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

Identification of both GABAA receptors and voltage-activated Na(+) channels as molecular targets of anticonvulsant α-asarone

Alpha (α)-asarone, a major effective component isolated from the Chinese medicinal herb Acorus tatarinowii, is clinically used as medication for treating epilepsy, cough, bronchitis, and asthma. In the present study, we demonstrated that α-asarone targets central nervous system GABAA receptor as well as voltage-gated Na(+) channels. Using whole-cell patch-clamp recording, α-asarone inhibited spontaneous firing of output neurons, mitral cells (MCs), in mouse olfactory bulb brain slice preparation and hyperpolarized the membrane potential of MCs. The inhibitory effect of α-asarone persisted in the presence of ionotropic glutamate receptor blockers but was eliminated after adding a GABAA receptor blocker, suggesting that GABAA receptors mediated the inhibition of MCs by α-asarone. This hypothesis was supported by the finding that α-asarone evoked an outward current, but did not influence inhibitory postsynaptic currents (IPSCs). In addition to inhibiting spontaneous firing, α-asarone also inhibited the Nav1.2 channel, a dominant rat brain Na(+) channel subtype. The effects of α-asarone on a defined Nav1.2 were characterized using transfected cells that stably expressed the Nav1.2 channel isoform. α-Asarone displayed strong tonic inhibition of Nav1.2 currents in a concentration- and membrane potential-dependent fashion. α-Asarone reduced channel availability in steady-state inactivation protocols by enhancing or stabilizing Na(+) channel inactivation. Both Na(+) channel blockade and activation of GABAA receptors provide a possible mechanism for the known anti-epileptic effects of α-asarone. It also suggests that α-asarone could benefit patients with cough possibly through inhibiting a Na(+) channel subtype to inhibit peripheral and/or central sensitization of cough reflexes.

Regulation of persistent Na current by interactions between beta subunits of voltage-gated Na channels

The beta subunits of voltage-gated Na channels (Scnxb) regulate the gating of pore-forming alpha subunits, as well as their trafficking and localization. In heterologous expression systems, beta1, beta2, and beta3 subunits influence inactivation and persistent current in different ways. To test how the beta4 protein regulates Na channel gating, we transfected beta4 into HEK (human embryonic kidney) cells stably expressing Na(V)1.1. Unlike a free peptide with a sequence from the beta4 cytoplasmic domain, the full-length beta4 protein did not block open channels. Instead, beta4 expression favored open states by shifting activation curves negative, decreasing the slope of the inactivation curve, and increasing the percentage of noninactivating current. Consequently, persistent current tripled in amplitude. Expression of beta1 or chimeric subunits including the beta1 extracellular domain, however, favored inactivation. Coexpressing Na(V)1.1 and beta4 with beta1 produced tiny persistent currents, indicating that beta1 overcomes the effects of beta4 in heterotrimeric channels. In contrast, beta1(C121W), which contains an extracellular epilepsy-associated mutation, did not counteract the destabilization of inactivation by beta4 and also required unusually large depolarizations for channel opening. In cultured hippocampal neurons transfected with beta4, persistent current was slightly but significantly increased. Moreover, in beta4-expressing neurons from Scn1b and Scn1b/Scn2b null mice, entry into inactivated states was slowed. These data suggest that beta1 and beta4 have antagonistic roles, the former favoring inactivation, and the latter favoring activation. Because increased Na channel availability may facilitate action potential firing, these results suggest a mechanism for seizure susceptibility of both mice and humans with disrupted beta1 subunits.

Localization and targeting of voltage-dependent ion channels in mammalian central neurons

The intrinsic electrical properties and the synaptic input-output relationships of neurons are governed by the action of voltage-dependent ion channels. The localization of specific populations of ion channels with distinct functional properties at discrete sites in neurons dramatically impacts excitability and synaptic transmission. Molecular cloning studies have revealed a large family of genes encoding voltage-dependent ion channel principal and auxiliary subunits, most of which are expressed in mammalian central neurons. Much recent effort has focused on determining which of these subunits coassemble into native neuronal channel complexes, and the cellular and subcellular distributions of these complexes, as a crucial step in understanding the contribution of these channels to specific aspects of neuronal function. Here we review progress made on recent studies aimed to determine the cellular and subcellular distribution of specific ion channel subunits in mammalian brain neurons using in situ hybridization and immunohistochemistry. We also discuss the repertoire of ion channel subunits in specific neuronal compartments and implications for neuronal physiology. Finally, we discuss the emerging mechanisms for determining the discrete subcellular distributions observed for many neuronal ion channels.

