Batrachotoxin acts as a stent to hold open homotetrameric prokaryotic voltage-gated sodium channels.

Batrachotoxin (BTX), an alkaloid from skin secretions of dendrobatid frogs, causes paralysis and death by facilitating activation and inhibiting deactivation of eukaryotic voltage-gated sodium (Nav) channels, which underlie action potentials in nerve, muscle, and heart. A full understanding of the mechanism by which BTX modifies eukaryotic Nav gating awaits determination of high-resolution structures of functional toxin-channel complexes. Here, we investigate the action of BTX on the homotetrameric prokaryotic Nav channels NaChBac and NavSp1. By combining mutational analysis and whole-cell patch clamp with molecular and kinetic modeling, we show that BTX hinders deactivation and facilitates activation in a use-dependent fashion. Our molecular model shows the horseshoe-shaped BTX molecule bound within the open pore, forming hydrophobic H-bonds and cation-π contacts with the pore-lining helices, leaving space for partially dehydrated sodium ions to permeate through the hydrophilic inner surface of the horseshoe. We infer that bulky BTX, bound at the level of the gating-hinge residues, prevents the S6 rearrangements that are necessary for closure of the activation gate. Our results reveal general similarities to, and differences from, BTX actions on eukaryotic Nav channels, whose major subunit is a single polypeptide formed by four concatenated, homologous, nonidentical domains that form a pseudosymmetric pore. Our determination of the mechanism by which BTX activates homotetrameric voltage-gated channels reveals further similarities between eukaryotic and prokaryotic Nav channels and emphasizes the tractability of bacterial Nav channels as models of voltage-dependent ion channel gating. The results contribute toward a deeper, atomic-level understanding of use-dependent natural and synthetic Nav channel agonists and antagonists, despite their overlapping binding motifs on the channel proteins.

An important role of a pyrethroid-sensing residue F1519 in the action of the N-alkylamide insecticide BTG 502 on the cockroach sodium channel.

Deltamethrin, a pyrethroid insecticide, and BTG 502, an alkylamide insecticide, target voltage-gated sodium channels. Deltamethrin binds to a unique receptor site and causes prolonged opening of sodium channels by inhibiting deactivation and inactivation. Previous (22)Na(+) influx and receptor binding assays using mouse brain synaptoneurosomes showed that BTG 502 antagonized the binding and action of batrachotoxin (BTX), a site 2 sodium channel neurotoxin. However, the effect of BTG 502 has not been examined directly on sodium channels expressed in Xenopus oocytes. In this study, we examined the effect of BTG 502 on wild-type and mutant cockroach sodium channels expressed in Xenopus oocytes. Toxin competition experiments confirmed that BTG 502 antagonizes the action of BTX and possibly shares a common receptor site with BTX. However, unlike BTX which causes persistent activation of sodium channels, BTG 502 reduces the amplitude of peak sodium current. A previous study showed that BTG 502 was more toxic to pyrethroid-resistant house flies possessing a super-kdr (knockdown resistance) mechanism than to pyrethroid-susceptible house flies. However, we found that the cockroach sodium channels carrying the equivalent super-kdr mutations (M918T and L1014F) were not more sensitive to BTG 502 than the wild-type channel. Instead, a kdr mutation, F1519I, which reduces pyrethroid binding, abolished the action of BTG 502. These results provide evidence the actions of alkylamide and pyrethroid insecticides require a common sodium channel residue.

Hoiamide a, a sodium channel activator of unusual architecture from a consortium of two papua new Guinea cyanobacteria.

