Two types of Na(+)-currents in cultured rat optic nerve astrocytes: changes with time in culture and with age of culture derivation

Astrocytes: The Housekeepers and Guardians of the CNS

Astroglia are a diverse group of cells in the central nervous system. They are of the ectodermal, neuroepithelial origin and vary in morphology and function, yet, they can be collectively defined as cells having principle function to maintain homeostasis of the central nervous system at all levels of organisation, including homeostasis of ions, pH and neurotransmitters; supplying neurones with metabolic substrates; supporting oligodendrocytes and axons; regulating synaptogenesis, neurogenesis, and formation and maintenance of the blood-brain barrier; contributing to operation of the glymphatic system; and regulation of systemic homeostasis being central chemosensors for oxygen, CO₂ and Na⁺. Their basic physiological features show a lack of electrical excitability (inapt to produce action potentials), but display instead a rather active excitability based on variations in cytosolic concentrations of Ca²⁺ and Na⁺. It is expression of neurotransmitter receptors, pumps and transporters at their plasmalemma, along with transports on the endoplasmic reticulum and mitochondria that exquisitely regulate the cytosolic levels of these ions, the fluctuation of which underlies most, if not all, astroglial homeostatic functions.

Ion Channels and Electrophysiological Properties of Astrocytes: Implications for Emergent Stimulation Technologies

Astrocytes comprise a heterogeneous cell population characterized by distinct morphologies, protein expression and function. Unlike neurons, astrocytes do not generate action potentials, however, they are electrically dynamic cells with extensive electrophysiological heterogeneity and diversity. Astrocytes are hyperpolarized cells with low membrane resistance. They are heavily involved in the modulation of K⁺ and express an array of different voltage-dependent and voltage-independent channels to help with this ion regulation. In addition to these K⁺ channels, astrocytes also express several different types of Na⁺ channels; intracellular Na⁺ signaling in astrocytes has been linked to some of their functional properties. The physiological hallmark of astrocytes is their extensive intracellular Ca²⁺ signaling cascades, which vary at the regional, subregional, and cellular levels. In this review article, we highlight the physiological properties of astrocytes and the implications for their function and influence of network and synaptic activity. Furthermore, we discuss the implications of these differences in the context of optogenetic and DREADD experiments and consider whether these tools represent physiologically relevant techniques for the interrogation of astrocyte function.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Nav1.5 in astrocytes plays a sex-specific role in clinical outcomes in a mouse model of multiple sclerosis

Astrogliosis is a hallmark of neuroinflammatory disorders such as multiple sclerosis (MS). A detailed understanding of the underlying molecular mechanisms governing astrogliosis might facilitate the development of therapeutic targets. We investigated whether Nav1.5 expression in astrocytes plays a role in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), a murine model of MS. We created a conditional knockout of Nav1.5 in astrocytes and determined whether this affects the clinical course of EAE, focal macrophage and T cell infiltration, and diffuse activation of astrocytes. We show that deletion of Nav1.5 from astrocytes leads to significantly worsened clinical outcomes in EAE, with increased inflammatory infiltrate in both early and late stages of disease, unexpectedly, in a sex-specific manner. Removal of Nav1.5 in astrocytes leads to increased inflammation in female mice with EAE, including increased astroglial response and infiltration of T cells and phagocytic monocytes. These cellular changes are consistent with more severe EAE clinical scores. Additionally, we found evidence suggesting possible dysregulation of the immune response-particularly with regard to infiltrating macrophages and activated microglia-in female Nav1.5 KO mice compared with WT littermate controls. Together, our results show that deletion of Nav1.5 from astrocytes leads to significantly worsened clinical outcomes in EAE, with increased inflammatory infiltrate in both early and late stages of disease, in a sex-specific manner.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Sodium channels in astroglia and microglia

Voltage-gated sodium channels are required for electrogenesis in excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons, cardiac, and skeletal muscle cells. Cells that have not traditionally been considered to be excitable (nonexcitable cells), including glial cells, also express sodium channels in physiological conditions as well as in pathological conditions. These channels contribute to multiple functional roles that are seemingly unrelated to the generation of action potentials. Here, we discuss the dynamics of sodium channel expression in astrocytes and microglia, and review evidence for noncanonical roles in effector functions of these cells including phagocytosis, migration, proliferation, ionic homeostasis, and secretion of chemokines/cytokines. We also examine possible mechanisms by which sodium channels contribute to the activity of glial cells, with an eye toward therapeutic implications for central nervous system disease. GLIA 2016;64:1628-1645.

Dynamics of sodium channel Nav1.5 expression in astrocytes in mouse models of multiple sclerosis

Astrocytes actively participate in the response of the central nervous system to injury, including in multiple sclerosis. Astrocytes can play both beneficial and detrimental roles in response to neuroinflammation; however, in extreme cases, astrogliosis can result in the formation of a glial scar, which can impede the regeneration of injured neurons. Although astrocytes do not express the voltage-gated sodium channel Nav1.5 in the nonpathological human brain, they exhibit robust upregulation of Nav1.5 within acute and chronic multiple sclerosis lesions. Recent work has indicated that Nav1.5 contributes to the pathways that regulate glial scar formation in vitro through modulation of intracellular Ca levels. However, the temporal dynamics of astrocytic Nav1.5 channel expression in response to neuroinflammatory pathologies has not been investigated. We examined astrocytes from mice with monophasic and chronic-relapsing (CR) experimental autoimmune encephalomyelitis (EAE) by immunohistochemical analysis to determine whether Nav1.5 is expressed in these cells, and whether the expression correlates with the severity of disease and/or phases of relapse and remission. Our results demonstrate that Nav1.5 is upregulated in astrocytes in situ in a temporal manner that correlates with disease severity in both monophasic and CR EAE. Further, in CR EAE, Nav1.5 expression is upregulated during relapses and subsequently attenuated during periods of remission. These observations are consistent with the suggestion that Nav1.5 can play a role in the response of astrocytes to inflammatory pathologies in the central nervous system and suggest Nav1.5 may be a potential therapeutic target to modulate reactive astrogliosis in vivo.

