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

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.

Nav1.7 and Nav1.3 are the only tetrodotoxin-sensitive sodium channels expressed by the adult guinea pig enteric nervous system

The types of sodium channels that are expressed by neurons shape the rising phase of action potentials and influence patterns of action potential discharge. With regard to the enteric nervous system (ENS), there is uncertainty about which channels are expressed, and in particular it is unknown whether Na(v)1.7 is present. We designed specific probes for the guinea pig Na(v)1.7 alpha subunit as well as for the other tetrodotoxin (TTX)-sensitive alpha subunits (Na(v)1.1, Na(v)1.2, Na(v)1.3, and Na(v)1.6) in order to perform in situ hybridization (ISH) histochemistry on guinea pig myenteric ganglia. We established that only Na(v)1.7 mRNA and Na(v)1.3 mRNA are expressed in these ganglia. The ISH signal for Na(v)1.7 transcripts was found in seemingly all the myenteric neurons. The expression of the Na(v)1.3 alpha subunit was confirmed by immunohistochemistry in a large proportion (62%) of the myenteric neuron population. This population included enteric sensory neurons. Na(v)1.6 immunoreactivity, absent from myenteric neurons, was detected in glial cells only when a high anti-Na(v)1.6 antibody concentration was used. This suggests that the Na(v)1.6 alpha subunit and mRNA are present only at low levels, which is consistent with the fact that no Na(v)1.6 mRNA could be detected in the ENS by ISH. The fact that adult myenteric neurons are endowed with only two TTX-sensitive alpha subunits, namely, Na(v)1.3 and Na(v)1.7, emphasizes the singularity of the ENS. Both these subunits, known to have slow-inactivation kinetics, are well adapted for generating action potentials from slow excitatory postsynaptic potentials, a mode of synaptic transmission that applies to all ENS neuron types.

Effect of chronically elevated CO2 on CA1 neuronal excitability

To study the effect of chronically elevated CO(2) on the excitability and function of neurons, we exposed mice to 7.5-8% CO(2) for approximately 2 wk (starting at 2 days of age) and examined the properties of freshly dissociated hippocampal neurons. Neurons from control mice (CON) and from mice exposed to chronically elevated CO(2) had similar resting membrane potentials and input resistances. CO(2)-exposed neurons, however, had a lower rheobase and a higher Na(+) current density (580 +/- 73 pA/pF; n = 27 neurons studied) than did CON neurons (280 +/- 51 pA/pF, n = 34; P < 0.01). In addition, the conductance-voltage curve was shifted in a more negative direction in CO(2)-exposed than in CON neurons (midpoint of the curve was -46 +/- 3 mV for CO(2) exposed and -34 +/- 3 mV for CON, P < 0.01), while the steady-state inactivation curve was shifted in a more positive direction in CO(2)-exposed than in CON neurons (midpoint of the curve was -59 +/- 2 mV for CO(2) exposed and -68 +/- 3 mV for CON, P < 0.01). The time constant for deactivation at -100 mV was much smaller in CO(2)-exposed than in CON neurons (0.8 +/- 0.1 ms for CO(2) exposed and 1.9 +/- 0.3 ms for CON, P < 0.01). Immunoblotting for Na(+) channel proteins (subtypes I, II, and III) was performed on the hippocampus. Our data indicate that Na(+) channel subtype I, rather than subtype II or III, was significantly increased (43%, n = 4; P < 0.05) in the hippocampi of CO(2)-exposed mice. We conclude that in mice exposed to elevated CO(2), 1) increased neuronal excitability is due to alterations in Na(+) current and Na(+) channel characteristics, and 2) the upregulation of Na(+) channel subtype I contributes, at least in part, to the increase in Na(+) current density.

