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

Effects of BmK AS on Nav1.2 expressed in Xenopus laevis oocytes

In the present study, the pharmacological effects of BmK AS, a beta-like scorpion toxin on rNav1.2 alpha-subunit expressed in Xenopus laevis oocytes were investigated using a two-electrode voltage-clamp recording. It was found that the voltage dependence of rNav1.2 inactivation was significantly shifted towards positive membrane potential by 500 nM BmK AS, whereas the activation curves of rNav1.2 were unruffled at the same dosage. The inactivation curves of both slow and fast inactivation currents were positively moved about 12.8 and 9.7 mV, respectively. In addition, the persistent currents of rNav1.2 were invariable. The effects of BmK AS on the rNav1.2 inactivation were opposite to the previous results found in the peripheral sensory neurons. The results suggested that Nav1.2 might be the target of BmK AS in the central nervous system, and BmK AS might have an excitatory effect on the central neuron through enhancing Nav1.2.

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.

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.

NaG: a sodium channel-like mRNA shared by Schwann cells and other neural crest derivatives

NaG, a member of subfamily 2, was originally characterized as a "glial" sodium channel, and is expressed at high levels in Schwann cells in vivo. However, NaG has also been shown by in situ hybridization to be highly expressed in rat dorsal root ganglion (DRG) neurons, which, like Schwann cells, are neural crest derivatives. In the present study, we used non-isotopic in situ hybridization with a riboprobe for NaG, in conjunction with RT-PCR, to determine whether NaG is expressed in tissues related to the DRG either by neural crest origin or sensory function. We found that the expression of significant levels of NaG mRNA was restricted to derivatives of the neural crest (neurons of DRG, trigeminal ganglion and mesencephalic nucleus of the trigeminal nerve, and Schwann and satellite cells); the absence of NaG from superior cervical ganglion neurons and adrenal medulla chromaffin cells indicates that not all neural crest derived neural elements express NaG. NaG was not observed in sensory neurons (retina, vestibular ganglion, spiral ganglion, olfactory epithelium) that are not of neural crest origin. Our results indicate that NaG mRNA is expressed not only in Schwann cells, but also in a spectrum of neuronal cell types with afferent function of neural crest origin. NaG thus appears to represent a transcript encoding a sodium channel or sodium channel-like protein that is uniquely expressed by both Schwann cells and afferent neurons of neural crest origin.

Differential expression of sodium channel genes in retinal ganglion cells

Action potential electrogenesis in the axons of retinal ganglion cells is supported by voltage-gated sodium channels, and a tetrodotoxin (TTX)-inhibitable sodium conductance participates in anoxic injury of these axons within the optic nerve. However, the subtypes of sodium channels expressed in retinal ganglion cells have not been identified. In this study, we used reverse transcription-polymerase chain reaction (RT-PCR) and restriction enzyme mapping, together with in situ hybridization, to examine the expression of transcripts for sodium channel alpha-subunits I, II, III, NaG, Na6, hNE/PN1 and SNS, and beta-subunits 1 and 2, in the retina of the adult rat. RT-PCR yielded high levels of amplification of I, II, III, Na6, beta1 and beta2 transcripts. In situ hybridization demonstrated the presence of all these mRNAs in the cell bodies of retinal ganglion cells. Retinal ganglion cells thus express multiple sodium channel mRNAs, suggesting that they deploy several different types of sodium channels.

Sodium channel alpha-subunit mRNAs I, II, III, NaG, Na6 and hNE (PN1): different expression patterns in developing rat nervous system

