Different spatial patterns of [Ca2+] increase caused by N- and L-type Ca2+ channel activation in frog olfactory bulb neurones

Deep two-photon brain imaging with a red-shifted fluorometric Ca2+ indicator

In vivo Ca2+ imaging of neuronal populations in deep cortical layers has remained a major challenge, as the recording depth of two-photon microscopy is limited because of the scattering and absorption of photons in brain tissue. A possible strategy to increase the imaging depth is the use of red-shifted fluorescent dyes, as scattering of photons is reduced at long wavelengths. Here, we tested the red-shifted fluorescent Ca2+ indicator Cal-590 for deep tissue experiments in the mouse cortex in vivo. In experiments involving bulk loading of neurons with the acetoxymethyl (AM) ester version of Cal-590, combined two-photon imaging and cell-attached recordings revealed that, despite the relatively low affinity of Cal-590 for Ca2+ (Kd=561 nM), single-action potential-evoked Ca2+ transients were discernable in most neurons with a good signal-to-noise ratio. Action potential-dependent Ca2+ transients were recorded in neurons of all six layers of the cortex at depths of up to -900 µm below the pial surface. We demonstrate that Cal-590 is also suited for multicolor functional imaging experiments in combination with other Ca2+ indicators. Ca2+ transients in the dendrites of an individual Oregon green 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-1 (OGB-1)-labeled neuron and the surrounding population of Cal-590-labeled cells were recorded simultaneously on two spectrally separated detection channels. We conclude that the red-shifted Ca2+ indicator Cal-590 is well suited for in vivo two-photon Ca2+ imaging experiments in all layers of mouse cortex. In combination with spectrally different Ca2+ indicators, such as OGB-1, Cal-590 can be readily used for simultaneous multicolor functional imaging experiments.

From lamprey to salamander: an exploratory modeling study on the architecture of the spinal locomotor networks in the salamander

The evolutionary transition from water to land required new locomotor modes and corresponding adjustments of the spinal "central pattern generators" for locomotion. Salamanders resemble the first terrestrial tetrapods and represent a key animal for the study of these changes. Based on recent physiological data from salamanders, and previous work on the swimming, limbless lamprey, we present a model of the basic oscillatory network in the salamander spinal cord, the spinal segment. Model neurons are of the Hodgkin-Huxley type. Spinal hemisegments contain sparsely connected excitatory and inhibitory neuron populations, and are coupled to a contralateral hemisegment. The model yields a large range of experimental findings, especially the NMDA-induced oscillations observed in isolated axial hemisegments and segments of the salamander Pleurodeles waltlii. The model reproduces most of the effects of the blockade of AMPA synapses, glycinergic synapses, calcium-activated potassium current, persistent sodium current, and [Formula: see text]-current. Driving segments with a population of brainstem neurons yields fast oscillations in the in vivo swimming frequency range. A minimal modification to the conductances involved in burst-termination yields the slower stepping frequency range. Slow oscillators can impose their frequency on fast oscillators, as is likely the case during gait transitions from swimming to stepping. Our study shows that a lamprey-like network can potentially serve as a building block of axial and limb oscillators for swimming and stepping in salamanders.

Establishment of retinal progenitor cell clones by transfection with Pax6 gene of mouse induced pluripotent stem (iPS) cells

We previously reported that transfection of Pax6 gene which regulated early events in eye development into mouse ES cells brought about their differentiation into retinal progenitors. Here, we attempted to establish cloned retinal progenitors which had ability to further differentiate into photoreceptor like cells by transfecting mouse induced pluripotent stem (iPS) cells with Pax6 gene. Undifferentiated iPS cells were transfected with Pax6 cDNA, followed by selection with G418. After limiting dilution culture, we selected cloned Pax6-transfected cells, which simultaneously expressed mRNAs of Nestin, Musashi1, Six3 and Chx10 for further characterization. We obtained totally 8 clonally expanding Pax6-transfected cells. They started to express mRNAs of Brn3b, Cone-rod homeobox (Crx), pkc, CD73, rhodopsin and the γ-subunit of rod cGMP phosphodiesterase (PDEγ). Flow cytometric analysis revealed that almost half of the cells were CD73+, a marker of photoreceptor precursors. Western blotting confirmed cytoplasmic protein expression of rhodopsin. High KCl stimulation increased free Ca influx into the cells on Ca(2+) imaging. iPS cells transfected with Pax6 gene, followed by subsequent limiting dilution culture became retinal progenitors including photoreceptor like cells. The cloned cell lines may be useful for analyzing differentiation requirement of retinal progenitors.

