Postsynaptic activation at the squid giant synapse by photolytic release of L-glutamate from a 'caged' L-glutamate

Blue and Green Light Responsive Caged Glutamate

Glutamate (Glu) is an excitatory neurotransmitter that plays a critical role in memory. Brain mapping activities of such pathways relied heavily on the ability to release Glu with spatiotemporal precision. Several photo-protecting groups (PPGs), referred to as photocages or cages, were designed to accomplish the release of Glu upon irradiation. Previously reported Glu cages responded to UV upon irradiation with single photons, which limited their use in vivo experiments due to cytotoxicity. Other caged designs suffered from lower quantum efficiency (QE) of release necessitating higher concentrations and/or longer photoirradiation times. There have been limited examples of cages that respond to visible light with single photon irradiation. Herein, we report the efficient preparation of 11 caged Glu examples that respond to two visible wavelengths, 467 nm (thiocoumarin based) and 515-540 nm (BODIPY based). The kinetics of photouncaging were studied for all caged designs, and we report all quantum efficiencies, i.e., quantum yields (Φ), that ranged from 0.0001-0.65. Two of the BODIPY cages are reported here for the first time, and one, Me-BODIPY-Br-Glu, shows the most efficient Glu release with a QE of 0.65. Similar caged designs can be extended to the inhibitory neurotransmitter, GABA. This would enable the use of two visible wavelengths to modulate the release of excitatory and inhibitory neurotransmitters upon demand via optical control.

Sensitized 1-Acyl-7-nitroindolines with Enhanced Two-Photon Cross Sections for Release of Neurotransmitters

Precise photochemical control, using two-photon excitation (2PE), of the timing and location of activation of glutamate is useful for studying the molecular and cellular physiology of the brain. Antenna-based light harvesting strategies represent a general method to increase the sensitivity to 2PE of otherwise insensitive photoremovable protecting groups (PPGs). This was applied to the most commonly used form of "caged" glutamate, MNI-Glu. Computational investigation showed that a four- or six-carbon linker attached between the 4-position of thioxanthone (THX) and the 4-position of the 5-methyl derivative of MNI-Glu (MMNI-Glu) would position the antenna and PPG close to one another to enable Dexter energy transfer. Nine THX-MMNI-Glu conjugates were prepared and their photochemical properties determined. Installation of the THX antenna resulted in a red shift of the absorption (λ_max = 385-405 nm) along with increased quantum yield compared to the parent compound MNI-Glu (λ_max = 347 nm). The THX-MMNI-Glu conjugate with a four-carbon linker and attachment to the 4-position of THX underwent photolysis via 1PE at 405 and 430 nm and via 2PE at 770 and 860 nm, yielding glutamate. The two-photon uncaging action cross section (δ_u) was 0.11 and 0.29 GM at 770 and 860, respectively, which was greater than for MNI-Glu (0.06 and 0.072 GM at 720 and 770 nm, respectively). The THX sensitizer harvested the light via 2PE and transferred its resulting triplet energy to MMNI-Glu. Release of glutamate through 2PE at 860 nm from the compound (100 μM) activated iGluSnFR, a genetically encoded, fluorescent glutamate sensor, on the surface of cells in culture, portending its usefulness in studies of neurophysiology in acute brain slice.

Immunocytochemical localization of the AMPA glutamate receptor subtype GluR2/3 in the squid optic lobe

As a major excitatory neurotransmitter in the cephalopod visual system, glutamate signaling is facilitated by ionotropic receptors, such as α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPAR). In cephalopods with large and well-developed brains, the optic lobes (OL) mainly process visual inputs and are involved in learning and memory. Although the presence of AMPAR in squid OL has been reported, the organization of specific AMPAR-containing neurons remains unknown. This study aimed to investigate the immunocytochemical localization of the AMPA glutamate receptor subtype 2/3-immunoreactive (GluR2/3-IR) neurons in the OL of Pacific flying squid (Tordarodes pacificus). Morphologically diverse GluR2/3-IR neurons were predominantly located in the tangential zone of the medulla. Medium-to-large GluR2/3-IR neurons were also detected. The distribution patterns and cell morphologies of calcium-binding protein (CBP)-IR neurons, specifically calbindin-D28K (CB)-, calretinin (CR)-, and parvalbumin (PV)-IR neurons, were similar to those of GluR2/3-IR neurons. However, two-color immunofluorescence revealed that GluR2/3-IR neurons did not colocalize with the CBP-IR neurons. Furthermore, the specific localizations and diverse types of GluR2/3-IR neurons that do not express CB, CR, or PV in squid OL were determined. These findings further contribute to the existing data on glutamatergic visual systems and provide new insights for understanding the visual processing mechanisms in cephalopods.

Photoactivatable AMPA for the study of glutamatergic neuronal transmission using two-photon excitation

We report a photoactivatable agonist of the AMPA subtype of ionotropic glutamate receptors, TMP-CyHQ-AMPA, which was designed to study the fast excitatory transmission between neurons. Upon visible light excitation, TMP-CyHQ-AMPA quantitatively released AMPA in high quantum yield on an ultra-short timescale. Intriguingly, the photolyisis can be carried out using 2-photon excitation (2PE) with remarkable efficiency, giving a two-photon uncaging action cross section (δu) value of 1.71 GM. TMP-CyHQ-AMPA is soluble in pysiological buffer and no hydrolysis was detected in the absence of light. Molecular docking experiments indicated that the photocaging strategy abolishes the affinity of AMPA for the GluR2 receptor and no GABAergic effects (as commonly observed in caged glutamates) are expected. TMP-CyHQ-AMPA can be used to study glutamatergic neuronal transmission with exceptional spatial-temporal resolution in complex tissue preparations.

