Acutely isolated astrocytes as models to probe astrocyte functions

On the electrical passivity of astrocyte potassium conductance

Predominant expression of leak-type K⁺ channels provides astrocytes a high membrane permeability to K⁺ ions and a hyperpolarized membrane potential that are crucial for astrocyte function in brain homeostasis. In functionally mature astrocytes, the expression of leak K⁺ channels creates a unique membrane K⁺ conductance that lacks voltage-dependent rectification. Accordingly, the conductance is named ohmic or passive K⁺ conductance. Several inwardly rectifying and two-pore domain K⁺ channels have been investigated for their contributions to passive conductance. Meanwhile, gap junctional coupling has been postulated to underlie the passive behavior of membrane conductance. It is now clear that the intrinsic properties of K⁺ channels and gap junctional coupling can each act alone or together to bring about a passive behavior of astrocyte conductance. Additionally, while the passive conductance can generally be viewed as a K⁺ conductance, the actual representation of this conductance is a combined expression of multiple known and unknown K⁺ channels, which has been further modified by the intricate morphology of individual astrocytes and syncytial gap junctional coupling. The expression of the inwardly rectifying K⁺ channels explains the inward-going component of passive conductance disobeying Goldman-Hodgkin-Katz constant field outward rectification. However, the K⁺ channels encoding the outward-going passive currents remain to be determined in the future. Here, we review our current understanding of ion channels and biophysical mechanisms engaged in the passive astrocyte K⁺ conductance, propose new studies to resolve this long-standing puzzle in astrocyte physiology, and discuss the functional implication(s) of passive behavior of K⁺ conductance on astrocyte physiology.

Reactive Astrocytes as Therapeutic Targets for Brain Degenerative Diseases: Roles Played by Metabotropic Glutamate Receptors

Astrocytes are well known to play critical roles in the development and maintenance of the central nervous system (CNS). Moreover, recent reports indicate that these cells are heterogeneous with respect to the molecules they express and the functions they exhibit in the quiescent or activated state. Because astrocytes also contribute to pathology, promising new results raise the possibility of manipulating specific astroglial populations for therapeutic roles. In this mini-review, we highlight the function of metabotropic glutamate receptors (mGluRs), in particular mGluR3 and mGluR5, in reactive astrocytes and relate these to three degenerative CNS diseases: multiple sclerosis, Alzheimer's disease and Amyotrophic Lateral Sclerosis. Previous studies demonstrate that effects of these receptors may be beneficial, but this varies depending on the subtype of receptor, the state of the astrocytes, and the specific disease to which they are exposed. Elucidating the role of mGluRs on astrocytes at specific times during development and disease will provide novel insights in understanding how to best use these to serve as therapeutic targets.

Astrocytes Grown in Alvetex(®) Three Dimensional Scaffolds Retain a Non-reactive Phenotype

Protocols which permit the extraction of primary astrocytes from either embryonic or postnatal mice are well established however astrocytes in culture are different to those in the mature CNS. Three dimensional (3D) cultures, using a variety of scaffolds may enable better phenotypic properties to be developed in culture. We present data from embryonic (E15) and postnatal (P4) murine primary cortical astrocytes grown on coated coverslips or a 3D polystyrene scaffold, Alvetex. Growth of both embryonic and postnatal primary astrocytes in the 3D scaffold changed astrocyte morphology to a mature, protoplasmic phenotype. Embryonic-derived astrocytes in 3D expressed markers of mature astrocytes, namely the glutamate transporter GLT-1 with low levels of the chondroitin sulphate proteoglycans, NG2 and SMC3. Embryonic astrocytes derived in 3D show lower levels of markers of reactive astrocytes, namely GFAP and mRNA levels of LCN2, PTX3, Serpina3n and Cx43. Postnatal-derived astrocytes show few protein changes between 2D and 3D conditions. Our data shows that Alvetex is a suitable scaffold for growth of astrocytes, and with appropriate choice of cells allows the maintenance of astrocytes with the properties of mature cells and a non-reactive phenotype.

