Scanning laser photostimulation: a new approach for analyzing brain circuits

Diversity and cell type specificity of local excitatory connections to neurons in layer 3B of monkey primary visual cortex

In the primary visual cortex of macaque monkeys, laminar and columnar axonal specificity are correlated with functional differences between locations. We describe evidence that embedded within this anatomical framework is finer specificity of functional connections. Photostimulation-based mapping of functional input to 31 layer 3B neurons revealed that input sources to individual cells were highly diverse. Although some input differences were correlated with neuronal anatomy, no 2 neurons received excitatory input from the same cortical layers. Thus, input diversity reveals far more cell types than does anatomical diversity. This implies relatively little functional redundancy; despite trends related to laminar or columnar position, pools of neurons contributing uniquely to visual processing are likely relatively small. These results also imply that similarities in the anatomy of circuits in different cortical areas or species may not indicate similar functional connectivity.

Mossy fiber-granule cell synapses in the normal and epileptic rat dentate gyrus studied with minimal laser photostimulation

Dentate granule cells become synaptically interconnected in the hippocampus of persons with temporal lobe epilepsy, forming a recurrent mossy fiber pathway. This pathway may contribute to the development and propagation of seizures. The physiology of mossy fiber-granule cell synapses is difficult to characterize unambiguously, because electrical stimulation may activate other pathways and because there is a low probability of granule cell interconnection. These problems were addressed by the use of scanning laser photostimulation in slices of the caudal hippocampal formation. Glutamate was released from a caged precursor with highly focused ultraviolet light to evoke action potentials in a small population of granule cells. Excitatory synaptic currents were recorded in the presence of bicuculline. Minimal laser photostimulation evoked an apparently unitary excitatory postsynaptic current (EPSC) in 61% of granule cells from rats that had experienced pilocarpine-induced status epilepticus followed by recurrent mossy fiber growth. An EPSC was also evoked in 13-16% of granule cells from the control groups. EPSCs from status epilepticus and control groups had similar peak amplitudes ( approximately 30 pA), 20-80% rise times (approximately 1.2 ms), decay time constants ( approximately 10 ms), and half-widths (approximately 8 ms). The mean failure rate was high (approximately 70%) in both groups, and in both groups activation of N-methyl-D-aspartate receptors contributed a small component to the EPSC. The strong similarity between responses from the status epilepticus and control groups suggests that they resulted from activation of a similar synaptic population. No EPSC was recorded when the laser beam was focused in the dentate hilus, suggesting that indirect activation of hilar mossy cells contributed little, if at all, to these results. Recurrent mossy fiber growth increases the density of mossy fiber-granule cell synapses in the caudal dentate gyrus by perhaps sixfold, but the new synapses appear to operate very similarly to preexisting mossy fiber-granule cell synapses.

Precisely localized LTD in the neocortex revealed by infrared-guided laser stimulation

In a direct approach to elucidate the origin of long-term depression (LTD), glutamate was applied onto dendrites of neurons in rat neocortical slices. An infrared-guided laser stimulation was used to release glutamate from caged glutamate in the focal spot of an ultraviolet laser. A burst of light flashes caused an LTD-like depression of glutamate receptor responses, which was highly confined to the region of "tetanic" stimulation (<10 micrometers). A similar depression of glutamate receptor responses was observed during LTD of synaptic transmission. A spatially highly specific postsynaptic mechanism can account for the LTD induced by glutamate release.