Sieve element Ca2+ channels as relay stations between remote stimuli and sieve tube occlusion in Vicia faba

Damage induces remote occlusion of sieve tubes in Vicia faba by forisome dispersion, triggered during the passage of an electropotential wave (EPW). This study addresses the role of Ca2+ channels and cytosolic Ca2+ elevation as a link between EPWs and forisome dispersion. Ca2+ channel antagonists affect the initial phase of the EPW as well as the prolonged plateau phase. Resting levels of sieve tube Ca2+ of approximately 50 nM were independently estimated using Ca2+-selective electrodes and a Ca2+-sensitive dye. Transient changes in cytosolic Ca2+ were observed in phloem tissue in response to remote stimuli and showed profiles similar to those of EPWs. The measured elevation of Ca2+ in sieve tubes was below the threshold necessary for forisome dispersion. Therefore, forisomes need to be associated with Ca2+ release sites. We found an association between forisomes and endoplasmic reticulum (ER) at sieve plates and pore-plasmodesma units where high-affinity binding of a fluorescent Ca2+ channel blocker mapped an increased density of Ca2+ channels. In conclusion, propagation of EPWs in response to remote stimuli is linked to forisome dispersion through transiently high levels of parietal Ca2+, release of which depends on both plasma membrane and ER Ca2+ channels.

Effects of clonidine on lidocaine-induced inhibition of axonal transport in cultured mouse dorsal root ganglion neurones

BACKGROUND

The alpha(2)-adrenoceptor agonist clonidine is used in combination with lidocaine for anaesthesia. Lidocaine inhibits axonal transport and neurite growth, whereas alpha(2)-adrenoceptor agonists have neurotrophic effects. Here we have investigated whether clonidine reduces lidocaine-induced inhibition of axonal transport in cultured mouse dorsal root ganglion neurones.

METHODS

Axonal transport of organelles and neurite growth were assessed by video microscopy in cells treated with clonidine and lidocaine for 1 h. Stable responses were achieved within this period.

RESULTS

Clonidine (10 and 100 microM) increased and lidocaine (10, 100 microM, and 1 mM) decreased axonal transport. The inhibitory effects of lidocaine were reduced by simultaneous treatment with clonidine. The actions of clonidine were antagonized by the alpha(2)-adrenoceptor antagonist yohimbine. Since clonidine was reported to block N-type channels, we further investigated the role of ion channels in the antagonistic action of clonidine on the lidocaine response. The action of lidocaine on axonal transport was not mimicked by the Na+ channel blocker tetrodotoxin and not blocked by the Na+ channel activator veratridine. The action of lidocaine was not blocked by the L-type Ca2+ channel blocker nifedipine, but was blocked by the N-type channel blocker omega-conotoxin MVIIA. These effects on axonal transport correlated with the effects on neurite growth.

CONCLUSIONS

Inhibition of axonal transport induced by lidocaine, which may be mediated by N-type channel activation, can be blocked by clonidine. Clonidine may alleviate the effects of lidocaine on neuronal dysfunction.

New statistical methods enhance imaging of cameleon fluorescence resonance energy transfer in cultured zebrafish spinal neurons

Cameleons are genetically encoded fluorescence resonance energy transfer (FRET)-based Ca(2+) indicators. Attempts to use cameleons to detect neural activity in vertebrate systems have been largely frustrated by the small FRET signal, in contradistinction to the higher signals seen in Drosophila and Caenorhabditis elegans. We have developed a statistical optimization method capable of detecting small ratiometric signals in noisy imaging data, called statistical optimization for the analysis of ratiometric signals. Using this method, we can detect and estimate anticorrelated ratiometric signals with subcellular resolution in cultured, dissociated zebrafish spinal neurons expressing cameleon or loaded with fluo-4 and fura-red. This method may make it possible to use yellow cameleons for measuring neural activity at high resolution in transgenic animals.

Introduction of the MASH1 gene into mouse embryonic stem cells leads to differentiation of motoneuron precursors lacking Nogo receptor expression that can be applicable for transplantation to spinal cord injury

ES cells transfected with the MASH1 gene yielded purified spinal motoneuron precursors expressing HB9 and Islet1. The cells lacked the expression of Nogo receptor that was of great advantage for axon growth after transplantation to an injured spinal cord. After transplantation, mice with the complete transection of spinal cord exhibited excellent improvement of the motor functions. Electrophysiological assessment confirmed the quantitative recovery of motor-evoked potential in the transplanted spinal cord. In the grafted spinal cord, gliosis was inhibited and Nogo receptor expression was scarcely detected. The transplanted cells labeled with GFP showed extensive outgrowth of axons positive for neurofilament middle chain, connected to each other and expressed Synaptophysin, Lim1/2 and Islet1. Thus, the in vivo differentiation into mature spinal motoneurons and the reconstitution of neuronal pathways were suggested. The grafted cell population was purified for neurons and was free from teratoma development. These therapeutic strategies may contribute to a potent treatment for spinal cord injury in future.

Finite-particle tracking reveals submicroscopic-size changes of mitochondria during transport in mitral cell dendrites

The mechanisms of molecular motor regulation during bidirectional organelle transport are still uncertain. There is, for instance, the unsettled question of whether opposing motor proteins can be engaged in a tug-of-war. Clearly, any non-synchronous activation of the molecular motors of one cargo can principally lead to changes in the cargo's shape and size; the cargo's size and shape parameters would certainly be observables of such changes. We therefore set out to measure position, shape and size parameters of fluorescent mitochondria (during their transport) in dendrites of cultured neurons using a finite-particle tracking algorithm. Our data clearly show transport-related submicroscopic-size changes of mitochondria. The observed displacements of the mitochondrial front and rear ends are consistent with a model in which microtubule plus- and minus-end-directed motor proteins or motors of the same type but moving along anti-parallel microtubules are often out-of-phase and occasionally engaged in a tug-of-war. Mostly the leading and trailing ends of mitochondria undergo similar characteristic movements but with a substantial time delay between the displacements of both ends, a feature reminiscent of an inchworm-like motility mechanism. More generally, we demonstrate that observing the position, shape and size of actively transported finite objects such as mitochondria can yield information on organelle transport that is generally not accessible by tracking the organelles' centroid alone.

