Calcium hotspots caused by L-channel clustering promote morphological changes in neuronal growth cones

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

Spatial organization of calcium dynamics in growth cones of sensory neurones

The concentration of calcium ions in the cytosol ([Ca2+]i) has a dominant influence on neuronal development. A [Ca2+]i rise can, depending on the amplitude and location, promote outgrowth or dramatically inhibit it. We have used the fluorescent calcium indicators Fura-2 and Fura-2 dextran to measure [Ca2+]i dynamics in sensory neurones from the adult rat. [Ca2+]i was low and uniform in advancing growth cones, even during specific behaviours such as protrusion, filling and consolidation. A brief train of action potentials caused [Ca2+]i to rise at the extreme leading edge of the growth cone. [Ca2+]i changes in more proximal regions of the growth cone were much smaller. This spatially organized [Ca2+]i change, which may result from a concentration of calcium channels at the growth cone leading edge, is likely to function in spontaneously active regenerating axons in vivo to specifically activate calcium-dependent processes at the growth cone tip.

Calcium channels in neuroblastoma cell growth cones

The concentration of free calcium ions in the cytosol has been shown to influence many components of growth cone behaviour, including the extension of filopodia and veils, the addition of new membrane to the plasmalemma, the retraction and disappearance of filopodia, and gross collapse and retraction of the growth cone. A spatially localized modulation of these processes by very local calcium changes has been proposed to underlie the steering of growth cones by gradients of neurotransmitters, voltage and cell adhesion molecules. Such local control can be studied in mouse neuroblastoma cells, where depolarization causes calcium to rise in a limited number of spatially restricted hotspots, triggering a localized advance. We have studied the simple, club-shaped growth cones that are characteristically found on advancing neurites. Depolarization caused calcium to increase most at the distal, leading tip. Agents that disrupt calcium-induced calcium release do not affect growth cone calcium dynamics, ruling out a local release of calcium at the tip as a cause of the gradient. Using cell-attached patch recording, we find that L-type calcium channels are present at a higher density at the distal tip than in the proximal growth cone. Our results show that the calcium gradients seen in depolarized growth cones are a direct consequence of a gradient of calcium channel density.

Cytosolic Ca2+ domain-dependent protective action of adenosine in cardiomyocytes

Recently, in beating cardiac cells heterogeneous spatiotemporal patterns in cytosolic Ca2+ distribution have been visualized, and associated with cell contraction. In non-beating cardiomyocytes, spatial heterogeneity of intracellular Ca2+ distribution has also been observed, yet its functional implication in resting cardiac cells is not known. Herein, distinct domains of lower versus higher concentrations of cytosolic Ca2+ (0.17 and 0.37 microM, respectively) were observed using epifluorescent digital imaging in single, non-beating, fluo-3-loaded cardiomyocytes. Extracellular K+ (16 mM) induced a uniform increase of cytosolic Ca2+, despite the initial presence of distinct domains of cytosolic Ca2+ (from 0.17 to 1.82 microM in domains with lower, and from 0.37 to 2.03 microM in domains with higher Ca2+ concentration, respectively). In contrast, adenosine (1 mM) prevented exracellular K+ to induce cytosolic Ca2+ loading selectively within domains with lower (from 0.17 to 0.18 microM), but not in domains with higher (from 0.37 to 1.4 microM) basal Ca2+ concentration. Thus, the response of a cardiomyocyte to the protective action of adenosine is heterogeneous within a resting single cell. The domain-distinct cytoprotective action of adenosine appears to be set by the basal Ca2+ concentration within a cytosolic domain.

Block of P-type Ca2+ channels by the NMDA receptor antagonist eliprodil in acutely dissociated rat Purkinje cells

The effect of eliprodil on P-type Ca2+ channels was investigated in acutely dissociated rat Purkinje neurons, by using the whole-cell patch-clamp technique. Eliprodil inhibited in a reversible manner the omega-agatoxin-IVA-sensitive Ba2+ current elicited by step depolarizations from a -80 mV holding voltage (IC50 = 1.9 microM). The Ba2+ current showed steady-state inactivation (V1/2 = -61 mV) which was shifted toward more positive values when the intracellular Ca2+ buffering was increased. In these conditions, the potency of eliprodil was decreased (IC50 = 8.2 microM), suggesting a modulation by intracellular Ca2+ of the eliprodil blockade. The potency of eliprodil was not modified at more depolarized holding potentials and was not dependent on the frequency at which the step-depolarizations were applied (0-0.2 Hz) indicating a lack of voltage and use dependence of the eliprodil blockade. When eliprodil was applied in the patch-pipette at a concentration which causes full block when applied externally, the Ba2+ current amplitude was not affected and external application of eliprodil was still efficacious, indicating an extracellular location of the binding site. Analysis of the time course of recovery from Ca2+ channel blockade obtained by concomitant application of eliprodil with Cd2+, omega-agatoxin-IVA or fluspirilene, indicated that these later compounds did not interact with eliprodil, suggesting that eliprodil acts at a different site. These results demonstrate that eliprodil blocks P-type Ca2+ channels in cerebellar Purkinje neurons and suggest that this property may contribute to its neuroprotective activity.

Fluorescence imaging of local membrane electric fields during the excitation of single neurons in culture

The spatial distribution of depolarized patches of membrane during the excitation of single neurons in culture has been recorded with a high spatial resolution (1 micron2/pixel) imaging system based on a liquid-nitrogen-cooled astronomical camera mounted on an inverted microscope. Images were captured from rat nodose neurons stained with the voltage-sensitive dye RH237. Conventional intracellular microelectrode recordings were made in synchrony with the images. During an action potential the fluorescence changes occurred in localized, unevenly distributed membrane areas, which formed clusters of depolarized sites of different sizes and intensities. When fast conductances were blocked by the addition of tetrodotoxin, a reduction in the number and the intensities of the depolarized sites was observed. The blockade by tetrodotoxin of voltage-clamped neurons also reduced the number of depolarized sites, although the same depolarizing voltage step was applied. Similarly, when a voltage-clamped neuron was depolarized by a constant-amplitude voltage step, the number of depolarized sites varied according to the degree of activation of the voltage-sensitive channels, which was modified by changing the holding potential. These results suggest that the spatial patterns of depolarization observed during excitation are related to the operations of ionic channels in the membrane.