Spatio-temporal characteristics of metabotropic glutamate receptor 5 traffic at or near the plasma membrane in astrocytes

Astrocytes can sense extracellular glutamate and respond to it by elevating their intracellular Ca(2+) levels via the activation of G-protein coupled receptors, such as metabotropic glutamate receptor 5 (mGluR5), which, during early postnatal development, is the primary receptor responsible for glutamatergic signaling in astrocytes. However, the detailed spatio-temporal characteristics of mGluR5 traffic at or near the plasma membrane of astrocytes are not well understood. To address this issue, we expressed recombinant fluorescent protein chimera of mGluR5 and used total internal reflection fluorescence microscopy on rat visual cortical astrocytes in culture. We used astrocytes lacking major processes, otherwise posing as a diffusion barrier, to infer into the general dynamics of this receptor. We found that plasmalemmal mGluR5 clusters in distinct areas, the size, and initial spatio-temporal level of occupancy of which dictated mGluR5 trafficking characteristics upon glutamate stimulation. These findings will be valuable in the interpretation of point-to-point information transfer and volume transmission between astrocytes and neurons, as well as that of paracrine signaling within astrocytic networks.

Freshly dissociated mature hippocampal astrocytes exhibit passive membrane conductance and low membrane resistance similarly to syncytial coupled astrocytes

Mature astrocytes exhibit a linear current-to-voltage K(+) membrane conductance (passive conductance) and an extremely low membrane resistance (Rm) in situ. The combination of these electrophysiological characteristics establishes a highly negative and stable membrane potential that is essential for basic functions, such as K(+) spatial buffering and neurotransmitter uptake. However, astrocytes are coupled extensively in situ. It remains to be determined whether the observed passive behavior and low Rm are attributable to the intrinsic properties of membrane ion channels or to gap junction coupling in functionally mature astrocytes. In the present study, freshly dissociated hippocampal tissues were used as a new model to examine this basic question in young adult animals. The morphologically intact single astrocytes could be reliably dissociated from animals postnatal day 21 and older. At this animal age, dissociated single astrocytes exhibit passive conductance and resting membrane potential similar to those exhibited by astrocytes in situ. To precisely measure the Rm from single astrocytes, dual-patch single-astrocyte recording was performed. We show that dissociated single astrocytes exhibit a low Rm similarly to syncytial coupled astrocytes. Functionally, the symmetric expression of high-K(+) conductance enabled rapid change in the intracellular K(+) concentrations in response to changing K(+) drive force. Altogether, we demonstrate that freshly dissociated tissue preparation is a highly useful model for study of the functional expression and regulation of ion channels, receptors, and transporters in astrocytes and that passive behavior and low Rm are the intrinsic properties of mature astrocytes.

Studying subcellular detail in fixed astrocytes: dissociation of morphologically intact glial cells (DIMIGs)

Studying the distribution of astrocytic antigens is particularly hard when they are localized in their fine, peripheral astrocyte processes (PAPs), since these processes often have a diameter comparable to vesicles and small organelles. The most appropriate technique is immunoelectron microscopy, which is, however, a time-consuming procedure. Even in high resolution light microscopy, antigen localization is difficult to detect due to the small dimensions of these processes, and overlay from antigen in surrounding non-glial cells. Yet, PAPs frequently display antigens related to motility and glia-synaptic interaction. Here, we describe the dissociation of morphologically intact glial cells (DIMIGs), permitting unambiguous antigen localization using epifluorescence microscopy. Astrocytes are dissociated from juvenile (p13-15) mouse cortex by applying papain treatment and cytospin centrifugation to attach the cells to a slide. The cells and their complete processes including the PAPs is thus projected in 2D. The entire procedure takes 2.5-3 h. We show by morphometry that the diameter of DIMIGs, including the PAPs is similar to that of astrocytes in situ. In contrast to cell culture, results derived from this procedure allow for direct conclusions relating to (1) the presence of an antigen in cortical astrocytes, (2) subcellular antigen distribution, in particular when localized in the PAPs. The detailed resolution is shown in an exemplary study of the organization of the astrocytic cytoskeleton components actin, ezrin, tubulin, and GFAP. The distribution of connexin 43 in relation to a single astrocyte's process tree is also investigated.