Immunolocalization of NaV1.2 channel subtypes in rat and cat brain and spinal cord with high affinity antibodies

High titer polyclonal antibodies were produced in rabbit against a peptide unique to NaV1.2 sodium channels. NaV1.2 antibodies displayed 500,000-fold greater affinity for the NaV1.2 peptide compared with NaV1.1 or NaV1.3 peptides from the same region. These antibodies, when coupled to Sepharose beads, retained saxitoxin binding sites from solubilized rat brain membranes. Eluted protein from this antibody-affinity column was recognized by antibodies directed against neuronal voltage-gated sodium channels. Rabbit antibodies, which had been partially purified, were used in immunocytochemical localization of the NaV1.2 channel in 50 microm rat brain slices at dilutions of 1:1000 or 1:2000. NaV1.2 channels were predominately localized in unmyelinated fibers in the cortex, hippocampus, spinal cord and hypothalamus. Varicosities were seen in fiber staining which may reflect true varicosities in the fiber or simply varying densities of sodium channels along the fiber. Cell body staining with the NaV1.2 antibody was primarily observed in the hypothalamus. Antibody staining in the cerebellum was complex, with staining observed primarily in posterior lobes and considerably lower amounts of staining observed in anterior lobes. Specific staining was limited to fibers located in the granule and molecular layer, in an orientation consistent with granule cell unmyelinated axon labeling.

Physiological and morphological development of the rat cerebellar Purkinje cell

Cerebellar Purkinje cells integrate multimodal afferent inputs and, as the only projection neurones of the cerebellar cortex, are key to the coordination of a variety of motor- and learning-related behaviours. In the neonatal rat the cerebellum is undeveloped, but over the first few postnatal weeks both the structure of the cerebellum and cerebellar-dependent behaviours mature rapidly. Maturation of Purkinje cell physiology is expected to contribute significantly to the development of cerebellar output. However, the ontogeny of the electrophysiological properties of the Purkinje cell and its relationship to maturation of cell morphology is incompletely understood. To address this problem we performed a detailed in vitro electrophysiological analysis of the spontaneous and intracellularly evoked intrinsic properties of Purkinje cells obtained from postnatal rats (P0 to P90) using whole-cell patch clamp recordings. Cells were filled with neurobiotin to enable subsequent morphological comparisons. Three stages of physiological and structural development were identified. During the early postnatal period (P0 to approximately P9) Purkinje cells were characterized by an immature pattern of Na(+)-spike discharge, and possessed only short multipolar dendrites. This was followed by a period of rapid maturation (from approximately P12 to approximately P18), consisting of changes in Na(+)-spike discharge, emergence of repetitive bursts of Na(+) spikes terminated by Ca(2+) spikes (Ca(2+)-Na(+) bursts), generation of the trimodal pattern, and a significant expansion of the dendritic tree. During the final stage (> P18 to P90) there were minor refinements of cell output and a plateau in dendritic area. Our results reveal a rapid transition of the Purkinje cell from morphological and physiological immaturity to adult characteristics over a short developmental window, with a close correspondence between changes in cell output and dendritic growth. The development of Purkinje cell intrinsic electrophysiological properties further matches the time course of other measures of cerebellar structural and functional maturation.

Modulation of the beta1-3 voltage-gated sodium channels in rat vestibular and facial nuclei after unilateral labyrinthectomy and facial nerve section: an in situ hybridization study

We investigated whether the production of the mRNAs for the auxiliary beta subunits of the Na channels are modulated in deafferented medial vestibular nucleus (MVN) and in axotomized facial motoneurons. No beta1-3 mRNAs modulation was detected at any time following unilateral labyrinthectomy in the deafferented and intact medial vestibular nucleus. In contrast, beta1 gene expression in the axotomized facial nucleus decreased compared to controls as soon as day post-lesion 3.