Two-Photon Uncaging of Glutamate

Two-photon microscopy produces the excited singlet state of a chromophore with wavelengths approximately double that used for normal excitation. Two photons are absorbed almost simultaneously, via a virtual state, and this makes the excitation technique inherently non-linear. It requires ultra-fast lasers to deliver the high flux density needed to access intrinsically very short lived intermediates, and in combination with lenses of high numerical aperture, this confines axial excitation highly. Since the two-photon excitation volume is similar to a large spine head, the technique has been widely used to study glutamatergic transmission in brain slices. Here I describe the principles of two-photon uncaging of glutamate and provide a practical guide to its application.

A Caged Enkephalin Optimized for Simultaneously Probing Mu and Delta Opioid Receptors

Physiological responses to the opioid neuropeptide enkephalin often involve both mu and delta opioid receptors. To facilitate quantitative studies into opioid signaling, we previously developed a caged [Leu5]-enkephalin that responds to ultraviolet irradiation, but its residual activity at delta receptors confounds experiments that involve both receptors. To reduce residual activity, we evaluated side-chain, N-terminus, and backbone caging sites and further incorporated the dimethoxy-nitrobenzyl moiety to improve sensitivity to ultraviolet light-emitting diodes (LEDs). Residual activity was characterized using an in vitro functional assay, and the power dependence and kinetics of the uncaging response to 355 nm laser irradiation were assayed using electrophysiological recordings of mu opioid receptor-mediated potassium currents in brain slices of rat locus coeruleus. These experiments identified N-MNVOC-LE as an optimal compound. Using ultraviolet LED illumination to photoactivate N-MNVOC-LE in the CA1 region of hippocampus, we found that enkephalin engages both mu and delta opioid receptors to suppress inhibitory synaptic transmission.

A Visible-Light-Sensitive Caged Serotonin

Serotonin, or 5-hydroxytryptamine (5HT), is an important neurotransmitter in the nervous system of both vertebrates and invertebrates. Deficits in 5HT signaling are responsible for many disabling psychiatric conditions, and its molecular machinery is the target of many pharmaceuticals. We present a new 5HT phototrigger, the compound [Ru(bpy)₂(PMe₃)(5HT)]²⁺, where PMe₃ is trimethylphosphine. As with other ruthenium-bipyridyl based caged compounds, [Ru(bpy)₂(PMe₃)(5HT)]²⁺ presents activity in the visible region of the spectrum. We characterize and discuss the photochemical properties of the caged compound, and demonstrate its use by modulating the excitability of mouse prefrontal principal neurons.

A caged substrate peptide for matrix metalloproteinases

Based on the widely applied fluorogenic peptide FS-6 (Mca-Lys-Pro-Leu-Gly-Leu-Dpa-Ala-Arg-NH2; Mca = methoxycoumarin-4-acetyl; Dpa = N-3-(2,4-dinitrophenyl)l-α,β-diaminopropionyl) a caged substrate peptide Ac-Lys-Pro-Leu-Gly-Lys*-Lys-Ala-Arg-NH2 (*, position of the cage group) for matrix metalloproteinases was synthesized and characterized. The synthesis implies the modification of a carbamidated lysine side-chain amine with a photocleavable 2-nitrobenzyl group. Mass spectrometry upon UV irradiation demonstrated the complete photolytic cleavage of the protecting group. Time-resolved laser-flash photolysis at 355 nm in combination with transient absorption spectroscopy determined the biphasic decomposition with τa = 171 ± 3 ms (79%) and τb = 2.9 ± 0.2 ms (21%) at pH 6.0 of the photo induced release of the 2-nitrobenzyl group. The recombinantly expressed catalytic domain of human membrane type I matrix metalloproteinase (MT1-MMP or MMP-14) was used to determine the hydrolysis efficiency of the caged peptide before and after photolysis. It turned out that the cage group sufficiently shields the peptide from peptidase activity, which can be thus controlled by UV light.

Caged glutamates with π-extended 1,2-dihydronaphthalene chromophore: design, synthesis, two-photon absorption property, and photochemical reactivity

Caging and photochemical uncaging of the excitatory neurotransmitter l-glutamate (glu) offers a potentially valuable tool for understanding the mechanisms of neuronal processes. Designing water-soluble caged glutamates with the appropriate two-photon absorption property is an attractive strategy to achieve this. This paper describes the design, synthesis, and photochemical reactivity of caged glutamates with π-extended 1,2-dihydronaphthalene structures, which possess a two-photon cross-section of ∼120 GM and an excellent buffer solubility (up to 115 mM). High yields up to 99% glutamate were observed in the photolysis of two caged glutamates. Suzuki-Miyaura cross-coupling and Buchwald-Hartwig amination were used as the key reactions to synthesize the caged compounds.

Synthesis and biological evaluation of bis-CNB-GABA, a photoactivatable neurotransmitter with low receptor interference and chemical two-photon uncaging properties

Photoactivatable "caged" neurotransmitters allow optical control of neural tissue with high spatial and temporal precision. However, the development of caged versions of the chief vertebrate inhibitory neurotransmitter, γ-amino butyric acid (GABA), has been limited by the propensity of caged GABAs to interact with GABA receptors. We describe herein the synthesis and application of a practically useful doubly caged GABA analog, termed bis-α-carboxy-2-nitrobenzyl-GABA (bis-CNB-GABA). Uncaging of bis-CNB-GABA evokes inward GABAergic currents in cerebellar molecular layer interneurons with rise times of 2 ms, comparable to flash duration. Response amplitudes depend on the square of flash intensity, as expected for a chemical two-photon uncaging effect. Importantly, prior to uncaging, bis-CNB-GABA is inactive at the GABAA receptor, evoking no changes in holding current in voltage-clamped neurons and showing an IC50 of at least 2.5 mM as measured using spontaneous GABAergic synaptic currents. Bis-CNB-GABA is stable in solution, with an estimated half-life of 98 days in the light. We expect that bis-CNB-GABA will prove to be an effective tool for high-resolution chemical control of brain circuits.