Hoiamide A, a novel bioactive cyclic depsipeptide, was isolated from an environmental assemblage of the marine cyanobacteria Lyngbya majuscula and Phormidium gracile collected in Papua New Guinea. This stereochemically complex metabolite possesses a highly unusual structure, which likely derives from a mixed peptide-polyketide biogenetic origin, and includes a peptidic section featuring an acetate extended and S-adenosyl methionine modified isoleucine moiety, a triheterocyclic fragment bearing two alpha-methylated thiazolines and one thiazole, and a highly oxygenated and methylated C15-polyketide substructure. Pure hoiamide A potently inhibited [(3)H]batrachotoxin binding to voltage-gated sodium channels (IC(50) = 92.8 nM), activated sodium influx (EC(50) = 2.31 microM) in mouse neocortical neurons, and exhibited modest cytotoxicity to cancer cells. Further investigation revealed that hoiamide A is a partial agonist of site 2 on the voltage-gated sodium channel.

A neurotoxinological approach to the treatment of obstructive sleep apnoea.

Current treatment approaches to the problem of obstructive sleep apnoea (OSA) have limitations. Specifically, invasive anatomical-based surgery and dental appliances typically do not alleviate obstruction at an acceptable rate, and compliance to continuous positive airway pressure (CPAP) devices is frequently suboptimal. Neurotoxinological treatment approaches are widespread in the field of medicine, but as yet have not been evaluated as a treatment for sleep-disordered breathing. In this review, it is argued that despite widespread recognition of the loss of upper airway (UA) muscular tone and/or reflexes in the expression of OSA, most treatment interventions to date have focused on anatomical principles alone. Several hypothesised neurotoxinological interventions aimed at either enhancing UA neuromuscular tone and/or reflexes are proposed, and some preliminary data is presented. Although in its early infancy, with considerable toxicity studies in animals yet to be done, a neurotoxinological approach to the problem of OSA holds promise as a future treatment, with the potential for both high effectiveness and patient compliance.

Phosphorimaging detection and quantitation for isotopic ion flux assays.

A 96-well-microplate-based ion flux method utilizing readily available autoradiographic phosphorimaging detection is described. Nicotinic acetylcholine receptor-mediated (22)Na influx in four cultured cell lines provided satisfactory concentration-response data for epibatidine and several other nicotinic agonists. The data were consistent with data obtained using standard 6-well assays. Assays for nicotinic-receptor-mediated (86)Rb efflux produced data similar to data obtained with the (22)Na influx assay. However, assays for (45)Ca influx were not successful, although (45)Ca was readily detected and quantified. Voltage-gated sodium channel-mediated (22)Na influx in a neuroblastoma cell line allowed assay of the effects of such sodium channel activators as batrachotoxin and a pumiliotoxin B/scorpion venom combination. Phosphorimaging detection allows for reliable beta counting of up to 1,200 simultaneous samples with excellent sensitivity and is amenable for application to high-throughput screening.

State-dependent access to the batrachotoxin receptor on the sodium channel.

Batrachotoxin causes sustained opening of voltage-gated sodium channels. Toxin binds irreversibly to wild type channels; however, it dissociates rapidly from channels with mutation F1710C in transmembrane segment IVS6. This dissociation requires channel activation, suggesting that the activation gate guards the toxin-binding site. Here we show that activity-dependent toxin dissociation was not affected by external sodium, arguing against a binding site within the pore, and demonstrate that dissociation occurred only during the first few milliseconds after membrane depolarization, as if the toxin leaves its binding site during closed states that precede the final open state in the activation pathway. Toxin interaction with preopen states may facilitate subsequent channel opening, thus accounting for the batrachotoxin-induced negative shift in channel activation.

The batrachotoxin receptor on the voltage-gated sodium channel is guarded by the channel activation gate.

