epilepsy

Reactive gliosis is a prominent morphological feature of mesial temporal lobe epilepsy. Because astrocytes express glutamate receptors, we examined changes in metabotropic glutamate receptor (mGluR) 2/3, mGluR5 and transforming growth factor (TGF)-beta in glial cells of the hippocampal regions in an experimental rat model of spontaneous seizures. Rats that exhibited behavioural status epilepticus (SE) directly after 1 h of electrical angular bundle stimulation, displayed chronic spontaneous seizures after a latent period of 1-2 weeks as observed using continuous electrographic monitoring. SE resulted in hypertrophy of astrocytes and microglia activation throughout the hippocampus as revealed by immunolabelling studies. A dramatic, seizure intensity-dependent increase in vimentin immunoreactivity (a marker for reactive astrocytes) was revealed in CA3 and hilar regions where prominent neuronal loss occurs. Increased vimentin labelling was first apparent 24 h after onset of SE and persisted up to 3 months. mGluR2/3 and mGluR5 protein expression increased markedly in glial cells of CA3 and hilus by 1 week after SE, and persisted up to 3 months after SE. Double immunolabelling of brain sections with vimentin confirmed co-localization with glial fibrillary acidic protein (GFAP), mGluR2/3 and mGluR5 in reactive astrocytes. TGF-beta, a cytokine implicated in mGluR3-mediated neuroprotection, was also upregulated during the first 3 weeks after SE throughout the hippocampus. This study demonstrates seizure-induced upregulation of two mGluR subtypes in reactive astrocytes, which - together with the increased production of TGF-beta - may represent a novel mechanism for modulation of glial function and for changes in glial-neuronal communication in the course of epileptogenesis.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Qualitative analysis of membrane currents in glial cells from normal and gliotic tissue in situ: down-regulation of Na+ current and lack of P2 purinergic responses

To date, the electrophysiological properties of glial cells located in reactive scar tissue are unknown. To address this issue two subtypes of hippocampal glial cells, located in thin vital slices of normal or gliotic brain tissue, were analysed for their voltage controlled ion channels using the patch-clamp technique. Reactive gliosis was induced in adult rats by a single peritoneal injection of kainic acid. The intensity of the following seizures was rated ascending from 1 to 6. Rats which exhibited seizures of level 3 or higher showed, within three days, a marked loss of pyramidal cells (60% in CA1 and CA3) and an increase in the density of glial fibrillary acidic protein immunostaining, representing an apparent increase in the number and size of astrocytes in all layers of the hippocampal CA1 subfield. Reactive and normal astrocytes of one subtype, electrophysiologically characterized by time-independent potassium currents, did not significantly differ in membrane potential and potassium conductivity. Glutamine synthetase-positive, but mostly glial fibrillary acidic protein-negative, glial cells (presumably representing immature astrocytes) were also included in this study. This subtype of glial cells showed several voltage- and time-dependent potassium currents and, under control conditions, tetrodotoxin-sensitive voltage-gated Na+ channels, which were almost completely lost after reactive gliosis. Another part of this study focuses on the sensitivity of reactive and control glial cells for extracellular ATP. Several in vitro studies suggest that P2 purinergic receptors on glial cells could trigger the induction of reactive gliosis. In contrast to results described on cultured astrocytes, we found in situ that hippocampal glial cells were not sensitive to ATP or stable P2 receptor agonists in control or in gliotic brain slices. In summary, the presence of at least two different subtypes of hippocampal astrocytes was demonstrated for control as well as for gliotic brain tissue. A dramatic down-regulation of tetrodotoxin-sensitive sodium channels in one subpopulation of reactive astrocytes was shown. This result supports the hypothesis that the presence of active neurons could be required to maintain glial voltage-gated sodium channels. Furthermore, we conclude that there is no longtime expression of P2 purinoceptors on hippocampal astrocytes in situ, and therefore the involvement of astrocytic ATP receptors in the genesis of reactive gliosis is unlikely.

AMPA receptor subunits expressed by single astrocytes in the juvenile mouse hippocampus

The subunit composition of native AMPA receptor (AMPA-R) channels was recently described in several neuronal cell types but less information is available on glial cells. Evidence from recombinant receptor studies suggests that the expression of distinct subunits determines the specific functional properties of the receptor channel. In the present study, we combined the patch clamp technique with the reverse transcription-polymerase chain reaction (RT-PCR) to correlate the expression of gene transcripts with functional properties of AMPA-R in single identified glial cells of the hippocampus. The cells were freshly isolated from the stratum radiatum of the CA1 subregion. We focused on cells expressing AMPA-R with an intermediate Ca2+ permeability which were identified as immature astrocytes due to their morphological, immunocytochemical and electrophysiological characteristics. After recording, the cells were harvested and RT-PCR was performed with the same individual cell to investigate the composition of their AMPA-R transcripts. Our results suggest the expression of a heteromeric subunit architecture. In all cells, the GluR2 subunit was present, which is known to confer a low Ca2+ permeability to the receptor complex. Most frequently, we met co-expression of GluR2 and GluR4. This study demonstrates that astrocytes in the hippocampus express a distinct AMPA-R subunit composition which differs from neurons. The glial receptors might be involved in the modulation of gene expression as well as the regulation of proliferation and differentiation.

