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D W Tank

Publications and source records attributed to D W Tank.

At least 37 records · Page 2Linked to original sources
PubMed1994

Ca2+ accumulations in dendrites of neocortical pyramidal neurons: an apical band and evidence for two functional compartments.

Apical dendrites constitute a prominent feature of the microcircuitry in the neocortex, yet their function is poorly understood. Using fura-2 imaging of layer 5 pyramidal neurons from slices of rat somatosensory cortex, we have investigated the Ca2+ influx into dendrites under intracellular, antidromic, synaptic, and receptor-agonist stimulation. We find three spatial patterns of Ca2+ accumulations: an apical band in the apical dendrite approximately 500 microns from the soma, an accumulation restricted to the basal dendrites, soma, and proximal apical dendrite, and a combination of both of these. We show that the apical band can be activated antidromically and synaptically and that, under blocked Na+ and K+ conductances, it generates Ca2+ spikes. Thus, the apical band may serve as a dendritic trigger zone for regenerative Ca2+ spikes or as a current amplifier for distal synaptic events. Our results suggest that the distal apical dendrite should be considered a separate functional compartment from the rest of the cell.

Animals↗
PubMed1994

Dendritic calcium dynamics.

Further characterization of the biochemical components that contribute to calcium handling, together with advances in optical imaging of ion concentration, are providing quantitative information on the dynamics of calcium in the dendrites of neurons in tissue culture, brain slices and in vivo. It has recently been demonstrated that strong spatial gradients and transient calcium elevations result from the geometry and membrane properties of dendrites. These studies are adding to our understanding of calcium's role in synaptic plasticity and in shaping the electrophysiological properties of neurons.

Animals↗
PubMed1994

Dynamics of propagating waves in the olfactory network of a terrestrial mollusk: an electrical and optical study.

1. The procerebral (PC) lobe of the terrestrial mollusk Limax maximus contains a highly interconnected network of local olfactory interneurons that receives ipsilateral axonal projections from superior and inferior noses. This network exhibits an approximately 0.7-Hz intrinsic oscillation in its local field potential (LFP). 2. Intracellular recordings show that the lobe contains at least two classes of neurons with activity phase locked to the oscillation. Neurons in one class produce periodic bursts of spikes, followed by a period of hyperpolarization and subsequently a depolarizing afterpotential. There is a small but significant chance for a second burst to occur during the depolarizing afterpotential; this leads to a double event in the LFP. Bursting neurons constitute approximately 10% of the neurons in the lobe. 3. Neurons in the other class fire infrequently and do not produce periodic bursts of action potentials. However, they receive strong, periodic inhibitory input during every event in the LFP. These nonbursting cells constitute the major fraction of neurons in the lobe. There is a clear correlation between the periodic burst of action potentials in the bursting neurons and the hyperpolarization seen in nonbursting neurons. 4. Optical techniques are used to image the spatially averaged transmembrane potentials in preparations stained with voltage-sensitive dyes. The results of simultaneous optical and electrical measurements show that the major part of the optical signal can be interpreted as a superposition of the intracellular signals arising from the bursting and nonbursting neurons. 5. Successive images of the entire PC lobe show waves of electrical activity that span the width of the lobe and travel its full length along a longitudinal axis. The direction of propagation in the unperturbed lobe is always from the distal to the proximal end. The wavelength varies between preparations but is on the order of the length of the preparation. 6. One-dimensional images along the longitudinal axis of the lobe are used to construct a space-time map of the optical activity, from which we calculate the absolute contribution of bursting and nonbursting neurons to the optical signal. The contribution of the intracellular signals from the two cell types appears to vary systematically across the lobe; bursting cells dominate at middle and proximal locations, and nonbursting cells dominate at distal locations. 7. The direction and form of the waves can be perturbed either by microsurgical manipulation of the preparation or by chemical modulation of its synaptic and neuronal properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

A quantitative measurement of the dependence of short-term synaptic enhancement on presynaptic residual calcium.