Astrocytes within multiple sclerosis lesions upregulate sodium channel Nav1.5

Astrocytes are prominent participants in the response of the central nervous system to injury, including neuroinflammatory insults. Rodent astrocytes in vitro have been shown to express voltage-gated sodium channels in a dynamic manner, with a switch in expression of tetrodotoxin-sensitive to tetrodotoxin-resistant channels in reactive astrocytes. However, the expression of sodium channels in human astrocytes has not been studied, and it is not known whether there are changes in the expression of sodium channels in reactive astrocytes of the human central nervous system. Here, we demonstrate a focal and robust upregulation of sodium channel Nav1.5 in reactive astrocytes at the borders of, and within, active and chronic multiple sclerosis lesions. Nav1.5 was only detectable at very low levels in astrocytes within multiple sclerosis macroscopically normal-appearing white matter or in normal control brain. Nav1.1, Nav1.2, Nav1.3 and Nav1.6 showed little or no expression in astrocytes within normal control tissue and limited upregulation in active multiple sclerosis lesions. Nav1.5 was also expressed at high levels in astrocytes in tissue surrounding new and old cerebrovascular accidents and brain tumours. These results demonstrate the expression of Nav1.5 in human astrocytes and show that Nav1.5 expression is dynamic in these cells. Our observations suggest that the upregulated expression of Nav1.5 in astrocytes may provide a compensatory mechanism, which supports sodium/potassium pump-dependent ionic homoeostasis in areas of central nervous system injury.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

In vitro neurogenesis by progenitor cells isolated from the adult human hippocampus

Neurogenesis persists in the adult mammalian hippocampus. To identify and isolate neuronal progenitor cells of the adult human hippocampus, we transfected ventricular zone-free dissociates of surgically-excised dentate gyrus with DNA encoding humanized green fluorescent protein (hGFP), placed under the control of either the nestin enhancer (E/nestin) or the Talpha1 tubulin promoter (P/Talpha1), two regulatory regions that direct transcription in neural progenitor cells. The resultant P/Talpha1:hGFP+ and E/nestin:enhanced (E)GFP+ cells expressed betaIII-tubulin or microtubule-associated protein-2; many incorporated bromodeoxyuridine, indicating their genesis in vitro. Using fluorescence-activated cell sorting, the E/nestin:EGFP+ and P/Talpha1:hGFP+ cells were isolated to near purity, and matured antigenically and physiologically as neurons. Thus, the adult human hippocampus contains mitotically competent neuronal progenitors that can be selectively extracted. The isolation of these cells may provide a cellular substrate for re-populating the damaged or degenerated adult hippocampus.

Impairment of spatial memory and changes in astroglial responsiveness following loss of molar teeth in aged SAMP8 mice

In order to evaluate the mechanism(s) responsible for senile impairment of cognitive function as a result of reduced mastication, the effects of the loss of the molar teeth (molarless condition) on the hippocampal expression of glial fibrous acidic protein (GFAP) and on spatial memory in young adult and aged SAMP8 mice were studied using immunohistochemical and behavioral techniques. Aged molarless mice showed a significantly reduced learning ability in a water maze test compared with age-matched control mice, while there was no difference between control and molarless young adult mice. Immunohistochemical analysis showed that the molarless condition enhanced the age-dependent increase in the density and hypertrophy of GFAP-labeled astrocytes in the CA1 region of the hippocampus. These effects increased the longer the molarless condition persisted. When the extracellular K+ concentration ([K+]o) was increased from 4 to 40 mM for hippocampal slices in vitro, the mean increase in the membrane potential was about 57 mV for fine, delicate astrocytes, the most frequently observed type of GFAP-positive cell in the young adult mice, and about 44 mV for the hypertrophic astrocytes of aged mice. However, there was no significant difference in resting membrane potential between these cell types. The data suggest that an impairment of spatial memory and changes in astroglial responsiveness occur following the loss of molar teeth in aged SAMP8 mice.

Immunocytochemical localization of NaCh6 in cultured spinal cord astrocytes

The expression of the alpha-subunit of voltage-gated sodium channel 6 (NaCh6) was examined in cultures of astrocytes from E18 rat spinal cord by using an antibody specific for NaCh6. Stellate cells with processes and flat, pancake-like astrocytes are the two morphological types predominantly present in these cultures. The antibody to NaCh6 labeled clusters at the cell body and along the length of the processes in stellate, process-bearing cells. Weak staining was observed in the flat, pancake-like astrocytes. Together with previous studies (Black et al., Mol Brain Res 23:235-245, 1994, Glia 14:133-144, 1995) that show that stellate cells express NaChs II and III (but not NaCh I) and flat cells express NaCh II, these results support the conclusions that there are different patterns of sodium channel expression between flat and stellate astrocytes and that multiple channel isoforms are expressed within the same cell. This study also suggests that NaCh6 may contribute to the electrical properties found in stellate astrocytes.

Spinal cord astrocytes display a switch from TTX-sensitive to TTX-resistant sodium currents after injury-induced gliosis in vitro

Two distinct morphological subtypes of astrocytes have been shown to express Na+ currents that differ biophysically and pharmacologically. Using an in vitro model for reactive gliosis, we recently reported marked changes in Na+ and K+ channel expression by astrocytes induced to proliferate. Using this in vitro assay in which a confluent monolayer of astrocytes is mechanically scarred to induce gliosis, we now demonstrate that sodium currents of scar-associated cells, in addition to doubling in current density, also switch from being tetrodotoxin-sensitive(TTX-S, IC50 8 nM) to being approximately 40-fold more TTX-resistant (TTX-R,IC50 314 nM). These changes occurred within 6 h after injury and were not associated with any notable changes in cell morphology. Changes in biophysical properties were analyzed for the two current types. The activation curve for TTX-R currents demonstrated a significant depolarized shift versus that of TTX-S currents (P </= 0. 003), and TTX-R currents have more depolarized V1/2 of activation (-33 vs. -23 mV). The V1/2 of inactivation was slightly, but not significantly, more depolarized for TTX-R currents as compared to TTX-S (-63 vs. -68 mV). Most notably, TTX-R currents showed significantly slower inactivation kinetics at depolarized voltage potentials than TTX-S sodium currents (0.76 vs. 1.128 ms, at -10 mV; P < 0.0004).

