Calcium Signaling during Cortical Apical Dendrite Initiation: A Role for Cajal-Retzius Neurons

The apical dendrite of a cortical projection neuron (CPN) is generated from the leading process of the migrating neuron as the neuron completes migration. This transformation occurs in the cortical marginal zone (MZ), a layer that contains the Cajal-Retzius neurons and their axonal projections. Cajal-Retzius neurons (CRNs) are well known for their critical role in secreting Reelin, a glycoprotein that controls dendritogenesis and cell positioning in many regions of the developing brain. In this study, we examine the possibility that CRNs in the MZ may provide additional signals to arriving CPNs, that may promote the maturation of CPNs and thus shape the development of the cortex. We use whole embryonic hemisphere explants and multiphoton microscopy to confirm that CRNs display intracellular calcium transients of <1-min duration and high amplitude during early corticogenesis. In contrast, developing CPNs do not show high-amplitude calcium transients, but instead show a steady increase in intracellular calcium that begins at the time of dendritic initiation, when the leading process of the migrating CPN is encountering the MZ. The possible existence of CRN to CPN communication was revealed by the application of veratridine, a sodium channel activator, which has been shown to preferentially stimulate more mature cells in the MZ at an early developmental time. Surprisingly, veratridine application also triggers large calcium transients in CPNs, which can be partially blocked by a cocktail of antagonists that block glutamate and glycine receptor activation. These findings outline a model in which CRN spontaneous activity triggers the release of glutamate and glycine, neurotransmitters that can trigger intracellular calcium elevations in CPNs. These elevations begin as CPNs initiate dendritogenesis and continue as waves in the post-migratory cells. Moreover, we show that the pharmacological blockade of glutamatergic signaling disrupts migration, while forced expression of a bacterial voltage-gated calcium channel (CavMr) in the migrating neurons promotes dendritic growth and migration arrest. The identification of CRN to CPN signaling during early development provides insight into the observation that many autism-linked genes encode synaptic proteins that, paradoxically, are expressed in the developing cortex well before the appearance of synapses and the establishment of functional circuits.

Cytotoxicity Models in Chromaffin Cells to Evaluate Neuroprotective Compounds

Primary cultures of bovine chromaffin cells are considered a good model to evaluate potential neuroprotective compounds for two major reasons: (i) they share many common features to neurons as they synthesize, store, and release neurotransmitters; they are excitable cells that express voltage-dependent calcium, potassium, and sodium channels; they express different neuronal receptor subtypes; and (ii) they can be easily cultured in high quantities from adult animals; as adult para-neurons, they can be used to reproduce different neurodegenerative-like cytotoxicity models. In this chapter, we describe protocols to mimic calcium overload (veratridine and thapsigargin) and oxidative stress (rotenone plus oligomycin-A and 6-hydroxydopamine) to evaluate potential neuroprotective compounds.

Improving in vitro ciguatoxin and brevetoxin detection: selecting neuroblastoma (Neuro-2a) cells with lower sensitivity to ouabain and veratridine (OV-LS)

Marine biotoxins accumulating in seafood products pose a risk to human health. These toxins are often potent in minute amounts and contained within complex matrices; requiring sensitive, reliable, and robust methods for their detection. The mouse neuroblastoma (Neuro-2a) cytotoxicity assay (N2a-assay) is a sensitive, high-throughput, in vitro method effective for detecting sodium channel-specific marine biotoxins. The N2a-assay can be conducted to distinguish between specific effects on voltage-gated sodium (Na_V) channels, caused by toxins that activate (e.g., ciguatoxins (CTXs), brevetoxins (PbTxs)) or block (e.g., tetrodotoxins, saxitoxins) the target Na_V. The sensitivity and specificity of the assay to compounds activating the Na_V are achieved through the addition of the pharmaceuticals ouabain (O) and veratridine (V). However, these compounds can be toxic to Neuro-2a cells and their application at insufficient or excessive concentrations can reduce the effectiveness of this assay for marine toxin detection. Therefore, during growth incubation, Neuro-2a cells were exposed to O and V, and surviving cells exhibiting a lower sensitivity to O and V (OV-LS) were propagated. OV-LS Neuro-2a cells were selected for 60-80% survival when exposed to 0.22/0.022 mM O/V during the cytotoxicity assay. At these conditions, OV-LS N2a cells demonstrated a 3.5-fold higher survival rate 71% ± 7.9 SD (n = 232), and lower sensitivity to O/V, compared to the original Neuro-2a cells 20% ± 9.0 SD (n = 16). Additionally, OV-LS N2a cells were 1.3-2.6-fold more sensitive for detecting CTX3C 1.35 pg/ml, CTX1B 2.06 pg/ml, and PbTx-3 3.04 ng/ml compared to Neuro-2a cells using 0.1/0.01 mM O/V. Detection of CTX3C in a complex fish matrix using OV-LS cells was 0.0048 pg CTX3C/mg fish tissue equivalent. This work shows the potential for a significant improvement in sensitivity for CTX3C, CTX1B, and PbTx-3 using the OV-LS N2a-assay.