In situ background estimation in quantitative fluorescence imaging

Fluorescence imaging of bulk-stained tissue is a popular technique for monitoring the activities in a large population of cells. However, a precise quantification of such experiments is often compromised by an ambiguity of background estimation. Although, in single-cell-staining experiments, background can be measured from a neighboring nonstained region, such a region often does not exist in bulk-stained tissue. Here we describe a novel method that overcomes this problem. In contrast to previous methods, we determined the background of a given region of interest (ROI) using the information contained in the temporal dynamics of its individual pixels. Since no information outside the ROI is needed, the method can be used regardless of the staining profile in the surrounding tissue. Moreover, we extend the method to deal with background inhomogeneities within a single ROI, a problem not yet solved by any of the currently available tools. We performed computer simulations to demonstrate the accuracy of our method and give example applications in ratiometric calcium imaging of bulk-stained olfactory bulb slices. Converting the fluorescence signals into [Ca2+] gives resting values consistent with earlier single-cell staining results, and odorant-induced [Ca2+] transients can be quantitatively compared in different cells. Using these examples we show that inaccurate background subtraction introduces large errors (easily in the range of 100%) in the assessment of both resting [Ca2+] and [Ca2+] dynamics. The proposed method allows us to avoid such errors.

Establishment of primary cultures of rat olfactory bulb under serum-free conditions for studies of cellular injury

Olfactory dysfunction has been implicated in various neurodegenerative diseases including Parkinson's and Alzheimer's disease but, despite intense interest in the neurobiology of the olfactory bulb (OB), studies of neurodegenerative mechanisms have not been attempted in primary OB cultures. This study was aimed at developing a primary OB culture under serum-free conditions in order to investigate injury and excitotoxicity in vitro. Olfactory bulbs from rat pups were rapidly trypsinised and mechanically dissociated and the resultant single cell suspension was centrifuged through a high bovine serum albumin concentration gradient to reduce cellular debris before being seeded in multi-well culture plates. Cells were plated in neurobasal medium containing 0.5 mM glutamine, 25 mM K(+), 2% B27 and 10% fetal calf serum (FCS) for 24 h and, after 1 day in vitro (div1), were maintained without FCS. At div8, neurones exhibited extensive neuritic networks, were present as a monolayer and were mainly bipolar and immunopositive for gamma-aminobutyric acid indicating that they were intrinsic OB neurones. At div8, neurones (positive for microtubule-associated protein-2, 73%) predominated over astrocytes (positive for glial fibrillary acidic protein, 27%). Cellular injury produced by staurosporine, hydrogen peroxide and kainate, when assessed by morphological and biochemical procedures, was shown to be concentration-dependent and significantly reduced the numbers of neurones and astrocytes. Further analyses of kainate-induced injury revealed the presence of TUNEL-positive cells (indicative of apoptosis) and increases in intracellular free calcium, both of which were antagonised by CNQX. Thus, the serum-free culture developed here is amenable to morphological and high throughput neurochemical analyses of mechanisms contributing to the injury of OB neurones in vitro.

Synaptic integration in dendritic trees

Most neurons have elaborate dendritic trees that receive tens of thousands of synaptic inputs. Because postsynaptic responses to individual synaptic events are usually small and transient, the integration of many synaptic responses is needed to depolarize most neurons to action potential threshold. Over the past decade, advances in electrical and optical recording techniques have led to new insights into how synaptic responses propagate and interact within dendritic trees. In addition to their passive electrical and morphological properties, dendrites express active conductances that shape individual synaptic responses and influence synaptic integration locally within dendrites. Dendritic voltage-gated Na(+) and Ca(2+) channels support action potential backpropagation into the dendritic tree and local initiation of dendritic spikes, whereas K(+) conductances act to dampen dendritic excitability. While all dendrites investigated to date express active conductances, different neuronal types show specific patterns of dendritic channel expression leading to cell-specific differences in the way synaptic responses are integrated within dendritic trees. This review explores the way active and passive dendritic properties shape synaptic responses in the dendrites of central neurons, and emphasizes their role in synaptic integration.