Postnatal development of ionic currents in rat hippocampal astrocytes in situ

Developmental changes in ion channel expression and cell morphology were studied in glial cells with the use of whole cell patch-clamp recordings in rat [postnatal day (P)5-P50] hippocampal slices. Recordings were obtained from 234 cells, presumed to be glia, in stratum radiatum and stratum lacunosum-moleculare of the CA1 region. Of 66 recorded cells filled with Lucifer yellow, 48 stained positive for glial fibrillary acidic protein, which identified them as astrocytes. All glial cells studied were of a stellate morphology, and developmental changes primarily comprised an increase in the length and number of cell processes associated with an overall increase in cell size and membrane capacitance. Two distinct outward potassium currents could be identified: a transient 4-aminopyridine-sensitive current (Ia) and a persistent outward current sensitive to tetraethylammonium (Id). Ia activated at -40 mV, and steady-state activation and inactivation midpoints were -16 and -74 mV, respectively. Decay time constants ranged from 7 ms at -30 mV to 19 ms at +80 mV. Id activated at -30 mV. A third K+ current sensitive to cesium activated with hyperpolarizing command voltages and showed strong inward rectification. Transient, voltage-activated sodium currents (I(Na)) were tetrodotoxin sensitive (100 nM) and activated at about -40 mV, peaked at about -10 mV, and reversed at +63 mV. I(Na) was half-inactivated at -49 mV and half-activated at -19 mV. During the first 2 wk of postnatal development, the percentage of cells showing inwardly rectifying K+ current (Ir), Ia, and I(Na) increased significantly from 40% (at P5) to 90% (at P20-P50). By contrast, almost all cells independent of age expressed Id. Specific conductances for Ir (g(ir)) and Ia increased significantly between P5 and P20, concomitant with a decrease in input resistance. By contrast, specific conductance of the outwardly rectifying K+ current (g(d)) decreased threefold between P5 and P20. Specific Na+ conductance was always <1/4 of the total potassium conductance. These results indicate that CA1 hippocampal astrocytes are characterized by expression of voltage-activated Na+ channels and three types of K+ channels showing changes in their relative expression during early postnatal development: 1) the number of cells expressing Ia, Ir, and I(Na) increases significantly and 2) their specific conductance changes such that g(d), predominant at P5-P20, is gradually replaced by g(ir), the predominant conductance in adult astrocytes. Adult morphological and electrophysiological phenotypes are established at about P20. These data suggest that previous studies in which cultured or acutely isolated cells from immature or embryonic rats were used were not adequately reflecting the properties of hippocampal astrocytes in situ.

Different voltage-gated sodium currents are expressed by human neuroblastoma NB69 cells when cultured in defined serum-free and in astroglial-conditioned media

Voltage-gated Na+ currents (INa) were analysed with the whole-cell patch-clamp technique in human neuroblastoma NB69 cells plated in serum-free "defined" medium (DM) or in "astroglial-conditioned" medium (CM). Cells survived in both media and expressed the microtubule associated protein 1A, indicating neuron-like differentiation. Two INa types with different time-, voltage-dependent properties and tetrodotoxin (TTX) sensitivities were expressed in DM and CM. The INa in DM-plated cells was present from day 4 and its surface density increased from 11 pA/pF (days 5-7) to 68 pA/pF (days 15-30). The underlying conductance (GNa) half-activated (V0A) at -24 mV. INa inactivation was fitted by single exponentials with 7.5 ms time constant (th) at the -35 mV half-inactivation voltage (V0I). INa was not affected by 10 nM, was reduced (65%) by 100 nM, and not completely abolished (92%) by 300 nM tetrodotoxin (TTX). The INa of CM-plated cells appeared at day 3-4 and its surface density increased from 14 pA/pF (days 3-6) to 28 pA/pF (days 11-14). The GNa V0A was -29 mV and inactivation was fitted by single exponentials with 2.6 ms that the -58 mV V0I. This INa was reduced (55%) by 10 nM and totally abolished by 100 nM tetrodotoxin (TTX). In conclusion, NB69 cells displayed a slow, "TTX-resistant," or a fast, "TTX-sensitive" INa in DM and CM, respectively, suggesting that the CM contained diffusible trophic factors of astroglial origin that induced the expression of a different Na+ channel type. About half of the CM- and DM-plated cells also displayed a persistent Na+ current (INaP).

TTX-sensitive and -resistant Na+ currents, and mRNA for the TTX-resistant rH1 channel, are expressed in B104 neuroblastoma cells

To examine the molecular basis for membrane excitability in a neuroblastoma cell line, we used whole cell patch-clamp methods and reverse transcription-polymerase chain reaction (RT-PCR) to study Na+ currents and channels in B104 cells. We distinguished Tetrodotoxin (TTX)-sensitive and -resistant Na+ currents and detected the mRNA for the cardiac rH1 channel in B104 cells. Na+ currents could be recorded in 65% of cells. In the absence of TTX, mean peak Na+ current density was 126 +/- 19 pA/pF, corresponding to a channel density of 2.7 +/- 0.4/micron 2 (mean +/- SE). Time-to-peak (t-peak), activation (tau m), and inactivation time constants (tau h) for Na+ currents in B104 cells were 1.0 +/- 0.04, 0.4 +/- 0.06, and 0.9 +/- 0.04 ms at -10 mV. The peak conductance-voltage relationship had a V 1/2 of -39.8 +/- 1.5 mV. V 1/2 for steady-state inactivation was -81.6 +/- 1.5 mV. TTX-sensitive and -resistant components of the Na current had half-maximal inhibitions (IC50), respectively, of 1.2 nM and, minimally, 575.5 nM. The TTX-sensitive and -resistant Na+ currents were kinetically distinct; time-to-peak, tau m, and tau h for TTX-sensitive currents were shorter than for TTX-resistant currents. Steady-state voltage dependence of the two currents was indistinguishable. The presence of TTX-sensitive and -resistant Na+ currents, which are pharmacologically and kinetically distinct, led us to search for mRNAs known to be associated with TTX-resistant channels, in addition to the alpha subunit mRNAs, which have previously been shown to be expressed in these cells. Using RT-PCR and restriction enzyme mapping, we were unable to detect alpha SNS, but detected mRNA for rH1, which is known to encode a TTX-resistant channel, in B104 cells. B104 neuroblastoma cells thus express TTX-sensitive and -resistant Na+ currents. These appear to be encoded by neuronal-type and cardiac Na+ channel mRNAs including the RH1 transcript. This cell line may be useful for studies on the rH1 channel, which is known to be mutated in the long-QT syndrome.