X-ray scattering study of pike olfactory nerve: elastic, thermodynamic and physiological properties of the axonal membrane

The effects of several agents, sugars, isotonic KCl, and a variety of drugs, on the structure of the axonal membranes of unmyelinated pike olfactory nerve have been studied by synchrotron radiation X-ray scattering experiments. The main effects of the sugars are: (i) to increase the electron density of the extra-axonal space and thereby yield the absolute scale of the electron density profile; (ii) to osmotically stress the membrane and thus yield its elastic modulus of area compressibility, since the related strain, thickness dilation, is directly determined by the X-ray scattering experiments. Exposure to isotonic KCl, a depolarizing agent, induces membrane thickness to increase. The energy liberated in this process is a function of the amplitude of the dilation and of the elastic modulus of the membrane. This energy turns out to be close to the thermal energy liberated by the pike olfactory nerve during the initial phase of action potential that has previously been measured by others. Electrical depolarization thus seems to be accompanied by a thickness dilation of the axonal membrane. Another effect of isotonic KCl is to induce a large fraction of the membranes to pair by tight apposition of their extra-axonal faces. Local anaesthetics and some drugs have the effect of altering membrane thickness. All these observations are interpreted in terms of a modulation of the conformational disorder of the hydrocarbon chains of the lipid molecules.

Quantitation of paralytic shellfish toxins using mouse brain synaptoneurosomes

A membrane potential assay based on synaptoneurosomes prepared from mouse brain was evaluated further for its utility in estimating saxitoxin and related bioactives. Saxitoxin concentrations quantitated in mussel extracts by the synaptoneurosomal technique correlated well with spiked concentrations in these samples (r2 = 0.995; slope=1.048). Other experiments found that the synaptoneurosomal assay can detect saxitoxin-like bioactives in zooplankton samples and the concentrations measured were consistent with preliminary estimations of saxitoxin equivalents using the [3H] saxitoxin receptor binding technique. Veratrine, a mixture of alkaloids that activate sodium channels, had similar potential as a substitute for veratridine in the synaptoneurosomal assay. The results provide additional evidence that the mouse brain synaptoneurosomal membrane potential assay has excellent capability for quantitation of saxitoxin-like activity in shellfish tissues and may also be applied to zooplankton samples.

Construction of 1 mm microdialysis probe for amino acids dialysis in rats

We describe here a microdialysis probe with 1 mm opening for precise and confined dialysis area in the awake, freely moving rat. This probe is designed to allow the local diffusion of the perfusion medium to an area approximately 175 microm high, 266 microm wide (mediolateral direction), and 305 microm in rostrocaudal direction. In addition, the probe allows the local application of drugs to the same precise area of interest. The probe was constructed from a piece of 25 gauge tubing with 1 mm hallowed opening located 0.5 mm from the distal (inserting) end. The dialysis fiber which was inserted into the stainless steel 25 gauge tubing and cemented into place has 200 microm diameter and 5000 molecular weight cut off. We tested the probe diffusion extent by direct infusion of fluorogold through the dialysis cannula. Changes in the extracellular concentrations of amino acids were measured in response to infusion of veratridine a sodium channel activator. All amino acids tested showed a significant 80% times decrease in their recovery concentration when compared to their respective concentrations recovered through 2 mm probe constructed earlier in our laboratory (Renno et al., 1992). Tests in awake rats with probes in the ventrocaudal PAG showed stable amounts of 12 different amino acids during repeated (6-8 times) 12 min samples at 3-5 microl/min collecting rate. Depolarization with 75 microM veratridine resulted in significant elevation in extracellular gamma-aminobutyric acid (GABA), aspartate, glutamate, taurine, glycine and citrulline. This design enables us to apply drugs of interest and measure the concentrations of amino acid neurotransmitters to a more precise, delineated and premeasured areas in the CNS.