Expression and regulation of voltage-gated sodium channel beta1 subunit protein in human gliosis-associated pathologies

Auxiliary beta1 subunits of voltage-gated sodium channels (NaChs) critically regulate channel activity and may also act as cell adhesion molecules (CAMs). In a recent study we have shown that the expression of beta1 NaCh protein is increased in reactive astrocytes in a rat epilepsy model of mesial temporal lobe epilepsy. The present study was undertaken to examine whether changes of NaCh beta1 subunit protein expression are also associated with structural changes occurring in human reactive astrocytes under different pathological conditions in vivo, as well as in response to changing environmental conditions in vitro. Strong beta1 astroglial immunoreactivity was present in human brain tissue from patients with astrogliosis. The over-expression of beta1 protein in reactive glia was observed in both epilepsy-associated brain pathologies (temporal lobe epilepsy, cortical dysplasia), as well as non-epileptic (cerebral infarction, multiple sclerosis, amyotrophic lateral sclerosis, meningo-encephalitis) disorders. The up-regulation of beta1 subunit protein in astrocytes can be reproduced in vitro. beta1 protein is highly expressed in human astrocytes cultured in the presence of trophic factors, under conditions in which they show morphology similar to the morphology of cells undergoing reactive gliosis. The growth factor-induced overexpression of beta1 protein was abrogated by PD98059, which inhibits the mitogen-activated protein kinase pathway. These findings demonstrate that the expression of NaCh beta1 subunit protein in astrocytes is plastic, and indicate a novel mechanism for modulation of glial function in gliosis-associated pathologies.

Changes in the gene expression of six subtypes of P2X receptors in rat dorsal root ganglion after spinal nerve ligation

Increased purinergic sensitivity of injured sensory neurons suggests the possible involvement of purinoceptors for the generation of pain after nerve injury. To identify the purinoceptors that are involved, the changes in mRNA levels of 6 subtype purinoceptors were examined in the dorsal root ganglia (DRG) of the normal rat and after spinal nerve ligation, using RNase protection assay (RPA). In addition, the P2X(2) containing neurons were examined in the L5 DRG, using an immunohistochemical method. The relative amounts of mRNAs for the six purinoceptor subtypes were in the order of P2X(3)>>P2X(4)>P2X(6)>P2X(5) approximately P2X(2)>P2X(1) in the normal lumbar DRG. After nerve injury, the mRNA of P2X(5) was increased, those of P2X(3) and P2X(6) were decreased, and those of P2X(2) and P2X(4) were unchanged. Immunohistochemical studies, however, showed 23% of the total DRG neurons are P2X(2) positive in the normal L5 DRG, but that increased to 73% after nerve ligation. These data suggest that not only transcriptional but also posttranscriptional changes of multiple purinoceptors might be involved in the enhancement of purinergic sensitivity in injured sensory neurons.

Glial membrane channels and receptors in epilepsy: impact for generation and spread of seizure activity

Epilepsy is a condition in the brain characterized by repetitively occurring seizures. While various changes in neuronal properties have been reported to accompany or induce seizure activity in human or experimental epilepsy, other studies suggested that glial cells might be involved in epileptogenesis. Recent findings demonstrate that in the course of the disease, glial cells not only undergo structural alterations but also display distinct functional properties. Several studies identified reduced inwardly rectifying K(+) currents in astrocytes of epileptic tissue, which probably results in disturbances of the K(+) homeostasis. Other data hinted at an abnormal increase in [Ca(2+)](i) in astrocytes through enhanced activity of glial glutamate receptors. This review summarizes current knowledge of alterations of plasma membrane channels and receptors of macroglial cells in epilepsy and discusses the putative importance of these changes for the generation and spread of seizure activity.

The changes in expression of three subtypes of TTX sensitive sodium channels in sensory neurons after spinal nerve ligation