Microtransplantation of cellular membranes from squid stellate ganglion reveals ionotropic GABA receptors

The squid has been the most studied cephalopod, and it has served as a very useful model for investigating the events associated with nerve impulse generation and synaptic transmission. While the physiology of squid giant axons has been extensively studied, very little is known about the distribution and function of the neurotransmitters and receptors that mediate inhibitory transmission at the synapses. In this study we investigated whether γ-aminobutyric acid (GABA) activates neurotransmitter receptors in stellate ganglia membranes. To overcome the low abundance of GABA-like mRNAs in invertebrates and the low expression of GABA in cephalopods, we used a two-electrode voltage clamp technique to determine if Xenopus laevis oocytes injected with cell membranes from squid stellate ganglia responded to GABA. Using this method, membrane patches containing proteins and ion channels from the squid's stellate ganglion were incorporated into the surface of oocytes. We demonstrated that GABA activates membrane receptors in cellular membranes isolated from squid stellate ganglia. Using the same approach, we were able to record native glutamate-evoked currents. The squid's GABA receptors showed an EC(50) of 98 μmol l(-1) to GABA and were inhibited by zinc (IC(50) = 356 μmol l(-1)). Interestingly, GABA receptors from the squid were only partially blocked by bicuculline. These results indicate that the microtransplantation of native cell membranes is useful to identify and characterize scarce membrane proteins. Moreover, our data also support the role of GABA as an ionotropic neurotransmitter in cephalopods, acting through chloride-permeable membrane receptors.

A chemist and biologist talk to each other about caged neurotransmitters

Caged compounds are small organic molecules that can be photoactivated with brief pulses of light. They are widely used to study a great variety of biological processes by physiologists, cell biologists and neuroscientists. Initially made and invented by biologists in the late 1970s, they are now made mostly by chemists, often without any dialogue with the end users, the biologists. The idea for this review is to stimulate interaction between the two communities to further the creative development and application of these powerful optical probes.

New caged neurotransmitter analogs selective for glutamate receptor sub-types based on methoxynitroindoline and nitrophenylethoxycarbonyl caging groups

Photolysis is widely used in experimental neuroscience to isolate post-synaptic receptor activation from presynaptic processes, to determine receptor mechanisms in situ, for pharmacological dissection of signaling pathways, or for photostimulation/inhibition in neural networks. We have evaluated new caged neuroactive amino acids that use 4-methoxy-7-nitroindolinyl- (MNI) or 1-(2-nitrophenyl)ethoxycarbonyl (NPEC) photoprotecting groups to make caged ligands specific for glutamate receptor sub-types. Each was tested for interference with synaptic transmission and excitability and for receptor-specific actions in slice preparations. No adverse effects were found at glutamate receptors. At high concentration, MNI-caged, but not NPEC-caged ligands, interfered with GABA-ergic transmission. MNI-caged amino acids have sub-microsecond release times suitable for investigating mechanisms at fast synaptic receptors in situ. MNI-NMDA and MNI-kainate were synthesized and tested. MNI-NMDA showed stoichiometric release of chirally pure NMDA. Wide-field photolysis in cerebellar interneurons produced a fast-rising sustained activation of NMDA receptors, and localized laser photolysis gave a fast, transient response. Photolysis of MNI-kainate to release up to 4 μM kainate generated large inward currents at resting membrane potential in Purkinje neurons. Application of GYKI 53655 indicated that 40% of the current was due to AMPA receptor activation by kainate. Signaling via metabotropic glutamate receptors (mGluR) does not require fast release rates. NPEC cages are simpler to prepare but have slower photorelease. Photolysis of NPEC-ACPD or NPEC-DHPG in Purkinje neurons generated slow inward currents blocked by the mGluR type 1 antagonist CPCCOEt similar to the slow sEPSC seen with parallel fiber burst stimulation. NPEC-AMPA was also tested in Purkinje neurons and showed large sustained inward currents selective for AMPA receptors with little activation of kainate receptors. MNI-caged l-glutamate, NMDA and kainate inhibit GABA-A receptors with IC₅₀ concentrations close to the maximum concentrations useful in receptor signaling experiments.

The age of enlightenment: evolving opportunities in brain research through optical manipulation of neuronal activity

Optical manipulation of neuronal activity has rapidly developed into the most powerful and widely used approach to study mechanisms related to neuronal connectivity over a range of scales. Since the early use of single site uncaging to map network connectivity, rapid technological development of light modulation techniques has added important new options, such as fast scanning photostimulation, massively parallel control of light stimuli, holographic uncaging, and two-photon stimulation techniques. Exciting new developments in optogenetics complement neurotransmitter uncaging techniques by providing cell-type specificity and in vivo usability, providing optical access to the neural substrates of behavior. Here we review the rapid evolution of methods for the optical manipulation of neuronal activity, emphasizing crucial recent developments.

Optical delivery of liposome encapsulated chemical stimuli to neuronal cells

Spatially confined and precise time delivery of neuroactive molecules is an important issue in neurophysiology. In this work we developed a technique for delivering chemical stimuli to cultured neurons consisting in encapsulating the molecules of interest in liposomes. These vectors were then loaded in reservoirs consisting of glass capillaries. The reservoirs were placed in the recording chamber and single liposomes were trapped and transported out by optical tweezers to the site of stimulation on cultured neurons. Finally, the release of liposome content was induced by application of UV-pulses, breaking the liposome membrane. The efficiency of encapsulation and release were first evaluated by loading the liposomes with fluorescein. In order to test the effect of the UV-induced release, liposomes with diameter ranging from 1 to 10 μm (fL to pL volumes), were filled with KCl and tested on neuronal cells. Neuronal cultures, loaded with Ca(2+) dye, were monitored by imaging intracellular Ca(2+). An efficient release from the liposomes was demonstrated by detectable calcium signals, indicating stimulated depolarization of the neuronal cells by KCl. The present technique represents an alternative method for focal chemical stimulation of cultured cells that circumvents some of the limitations of microejection and photorelease of caged compounds.