Long-range inhibition within the zebra finch song nucleus RA can coordinate the firing of multiple projection neurons

The zebra finch forebrain song control nucleus RA (robust nucleus of the archistriatum) generates a phasic and temporally precise neural signal that drives vocal and respiratory motoneurons during singing. RA's output during singing predicts individual notes, even though afferent drive to RA from the song nucleus HVc is more tonic, and predicts song syllables, independent of the particular notes that comprise the syllable. Therefore RA's intrinsic circuitry transforms neural activity from HVc into a highly precise premotor output. To understand how RA's intrinsic circuitry effects this transformation, we characterized RA interneurons and projection neurons using intracellular recordings in brain slices. RA interneurons fired fast action potentials with steep current-frequency relationships and had small somata with thin aspinous processes that extended throughout large portions of the nucleus; the similarity of their fine processes to those labeled with a glutamic acid decarboxylase (GAD) antibody strongly suggests that these interneurons are GABAergic. Electrical stimulation revealed that RA interneurons receive excitatory inputs from RA's afferents, the lateral magnocellular nucleus of the anterior neostriatum (LMAN) and HVc, and from local axon collaterals of RA projection neurons. To map the functional connections that RA interneurons make onto RA projection neurons, we focally uncaged glutamate, revealing long-range inhibitory connections in RA. Thus these interneurons provide fast feed-forward and feedback inhibition to RA projection neurons and could help create the phasic pattern of bursts and pauses that characterizes RA output during singing. Furthermore, selectively activating the inhibitory network phase locks the firing of otherwise unconnected pairs of projection neurons, suggesting that local inhibition could coordinate RA output during singing.

Local excitatory circuits in the intermediate gray layer of the superior colliculus

We have used photostimulation and whole cell patch-clamp recording techniques to examine local synaptic interactions in slices from the superior colliculus of the tree shrew. Uncaging glutamate 10-75 microm from the somata of neurons in the intermediate gray layer elicited a long-lasting inward current, due to direct activation of glutamate receptors on these neurons, and brief inward currents caused by activation of presynaptic neurons. The synaptic responses occurred as individual currents or as clusters that lasted up to several hundred milliseconds. Excitatory synaptic responses, which reversed at membrane potentials near 0 mV, could be evoked by uncaging glutamate anywhere within 75 microm of an intermediate layer neuron. Our results indicate the presence of extensive local excitatory circuits in the intermediate layer of the superior colliculus and support the hypothesis that such intrinsic circuitry contributes to the development of presaccadic command bursts.

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

UV photolysis of caged molecules is a powerful method for studying cellular signaling. However, UV energy is often delivered through the microscope objective which can make certain experiments difficult. We have evaluated the utility of delivering UV pulses directly to the sample through an optical fiber. Visible (635 nm) and UV (337 nm) lasers were coupled into a UV transmitting optical fiber which was micromanipulated over the sample under investigation. Positioning of the fiber, and thus the photolysis beam, was achieved using the visible laser which acted much like a flashlight. By controlling the size of the optical fiber it is also possible to control the area of the sample which is exposed to UV light. After positioning the fiber we demonstrate that the UV beam exiting the optical fiber reliably photolysed NP-EGTA that had been loaded into cells, resulting in an elevation of intracellular calcium. Additionally, caged norepinephrine in the bathing saline was photo-released to activate receptor-operated calcium signaling pathways. Since the delivery of the UV energy is independent of microscope configuration, this approach can be readily incorporated into wide-field fluorescence imaging, confocal microscopy and electrophysiological applications.

NMDA and AMPA receptors on neocortical neurons are differentially distributed

The distribution of glutamate receptor subtypes on the surface of neurons is highly relevant for synaptic activation and signal processing in the neocortex. As a novel approach we have used infra-red videomicroscopy in combination with photostimulation or microiontophoresis in brain slices of rat neocortex to map the distribution of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors on pyramidal neurons of layer V. Both modes of application revealed a spatially distinct distribution of glutamate receptor subtypes: the soma and the proximal dendrite of neurons are highly sensitive to NMDA, whereas the more distal parts of the dendrite are more sensitive to AMPA. An implication is that NMDA receptors near the soma might regulate the amplification of synaptic signals resulting from AMPA receptor activation on remote dendritic sites.