Action potential backpropagation and multiglomerular signaling in the rat vomeronasal system

In the accessory olfactory bulb (AOB), sensory neurons expressing a given vomeronasal receptor (VR) gene send divergent projections to many glomeruli, and second-order neurons (mitral cells) link to multiple glomeruli via branched primary dendrites. We used calcium imaging and paired somadendritic patch-clamp recording to track backpropagated action potentials (APs) in rat AOB primary dendrites. In cells loaded with 150 microm Calcium Orange, somatic spikes elicited fluorescence transients over the entire primary dendritic tree, and the relative fluorescence increment DeltaF/F(0) increased along all branches from soma to glomeruli. Backpropagation was reliant on Na+ channels: in 1 microm TTX, somatic AP commands evoked dendritic Ca2+ transients that declined steeply with distance. In paired soma- dendritic whole-cell recordings, backpropagated APs were unattenuated up to approximately 200 microm from the soma, whereas subthreshold voltage transients decayed markedly. Computational modeling indicated that the large distal Ca2+ transients are consistent with active, not passive, backpropagation. Genetic tracing in the AOB has suggested homotypic connectivity with individual mitral cell dendritic arbors projecting only to glomeruli targeted by sensory neurons expressing the same VR gene. Non-decremental, non-dichotomous backpropagation in AOB primary dendrites ensures fast, reliable communication between mitral cells and their homotypic glomeruli, binding them into functional modules in accordance with their VR-coded inputs.

Plasticity of dendritic excitability

Dendrites are equipped with a plethora of voltage-gated ion channels that greatly enrich the computational and storage capacity of neurons. The excitability of dendrites and dendritic function display plasticity under diverse circumstances such as neuromodulation, adaptation, learning and memory, trauma, or disorders. This adaptability arises from alterations in the biophysical properties or the expression levels of voltage-gated ion channels-induced by the activity of neurotransmitters, neuromodulators, and second-messenger cascades. In this review we discuss how this plasticity of dendritic excitability could alter information transfer and processing within dendrites, neurons, and neural networks under physiological and pathological conditions.

Activation of REST/NRSF target genes in neural stem cells is sufficient to cause neuronal differentiation

REST/NRSF is a transcriptional repressor that acts at the terminal stage of the neuronal differentiation pathway and blocks the transcription of several differentiation genes. REST/NRSF is generally downregulated during induction of neuronal differentiation. The recombinant transcription factor REST-VP16 binds to the same DNA binding site as does REST/NRSF but functions as an activator instead of a repressor and can directly activate the transcription of REST/NRSF target genes. However, it is not known whether REST-VP16 expression is sufficient to cause formation of functional neurons from neural stem cells (NSCs). Here we show that regulated expression of REST-VP16 in a physiologically relevant NSC line growing under cycling conditions converted the cells rapidly to the mature neuronal phenotype. Furthermore, when grown in the presence of retinoic acid, REST-VP16-expressing NSCs activated their target, as well as other differentiation genes that are not their direct target, converting them to the mature neuronal phenotype and enabling them to survive in the presence of mitotic inhibitors, which is a characteristic of mature neurons. In addition, these neuronal cells were physiologically active. These results showed that direct activation of REST/NRSF target genes in NSCs with a single transgene, REST-VP16, is sufficient to cause neuronal differentiation, and the findings suggested that direct activation of genes involved in the terminal stage of differentiation may cause neuronal differentiation of NSCs.

Conversion of myoblasts to physiologically active neuronal phenotype

Repressor element 1 (RE1)-silencing transcription factor (REST)/neuron-restrictive silencer factor (NRSF) can repress several terminal neuronal differentiation genes by binding to a specific DNA sequence (RE1/neuron-restrictive silencer element [NRSE]) present in their regulatory regions. REST-VP16 binds to the same RE1/NRSE, but activates these REST/NRSF target genes. However, it is unclear whether REST-VP16 expression is sufficient to cause formation of functional neurons either from neural stem cells or from heterologous stem cells. Here we show that the expression of REST-VP16 in myoblasts grown under muscle differentiation conditions blocked entry into the muscle differentiation pathway, countered endogenous REST/NRSF-dependent repression, activated the REST/NRSF target genes, and, surprisingly, activated other neuronal differentiation genes and converted the myoblasts to a physiologically active neuronal phenotype. Furthermore, in vitro differentiated neurons produced by REST-VP16-expressing myoblasts, when injected into mouse brain, survived, incorporated into the normal brain, and did not form tumors. This is the first instance in which myoblasts were converted to a neuronal phenotype. Our results suggest that direct activation of REST/NRSF target genes with a single transgene, REST-VP16, is sufficient to activate other terminal neuronal differentiation genes and to override the muscle differentiation pathways, and they suggest that this approach provides an efficient way of triggering neuronal differentiation in myoblasts and possibly other stem cells.

Organization of glomeruli in the main olfactory bulb of Xenopus laevis tadpoles

Structural and functional investigations were carried out to study olfactory glomeruli in the main olfactory bulb (OB) in tadpoles of the clawed frog, Xenopus laevis. Calcium imaging of odor response patterns of OB neurons revealed that the synapses within the glomeruli are functional. Tracing axons of individual olfactory receptor neurons (ORNs), dendrites of mitral/tufted (M/T) cells and processes of periglomerular interneurons indicate that the glomerular architecture is solely determined by terminal branches of ORN axons and tufts of M/T primary dendrites. The small population of periglomerular neurons forms wide-field arborizations that always extend over many glomeruli, enter the glomeruli, but lack any glomerular tufts. Antibodies to synaptophysin indicate a high density of synapses within glomeruli, which was further confirmed at the ultrastructural level and quantified to approximately 0.5 synaptic sites per microm(2). Combining immunocytochemistry and ultrastructural investigations, we show that glomeruli in Xenopus laevis tadpoles lack any cellular borders. Glomeruli are surrounded neither by periglomerular somata nor by glial processes. Taken together, our results demonstrate that olfactory glomeruli in Xenopus laevis tadpoles (1) are fully functional, (2) are spheroidal neuropil aggregations of terminal tufts of ORNs and tufts of primary dendrites of M/T cells, and (3) are not enwrapped by a border formed by juxtaglomerular cells.