Backpropagation of action potentials generated at ectopic axonal loci: hypothesis that axon terminals integrate local environmental signals

This review deals with the fascinating complexity of presynaptic axon terminals that are characterized by a high degree of functional distinctiveness. In vertebrate and invertebrate neurons, all-or-none APs can take off not only from the axon hillock, but also from ectopic axonal loci including terminals. Invertebrate neurons display EAPs, for instance alternating with somatic APs, during survival functions. In vertebrate, EAPs have been recorded in the peripheral and central nervous systems in time relationship with physiological or pathological neuronal activities. In motor or sensory axon, EAP generation may be the cause of motor dysfunctioning or sensory perceptions and pain respectively. Locomotion is associated with rhythmic depolarizations of the presynaptic axonal membrane of primary afferents, which are ridden by robust EAP bursts. In central axons lying within an epileptic tissue EAP discharges, coinciding with paroxysmal ECoG waves, get longer as somatic discharges get shorter during seizure progression. Once invaded by an orthodromic burst, an ectopic axonal locus can display an EAP after discharge. Such loci can also fire during hyperpolarization or the postinhibitory excitatory period of the parent somata, but not during their tonic excitation. Neurons are thus endowed with electrophysiological intrinsic properties making possible the alternate discharges of somatic APs and EAPs. In invertebrate and vertebrate neurons, ectopic axonal loci fire while the parent somata stop firing, further suggesting that axon terminal networks are unique and individual functional entities. The functional importance of EAPs in the nervous systems is, however, not yet well understood. Ectopically generated axonal APs propagate backwards and forwards along the axon, thus acting as a retrograde and anterograde signal. In invertebrate neurons, somatically and ectopically generated APs cannot have the same effect on the postsynaptic membrane. As suggested by studies related to the dorsal root reflex, EAPs may not only be implied in the presynaptic modulation of transmitter release but also contribute significantly during their backpropagation to a powerful control (collision process) of incoming volleys. From experimental data related to epileptiform activities it is proposed that EAPs, once orthodromically conducted, might potentiate synapses, initiate, spread or maintain epileptic cellular processes. For instance, paroxysmal discharges of EAPs would exert, like a booster-driver, a powerful synchronizing synaptic drive upon a large number of excitatory and inhibitory postsynaptic neurons. We have proposed that, once backpropagated, EAPs are likewise capable of initiating (and anticipating) threshold and low-threshold somatodendritic depolarizations. Interestingly, an antidromic EAP can modulate the excitability of the parent soma.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of spontaneous calcium transients in nerve growth cones and their effect on growth cone migration

This study examines the mechanisms of spontaneous and induced [Ca2+]i spiking in nerve growth cones and the effect of spikes on growth cone migration. Over a 10-20 min observation period, 29% of DRG growth cones undergo spontaneous and transient elevations in physiological extracellular Ca2+ ((Ca2+)o; 2 mM), whereas 67% of growth cones exposed to 20 mM (Ca2+)o exhibit similar [Ca2+]i spikes. Spontaneous [Ca2+]i spiking was not observed in neuronal cell bodies or nonneuronal cells. Ca2+ influx through non-voltage-gated Ca2+ channels was required for spontaneous [Ca2+]i spikes in growth cones, since removal of (Ca2+)o, or addition of the general Ca2+ channel blockers La3+ or Ni2+, reversibly blocked [Ca2+]i spiking, while blockers of the voltage-gated Ca2+ channels did not. Experiments using agents that influence intracellular Ca2+ stores suggest that Ca2+ stores may buffer and release Ca2+ during growth cone [Ca2+]i spikes. Growth cone migration was immediately and transiently inhibited by [Ca2+]i spikes, but eventually returned to prespike rates.

Ca2+ waves in PC12 neurites: a bidirectional, receptor-oriented form of Ca2+ signaling

Spatial and temporal aspects of Ca2+ signaling were investigated in PC12 cells differentiated with nerve growth factor, the well known nerve cell model. Activation of receptors coupled to polyphosphoinositide hydrolysis gave rise in a high proportion of the cells to Ca2+ waves propagating non decrementally and at constant speed (2-4 microns/s at 18 degrees C and approximately 10-fold faster at 37 degrees C) along the neurites. These waves relied entirely on the release of Ca2+ from intracellular stores since they could be generated even when the cells were incubated in Ca(2+)-free medium. In contrast, when the cells were depolarized with high K+ in Ca(2+)-containing medium, increases of cytosolic Ca2+ occurred in the neurites but failed to evolve into waves. Depending on the receptor agonist employed (bradykinin and carbachol versus ATP) the orientation of the waves could be opposite, from the neurite tip to the cell body or vice versa, suggesting different and specific distribution of the responsible surface receptors. Cytosolic Ca2+ imaging results, together with studies of inositol 1,4,5-trisphosphate generation in intact cells and inositol 1,4,5-trisphosphate-induced Ca2+ release from microsomes, revealed the sustaining process of the waves to be discharge of Ca2+ from the inositol 1,4,5-trisphosphate- (and not the ryanodine-) sensitive stores distributed along the neurites. The activation of the cognate receptor appears to result from the coordinate action of the second messenger and Ca2+. Because of their properties and orientation, the waves could participate in the control of not only conventional cell activities, but also excitability and differential processing of inputs, and thus of electrochemical computation in nerve cells.

Nuclear calmodulin responds rapidly to calcium influx at the plasmalemma

We have studied the rate and extent of calcium binding to calmodulin in neuronal cytosol and nucleus during brief calcium influx across the plasmalemma. Rat sensory neurones were whole-cell patch clamped using a pipette containing a fluorescent analogue of calmodulin that reports when it has bound calcium. Cytosolic and nuclear signals were separated using a confocal microscope. Plasmalemmal calcium influx during a one second depolarization that activated L type calcium channels caused large fractions of calmodulin in both the cytosol and nucleus to bind calcium. Thus, contrary to previous predictions, nuclear processes that require the calcium:calmodulin complex will be activated readily by even brief cell stimulation.