Voltage-gated Na+ channels in glia: properties and possible functions

Glial cells are nervous-system cells that have classically been considered to be inexcitable. Despite their lack of electrical excitability, they can express voltage-activated Na+ channels with properties similar to the Na+ channels used by excitable cells to generate action potentials. The functional role that these voltage-activated Na+ channels play in glia is unclear. Three functions have been proposed: (1) glial cells might synthesize Na+ channels and donate them to adjacent neurons, thereby reducing the biosynthetic load of neurons; (2) Na+ channels might endow glial cells with the ability to sense electric activity of neighboring neurons, and might thus play a role in neuro-glial communication; and (3) Na+ influx through voltage-gated Na+ channels could be important to fuel the glial (Na+,K+)-ATPase, thereby facilitating and possibly modulating K+ uptake from the extracellular space.

Potassium currents in cultured glia of the frog optic nerve

The processes that participate in clearing increases in [K+]o produced by active neurons include KCl uptake, Na pump stimulation, and spatial buffering. The latter process requires glial cells to carry: 1) inward K+ currents in regions where K+ is elevated at a glial membrane potential more negative than EK; and 2) outward K+ currents at normal K+ and glial membrane potential more positive than EK (Orkand et al: J Neurophysiol 29:788, 1966). Techniques for isolation and culturing glial cells brought new possibilities for studying ionic channels involved in spatial buffering. However, they raised the question of the extent to which the properties of ionic channels are changed due to the process of culturing when glial cells are exposed to an artificial environment and deprived of direct interaction with neurons. We studied potassium currents in glial cells from the frog optic nerve that were cultured for 1-8 days. At 24-48 h, cells exhibited an inwardly rectifying Cs+ blocked current (IK(IN)) that increased in amplitude and shifted its threshold of activation to EK when [K+]o was increased from 3 to 6 or 10 mM. IK(IN), diminished after 3 days in culture and virtually disappeared after 5 days. At 24-48 h, a potassium delayed rectifier current (IKD) was relatively small but became large at 3 days, and was practically the only current present after 5 days. IKD was activated at -8.5 +/- 0.58 mV(SE, n = 48) and 58 +/- 2.2% (SE, n = 48) blocked by 20 mM tetraethylammonium. The results of this study support the idea that the inward rectifying potassium channels (Kir) are responsible for carrying K+ into glial cells whenever [K+]o increases. However, the delayed rectifier potassium channels (KD) cannot provide the pathway for outward K+ current during spatial buffering, and another mechanism must be involved in this process. Our study provides further evidence that culture conditions can greatly influence functional expression of ionic channels in glial cells.

Neuron-like physiological properties of cells from human oligodendroglial tumors

One of the most common symptoms of patients with oligodendrogliomas is the high frequency of epileptic seizures. We thus studied the physiological properties of cells in six human oligodendrogliomas and two oligoastrocytomas obtained from surgical material. The majority of tumor cells in living brain slices can generate action potentials as recorded with the patch-clamp technique indicating that this tissue is dominated by electrically excitable cells. In cultures from the same material, the action potential generating cells prevail within the first days and are subsequently replaced by electrically inexcitable cells. From histopathological and immunohistochemical data, the histogenesis of human oligodendroglial tumor is still uncertain. Our physiological study has not settled the debate on the origin of these tumors but revealed important findings with regard to this question. Since action potential generating glial cells have not been described in situ so far their occurrence in oligodendroglial tumors implies that oligodendroglial tumor cells may belong to the neuronal cell lineage.

Hyperpolarization-activated ion currents in cultured rat cortical and spinal cord astrocytes

Hyperpolarization-activated currents were recorded from rat brain cortical and spinal cord astrocytes maintained in culture. Spinal cord astrocytes expressed primarily an inward rectifier potassium current characterized by time-dependent inactivation, a strong dependence on extracellular Na+ and insensitivity to intracellular GTP-gamma-S (0.2 mM). In cortical astrocytes voltage clamp protocols aimed to elicit currents activated at, or negative to cell membrane potentials led to the development of two distinct ion currents. The most prominent current resembled the inward rectifier potassium current. This component was sensitive to blockade by extracellular cesium and was greatly reduced during recordings performed with GTP-gamma-S (0.2 Mm) added to the pipette solutions. The remaining current component was similar to the endothelial I ha current. I ha conductance was enhanced by extracellular potassium and the current reversal potential behaved as expected for a mixed cation, Na+/K+ current. I ha was nearly abolished after removal of extracellular Na. These results are consistent with the expression of a novel mixed cation conductance in glial cells, possibly involved in extracellular potassium buffering.

Type II brain sodium channel expression in non-neuronal cells: embryonic rat osteoblasts

Although voltage-sensitive sodium channels play a central role in electrogenesis in neurons, rat brain sodium channels are also present in some glial cells. To determine whether rat brain sodium channel alpha-subunit isotypes are expressed in other cell types, we examined osteoblasts within the embryonic day 17 (E17) vertebral column with in situ hybridization and immunocytochemical methods. For in situ hybridization studies, riboprobes hybridizing to isoform-specific sequences in the 3'-noncoding region of sodium channel mRNAs (NCI, NCII and NCIII) were utilized. Sodium channel mRNA I and III were not detectable in osteoblasts of the vertebra centrum or neural arches in E17 rats. In contrast, sodium channel mRNA II was moderately expressed by osteoblasts in the developing vertebral column of E17 rats. In immunocytochemical experiments, antipeptide antibodies directed against conserved and isotype-specific regions of the sodium channel alpha-subunit were used. Antibody SP20, which recognizes a conserved region of the sodium channel, intensely stains osteoblasts in both the vertebra centrum and neural arches. Antibody SP11-I, which recognizes sodium channel I, exhibited negligible-to-low levels of immunostaining in vertebral column osteoblasts. Osteoblasts reacted with antibody SP11-II, which recognizes sodium channel II, displayed moderate levels of immunostaining. Antibody SP32-III, which recognizes sodium channel III, displayed negligible levels of staining in osteoblasts within vertebra centrum and neural arches. These results demonstrate that osteoblasts in situ within E17 vertebral columns express sodium channel II mRNA and protein. Together with previous electrophysiological observations, the present results suggest that functional sodium channels are expressed in osteoblasts in vivo. These results extend the range of non-neuronal cells known to express rat brain sodium channels.