Comparative distribution of voltage-gated sodium channel proteins in human brain

Antisera directed against unique peptide regions from each of the human brain voltage-gated sodium channel alpha subunits were generated. In immunoblots these were found to be highly specific for the corresponding recombinant polypeptides and to recognise the native holoprotein in human brain membrane preparations. These antisera were used to perform a comparative immunohistochemical distribution analysis of all four brain sodium channel subtypes in selected human CNS regions. Distinct but heterogeneous distribution patterns were observed for each of the alpha subunits. In general, these were complimentary to that previously shown for the corresponding human mRNAs. A high degree of conservation with respect to the distribution found in rat was also evident. The human alpha subunit proteins exhibited distinct subcellular localisation patterns. Types I, III and VI immunoreactivity was predominantly in neuronal cell bodies and proximal processes, whereas type II was concentrated along axons. This is similar to rat brain and suggests the different the sodium channel subtypes have distinct functions which are highly conserved between human and rodents. A notable difference was that the type III protein was detected in all human brain regions examined, unlike in rat brain where expression in adults is very restricted. Also in contrast to rat brain, the human type VI protein was not detected in axons of unmyelinated neurons. These differences may reflect true species variation and could have important implications for understanding the function of the sodium channel subtypes and their roles in human disease.

Distribution of voltage-gated sodium channel alpha-subunit and beta-subunit mRNAs in human hippocampal formation, cortex, and cerebellum

The distribution of mRNAs encoding voltage-gated sodium channel alpha subunits (I, II, III, and VI) and beta subunits (beta1 and beta2) was studied in selected regions of the human brain by Northern blot and in situ hybridisation experiments. Northern blot analysis showed that all regions studied exhibited heterogenous expression of sodium channel transcripts. In situ hybridisation experiments confirmed these findings and revealed a predominantly neuronal distribution. In the parahippocampal gyrus, subtypes II and VI and the beta-subunit mRNAs exhibited robust expression in the granule cells of the dentate gyrus and pyramidal cell layer of the hippocampus. Subtypes I and III showed moderate expression in granule cells and low expression in the pyramidal cell layer. Distinct expression patterns were also observed in the cortical layers of the middle frontal gyrus and in the entorhinal cortex. In particular, all subtypes exhibited higher levels of expression in cortical layers III, V, and VI compared with layers I and II. All subtypes were expressed in the granular layer of the cerebellum, whereas specific expression of subtypes I, VI, beta1, and beta2 mRNAs was observed in Purkinje cells. Subtypes I, VI, and beta1 mRNAs were expressed, at varying levels, in the pyramidal cells of the deep cerebellar nuclei. These data indicate that, as in rat, human brain sodium channel mRNAs have a distinct regional distribution, with individual cell types expressing different compliments of sodium channels. The differential distribution of sodium channel subtypes suggest that they have distinct roles that are likely to be of paramount importance in maintaining the functional heterogeneity of central nervous system neurons.

beta 3: an additional auxiliary subunit of the voltage-sensitive sodium channel that modulates channel gating with distinct kinetics

The voltage-sensitive sodium channel confers electrical excitability on neurons, a fundamental property required for higher processes including cognition. The ion-conducting alpha-subunit of the channel is regulated by two known auxiliary subunits, beta1 and beta2. We have identified rat and human forms of an additional subunit, beta3. It is most closely related to beta1 and is the product of a separate gene localized to human chromosome 11q23.3. When expressed in Xenopus oocytes, beta3 inactivates sodium channel opening more slowly than beta1 does. Structural modeling has identified an amino acid residue in the putative alpha-subunit binding site of beta3 that may play a role in this difference. The expression of beta3 within the central nervous system differs significantly from beta1. Our results strongly suggest that beta3 performs a distinct neurophysiological function.

Spatial distribution of NA+ and K+ channels in spinal dorsal horn neurones: role of the soma, axon and dendrites in spike generation

Spinal dorsal horn neurones play an important role in processing sensory information received from primary afferent fibers. The application of the patch-clamp technique to thin slices of rat spinal cord has enabled the study of ionic channels in visually identified dorsal horn neurones. The small soma of these neurones isolated from the slice by means of a novel method of 'entire soma isolation' has become a convenient model for investigating the properties and distributions of ionic channels. The present review summarizes results of recent experiments studying different types of voltage-gated Na+ and K+ channels expressed in dorsal horn neurones. Uneven distribution of the channels between the soma. axon and dendrites appears to play a major role in determining the neuronal excitability. The contribution of the soma, axon and dendrites to generation and propagation of the action potentials in central neurones is discussed.

cAMP-dependent phosphorylation of the tetrodotoxin-resistant voltage-dependent sodium channel SNS