Analysing functional connectivity in brain slices by a combination of infrared video microscopy, flash photolysis of caged compounds and scanning methods

We evaluate a novel set-up for scanning functional connectivity in brain slices from the somatosensory cortex of the rat. Upright infrared video microscopy for targeted placement of electrodes is combined with rapid photolysis of bath-applied caged neurotransmitter induced by a xenon flash lamp. Flash photolysis of caged glutamate and electrical stimulation produce comparable field potential responses and demonstrate that the viability of the submerged slices exceeds several hours. Glutamate release leads to field potential responses whose two phases are differentially affected by selective blockade of N-methyl-D-aspartate- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptors with DL-2-amino-5-phosphonovaleric acid and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulphonamide, respectively. Rapid computer-controlled scanning of hundreds of distinct stimulation sites with simultaneous recordings at a fixed reference site allows construction of functional input maps from peak amplitudes and delays to peak of field potential responses. Selective laminar expansion of the functional input maps after bicuculline application demonstrates that the combination of this conveniently assembled set-up with pharmacological and physical manipulations can provide insights into the determinants of functional connectivity in brain slices.

A new slicing method for the lower vertebrate brain: brain mould and adjustable stage

This paper describes a new method for slicing the lower vertebrate brain which is too small and soft to be sliced using conventional methods. The brain is sliced in a pre-prepared agar mould glued to a special stage placed in a conventional vibratome. The mould is constructed from a plaster model prepared by embedding a paraformaldehyde fixed brain in a paraffin and vaseline mixture. The brain to be sliced is placed within two prepared agar half cylinders which are in turn placed in a pre-prepared larger agar collar glued to a special stage. The study describes in detail the preparation of the interlocking collar and inner cylinders of agar. The plexiglass stage allows the agar block containing the brain, to be rotated and inclined to improve the angle of cut. By using this method, small and soft brain even with thin walls and large ventricles could be sliced coronally, sagittally, horizontally or obliquely. Brain slices obtained by this method have good viability showing spontaneous and evoked activity.

Calcium imaging and multielectrode recordings of global patterns of activity in the developing nervous system

Complex but coordinated interactions involving ensembles of neuronal cells result in the accurate processing of information in the adult central nervous system. However, recent studies monitoring the global patterns of activity of neuronal populations have demonstrated that immature neurons also interact to produce coordinated patterns of activity during the early stages of development. In particular, these patterns of coordinated activity occur during the period when neuronal connections are established, thus leading us to believe that such activity patterns might underlie the precision to which many neural pathways are wired up. Multielectrode recording and calcium imaging are two of the techniques that have been instrumental in revealing the spatial and temporal properties of the coordinated activity of developing neural networks in vitro. While multielectrode arrays measure the action potential activity of the cells, calcium imaging permits changes in intracellular calcium levels to be monitored over time. Both techniques have been used successfully to monitor the activity of cellular networks in culture, but they have also been applied in assessing the patterns of activity in intact or semi-intact pieces of neural tissues, such as the developing retina, neocortex and spinal cord. More recently, it has also been possible to correlate the structure and function of the cellular components of the networks by combining intracellular dye filling with the multineuronal recordings. In this review, brief descriptions and the applications of the two techniques will be presented, and the advantages and limitations of multielectrode array will be compared with that of calcium imaging using recordings of the developing mammalian retina as the primary example.

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.

Mechanism and kinetics of heterosynaptic depression at a cerebellar synapse

High levels of activity at a synapse can lead to spillover of neurotransmitter from the synaptic cleft. This extrasynaptic neurotransmitter can diffuse to neighboring synapses and modulate transmission via presynaptic receptors. We studied such modulation at the synapse between granule cells and Purkinje cells in rat cerebellar slices. Brief tetanic stimulation of granule cell parallel fibers activated inhibitory neurons, leading to a transient elevation of extracellular GABA, which in turn caused a short-lived heterosynaptic depression of the parallel fiber to Purkinje cell EPSC. Fluorometric calcium measurements revealed that this synaptic inhibition was associated with a decrease in presynaptic calcium influx. Heterosynaptic inhibition of synaptic currents and calcium influx was eliminated by antagonists of the GABAB receptor. The magnitude and time course of the depression of calcium influx were mimicked by the rapid release of an estimated 10 microM GABA using the technique of flash photolysis. We found that inhibition of presynaptic calcium influx peaked within 300 msec and decayed in <3 sec at 32 degrees C. These results indicate that presynaptic GABAB receptors can sense extrasynaptic GABA increases of several micromolar and that they rapidly regulate the release of neurotransmitter primarily by modulating voltage-gated calcium channels.