Batrachotoxin (BTX), from South American frogs of the genus Phyllobates, irreversibly activates voltage-gated sodium channels. Previous work demonstrated that a phenylalanine residue approximately halfway through pore-lining transmembrane segment IVS6 is a critical determinant of channel sensitivity to BTX. In this study, we introduced a series of mutations at this site in the Na(v)1.3 sodium channel, expressed wild-type and mutant channels in Xenopus laevis oocytes, and examined their sensitivity to BTX using voltage clamp recording. We found that substitution of either alanine or isoleucine strongly reduced channel sensitivity to toxin, whereas cysteine, tyrosine, or tryptophan decreased toxin action only modestly. These data suggest an electrostatic ligand-receptor interaction at this site, possibly involving a charged tertiary amine on BTX. We then used a mutant channel (mutant F1710C) with intermediate toxin sensitivity to examine the properties of the toxin-receptor reaction in more detail. In contrast to wild-type channels, which bind BTX almost irreversibly, toxin dissociation from mutant channels was rapid, but only when the channels were open, not when they were closed. These data suggest the closed activation gate trapped bound toxin. Although BTX dissociation required channel activation, it was, paradoxically, slowed by strong membrane depolarization, suggesting additional state-dependent and/or electrostatic influences on the toxin binding reaction. We propose that BTX moves to and from its receptor through the cytoplasmic end of the open ion-conducting pore, in a manner similar to that of quaternary local anesthetics like QX314.

Rapid and slow voltage-dependent conformational changes in segment IVS6 of voltage-gated Na(+) channels.

Mutations in segment IVS6 of voltage-gated Na(+) channels affect fast-inactivation, slow-inactivation, local anesthetic action, and batrachotoxin (BTX) action. To detect conformational changes associated with these processes, we substituted a cysteine for a valine at position 1583 in the rat adult skeletal muscle sodium channel alpha-subunit, and examined the accessibility of the substituted cysteine to modification by 2-aminoethyl methanethiosulfonate (MTS-EA) in excised macropatches. MTS-EA causes an irreversible reduction in the peak current when applied both internally and externally, with a reaction rate that is strongly voltage-dependent. The rate increased when exposures to MTS-EA occurred during brief conditioning pulses to progressively more depolarized voltages, but decreased when exposures occurred at the end of prolonged depolarizations, revealing two conformational changes near site 1583, one coupled to fast inactivation, and one tightly associated with slow inactivation. Tetraethylammonium, a pore blocker, did not affect the reaction rate from either direction, while BTX, a lipophilic activator of sodium channels, completely prevented the modification reaction from occurring from either direction. We conclude that there are two inactivation-associated conformational changes in the vicinity of site 1583, that the reactive site most likely faces away from the pore, and that site 1583 comprises part of the BTX receptor.

Neuroprotective action of a novel compound--M50463--in primary cultured neurons.

The neuroprotective effects of a novel synthetic compound, M50463, have been determined by using embryonic rat neocortical neurons in various culture conditions. M50463 was initially characterized as a potent specific ligand for a voltage-dependent sodium channel by radioligand binding studies. In fact, M50463 inhibited neuronal cell death induced by veratrine and inhibited an increase of the intracellular calcium level in neurons evoked by veratrine. In addition to such expected effects, M50463 had the ability to prevent glutamate neurotoxicity, to promote the neuronal survival in serum-deprived medium and to prevent nitric oxide-induced neurotoxicity. These results suggested that M50463 is not a simple sodium channel blocker, but a neuroprotective agent which has some crucial mechanism of action on neuronal death occurring in various situations, and it is a novel, innovative candidate for neuroprotective therapy for various neurodegenerative disorders.

Interactions of delta-conotoxins with alkaloid neurotoxins reveal differences between the silent and effective binding sites on voltage-sensitive sodium channels.