Quinolinic acid-induced increases in calbindin D28k immunoreactivity in rat striatal neurons in vivo and in vitro mimic the pattern seen in Huntington's disease

In Huntington's disease striatal neurons undergo marked changes in dendritic morphology and coincidently exhibit an increase in immunoreactive calbindin D28k (calbindin), a cytosolic calcium-binding protein which is highly abundant in these neurons. Previous studies in the rat striatum have shown that excitotoxic injury, which is linked to a rise in intracellular Ca2+, mimics many of the neurochemical and neuropathological characteristics of Huntington's disease. We speculated, therefore, that the apparent increase in calbindin labeling in Huntington's disease spiny neurons may signal the response to an excitotoxic process. To investigate this possibility, we compared the cellular features of calbindin immunoreactivity in grade 1-4 Huntington's disease cases with those seen in rat striatal neurons in vivo and in vitro following treatment with N-methyl-D-aspartate (NMDA) receptor agonist, quinolinic acid. In human post mortem control cases calbindin immunoreactivity was seen primarily in the somata and proximal dendrites of striatal neurons. In the Huntington's disease cases, calbindin labeling was markedly increased throughout the second and third order dendrites and in spines, and this change was more prevalent in advanced cases (grades 3-4). In the rat brain, two weeks after intrastriatal injection of quinolinic acid (6-20 ng), surviving medium-spiny neurons in the transition zone around the lesion core exhibited a marked increase in calbindin immunoreactivity similar to that seen in Huntington's disease spiny neurons. In more peripheral areas away from the lesion and on the contralateral unlesioned side, calbindin immunostaining was confirmed to somata and proximal dendrites. In situ hybridization histochemistry with an 35S-labeled oligonucleotide probe showed no change or a decrease in calbindin mRNA levels in neurons within the transition zone, suggesting that the observed increase in calbindin staining was not the result of increased transcription. In 12 day old postnatal striatal cultures, 2-6 h exposures to quinolinic acid (0.5 mM) significantly increased the length of neurites exhibiting calbindin immunoreactivity when compared to untreated controls. This effect was blocked by the selective NMDA receptor blocker (+/-)-2-amino-5-phosphonopentanoic acid (AP-5), indicating that an NMDA receptor-mediated mechanism contributed to the change in staining pattern. Results in rats suggest that the subcellular redistribution of calbindin immunoreactivity observed in Huntington's disease spiny neurons may be related to an NMDA receptor-induced excitotoxic process. An increased availability of calbindin protein at dendrites and spines may reflect a greater demand for Ca2+ buffering precipitated by an abnormal rise in in intracellular Ca2+.

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.

Targeted disruption of Ca(2+)-calmodulin signaling in Drosophila growth cones leads to stalls in axon extension and errors in axon guidance

Ca(2+)-calmodulin (CaM) function was selectively disrupted in a specific subset of growth cones in transgenic Drosophila embryos in which a specific enhancer element drives the expression of the kinesin motor domain fused to a CaM antagonist peptide (kinesin-antagonist or KA, which blocks CaM binding to target proteins) or CaM itself (kinesin-CaM or KC, which acts as a Ca(2+)-binding protein). In both KA and KC mutant embryos, specific growth cones exhibit dosage-dependent stalls in axon extension and errors in axon guidance, including both defects in fasciculation and abnormal crossings of the midline. These results demonstrate an in vivo function for Ca(2+)-CaM signaling in growth cone extension and guidance and suggest that Ca(2+)-CaM may in part regulate specific growth cone decisions, including when to defasciculate and whether or not to cross the midline.

Physiological cytosolic Ca2+ transients evoke concurrent mitochondrial depolarizations

Calcium, a ubiquitous second messenger, stimulates the activity of several mitochondrial dehydrogenases. This has led to the suggestion that the same messenger that signals cell activation could also activate mitochondrial electron/proton transport, thereby meeting demands for increased cellular energy. To test this in live cells, quantitative three-dimensional microscopy and ratio imaging were used to measure membrane potential of individual mitochondria and cytosolic calcium distribution. Mitochondria reversibly depolarized as cytosolic calcium rose and then fell following physiological stimulation. Thus, the dominant response of the mitochondrion to a rise in cytosolic [Ca2+] is to draw on the electrochemical potential, possibly to accelerate processes directly involved in ATP synthesis and calcium homeostasis.

Distinct electric potentials in soma and neurite membranes

Structurally similar voltage-dependent ion channels may behave differently in different locations along the surface of a neuron. A possible reason could be that channels experience nonuniform electrical potentials along the plasmalemma. Here, we map the electrical potentials along the membrane of differentiated N1E-115 neuroblastoma cells with a potential-sensitive dye. We find that the intramembrane potential gradient is indeed more positive in the membranes of neurites than in the membranes of somata. This is not attributable to differences in ion conductances or surface charge densities between the membranes of neurites and somata; instead, it can be explained by differences in lipid composition. The spatial variation in intramembrane electrical potential may help account for electrophysiological and functional differences between neurites and somata.

The role of calcium and cAMP in the mechanism of action of two melanocortins: alpha MSH and the ACTH4-9 analogue Org 2766

Melanocortins accelerate functional recovery after nerve crush and enhance neurite outgrowth in vitro. To get more insight in the mechanism of action of melanocortins, we studied the effects of two neurotrophic peptides: alpha-melanocyte-stimulating hormone (alpha MSH) and an adrenocorticotropin4-9 analogue Org 2766 on second messengers in cultures of spinal cord (SC), dorsal root ganglion (DRG) and Schwann cells. alpha MSH (10 microM) enhanced the forskolin-induced cAMP production in SC- (45%) and in DRG-cells (35%). Org 2766 (1 microM) induced an increase in cAMP only in SC-cells (39%). The peptides did not affect the cAMP levels in Schwann cells. Neither peptide evoked significant changes in the intracellular free calcium concentration ([Ca2+]i) in batch-measurements of all cell types, however, Ca(2+)-imaging revealed an infrequent occurrence of large [Ca2+]i-elevations in individual SC-neurons. The results indicate that SC- and DRG-cells are targets for both peptides, while Schwann cells are not or exploit different pathways. We observed for alpha MSH that cAMP production always coincides with outgrowth stimulation, whereas for Org 2766 cAMP production and outgrowth stimulation appear not causally related. These differences in second messenger stimulation could be explained by receptor heterogeneity. We suggest that alpha MSH and Org 2766 act through different receptors, each with its own signalling pathways.