We simultaneously measured presynaptic free calcium ion concentration ([Ca2+]i) and synaptic strength at the crayfish claw opener neuromuscular junction (nmj) under a variety of experimental conditions. Our experiments were designed both to test the hypothesis that elevated [Ca2+]i is necessary and sufficient for the induction of a form of synaptic enhancement that persists for several seconds after tetanic stimulation--augmentation--and to determine the quantitative relationship between elevated [Ca2+]i and this enhancement. Action potential trains increased [Ca2+]i and enhanced transmission. During the decay phase of synaptic enhancement known as augmentation (time constant of decay approximately 7 sec at 20 degrees C with < 200 microM fura-2 in terminals), [Ca2+]i was elevated 700 nM or less above rest and an essentially linear relationship between [Ca2+]i and enhancement was observed. Introduction of exogenous Ca2+ buffers into the presynaptic terminal slowed the buildup and recovery kinetics of both [Ca2+]i and the component of synaptic enhancement corresponding to augmentation. The slope of the relationship relating delta [Ca2+]i to augmentation was not changed. The time course of augmentation and recovery of [Ca2+]i remained correlated as the temperature of the preparation was changed from about 10 degrees C to 20 degrees C, but the quantitative relationship of enhancement to [Ca2+]i was increased more than two- to threefold. During moderate frequency trains of action potentials, a slowly developing component of the total synaptic enhancement was approximately linearly related to residual [Ca2+]i measured with fura-2. The quantitative relationship between [Ca2+]i and this component of synaptic enhancement during trains was the same as that during synaptic augmentation after trains.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1994

The role of presynaptic calcium in short-term enhancement at the hippocampal mossy fiber synapse.

The mossy fiber synapse between dentate granule cells and CA3 pyramidal cells in the guinea pig hippocampus shows a robust short-term synaptic enhancement. We have simultaneously measured presynaptic residual free calcium ([Ca2+]i) and postsynaptic field potentials at this synapse to examine the role of [Ca2+]i in this enhancement. Single action potentials produced an increase in [Ca2+]i of 10-50 nM that decayed to resting levels with a time constant of about 1 sec. Trains of action potentials produced larger [Ca2+]i increases that returned more slowly to resting levels. Following the onset of moderate frequency stimulus trains (0.1-5 Hz), synaptic transmission and [Ca2+]i both increased and eventually plateaued. During the steady-state phase a linear relationship between [Ca2+]i and synaptic enhancement was observed. During the initial buildup, however, [Ca2+]i rose more rapidly than synaptic enhancement. Similarly, during the decay phase immediately following termination of a stimulus train, [Ca2+]i returned to prestimulus levels faster than synaptic enhancement. High concentrations of the calcium buffer EGTA in the presynaptic terminal slowed the buildup and decay of both [Ca2+]i and synaptic enhancement produced by stimulus trains. Under these conditions, the time course of [Ca2+]i and synaptic enhancement were well matched. This suggests that, despite the differences in kinetic rates observed for normal buffering conditions, increases in [Ca2+]i play a causal role in short-term enhancement. An increase in [Ca2+]i of 10-30 nM produced a twofold enhancement. We propose a simple kinetic model to explain these results. The model assumes that synaptic enhancement is controlled by a Ca-dependent first-order reaction. According to this scheme, a change in [Ca2+]i alters neurotransmitter release, but the slow kinetics of the underlying reaction introduces a temporal filter, producing a delay in the change in synaptic enhancement.

Animals↗
PubMed1993

Tesla gradient recalled echo characteristics of photic stimulation-induced signal changes in the human primary visual cortex.

Multi-echo measurements of photic stimulation-induced signal changes in human visual cortex were made at 4 Tesla in order to quantify the nature of the signal change and its vascular origin, and to determine the optimum echo time for detection of the changes. Utilizing high resolution images, two distinct regions (ascribed to be microvasculature and visible venous vessels) were identified as giving rise to the signal increase. The fractional signal changes in gray matter areas depended linearly on echo time (TE) in the range of 10 to 60 ms and extrapolated to virtually zero for TE = 0, indicating that in-flow effects secondary to stimulation-induced blood flow increases were negligible in our functional imaging studies; instead, signal change due to photic stimulation originated from the increase in the apparent transverse relaxation rate, 1/T2*. This decrease in (1/T2*), brought about by the alterations in hemodynamic parameters, was 1.3 +/- 0.4 s-1 for gray matter and 3.0 +/- 0.7 s-1 (averaged over 10 individuals) for venous vessels visible in the images. The optimum choice of echo time was found to be TE > or = T2*.

Humans↗
PubMed1993

Functional brain mapping by blood oxygenation level-dependent contrast magnetic resonance imaging. A comparison of signal characteristics with a biophysical model.

