UV photolysis using a micromanipulated optical fiber to deliver UV energy directly to the sample

Astrocytes display complex and localized calcium responses to single-neuron stimulation in the hippocampus

Astrocytes show a complex structural and physiological interplay with neurons and respond to neuronal activation in vitro and in vivo with intracellular calcium elevations. These calcium changes enable astrocytes to modulate synaptic transmission and plasticity through various mechanisms. However, the response pattern of astrocytes to single neuronal depolarization events still remains unresolved. This information is critical for fully understanding the coordinated network of neuron-glial signaling in the brain. To address this, we developed a system to map astrocyte calcium responses along apical dendrites of CA1 pyramidal neurons in hippocampal slices using single-neuron stimulation with channelrhodopsin-2. This technique allowed selective neuronal depolarization without invasive manipulations known to alter calcium levels in astrocytes. Light-evoked neuronal depolarization was elicited and calcium events in surrounding astrocytes were monitored using the calcium-sensitive dye Calcium Orange. Stimulation of single neurons caused calcium responses in populations of astrocytes along the apical axis of CA1 cell dendrites. Calcium responses included single events that were synchronized with neuronal stimulation and poststimulus changes in calcium event frequency, both of which were modulated by glutamatergic and purinergic signaling. Individual astrocytes near CA1 cells showed low ability to respond to repeated neuronal depolarization events. However, the response of the surrounding astrocyte population was remarkably accurate. Interestingly, the reliability of responses was graded with respect to astrocyte location along the CA1 cell dendrite, with astrocytes residing in the primary dendrite subregion being most responsive. This study provides a new perspective on the dynamic response property of astrocyte ensembles to neuronal activity.

Two-photon microscope for multisite microphotolysis of caged neurotransmitters in acute brain slices

We developed a two-photon microscope optimized for physiologically manipulating single neurons through their postsynaptic receptors. The optical layout fulfills the stringent design criteria required for high-speed, high-resolution imaging in scattering brain tissue with minimal photodamage. We detail the practical compensation of spectral and temporal dispersion inherent in fast laser beam scanning with acousto-optic deflectors, as well as a set of biological protocols for visualizing nearly diffraction-limited structures and delivering physiological synaptic stimuli. The microscope clearly resolves dendritic spines and evokes electrophysiological transients in single neurons that are similar to endogenous responses. This system enables the study of multisynaptic integration and will assist our understanding of single neuron function and dendritic computation.

Femtosecond Laser Microfabrication of an Integrated Device for Optical Release and Sensing of Bioactive Compounds

Flash photolysis of caged compounds is one of the most powerful approaches to investigate the dynamic response of living cells. Monolithically integrated devices suitable for optical uncaging are in great demand since they greatly simplify the experiments and allow their automation. Here we demonstrate the fabrication of an integrated bio-photonic device for the optical release of caged compounds. Such a device is fabricated using femtosecond laser micromachining of a glass substrate. More in detail, femtosecond lasers are used both to cut the substrate in order to create a pit for cell growth and to inscribe optical waveguides for spatially selective uncaging of the compounds present in the culture medium. The operation of this monolithic bio-photonic device is tested using both free and caged fluorescent compounds to probe its capability of multipoint release and optical sensing. Application of this device to the study of neuronal network activity can be envisaged.

Novel approach to real-time flash photolysis and confocal [Ca2+] imaging

Flash photolysis of "caged" compounds using ultraviolet light is a powerful experimental technique for producing rapid changes in concentrations of bioactive signaling molecules. Studies that employ this technique have used diverse strategies for controlling the spatial and temporal application of light to the specimen. In this paper, we describe a new system for flash photolysis that delivers light from a pulsed, adjustable intensity laser through an optical fiber coupled into the epifluorescence port of a commercial confocal microscope. Photolysis is achieved with extremely brief (5 ns) pulses of ultraviolet light (355 nm) that can be synchronized with respect to confocal laser scanning. The system described also localizes the UV intensity spatially so that uncaging only occurs in defined subcellular regions; moreover, because the microscope optics are used in localization, the photolysis volume can be easily adjusted. Experiments performed on rat ventricular myocytes loaded with the Ca(2+) indicator fluo-3 and the Ca(2+) cage o-nitrophenyl ethylene glycol bis(2-aminoethyl ether)-N,N,N'N'-tetraacetic acid (NP-EGTA) demonstrate the system's capabilities. Localized intracellular increases in [Ca(2+)] can trigger sarcoplasmic reticular Ca(2+) release events such as Ca(2+) sparks and, under certain conditions, regenerative Ca(2+) waves. This relatively simple and inexpensive system is, therefore, a useful tool for examining local signaling in the heart and other tissues.

