L-type calcium channels may regulate neurite initiation in cultured chick embryo brain neurons and N1E-115 neuroblastoma cells

Synergistic Cellular Responses Conferred by Concurrent Optical and Magnetic Stimulation Are Attenuated by Simultaneous Exposure to Streptomycin: An Antibiotic Dilemma

Concurrent optical and magnetic stimulation (COMS) combines extremely low-frequency electromagnetic and light exposure for enhanced wound healing. We investigated the potential mechanistic synergism between the magnetic and light components of COMS by comparing their individual and combined cellular responses. Lone magnetic field exposure produced greater enhancements in cell proliferation than light alone, yet the combined effects of magnetic fields and light were supra-additive of the individual responses. Reactive oxygen species were incrementally reduced by exposure to light, magnetics fields, and their combination, wherein statistical significance was only achieved by the combined COMS modality. By contrast, ATP production was most greatly enhanced by magnetic exposure in combination with light, indicating that mitochondrial respiratory efficiency was improved by the combination of magnetic fields plus light. Protein expression pertaining to cell proliferation was preferentially enhanced by the COMS modality, as were the protein levels of the TRPC1 cation channel that had been previously implicated as part of a calcium-mitochondrial signaling axis invoked by electromagnetic exposure and necessary for proliferation. These results indicate that light facilitates functional synergism with magnetic fields that ultimately impinge on mitochondria-dependent developmental responses. Aminoglycoside antibiotics (AGAs) have been previously shown to inhibit TRPC1-mediated magnetotransduction, whereas their influence over photomodulation has not been explored. Streptomycin applied during exposure to light, magnetic fields, or COMS reduced their respective proliferation enhancements, whereas streptomycin added after the exposure did not. Magnetic field exposure and the COMS modality were capable of partially overcoming the antagonism of proliferation produced by streptomycin treatment, whereas light alone was not. The antagonism of photon-electromagnetic effects by streptomycin implicates TRPC1-mediated calcium entry in both magnetotransduction and photomodulation. Avoiding the prophylactic use of AGAs during COMS therapy will be crucial for maintaining clinical efficacy and is a common concern in most other electromagnetic regenerative paradigms.

CaBP1 regulates Cav1 L-type Ca2+ channels and their coupling to neurite growth and gene transcription in mouse spiral ganglion neurons

CaBP1 is a Ca²⁺ binding protein that is widely expressed in neurons in the brain, retina, and cochlea. In heterologous expression systems, CaBP1 interacts with and regulates voltage-gated Ca_v Ca²⁺ channels but whether this is the case in neurons is unknown. Here, we investigated the cellular functions of CaBP1 in cochlear spiral ganglion neurons (SGNs), which express high levels of CaBP1. Consistent with the role of CaBP1 as a suppressor of Ca²⁺-dependent inactivation (CDI) of Ca_v1 (L-type) channels, Ca_v1 currents underwent greater CDI in SGNs from mice lacking CaBP1 (C-KO) than in wild-type (WT) SGNs. The coupling of Ca_v1 channels to downstream signaling pathways was also disrupted in C-KO SGNs. Activity-dependent repression of neurite growth was significantly blunted and unresponsive to Ca_v1 antagonists in C-KO SGNs in contrast to WT SGNs. Moreover, Ca_v1-mediated Ca²⁺ signals and phosphorylation of cAMP-response element binding protein were reduced in C-KO SGNs compared to WT SGNs. Our findings establish a role for CaBP1 as an essential regulator of Ca_v1 channels in SGNs and their coupling to downstream pathways controlling activity-dependent transcription and neurite growth.

L-type Ca(2+) channel blocker inhibits mossy fiber sprouting and cognitive deficits following pilocarpine seizures in immature mice