Multiple subtypes of voltage-gated calcium currents in the Edinger-Westphal nucleus

Avian Edinger-Westphal (EW) neurons provide a unique opportunity to compare electrophysiologically the membranes of cell bodies and terminals in the same population of neurons. Axons that originate from neurons in the lateral region of the EW nucleus form a morphologically distinct presynaptic terminal, known as a calyx, on ciliary ganglion neurons. Several studies have shown that calyciform terminals in the ciliary ganglion exhibit predominantly N-type, high-voltage-activated (HVA) calcium channels. The goal of this study was to characterize and compare the calcium currents expressed in EW cell somas with those reported in the terminals. Whole-cell patch-clamp techniques were used to record from cell bodies in the lateral EW nucleus in slice preparations. Slices were obtained from embryonic day 16 chicks, matching the age of the embryos in which calyces were studied. Recordings were localized to the lateral region of the EW nucleus using Lucifer yellow fills. Voltage-step commands from -70 to 0 mV produced calcium currents with both a sustained and an inactivating component. Depolarization steps to 0 mV from a holding potential of -40 mV eliminated the inactivating component. These recordings suggested the presence of both LVA and HVA calcium currents. Application of 0.1 mM NiCl2 produced a reversible decrease in the amplitude of the whole-cell calcium current, preferentially affecting the inactivating component. The Ni2+(-)sensitive current activated and inactivated rapidly in a voltage-dependent manner. Treatment with 0.1 mM cadmium chloride caused a reversible reduction in the amplitude of the calcium current.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatial changes in calcium signaling during the establishment of neuronal polarity and synaptogenesis

Calcium imaging techniques were used to obtain a clear although indirect evidence about the distribution of functional glutamate receptors of NMDA and non-NMDA type in cultured hippocampal neurons during establishment of polarity and synaptogenesis. Glutamate receptors were expressed and were already functional as early as one day after plating. At this stage NMDA and non-NMDA receptors were distributed in all plasmalemmal areas. During the establishment of neuronal polarity, responses to either types of glutamate receptors became restricted to the soma and dendrites. Compartmentalization of glutamate receptors occurred at stages of development when synaptic vesicles were already fully segregated to the axon. Formation of synapses was accompanied by a further redistribution of receptors, which segregated to synapse-enriched portions of dendrites. Receptor compartmentalization and dendritic redistribution as well as accumulation of synaptic vesicles at synaptic sites occurred also in neurons cultured in the presence of either the sodium channel blocker tetrodotoxin or glutamate receptor antagonists. These results indicate that signals generated by neuronal electrical activity or receptor activation are not involved in the establishment of neuronal polarity and synaptogenesis.

Local communication within dendritic spines: models of second messenger diffusion in granule cell spines of the mammalian olfactory bulb

Dendritic spines are generally believed to play a role in modulating synaptically induced electrical events. In addition, they may also confine second messengers and thus topologically limit the distance over which second messenger cascades may be functionally significant. In order to address this possibility, computer simulations of transient second messenger concentration changes were performed. The results show the importance of spine morphology and binding and extrusion mechanisms in controlling second messenger transients. In the presence of intrinsic cytoplasmic binding sites and kinetic rates similar to that expected for calcium, second messengers were confined to the spine head. In the absence of binding/extrusion mechanisms, the size and time course of the input transient to the spine head influenced the second messenger transients that might be seen at the base of the spine neck and in other spines. Large and/or sustained increases in second messenger concentration in the spine head were communicated to the spine base and to other spine heads. The results emphasize the importance of a knowledge of breakdown pathways, concentrations and kinetics of binding sites, and extrusion mechanisms for understanding the dynamics of local chemical changes for dendritic spine function.

The nucleus is insulated from large cytosolic calcium ion changes

Extracellular events regulate functions in the cell nucleus by means of calcium ions acting through effector enzymes. Recently, the traditional view of the nuclear pore as freely permeable to small ions has been questioned as a result of reports that nuclear calcium can be regulated independently of cytosolic calcium. We have used confocal microscopy of fluorescent Ca2+ indicators to investigate the Ca2+ dynamics between cytosol and nucleus in neurons. We find that a previously reported amplification of Ca2+ changes in the nucleus is a measurement artefact. Small changes of cytosolic Ca2+ cause equally rapid changes in nuclear Ca2+, consistent with the free diffusion of Ca2+ through nuclear pores. In contrast, large cytosolic Ca2+ increases (above 300 nM) are attenuated in the nucleus. Our results show the nuclear envelope shapes but does not block the passage of Ca2+ signals from cytosol to nucleus.

The development of parvalbumin and calbindin-D28k immunoreactive interneurons in kitten visual cortical areas

Calbindin-D and parvalbumin are calcium binding proteins which are found in non-overlapping subpopulations of GABA-ergic interneurons in mammalian neocortex. We studied the development of these calcium-binding proteins in interneurons of cat striate and extrastriate cortical areas which have differing patterns of connectivity and follow different developmental timetables. We examined primary visual areas 17 and 18, secondary visual area 19, medial lateral suprasylvian and lateral suprasylvian areas (MLS and LLS) and association areas 7 and the splenial visual area from the day of birth (P0) through P101. Parvalbumin-immunoreactive (ir) interneurons followed the inside-out pattern of maturation of cortical laminae. They were located only in infragranular layers at the earliest ages and were not observed in the overlying cortical plate. At 3 weeks of age, when cortical lamination is mature, parvalbumin stained cells were found in all cortical layers except layer I. The number of stained secondary and tertiary dendrites in the parvalbumin-ir interneuronal population decreased with age. This change was associated with a shift in the molecular weight of parvalbumin detected on Western blots. During the first postnatal week, the area 17/18 border contained more parvalbumin-ir neurons than other visual areas. The developmental pattern of calbindin staining differed considerably from the parvalbumin staining pattern. Very few calbindin-ir interneurons were seen in area 17 during the first 2 weeks of life. In lateral cortical areas, calbindin-ir neurons were located in cortical plate, infragranular layers of cortex and white matter/subplate. Calbindin-ir neurons increased in supragranular layers of secondary cortical areas by P7 and in area 17 by P20. In the mature cortex, the calbindin staining pattern was bilaminar, with a dense band of calbindin-ir cells in layer II and a second band in layers V-VI. There was no difference in the distribution of calbindin-ir neurons among visual areas at maturity.