Our previous studies showed that the ectopic discharges in injured sensory neurons and mechanical allodynia that developed after spinal nerve ligation were significantly reduced by application of a low concentration of tetrodotoxin (TTX) to the corresponding dorsal root ganglion (DRG) of the ligated spinal nerve. Based on these data, we hypothesized that expression of TTX-sensitive sodium channels is up-regulated in the injured sensory neurons and that such up-regulation plays an important role in the generation of ectopic discharges and thus pain behaviors in spinal nerve ligated neuropathic rats. To test this hypothesis, the present study examined the changes in three subtypes of TTX-sensitive sodium channels in the DRG after spinal nerve ligation. The changes in the total amount of mRNA for alpha-subunits of sodium channel brain type I (type I), brain type II (type II) and brain type III (type III) were determined by RNase protection assays (RPA). The population of DRG neurons expressing type III sodium channel protein was examined by an immunohistochemical method with antibodies to type III sodium channels. In the normal DRG, the level of mRNA for the type I sodium channel is high while that for type II and type III is very low. After spinal nerve ligation, the expression of type III mRNA was significantly increased at 16-h postoperatively (PO), doubled by 3 days PO and then was maintained at this high level until the end of the experiment (7 days PO). By contrast, the amount of mRNA for type I and type II sodium channels started to decrease at 1 day PO and were reduced to 25-50% of the normal control levels by 7 days after nerve ligation. Neurons showing positive immunostaining for type III sodium channels were rare ( approximately 3.2% of total population) in the normal DRG but increased after nerve ligation to 21% and 15% of the total neuronal population by 1 day and 7 days PO, respectively. Type III immunoreactivity was found preferentially in medium to large sized neurons. Thus the majority of neurons with cell bodies having diameters > or =40 microm became type III-positive after nerve ligation. The data indicate that the increased expression of type III sodium channels in axotomized sensory neurons may be the critical factor for the TTX sensitivity of ectopic discharges in injured sensory neurons and thus the generation of ectopic discharges and neuropathic pain behaviors in spinal nerve ligated rats.

Resurgence of sodium channel research

A variety of isoforms of mammalian voltage-gated sodium channels have been described. Ten genes encoding sodium channel alpha subunits have been identified, and nine of those isoforms have been functionally expressed in exogenous systems. The alpha subunit is associated with accessory beta subunits in some tissues, and three genes encoding different beta subunits have been identified. The alpha subunit isoforms have distinct patterns of development and localization in the nervous system, skeletal and cardiac muscle. In addition, many of the isoforms demonstrate subtle differences in their functional properties. However, there are no clear subfamilies of the channels, unlike the situation with potassium and calcium channels. The subtle differences in the functional properties of the sodium channel isoforms result in unique conductances in specific cell types, which have important physiological effects for the organism. Small alterations in the electrophysiological properties of the channel resulting from mutations in specific isoforms cause human diseases such as periodic paralysis, long QT syndrome, and epilepsy.

The cardiac sodium channel mRNA is expressed in the developing and adult rat and human brain

Expression of the rat (RH-I/SkM2) and human (hH1/SCN5A) tetrodotoxin-resistant (TTX-R), voltage-sensitive sodium channels is thought to be specific to cardiac tissue. We detected RH-I/SkM2 mRNA in newborn rat brain using both RNase protection assay analysis and in situ hybridization and in adult rat brain using RNase protection assay analysis. This expression was observed primarily in developing limbic structures of the cerebrum and diencephalon, and in the medulla of the brain stem. Using RT-PCR analysis, we detected hH1/SCN5A mRNA in both fetal and adult human brain. Interestingly, mutations in the human cardiac sodium channel are known to lead to cardiac abnormalities, which result in arrhythmias and frequently in sudden cardiac death. If these mutant channels were also expressed in limbic regions of the brain, alterations in channel function could have drastic effects on the brain's signaling ability, possibly promoting seizure activity.

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.

Diversity of mammalian voltage-gated sodium channels

A variety of different isoforms of mammalian voltage-gated sodium channels have been identified. These channels can be classified into three different types. Eight type 1 isoforms have been identified in the CNS, PNS, skeletal muscle, and heart. All of these channels have been expressed in exogenous systems, and all of the genes have been mapped. Three type 2 isoforms have been identified in heart, uterus, and muscle. These channels diverge from the type 1 channels in critical regions, and have not been functionally expressed, so their significance is unknown. A single isoform identified in the PNS may represent a third class of channels, in that it diverges from both type 1 and 2 channels. The type 3 channel has not been functionally expressed.

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.