Dendrodendritic inhibition and simulated odor responses in a detailed olfactory bulb network model

In the olfactory bulb, both the spatial distribution and the temporal structure of neuronal activity appear to be important for processing odor information, but it is currently impossible to measure both of these simultaneously with high resolution and in all layers of the bulb. We have developed a biologically realistic model of the mammalian olfactory bulb, incorporating the mitral and granule cells and the dendrodendritic synapses between them, which allows us to observe the network behavior in detail. The cell models were based on previously published work. The attributes of the synapses were obtained from the literature. The pattern of synaptic connections was based on the limited experimental data in the literature on the statistics of connections between neurons in the bulb. The results of simulation experiments with electrical stimulation agree closely in most details with published experimental data. This gives confidence that the model is capturing features of network interactions in the real olfactory bulb. The model predicts that the time course of dendrodendritic inhibition is dependent on the network connectivity as well as on the intrinsic parameters of the synapses. In response to simulated odor stimulation, strongly activated mitral cells tend to suppress neighboring cells, the mitral cells readily synchronize their firing, and increasing the stimulus intensity increases the degree of synchronization. Preliminary experiments suggest that slow temporal changes in the degree of synchronization are more useful in distinguishing between very similar odorants than is the spatial distribution of mean firing rate.

Neuronal representation of odourants in the olfactory bulb of Xenopus laevis tadpoles

When an odourant enters the nose, olfactory receptor neurons (ORNs) convey information about it to the olfactory bulb (OB), where this information is processed and where the first central representations of the odourant are generated. In this paper we show how odourants are represented by ensembles of OB neurons, in particular mitral cells (MCs) which are the output neurons of the OB. We were able to demonstrate for the first time that the intracellular calcium concentrations ([Ca2+]i) in the somata of these neurons undergo specific changes and that different stimuli are represented by different neuronal [Ca2+]i patterns. The similarity of patterns was assessed by cross-correlation analysis. We further show that noradrenaline (NA), which is reported to be involved in olfactory memory formation and to modulate synaptic transmission at dendrodendritic synapses in the OB, profoundly changes the representation of odourants at the level of MCs.

Polarized and compartment-dependent distribution of HCN1 in pyramidal cell dendrites

An ion channel's function depends largely on its location and density on neurons. Here we used high-resolution immunolocalization to determine the subcellular distribution of the hyperpolarization-activated and cyclic-nucleotide-gated channel subunit 1 (HCN1) in rat brain. Light microscopy revealed graded HCN1 immunoreactivity in apical dendrites of hippocampal, subicular and neocortical layer-5 pyramidal cells. Quantitative comparison of immunogold densities showed a 60-fold increase from somatic to distal apical dendritic membranes. Distal dendritic shafts had 16 times more HCN1 labeling than proximal dendrites of similar diameters. At the same distance from the soma, the density of HCN1 was significantly higher in dendritic shafts than in spines. Our results reveal the complex cell surface distribution of voltage-gated ion-channels, and predict its role in increasing the computational power of single neurons via subcellular domain and input-specific mechanisms.

Anisotropic diffusion in mitral cell dendrites revealed by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) can be used to measure kinetic properties of single molecules in drops of solution or in cells. Here we report on FCS measurements of tetramethylrhodamine (TMR)-dextran (10 kDa) in dendrites of cultured mitral cells of Xenopus laevis tadpoles. To interpret such measurements correctly, the plasma membrane as a boundary of diffusion has to be taken into account. We show that the fluorescence data recorded from dendrites are best described by a model of anisotropic diffusion. As compared to diffusion in water, diffusion of the 10-kDa TMR-dextran along the dendrite is slowed down by a factor 1.1-2.1, whereas diffusion in lateral direction is 10-100 times slower. The dense intradendritic network of microtubules oriented parallel to the dendrite is discussed as a possible basis for the observed anisotropy. In somata, diffusion was found to be isotropic in three dimensions and 1.2-2.6 times slower than in water.

Glutamate receptor desensitization block potentiates the stimulated GABA release through external Ca2+-independent mechanisms from granule cells of olfactory bulb

Glutamate stimulated release of [3H]GABA was studied, during receptor desensitization block and its modulation by voltage gated Ca2+ channels, internal Ca2+ mobilization and GABA transport inhibitors from olfactory bulb slices. Under control conditions, glutamate and agonists induced release was strongly inhibited by Mg/0 Ca2+ Krebs and Cd2+ and partially inhibited by Ni2+ and nifedipine. Cyclothiazide, which blocks desensitization of glutamate receptors, potentiated glutamate, kainate, AMPA and quisqualate induced release. This effect was less dependent of entry of external Ca2+, but was inhibited by trifluoperazine and thapsigargin, inhibitors of Ca2+-calmodulin and endoplasmatic Ca2+ ATPase respectively. Nipecotic acid and NO-711, inhibitors of the GABA transporter, were also able to reduce cyclothiazide potentiated release induced by the 4 secretagogues. Under control conditions, glutamate stimulates the release of GABA in cooperation with VDCC. However, during receptor desensitization block, glutamate stimulated GABA release is mainly modulated through mechanisms dependent on internal Ca2+ mobilization and reversal of the GABA transporter.