Flash photolysis using a light emitting diode: An efficient, compact, and affordable solution

Flash photolysis has become an essential technique for dynamic investigations of living cells and tissues. This approach offers several advantages for instantly changing the concentration of bioactive compounds outside and inside living cells with high spatial resolution. Light sources for photolysis need to deliver pulses of high intensity light in the near UV range (300-380 nm), to photoactivate a sufficient amount of molecules in a short time. UV lasers are often required as the light source, making flash photolysis a costly approach. Here we describe the use of a high power 365 nm light emitting diode (UV LED) coupled to an optical fiber to precisely deliver the light to the sample. The ability of the UV LED light source to photoactivate several caged compounds (CMNB-fluorescein, MNI-glutamate, NP-EGTA, DMNPE-ATP) as well as to evoke the associated cellular Ca(2+) responses is demonstrated in both neurons and astrocytes. This report shows that UV LEDs are an efficient light source for flash photolysis and represent an alternative to UV lasers for many applications. A compact, powerful, and low-cost system is described in detail.

Light-Mediated Inhibition of Protein Synthesis

The regulation of protein synthesis is vital for a host of cell biological processes, but investigating roles for protein synthesis have been hindered by the inability to selectively interfere with it. To inhibit protein synthesis with spatial and temporal control, we have developed a photo-releasable anisomycin compound, N-([6-bromo-7-hydroxycoumarin-4-yl]methyloxycarbonyl)anisomycin (Bhc-Aniso), that can be removed through exposure to UV light. The area of protein synthesis inhibition can be restricted to a small light-exposed region or, potentially, the volume of two-photon excitation if a pulsed IR laser is the light source. We have tested the compound's effectiveness with an in vitro protein-translation system, CHO cells, HEK293 cells, and neurons. The photo-released anisomycin can inhibit protein synthesis in a spatially restricted manner, which will enable the specific inhibition of protein synthesis in subsets of cells with temporal and spatial precision.

C(a2+)-dependent glutamate release involves two classes of endoplasmic reticulum Ca(2+) stores in astrocytes

Astrocytes can modulate synaptic transmission by releasing glutamate in a Ca(2+)-dependent manner. Although the internal Ca(2+) stores have been implicated as the predominant source of Ca(2+) necessary for this glutamate release, the contribution of different classes of these stores is still not well defined. To address this issue, we cultured purified solitary cortical astrocytes and monitored changes in their internal Ca(2+) levels and glutamate release into the extracellular space. Ca(2+) levels were monitored by using the Ca(2+) indicator fluo-3 and quantitative fluorescence microscopy. Glutamate release was monitored by an L-glutamate dehydrogenase-linked detection system. Astrocytes were mechanically stimulated with a glass pipette, which reliably caused an increase in internal Ca(2+) levels and glutamate release into the extracellular space. Although we find that the presence of extracellular Cd(2+), a Ca(2+) channel blocker, significantly reduces mechanically induced glutamate release from astrocytes, we confirm that internal Ca(2+) stores are the predominant source of Ca(2+) necessary for this glutamate release. To test the involvement of different classes of internal Ca(2+) stores, we used a pharmacological approach. We found that diphenylboric acid 2-aminoethyl ester, a cell-permeable inositol 1,4,5-trisphosphate (IP(3)) receptor antagonist, greatly reduced mechanically induced glutamate release. Additionally, the preincubation of astrocytes with caffeine or ryanodine also reduced glutamate release. Taken together, our data are consistent with dual IP(3)- and caffeine/ryanodine-sensitive Ca(2+) stores functioning in the control of glutamate release from astrocytes.

Electroporation loading and photoactivation of caged InsP3: tools to investigate the relation between cellular ATP release in response to intracellular InsP3 elevation

Photolytic liberation of InsP(3) in single cells triggers cell-to-cell propagating calcium changes that are communicated by a gap junctional and a paracrine purinergic pathway involving InsP(3)-triggered ATP release. We investigated the relation between the InsP(3) stimulus and the resulting ATP release in ECV304 cells using UV photolysis of caged compounds and bioluminescent ATP measurements. Careful consideration of all steps, starting from caged InsP(3) loading into the cells by electroporation, up to photoliberation upon UV exposure, allowed to derive a dose-response relation that revealed a first part with a flattening ATP release response in the below 10microM InsP(3) concentration range and a second phase of steeply increasing ATP release in response to above 10microM InsP(3) stimulation. ATP release triggered by below 10microM InsP(3) concentrations attained a level in the order of 30% above baseline ATP release, while the steeply increasing response to high InsP(3) concentrations attained values in the order of 150% above baseline. Our data indicate the involvement of low affinity InsP(3) receptor sites in the pathway leading to triggered ATP release, with activation of these receptors causing the release of 1-2% of the total cellular ATP pool.