1. Protein kinase A (PKA) modulation of tetrodotoxin-resistant (TTX-r) voltage-gated sodium channels may underly the hyperalgesic responses of mammalian sensory neurones. We have therefore examined PKA phosphorylation of the cloned alpha-subunit of the rat sensory neurone-specific TTX-r channel SNS. Phosphorylation of SNS was compared with that of a mutant channel, SNS(SA), in which all five PKA consensus sites (RXXS) within the intracellular I-II loop had been eliminated by site-directed mutagenesis (serine to alanine). 2. In vitro PKA phosphorylation and tryptic peptide mapping of SNS and mutant SNS(SA) I-II loops expressed as glutathione-S-transferase (GST) fusion proteins confirmed that the five mutated serines were the major PKA substrates within the SNS I-II loop. 3. SNS and SNS(SA) channels were transiently expressed in COS-7 cells and their electrophysiological properties compared. In wild-type SNS channels, forskolin and 8-bromo cAMP produced effects consistent with PKA phosphorylation. Mutant SNS(SA) currents, however, were not significantly affected by either agent. Thus, elimination of the I-II loop PKA consensus sites caused a marked reduction in PKA modulation of wild-type channels. 4. Under control conditions, the voltage dependence of activation of SNS(SA) current was shifted to depolarized potentials compared with SNS. This was associated with a slowing of SNS(SA) current inactivation at hyperpolarized potentials and suggested a tonic PKA phosphorylation of wild-type channels under basal conditions.5. We conclude that the major substrates involved in functional PKA modulation of the SNS channel are located within the intracellular I-II loop.

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.

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.

Functional distribution of three types of Na+ channel on soma and processes of dorsal horn neurones of rat spinal cord

1. Voltage-gated Na+ channels and their distribution were studied by the patch-clamp technique in intact dorsal horn neurones in slices of newborn rat spinal cord and in neurones isolated from the slice by slow withdrawal of the recording pipette. This new method of neurone isolation was further used to study the roles of soma and axon in generation of action potentials. 2. Tetrodotoxin (TTX)-sensitive Na+ currents in intact neurones consisted of three components. A fast component with an inactivation time constant (tau f) of 0.6-2.0 ms formed the major part (80-90%) of the total Na+ current. The remaining parts consisted of a slowly inactivating component (tau s of 5-20 ms) and a steady-state component. 3. Single fast and slow inactivating Na+ channels with conductances of 11.6 and 15.5 pS, respectively, were identified in the soma of intact neurones in the slice. Steady-state Na+ channels were not found in the soma, suggesting an axonal or dendritic localization of these channels. 4. In the whole-cell recording mode, the entire soma of a dorsal horn neurone could be isolated from the slice by slow withdrawal of the recording pipette, leaving all or nearly all of its processes in the slice. The isolated structure was classified as: (1) 'soma' if it lost all of its processes, (2) 'soma+axon' complex if it preserved one process and at least 85% of its original peak Na+ current or (3) 'soma+dendrite' complex if it preserved one process but the remaining Na+ current did not exceed those observed in the isolated 'somata'. 5. The spatial distribution of Na+ channels in the neurone was studied by comparing Na+ currents recorded before and after isolation. The isolated 'soma' contained 13.8 +/- 1.3% of inactivating Na+ current but no steady-state Na+ current. 'Soma+axon' complexes contained 93.6 +/- 1.4% of inactivating and 46% of steady-state Na+ current. 6. In current-clamp experiments, the intact neurones and isolated 'soma+axon' complexes responded with 'all-or-nothing' action potentials to current injections. In contrast, isolated 'somata' showed only passive or local responses and were unable to generate action potentials. 7. It is concluded that dorsal horn neurones of the spinal cord possess three types of TTX-sensitive voltage-gated Na+ channels. The method of entire soma isolation described here shows that the majority of inactivating Na+ channels are localized in the axon hillock and only a small proportion (ca 1/7) are distributed in the soma. Steady-state Na+ channels are most probably expressed in the axonal and dendritic membranes. The soma itself is not able to generate action potentials. The axon or its initial segment plays a crucial role in the generation of action potentials.