Vasoactive intestinal polypeptide stimulates cyclic AMP production in mouse N1E-115 neuroblastoma cells: modulation by a protein kinase C activator and ionomycin

In this study, we investigated the vasoactive intestinal polypeptide (VIP)-stimulated cAMP production and its interaction with protein kinase C activation and elevation of intracellular Ca2+ in N1E-115 neuroblastoma cells. VIP treatment caused a 55-fold increase in cAMP accumulation. Addition of 4 beta-phorbol 12-myristate 13-acetate reduced VIP- but not forskolin-stimulated cAMP response. In comparison, ionomycin potentiated both VIP- and forskolin-induced cAMP accumulation. Our results indicate that VIP stimulates cAMP accumulation in N1E-115 cells, and that although activation of protein kinase C inhibits the VIP-stimulated cAMP response, elevation of intracellular Ca2+ potentiates this signaling pathway.

Calcium channels and GABA receptors in the early embryonic chick retina

The properties of calcium channels were studied at the period of neurogenesis in the early embryonic chick retina. The whole neural retina was isolated from embryonic day 3 (E3) chick and loaded with a Ca(2+)-sensitive fluorescent dye (Fura-2). The retinal cells were depolarized by puff application of high-K+ solutions. Increases in intracellular Ca2+ concentrations were evoked by the depolarization through calcium channels. The type of calcium channel was identified as L-type by the sensitivity to dihydropyridines. The Ca2+ response was completely blocked by 10 microM nifedipine, whereas it was remarkably enhanced by 5 microM Bay K 8644. Then we sought a factor to activate the calcium channel and found that GABA could activate it by membrane depolarization at the E3 chick retina. Puff application of 100 microM GABA raised intracellular Ca2+ concentrations, and this Ca2+ response to GABA was also sensitive to the two dihydropyridines. Intracellular potential recordings verified clear depolarization by bath-applied 100 microM GABA. The Ca2+ response to GABA was mediated by GABAA receptors, since the GABA response was blocked by 10 microM bicuculline or 50 microM picrotoxin, and mimicked by muscimol but not by baclofen. Neither glutamate, kainate, nor glycine evoked any Ca2+ response. We conclude that L-type calcium channels and GABAA receptors are already expressed before differentiation of retinal cells and synapse formation in the chick retina. A possibility is proposed that GABA might act as a trophic factor by activating L-type calcium channels via GABAA receptors during the early period of retinal neurogenesis.

Localized calcium spikes and propagating calcium waves

Ca2+ signals control or modulate diverse cellular processes such as cell growth, muscle contraction, hormone secretion, and neuronal plasticity. Elevations in intracellular Ca2+ concentrations can be highly localized to micron and submicron domains or propagated as intra- and intercellular waves over distances as large as 1 mm. Localized, subcellular Ca2+ spikes are thought to selectively activate effector systems such as Ca2+ activated chloride currents in pancreatic acinar cells, neurotransmitter release in synaptic nerve terminals, and morphological changes in neural growth cones. In contrast, long-ranged Ca2+ waves synchronize the activities of different cytoplasmic regions of a single cell, such as cortical granule exocytosis after egg fertilization or coordinate the activities of many cells, such as ciliary beating in pulmonary epithelium. The purpose of this review is to delineate the role of Ca2+ in the generation of localized, subcellular Ca2+ spikes and long-ranged intracellular and intercellular Ca2+ waves.

Spatial localization of agonist-induced Ca2+ entry in bovine adrenal chromaffin cells. Different patterns induced by histamine and angiotensin II, and relationship to catecholamine release

The spatial organization of agonist-induced Ca2+ entry in single bovine adrenal chromaffin cells has been investigated using video-imaging techniques to visualize fura-2 quenching by the Ca2+ surrogate, Mn2+. The potent secretagogue histamine, in addition to releasing Ca2+ from intracellular stores, resulted in a large influx of external Mn2+ that occurred over the entire surface of the cell. The influx of Ca2+ that this mirrors was found to be an obligatory requirement for the triggering of catecholamine release by histamine, which suggests that such a global influx of Ca2+ into the cell probably underlies the ability of this agonist to stimulate a large secretory response. By contrast, the weaker secretagogue angiotensin II, which also acts through the second messenger inositol trisphosphate, produced a localized entry of external Mn2+ in 64% of cells. In these cells, localized Mn2+ entry always occurred at the pole of the cell in which the angiotensin II-induced rise in [Ca2+]i was largest. Since exocytosis in response to angiotensin II has previously been shown to be restricted to this same pole of the cell (Cheek et al. (1989). J. Cell Biol. 109, 1219–1227), these results suggest that localized influx of Ca2+ in response to angiotensin II could underlie the polarized exocytotic response observed with this stimulus. These results directly demonstrate that different agonists can induce different patterns of divalent cation influx in the same cells and, furthermore, suggest how these different patterns can have a direct influence on cellular function.

Development of NMDA receptor-channel complex and L-type calcium channels in mouse hippocampus

In vitro binding autoradiography was used to examine the pattern and intensity of binding of [3H]glutamate to NMDA receptors, [3H]MK 801 to NMDA receptor associated channels, and [3H]PN-200 110 to L-type calcium channels in the hippocampus of mice aged 3-70 days. The distribution of NMDA receptors and NMDA receptor associated channels was similar but not identical at the tested ages. Beginning with postnatal day 8, high binding levels were confined mostly to the hippocampal strata: the oriens and radiatum (CA1 and CA3 with [3H]MK 801 labeling but only CA1 with NMDA displaced [3H]glutamate labeling), the moleculare (higher labeling with [3H]MK 801 than with NMDA displaced [3H]glutamate binding), and the lucidum. The binding values for NMDA receptor-channel complex rose in the examined period (especially within the second and third week), reaching a plateau at the end of the third postnatal week. Sharp growth of binding within the second and third week of life was about 50% greater with [3H]MK 801 than with NMDA displaced [3H] glutamate labeling. L-type calcium channels were found to be most abundant in the strata: the oriens of the CA3 field, the moleculare, and the lucidum. The time course of binding value changes for the calcium channel was similar to the time course found for the NMDA receptor-channel complex.

A sensory role for neuronal growth cone filopodia

The dynamic nature of neuronal growth cone filopodia led to the suggestion that the primary function of filopodia is to sample their immediate environment, responding to and transducing environmental signals that affect growth cone behaviour and shape. Filopodia seem well suited to serve as antenna-like sensors, their broad span allows sampling of information over a greatly enhanced radius, and forward-projecting filopodia encounter potential cues in the molecular terrain long before the advancing growth cone itself. Filopodia in culture can serve structural roles, exert mechanical tension and selectively adhere to their surrounding. Whether or not filopodia have a general sensory role has not been tested directly, largely because of their small size, which limits an electrophysiological approach, and their integral relationship with the parent growth cone, which prevents resolution of their different functions. Here we use surgical procedures to isolate individual filopodia from their parent growth cone and, by monitoring their morphology and calcium second messenger systems, we show that neuronal growth cone filopodia contain signal transduction mechanisms that allow autonomous responses and the transmission of distant environmental information to their parent growth cone.