Isoform-specific expression of sodium channels in astrocytes in vitro: immunocytochemical observations

The expression of sodium channel alpha-subunit isoforms in astrocytes cultured from P-0 rat spinal cord and P-7 rat optic nerve was examined utilizing immunocytochemical methods with antibodies generated against conserved and isoform-specific amino acid sequences of the rat brain sodium channel. In spinal cord cultures at 5 days in vitro (DIV), both stellate and flat astrocytes were immunostained with antibody SP20, which recognizes a conserved sequence common to sodium channel types I, II/IIA, and III. Antibody SP11-I, which is directed against a subtype-specific sequence in sodium channel I, did not yield detectable staining in spinal cord astrocytes. Antibody SP11-II, which is directed against a subtype-specific sequence in sodium channel II, immunostained both stellate and flat spinal cord astrocytes, although with less intensity than SP20. Antibody SP32-III, which is directed against a subtype-sequence in sodium channel III, immunostained stellate but not flat spinal cord astrocytes. SP20, SP11-II, and SP32-III staining persisted in stellate spinal cord astrocytes through 14-21 DIV, while SP20 and SP11-II immunostaining in flat spinal cord astrocytes was attenuated with time in culture. In optic nerve cultures at 5 DIV, SP20 staining was present in both stellate and flat astrocytes, but at reduced levels compared to spinal cord astrocytes. With increased time in culture SP20 staining was maintained in stellate optic nerve astrocytes but was gradually lost in flat optic nerve astrocytes. Stellate optic nerve astrocytes exhibited low levels of staining with SP11-I, SP11-II, and SP32-III. Flat optic nerve astrocytes lacked or displayed very low SP11-II staining, and SP11-I and SP32-III staining was not detectable. These observations demonstrate that cultures astrocytes are immunoreactive to antibodies generated against conserved and isotype-specific peptide sequences of rat brain sodium channels, and further suggest that there are different patterns of sodium channel expression between flat vs. stellate astrocytes and in astrocytes derived from different regions of the CNS.

Differential Na+ channel beta 1 subunit mRNA expression in stellate and flat astrocytes cultured from rat cortex and cerebellum: a combined in situ hybridization and immunocytochemistry study

Astrocytes have been shown to express voltage-sensitive Na+ channels, but the molecular structure of these channels is not yet known. Recent studies have demonstrated the expression of rat brain voltage-sensitive Na+ channel mRNAs in astrocytes. In this study, we used a combined non-radioactive in situ hybridization/immunocytochemistry method to investigate the expression of voltage-sensitive Na+ channel beta 1 subunit (Na beta 1) mRNA in definitively identified, GFAP-positive astrocytes cultured from two different regions of the rat brain, cerebrum and cerebellum. In general, two morphologically distinct types of GFAP-positive astrocytes were observed in culture: flat, fibroblast-like and stellate, process-bearing. We observed a differential expression of Na beta 1 mRNA in GFAP-positive astrocytes: 1) stellate astrocytes expressed Na beta 1 mRNA, although the level of Na beta 1 mRNA expression was variable, and 2) flat astrocytes generally did not express Na beta 1 mRNA. Moreover, Bergmann-like cells from cerebellum did not express Na beta 1 mRNA, while the granule cells associated with Bergmann-like cell expressed Na beta 1 mRNA. These observations indicate that Na beta 1 mRNA is differentially expressed in rat astrocytes with various morphologies in vitro.

Type II sodium channels in spinal cord astrocytes in situ: immunocytochemical observations

The expression of sodium channel alpha-subunit isoforms in astrocytes in adult rat spinal cord and optic nerve was examined utilizing immunocytochemical methods with antibodies generated against conserved and subtype-specific sequences of the sodium channel. In adult rat spinal cord, astrocytes within the dorsal and ventral funiculi were immunolabelled with antibody SP20, which recognizes a conserved sequence within sodium channel types I, II, and III. In addition, astrocytes within these spinal cord white matter tracts were immunostained with antibody SP11-II, which recognizes sodium channel type II. Antibodies SP11-I and SP32-III, which are directed against subtype-specific sequences in sodium channel types I and III, respectively, did not label astrocytes in the dorsal and ventral funiculi of the spinal cord. In optic nerves, astrocytes were immunostained with antibody SP20. However, no detectable labelling of cells within the optic nerve was observed with antibodies SP11-I, SP11-II, and SP32-III. These observations demonstrate that sodium channel II is expressed by astrocytes in spinal cord white matter. Moreover, these data suggest that regional factors regulate the level of sodium channel isoform expression in astrocytes.

The beta 1 subunit mRNA of the rat brain Na+ channel is expressed in glial cells

Although the molecular characteristics of glial Na+ channels are not well understood, recent studies have shown the presence of mRNA for rat brain Na+ channel alpha subunits in astrocytes and Schwann cells. In this study, we asked whether the mRNA for the rat brain Na+ channel beta 1 subunit is expressed in glial cells. We performed in situ hybridization using a complementary RNA probe for the coding regions of the rat brain Na+ channel beta 1 subunit mRNA and detected beta 1 subunit mRNA in cultured rat optic nerve astrocytes and sciatic nerve Schwann cells. The beta 1 subunit was amplified by reverse transcription-polymerase chain reaction in rat optic and sciatic nerves, which lack neuronal somata but contain astrocytes and Schwann cells, respectively. Doublet bands of the beta 1 subunit mRNA were amplified from both optic and sciatic nerves. Through the cloning and sequencing of these bands, we confirmed the amplification of a mRNA highly homologous to the previously cloned rat brain Na+ channel beta 1 subunit (beta 1.1) and a novel form of the beta 1 subunit mRNA (beta 1.2), which is closely homologous to beta 1.1 but contains an additional 86-nucleotide insert in 3' noncoding regions. Two beta 1 subunit mRNAs were also amplified from rat brain and skeletal muscle, but not from rat liver or kidney. These results indicate that rat brain Na+ channel beta 1 subunit mRNAs are expressed in glial cells as well as in neurons.