Sodium current amplitude increases dramatically in human retinal glial cells during diseases of the eye

Müller cells, the main macroglial cells of the retina, express several types of voltage and ligand-activated ion channels, including Na+ channels. Using the whole-cell voltage-clamp technique, we studied the expression of Na+ currents in acutely isolated, non-cultivated human Müller cells from retinas of healthy organ donors and patients suffering from different eye diseases. In both types of retinas transient Na+ currents could be recorded from Müller cells. The tetrodotoxin-resistant Na+ currents, which were not completely blocked even at a concentration of 10 microM tetrodotoxin, had a mean current density of 3.0 +/- 3.0 pA/pF (mean +/- SD, n = 10) in Müller cells from donor retinas and of 12.2 +/- 9.6 pA/pF (n = 74) in Müller cells from patient retinas. Only 33.3% of healthy but 88.4% of pathological Müller cells depicted such currents. The GNa+/GK+ ratio was very high in several Müller cells from patient retinas, such that action potential-like activity could be generated after prehyperpolarizing current injection in some of these cells. Apparently, the Na+ channels, due to their negative steady-state inactivation curve (Vh = -84.5 mV), do not influence the lowered membrane potential of the pathological cells, since they are inactivated at these voltages. Currently, we do not have an explanation for the increase in amplitude and frequency of Na+ currents in human Müller cells under pathological conditions. However, the up-regulation of Na+ channels may mirror a basic glial response to pathological conditions, since it has also been found previously in human hippocampal astrocytes from epileptic foci and in rat cortex stab wounds lined by an astrocytic scar.

Identified glial cells in the early postnatal mouse hippocampus display different types of Ca2+ currents

Based on their typical pattern of membrane currents, four populations of glial cells could be identified in thin brain slices of the postnatal hippocampus. In the present study, we applied the patch-clamp technique to glial cells in the hippocampal CA1 region, which are characterized by a complex pattern of different Na+ and K+ currents ("complex" cells). These cells were identified as non-neuronal cells, most likely astrocytes, by their glutamine synthetase immunoreactivity. Two types of glial Ca2+ currents could be identified that differed in their kinetics and pharmacological properties. A low-voltage activated (LVA), fast inactivating component was activated at membrane potentials positive to -60 mV and reached maximum current amplitudes at about -20 mV. This current was sensitive to amiloride and thus displayed properties of neuronal LVA currents. The threshold potential of the second Ca2+ current component was at about -40 mV, and peak currents were observed at 0 mV. In contrast to the LVA component, the inactivation of these high-voltage activated (HVA) currents slowed down with increasing depolarizations. This current was sensitive to low concentrations of Cd2+ but was not affected by amiloride. A small fraction of the HVA currents was sensitive to nifedipine, and omega-conotoxin GVIA (omega-CgTx) was also found to reduce the glial HVA component. The study provides electrophysiological and pharmacological characterization of different types of Ca2+ currents in gray matter glial cells in situ.

Long, interlaminar astroglial cell processes in the cortex of adult monkeys

At variance with current descriptions stressing the stellate geometry of cortical astrocytes in the brain of adult mammals, GFAP-immunoreactive astrocytes from prefrontal and rostral cingulate cortices in two adult New World monkey species, Cebus apella and Saimiri sciureus, were found to have long cellular processes traversing several cortical lamina. These unreported features of cortical astroglial cells in adult nonhuman primates pose new issues for the understanding of iso- and allocortical organization and processing in higher mammals.

The possible role of glia in nociceptive processing and hyperalgesia in the spinal cord of the rat

Recent studies have suggested that glia might play a more active role in synaptic function than previously thought. Therefore, the present studies have evaluated the potential role of spinal cord glia in acute nociceptive processing and in the thermal and mechanical hyperalgesia produced by peripheral injury. In the present experiments, we found that: (1) selective inhibition of glia metabolism with intrathecal (i.t.) administration of fluorocitrate (1 nmol) results in a marked, but reversible, attenuation of the persistent thermal and mechanical hyperalgesia produced by intraplantar zymosan (5 mg); (2) selective inhibition of the inducible form of nitric oxide synthase (iNOS) with i.t. aminoguanidine (1 pmol-1 nmol) resulted in a dose-dependent inhibition of the persistent thermal, but not mechanical hyperalgesia produced by intraplantar zymosan (5 mg); (3) i.t. coadministration of interleukin 1 beta (IL1 beta; 10 ng) and interferon gamma (IFN; 1000 U) resulted in expression of the message for iNOS 8 hr after administration assessed using reverse-transcription polymerase chain reaction (RT-PCR) and Southern blot analysis; and (4) i.t. administration of lipopolysaccharide (LPS; 150 micrograms) produced a time-dependent thermal hyperalgesia compared with saline treated-rats (15 microliters). There was no change in mechanical withdrawal thresholds over time following any treatment, except fluorocitrate. We have previously shown that NO plays a significant role in mechanisms of hyperalgesia. In the present experiments we have extended these observations and have now shown a role for iNOS, expressed by glia, in mechanisms of hyperalgesia. These results suggest an unexplored avenue for the development of potential new and novel therapies for pain control.