Coupling of L-type voltage-sensitive calcium channels to P2X(2) purinoceptors in PC-12 cells

Extracellular ATP elevates cytosolic Ca(2+) by activating P2X and P2Y purinoceptors and voltage-sensitive Ca(2+) channels (VCCCs) in PC-12 cells, thereby facilitating catecholamine secretion. We investigated the mechanism by which ATP activates VSCCs. 2-Methylthioadenosine 5'-triphosphate (2-MeS-ATP) and UTP were used as preferential activators of P2X and P2Y, respectively. Nifedipine inhibited the ATP- and 2-MeS-ATP-evoked cytosolic Ca(2+) concentration increase and [(3)H]norepinephrine secretion, but not the UTP-evoked responses. Studies with Ca(2+) channel blockers indicated that L-type VSCCs were activated after the P2X activation. Mn(2+) entry profiles and studies with thapsigargin revealed that Ca(2+) entry, rather than Ca(2+) release, was sensitive to nifedipine. Although P2X(2) and P2X(4) receptor mRNAs were detected, studies with pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid revealed that P2X(2) was mainly coupled to the L-type VSCCs. The inhibitory effect of nifedipine did not occur in the absence of extracellular Na(+), suggesting that Na(+) influx, which induces depolarization, was essential for the P2X(2)-mediated activation of VSCCs. We report that depolarization induced by Na(+) entry through the P2X(2) purinoceptors effectively activates L-type VSCCs in PC-12 cells.

Noradrenergic modulation of calcium currents and synaptic transmission in the olfactory bulb of Xenopus laevis tadpoles

Norepinephrine (NE) has various modulatory roles in both the peripheral and the central nervous systems. Here we investigate the function of the locus coeruleus efferent fibres in the olfactory bulb of Xenopus laevis tadpoles. In order to distinguish unambiguously between mitral cells and granule cells of the main olfactory bulb and the accessory olfactory bulb, we used a slice preparation. The two neuron types were distinguished on the basis of their location in the slice, their typical branching pattern and by electrophysiological criteria. At NE concentrations lower than 5 microM there was only one effect of NE upon voltage-gated conductances; NE blocked a high-voltage-activated Ca(2+)-current in mitral cells of both the main and the accessory olfactory bulbs. No such effect was observed in granule cells. The effect of NE upon mitral cell Ca(2+)-currents was mimicked by the alpha(2)-receptor agonists clonidine and alpha-methyl-NE. As a second effect, NE or clonidine blocked spontaneous synaptic activity in granule cells of both the main and the accessory olfactory bulbs. NE or clonidine also blocked the spontaneous synaptic activity in mitral cells of either olfactory bulb. The amplitude of glutamate-induced currents in granule cells was modulated neither by clonidine nor by alpha-methyl-NE. Taken together, the main effect of the noradrenergic, presynaptic, alpha(2)-receptor-mediated block of Ca(2)+-currents in mitral cells appeared to be a wide-spread disinhibition of mitral cells in the accessory olfactory bulb as well as in the main olfactory bulb.

Fluorescence correlation spectroscopy in small cytosolic compartments depends critically on the diffusion model used

Fluorescence correlation spectroscopy (FCS) is a powerful technique for measuring low concentrations of fluorescent molecules and their diffusion constants. In the standard case, fluorescence fluctuations are measured in an open detection volume defined by the confocal optics. However, if FCS measurements are carried out in cellular processes that confine the detection volume, the standard FCS model leads to erroneous results. In this paper, we derive a modified FCS model that takes into account the confinement of the detection volume. Using this model, we have carried out the first FCS measurements in dendrites of cultured neurons. We further derive, for the case of confined diffusion, the limits within which the standard two- and three-dimensional diffusion models give reliable results.

BK channel activation by brief depolarizations requires Ca2+ influx through L- and Q-type Ca2+ channels in rat chromaffin cells

BK channel activation by brief depolarizations requires Ca2+ influx through L- and Q-type Ca2+ channels in rat chromaffin cells. Ca2+- and voltage-dependent BK-type K+ channels contribute to action potential repolarization in rat adrenal chromaffin cells. Here we characterize the Ca2+ currents expressed in these cells and identify the Ca2+ channel subtypes that gate the activation of BK channels during Ca2+ influx. Selective Ca2+ channel antagonists indicate the presence of at least four types of high-voltage-gated Ca2+ channels: L-, N-, P, and Q type. Mean amplitudes of the L-, N-, P-, and Q-type Ca2+ currents were 33, 21, 12, and 24% of the total Ca2+ current, respectively. Five-millisecond Ca2+ influx steps to 0 mV were employed to assay the contribution of Ca2+ influx through these Ca2+ channels to the activation of BK current. Blockade of L-type Ca2+ channels by 5 microM nifedipine or Q-type Ca2+ channels by 2 microM Aga IVA reduced BK current activation by 77 and 42%, respectively. In contrast, blockade of N-type Ca2+ channels by brief applications of 1-2 microM CnTC MVIIC or P-type Ca2+ channels by 50-100 nM Aga IVA reduced BK current activation by only 11 and 12%, respectively. Selective blockade of L- and Q-type Ca2+ channels also eliminated activation of BK current during action potentials, whereas almost no effects were seen by the selective blockade of N- or P-type Ca2+ channels. Finally, the L-type Ca2+ channel agonist Bay K 8644 promoted activation of BK current by brief Ca2+ influx steps by more than twofold. These data show that, despite the presence of at least four types of Ca2+ channels in rat chromaffin cells, BK channel activation in rat chromaffin cells is predominantly coupled to Ca2+ influx through L- and Q-type Ca2+ channels.