Behavioral and cognitive deficits are one of the most frequent sequelae of childhood epilepsy. Accumulating evidence indicates that epilepsy induces aberrant development of the mossy fibers in the hippocampus, the region that is commonly accepted to play a key role in learning and memory. We have therefore proposed that such abnormal maturation of the central nervous system may cause the adverse prognoses following epilepsy. Based on this hypothesis, using primary cultures of the dentate granule cells, we showed that the L-type Ca(2+) channel blocker nicardipine was neuroprotective against excessive mossy fiber synaptogenesis induced by prolonged depolarization that was assumed to mimic epileptiform conditions. Therefore, we evaluated the in vivo effect of nicardipine on aversive sequelae following epileptiform seizures. We found aberrant sprouting of the mossy fibers and poor performance of spatial and contextual tasks in the mice that had received treatment with pilocarpine at their early postnatal age. Repetitive administration of nicardipine prevented the mossy fiber sprouting and ameliorated the cognitive deterioration, although it did not show anticonvulsant actions against pilocarpine seizures. In the present study, we proposed two in vitro and in vivo models for evaluating epilepsy sequelae and noticed that L-type Ca(2+) channel blocker nicardipine was effective in both models. L-type Ca(2+) channel blocker may be a good candidate for a preventive for childhood epilepsy sequelae. Likewise, these useful systems will disclose additional candidates in future.

Developmental effects of chronic low-level lead exposure on voltage-gated calcium channels in brain synaptosomes obtained from the neonatal and the adult rats

Effects of chronic low level (1 mg/kg/day) lead exposure were studied on (1) the density and the binding properties of L, N, and P type voltage-gated Ca2+ influx channels (VGCCs), and (2) the depolarization-induced rise in [Ca2+]i in synaptosomes obtained from the brains of the neonatal (postnatal-day-5) and the adult (postnatal-week-20) rats. Lead exposure started prenatally and continued for either up to postnatal-day-5 or up to postnatal-week-20. The KD and the Bmax values for the binding of nifedipine (antagonist of L type channels), omega-CgTx (a specific antagonist of N type channels) and omega-AgaTx (antagonist of P type channels) to VGCCs in the neonatal samples were less then those in the adult samples. Depolarization increased (1) the density and the antagonist binding-affinity of VGCCs and (2) increased [Ca2+]i in both the neonatal and the adult samples. The depolarization-induced increase in [Ca2+]i in the neonatal samples was lower than that in the adult samples. Chronic low-level lead exposure decreased the densities of L, N, and P type VGCCs and attenuated the depolarization-induced increase in [Ca2+]i in synaptosomes. Chronic low-level lead exposure, however, did not affect the relative ratio of L, N, and P channels, the affinity of VGCCs for antagonists, and the depolarization-induced increase in antagonist binding to VGCCs in synaptosomes. Thus chronic low-level lead exposure during early development and adulthood may decrease the synthesis of VGCCs but not their antagonist binding-affinity in both the neonatal and the adult rats.

Calcium currents in dissociated cochlear neurons from the chick embryo and their modification by neurotrophin-3

Calcium entry through voltage-dependent channels play a critical role in neuronal development. Using patch-clamp techniques we have identified the components of the macroscopic Ca2+ current in acutely-isolated chick cochlear ganglion neurons and analysed their functional expression throughout embryonic development. With Ba2+ as a charge carrier, the currents exhibited two main components, both with a high activation threshold but differing in their inactivation kinetics. One component showed inactivation with a time constant around 100 ms (transient) whereas the other hardly inactivated (sustained). The currents were sensitive to omega-Conotoxin GVIA and dihydropyridines, blocked by 20 microM Cd2+, but unaffected by omega-Agatoxin IVA. In a few cases, only with Ca2+ as a charge carrier, an additional component with low activation threshold and fast inactivation (time constant of 20 ms), was observed. Currents were first detected at day 7 of embryonic development. Current density (amplitude/cell capacitance) increased through embryonic day 9, when early synaptic contacts are established, and decreased thereafter to lower steady values. The effect of neurotrophin-3, a neurotrophic factor required for survival and differentiation of cochlear ganglion neurons, was also examined. Neurons isolated at embryonic day 7 or day 11 and maintained two days in culture with 2 ng/ml neurotrophin-3 showed a substantial increase in Ca2+ current density, particularly in the transient component. These findings indicate that the expression of neuronal Ca2+ channels is predominant at the time of synapse formation between transducing hair cells and their primary afferents. Besides its effects on survival and neuritogenesis, neurotrophin-3 enhances the expression of Ca2+ channels in cultured neurons. Taken together these results suggest that the functional expression of Ca2+ channels is regulated during embryonic development of cochlear neurons by the release of neurotrophin-3 from the differentiating sensory epithelium of the cochlea.