L-glutamate and its ionotropic receptors in the nervous system of cephalopods

In several species of cephalopod molluscs there is good evidence for the presence of L-glutamate in the central and peripheral nervous system and evidence for both classes of ionotropic receptor, AMPA/kainate and NMDA.The best evidence for glutamate being a transmitter in cephalopods comes from pharmacological, immunohistochemical and molecular investigations on the giant fibre system in the squid stellate ganglion. These studies confirm there are AMPA/kainate-like receptors on the third-order giant axon. In the (glial) Schwann cells associated with the giant axons both classes of glutamate receptor occur.Glutamate is an excitatory transmitter in the chromatophores and in certain somatic muscles and its action is mediated primarily via AMPA/kainate-like receptors, but at some chromatophores there are NMDA-like receptors.In the statocysts the afferent crista fibres are also glutamatergic, acting at non-NMDA receptors.In the brain (of Sepia) a neuronal NOS is activated by glutamate with subsequent production of nitric oxide and elevation of cGMP levels. This signal transduction pathway is blocked by D-AP-5, a specific antagonist of the NMDA receptor.Recently immunohistochemical analysis has demonstrated (in Sepia and Octopus) the presence of NMDAR2A /B - like receptors in motor centres, in the visual and olfactory systems and in the learning system. Physiological experiments have shown that glutamatergic transmission is involved in long term potentation (LTP) in the vertical lobe of Octopus, a brain area involved in learning. This effect seems to be mediated by non-NMDA receptors. Finally in the CNS of Sepia NMDA-mediated nitration of tyrosine residues of cytoskeletal protein such as alpha-tubulin, has been demonstrated.

Pre- and postsynaptic excitation and inhibition at octopus optic lobe photoreceptor terminals; implications for the function of the 'presynaptic bags'

Synaptic transmission was examined in the plexiform zone of Octopus vulgaris optic lobes using field-potential recording from optic lobe slices. Stimulation of the optic nerve produced pre- and postsynaptic field potentials. Transmission was abolished in calcium-free seawater, L- glutamate or the AMPA/Kainate receptor blocker CNQX (EC(50), 40 microm), leaving an intact presynaptic field potential. ACh markedly reduced or blocked and d-tubocurarine augmented both pre- and postsynaptic field potentials, while alpha-bungarotoxin and atropine were without effect. Paired-pulse stimulation showed short-term depression of pre- and postsynaptic components with a half-time of recovery of approximately 500 ms. The depression was partially relieved in the presence of d-tubocurarine (half-time of recovery, 350 ms). No long-term changes in synaptic strength were induced by repetitive stimulation. A polyclonal antibody raised against a squid glutamate receptor produced positive staining in the third radial layer of the plexiform zone. No positive staining was observed in the other layers. Taking into account previous morphological data and our results, we propose that the excitatory terminations of the photoreceptors are in the innermost layer of the plexiform zone where the transmitter is likely to be glutamate and postsynaptic receptors are AMPA/kainate-like. Thus, the function of the terminal bags is to provide a location for a presynaptic cholinergic inhibitory shunt. The results imply that this arrangement provides a temporal filter for visual processing and enhances the perception of moving vs. stationary objects.

Optical switches and triggers for the manipulation of ion channels and pores

Like fluorescence sensing techniques, methods to manipulate proteins with light have produced great advances in recent years. Ion channels have been one of the principal protein targets of photoswitched manipulation. In combination with fluorescence detection of cell signaling, this has enabled non-invasive, all-optical experiments on cell and tissue function, both in vitro and in vivo. Optical manipulation of channels has also provided insights into the mechanism of channel function. Optical control elements can be classified according to their molecular reversibility as non-reversible phototriggers where light breaks a chemical bond (e.g. caged ligands) and as photoswitches that reversibly photoisomerize. Synthetic photoswitches constitute nanoscale actuators that can alter channel function using three different strategies. These include (1) nanotoggles, which are tethered photoswitchable ligands that either activate channels (agonists) or inhibit them (blockers or antagonists), (2) nanokeys, which are untethered (freely diffusing) photoswitchable ligands, and (3) nanotweezers, which are photoswitchable crosslinkers. The properties of such photoswitches are discussed here, with a focus on tethered photoswitchable ligands. The recent literature on optical manipulation of ion channels is reviewed for the different channel families, with special emphasis on the understanding of ligand binding and gating processes, applications in nanobiotechnology, and with attention to future prospects in the field.

Kinetics of proton binding to the sarcoplasmic reticulum Ca-ATPase in the E1 state

A new caged proton, 2-methoxy-5-nitrophenyl sulfate, was synthesized and used in time-resolved pH jump experiments to study proton binding in the sarcoplasmic reticulum Ca-ATPase. The major advantage of this compound is that it does not produce significant artifacts in experiments in which the fluorescent styryl dye 2BITC is used to monitor ion movements in the Ca pump. Two rate-limiting processes were resolved and their dependence on pH, Ca(2+) concentration, and temperature investigated. The faster process showed a relaxation time between 4 and 8 ms independent on pH and Ca(2+) concentration, and the time constant of the slower process varied between 31 ms (0 Ca(2+)) and 100 ms (100 microM Ca(2+)). A consistent mechanism to explain the results was derived in agreement with previous studies and the generally accepted Post-Albers scheme of the pump cycle. This mechanism requires that under physiological conditions the ion-binding sites are always occupied and two protons and a Ca(2+) ion replace each other. In the absence of ATP at low pH a nonphysiological state can be induced in which up to four protons bind to the Ca pump in the E(1) conformation. So far it could not be verified whether these additional protons bind to amino acid side chains or are coordinated as hydronium ions.