Expression of the mRNA for the beta 2 subunit of the voltage-dependent sodium channel in rat CNS

Expression of the voltage-dependent sodium channel has been analysed in adult rat central nervous system by Northern blotting and in situ hybridization. Northern blots showed that all the territories studied express beta 2 transcripts, albeit with widely varying levels (with cerebellum >> hippocampus > brain > brainstem > spinal cord). In situ hybridization confirmed that in these structures, all the neuronal cell bodies contain beta 2 mRNA; expression was particularly high in the granule cells of the cerebellum, in both pyramidal cell layer and dentate gyrus in the hippocampus, and in spinal cord motor neurons. Northern blots also showed that RNA extracted from optic nerve and cultured cortical astrocytes contained beta 2 mRNA, while it was totally absent from sciatic nerve. In situ hybridization evidenced the presence of a numerous population of beta 2-positive cells in cerebellum white matter, spinal cord white matter, and in corpus callosum, where frontal sections showed labelled cells arranged in the chain-like or row pattern typical of interfascicular oligodendrocytes. Combination of antiglial fibrillary acid protein (GFAP) immunofluorescent histochemistry with detection of beta 2 mRNA evidenced that expression of the transcripts was indeed restricted to GFAP-negative cells in white matter.

Novel splice variants of the voltage-sensitive sodium channel alpha subunit

In this study, we have identified novel splice variants of the Na+ channel alpha subunit mRNA from cultured rat astrocytes and neuroblastoma cells. These splice variants are characterized by premature truncation or deletion of a segment in the third domain of the Na+ channel alpha subunit. The expression of three splice variants was upregulated by exposure to 1 mM dibutyryl cAMP in spinal cord astrocytes but not in cerebral astrocytes and in B50 and B104 neuroblastoma cells. The calcium ionophore 1 microM A23187, did not influence the expression of splice variants in either astrocytes or neuroblastoma cells. These findings suggest that spinal cord astrocytes may maintain a unique regulatory pathway that participates in the control of Na+ channel mRNA expression.

Effects of increased extracellular potassium on influx of sodium ions in cultured rat astroglia and neurons

Membrane depolarization by elevated extracellular K+ concentration ([K+]o) causes rapid Na+ influx through voltage-sensitive Na+ channels into excitable cells. The consequent increases in intracellular Na+ concentration ([Na+]i) and/or [K+]o stimulate Na+,K+-ATPase activity, which in turn stimulates energy metabolism and rates of glucose utilization (CMR[glc]) in neurons. We previously reported that in cultured cells elevated [K+]o stimulated CMR(glc) in neurons but not astroglia; but increasing [Na+]i by opening voltage-sensitive Na+ channels with veratridine stimulated CMR(glc) in both. These results indicated that Na+ influx plays a key role in the regulation of energy metabolism in neurons and astroglia, but that depolarization of astroglial membranes by elevated [K+]o does not open voltage-sensitive Na+ channels as it does in neurons. To examine this possibility directly we have measured the effects of increased [K+]o and of veratridine on Na+ influx into cultured rat astroglia and neurons. Cells were incubated in bicarbonate buffer containing ouabain (1 mM), tracer amounts of 22NaCl, and various concentrations (5.4, 28, 56 mM) of K+ or 75 microM veratridine for 0-60 min. Cells were digested and assayed for intracellular 22Na+ content. Elevated extracellular K+ stimulated tetrodotoxin-sensitive 22Na+ accumulation in cultured neurons but inhibited 22Na+ influx in astroglia. Veratridine-stimulated Na+ influx in both astroglia and neurons (144% and 133%, respectively), and these effects were completely blocked by 10 microM tetrodotoxin. These results indicate that increased [K+]o does not open voltage-sensitive Na+ channels and may inhibit Na+ influx in astroglia.

Pharmacological characterization of Na+ influx via voltage-gated Na+ channels in spinal cord astrocytes