It recently has been demonstrated that magnetic resonance imaging can be used to map changes in brain hemodynamics produced by human mental operations. One method under development relies on blood oxygenation level-dependent (BOLD) contrast: a change in the signal strength of brain water protons produced by the paramagnetic effects of venous blood deoxyhemoglobin. Here we discuss the basic quantitative features of the observed BOLD-based signal changes, including the signal amplitude and its magnetic field dependence and dynamic effects such as a pronounced oscillatory pattern that is induced in the signal from primary visual cortex during photic stimulation experiments. The observed features are compared with the results of Monte Carlo simulations of water proton intravoxel phase dispersion produced by local field gradients generated by paramagnetic deoxyhemoglobin in nearby venous blood vessels. The simulations suggest that the effect of water molecule diffusion is strong for the case of blood capillaries, but, for larger venous blood vessels, water diffusion is not an important determinant of deoxyhemoglobin-induced signal dephasing. We provide an expression for the apparent in-plane relaxation rate constant (R2*) in terms of the main magnetic field strength, the degree of the oxygenation of the venous blood, the venous blood volume fraction in the tissue, and the size of the blood vessel.

Biophysical Phenomena↗
PubMed1993

Timing of odor stimulation does not alter patterning of olfactory bulb unit activity in freely breathing rats.

1. The effect of the timing between nasal airflow and ongoing tracheal respiration on single-unit activity in the olfactory bulb (OB) of the rat was examined. Nasal and tracheal breathing were dissociated with the use of a double tracheal cannulation technique that allowed independent control of nasal airflow and control of the synchronization of nasal airflow and tracheal breathing. 2. When amyl acetate-saturated air was presented to the nose, OB units showed a distinct reorganization of activity known as respiratory patterning. Of 43 cells examined, 29 fired maximally after inspiration, and 14 fired maximally after expiration. In all 43 cells the patterning of OB activity was synchronized with the time course of the nasal stimulation. This synchronization was independent of the point in the ongoing respiratory cycle (tracheal breathing) at which the odor stimulation was applied. 3. Patterning of OB single-unit activity was also observed when odor was applied as a series of "inspirations" without intervening expirations. Patterning was observed to follow the time course of the odor stimulation even when this was considerably longer and slower than normal breathing. No patterning of activity was observed during continuous odor stimulation or in the absence of odor stimulation. 4. It is concluded that respiratory patterning of OB single-unit activity in the rat is not directly dependent on centrifugal inputs synchronized to respiration. Rather, the observed pattern of neural activity reflects the phasic stimulation of the olfactory receptors with each inspiration and the dynamics of the circuitry intrinsic to the bulb itself.

Action Potentials↗
PubMed1993

Coherent network oscillations by olfactory interneurons: modulation by endogenous amines.

1. The procerebral (PC) lobe of the terrestrial mollusk Limax maximus contains a highly interconnected network of local olfactory interneurons that receives direct axonal projections from the two pairs of noses. This olfactory processing network generates a 0.7-Hz oscillation in its local field potential (LFP) that is coherent throughout the network. The oscillating LFP is modulated by natural odorants applied to the neuroepithelium of the superior nose. 2. Two amines known to be present in the PC lobe, dopamine and serotonin, increase the frequency of the PC lobe oscillation and alter its waveform. 3. Glutamate, another putative neurotransmitter known to be present in the lobe, suppresses the PC lobe oscillation by a quisqualate-type receptor and appears to be used by one of the two classes of neurons in the PC lobe to generate the basic LFP oscillation. 4. The known activation of second messengers in Limax PC lobe by dopamine and serotonin together with their effects on the oscillatory rhythm suggest the hypothesis that these amines augment mechanisms mediating synaptic plasticity in the olfactory network, similar to hypothesized effects of amines in vertebrate olfactory systems. 5. The use of a distributed network of interneurons showing coherent oscillations may relate to the highly developed odor recognition and odor learning ability of Limax.

Animals↗
PubMed1993

Cultured olfactory interneurons from Limax maximus: optical and electrophysiological studies of transmitter-evoked responses.