2,5-Dimethylphenacyl carbamate: a photoremovable protecting group for amines and amino acids

2,5-Dimethylphenacyl (DMP) carbamates (1a-c) released the corresponding free amines or amino acids in high chemical yields, albeit with quantum yields Phi of only 0.04-0.09, upon irradiation in either aprotic or protic solvents. The photoreaction proceeded principally from the triplet excited state via the E-photoenol. The lifetimes of the triplet enol and the E- and Z-enols in the ground state were determined by laser flash photolysis. The primary photoinitiated transformation liberated a carbamic acid derivative, which subsequently decarboxylated to the amino group-containing compound. Exhaustive irradiation of a DMP-protected aniline (1a) in acetonitrile did not provide aniline in quantitative chemical yields, because it was involved in reductive cleavage of the starting material as an electron donor, thereby decreasing the overall deprotection yield (86%). Phenylalanine methyl ester, liberated from 1c, was, however, obtained in excellent chemical yield (97%). It was also found that the carbamates, while thermally stable, released amines with higher quantum yields in acidic methanol solutions. The DMP chromophore is proposed as an excellent photoremovable protecting group for amino acids and, under specific conditions, for amines in organic synthesis and biochemistry.

Modulation of an AMPA-like glutamate receptor (SqGluR) gating by L- and D-aspartic acids

L- and D-aspartic acids (L-Asp and D-Asp) are present in the majority of nervous systems. In phylogeny, significant levels have been reported in mollusc brains, particularly cephalopods. To examine the role of L- and D-Asp on a cephalopod receptor, we studied ligand gating of a squid glutamate receptor (SqGluR) expressed in HEK 239 (human embryonic kidney) cells. Under voltage clamp, application of L-glutamate (L-Glu; 1-30 mM), but not D-glutamate (D-Glu), or L- or D-Asp, evoked an inward current of 0.1 nA. L- or D-Asp (200 microM) applied with 20 mM L-Glu, slowed the time course of activation and inactivation of the L-Glu gated current (time constant increased from 1 s (L-Glu alone) to 3 s (D-Asp and L-Glu) and to 19 s (L-Asp and L-Glu)). Our results suggest that in molluscan systems, aspartic acid could act as a neuromodulator during glutamatergic transmission and could significantly alter synaptic integration by slowing glutamate receptor gating.

N-methyl-D-aspartate receptor-like immunoreactivity in the brain of Sepia and Octopus

Ionotropic glutamate receptors have been subdivided into N-methyl-D-aspartate (NMDA) and AMPA/kainate classes. NMDA receptor subunit 2A and 2B immunoreactivity is shown to be present in specific regions of the central nervous system (CNS) of the cephalopod molluscs Sepia officinalis and Octopus vulgaris. An antibody that recognizes both mammalian NMDAR2A and NMDAR2B subunits equally was used. SDS-PAGE/Western blot analysis performed on membrane proteins revealed an immunoreactive band at 170 kDa for both species. Immunoreactive bands from both Octopus and Sepia brains disappeared when the antibody was preabsorbed with membrane proteins from rat hippocampus or from their own brains. The same antibody was then used for immunohistochemical staining of serial sections of the CNS to reveal localized specific staining of cell bodies and fibers in several lobes of the brain. Staining was found in lower motor centers, in some higher motor centers, in learning centers, and in the optic lobes. Immunopositivity was also found in the areas of brain that control the activity of the optic gland, a gonadotropic endocrine gland. These findings suggest that glutamate, via NMDA receptors, may be involved as a signaling molecule in motor, learning, visual, and olfactory systems in the cephalopod brain.

Flash photolysis reveals a diversity of ionotropic glutamate receptors on the mitral cell somatodendritic membrane

It is widely held that the soma and basal dendrites of olfactory bulb mitral cells receive exclusively inhibitory synaptic input from local interneurons. However, the mitral somatodendritic membrane exhibits immunoreactivity for a variety of glutamate receptors, and blocking GABA receptors unmasks mitral cell self-excitation. This excitation is proposed to be mediated either by diffuse spillover of the mitral cells' own released glutamate, or by punctate transmission from glutamate-releasing granule cells. This study examined the pharmacology and kinetics of glutamate sensitivity of mitral cells by flash photolysis of nitroindoline caged glutamates, which facilitate reliable activation of receptors in the synaptic cleft. Wide-field laser uncaging (3.5-ms flash) of approximately 0.5-1 mM glutamate onto the soma activated large currents with fast (3.4-ms rise, 7.5-ms decay) and slow (64-ms rise, >10-s decay) components. In 100 microM APV, slow currents were reduced to 53% of control (257-ms rise, 2-s decay), displayed outward rectification in 1.3 mM Mg2+, and blocked by 15 microM 5,7-dichlorokynurenate. Responses to less, similar 100 microM glutamate were fully antagonized by 100 microM APV, consistent with competitive inhibition at high-affinity NMDA receptors. An APV-resistant NMDA receptor was not observed, refuting the punctate transmission model. Fast currents were blocked by 10 microM NBQX, boosted 3.28-fold by 100 microM cyclothiazide, and resolved into AMPA (40%) and kainate (60%) receptor components by 100 microM SYM2206. The results suggest that self-excitation depends on AMPA, kainite, and conventional NMDA autoreceptors on the mitral cell.