Spinal cord astrocytes display a high density of voltage-gated Na+ channels. To study the contribution of Na+ influx via these channels to Na+ homeostasis in cultured spinal cord astrocytes, we measured intracellular Na+ concentration ([Na+]i) with the fluorescent dye sodium-binding benzofuran isophthalate. Stellate and nonstellate astrocytes, which display Na+ currents with different properties, were differentiated. Baseline [Na+]i was 8.5 mM in these cells and was not altered by 100 microM tetrodotoxin (TTX). Inhibition of Na+ channel inactivation by veratridine (100 microM) evoked a [Na+]i increase of 47.1 mM in 44% of stellate and 9.7 mM in 64% of nonstellate astrocytes. About 30% of cells reacted to veratridine with a [Na+]i decrease of approximately 2 mM. Qualitatively similar [Na+]i changes were caused by aconitine. The effects of veratridine were blocked by TTX, amplified by (alpha-)scorpion toxin and usually were readily reversible. Veratridine-induced [Na+]i increases were reduced upon membrane depolarization with elevated extracellular [K+]. Recovery to baseline [Na+]i was unaltered during blocking of K+ channels with 4-aminopyridine. [Na+]i increases evoked by the ionotropic non-N-methyl--aspartate receptor agonist kainate were not altered by TTX. Our results indicate that influx of Na+ via voltage- gated Na+ channels is not a prerequisite for glial Na+,K+-ATPase activity in spinal cord astrocytes at rest nor does it seem to be involved in [Na+]i increases evoked by kainate. During pharmacological inhibition of Na+ channel inactivation, however, Na+ channels can serve as prominent pathways of Na+ influx and mediate large perturbations in [Na+]i, suggesting that Na+ channel inactivation plays an important functional role in these cells.

Regulation of Na+ channel beta 1 and beta 2 subunit mRNA levels in cultured rat astrocytes

Using quantitative competitive reverse transcription-polymerase chain reaction (RT-PCR), we found an increased level of Na+ channel beta 1 (Na beta 1) mRNAs in spinal cord astrocytes and in the B50 neuroblastoma cell line after exposure to 1 mM dibutyryl cAMP. In contrast, the calcium ionophore (1 microM A23187) did not affect Na beta 1 mRNA levels in these cells. Further, we amplified full length coding region of Na+ channel beta 2 (Na beta 2) mRNA from rat optic nerve and cultured astrocytes using RT-PCR. It appeared that the Na beta 2 mRNA level was increased by dibutyryl cAMP, but not by A23187, in spinal cord astrocytes. These findings suggest that the Na beta 1 and Na beta 2 mRNA levels in spinal cord astrocytes are influenced by increased cAMP but not by calcium.

Increase in mRNAs encoding neonatal II and III sodium channel alpha-isoforms during kainate-induced seizures in adult rat hippocampus

Subtypes I, II and III of sodium channel alpha-subunit mRNAs were analyzed in adult rat brain areas after kainate-induced seizures. Tissue samples were microdissected from occipital neocortex, CA1 and CA3 hippocampus areas and dentate gyrus. Three reverse transcriptase-polymerase chain reaction (RT-PCR) protocols were undertaken to amplify these mRNAs. Amplification products were then distinguished after digestion by restriction enzymes, electrophoresis separation and densitometric analysis of gel profiles. PCR 1 evidenced the relative percentage of mRNAs I, II and III as well as neonatal II and III subtype isoforms, which resulted from an alternative splicing. PCR 2 and 3 were performed to focus on the neonatal vs. adult ratio in II and III subtypes, respectively. Seizures were shown to induce an increase in both neonatal subtypes, which suggested an alteration at the splicing level. These changes exhibited a peculiar brain regional distribution, the maximal effect being observed in dentate gyrus and hippocampus CA1 area. In situ hybridization experiments, using a digoxigenin-labeled oligonucleotide probe-specific for neonatal II and III mRNAs, confirmed this increase in neonatal mRNA subtypes. These changes were transient, reaching a maximum 6 h after drug injection, then disappearing between 12 and 48 h. They were prevented by a pre-treatment of animals by MK-801, a non-competitive antagonist of NMDA receptors. This work, thus, suggested that KA-induced seizures can be accompanied by transient alteration in the splicing pattern of sodium channel alpha-subunit mRNAs which resulted in an increase in expression of their neonatal isoforms within localized areas of adult rat brain.

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.