Blocking Ca2+ mobilization with thapsigargin reduces neurite initiation in cultured adult rat DRG neurons

Adult rat DRG neurons rapidly extend extensive neuritic arbors after a 1-2-day delay in culture and generate large depolarization-induced calcium signals during this time period that are derived primarily from intracellular calcium release. To assess whether intracellular calcium mobilization is required for neurite initiation, calcium stores were depleted by brief exposure to the irreversible endoplasmic reticulum calcium ATPase inhibitor thapsigargin; cultures were then maintained for 3 days, immunostained for neurofilament and scored for percentage of neurons with neurites at least twice as long as the cell body. Brief thapsigargin treatment (20 min) during the first 24 h in culture resulted in a substantial decrease in neurite initiation frequency without affecting neuronal or nonneuronal cell survival, suggesting that intracellular calcium mobilization is necessary for triggering neurite initiation in these neurons.

The effects of triethyl lead on the development of hippocampal neurons in culture

Triethyl lead is the major metabolite of tetraethyl lead, which is used in industrial processes and as an antiknock additive to gasoline. We tested the hypothesis that low levels of triethyl lead (0.1 nmol/L to 5 mumol/L) interfere with the normal development of cultured E18 rat hippocampal neurons, possibly through increases in intracellular free calcium ion concentration, [Ca2+]in. The study assessed survival and differentiation using morphometric analysis of individual neurons. We also looked at short-term (up to 3.75-h) changes in intracellular calcium using the calcium-sensitive dye fura-2. Survival of neurons was significantly reduced at 5 mumol/L, and overall production of neurites was reduced at > or = 2 mumol/L. The length of axons and the number of axons and dendrites were reduced at > or = 1 mumol/L. Neurite branching was inhibited at 10 nmol/L for dendrites and 100 nmol/L for axons. Increases in intracellular calcium were observed during a 3.75-h exposure of newly plated neurons to 5 mumol/L triethyl lead. These increases were prevented by BAPTA-AM; which clamps [Ca2+]in at about 100 nmol/L. Culturing neurons with BAPTA-AM and 5 mumol/L triethyl lead did not reverse the effects of triethyl lead, suggesting that elevation of [Ca2+]in is not responsible for decreases in survival and neurite production. Triethyl lead has been shown to disrupt cytoskeletal elements, particularly neurofilaments, at very low levels, suggesting a possible mechanism for its inhibition of neurite branching at nanomolar concentrations.

Resting membrane potentials and excitability at different regions of rat dorsal root ganglion neurons in culture

To study the role of electrical membrane processes in neuronal regeneration and growth, resting membrane potentials and action potentials of sensory (dorsal root ganglion) neurons growing in culture were measured at the soma, neurite and growth cone using the whole-cell patch-clamp technique. Our results show that resting membrane potentials measured at the soma (-56.8 +/- 8.8 mV), neurite varicosity (-55.8 +/- 5.2 mV) and growth cone (-57.2 +/- 4.1 mV) of growing neurons were not statistically different. The membrane resistance measured around the resting membrane potential at the neurite varicosity (160 +/- 70 M omega) was smaller than those at the soma (687 +/- 540 M omega) and growth cone (922 +/- 825 M omega). The resting membrane potential measured at the soma using a perforated patch (-60.3 +/- 4.4 mV) was not different from that measured in the normal whole cell. In both configurations, isotonic KCl (140 mM) depolarized the membrane potential to above 0 mV. The K+ channel blockers quinine, Cs+, 4-aminopyridine and tetraethylammonium depolarized the membrane potential by 10-40 mV, while Na(+)-free extracellular solution hyperpolarized it by about 10 mV. Extracellularly applied ouabain, intracellular Na(+)-free or low Cl(-)-containing solutions did not affect the resting membrane potential. Similar results were obtained for growth cones. Action potentials could be evoked by current pulses in 81% of somata and in all growth cones, but not in neurite varicosities. Current-induced repetitive firing was found in 19% of somata and in 65% of growth cones.(ABSTRACT TRUNCATED AT 250 WORDS)

Localized membrane depolarizations and localized calcium influx during electric field-guided neurite growth

Our study explores the mechanisms behind neurite galvanotropism. Using phase, differential interference contrast and ratiometric fluorescence microscopy, we reveal four responses of N1E-115 mouse neuroblastoma cells to 0.1-1.0 mV/microns uniform DC electric fields: cathode-directed neurite initiation and elongation, cathode-biased growth cone filopodial protrusions, transient cathode-localized calcium increases, and persistent cathode-localized membrane depolarizations. These newly demonstrated events are temporally and spatially correlated, suggesting that they are causally related. The calcium increases are prevented by calcium channel blockers and by the removal of extracellular calcium. We therefore propose that the observed field-induced membrane depolarizations activate voltage-dependent calcium channels, resulting in cathode-localized calcium influx. This, in turn, may initiate the observed cathode-biased growth cone filopodial protrusions, followed by the cathode-directed neurite elongation.