Cortical type 2 astrocytes are not dye coupled nor do they express the major gap junction genes found in the central nervous system

The O-2A progenitor cell first described from the rat optic nerve is a bipotential precursor of oligodendrocytes and type 2 astrocytes. Each cell expresses specific markers that distinguish them as unique cell types. O-2A progenitors cultured in high serum preferentially differentiate into type 2 astrocytes and when exposed to defined medium or low serum develop along the oligodendrocyte lineage. We analyzed the gap junction gene expression of type 2 astrocytes to determine if they are coupled to form a syncytium, like their type 1 astrocyte counterparts. Dye coupling experiments demonstrated that cortical type 2 astrocytes are not coupled, while type 1 astrocytes in the same culture dish are highly coupled. Immunocytochemistry revealed the presence of Cx43 in type 1 astrocytes but we could not detect Cx26, 32, or 43 protein in type 2 astrocytes. In situ hybridization did not detect mRNA for any of the three connexin genes in type 2 astrocytes. These data demonstrate that type 2 astrocytes do not express the major gap junction genes found in the central nervous system. The precise function of type 2 astrocytes is not known but the lack of gap junction genes expression suggests that their functions are different from the spatial buffering capacity of type 1 astrocytes.

Voltage-dependent ion channels in glial cells

Glial cells, although non-excitable, express a wealth of voltage-activated ion channels that are typically characteristic of excitable cells. Since these channels are also observed in acutely isolated cells and in brain slices, they have to be considered functional in the intact brain. Numerous studies over the past 10 years have yielded detailed characterizations of glial channels permitting comparison of their properties to those of their neuronal counterparts. While for the most part such comparisons have demonstrated a high degree of similarity, they also provide evidence for the expression of some uniquely glial ion channels. An increasing number of studies indicate that the expression of "glial" channels is influenced by the cells' microenvironment. For example, the presence of neurons can induce or inhibit (depending on the preparation and type of channel studied) the expression of glial ion channels. Like ion channels in excitable cells, glial channels can be functionally regulated by activation of second-messenger pathways, allowing for short-term modulation of their membrane properties. Although the extent to which most of the characterized ion channels are involved in glial function is presently unclear, a growing body of data suggests that certain channels play an active role in glial function. Thus inwardly rectifying K+ channels in concert with delayed rectifying K+ channels are thought to be involved in the removal and redistribution of excess K+ in the brain, a process referred to as "spatial buffering". Glial K+ channels may also be crucial in modulating glial proliferation. Cl- channels and stretch-activated cation channels are believed to be involved in volume regulation. Na+ channels appear to be important in fueling the glial Na+/K(+)-pump, and Ca2+ channels are likely involved in numerous cellular events in which intracellular Ca2+ is a critical second messenger.

Anoxic injury of rat optic nerve: ultrastructural evidence for coupling between Na+ influx and Ca(2+)-mediated injury in myelinated CNS axons

Physiological studies in the anoxic rat optic nerve indicate that irreversible loss of function, measured by the compound action potential, is due to depolarization and run-down of the transmembrane Na+ gradient which triggers Ca2+ entry through reverse Na(+)-Ca2+ exchange. EM studies in the anoxic optic nerve have demonstrated characteristic changes, including mitochondrial swelling and dissolution of cristae, submyelinic vacuoles, detachment of perinodal oligodendrocyte-axon loops, and severe cytoskeletal damage with loss of microtubules and neurofilaments within the axoplasm. To further examine the coupling between Na+ influx and Ca(2+)-mediated injury in myelinated axons within anoxic white matter, we have examined the ultrastructural effects of tetrodotoxin (TTX), in the anoxic optic nerve. Optic nerves, maintained in an interface brain slice chamber, were exposed to a 60-min period of anoxia. TTX (1 microM) was introduced 10 min before the onset of anoxia. Nerves were examined at the end of the anoxic period, or after 80 min in 1 microM TTX for normoxic controls. Under normoxic conditions, optic nerve axons exposed to TTX exhibited a normal ultrastructure. In optic nerves exposed to TTX studied at the end of a 60-min period of anoxia, mitochondria showed swelling and loss of cristae, and terminal oligodendroglial loops were detached from the nodal axon membrane. Cytoskeletal architecture was preserved in anoxic optic nerve axons treated with TTX, and axonal microtubules and neurofilaments maintained their continuity. Submyelinic empty spaces were not present. Perinodal astrocyte processes often appeared to be replaced by cellular remnants containing multiple membranous profiles; clusters of shrunken astrocytic processes were present between myelinated axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium channel mRNAs in cultured spinal cord astrocytes: in situ hybridization in identified cell types

The expression of rat brain sodium channel alpha-subunit mRNAs I, II and III and a putative glial cell-specific sodium channel (NaG) mRNA was examined in cultured astrocytes from P-0 rat spinal cord by RNA blot hybridization and by non-isotope in situ hybridization cytochemistry utilizing two independent sets of isoform-specific RNA probes. Sodium channel mRNA I was not detectable in the cultured astrocytes by RNA blot or in situ hybridization. Sodium channel mRNA II showed negligible-to-low levels of expression in flat, fibroblast-like and 'pancake' astrocytes at 4 days in vitro (div), while stellate, process-bearing astrocytes exhibited low-to-moderate levels of mRNA II expression. At 7 div, mRNA II expression ranged from low-to-moderate in flat astrocytes and was moderately high in most process-bearing astrocytes. In RNA blots, a weak band was observed at 9.5 kb. Sodium channel mRNA III expression was negligible in flat astrocytes and was detectable in low-to moderate levels in stellate astrocytes beginning at 4 div; by 7 div, mRNA III was detectable in low levels in flat astrocytes and low-to-moderate levels in stellate astrocytes. RNA blots showed two bands of nearly equal intensity, one at 9.0 kb and one at 7.2 kb. NaG mRNA showed increased expression with time in culture, being detectable in flat and stellate astrocytes at 4 div and becoming very prominent in flat astrocytes at extended times in culture. In RNA blots of cultured astrocytes at 7 div, a strong hybridizing signal with the NaG probe was observed. These observations demonstrate that flat and stellate astrocytes cultured from rat spinal cord express rat brain sodium channel mRNA II and III, and NaG, and suggest that astrocytes in vitro may co-express multiple forms of sodium channel mRNA.