Primary cultures of astrocytes: their value in understanding astrocytes in health and disease

During the past few decades of astrocyte research it has become increasingly clear that astrocytes have taken a central position in all central nervous system activities. Much of our new understanding of astrocytes has been derived from studies conducted with primary cultures of astrocytes. Such cultures have been an invaluable tool for studying roles of astrocytes in physiological and pathological states. Many central astrocytic functions in metabolism, amino acid neurotransmission and calcium signaling were discovered using this tissue culture preparation and most of these observations were subsequently found in vivo. Nevertheless, primary cultures of astrocytes are an in vitro model that does not fully mimic the complex events occurring in vivo. Here we present an overview of the numerous contributions generated by the use of primary astrocyte cultures to uncover the diverse functions of astrocytes. Many of these discoveries would not have been possible to achieve without the use of astrocyte cultures. Additionally, we address and discuss the concerns that have been raised regarding the use of primary cultures of astrocytes as an experimental model system.

Bicarbonate efflux via GABA(A) receptors depolarizes membrane potential and inhibits two-pore domain potassium channels of astrocytes in rat hippocampal slices

Increasing evidence indicates the functional expression of ionotropic γ-aminobutyric acid receptor (GABA(A) -R) in astrocytes. However, it remains controversial in regard to the intracellular Cl(-) concentration ([Cl(-) ](i) ) and the functional role of anion-selective GABA(A) -R in astrocytes. In gramicidin perforated-patch recordings from rat hippocampal CA1 astrocytes, GABA and GABA(A) -R-specific agonist THIP depolarized astrocyte membrane potential (V(m) ), and the THIP-induced currents reversed at the voltages between -75.3 and -78.3 mV, corresponding to a [Cl(-) ](i) of 3.1-3.9 mM that favored a passive distribution of Cl(-) anions across astrocyte membrane. Further analysis showed that GABA(A) -R-induced V(m) depolarization was ascribed to HCO(3) (-) efflux, while a passively distributed Cl(-) mediated no net flux or influx of Cl(-) that leads to an unchanged or hyperpolarized V(m) . In addition to a rapidly activated GABA(A) -R current component, GABA and THIP also induced a delayed inward current (DIC) in 63% of astrocytes. The DIC became manifest after agonist withdrawal and enhanced in amplitude with increasing agonist application duration or concentrations. Astrocytic two-pore domain K(+) channels (K2Ps), especially TWIK-1, appeared to underlie the DIC, because (1) acidic intracellular pH, as a result of HCO(3) (-) efflux, inhibited TWIK-1, (2) the DIC remained in the Cs(+) recording solutions that inhibited conventional K(+) channels, and (3) the DIC was completely inhibited by 1 mM quinine but not by blockers for other cation/anion channels. Altogether, HCO(3) (-) efflux through activated GABA(A) -R depolarizes astrocyte V(m) and induces a delayed inhibition of K2Ps K(+) channels via intracellular acidification.

Riluzole and gabapentinoids activate glutamate transporters to facilitate glutamate-induced glutamate release from cultured astrocytes