A tetrodotoxin-resistant voltage-gated sodium channel expressed by sensory neurons

Dorsal root ganglion sensory neurons associated with C-fibres, many of which are activated by tissue-damage, express an unusual voltage-gated sodium channel that is resistant to tetrodotoxin. We report here that we have identified a 1,957 amino-acid sodium channel in these cells that shows 65% identity with the rat cardiac tetrodotoxin-insensitive sodium channel, and is not expressed in other peripheral and central neurons, glia or non-neuronal tissues. In situ hybridization shows that the channel is expressed only by small-diameter sensory neurons in neonatal and adult dorsal root and trigeminal ganglia. The channel is resistant to tetrodotoxin when expressed in Xenopus oocytes. The electrophysiological and pharmacological properties of the expressed channel are similar to those described for the small-diameter sensory neuron tetrodotoxin-resistant sodium channels. As some noxious input into the spinal cord is resistant to tetrodotoxin, block of expression or function of such a C-fibre-restricted sodium channel may have a selective analgesic effect.

Na+ channel beta 1 subunit mRNA expression in developing rat central nervous system

The sodium channel beta 1 subunit (Na beta 1) is a component of the rat brain voltage-dependent sodium channel. We have used nonradioactive in situ hybridization cytochemical techniques to demonstrate that transcript levels of Na beta 1 are differentially upregulated during postnatal development of several CNS regions, with selective labeling of specific neuronal populations. In the hippocampus, labeling of the pyramidal cell layer (particularly in the CA3 region) and dentate granule cells was initially observed at postnatal day 2 (P2) and P10, respectively, and became progressively more intense with maturation. Labeled cells were first observed in the hilus at P10. In the developing cerebellum, transient labeling was observed in the external granule cell layer beginning at P1 while label increased in the internal granule cell layer up to P21. Purkinje cells showed significant label beginning at P4 and increasing up to P21. Weak signal was seen in neurons of deep nuclei at P1 and increased up to P21. Na beta 1 labeling in the spinal cord was first observed in the ventral horn at P2, and the intensity of labeling in these large motoneurons gradually increased. In addition, there was a ventral-dorsal gradient in this region, with label appearing subsequently in neurons of Rexed laminae IX, VII and VIII, and in the dorsal horn (Rexed laminae I-VI). In these regions, the labeling reached a plateau within the first 2-3 weeks after birth and persisted into the adult rat. The time course and regional heterogeneity of Na beta 1 expression are consistent with the hypothesis that the expression of mature Na+ channels, including Na beta 1, contributes to the development of circuitry that supports complex patterns of electrogenesis.

Expression of sodium channel alpha- and beta-subunits in the nervous system of the myelin-deficient rat

Using subtype-specific riboprobes and a non-isotope in situ hybridization technique, the pattern of expression of the mRNAs for voltage dependent sodium channel alpha-subunits I, II, III and NaG, and the beta 1-subunit were compared in myelin-deficient rats and unaffected male littermates. Tissues examined included the hippocampus, cerebellum, spinal cord and dorsal root ganglia. Previous studies have demonstrated that the expression of sodium channel alpha- and beta 1-subunits follows a distinct temporal and spatial pattern during development, characterized in part by greater expression of alpha-subunit III and its mRNA during development than in the adult. We examined animals of 20-22 days of age, a time when, according to earlier reports, the unaffected animals should nearly have reached an adult expression pattern. Normal male littermates were indeed found to express a sodium channel subunit mRNA pattern generally consistent with previous reports on adult rats. Myelin-deficient animals exhibited an expression pattern identical to the unaffected littermates, indicating that myelination is not required for the progression from the embryonic to the adult expression pattern of sodium channel subunits.

Na+ channel beta 1 subunit mRNA: differential expression in rat spinal sensory neurons

The brain Na+ channel beta 1 subunit (Na beta 1) mRNA has recently been localized within rat central nervous system where it is expressed at differing levels in different types of neurons. In the present study, we have studied the expression pattern of Na beta 1 mRNA in rat dorsal root ganglion (DRG) neurons using non-radioactive in situ hybridization histochemistry. Na beta 1 mRNA is differentially expressed in adult DRG, with higher levels in intermediate-to-large (> approximately 25 microns in diameter) DRG neurons than in small (< 25 microns) DRG neurons. This cell body size-related Na beta 1 mRNA expression is consistently observed beginning at postnatal day 4 and continues throughout development to adulthood. The present results indicate that (i) Na beta 1 mRNA is expressed in neurons in the peripheral nervous system and (ii) Na beta 1 gene expression is differentially regulated in DRG neurons in relation to their cell body sizes.

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.