Expression of mRNA for a sodium channel in subfamily 2 in spinal sensory neurons

RNA blot analysis and non-isotopic in situ hybridization cytochemistry were used to study the expression of the mRNA for the glial sodium channel NaG, belonging to Na+ channel subfamily 2, in rat dorsal root ganglia (DRG). mRNA hybridizing at high stringency with an antisense riboprobe against the NaG sequence was observed in both Schwann cells and spinal sensory neurons in situ within DRG, but was expressed at higher levels in the latter. In contrast, hybridization was not detectable in neurons within hippocampus, cerebellum and spinal cord. The expression of the mRNA hybridizing with the NaG probe appears to be developmentally regulated in both Schwann cells and DRG neurons, with levels increasing as development proceeds. Thus, in addition to the mRNAs for types I and II/IIA alpha-subunits and beta 1-subunit in DRG neurons and types II/IIA and III alpha-subunits beta 1-subunit in Schwann cells, the mRNA for an additional sodium channel belonging to subfamily 2 is expressed in these cells in situ.

Altered brain sodium channel transcript levels in human epilepsy

Normal, and perhaps pathological, characteristics of neuronal excitability are related to the distribution and density of voltage-gated ion channels such as the sodium channel. We studied normal and epileptic human brain using the ligase detection reaction to measure the relative quantities of mRNAs encoding sodium channel subtypes 1 and 2. Normal brains exhibited characteristic 1:2 ratios which varied by brain region, but the ratios were invariate among individuals. These normal values were altered as much as threefold in anatomically corresponding regions of epileptic brain tissues. Changes of this magnitude in such a highly conserved value support a potential role for sodium channels in the pathophysiology of epilepsy.

Functional expression of voltage dependent sodium channels in Xenopus oocytes injected with mRNA from neonatal or adult rat brain

The two electrode voltage clamp technique was used to study voltage-dependent sodium currents (INa) in Xenopus laevis oocytes previously injected with mRNA extracted from adult (A) or neonatal (N, < 5 days old) rat brains. In the presence of niflumic acid (300 microM) to block endogenous Ca(2+)-activated Cl- currents, depolarizing voltage steps from a holding potential of -100 mV to various voltages elicited in both groups of oocytes fast inward sodium currents which peaked at approximately 0 mV and then slowly declined to approximately 75% of the maximum current at +40 mV. At the peak, A INa was significantly larger than N INa (296 +/- 59 nA vs. 147 +/- 32 nA). Inactivation kinetics of N INa was best fit with one exponential component whereas A INa with two exponential components. A significant difference in the voltage dependence of inactivation was found between A INa or N INa. The values of Vh were -53 +/- 0.9 mV or -59.8 +/- 0.7 mV for A INa or N INa respectively. The recovery from inactivation was fitted in both groups with two exponential functions (tau f and tau s) whose values were not significantly different. However the ratio between tau f and tau s was significantly higher for N INa comparing to A INa (5.7 vs. 2.1). TTX reversibly blocked INa. The IC50 value was 58.2 +/- 6.3 nM for A INa and 20.4 +/- 2.2 nM for N INa. These results suggest that different isoforms of TTX-sensitive, voltage-dependent sodium channel subunits are functionally expressed, may be in different proportions in oocytes injected with A or N mRNA.

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.

Rat brain Na+ channel mRNAs in non-excitable Schwann cells

The expression of rat brain voltage-sensitive Na+ channel mRNAs in Schwann cells was examined using in situ hybridization cytochemistry and RT-PCR. The mRNAs of rat brain Na+ channel subtype II and III, but not subtype I, were detected in cultured Schwann cells from sciatic nerve and in intact sciatic nerve, which contains Schwann cells but not neuronal cell bodies. These results indicate that rat brain Na+ channel mRNAs, which have been considered as mainly neuronal-type messages, are also expressed in glial cells in vitro and in vivo.

In situ hybridization localization of the Na+ channel beta 1 subunit mRNA in rat CNS neurons

Localization of Na+ channel beta 1 subunit (Na beta 1) mRNA was examined in adult rat hippocampus, cerebellum and spinal cord by in situ hybridization histochemistry. In hippocampus, Na beta 1 mRNA was strongly expressed by CA3 followed by CA1 pyramidal cells and dentate granule cells. In cerebellum, strong Na beta 1 mRNA expression was observed in Purkinje cells and moderate expression in granule cells and scattered cells of the molecular layer. In spinal cord, neurons in gray matter exhibited moderate to strong expression of Na beta 1 mRNA. These results provide the first localization study of Na beta 1 mRNA in the CNS, demonstrating a differential expression in different neurons.

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.