The expression of sodium channel alpha-subunit mRNAs I, II, III, NaG, Na6 and hNE (PN1) was examined in developing (E17-P30) hippocampus, cerebellum, spinal cord and dorsal root ganglia using non-isotopic in situ hybridization cytochemistry. The results showed distinct patterns of expression for each of the sodium channel mRNAs with maturation of the nervous system. In the hippocampus, sodium channel mRNA I was not detected at any developmental time, while mRNA II showed increasing hybridization signal between E17 and P30. Sodium channel mRNA III was more prevalent at late embryonic and early postnatal times, and was barely detectable at P30. The transcript for NaG showed transient expression between P2 and P15, being expressed at low levels at E17 and not being detectable at P30. Sodium channel mRNA Na6 exhibited a high level of expression between E17 and P15 in the hippocampal formation, with an attenuation of the signal by P30. hNE (PN1) mRNA was not detected in the hippocampus at any time examined. In the cerebellum, sodium channel mRNA I was not detected at E17 or P2, but became detectable in Purkinje cells at P15 and continued to show a low level of expression in these cells at P30. mRNA I was not detected at any time examined in granule cells of the cerebellum. Sodium channel mRNA II exhibited increasing expression in the developing cerebellum, and showed increasing signal in Purkinge cells beginning on P2 and granule cells on P15. Sodium channel mRNA III was down-regulated with development in the cerebellum, although mRNA III was readily detected at E17, it was not detected in any layers of the cerebellum by P15. NaG mRNA showed a peak of expression at P2, and was present at low levels at E17 and P15 and not detectable at P30. Na6 mRNA was highly expressed in the E17 cerebellum; this mRNA was present at high levels in Purkinje cells throughout development, although in granule cells the signal was attenuated at P15-P30. Sodium channel hNE (PN1) mRNA was not detected in the cerebellum at any time in development. In the spinal cord, sodium channel mRNA I showed increasing expression beginning at P2 and was highly expressed, particularly in ventral motor neurons, by P30. Sodium channel II mRNA was detected at all stages of development in the spinal cord; in contrast, mRNA III was detected at E17 and P2, but showed very low levels of expression by P30. NaG mRNA exhibited a transient expression in spinal cord at P2, but was not detectable at E17 and P30. Na6 mRNA was detectable at very low levels at E17 and became highly expressed at P2, prior to a reduction of the signal at P15 and P30. hNE (PN1) mRNA was not detected in the spinal cord at any time in development. In the dorsal root ganglia, sodium channel I mRNA hybridization signal was detected in DRG neurons at P2, with slightly increased levels at P15 and P30. Sodium channel II mRNA exhibited a relatively constant, moderate level of expression at all developmental ages. Sodium channel III mRNA was highly expressed in DRG neurons at E17 but was down-regulated with further development so that it was not detectable by P30. NaG mRNA was strongly expressed by some DRG neurons at all stages of development from E17 to P30; in general the level of NaG labelling was greater in larger neurons than in smaller neurons. Na6 mRNA showed increasing expression with development in DRG neurons; at E17, low levels of Na6 mRNA were detected and by P15 to P30 high levels of expression were present in some neurons. hNE (PN1) mRNA was present in DRG neurons at P2, and was up-regulated with further development so that by P30 hNE (PN1) was expressed in all DRG neurons sizes. These results demonstrate that sodium channel alpha-subunit mRNAs I, II, III, NaG, Na6 and hNE (PN1) exhibit distinct spatial and temporal patterns of expression in nervous tissue, and suggest that the expression of the sodium channel alpha-subunits is differentially regulated. (ABSTRACT TRUNCATED)

Spinal sensory neurons express multiple sodium channel alpha-subunit mRNAs

The expression of sodium channel alpha-, beta 1- and beta 2-subunit mRNAs was examined in adult rat DRG neurons in dissociated culture at 1 day in vitro and within sections of intact ganglia by in situ hybridization and reverse transcription polymerase chain reaction (RT-PCR). The results demonstrate that sodium channel alpha-subunit mRNAs are differentially expressed in small (< 25 microns diam), medium (25-45 microns diam.) and large (> 45 microns diam.) cultured DRG neurons at 1 day in vitro (div). Sodium channel mRNA I is expressed at higher levels in large neurons than small DRG neurons, while sodium channel mRNA II is variably expressed, with most cells lacking or exhibiting low levels of detectable signal of these mRNAs and limited numbers of neurons with moderate expression levels. DRG neurons generally exhibit negligible or low levels of hybridization signal for sodium channel mRNA III. Sodium channel mRNAs Na6 and NaG show similar patterns of expression, with most large and many medium DRG neurons exhibiting high levels of expression. The mRNA for the rat cognate of human sodium channel hNE-Na is detected in virtually every DRG neuron; most cells in all size classes exhibit moderate or high levels of hNE-Na expression. Sodium channel SNS mRNA is expressed in all size classes of DRG neurons, but shows greater expression in small and medium DRG neurons than in large neurons. The mRNA for the rat cognate of mouse sodium channel mNa 2.3 is not detected, or is detected at low levels, in most DRG neurons, regardless of size, although moderate expression is detected in some neurons. Sodium channel beta 1- and beta 2-subunit mRNAs exhibit similar expression patterns; they are detected in most DRG neurons, although the level of expression tends to be greater in large neurons than in small neurons. RT-PCR and in situ hybridization of intact adult DRG showed a similar pattern of expression of sodium channel mRNAs to that observed in DRG neurons in vitro. These results demonstrate that adult DRG neurons express multiple sodium channel mRNAs in vitro and in situ and suggest a molecular basis for the biophysical heterogeneity of sodium currents observed in these cells.