Tyrosine hydroxylase expression in primary cultures of olfactory bulb: role of L-type calcium channels

Sensory activity mediates regulation of tyrosine hydroxylase (TH), the first enzyme in the dopamine biosynthetic pathway, in the rodent olfactory bulb. The current studies established for the first time primary cultures of neonatal mouse olfactory bulb expressing TH and tested whether L-type calcium channels mediate the activity-dependent regulation of the dopamine phenotype. After 1 d in vitro (DIV), a small population of TH-immunostained neurons that lacked extensive processes could be demonstrated. After an additional 2 DIV in serum-free medium, the number of TH neurons had doubled, and they exhibited long interdigitating processes. Membrane depolarization for 48 hr with 50 mM KCl produced a further 2.4-fold increase in the number of TH-immunoreactive neurons compared with control cultures. Increased TH neuron number required at least 36 hr of exposure to KCl. Forskolin, which increases intracellular cAMP levels, induced a 1.5- to 1.6-fold increase in the number of TH-immunostained neurons. Combined treatment with KCl and forskolin was not additive. Nifedipine, an L-type calcium channel blocker, completely prevented the depolarization-mediated increase in TH expression but did not block the response to forskolin. Treatment with Bay K8644, an L-type calcium channel agonist, also significantly increased the number of TH-expressing neurons. Depolarization also induced alterations in neuritic outgrowth, resulting in a stellate versus an elongate morphology that, in contrast, was not prevented by nifedipine. These results are the first demonstration that in vitro, as in vivo, depolarization increases TH expression in olfactory bulb and that L-type calcium channels mediate this activity-dependent regulation of the dopamine phenotype.

Tyrosine hydroxylase expression in primary cultures of olfactory bulb: role of L-type calcium channels

Sensory activity mediates regulation of tyrosine hydroxylase (TH), the first enzyme in the dopamine biosynthetic pathway, in the rodent olfactory bulb. The current studies established for the first time primary cultures of neonatal mouse olfactory bulb expressing TH and tested whether L-type calcium channels mediate the activity-dependent regulation of the dopamine phenotype. After 1 d in vitro (DIV), a small population of TH-immunostained neurons that lacked extensive processes could be demonstrated. After an additional 2 DIV in serum-free medium, the number of TH neurons had doubled, and they exhibited long interdigitating processes. Membrane depolarization for 48 hr with 50 mM KCl produced a further 2.4-fold increase in the number of TH-immunoreactive neurons compared with control cultures. Increased TH neuron number required at least 36 hr of exposure to KCl. Forskolin, which increases intracellular cAMP levels, induced a 1.5- to 1.6-fold increase in the number of TH-immunostained neurons. Combined treatment with KCl and forskolin was not additive. Nifedipine, an L-type calcium channel blocker, completely prevented the depolarization-mediated increase in TH expression but did not block the response to forskolin. Treatment with Bay K8644, an L-type calcium channel agonist, also significantly increased the number of TH-expressing neurons. Depolarization also induced alterations in neuritic outgrowth, resulting in a stellate versus an elongate morphology that, in contrast, was not prevented by nifedipine. These results are the first demonstration that in vitro, as in vivo, depolarization increases TH expression in olfactory bulb and that L-type calcium channels mediate this activity-dependent regulation of the dopamine phenotype.

Action potential propagation into the presynaptic dendrites of rat mitral cells

1. Dendritic patch-clamp recordings were obtained from mitral cells in rat olfactory bulb slices, up to 350 microns from the soma. Simultaneous dendritic and somatic whole-cell recordings indicated that action potentials (APs) evoked by somatic or dendritic current injection were initiated near the soma. Both the large amplitude (100.7 +/- 1.1 mV) and the short duration (1.38 +/- 0.07 ms) of the AP were maintained as the AP propagated back into the primary mitral cell dendrites. 2. Outside-out patches isolated from mitral cell dendrites contained voltage-gated Na+ channels (peak conductance density, 90 pS micron-2 at -10 mV). When an AP was used as a somatic voltage-clamp command in the presence of 1 microM tetrodotoxin (TTX), the amplitude of the dendritic potential was attenuated to 48 +/- 14 mV. This shows that dendritic Na+ channels support the active back-propagation of APs. 3. Dendritic patches contained voltage-gated K+ channels with high density (conductance density, 513 pS micron-2 at 30 mV). Dendritic K+ currents were reduced to 35% by 1 mM external tetraethylammonium chloride (TEACl). When an AP was used as a somatic voltage-clamp command in the presence of TEACl, the dendritic potential was markedly prolonged. This indicates that dendritic K+ channels mediate the fast repolarization of dendritic APs. 4. We conclude that voltage-gated Na+ and K+ channels support dendritic APs with large amplitudes and short durations that may trigger fast transmitter release at dendrodendritic synapses in the olfactory bulb.