Relationships between local synaptic connections and orientation domains in primary visual cortex

Combined optical imaging of ferret primary visual cortex in vivo and scanning laser photostimulation in brain slices were used to determine the spatial relationships between synaptic inputs onto individual neurons and the pattern of orientation columns. In the upper cortical layers, both excitatory and inhibitory inputs originated primarily from regions with orientation tuning similar to that of the recorded neurons; the shapes of the input tuning curves were indistinguishable. The orientation distributions of both types of inputs centered around the orientation of the recorded neurons, and no evidence for preferential cross-orientation inputs, either excitatory or inhibitory, was observed. These patterns of synaptic connectivity are most consistent with feedforward models for generation of orientation selectivity and are inconsistent with the patterns required by models based on cross-orientation inhibition.

Chemical two-photon uncaging: a novel approach to mapping glutamate receptors

Functional mapping of neurotransmitter receptors requires rapid and localized application of transmitter. The usefulness of caged glutamate for this purpose has been limited, because photolysis by unfocused light above and below the target cell limits depth resolution. This problem is eliminated by using a double-caged glutamate that requires absorption of two photons for conversion to active glutamate, resulting in a substantial improvement in spatial resolution over conventional caged glutamate. This method was used to map the distribution of glutamate receptors on hippocampal pyramidal neurons. A higher density of AMPA receptors was found on distal apical dendrites than on basal or primary apical dendrites, suggesting that synaptic efficacy is locally heterogeneous. Such "chemical two-photon uncaging" offers a simple, general, and economical strategy for spatially localized photolysis of caged compounds.

Identification of chemical synapses in the pharynx of Caenorhabditis elegans

The rhythmic contraction of the Caenorhabditis elegans pharynx is unique in that the network of 12 neurons, including two M3 neurons, that regulate the contraction is known. The neurotransmitters secreted by these cells, and the target cells responding to these chemical signals, are not known. Here, we describe an approach to obtain this missing information and use the M3 cells as an example. Electrical recordings (electropharyngeograms) were used in conjunction with temporally and spatially defined application of neurotransmitters via photolysis of inactive, photolabile precursors. To illustrate the technique we used pharyngeal preparations in which the two M3 neurons are intact and preparations in which they were removed by laser irradiation. Removal of M3 neurons results in the loss of the small negative peaks in the electropharyngeograms and an increase in time during which the pharynx remains contracted. We demonstrate that the application of glutamate by photolysis of caged glutamate to a pharynx from which the two M3 neurons were removed produces effects similar to those observed before removal of the M3 neurons. In control experiments, photolytic release from photolabile precursors of carbamoylcholine, a stable and well characterized analog of acetylcholine, or of gamma-aminobutyric acid, from photolabile precursors did not have this effect. The response depended on the amount of glutamate released. By reducing the size of the photolytic beam, glutamate was released at several different locations of the pharynx. Two areas of the pharynx mainly respond to the application of glutamate; one corresponds to the pm4 muscle cells in the metacorpus, and the other to the junction between muscle cells pm5 in the isthmus and pm6 in the terminal bulb.