Cloning and characterization of an ionotropic glutamate receptor subunit expressed in the squid nervous system

In this paper we describe the cloning of a putative ionotropic glutamate receptor subunit, SqGluR, and its distribution in the nervous system of the squid. A full-length cDNA was assembled from a cDNA library of the stellate ganglion/giant fibre lobe complex of Loligo opalescens. The deduced amino acid sequence of the mature SqGluR displayed 44-46% amino acid identity with mammalian GluR1-GluR4 and 53% with Lym-eGluR1 from Lymnaea stagnalis. In situ hybridizations in adult squid confirmed that the SqGluR mRNA is abundant in giant fibre lobe neurons, in large, presumptive motor neurons of the stellate ganglion proper and in the supraoesophageal and optic lobes of the central nervous system. In newborn squid, SqGluR mRNA expression was detected throughout the nervous system but not elsewhere. A synthetic peptide corresponding to the last 15 amino acids of the SqGluR C-terminus was used to generate polyclonal antibodies, which were used for immunoblot analysis to demonstrate widespread expression in the squid central and peripheral nervous systems. Injection of the synthetic peptide into the postsynaptic side of the giant synapse inhibited synaptic transmission.

Photochemical and pharmacological evaluation of 7-nitroindolinyl-and 4-methoxy-7-nitroindolinyl-amino acids as novel, fast caged neurotransmitters

Reagents capable of rapid and efficient release of neuroactive amino acids (L-glutamate, GABA and glycine) upon flash photolysis of thermally stable, inert precursors have been elusive. 7-Nitroindolinyl (NI)-caged and 4-methoxy-7-nitroindolinyl (MNI)-caged compounds that fulfil these criteria are evaluated here. These caged precursors are highly resistant to hydrolysis. Photolysis is fast (half time< or =0.26 ms) and the conversion achieved with a xenon flashlamp is about 15% for the NI-caged L-glutamate and about 35% for the MNI-caged L-glutamate. A procedure is described for calibration of photolysis in a microscope-based experimental apparatus. NI-caged L-glutamate itself showed no agonist or antagonist effects on AMPA and NMDA receptors in cultured neurones, and had no effect on climbing fibre activation of Purkinje neurones. A control compound with identical photochemistry that generated an inert phosphate upon photolysis was used to confirm that the intermediates and by-products of photolysis have no deleterious effects. MNI-caged L-glutamate is as stable and fast as NI-caged L-glutamate and similarly inert at glutamate receptors, but about 2.5 times more efficient. However, NI-caged GABA is an antagonist at GABA(A) receptors and NI-glycine an antagonist at glycine receptors. The results show the utility and limitations of these fast and stable caged neurotransmitters in the investigation of synaptic processes.

Determinants of chemotactic signal amplification in Escherichia coli

A well-characterized protein phosphorelay mediates Escherichia coli chemotaxis towards the amino acid attractant aspartate. The protein CheY shuttles between flagellar motors and methyl-accepting chemoreceptor (MCP) complexes containing the linker CheW and the kinase CheA. CheA-CheY phosphotransfer generates phospho-CheY, CheY-P. Aspartate triggers smooth swim responses by inactivation of the CheA bound to the target MCP, Tar; but this mechanism alone cannot explain the observed response sensitivity. Here, we used behavioral analysis of mutants deleted for CheZ, a catalyst of CheY-P dephosphorylation, or the methyltransferase CheR and/or the methylesterase CheB to examine the roles of accelerated CheY-P dephosphorylation and MCP methylation in enhancement of the chemotactic response. The extreme motile bias of the mutants was adjusted towards wild-type values, while preserving much of the aspartate response sensitivity by expressing fragments of the MCP, Tsr, that either activate or inhibit CheA. We then measured responses to small jumps of aspartate, generated by flash photolysis of photo-labile precursors. The stimulus-response relation for Delta cheZ mutants overlapped that for the host strains. Delta cheZ excitation response times increased with stimulus size consistent with formation of an occluded CheA state. Thus, neither CheZ-dependent or independent increases in CheY-P dephosphorylation contribute to the excitation response. In Delta cheB Delta cheR or Delta cheR mutants, the dose for a half-maximal response, [Asp](50), was ca 10 microM; but was elevated to 100 microM in Delta cheB mutants. In addition, the stimulus-response relation for these mutants was linear, consistent with stoichiometric inactivation, in contrast to the non-linear relation for wild-type E. coli. These data suggest that response sensitivity is controlled by differential binding of CheR and/or CheB to distinct MCP signaling conformations.

Localization of L-glutamate and glutamate-like receptors at the squid giant synapse

HPLC analysis of the amino acid contents of the second- and third-order giant fibres at the giant synapse in the stellate ganglion of the squid Loligo vulgaris shows that there are significantly higher amounts of L-glutamate and L-aspartate in the second-order (presynaptic) fibre than in the third-order (postsynaptic) fibre. Immunocytochemical staining of sections of the ganglion with an antibody raised against L-glutamate produces specific positive staining in the synaptic region of the second-order fibre. In contrast, staining with antibodies raised against glutamate-receptors (mammalian GluR1 with GluR2/3) produces positive staining in the third-order fibre at the postsynaptic region. These data provide further evidence for the hypothesis that L-glutamate is an excitatory transmitter at the giant synapse.

Elucidation of a PTS-carbohydrate chemotactic signal pathway in Escherichia coli using a time-resolved behavioral assay

Chemotaxis of Escherichia coli toward phosphotransferase systems (PTSs)-carbohydrates requires phosphoenolpyruvate-dependent PTSs as well as the chemotaxis response regulator CheY and its kinase, CheA. Responses initiated by flash photorelease of a PTS substrates D-glucose and its nonmetabolizable analog methyl alpha-D-glucopyranoside were measured with 33-ms time resolution using computer-assisted motion analysis. This, together with chemotactic mutants, has allowed us to map out and characterize the PTS chemotactic signal pathway. The responses were absent in mutants lacking the general PTS enzymes EI or HPr, elevated in PTS transport mutants, retarded in mutants lacking CheZ, a catalyst of CheY autodephosphorylation, and severely reduced in mutants with impaired methyl-accepting chemotaxis protein (MCP) signaling activity. Response kinetics were comparable to those triggered by MCP attractant ligands over most of the response range, the most rapid being 11.7 +/- 3.1 s-1. The response threshold was <10 nM for glucose. Responses to methyl alpha-D-glucopyranoside had a higher threshold, commensurate with a lower PTS affinity, but were otherwise kinetically indistinguishable. These facts provide evidence for a single pathway in which the PTS chemotactic signal is relayed rapidly to MCP-CheW-CheA signaling complexes that effect subsequent amplification and slower CheY dephosphorylation. The high sensitivity indicates that this signal is generated by transport-induced dephosphorylation of the PTS rather than phosphoenolpyruvate consumption.