1. The olfactory processing network in the procerebral (PC) lobe of the terrestrial mollusk Limax maximus exhibits a coherent oscillation of local field potential that is modulated by odor input. To understand the cellular basis of this oscillation, we developed a cell culture preparation of isolated PC neurons and studied the responses of isolated cells to stimulation with neurotransmitters known to be present in the PC lobe. 2. The distribution of PC soma diameters suggests at least two different populations of neurons. Approximately 95% of isolated cells had soma diameters of 7-8 microns, with the remaining cells having larger diameters (10-15 microns). 3. Extracellular measurements of action potentials and optical measurements of intracellular calcium concentrations in fura-2-loaded cells were made. Serotonin and dopamine excited PC neurons and promoted transitions from steady to bursty activity. Both amines elicited increases in intracellular calcium, presumably concomitant with the increase in action-potential frequency. 4. Glutamate suppressed action-potential firing and reduced intracellular calcium. This effect was seen most clearly when glutamate was applied to cells excited by high potassium medium. Quisqualate is an effective glutamate agonist in this system, whereas kainate is not. 5. Combined with anatomic and biochemical data and with studies of the effects of these neurotransmitters on the oscillating local field potential of the intact PC network, the data from isolated PC neurons are consistent with the hypothesis that dopamine and serotonin modulate network dynamics, whereas glutamate is involved in generating the basic oscillation of local field potential in the PC. 6. The optical studies of fura-2-loaded cells showed that several treatments that increase the rate of action-potential production lead to elevations in intracellular calcium. Optical studies of intracellular calcium may be useful for multisite measurements of activity in the intact, oscillating PC lobe network.

Action Potentials↗
PubMed1992

Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging.

We report that visual stimulation produces an easily detectable (5-20%) transient increase in the intensity of water proton magnetic resonance signals in human primary visual cortex in gradient echo images at 4-T magnetic-field strength. The observed changes predominantly occur in areas containing gray matter and can be used to produce high-spatial-resolution functional brain maps in humans. Reducing the image-acquisition echo time from 40 msec to 8 msec reduces the amplitude of the fractional signal change, suggesting that it is produced by a change in apparent transverse relaxation time T*2. The amplitude, sign, and echo-time dependence of these intrinsic signal changes are consistent with the idea that neural activation increases regional cerebral blood flow and concomitantly increases venous-blood oxygenation.

Brain Chemistry↗
PubMed1992

Functional brain mapping using magnetic resonance imaging. Signal changes accompanying visual stimulation.

Easily detectable (5%-20%) transient increases in the intensity of water proton magnetic resonance (MR) signals in human primary visual cortex were observed during visual stimulation in gradient echo images at 4-T field strength. The signal intensity increases were predominantly restricted to areas containing gray matter and were used to produce high-spatial-resolution human functional brain maps. Time dependence of the functional brain maps also was monitored during visual stimulation using images acquired every approximately 5 seconds; these images with high spatial and temporal resolution demonstrated that photic stimulation first resulted in signal increases in a large area of the visual cortex followed by a reduction in the size of the area, and that signal intensity increases in the gray matter were time dependent. Reducing the image acquisition echo times reduced the amplitude of the fractional signal change, suggesting that it is produced by a change in T2 or T2*. The amplitude, sign, and echo time dependence of these intrinsic signal changes are consistent with the idea that neural activation increases regional cerebral with the idea that neural activation increases regional cerebral blood flow (rCBF) with a concomitant increase in venous blood oxygenation.

Brain Mapping↗
PubMed1992

Calcium concentration dynamics produced by synaptic activation of CA1 hippocampal pyramidal cells.