Photostimulation with caged neurotransmitters using fiber optic lightguides

'Caged' neurotransmitters are molecules that are transformed to a neuroactive state by exposure to light of an appropriate wavelength and intensity. Use of these substances has centered on in vitro bath application and subsequent activation using light from lasers or flashlamps that is delivered into the preparation through microscope optics. We have tested a new and simpler method, using finely tapered fiberoptic lightguides, that promises to expand the use of caged compounds for in vitro and in vivo experimentation. We demonstrated the feasibility and flexibility of this method for caged neurotransmitter delivery using a range of ex vitro, in vitro and in vivo approaches. The degree and timing of uncaging could be controlled by manipulating the wavelength, intensity and timing of the light projected into the optical fiber. Because of the small size of the light guide and the ability to control light exposure at the source, this new method promises greater control over the spatial and temporal delivery of neuroactive substances than simple bath or iontophoretic application, and enables delivery of conventional neurotransmitters with a spatial and temporal resolution closer to that of the natural neuronal circuitry. In addition, this new method allows the application of normally labile substances, such as the free radical gas nitric oxide, by the photoconversion of photosensitive precursors.

Caged compounds: tools for illuminating neuronal responses and connections

A number of new 'caged' intracellular second messengers and neurotransmitters have been developed using the photolabile o-nitrobenzyl group. This chemistry has also recently been exploited in novel ways, including the development of caged enzyme substrates and caged proteins. Although caged compounds continue to be used primarily for mechanistic (kinetic) studies of processes mediated by transmitters or second messengers, the spatial resolution afforded by the use of light to effect changes in transmitter concentrations has now been clearly demonstrated. The increased availability of caged compounds and of the technologies required to exploit them provides neurobiologists with powerful tools for probing neuronal response properties and connectivity patterns.

Convergence of magno- and parvocellular pathways in layer 4B of macaque primary visual cortex

Early visual processing is characterized by two independent parallel pathways: the magnocellular stream, which carries information useful for motion analysis, and the parvocellular stream, which carries information useful for analyses of shape and colour. Although increasing anatomical and physiological evidence indicates some degree of convergence of the two streams, the pathway through layer 4B of primary visual cortex (VI) and on to higher cortical areas is usually considered to carry only magnocellular input. This is inferred from anatomical descriptions of local circuitry in V1, and functional studies of area MT, which receives input from layer 4B. We have directly measured the sources of local functional input to individual layer 4B neurons by combining intracellular recording and biocytin labelling with laser-scanning photostimulation. We found that most layer 4B neurons receive strong input from both magnocellular-stream-recipient layer 4Calpha neurons and parvocellular-stream-recipient layer 4Cbeta neurons. Thus higher cortical areas that receive input either directly or indirectly from layer 4B are likely to be more strongly influenced by the parvocellular pathway than previously believed.

High-resolution immunogold localization of AMPA type glutamate receptor subunits at synaptic and non-synaptic sites in rat hippocampus