In vitro induction of microcyst-like structures in the superior olivary complex

To investigate the etiology of hole formation in the gerbil and rat central auditory system, organotypic cultures were grown in control and veratridine-containing media. The latter condition is known to increase neuronal activity. Tissue was obtained at postnatal day 6 and grown for 6-9 days in vitro, a period prior to the formation of holes in vivo. In both rats and gerbils, veratridine led to the appearance of large numbers of holes, and these were phenotypically similar to those found in vivo. These results support the idea that hole formation is an activity-dependent phenomenon, and suggest that it is not restricted to the mature gerbil auditory system.

Epileptiform activity increases the level of nerve growth factor in cerebrospinal fluid of epileptic patients and in hippocampal neurons in tissue culture

A neurite-stimulating effect was induced by both the cerebrospinal fluid of epileptic patients and the media of co-cultures of rat hippocampus and chick embryo sensory neurons after veratridine treatment. Cerebrospinal fluid from patients with epilepsy stimulated extensive neurite growth in the organotypic culture of chick embryo dorsal root ganglia. The anti-nerve growth factor antibody partly blocked the neurite-stimulating effect of the cerebrospinal fluid. Co-cultures of newborn rat hippocampus and chick embryo dorsal root ganglia were used to investigate the involvement of neurotrophic factors into the processes which are activated by neuronal activity. The data obtained suggest that veratridine, an epileptiform agent, gave rise to an elevation in the level of neurotrophic factors in the culture media and neurite outgrowth of dorsal root ganglia sensory neurons. The anti-nerve growth factor antibody was shown to block the neurite-stimulating effect mediated by veratridine. These results indicate that the epileptiform activity of neurons evokes the expression of neurotrophins.

Evidence of glutamatergic deficiency in schizophrenia

Studies of amino acid release were carried out using frozen sections from brains of schizophrenics and controls. Synaptosomes were prepared via differential centrifugation in Ficoll allowing the veratridine-induced release of aspartate, glutamate, glycine, and GABA to be measured. The release of glutamate and gamma-aminobutyric acid (GABA) was reduced in the synaptosomes from schizophrenics. This decrease could be reversed partially by pre-incubation of the synaptosomes with haloperidol. Additionally, the activity of glutamate decarboxylase was decreased and partially restored by haloperidol pre-incubation. These data are consistent with the hypothesis of a glutamatergic/GABAergic deficit in schizophrenia.

Activation of the action potential Na+ ionophore of cultured neuroblastoma cells by veratridine and batrachotoxin

The activation of the action potential Na+ ionophore by veratridine and batrachotoxin is time- and concentration-dependent and completely reversible. Batrachotoxin acts more slowly than veratridine. The concentration dependence of activation at equilibrium suggests reversible interaction of each toxin with a single class of independent sites having dissociation constants at physiologic ion concentrations of 80 plus or minus 13 muM for veratridine and 0.4 plus or minus muM for batrachotoxin. The maximum velocity of Na+ uptake at 50 mM Na+ is 128 plus or minus 12 nmol/min/mg in the presence of batrachotoxin compared to 48 plus or minus 4 nmol/min/mg in the presence of veratridine. Treatment of cells with excess veratridine in addition to batrachotoxin inhibits batrachotoxin-dependent 22-Na+ uptake. The concentration dependence of this inhibition suggests that it reflects competitive displacement of batrachotoxin from its binding site by veratridine. The activation by veratridine and batrachotoxin is inhibited in a competitive manner by divalent cations. The inhibition by divalent cations exhibits significant ion specificity with Mn-2+ greater than Co-2+ greater than Ni-2+ greater than Ca-2+ greater than Mg-2+ greater than Sr-2+. The inhibition constants (KI) for Ca-2+ are 0.84 mM for veratridine-dependent 22-Na+ uptake and 1.2 mM for batrachotoxin-dependent 22-Na+ uptake. The activation by veratridine and batrachotoxin is inhibited in a noncompetitive manner by tetrodotoxin. The apparent KD for tetrodotoxin as 11 plus or minus 1 nM in the presence of 150 mM Na+ and approximately 8.5 nM in 50 mM Na+. Divalent cations do not affect the apparent KD for tetrodotoxin. A hypothesis is presented which suggests that batrachotoxin, veratridine, and divalent cations interact with an activation site associated with the action potential Na+ ionophore, whereas tetrodotoxin interacts with a physically and functionally independent site involved in the transport of monovalent cations by the ionophore.