The expression of rat brain voltage-sensitive Na+ channel mRNAs in astrocytes

Astrocytes from various regions of CNS have been shown to express voltage-activated Na+ currents. To date, three distinct subtypes (I, II and III) of Na+ channels have been cloned from rat brain. We have applied a combined technique of reverse transcription and polymerase chain reaction (RT-PCR) to examine the expression of rat brain Na+ channels in rat astrocytes in vivo and in vitro. Five PCR primer sets were used to amplify coding or 3' non-coding regions of subtype I, II, and III Na+ channels. We were able to amplify all three of these rat brain Na+ channel subtypes from rat optic nerve, which does not have neuronal cell bodies but does contain astrocytes known to express voltage-sensitive Na+ channels. In studies on cultured spinal cord astrocytes, we were also able to amplify all three subtypes of rat brain Na+ channel mRNAs. In control experiments, RT-PCR was performed on RNAs prepared from several rat tissues, including brain, skeletal muscle, and liver. Rat brain was shown to express the three Na+ channel subtypes as expected. In rat skeletal muscle, subtype I and III Na+ channel mRNAs, but not subtype II, were amplified. In rat liver, Na+ channel messages were not detectable. The present study provides the first direct evidence that astrocytes in vivo and in vitro express rat brain voltage-sensitive Na+ channel mRNAs, which have been considered as mainly neuronal-type Na+ channel messages.

Na+ channels of Müller (glial) cells isolated from retinae of various mammalian species including man

Within the last few years, the expression of voltage-dependent, TTX-sensitive Na+ channels has been demonstrated in several types of neuroglial cells such as astrocytes and Schwann cells. Recently, we reported the occurrence of such Na+ currents in retinal Müller (glial) cells from dog and cat. This paper deals with the description of the properties of Na+ currents in Müller cells isolated from retinae of several mammalian species, as well as from human retinae. These Na+ currents were eliminated by TTX (1 microM), and by exposure to sodium-free extracellular solution; typically, they were demonstrable only after blocking most of the K+ conductance by Ba2+ (1 mM). Voltage-dependent activation and inactivation characteristics and time constants of the Na+ currents were similar to those of currents carried by neuronal Na+ channels. The estimated number of sodium channels per cell was low (about 1,500 channels per 7,500 microns 2), and the K+ conductance exceeded the peak Na+ conductance by an average factor of 5. Thus, the cells were incapable of generating action-potential-like responses under current clamp. Modelling estimations show that triggering of glial Na+ currents under physiological conditions, if any, can at best occur by emhaptic transmission at perinodal sites of optic axons. It is speculated that glial Na+ channels might be involved in neuroglial signalling events.

Fibrous and protoplasmic astrocytes express GABAA receptors that differ in benzodiazepine pharmacology

Astrocytes cultured from spinal cord contain two morphologically distinguishable types of astrocytes: fibrous and protoplasmic cells. Both astrocyte subtypes, in culture, are able to express GABAA receptors, and their activation results in inward currents at the resting potential. Using patch-clamp electrophysiology we characterized their basic receptor pharmacology and compared it to spinal cord neurons that were also present in small numbers in these cultures. As in neuronal GABAA receptors, the local anesthetic pentobarbital effectively potentiated GABA-induced currents in both astrocyte subtypes. Similarly, the benzodiazepine diazepam, on average doubled GABA-induced currents in both astrocytes subtypes. In contrast to these effects that were similar in both astrocytes types and similar to spinal cord neurons, the response to the convulsant methyl-4-ethyl-6,7-dimethoxy-beta-carboline-3-carboxylate (DMCM), which is an inverse benzodiazepine agonist differs between astrocyte subtypes. DMCM reduced GABA-induced currents by about 50% in fibrous astrocytes as we also observed with spinal cord neurons. In contrast, DMCM increased GABA currents in protoplasmic astrocytes by up to 150%, an effect never observed in neurons. DMCM potentiations of GABA currents have recently been attributed to differences in receptor subunit composition. Our results thus indicate that subtypes of astrocytes express GABAA receptors that differ pharmacologically and likely differ also in subunit composition.

Spinal cord astrocytes in vitro: phenotypic diversity and sodium channel immunoreactivity

The expression of sodium channels in morphologically and antigenically distinct astrocytes derived from neonatal rat spinal cords was examined at various times in culture. During the course of this study [2-40 days in vitro (DIV)], nine morphologies of glial fibrillary acidic protein (GFAP)+ cells were distinguished: 1a) flat, fibroblast-like; 1b) elongated, with generally few, short processes; 1c) triangular soma with three short, stubby processes; 1d) bipolar with long, slender processes; 1e) bipolar with broad, flared processes; 1f) stellate with radially oriented slender processes extending from a small to moderate-sized soma; 1g) multiple short, stubby processes extending from a moderate-sized soma; 1h) flat, roundish shape with either a smooth edge ("pancake"-like) or numerous very short processes; and 1i) broad, elongated cell body with orthogonally oriented short, spike-like processes. Not all cell types were present at all times in culture. Each type of astrocyte displayed sodium channel immunoreactivity at some time in culture; however, different types of astrocytes exhibited different patterns, over time, of sodium channel staining. Sodium channel immunoreactivity in all astrocyte types was reduced to low levels by 14 DIV, and was not detectable at 40 DIV. Except for types 1b and 1e, A2B5 staining was present on all astrocyte morphologies at some time in culture, and was generally attenuated with longer times in vitro; in contrast to cultures derived from neonatal rat optic nerve, A2B5 staining does not distinguish unequivocally between the various classes of morphologically different astrocytes derived from spinal cord. O4 immunoreactivity was consistently observed only on bipolar, elongated, and process-bearing astrocytes, though not all process-bearing astrocytes were O4+. These results demonstrate that astrocytes derived from neonatal spinal cord are morphologically and antigenically heterogeneous. Moreover, while spinal cord astrocytes express sodium channels, these astrocytes exhibit a time-course of channel expression that is different from astrocytes derived from several other CNS regions where sodium channel staining is maintained even for extended times in culture, suggesting a regional modulation of astrocyte function.