The delta-conotoxin-TxVIA from Conus textile (delta TxVIA) is a mollusk-specific conotoxin that slows sodium channel inactivation exclusively in mollusk neuronal membranes but reveals high-affinity binding to both mollusk (effective binding) and rat brain (silent binding) neuronal membranes, despite not having any toxic effect in vertebrates in vivo and in vitro. Using binding studies with radioactive delta TxVIA we demonstrate that a different mollusk-specific conotoxin, delta-conotoxin-GmVIA from the venom of Conus gloriamaris, possesses "silent" and effective binding properties in rat brain and mollusk sodium channels, respectively. Binding studies and electrophysiological tests with both vertebrate muscle and insect neuronal preparations have indicated that the silent binding sites of delta TxVIA are highly conserved in a wide range of distinct vertebrate and insect sodium channels. Direct probing of receptor site 2 by a tritiated derivative of batrachotoxin ([3H]BTX-B) revealed that [3H]BTX-B binding in mollusk sodium channels is of high affinity with no addition of enhancing ligands, unlike [3H]BTX-B binding in rat brain. In contrast to the negative allosteric modulation of delta TxVIA binding by veratridine, delta TxVIA is not able to affect the binding of [3H]BTX-B in mollusk neuronal membranes but reduces [3H]BTX-B binding in rat brain in the presence of alpha-scorpion toxins. The latter finding indicates the existence of a pharmacological distinction between the silent and effective binding sites of delta TxVIA and points out possible functionally important structural differences between molluscan and rat brain sodium channels.

Mechanism of thiamine transport in neuroblastoma cells. Inhibition of a high affinity carrier by sodium channel activators and dependence of thiamine uptake on membrane potential and intracellular ATP.

Nerve cells are particularly sensitive to thiamine deficiency. We studied thiamine transport in mouse neuroblastoma (Neuro 2a) cells. At low external concentration, [14C]thiamine was taken up through a saturable high affinity mechanism (Km = 35 nM). This was blocked by low concentrations of the Na+ channel activators veratridine (IC50 = 7 +/- 4 microM) and batrachotoxin (IC50 = 0.9 microM). These effects were not antagonized by tetrodotoxin and were also observed in cell lines devoid of Na+ channels, suggesting that these channels are not involved in the mechanism of inhibition. At high extracellular concentrations, thiamine uptake proceeds essentially via a low affinity carrier (Km = 0.8 mM), insensitive to veratridine but blocked by divalent cations. In both cases, the uptake was independent on external sodium, partially inhibited (10-35%) by depolarization and sensitive to metabolic inhibitors. A linear relationship between the rate of thiamine transport and intracellular ATP concentration was found. When cells grown in a medium of low thiamine concentration (6 nM) were exposed to 100 nM extracellular thiamine, a 3-fold increase in intracellular thiamine diphosphate was observed after 2 h while the concomitant increase in intracellular free thiamine was barely significant. These data suggest a secondary active transport of thiamine, the main driving force being thiamine phosphorylation rather than the sodium gradient.

Grayanotoxin-I-modified eel electroplax sodium channels. Correlation with batrachotoxin and veratridine modifications.

To probe the structure-function relationships of voltage-dependent sodium channels, we have been examining the mechanisms of channel modification by batrachotoxin (BTX), veratridine (VTD), and grayanotoxin-I (GTX), investigating the unifying mechanisms that underlie the diverse modifications of this class of neurotoxins. In this paper, highly purified sodium channel polypeptides from the electric organ of the electric eel were incorporated into planar lipid bilayers in the presence of GTX for comparison with our previous studies of BTX (Recio-Pinto, E., D. S. Duch, S. R. Levinson, and B. W. Urban. 1987. J. Gen. Physiol. 90:375-395) and VTD (Duch, D. S., E. Recio-Pinto, C. Frenkel, S. R. Levinson, and B. W. Urban. 1989. J. Gen. Physiol. 94:813-831) modifications. GTX-modified channels had a single channel conductance of 16 pS. An additional large GTX-modified open state (40-55 pS) was found which occurred in bursts correlated with channel openings and closings. Two voltage-dependent processes controlling the open time of these modified channels were characterized: (a) a concentration-dependent removal of inactivation analogous to VTD-modified channels, and (b) activation gating similar to BTX-modified channels, but occurring at more hyperpolarized potentials. The voltage dependence of removal of inactivation correlated with parallel voltage-dependent changes in the estimated K1/2 of VTD and GTX modifications. Ranking either the single channel conductances or the depolarization required for 50% activation, the same sequence is obtained: unmodified > BTX > GTX > VTD. The efficacy of the toxins as activators follows the same ranking (Catterall, W. A. 1977. J. Biol. Chem. 252:8669-8676).