Effects of potassium, veratridine, and scorpion venom on calcium accumulation and transmitter release by nerve terminals in vitro

1. 45-Ca uptake by pinched-off nerve terminals (synaptosomes) of rat brain incubated in standard physiological saline (including 132 mM-Na + 5mM-K + 1-2 mM-Ca) at 30 degrees C averages about 0-5 mumole Ca per g protein per minute. This may be equivalent to a Ca influx of about 0-03 p-mole/cm-2 sec. 2. The rate of 45-Ca uptake is increased when the concentration of K in the medium is increased above 15-20 mM, K replacing Na isosmotically. Maximum stimulation, a three- to six-fold increase in the rate of Ca uptake, occurs when [K]o is about 60 mM. The effect of increased [K]o is reversible. 3. The K-stimulated Ca uptake is associated primarily with the nerve terminal fraction of brain homogenates. The entering Ca is not accompanied by extracellular markers such as mannitol or inulin. Replacement of external chloride by methylsulphate or sulphate does not prevent the stimulation by K. 4. The effects of external K are quantitatively mimicked by Rb. Caesium also stimulates Ca uptake, but is only about one fifth as effective as K or Rb; Li is ineffective. 5. Two other depolarizing agents also stimulate Ca uptake by synaptosomes: veratridine (7-5 times 10- minus 6 to 7-5 times 10- minus 5 M) and scorpion (Leirus quinquestriatus) venom (6-7 times 10- minus 7 to 6-7 times 10- minus g/ml.). The stimulatory effects of veratridine and scorpion venom, but not of increased [K] are blocked by 2 times 10- minus 7 M tetrodotoxin. 6. Internal K also influences the rate of 45-Ca uptake by synaptosomes: lowering [K]i reduces the stimulatory effect of external K and veratridine. 7. Replacement of external Na by choline markedly inhibits the response to veratridine, but has a much smaller effect on the response to increased [K]o. 8. The Ca uptake mechanism has an apparent dissociation constant for Ca (KCa) of about 0-8 mM. Increasing [K]o increases the maximal rate of Ca uptake, but has no effect on KCa. The K-induced 45-Ca uptake is competitively inhibited by Mg-2+, Mn-2+ and La-3+. 9. The release of acetylcholine and noradrenaline was also studied. Increasing [K]o stimulates external Ca-dependent acetylcholine release. Scorpion venom stimulates noradrenaline release from synaptosomes; this effect could be prevented by adding tetrodotoxin or removing external Ca. 10. These results indicate that synaptosomes may increase their permeability to Ca, accumulate Ca and release neural transmitter substances, when stimulated by depolarizing agents under appropriate physiological conditions.

RENAL EFFECTS OF VERATRIDINE

Veratridine hydrochloride injected subcutaneously into unanaesthetized rats inhibited water diuresis. A linear relationship between log dose and antidiuretic effect could be established over the dose range 50 to 200 mug./100 g. of body weight. When veratridine hydrochloride was injected intravenously in doses from 10 to 30 mug./100 g., this relationship was also linear. In terms of its antidiuretic action, the alkaloid was approximately five times as effective when given intravenously. Rats anaesthetized with urethane responded to an intravenous injection with a more pronounced inhibition than unanaesthetized animals. Protoveratrine injected intravenously into unanaesthetized rats showed no clear relationship between dose and magnitude of antidiuretic effect. Veratridine hydrochloride injected intravenously had a pronounced hypotensive effect in both anaesthetized and unanaesthetized rats. Treatment with atropine did not affect this hypotensive action significantly. Atropine given subcutaneously 30 min. before an intravenous injection of veratridine hydrochloride abolished or diminished the inhibitory effect of veratridine on water diuresis. Veratridine hydrochloride injected intravenously into unanaesthetized rats caused a marked depression of the clearance of inulin and p-aminohippurate. In unanaesthetized rats with an osmotic diuresis, veratridine hydrochloride produced its usual antidiuretic effect. The urine of rats injected with veratridine hydrochloride produced an antidiuretic effect when injected intravenously into other animals. The antidiuretic potency of such urines was not affected by treatment with thioglycollate. Animals injected with veratridine excreted small amounts of a veratridine-like substance in the urine. These results do not suggest that veratridine in antidiuretic and hypotensive doses stimulated the neurohypophysis in the rat.

Clinical Studies on Veratrum Alkaloids

The action of two pure veratrum alkaloids, veratridine and protoveratrine, in human hypertension is described for the first time. The vasodepressor reflex pathway involved in the response to these ester alkaloids is reviewed. Protoveratrine is found to produce a striking fall in blood pressure in both essential and renal hypertension after intravenous administration.