Modulation of spike-mediated synaptic transmission by presynaptic background Ca2+ in leech heart interneurons

At the core of the rhythmically active leech heartbeat central pattern generator are pairs of mutually inhibitory interneurons. Synaptic transmission between these interneurons consists of spike-mediated and graded components, both of which wax and wane on a cycle-by-cycle basis. Low-threshold Ca2+ currents gate the graded component. Ca imaging experiments indicate that these low-threshold currents are widespread in the neurons and that they contribute to neuron-wide changes in internal background Ca2+ concentration (Ivanov and Calabrese, 2000). During normal rhythmic activity, background Ca2+ concentration oscillates, and thus graded synaptic transmission waxes and wanes as the neurons move from the depolarized to the inhibited phases of their activity. Here we show that in addition to gating graded transmitter release, the background Ca2+ concentration changes evoked by low-threshold Ca2+ currents modulate spike-mediated synaptic transmission. We develop stimulation paradigms to simulate the changes in baseline membrane potential that accompany rhythmic bursting. Using Ca imaging and electrophysiological measurements, we show that the strength of spike-mediated synaptic transmission follows the changes in background Ca2+ concentration that these baseline potential changes evoke and that it does not depend on previous spike activity. Moreover, we show using internal EGTA and photo-release of caged Ca2+ and caged Ca2+ chelator that changes in internal Ca2+ concentration modulate spike-mediated synaptic transmission. Thus activity-dependent changes in background Ca2+, which have been implicated in homeostatic regulation of intrinsic membrane currents and synaptic strength, may also regulate synaptic transmission in an immediate way to modulate synaptic strength cycle by cycle in rhythmically active networks.

High-resolution iontophoresis for single-synapse stimulation

We present a technique for stimulating post-synaptic receptors with neurotransmitter locally at a single synapse and with a concentration profile that is comparable to endogenous stimulation. We modify the technique of iontophoresis to use a 0.1 microm electrode tip for local stimulation, and we combine it with fast capacitance compensation to achieve high-speed application from a high-resistance tip. Ejection of fluorescent dye from the electrode shows that transmitter can be limited to the width of a single synapse and to a time scale similar to an endogenous event. The speed and localization of transmitter is confirmed by iontophoretically stimulating single labeled synapses in cultured hippocampal neurons held under voltage clamp. The amount of transmitter ejected is linear and reproducible over a physiologically relevant range, making this technique useful for examining receptor kinetics and receptor insertion/removal. The system should be capable of delivering any charged neurotransmitter, and we show examples using glutamate and GABA. The technique is also combined with computer-controlled manipulation to study the strength and plasticity of multiple synapses in real-time.

Functional visualization of the separate but interacting calcium stores sensitive to NAADP and cyclic ADP-ribose

Cells possess multiple Ca(2+) stores and their selective mobilization provides the spatial-temporal Ca(2+) signals crucial in regulating diverse cellular functions. Except for the inositol trisphosphate (IP(3))-sensitive Ca(2+) stores, the identities and the mechanisms of how these internal stores are mobilized are largely unknown. In this study, we describe two Ca(2+) stores, one of which is regulated by cyclic ADP-ribose (cADPR) and the other by nicotinic acid adenine dinucleotide phosphate (NAADP). We took advantage of the large size of the sea urchin egg and stratified its organelles by centrifugation. Using photolysis to produce either uniform or localized increases of cADPR and NAADP from their respective caged analogs, the two separate stores could be visually identified by Ca(2+) imaging and shown to be segregated to the opposite poles of the eggs. The cADPR-pole also contained the IP(3)-sensitive Ca(2+) stores, the egg nucleus and the endoplasmic reticulum (ER); the latter was visualized using Bodipy-thapsigargin. On the other hand, the mitochondria, as visualized by rhodamine 123, were segregated to the opposite pole together with the NAADP-sensitive calcium stores. Fertilization of the stratified eggs elicited a Ca(2+) wave starting at the cADPR-pole and propagating toward the NAADP-pole. These results provide the first direct and visual evidence that the NAADP-sensitive Ca(2+) stores are novel and distinct from the ER. During fertilization, communicating signals appear to be transmitted from the ER to NAADP-sensitive Ca(2+) stores, leading to their activation.

Physiological astrocytic calcium levels stimulate glutamate release to modulate adjacent neurons

Astrocytes can release glutamate in a calcium-dependent manner and consequently signal to adjacent neurons. Whether this glutamate release pathway is used during physiological signaling or is recruited only under pathophysiological conditions is not well defined. One reason for this lack of understanding is the limited knowledge about the levels of calcium necessary to stimulate glutamate release from astrocytes and about how they compare with the range of physiological calcium levels in these cells. We used flash photolysis to raise internal calcium in astrocytes, while monitoring astrocytic calcium levels and glutamate, which evoked slow inward currents that were recorded electrophysiologically from single neurons grown on microislands of astrocytes. With this approach, we demonstrate that modest changes of astrocytic calcium, from 84 to 140 nM, evoke substantial glutamatergic currents in neighboring neurons (-391 pA), with a Hill coefficient of 2.1 to 2.7. Because the agonists glutamate, norepinephrine, and dopamine all raise calcium in astrocytes to levels exceeding 1.8 microM, these quantitative studies demonstrate that the astrocytic glutamate release pathway is engaged at physiological levels of internal calcium. Consequently, the calcium-dependent release of glutamate from astrocytes functions within an appropriate range of astrocytic calcium levels to be used as a signaling pathway within the functional nervous system.