We have recently demonstrated that the glutamate transporter activator riluzole paradoxically enhanced glutamate-induced glutamate release from cultured astrocytes. We further showed that both riluzole and the α(2)δ subunit ligand gabapentin activated descending inhibition in rats by increasing glutamate receptor signaling in the locus coeruleus and hypothesized that these drugs share common mechanisms to enhance glutamate release from astrocytes. In the present study, we examined the effects of riluzole and gabapentin on glutamate uptake and release and glutamate-induced Ca(2+) responses in primary cultures of astrocytes. Riluzole and gabapentin facilitated glutamate-induced glutamate release from astrocytes and significantly increased glutamate uptake, the latter being completely blocked by the non-selective glutamate transporter blocker DL-threo-β-benzyloxyaspartic acid (DL-TBOA). Riluzole and gabapentin also enhanced the glutamate-induced increase in intracellular Ca(2+) concentrations. Some α(2)δ subunit ligands, pregabalin and L-isoleucine, enhanced the glutamate-induced Ca(2+) response, whereas another, 3-exo-aminobicyclo[2.2.1]heptane-2-exo-carboxylic acid (ABHCA), did not. The enhancement of glutamate-induced intracellular Ca(2+) response by riluzole and gabapentin was blocked by the DL-TBOA and an inhibitor of Na(+)/Ca(2+) exchange, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiurea (KB-R7943). Gabapentin's enhancement of Ca(2+) increase was specific to glutamate stimulation, as it was not mimicked with stimulation by ATP. These results suggest that riluzole and gabapentin enhance Na(+)-glutamate co-transport through glutamate transporters, induce subsequent Ca(2+) influx via the reverse mode of Na(+)/Ca(2+) exchange, and thereby facilitate Ca(2+)-dependent glutamate release by glutamate in astrocytes. The present study also demonstrates a novel target of gabapentinoid action in astrocytes other than α(2)δ subunits in neurons.

Dynamic imaging of cannabinoid receptor 1 vesicular trafficking in cultured astrocytes

Astrocytes possess GPCRs (G-protein-coupled receptors) for neuroactive substances and can respond via these receptors to signals originating from neurons as well as astrocytes. Like many transmembrane proteins, GPCRs exist in a dynamic equilibrium between receptors expressed at the plasma membrane and those present within intracellular trafficking compartments. The characteristics of GPCR trafficking within astrocytes have not been investigated. We therefore monitored the trafficking of recombinant fluorescent protein chimeras of the CB1R (cannabinoid receptor 1) that is thought to be expressed natively in astrocytes. CB1R chimeras displayed a marked punctate intracellular localization when expressed in cultured rat visual cortex astrocytes, an expression pattern reminiscent of native CB1R expression in these cells. Based upon trafficking characteristics, we found the existence of two populations of vesicular CB1R puncta: (i) relatively immobile puncta with movement characteristic of diffusion and (ii) mobile puncta with movement characteristic of active transport along cytoskeletal elements. The predominant direction of active transport is oriented radially to/from the nuclear region, which can be abolished by disruption of the microtubule cytoskeleton. CB1R puncta are localized within intracellular acidic organelles, mainly co-localizing with endocytic compartments. Constitutive trafficking of CB1R to and from the plasma membrane is an energetically costly endeavour whose function is at present unclear in astrocytes. However, given that intracellular CB1Rs can engage cell signalling pathways, it is likely that this process plays an important regulatory role.

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.

Ca2+ entry through TRPC1 channels contributes to intracellular Ca2+ dynamics and consequent glutamate release from rat astrocytes

Astrocytes can respond to a variety of stimuli by elevating their cytoplasmic Ca2+ concentration and can in turn release glutamate to signal adjacent neurons. The majority of this Ca2+ is derived from internal stores while a portion also comes from outside of the cell. Astrocytes use Ca2+ entry through store-operated Ca2+ channels to refill their internal stores. Therefore, we investigated what role this store-operated Ca2+ entry plays in astrocytic Ca2+ responses and subsequent glutamate release. Astrocytes express canonical transient receptor potential (TRPC) channels that have been implicated in mediating store-operated Ca2+ entry. Here, we show that astrocytes in culture and freshly isolated astrocytes from visual cortex express TRPC1, TRPC4, and TRPC5. Indirect immunocytochemistry reveals that these proteins are present throughout the cell; the predominant expression of functionally tested TRPC1, however, is on the plasma membrane. Labeling in freshly isolated astrocytes reveals changes in TRPC expression throughout development. Using an antibody against TRPC1 we were able to block the function of TRPC1 channels and determine their involvement in mechanically and agonist-evoked Ca2+ entry in cultured astrocytes. Blocking TRPC1 was also found to reduce mechanically induced Ca2+-dependent glutamate release. These data indicate that Ca2+ entry through TRPC1 channels contributes to Ca2+ signaling in astrocytes and the consequent glutamate release from these cells.