The spatial and temporal dynamics of many electrophysiological and biochemical processes in nerve cells are in turn dependent on the concentration dynamics of the second messenger calcium. We have used microfluorimetry of the calcium indicator fura-2 (Grynkiewicz et al., 1985) to measure and characterize synaptically activated calcium changes in individual CA1 pyramidal cells contained within guinea pig hippocampal slices. One component of the calcium changes was largely produced by influx through voltage-dependent Ca2+ channels (VDCCs). It consisted of large transient accumulations in the proximal-apical and basal dendrites; the amplitude was smaller in the distal-apical dendrites and the soma. This spatial profile was insensitive to the method of cell activation: stimulation of inputs located at different positions on the dendritic tree as well as antidromic stimulation produced only slight modifications. This component was not blocked by the NMDA antagonist 5-amino-4-phosphonovalerate (AP5) (Collingridge et al., 1983), was greatly reduced by Cd2+, partially reduced by nifedipine, and was increased by Bay-K 8644, providing the evidence that it was largely produced by influx through VDCCs. Blocking postsynaptic Na+ channels with QX-314 greatly reduced the accumulation amplitude, and spatial differences between proximal-dendritic and distal-dendritic regions were less pronounced, suggesting that active sodium conductances contribute significantly to the spatial activation of calcium conductances. Residual spatial differences that persist in QX-314 experiments are consistent with the idea that VDCCs have decreased density on distal-apical dendrites. A second component of accumulation was induced by ionic currents through NMDA receptor channels. It was blocked by AP5, unaffected by QX-314, attenuated and slowed down by elevated calcium buffering, and spatially localized to regions receiving activated synaptic inputs. The magnitude of this component was strongly dependent on the frequency and amplitude of synaptic activation. At high frequency, it was generally very large, often saturating the fura-2 (> 2 microM). Measurements made with the indicator furaptra also showed large localized AP5-sensitive fluorescence changes. Our results suggest that in dendritic regions near activated input fibers calcium levels may reach 2-10 microM. In general, our measurements of calcium dynamics provide an experimental basis for evaluating the spatial distribution of calcium conductances, the spatial distribution of calcium-activated electrophysiological and biochemical processes, and the spatial uniformity of calcium buffering and removal systems in CA1 hippocampal pyramidal cells. The time course and amplitude of Ca2+ transients we measured suggest that activation of Ca(2+)-dependent conductances [e.g., IK(Ca)] will be markedly different for different cellular regions.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1991

Selective fura-2 loading of presynaptic terminals and nerve cell processes by local perfusion in mammalian brain slice.

We describe a method for filling presynaptic terminals and cell dendrites in adult brain slices with the fluorescent calcium indicator fura-2 by localized perfusion of the acetoxymethyl (AM) ester derivative. The method provides labeling selectivity, similar to that produced by intracellular microinjection of fura-2, with the simplicity of bath application of membrane-permeant AM esters. Application of the method to mossy fiber tracts in hippocampal region CA3 and parallel fiber tracts in cerebellum resulted in distant presynaptic terminals well labelled with fura-2 without concomitant postsynaptic labelling, allowing optical measurements of calcium concentration in individual presynaptic terminals. Application of the method to CA1 pyramidal cells produced intracellular loading of apical dendrites with fura-2. Dendritic calcium changes produced by afferent fiber stimulation were similar to those determined from cells filled with fura-2 by intracellular microinjection. The method appears to be general, and should provide a means to fill projecting axons and dendritic processes in many areas of the brain with fluorescent indicators, allowing optical measurements of ion concentration dynamics to be performed in brain slice that were previously impractical.

Animals↗
PubMed1991

The maintenance of LTP at hippocampal mossy fiber synapses is independent of sustained presynaptic calcium.

We have examined the role of presynaptic residual calcium in maintaining long-term changes in synaptic efficacy observed at mossy fiber synapses between hippocampal dentate granule cells and CA3 pyramidal cells. Calcium concentrations in individual mossy fiber terminals in hippocampal slice were optically measured with the calcium indicator fura-2 while stimulating the mossy fiber pathway and recording excitatory postsynaptic potentials extracellularly. Short-term synaptic enhancement was accompanied by increased presynaptic residual calcium concentration. A 2-fold enhancement of transmitter release was accompanied by a 10-30 nM increase in residual calcium. Following induction of mossy fiber LTP, transiently elevated presynaptic calcium decayed to prestimulus levels, whereas enhancement of synaptic transmission persisted. Our results demonstrate that, despite an apparent strong sensitivity of synaptic enhancement to presynaptic residual calcium levels, sustained increases in presynaptic residual calcium levels are not responsible for the maintained synaptic enhancement observed during mossy fiber LTP.

Animals↗
PubMed1991

Presynaptic calcium in transmitter release and posttetanic potentiation.

This review gives some indication of the progress that has been made in understanding synaptic transmission by use of new methods for measuring and controlling presynaptic [Ca2+]i. Many unsolved problems remain. We still do not have a clear idea of the exact relationship between [Ca2+]i and transmitter release and whether this relationship is the same under all circumstances. The apparently different [Ca2+]i-dependence of evoked transmitter release and of PTP suggest multiple molecular sites of calcium action that remain to be identified. A complete and comprehensive model of transmitter release has yet to be devised, and questions raised by our experiments may indicate that it is still too early to try to construct a precise model. We also do not know just how serotonin acts to modulate transmitter release, only that it does not appear to alter either resting or entering calcium. Some of these questions may be approachable with the techniques described here; others are not and require different methods for their resolution. The work continues.

Action Potentials↗