Distribution of functional glutamate receptors in cultured embryonic Drosophila myotubes revealed using focal release of L-glutamate from caged compound by laser

During the formation of neuromuscular junctions in Drosophila embryos, glutamate receptors undergo a drastic change in distribution. To study the underlying mechanism of this developmental process, it is desirable to map the distribution of functional receptors with accurate spatial resolution. Since glutamate receptors desensitize within several milliseconds, the agonist must be applied rapidly. To fulfil these requirements we used laser stimulation of a caged compound to release L-glutamate at a focal spot. Since the glutamate receptor channel is permeable to Ca2+, we assayed the change in internal Ca2+ concentration using a Ca2+ indicator, fluo-3. Using this approach, we mapped the distribution of functional glutamate receptors in cultured embryonic Drosophila myotubes and myoblasts. Consistent with previous immunofluorescence studies using an antibody against a glutamate receptor subunit, a large increase of internal Ca2+ concentration was observed when laser stimulation was located close to some nuclei in the myotube. No change was detected when the laser stimulus was applied over any regions of the myoblasts. No increase of the internal Ca2+ concentration in myotubes was observed when the external solution contained either glutamate at a desensitizing concentration (1 mM) or a glutamate receptor channel blocker, argiotoxin (1 microg/ml). These results indicate that a rise in intracellular Ca2+ concentration can be used to show the distribution of the functional receptor on the muscle surface membrane.

Regulation of neurotransmitter release kinetics by NSF

NSF (N-ethylmaleimide-sensitive factor) is an adenosine triphosphatase (ATPase) that contributes to a protein complex essential for membrane fusion. The synaptic function of this protein was investigated by injecting, into the giant presynaptic terminal of squid, peptides that inhibit the ATPase activity of NSF stimulated by the soluble NSF attachment protein (SNAP). These peptides reduced the amount and slowed the kinetics of neurotransmitter release as a result of actions that required vesicle turnover and occurred at a step subsequent to vesicle docking. These results define NSF as an essential participant in synaptic vesicle exocytosis that regulates the kinetics of neurotransmitter release and, thereby, the integrative properties of synapses.

N-Nmoc-L-glutamate, a new caged glutamate with high chemical stability and low pre-photolysis activity

We report the synthesis, the physicochemical characterization, and biological evaluation of a new caged glutamate, N-(o-nitromandelyl)oxycarbonyl-L-glutamic acid (Nmoc-Glu), that liberates free glutamate on photolysis. The low affinity of certain glutamate receptors and their rapid entry into desensitization have effectively prevented the creation of an ideal caged glutamate. In the absence of an ideal compound, Nmoc-Glu was designed to resist spontaneous hydrolysis while maintaining reasonable photorelease yield and kinetics. Chemical and physiological analyses reveal that Nmoc-Glu, indeed, has exceptionally low residual activity and high chemical stability. The quantum yield of Nmoc-Glu is 0.11. Photolytic uncaging and release of free glutamate occur in two steps, consisting of an initial light-induced cleavage that proceeds on the sub-millisecond time scale, and a subsequent light-independent, pH-dependent decarboxylation step that proceeds on the millisecond time scale. The low residual activity and high chemical stability of Nmoc-Glu are important advantages in applications where pre-photolysis Glu receptor activation and desensitization must be minimized.

Activation of NMDA receptors is necessary for the induction of associative long-term potentiation in area CA1 of the rat hippocampal slice

1. It is commonly assumed that the role of the strongly activated heterosynaptic input during the induction of associative long-term potentiation (LTP) is to relieve the magnesium blockade of NMDA receptors located at the weakly stimulated synapses and thereby allow the weak input to undergo potentiation. We tested this assumption by using a caged form of the NMDA receptor antagonist, D-(-)-2-amino-5-phosphonopentanoic acid (D-AP5) to block the activation of NMDA receptors at the weak input in a conditioning protocol for the induction of associative LTP in area CA1 of the rat hippocampal slice. 2. The effect of releasing D-AP5 by flash photolysis of 100 microM caged D-AP5 (N-[1-(2-nitrophenyl)ethoxycarbonyl]-D-AP5) on pharmacologically isolated NMDA receptor-mediated field EPSPs was examined in area CA1. The slope of the EPSP was reduced by 71% within 50 ms of the initiation of the photolytic reaction when the concentration of released D-AP5 had reached 2.0-2.5 microM and was reduced by 95% within 1 min (10 microM D-AP5 released). 3. Associative LTP was induced by pairing a strong tetanus to one input with a weak tetanus (subthreshold for homosynaptic LTP) to a second input. The strong tetanus preceded the weak by 50 ms. Rapid application of D-AP5, by flash photolysis of caged D-AP5, coincident with the last shock of the strong tetanus, resulted in the blockade of NMDA receptor activation during the period of the weak tetanus. Associative LTP was blocked by photolysis of caged D-AP5 but was normally expressed in experiments using caged L-AP5. 4. We conclude that activation of NMDA receptors at the weakly activated input is an essential requirement for synaptically induced associative LTP.