The cellular and subcellular localization of the GluRA, GluRB/C and GluRD subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type glutamate receptor was determined in the rat hippocampus using polyclonal antipeptide antibodies in immunoperoxidase and immunogold procedures. For the localization of the GluRD subunit a new polyclonal antiserum was developed using the C-terminal sequence of the protein (residues 869-881), conjugated to carrier protein and absorbed to colloidal gold for immunization. The purified antibodies immunoprecipitated about 25% of 3[H]AMPA binding activity from the hippocampus, cerebellum or whole brain, but very little from neocortex. These antibodies did not precipitate a significant amount of 3[H]kainate binding activity. The antibodies also recognize the GluRD subunit, but not the other AMPA receptor subunits, when expressed in transfected COS-7 cells and only when permeabilized with detergent, indicating an intracellular epitope. All subunits were enriched in the neuropil of the dendritic layers of the hippocampus and in the molecular layer of the dentate gyrus. The cellular distribution of the GluRD subunit was studied more extensively. The strata radiatum, oriens and the dentate molecular layer were more strongly immunoreactive than the stratum lacunosum moleculare, the stratum lucidum and the hilus. However, in the stratum lucidum of the CA3 area and in the hilus the weakly reacting dendrites were surrounded by immunopositive rosettes, shown in subsequent electron microscopic studies to correspond to complex dendritic spines. In the stratum radiatum, the weakly reacting apical dendrites contrasted with the surrounding intensely stained neuropil. The cell bodies of pyramidal and granule cells were moderately reactive. Some non-principal cells and their dendrites in the pyramidal cell layer and in the alveus also reacted very strongly for the GluRD subunit. At the subcellular level, silver intensified immunogold particles for the GluRA, GluRB/C and GluRD subunits were present at type 1 synaptic membrane specializations on dendritic spines of pyramidal cells throughout all layers of the CA1 and CA3 areas. The most densely labelled synapses tended to be on the largest spines and many smaller spines remained unlabelled. Immunoparticle density at type 1 synapses on dendritic shafts of some non-principal cells was consistently higher than at labelled synapses of dendritic spines of pyramidal cells. Synapses established between dendritic spines and mossy fibre terminals, were immunoreactive for all studied subunits in stratum lucidum of the CA3 area. The postembedding immunogold method revealed that the AMPA type receptors are concentrated within the main body of the anatomically defined type 1 (asymmetrical) synaptic junction. Often only a part of the membrane specialization showed clustered immunoparticles. There was a sharp decrease in immunoreactive receptor density at the edge of the synaptic specialization. Immunolabelling was consistently demonstrated at extrasynaptic sites on dendrites, dendritic spines and somata. The results demonstrate that the GluRA, B/C and D subunits of the AMPA type glutamate receptor are present in many of the glutamatergic synapses formed by the entorhinal, CA3 pyramidal and mossy fibre terminals. Some interneurons have a higher density of AMPA type receptors in their asymmetrical afferent synapses than pyramidal cells. This may contribute to a lower activation threshold of interneurons as compared to principal cells by the same afferents in the hippocampal formation.

Confocal imaging and local photolysis of caged compounds: dual probes of synaptic function

Chemical signals generated at synapses are highly limited in both spatial range and time course, so that experiments studying such signals must measure and manipulate them in both these dimensions. We describe an optical system that combines confocal laser scanning microscopy, to measure such signals, with focal photolysis of caged compounds. This system can elevate neurotransmitter and second messenger levels in femtoliter volumes of single dendrites within a millisecond. The method is readily combined with whole-cell patch-clamp measurements of electrical signals in brain slices. In cerebellar Purkinje cells, photolysis of caged IP3 causes spatially restricted intracellular release of Ca2+, and photolysis of a caged Ca2+ compound locally opens Ca(2+)-dependent K+ channels. Furthermore, localized photolysis of the caged neurotransmitter GABA transiently activates GABA receptors. The use of focal uncaging can yield new information about the spatial range of signaling actions at synapses.

Combining patch-clamp and optical methods in brain slices

Combining patch-clamp and optical imaging techniques in brain slices offers several advantages for physiological studies of nerve cells. Numerous practical considerations weigh heavily in this design of an apparatus suitable for such combined measurements. These considerations include the thickness of the slices, the type of microscope to be used for imaging and the kind of optical signal to be measured. A system that combine optical and patch-clamp methods can be modified readily to permit studies of intracellular and extracellular signaling pathways via flash photolysis of caged compounds.

Rearrangements of synaptic connections in visual cortex revealed by laser photostimulation

Assessing patterns of synaptic connections in the developing mammalian neocortex has relied primarily on anatomical studies. In a physiological approach described here, the patterns of synaptic connections in slices of developing ferret visual cortex were determined with scanning laser photostimulation. Functional synaptic inputs to pyramidal cells in cortical layers 2 and 3 originating from sites close to the neuronal cell body appeared at least 2 weeks before eye opening, prior to the formation of long-range horizontal connections. Extensive long-range horizontal connections appeared in the next 10 days of development. The number of local connections peaked at the time of eye opening; the number of these connections subsequently declined to the level found in the adult while the specificity of long-distance connections increased. Thus, the relative influence of local connections on the activity of layer 2 and layer 3 neurons declines as the cortex matures while the influence of longer range connections increases substantially.