Vesicular glutamate transporter-dependent glutamate release from astrocytes

Astrocytes exhibit excitability based on variations of their intracellular Ca2+ concentrations, which leads to glutamate release, that in turn can signal to adjacent neurons. This glutamate-mediated astrocyte-neuron signaling occurs at physiological intracellular Ca2+ levels in astrocytes and includes modulation of synaptic transmission. The mechanism underlying Ca2+-dependent glutamate release from astrocytes is most likely exocytosis, because astrocytes express the protein components of the soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptors complex, including synaptobrevin 2, syntaxin, and synaptosome-associated protein of 23 kDa. Although these proteins mediate Ca2+-dependent glutamate release from astrocytes, it is not well understood whether astrocytes express functional vesicular glutamate transporters (VGLUTs) that are critical for vesicle refilling. Here, we find in cultured and freshly isolated astrocytes the presence of brain-specific Na+-dependent inorganic phosphate cotransporter and differentiation-associated Na+-dependent inorganic phosphate cotransporter that have recently been identified as VGLUTs 1 and 2. Indirect immunocytochemistry showed a punctate pattern of VGLUT immunoreactivity throughout the entire cell body and processes, whereas pharmacological inhibition of VGLUTs abolished mechanically and agonist-evoked Ca2+-dependent glutamate release from astrocytes. Taken together, these data indicate that VGLUTs play a functional role in exocytotic glutamate release from astrocytes.

Functional NMDA receptor subtype 2B is expressed in astrocytes after ischemia in vivo and anoxia in vitro

NMDA-type glutamate receptors play a critical role in neuronal synaptogenesis, plasticity, and excitotoxic death. Recent studies indicate that functional NMDA receptors are also expressed in certain glial populations in the normal brain. Using immunohistochemical methods, we detected the presence of the NMDA receptor 2B (NR2B) subunit of the NMDA receptor in neurons but not astrocytes in the CA1 and subicular regions of the rat hippocampus. However, after ischemia-induced neuronal death in these regions, double immunohistochemical labeling revealed that NR2B subunits colocalized with the astrocyte marker glial fibrillary acid protein and with NR1 subunits that are required for functional NMDA receptors. NR2B expression was first observed 3 d after ischemia and reached a peak at 28 d. At 56 d, only a few NR2B-expressing astrocytes were still present. In vitro, when postnatal hippocampal cultures were subjected to 5 min of anoxia, it resulted in NR2B expression on astrocytes in the glial feed layer. Imaging of intracellular calcium with postanoxic cultures and astrocytes isolated acutely from the ischemic hippocampus revealed a rise in intracellular [Ca2+] after stimulation with the specific agonist NMDA. The response could be blocked reversibly with the competitive antagonist 2-amino-5-phosphonovalerate and attenuated by the NR2B-selective antagonist ifenprodil. Control astrocytes were not responsive to NMDA but responded to glutamate. An understanding of the role of astrocytes that express functional NMDA receptors in response to ischemia may guide development of novel stroke therapies.

Development of depolarization-induced calcium transients in insect glial cells is dependent on the presence of afferent axons

Changes in the intracellular Ca(2+) concentration ([Ca(2+)](i)) induced by depolarization have been measured in glial cells acutely isolated from antennal lobes of the moth Manduca sexta at different postembryonic developmental stages. Depolarization of the glial cell membrane was elicited by increasing the external K(+) concentration from 4 to 25 mM. At midstage 5 and earlier stages, less than 20% of the cells responded to 25 mM K(+) (1 min) with a transient increase in [Ca(2+)](i) of approximately 40 nM. One day later, at late stage 5, 68% of the cells responded to 25 mM K(+), the amplitude of the [Ca(2+)](i) transients averaging 592 nM. At later stages, all cells responded to 25 mM K(+) with [Ca(2+)](i) transients with amplitudes not significantly different from those at late stage 5. In stage 6 glial cells isolated from deafferented antennal lobes, i.e., from antennal lobes chronically deprived of olfactory receptor axons, only 30% of the cells responded with [Ca(2+)](i) transients. The amplitudes of these [Ca(2+)](i) transients averaged 93 nM and were significantly smaller than those in normal stage 6 glial cells. [Ca(2+)](i) transients were greatly reduced in Ca(2+)-free, EGTA-buffered saline, and in the presence of the Ca(2+) channel blockers cadmium and verapamil. The results suggest that depolarization of the cell membrane induces Ca(2+) influx through voltage-activated Ca(2+) channels into antennal lobe glial cells. The development of the depolarization-induced Ca(2+) transients is rapid between midstage 5 and stage 6, and depends on the presence of afferent axons from the olfactory receptor cells in the antenna.