Specifying the path of the intersegmental nerve of the Drosophila embryo: a role for Delta and Notch

The intersegmental nerve (ISN) of the Drosophila embryo follows a reproducible course near the anterior border of each segment. Based on the experiments reported here, we suggest that growth of the axons constituting the nerve is guided, in part, by the transmembrane proteins Delta and Notch. In particular, we suggest that expression of Delta protein on a branch of the trachea provides a path for the nerve through the lateral part of the embryo, and that the growing axons use the Notch protein on their surfaces to recognize this path. Consistent with this idea, we show that disruption of the trachea abolishes the ability of the ISN to extend through this part of the embryonic periphery. Finally, we argue that the same regulatory network that directs these peripheral axons also specifies the trajectory of part of the axonal scaffold of the central nervous system.

Role of intracellular calcium in NI-35-evoked collapse of neuronal growth cones

A myelin-associated protein from the central nervous system, the neurite growth inhibitor NI-35, inhibits regeneration of lesioned neuronal fiber tracts in vivo and growth of neurites in vitro. Growth cones of cultured rat dorsal root ganglion neurons arrested their growth and collapsed when exposed to liposomes containing NI-35. Before morphological changes, the concentration of free intracellular calcium ([Ca2+]i) showed a rapid and large increase in growth cones exposed to liposomes containing NI-35. Neither an increase in [Ca2+]i nor collapse of growth cones was detected in the presence of antibodies to NI-35. Dantrolene, an inhibitor of calcium release from caffeine-sensitive intracellular calcium stores, protected growth cones from collapse evoked by NI-35. Depletion of these caffeine-sensitive intracellular calcium stores prevented the increase in [Ca2+]i evoked by NI-35. The NI-35-evoked cascade of intracellular messengers that mediates collapse of growth cones includes the crucial step of calcium release from intracellular stores.

Voltage, calcium, and stretch activated ionic channels and intracellular calcium in bone cells

Embryonic chick bone cells express various types of ionic channels in their plasma membranes for as yet unresolved functions. Chick osteoclasts (OCL) have the richest spectrum of channel types. Specific for OCL is a K+ channel, which activates (opens) when the inside negative membrane potential (Vm) becomes more negative (hyperpolarization). This is consistent with findings of others on rat OCL. The membrane conductance constituted by these channels is called the inward rectifying K+ conductance (GKi), or inward rectifier, because the hyperpolarization-activated channels cause cell-inward K+ current to pass more easily through the membrane than outward K+ current. Besides GKi channels, OCL may express two other types of voltage-activated K+ channels. One constitutes the transient outward rectifying K+ conductance (GKto), which is activated upon making the membrane potential less negative (depolarization) but has a transient nature. This conductance favors transient K+ conduction in the cell-outward direction. The GKto also occurs in a small percentage of cells in osteoblast (OBL) and periosteal fibroblast (PFB) cultures. The other OCL K+ conductance, the GKCa, is activated by both membrane depolarization and a rise in [Ca2+]i. GKCa channels are also present in the other chick bone cell types, that is, OBL, osteocytes (OCY), and PFB. Furthermore, in excised patches of all bone cell types, channels have been found that conduct anions, including Cl- and phosphate ions. These channels are only active around Vm = 0 mV. While searching for a membrane mechanism for adaptation of bone to mechanical loading, we found stretch-activated channels in chick osteoclasts; other investigators have found stretch-activated cation channels (K+ or aselective) in rat and human osteogenic cell lines. In contrast to other studies on cell lines or OBL from other species, we have not found any of the classic macroscopic voltage-activated calcium conductances (GCa) in any of the chick bone cells under our experimental conditions. However, our fluorescence measurements of [Ca2+]i in single cells indicate the presence of Ca2+ conductive pathways through the plasma membrane of osteoblastic cells and osteoclasts, consistent with other studies. We discuss possible roles for GKi, GKCa, and anion channels in acid secretion by OCL and for stretch-activated channels in OCL locomotion.

Analysis of dynamic and stationary pattern formation in the cell cortex

We study a sol-gel mechanochemical model for cellular cytoplasm. Using conservation equations and a force balance equation, we derive equations for the sol-gel dynamics. Regular perturbation analysis suggests the growth of patterns which may be either dynamic or stationary, depending on parameter values. Nonlinear analysis, which indicates that these patterns remain bounded, is confirmed by numerically solving the mechanochemical equations. We use these analytical and numerical results to model two different biological problems: the dynamic formation of filopodia in nerve growth cones, and the growth of microvilli in epithelial cells.

Calcium influx into neurons can solely account for cell contact-dependent neurite outgrowth stimulated by transfected L1

We have used monolayers of control 3T3 cells and 3T3 cells expressing transfected human L1 as a culture substrate for rat PC12 cells and rat cerebellar neurons. PC12 cells and cerebellar neurons extended longer neurites on human L1 expressing cells. Neurons isolated from the cerebellum at postnatal day 9 responded equally as well as those isolated at postnatal day 1-4, and this contrasts with the failure of these older neurons to respond to the transfected human neural cell adhesion molecule (NCAM). Human L1-dependent neurite outgrowth could be blocked by antibodies that bound to rat L1 and, additionally, the response could be fully inhibited by pertussis toxin and substantially inhibited by antagonists of L- and N-type calcium channels. Calcium influx into neurons induced by K+ depolarization fully mimics the L1 response. Furthermore, we show that L1- and K+(-)dependent neurite outgrowth can be specifically inhibited by a reduction in extracellular calcium to 0.25 microM, and by pretreatment of cerebellar neurons with the intracellular calcium chelator BAPTA/AM. In contrast, the response was not inhibited by heparin or by removal of polysialic acid from neuronal NCAM both of which substantially inhibit NCAM-dependent neurite outgrowth. These data demonstrate that whereas NCAM and L1 promote neurite outgrowth via activation of a common CAM-specific second messenger pathway in neurons, neuronal responsiveness to NCAM and L1 is not coordinately regulated via posttranslational processing of NCAM. The fact that NCAM- and L1-dependent neurite outgrowth, but not adhesion, are calcium dependent provides further evidence that adhesion per se does not directly contribute to neurite outgrowth.