Voltage-clamp study of calcium currents during differentiation in the NCB-20 neuronal cell line

1. Calcium currents (ICa) were studied in voltage-clamped NCB-20 cells. In undifferentiated cells, voltage steps from hyperpolarized potentials (-80/-100 mV) essentially revealed transient ICa showing characteristics classically described for "T-type" channels. In about 50% of the cells, there was a residual current at the end of the step; no ICa was elicited from a holding potential of -50 mV. 2. In contrast, 100% of the cells differentiated with dibutyryl cyclic AMP (cAMP) displayed a residual current in addition to the transient one, and depolarizing steps from a holding potential of -50 mV induced a sustained current. In these cells, Bay K 8644 elicited both a negative shift in voltage dependence and a moderate increase of the sustained component. 3. Although these changes in Ca2+ channel physiology result from chemically induced differentiation, they might not be directly related to the concomitant morphologic differentiation. 4. In undifferentiated NCB-20 cells, T-type Ca2+ currents can be elicited in relative isolation.

Voltage-dependent L-type calcium channels in the development and plasticity of mouse barrel cortex

Entry of calcium ions into the neuron is a triggering signal for initiation of several processes which may lead to modification of synaptic connectivity. The developmental changes of voltage-dependent L-type calcium channel (VDLCC) were studied using [3H]PN 200 110 nifedipine displaceable binding in the barrel cortex of mice, a model structure for studying cortical plasticity. In vitro binding autoradiography was used to examine quantitatively the pattern of [3H]PN 200 110 binding to brains of animals aged from 3 to 70 days. The binding values in the somatosensory cortex rose two-fold in the period examined, reaching a plateau in the 4th postnatal week. The laminar pattern of binding changed during development, with the locus of heaviest labeling shifting from layer IV to II/III in the third postnatal week and thin bands of labeling developing in layers IV and VI. A very faint barrel-like pattern of labeling in the barrel field was observed. Neither this pattern nor the binding values were altered by unilateral neonatal removal of all vibrissal follicles. Saturation studies of binding to crude synaptosomal fractions of cerebral cortex of mice aged 3, 15, 28 and 70 days revealed the presence of a single binding site, with Bmax increasing from 48.7 +/- 5.1 fmol/mg protein at postnatal day 3 to 191.7 +/- 9.6 fmol/mg protein at day 70. No developmental changes in KD values were found. No correlation was found between the critical period for cytoarchitectonic plasticity of the barrels and the time when high values of VDLCC binding were observed.

N-methyl-D-aspartate effects on the growth, morphology and cytoskeleton of individual neurons in vitro

Short-term (up to 5 h post-plating) cerebellar granule cell cultures were prepared from the week-old rat and maintained in a micro-incubator during time lapse video microscopy to examine normal and N-methyl-D-aspartate (NMDA)-evoked neurite extension. In untreated cultures growth of neurites was stochastic but proceeded at an average rate of 12.0 +/- 1.4 microns/h. Growth cone morphology was variable. The classical filopodia and lamellipodia possessing tips were motile or non-motile, while those processes ending in a club shape were rarely seen to extend. Individual growth cones passed through several different morphologies during growth. New processes extended more rapidly (13.0 +/- 1.7 microns/h) than those already present (9.0 +/- 0.5 microns/h). Addition of the NMDA receptor antagonist, aminophosphonovalerate (APV), caused a marked retraction of pre-existing processes. Stimulation of the receptor with 50 microM NMDA caused a marked increase in growth rate compared to controls (15.0 microns/h and 1.7 microns/h, respectively). When the presence of actin-rich structures was examined using rhodamine-phalloidin labelling it was found that NMDA increased the proportion of neuronal processes that possessed a growth cone by 28%. Conversely, inhibition of NMDA receptor activity with APV reduced the formation of lamellipodia from neuronal cell bodies.