Molecular dissection of the myelinated axon

The membrane of the myelinated axon expresses a rich repertoire of physiologically active molecules: (1) Voltage-sensitive NA+ channels are clustered at high density (approximately 1,000/microns 2) in the nodal axon membrane and are present at lower density (< 25/microns 2) in the internodal axon membrane under the myelin. Na+ channels are also present within Schwann cell processes (in peripheral nerve) and perinodal astrocyte processes (in the central nervous system) which contact the Na+ channel-rich axon membrane at the node. In some demyelinated fibers, the bared (formerly internodal) axon membrane reorganizes and expresses a higher-than-normal Na+ channel density, providing a basis for restoration of conduction. The presence of glial cell processes, adjacent to foci of Na+ channels in immature and demyelinated axons, suggests that glial cells participate in the clustering of Na+ channels in the axon membrane. (2) "Fast" K+ channels, sensitive to 4-aminopyridine, are present in the paranodal or internodal axon membrane under the myelin; these channels may function to prevent reexcitation following action potentials, or participate in the generation of an internodal resting potential. (3) "Slow" K+ channels, sensitive to tetraethylammonium, are present in the nodal axon membrane and, in lower densities, in the internodal axon membrane; their activation produces a hyperpolarizing afterpotential which modulates repetitive firing. (4) The "inward rectifier" is activated by hyperpolarization. This channel is permeable to both Na+ and K+ ions and may modulate axonal excitability or participate in ionic reuptake following activity. (5) Na+/K(+)-ATPase and (6) Ca(2+)-ATPase are also present in the axon membrane and function to maintain transmembrane gradients of Na+, K+, and Ca2+. (7) A specialized antiporter molecule, the Na+/Ca2+ exchanger, is present in myelinated axons within central nervous system white matter. Following anoxia, the Na+/Ca2+ exchanger mediates an influx of Ca2+ which damages the axon. The molecular organization of the myelinated axon has important pathophysiological implications. Blockade of fast K+ channels and Na+/K(+)-ATPase improves action potential conduction in some demyelinated axons, and block of the Na+/Ca2+ exchanger protects white matter axons from anoxic injury. Modification of ion channels, pumps, and exchangers in myelinated fibers may thus provide an important therapeutic approach for a number of neurological disorders.

Different Na+ currents in P0- and P7-derived hippocampal astrocytes in vitro: evidence for a switch in Na+ channel expression in vivo

Hippocampal astrocytes, derived from postnatal day zero (P0) rats, appear to be pluripotential with respect to sodium current expression in vitro, and display Na+ currents with h infinity midpoints close to -65 up to 5 days in vitro (DIV), and Na+ currents with midpoints close to -85 mV at 6 DIV and thereafter. These astrocytes also exhibit a biphasic pattern of Na+ current density, which is expressed at moderate levels at early times in vitro and decreases throughout the first 5 DIV, prior to expressing a second peak for the duration of time in culture. These observations have been interpreted as suggesting that astrocytes in these cultures display a 'switch' in Na+ channel biosynthesis, so that they express different types of Na+ current (with different h infinity curves) at early and later times in culture. To test the hypothesis that a similar switch in Na+ channel expression occurs in vivo, we have used patch-clamp methods to study Na+ current expression in astrocytes derived from rat hippocampus at various stages of postnatal development, P0, P4, P5 and P7. We observed a biphasic distribution of Na+ current density, which was highest in P0- and P7-derived astrocytes (18 pA/pF and 10.3 pA/pF, respectively); astrocytes derived at P4 and P5 did not express sodium currents. While P0-derived astrocytes show depolarized h infinity curves (midpoints close to -65 mV) at early times in culture, P7-derived astrocytes, studied at comparable times in vitro, display hyperpolarized h infinity curves (midpoints close to -85 mV).(ABSTRACT TRUNCATED AT 250 WORDS)

Taurine, GABA and GFAP immunoreactivity in the developing and adult rat optic nerve

The localizations of taurine, gamma-aminobutyric acid (GABA) and glial fibrillary acidic protein (GFAP) within the developing rat optic nerve were determined using immunocytochemical techniques on tissues from animals ranging in age from embryonic day 20 to postnatal 28 days. Mature nerves from 3-4-month-old adults were also examined. At the younger ages, taurine immunoreactivity was intense and localized specifically to the optic nerve axons, but by postnatal day 15 and thereafter its predominant localization was in macroglia. Some of these glia were astrocytes as indicated by the specific marker, GFAP. GABA immunoreactivity was present at the same time as taurine but was found only in macroglia. In mature nerves the patterns of taurine, GABA and GFAP distribution (within glia) were highly similar.

Sodium channel expression in optic nerve astrocytes chronically deprived of axonal contact

Immunocytochemical and electrophysiological methods were used to examine the effect of retinal ablation on the expression of sodium channels within optic nerve astrocytes in situ and in vitro. Enucleation was performed at postnatal day 3 (P3), and electron microscopy of the enucleated optic nerves at P28-P40 revealed complete degeneration of retinal ganglion axons, resulting in optic nerves composed predominantly of astrocytes. In contrast to control (non-enucleated) optic nerve astrocytes, which exhibited distinct sodium channel immunoreactivity following immunostaining with antibody 7493, the astrocytes in enucleated optic nerves did not display sodium channel immunoreactivity in situ. Cultures obtained from enucleated optic nerves consisted principally (greater than 90%) of glial fibrillary acidic protein (GFAP)+/A2B5- ("type-1") astrocytes, as determined by indirect immunofluorescence; GFAP+/A2B5+ ("type-2") astrocytes were not present, nor were GFAP-/A2B5+ (O-2A) progenitor cells. Sodium channel immunoreactivity was not present in GFAP+/A2B5- astrocytes obtained from enucleated optic nerves; in contrast, GFAP+/A2B5- astrocytes from control optic nerves exhibited 7493 immunostaining for the first 4-6 days in culture. Sodium current expression, studied using whole-cell patch-clamp recording, was attenuated in cultured astrocytes derived from enucleated optic nerves. Whereas 39 of 50 type-1 astrocytes cultured from intact optic nerves showed measurable sodium currents at 1-7 days in vitro, sodium currents were present in only 6 of 38 astrocytes cultured from enucleated optic nerves. Mean sodium current densities in astrocytes from the enucleated optic nerves (0.66 +/- 0.3 pA/pF) were significantly smaller than in astrocytes from control optic nerves (7.15 +/- 1.1 pA/pF). The h infinity-curves of sodium currents were similar in A2B5- astrocytes from enucleated and control rat optic nerves. These results suggest that there is neuronal modulation of sodium channel expression in type-1 optic nerve astrocytes, and that, following chronic loss of axonal association in vivo, sodium channel expression is down-regulated in this population of optic nerve astrocytes.