Signaling and gene expression in the neuron-glia unit during brain function and dysfunction: Holger Hydén in memoriam

Holger Hydén demonstrated almost 40 years ago that learning changes the base composition of nuclear RNA, i.e. induces an alteration in gene expression. An equally revolutionary observation at that time was that a base change occurred in both neurons and glia. From these findings, Holger Hydén concluded that establishment of memory is correlated with protein synthesis, and he demonstrated de novo synthesis of several high-molecular protein species after learning. Moreover, the protein, S-100, which is mainly found in glial cells, was increased during learning, and antibodies towards this protein inhibited memory consolidation. S-100 belongs to a family of Ca(2+)-binding proteins, and Holger Hydén at an early point realized the huge importance of Ca(2+) in brain function. He established that glial cells show more marked and earlier changes in RNA composition in Parkinson's disease than neurons. Holger Hydén also had the vision and courage to suggest that "mental diseases could as well be thought to depend upon a disturbance of processes in glia cells as in the nerve cells", and he showed that antidepressant drugs cause profound changes in glial RNA. The importance of Holger Hydén's findings and visions can only now be fully appreciated. His visionary concepts of the involvement of glia in neurological and mental illness, of learning being associated with changes in gene expression, and of the functional importance of Ca(2+)-binding proteins and Ca(2+) are presently being confirmed and expanded by others. This review briefly summarizes highlights of Holger Hydén's work in these areas, followed by a discussion of recent research, confirming his findings and expanding his visions. This includes strong evidence that glial dysfunction is involved in the development of Parkinson's disease, that drugs effective in mood disorders alter gene expression and exert profound effects on astrocytes, and that neuronal-astrocytic interactions in glutamate signaling, NO synthesis, Ca(2+) signaling, beta-adrenergic activity, second messenger production, protein kinase activities, and transcription factor phosphorylation control the highly programmed events that carry the memory trace through the initial, signal-mediated short-term and intermediate memory stages to protein synthesis-dependent long-term memory.

Tissue printing of astroglial interlaminar processes from human and non-human primate cerebral cortex

Astroglial interlaminar processes are unique features of the cerebral cortex of adult primates, including man. The functional role of these processes in the primate cerebral cortex is largely unknown. The development and standardization of procedures that could maximize the utilization of primate brain samples is required for the experimental analysis of the individual and collective dynamic properties of interlaminar glial processes. With this aim and in order to assess the relative stability of these glial processes in ex vivo conditions, "tissue printing" procedures were applied. "Tissue printing" allows for the acute transfer of cellular elements from fresh tissue onto an artificial substrate. Human, monkey (Cebus apella), and rat brain samples were subjected to "tissue printing" procedures followed by cell culture and immunohistochemistry. For the purpose of comparing the efficiency of this procedure on the transfer of other long glial processes, "tissue prints" of radial glial processes from neonatal rats and of Bergmann glia from cerebellar samples of adult rats were included. Nitrocellulose (with and without added fibronectin or laminin) produced the best attachment results. Interlaminar processes were not modified following 24-h incubation in a cell culture medium, with the addition of agents known to modify astroglial morphotypes in vitro (cyclic adenosine monophosphate, 40 mM K(+), or fetal calf serum). It is concluded that glia with interlaminar processes can be detached from fresh tissue using "tissue printing" procedures, can be maintained for at least 24 h in standard culture conditions, and showed a stable morphological phenotype.