Two types of actions of norepinephrine on identified auditory efferent neurons in rat brain stem slices

Whole cell voltage-clamp recordings were performed on auditory olivocochlear neurons in the ventral nucleus of the trapezoid body (VNTB) of brain stem slices from neonatal rats. Each neuron was identified by retrograde labeling with Fast Blue injected into the cochlea. Bath application of norepinephrine (NE; 1-10 microM) reversibly induced an inward current in 26 of 38 labeled neurons that were voltage clamped at -75 mV. This was responsible for the membrane depolarization to NE observed in current-clamp mode. The NE-induced inward current appeared to be more prominent at -55 mV than at -75 mV and reversed at around -100 mV. It was attenuated but not prevented by 20 mM tetraethylammonium, and it persisted when the perfusate contained 2 mM Cs+ or 100 microM Cd2+. However, the NE-induced inward current was attenuated to varying degrees in a zero-Ca2+ solution. Current-voltage plots revealed that NE caused a decrease in membrane K+ conductance. A suppression of voltage-gated Ca2+ currents by NE was also observed. The excitatory action of NE was blocked by the alpha-adrenoreceptor antagonist phentolamine. The alpha1-adrenoreceptor agonist phenylephrine had an effect similar to that of NE. In 6 of 38 labeled neurons, an inhibitory action of NE (1-10 microM) was observed that appeared to be due to an activation of an inwardly rectified K+ current, which caused hyperpolarization of resting membrane potentials in current-clamp mode. This inhibitory response was independent of external Ca2+ and was abolished by 2-5 mM Cs+ or 0.5 mM Ba2+ applied in the perfusate. The receptors involved in the inhibitory actions of NE are not clear. The effect was partially and reversibly blocked by propranolol (10 microM), a beta-adrenoreceptor antagonist. However, isoprenaline (10 microM), a beta-adrenoreceptor agonist, failed to induce any effect. On the other hand, the inhibitory effect was irreversibly blocked by pretreatment with phentolamine (5-10 microM). Phenylephrine (5-10 microM) had no effect.

A transient, RCK4-like K+ current in cultured Xenopus olfactory bulb neurons

A transient K+ current in cultured olfactory bulb neurons of Xenopus tadpoles was studied using the whole-cell patch-clamp technique. The current, which was resistant to 80 mM tetraethylammoniumchloride (TEA) and 10 nM charybdotoxin but blocked by 5 mM 4-aminopyridine (4-AP), activated between -60 and -40 mV and showed time- and voltage-dependent inactivation. Its peak amplitude was nearly independent of the extracellular K+ concentration ([K+]o) in the range of 0.05 to 10 mM, indicating that its conductance increased upon increasing [K+]o. The transient K+ current showed a slow recovery from inactivation with the time for half-maximum recovery from a conditioning pulse to 80 mV for 1 s varying from 100 ms to 500 ms. Complete recovery required as much as 5-10 s at -80 mV, but could be speeded up at hyperpolarized potentials. The current resembles the RCK4 (Kv1.4) current of rat neurons except that its recovery from inactivation was independent of [K+]o. High-frequency stimulation (20-67 Hz) of the neurons with short (5 ms) voltage pulses resulted in a frequency-dependent, progressive inactivation of the transient K+ current. This suggests that, during phasic responses of olfactory bulb neurons, inactivation of the transient K+ current occurs and may lead to lengthening of action potentials and facilitation of synaptic transmission.

Second messenger signaling in olfactory transduction

Olfactory receptor neurons respond to odorants with G-protein mediated increases in the concentration of cyclic adenosine 3',5'-monophosphate (cAMP) and/or inositol 1,4,5-trisphospahte (InsP3). These two second messengers directly regulate opening of cAMP- and InsP3-regulated conductances localized to the apical transduction compartments of the cell (cilia and olfactory knob). In the presence of physiological concentrations of extracellular Ca2+, these second messenger regulated conductances mediate influx of Ca2+ into the olfactory neuron resulting in large, localized increases in intracellular Ca2+ ([Ca2+]i). A significant advance in our understanding of the molecular mechanisms of olfaction is the recent realization that this increase in [Ca2+]i plays an important role as a "third messenger" in olfactory transduction. Second messenger dependent increases in [Ca2+]i cause opening of ciliary Ca(2+)-activated Cl-, cation and/ or K+ channels that can carry a large percentage of the generator current, thus amplifying the signal substantially. As a result of this sequence of events, the generator potential in olfactory neurons can be depolarizing, leading to excitation of the neuron, or hyperpolarizing, leading to suppression of basal action potential firing rate. This dual effect of odorants on olfactory neurons may play an important role in quality coding and in the ability to detect low concentrations of odorants, particularly in complex mixtures.