Peptidergic neurons of the crab, Cardisoma carnifex, in defined culture maintain characteristic morphologies under a variety of conditions

Peptidergic neurons dissociated from the neurosecretory cell group, the X-organ, of adult crabs (Cardisoma carnifex) show immediate outgrowth on unconditioned plastic dishes in defined medium. Most of the neurons can be categorized as small cells, branchers or veilers. A fourth type, "superlarge," found occasionally, has a soma diameter greater than 40 microns and multipolar outgrowth. We report here the effects on morphology that follow alterations of the standard defined culturing conditions. The three common types of neurons are present when cells are grown in crab saline or saline with L-glutamine and glucose (saline medium). Changes of pH between 7.0 to 7.9 have no effect. Osmolarity changes cause transient varicosities in small cells. In some veilers, pits rapidly appear in the veil and then disappear within 35 min. In cultures at 26 degrees C instead of 22 degrees C, veilers extend processes from the initial veil in a pattern similar to branchers, and the processes of adjacent veilers sometimes form appositions. Culturing in higher [K+]o medium ([K+]o = 15-110 mM; standard = 11 mM) has no long-term effect, but growth is arrested by [K+]o greater than 30 mM. Cultures were also grown in media in which [Ca2+]o ranged from 0.1 microM to 26 mM (standard = 13 mM). Outgrowth occurred from all neuronal types in all [Ca2+]o tested. Thus, the expression of different outgrowth morphologies occurs under a wide variety of culturing conditions.

Two pathways of cyclic GMP production through glutamate receptor-mediated nitric oxide synthesis

The selective agonists for the metabotropic glutamate receptor and the ionotropic non-N-methyl-D-aspartate (NMDA) glutamate receptor, (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) and (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), respectively, increased the cyclic GMP (cGMP) content in cerebellar slices prepared from adult rats. The ACPD-induced rise in cGMP level was blocked by compounds known to antagonize metabotropic glutamate receptors, such as DL-2-amino-3-phosphonopropionic acid and L-2-amino-4-phosphonobutyric acid, but not by ionotropic glutamate receptor antagonists, D-2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), whereas the AMPA-induced rise in cGMP level was suppressed by CNQX. Both rises in cGMP level involved nitric oxide synthase (NOS), because NG-methyl-L-arginine (NMLA), an inhibitor of NOS, blocked both cGMP level rises, and excess L-arginine reversed the effect of NMLA. After lithium chloride treatment, which could exhaust phosphatidylinositol phosphates, ACPD no longer increased cGMP levels, whereas AMPA was still effective. In a calcium-free medium, ACPD still induced a rise in cGMP level, whereas AMPA did not. When the molecular layer was isolated to determine the cGMP content separately from that in the rest of the cerebellar cortex, it was found that ACPD raised the cGMP level mainly in the molecular layer, whereas AMPA raised it in both sections.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of cellular mechanisms for modulation of calcium transients using a mathematical model of fura-2 Ca2+ imaging in Aplysia sensory neurons

A theoretical model of [Ca++]i diffusion, buffering, and extrusion was developed for Aplysia sensory neurons, and integrated with the measured optical transfer function of our fura-2 microscopic recording system, in order to fully simulate fura-2 video or photomultiplier tube measurements of [Ca++]i. This allowed an analysis of the spatial and temporal distortions introduced during each step of fura-2 measurements of [Ca++]i in cells. In addition, the model was used to evaluate the plausibility of several possible mechanisms for modulating [Ca++]i transients evoked by action potentials. The results of the model support prior experimental work (Blumenfeld, Spira, Kandel, and Siegelbaum, 1990. Neuron. 5: 487-499), suggesting that 5-HT and FMRFamide modulate action potential-induced [Ca++]i transients in Aplysia sensory neurons through changes in Ca++ influx, and not through changes in [Ca++]i homeostasis or release from internal stores.

Local increases in intracellular calcium elicit local filopodial responses in Helisoma neuronal growth cones

Highly localized changes in intracellular Ca2+ concentration ([Ca2+]i) can be evoked in neuronal growth cones; these are followed by local changes in filopodia. Focally applied electric fields evoked spatially restricted, high magnitude increases in growth cone [Ca2+]i. The earliest and greatest increases were localized to small regions within a growth cone. Such fields also produced characteristic changes in the disposition of filopodia: both filopodial length and number were significantly increased on the cathode side of growth cones. The requirement for extracellular Ca2+ and the strong correlation between the evoked rise in [Ca2+]i and the changes in filopodia (r = 0.98) indicate that cathode stimulation results in local Ca2+ influx, leading to locally increased [Ca2+]i and local changes in filopodial behavior.

Polarized efflux of 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein from cultured epithelial cell monolayers

We have investigated the polarity of the efflux of the intracellular pH fluorochrome 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) from layers of epithelial Madin-Darby canine kidney (MDCK, Strains I and II) and human intestinal (Caco-2, HCT-8 and T84) cells grown on porous membranes. In Strain I MDCK cells, BCECF efflux was effectively reduced by indomethacin (50% inhibition with 100 microM) and 5-nitro-2-(3-phenylpropyl-amino)-benzoate (NPPB; 50% inhibition with 10 microM). Replacement of external Cl- with bromide, iodide or nitrate did not alter BCECF efflux, while substitution with methanesulphonate resulted in a small but significant reduction. All five cell lines form confluent epithelial layers when grown on porous membranes. Efflux of BCECF from Strain I MDCK epithelial layers into the apical solution was approximately three times greater than into the basal solution. Addition of indomethacin to the apical solution attenuated efflux into the apical but not the basal solution, while basal indomethacin was effective against basal efflux. NPPB has a similar specificity of action. Adrenaline, a stimulant of electrogenic Cl- secretion, did not alter the pattern of BCECF efflux. BCECF efflux was also polarized, with apical efflux greater than basal efflux, in MDCK Strain II and Caco-2 epithelial layers. In contrast, BCECF efflux into the basal and apical media was equivalent in layers formed from HCT-8 and T84 cells. However, indomethacin reduced efflux in all five epithelial lines, although the relative sensitivities of the apical and basal efflux rates to indomethacin varied, as did the sensitivity to the sidedness of application of indomethacin. In MDCK and HCT-8 epithelial layers, transepithelial vinblastine secretion mediated by P-glycoprotein was not inhibited by indomethacin. The data are consistent with the hypothesis that BCECF efflux is a manifestation of a novel ATP-dependent xenobiotic secretory efflux mechanism in renal and gastrointestinal epithelia. The factors regulating the polarity of BCECF efflux, both the indomethacin-sensitive and -insensitive components, have yet to be elucidated.