Voltage-dependent tetrodotoxin binding to single batrachotoxin-modified Na channels recorded from intact neuroblastoma cells.

To clarify the voltage-dependent actions of tetrodotoxin (TTX) on Na channels in cellular membranes, we examined the TTX block of single batrachotoxin-modified Na channels in neuroblastoma cells. We found these Na channels had a high affinity for TTX which decreased e-fold per 35.5 mV depolarization. The decrease in affinity resulted primarily from a decrease in the blocking rate for TTX; the unblocking rate increased slightly with depolarization. While the voltage-dependence of TTX binding to neuroblastoma Na channels was similar to that reported in purified Na channels incorporated in bilayers, the magnitude and voltage-dependence of the rate constants were quite different.

Regulation by batrachotoxin, veratridine, and monensin of basal and carbachol-induced phosphoinositide hydrolysis in neurohybrid NCB-20 cells.

Batrachotoxin (BTX), veratridine and monensin induced a time- and dose-dependent increase of [3H]-inositol monophosphate (3H-IP1) accumulation in the presence of lithium in prelabeled neurohybrid NCB-20 cells. A decrease of NaCl concentration to less than 30 mM markedly increased basal 3H-IP1 accumulation; however, the percentage of stimulation induced by these three agents remained unchanged even in the complete absence of sodium. The stimulation of phosphoinositide hydrolysis induced by these agents was detected in the absence of lithium but was largely prevented in the calcium-free medium. Tetradotoxin (TTX) blocked effects of BTX and veratridine (IC50 approximately 20nM), but not that stimulated by monensin. Thus, calcium-dependent activation of phospholipase C by these agents did not involve the entry of sodium or lithium. BTX and monensin also induced greater than additive effects on carbachol-induced 3H-IP1 accumulation. These effects were also TTX-sensitive and involved an increase in the Vmax and a decrease in the EC50 for carbachol. Veratridine provoked strikingly different effects on carbachol-dependent phosphoinositide turnover, depending on the passage number of the cells.

Human natural killer cells express Na+ channels. A pharmacologic flow cytometric study.

Voltage-gated excitability of purified human NK cells was studied by using flow cytometry and the voltage-sensitive dye, oxonol. Highly purified human NK cells (CD16 = 95 +/- 1%) from normal volunteers were prepared by using a negative panning technique. The Na(+)-channel agonists batrachotoxin (BTX) (1 to 4 microM) and veratridine (Ver) (100 to 400 microM) depolarized a population of highly purified human NK cells as determined by flow cytometry. BTX and Ver responses were concentration-, time-, temperature-, and Na(+)-dependent. The Na+ channel antagonist tetrodotoxin (1 microM) blocked BTX and Ver responses. Ver (100 microM) produced significant inhibition of cytotoxicity when purified NK cells were incubated with K562 tumor target cells in a 4-h 51Cr release cytotoxicity assay. The effect was blocked by tetrodotoxin. These results strongly suggest presence of functional Na+ channels in NK cells. Activation of voltage-dependent Na+ channels depolarizes cells and reduces their in vitro cytotoxic function.

Veratridine modification of the purified sodium channel alpha-polypeptide from eel electroplax.