Developmental expression of metabotropic P2Y(1) and P2Y(2) receptors in freshly isolated astrocytes from rat hippocampus

There are at least three subtypes of cloned metabotropic P2 receptors linked to intracellular Ca(2+) rises in rat brain cells, namely, P2Y(1), P2Y(2) and P2Y(4). In this study we explore the subtypes of the metabotropic P2 receptors seen in freshly isolated astrocytes (FIAs) from P8-P25 rats. We found by single cell RT-PCR that in process-bearing FIAs from hippocampi of P8-P12 rats, 31% of the glial fibrillary acidic protein (GFAP) mRNA (+) cells expressed P2Y(1) mRNA while only 5% of the cells tested expressed P2Y(2) mRNA. The expression of P2Y(1) receptor mRNA was not changed in FIAs from the hippocampi of P18-P25 rats, but 38% of the GFAP mRNA (+) cells in the P18-P25 age group then showed P2Y(2) mRNA. We also studied whether the mRNA was expressing functional receptor protein by measuring Ca(2+) responses to specific agonists for P2Y(1) and P2Y(2). We found that similar proportions of GFAP mRNA (+) FIAs responded to ATP or UTP as showed mRNAs for P2Y (1) and P2Y(2,) respectively. Total tissue RNA from P9 and P24 rat hippocampus showed a 2.8-fold increase in P2Y(2) mRNA levels from P9 to P24 with a decrease in P2Y(1) mRNA. Thus, this study shows a marked up-regulation of mRNA for P2Y(2) from 9 to 24 days in rat hippocampus, and some of this increase is likely due to the protoplasmic astrocytes which is being translated into functional receptor protein in these cells.

Freshly isolated astrocytes from rat hippocampus show two distinct current patterns and different [K(+)](o) uptake capabilities

Whether astrocytes predominantly express ohmic K(+) channels in vivo, and how expression of different K(+) channels affects [K(+)](o) homeostasis in the CNS have been long-standing questions for how astrocytes function. In the present study, we have addressed some of these questions in glial fibrillary acidic protein [GFAP(+)], freshly isolated astrocytes (FIAs) from CA1 and CA3 regions of P7-15 rat hippocampus. As isolated, these astrocytes were uncoupled allowing a higher resolution of electrophysiological study. FIAs showed two distinct ion current profiles, with neither showing a purely linear I-V relationship. One population of astrocytes had a combined expression of outward potassium currents (I(Ka), I(Kd)) and inward sodium currents (I(Na)). We term these outwardly rectifying astrocytes (ORA). Another population of astrocytes is characterized by a relatively symmetric potassium current pattern, comprising outward I(Kdr), I(Ka), and abundant inward potassium currents (I(Kin)), and a larger membrane capacitance (C(m)) and more negative resting membrane potential (RMP) than ORAs. We term these variably rectifying astrocytes (VRA). The I(Kin) in 70% of the VRAs was essentially insensitive to Cs(+), while I(Kin) in the remaining 30% of VRAs was sensitive. The I(Ka) of VRAs was most sensitive to 4-aminopyridine (4-AP), while I(Kdr) of ORAs was more sensitive to tetraethylammonium (TEA). ORAs and VRAs occurred approximately equally in FIAs isolated from the CA1 region (52% ORAs versus 48% VRAs), but ORAs were enriched in FIAs isolated from the CA3 region (71% ORAs versus 29% VRAs), suggesting an anatomical segregation of these two types of astrocytes within the hippocampus. VRAs, but not ORAs, showed robust inward currents in response to an increase in extracellular K(+) from 5 to 10 mM. As VRAs showed a similar current pattern and other passive membrane properties (e.g., RMP, R(in)) to "passive astrocytes"in situ (i.e., these showing linear I-V curves), such passive astrocytes possibly represent VRAs influenced by extensive gap-junction coupling in situ. Thus, our data suggest that, at least in CA1 and CA3 regions from P7-15 rats, there are two classes of GFAP(+) astrocytes which possess different K(+) currents. Only VRAs seem suited to uptake of extracellular K(+) via I(Kin) channels at physiological membrane potentials and increases of [K(+)](o). ORAs show abundant outward potassium currents with more depolarized RMP. Thus VRAs and ORAs may cooperate in vivo for uptake and release of K(+), respectively.