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.

Developmental switch in the pharmacology of Ca2+ channels coupled to acetylcholine release

The pharmacological specificity of Ca2+ channel-secretion coupling in acetylcholine (ACh) and somatostatin (SOM) release was studied in the chick eye choroid neuromuscular junctions and in dissociated ciliary ganglion (CG) neurons. ACh secretion changes in development from stage (St) 40, when release is dihydropyridine (DHP) and partially omega-conotoxin (omega-CgTX) sensitive, to posthatch, when release is insensitive to DHPs but sensitive to omega-CgTX. St 40 CG neurons cultured with striated muscle have release properties similar to those of St 40 iris and choroid but different from those of St 34 neurons, which are neither DHP nor omega-CgTX sensitive. SOM (also coreleased from posthatch choroid terminals) can inhibit ACh release in both posthatch and St 40 choroids, suggesting that the SOM receptor interacts with both DHP-sensitive and -insensitive channels.

Ca(2+)-dependent changes in the mitochondrial energetics in single dissociated mouse sensory neurons

Depolarization of neurons promotes Ca2+ influx through voltage-activated channels, raising the intracellular Ca2+ concentration ([Ca2+]i). The consequences of such changes in [Ca2+]i for mitochondrial function were assessed in single, freshly dissociated mammalian neurons. Microfluorimetric techniques were used to measure [Ca2+]i, mitochondrial membrane potential [delta psi m, Rhodamine 123 (Rh 123) fluorescence], NAD(P)H/NAD(P)+ autofluorescence and flavoprotein autofluorescence combined with whole-cell voltage-clamp techniques. Brief (100-500 ms) depolarization of the cell membrane by high K+ or by voltage commands raised [Ca2+]i and depolarized delta psi m. The change in delta psi m was dependent on extracellular Ca2+. Under voltage-clamp control of the cell membrane, the voltage-dependence of the change in Rh 123 fluorescence reflected that of the Ca2+ current. The response was reduced by Ca2+ buffers introduced into the cell. The behaviour of this signal is thus consistent with a mitochondrial response to raised [Ca2+]i and does not reflect the change in cell membrane potential per se. Similar stimuli caused a rapid decrease of NAD(P)H autofluorescence, followed by an increase which could last several minutes. Flavoprotein fluorescence increased transiently, followed by a decrease lasting for several minutes. These signals indicate an initial oxidation of NAD(P)H and FADH, followed by a prolonged increase in the reduced state of both coenzymes. All these changes were dependent on extracellular [Ca2+]. Raising [Ca2+]i again during the period of NAD+ reduction caused an oxidizing response. Ruthenium Red applied to the cells (i) reduced both the Ca2+ current and the depolarization-induced [Ca2+]i transient and (ii) directly quenched Rh 123 fluorescence. When introduced into the cells with patch pipettes, it prevented the changes in autofluorescence without interfering with the Ca2+ conductance. Oligomycin blocked neither the response of delta psi m nor of NADH autofluorescence, suggesting that the signals do not reflect a response to falling ATP/ADP.Pi ratios as a consequence of the high [Ca2+]i. The changes in NADH autofluorescence were sustained in the presence of iodoacetic acid with pyruvate as substrate. Thus brief physiological elevations of [Ca2+]i depolarize delta psi m, probably through Ca2+ cycling across the mitochondrial inner membrane. The changes in autofluorescence are consistent with (i) increased respiration which could result from the depolarization of delta psi m, followed rapidly by (ii) increased activity of the Ca(2+)-dependent intramitochondrial enzymes. Changes in [Ca2+]i within a physiological range may thus promote significant and long-lasting changes in mitochondrial energy production.

Intracellular ion imaging using fluorescent dyes: artefacts and limits to resolution

Development of highly efficient fluorescent ratio indicators has made imaging of ion concentrations within individual cells possible (Grynkiewicz et al. 1985; Tsien and Poenie 1986). Ion imaging is a complex technique and is therefore prone to artefacts. In this paper we investigate the limits of the technique and its potential pitfalls. The spatial resolution of an imaging system is determined for different cell geometries. We describe a technique to increase the time resolution of existing systems by using a single excitation wavelength to measure changes in ion concentration. We demonstrate examples of potential artefacts arising from hardware limitations, image processing and fundamental optics. Methods for recognition and minimization of these problems are discussed.

Intracellular cyclic AMP produces effects opposite to those of cyclic GMP and calcium on shape and motility of neuroblastoma cells

We have directly evaluated the effects of various intracellular second messengers including cyclic nucleotides, calcium ion, and inositol polyphosphates on shape and motility of differentiating mouse neuroblastoma cells. The messengers were microinjected into cells and the responses of the soma, neurite, and growth cone were monitored using time-lapse video microscopy. Each messenger altered cell shape and motility in a characteristic manner. Cyclic AMP promoted lamellipodial expansion, neurite outgrowth, and motility. The other injected messengers opposed motility. Cyclic GMP caused motile structures to freeze and to retract permanently, while the inhibitory effects of calcium injection were concentration-dependent. Small calcium injections affected specifically actin-containing motile structures which froze and retracted temporarily. Intermediate calcium injections caused a strong contraction at the site of injection in all cells. With large injections, cells retracted long neurites, rounded up, and frequently began vigorous blebbing that continued to cell death. Injections of the inositol polyphosphates IP3(1,4,5) and IP4(1,4,5,6) mimicked the effects of small calcium injections, as did electrical stimulation that elicited action potentials. The results suggest that in mouse neuroblastoma cells, intracellular cAMP elevation increases cytoskeletal organization and promotes neurite extension perhaps through an enhancement of cell-substratum adhesion. On the other hand, a rise of intracellular cGMP or intracellular calcium interferes directly with the function and organization of the actin-microfilament system. The integrated action of these second messenger systems may, therefore, operate in vivo to allow substances released from neighboring cells to regulate neuronal architecture.