In the interest of continuing structure-function studies, highly purified sodium channel preparations from the eel electroplax were incorporated into planar lipid bilayers in the presence of veratridine. This lipoglycoprotein originates from muscle-derived tissue and consists of a single polypeptide. In this study it is shown to have properties analogous to sodium channels from another muscle tissue (Garber, S. S., and C. Miller. 1987. Journal of General Physiology. 89:459-480), which have an additional protein subunit. However, significant qualitative and quantitative differences were noted. Comparison of veratridine-modified with batrachotoxin-modified eel sodium channels revealed common properties. Tetrodotoxin blocked the channels in a voltage-dependent manner indistinguishable from that found for batrachotoxin-modified channels. Veratridine-modified channels exhibited a range of single-channel conductance and subconductance states. The selectivity of the veratridine-modified sodium channels for sodium vs. potassium ranged from 6-8 in reversal potential measurements, while conductance ratios ranged from 12-15. This is similar to BTX-modified eel channels, though the latter show a predominant single-channel conductance twice as large. In contrast to batrachotoxin-modified channels, the fractional open times of these channels had a shallow voltage dependence which, however, was similar to that of the slow interaction between veratridine and sodium channels in voltage-clamped biological membranes. Implications for sodium channel structure are discussed.

Stimulation of phosphoinositide breakdown in brain synaptoneurosomes by agents that activate sodium influx: antagonism by tetrodotoxin, saxitoxin, and cadmium.

Agents that increase intracellular concentrations of Na+ stimulate phosphoinositide breakdown in guinea pig cerebral cortical synaptoneurosomes. When combined, these agents did not have additive effects on phosphoinositide breakdown but did have additive or greater than additive effects with carbamylcholine. Scorpion venom (Leiurus quinquestriatus) and pumiliotoxin B, which induce small increases in influx of 22Na+ in synaptoneurosomes, stimulate phosphoinositide breakdown by about 6- and 3-fold, respectively; both effects are inhibited by tetrodotoxin (TTX). Batrachotoxin (BTX) and veratridine, which cause a large increase in influx of 22Na+ through activation of voltage-dependent sodium channels, induce a 5- to 6-fold dose-dependent increase in phosphoinositide breakdown, which appears competitively inhibited by 5 microM TTX. BTX- and veratridine-elicited influx of 22Na+ into synaptoneurosomes is virtually completely blocked by 5 microM TTX. Agents that block voltage-dependent calcium channels, such as D-600, nifedipine, and Co2+, do not inhibit either influx of 22Na+ or stimulation of phosphoinositide breakdown elicited by scorpion venom, pumiliotoxin B, or BTX. Cadmium ions (200 microM), which are known to block TTX-resistant sodium channels, block phosphoinositide breakdown induced by agents that activate sodium influx through sodium channels. Cadmium blocks BTX-induced phosphoinositide breakdown with an IC50 value of 48 microM, while blocking BTX-induced 22Na+ influx in synaptoneurosomes with a 13-fold lower potency (IC50, 610 microM). In the presence of 0.5 microM TTX, the IC50 for Cd2+ inhibition of BTX-induced 22Na+ influx is now 430 microM. Neither TTX nor Cd2+ antagonize neurotransmitter- or monensin-induced phosphoinositide breakdown. It appears that BTX-induced phosphoinositide breakdown in guinea pig synaptoneurosomes is dependent primarily on activation of TTX-resistant, Cd2+-sensitive sodium channels that account for only a small fraction of the total sodium influx induced by BTX in synaptoneurosomes. However, cadmium also may in some way inhibit phosphoinositide breakdown elicited by sodium channel agents at a point subsequent to sodium influx.

Purified and unpurified sodium channels from eel electroplax in planar lipid bilayers.