Kinetics of force generation by single kinesin molecules activated by laser photolysis of caged ATP

To relate transients of force by single kinesin molecules with the elementary steps of the ATPase cycle, we measured the time to force generation by kinesin after photorelease of ATP from caged ATP. Kinesin-coated beads were trapped by an infrared laser and brought onto microtubules fixed to a coverslip. Tension was applied to a kinesin-microtubule rigor complex using the optical trap, and ATP was released by flash photolysis of caged ATP with a UV laser. Kinesin started to generate force and move stepwise with a step size of 8 nm at average times of 31, 45, and 79 ms after photorelease of 450, 90, and 18 microM ATP, respectively. The kinetics of force generation were consistent with a two-step reaction: ATP binding, with an apparent second-order rate constant of 0.7 microM-1.s-1, followed by force generation at 45 s-1 per kinesin molecule. The transient rate of force generation was close to the rate of the ATPase cycle in solution, suggesting that the rate-limiting step of ATPase cycle is involved with the force generation.

Glutamate as a putative neurotransmitter in the mollusc, Lymnaea stagnalis

Bath-applied glutamate (10-1000 microM) produced excitatory and inhibitory responses on numerous identified neurons of the mollusc Lymnaea stagnalis. Using both in situ and in vitro preparations, glutamate or glutamate agonists produced a depolarization in identified neurons right pedal dorsal 1 and right pedal dorsal 2 and 3. However, attempts to block glutamate-evoked responses with glutamate antagonists were unsuccessful. We examined a potential glutamatergic neuron, visceral dorsal 4. Exogenous application of the peptides (GDPFLRFamide and SDPFLRFamide) could mimic the inhibitory, but not the excitatory effects of visceral dorsal 4 on its postsynaptic cells, implying the presence of a second transmitter. We tested the possibility that glutamate is this second neurotransmitter by using excitatory synapses between visceral dorsal 4 and postsynaptic cells right pedal dorsal 2 and 3, right pedal dorsal 1, visceral F group and right parietal B group neurons. Of all the putative neurotransmitters tested, only glutamate had consistent excitatory effects on these postsynaptic cells. Also, the amplitude of the right pedal dorsal 2 and 3 excitatory postsynaptic potentials was reduced in the presence of N-methyl-D-aspartate and other glutamate agonists, suggesting desensitization of the endogenous transmitter receptor. In conclusion, some identified Lymnaea neurons respond to glutamate via a receptor with novel pharmacological properties. Furthermore, a Lymnaea interneuron may employ glutamate as a transmitter at excitatory synapses.

Adaptation of Ca(2+)-triggered exocytosis in presynaptic terminals

Rapid increases in Ca2+ concentration, produced by photolysis of caged Ca2+, triggered exocytosis in squid nerve terminals. This exocytosis was transient in nature, decaying with a time constant of approximately 30 ms. The decay could not be explained by a decline in presynaptic Ca2+ concentration, depletion of synaptic vesicles, or desensitization of postsynaptic receptors. Experiments in which Ca2+ was increased either in a series of steps or continuously at different rates suggested that the decay is caused by adaptation of the exocytotic Ca2+ receptor to higher levels of Ca2+. This adjustable sensitivity to Ca2+ represents a novel property of the triggering mechanism that can be used to evaluate molecular models of exocytosis. Adaptation can limit the amount of transmitter released by a nerve terminal and permit the speed of a presynaptic Ca2+ rise to serve as a critical determinant of synaptic efficacy.

Laser photolysis of caged calcium: rates of calcium release by nitrophenyl-EGTA and DM-nitrophen

Nitrophenyl-EGTA and DM-nitrophen are Ca2+ cages that release Ca2+ when cleaved upon illumination with near-ultraviolet light. Laser photolysis of nitrophenyl-EGTA produced transient intermediates that decayed biexponentially with rates of 500,000 s-1 and 100,000 s-1 in the presence of saturating Ca2+ and 290,000 s-1 and 68,000 s-1 in the absence of Ca2+ at pH 7.2 and 25 degrees C. Laser photolysis of nitrophenyl-EGTA in the presence of Ca2+ and the Ca2+ indicator Ca-orange-5N produced a monotonic increase in the indicator fluorescence, which had a rate of 68,000 s-1 at pH 7.2 and 25 degrees C. Irradiation of DM-nitrophen produced similar results with somewhat slower kinetics. The transient intermediates decayed with rates of 80,000 s-1 and 11,000 s-1 in the presence of Ca2+ and 59,000 s-1 and 3,600 s-1 in the absence of Ca2+ at pH 7.2 and 25 degrees C. The rate of increase in Ca(2+)-indicator fluorescence produced upon photolysis of the DM-nitrophen: Ca2+ complex was 38,000 s-1 at pH 7.2 and 25 degrees C. In contrast, pulses in Ca2+ concentration were generated when the chelator concentrations were more than the total Ca2+ concentration. Photoreleased Ca2+ concentration stabilized under these circumstances to a steady state within 1-2 ms.

Scanning laser photostimulation: a new approach for analyzing brain circuits

A new technique for understanding the organization of complex circuits in the vertebrate brain, scanning laser photostimulation, is described. This approach is based on the photolysis of a caged form of the excitatory neurotransmitter glutamate. Computer-controlled photostimulation and whole cell recording in brain slices allow the construction of detailed maps of the position, strength, sign and number of inputs converging on a single postsynaptic neuron. Scanning laser photostimulation offers many advantages over current techniques: spatial resolution is superb, fibers of passage are not activated, and thousands of presynaptic locations can be stimulated. This review describes the technique of photostimulation, outlines the instrumentation, necessary to implement it, and discusses the interpretation of photostimulation-derived data. Several examples of applications, ranging from mapping circuits in the mammalian visual cortex to determining receptor distributions on single neurons are considered. Although still in its early stages, scanning laser photostimulation offers neuroscientists a powerful tool for determining the organization and function of local brain circuits.