Highly purified sodium channel protein from the electric eel, Electrophorus electricus, was reconstituted into liposomes and incorporated into planar bilayers made from neutral phospholipids dissolved in decane. The purest sodium channel preparations consisted of only the large, 260-kD tetrodotoxin (TTX)-binding polypeptide. For all preparations, batrachotoxin (BTX) induced long-lived single-channel currents (25 pS at 500 mM NaCl) that showed voltage-dependent activation and were blocked by TTX. This block was also voltage dependent, with negative potentials increasing block. The permeability ratios were 4.7 for Na+:K+ and 1.6 for Na+:Li+. The midpoint for steady state activation occurred around -70 mV and did not shift significantly when the NaCl concentration was increased from 50 to 1,000 mM. Veratridine-induced single-channel currents were about half the size of those activated by BTX. Unpurified, nonsolubilized sodium channels from E. electricus membrane fragments were also incorporated into planar bilayers. There were no detectable differences in the characteristics of unpurified and purified sodium channels, although membrane stability was considerably higher when purified material was used. Thus, in the eel, the large, 260-kD polypeptide alone is sufficient to demonstrate single-channel activity like that observed for mammalian sodium channel preparations in which smaller subunits have been found.

Nerve growth factor increases the number of functional Na channels and induces TTX-resistant Na channels in PC12 pheochromocytoma cells.

The PC12 clone is a line of rat pheochromocytoma cells that undergoes neuronal differentiation in the presence of NGF protein. In the absence of NGF, PC12 cells are electrically inexcitable, while after several weeks of NGF treatment they develope Na+ action potentials. Past estimates made by measuring binding of 3H-saxitoxin (STX) indicate that NGF treatment brings about a large increase in Na channel density that is of sufficient magnitude to account for the induction of excitability. We have now used 22Na uptake to measure the Na permeability of PC12 cells before and after long-term NGF treatment. Treatment with NGF does not change the resting Na+ permeability. The alkaloid toxins veratridine and batrachotoxin (BTX) and scorpion toxin were used to activate Na channels. Such studies demonstrate that these toxins induce TTX-sensitive Na uptake in both NGF-treated and untreated cells and reveal differences in functional Na channel numbers per cell and per unit of membrane area that are similar to those found in the STX binding studies. On the other hand, affinities for drugs that activate these channels are not affected by NGF treatment. We also find that NGF-treated PC12 cells contain a population of Na channels with low affinity for TTX. These channels account for 5-20% of total BTX or veratridine-stimulated flux. Thus, NGF has 2 effects regarding the Na channels of PC12 cells: it increases the number of functional Na channels that otherwise behave similarly to those present before NGF treatment, and it induces the presence of TTX-resistant Na channels. These findings indicate that the PC12 model system may serve to study the developmental regulation of Na channel expression and properties.

Development of sodium channel protein during chemically induced differentiation of neuroblastoma cells.

We have previously shown that the [3H]saxitoxin binding site of the sodium channel is expressed independently of the [125I]scorpion toxin binding site in chick muscle cultures and in rat brain. In the present work, we studied the development of the sodium channel protein during chemically induced differentiation of N1E-115 neuroblastoma cells, using [3H]saxitoxin binding, [125I]scorpion toxin binding, and 22Na uptake techniques. When grown in their normal culture medium, these cells are mostly undifferentiated, bind 90 +/- 10 fmol of [3H]saxitoxin/mg of protein and 112 +/- 14 fmol of [125I]scorpion toxin/mg protein, and, when stimulated with scorpion toxin and batrachotoxin, take up 70 +/- 5 nmol of 22Na/min/mg of protein. Cells treated with dimethyl sulfoxide (DMSO) or hexamethylene-bis-acetamide (HMBA) differentiate morphologically within 3 days. At this time, the [3H]saxitoxin binding, the [125I]scorpion toxin binding, and the 22Na uptake values are not very different from those of undifferentiated cells. With subsequent time in DMSO or HMBA, these values continue to increase, a result indicating that the main period of sodium channel expression occurs well after the cells have assumed the morphologically differentiated state. The data indicate that the expression of sodium channels and morphological differentiation are independently regulated neuronal properties, that the attainment of morphological differentiation is necessary but not in itself sufficient for full expression of the sodium channel proteins, and that, in contrast to the chick muscle cultures and rat brain, the [3H]saxitoxin site and [125I]scorpion toxin site appear to be coregulated in N1E-115 cells.