Scanning laser photostimulation: a new approach for analyzing brain circuits

Genetic dissection of neural circuits

Understanding the principles of information processing in neural circuits requires systematic characterization of the participating cell types and their connections, and the ability to measure and perturb their activity. Genetic approaches promise to bring experimental access to complex neural systems, including genetic stalwarts such as the fly and mouse, but also to nongenetic systems such as primates. Together with anatomical and physiological methods, cell-type-specific expression of protein markers and sensors and transducers will be critical to construct circuit diagrams and to measure the activity of genetically defined neurons. Inactivation and activation of genetically defined cell types will establish causal relationships between activity in specific groups of neurons, circuit function, and animal behavior. Genetic analysis thus promises to reveal the logic of the neural circuits in complex brains that guide behaviors. Here we review progress in the genetic analysis of neural circuits and discuss directions for future research and development.

Three-Dimensional Mapping of Unitary Synaptic Connections by Two-Photon Macro Photolysis of Caged Glutamate

To understand the precise microarchitecture of the cortical circuitry, it is crucial to know the distribution of synaptic connections and their synaptic strengths at the level of a single cell, rather than a group of cells. Here, we describe a new application of two-photon photolysis of caged glutamate that enabled us to induce an action potential in only a small number (about five) of pyramidal neurons by increasing the volume of two-photon excitation by reducing the effective numerical aperture of the objective. We performed whole cell patch-clamp recordings from layer 2/3 pyramidal neurons in the rat visual cortex and stimulated many neurons in a large three-dimensional space (∼600 × 600 × 100 μm) including neurons in layers 2/3 and 4 using this new technique. We mapped the density and amplitude of unitary excitatory postsynaptic currents and found that the basic microarchitecture of excitatory synaptic connections consists of two regions: a columnar, dense core region with a radius of 150 μm and an outer, sparse region. The dense core region includes the majority of strong synaptic connections in layer 2/3. Our results reveal the columnar organization of synaptic connectivity in the rat visual cortex, where functional columns have not been clearly demonstrated. Thus this technique will be a uniquely powerful tool for quantifying synaptic connectivity and manipulating neural activity at the single-cell level.

Light-activated ion channels for remote control of neural activity

Light-activated ion channels provide a new opportunity to precisely and remotely control neuronal activity for experimental applications in neurobiology. In the past few years, several strategies have arisen that allow light to control ion channels and therefore neuronal function. Light-based triggers for ion channel control include caged compounds, which release active neurotransmitters when photolyzed with light, and natural photoreceptive proteins, which can be expressed exogenously in neurons. More recently, a third type of light trigger has been introduced: a photoisomerizable tethered ligand that directly controls ion channel activity in a light-dependent manner. Beyond the experimental applications for light-gated ion channels, there may be clinical applications in which these light-sensitive ion channels could prove advantageous over traditional methods. Electrodes for neural stimulation to control disease symptoms are invasive and often difficult to reposition between cells in tissue. Stimulation by chemical agents is difficult to constrain to individual cells and has limited temporal accuracy in tissue due to diffusional limitations. In contrast, ion channels that can be directly activated with light allow control with unparalleled spatial and temporal precision. The goal of this chapter is to describe light-regulated ion channels and how they have been tailored to control different aspects of neural activity, and how to use these channels to manipulate and better understand development, function, and plasticity of neurons and neural circuits.

A chronic histopathological and electrophysiological analysis of a rodent hypoxic-ischemic brain injury model and its use as a model of epilepsy

Ischemic brain injury is one of the leading causes of epilepsy in the elderly, and there are currently no adult rodent models of global ischemia, unilateral hemispheric ischemia, or focal ischemia that report the occurrence of spontaneous motor seizures following ischemic brain injury. The rodent hypoxic-ischemic (H-I) model of brain injury in adult rats is a model of unilateral hemispheric ischemic injury. Recent studies have shown that an H-I injury in perinatal rats causes hippocampal mossy fiber sprouting and epilepsy. These experiments aimed to test the hypothesis that a unilateral H-I injury leading to severe neuronal loss in young-adult rats also causes mossy fiber sprouting and spontaneous motor seizures many months after the injury, and that the mossy fiber sprouting induced by the H-I injury forms new functional recurrent excitatory synapses. The right common carotid artery of 30-day old rats was permanently ligated, and the rats were placed into a chamber with 8% oxygen for 30 min. A quantitative stereologic analysis revealed that the ipsilateral hippocampus had significant hilar and CA1 pyramidal neuronal loss compared with the contralateral and sham-control hippocampi. The septal region from the ipsilateral and contralateral hippocampus had small but significantly increased amounts of Timm staining in the inner molecular layer compared with the sham-control hippocampi. Three of 20 lesioned animals (15%) were observed to have at least one spontaneous motor seizure 6-12 months after treatment. Approximately 50% of the ipsilateral and contralateral hippocampal slices displayed abnormal electrophysiological responses in the dentate gyrus, manifest as all-or-none bursts to hilar stimulation. This study suggests that H-I injury is associated with synaptic reorganization in the lesioned region of the hippocampus, and that new recurrent excitatory circuits can predispose the hippocampus to abnormal electrophysiological activity and spontaneous motor seizures.

Endomorphin-1 modulates intrinsic inhibition in the dorsal vagal complex

Mu-opioid receptor (MOR) agonists profoundly influence digestive and other autonomic functions by modulating neurons in nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMV). Whole cell recordings were made from NTS and DMV neurons in brain stem slices from rats and transgenic mice that expressed enhanced green fluorescent protein (EGFP) under the control of a GAD67 promoter (EGFP-GABA neurons) to identify opioid-mediated effects on GABAergic circuitry. Synaptic and membrane properties of EGFP-GABA neurons were assessed. The endogenous selective MOR agonist endomorphin-1 (EM-1) reduced spontaneous and evoked excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) in both rat and mouse DMV neurons. Electrical stimulation of the solitary tract evoked constant-latency EPSCs in approximately 50% of EGFP-GABA neurons, and the responses were reduced by EM-1 application. EM-1 reduced action potential firing, the frequency and amplitude of synaptic inputs in EGFP-GABA neurons and responses to direct glutamate stimulation. A subset of EGFP-GABA neurons colocalized mRFP1 after retrograde, transneuronal infection after gastric inoculation with PRV-614, indicating that they synapsed with gastric-projecting DMV neurons. Glutamate photolysis stimulation of intact NTS projections evoked IPSCs in DMV neurons, and EM-1 reduced the evoked response, most likely by activation of MOR on the soma of premotor GABA neurons in NTS. Naltrexone or H-d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), MOR antagonists, blocked the effects of EM-1. Our results show that GABA neurons in the NTS receive direct vagal afferent input and project to gastric-related DMV neurons. Furthermore, modulation by EM-1 of specific components of the vagal complex differentially suppresses excitatory and inhibitory synaptic input to the DMV by acting at different receptor locations.

Channelrhodopsin-2-assisted circuit mapping of long-range callosal projections

The functions of cortical areas depend on their inputs and outputs, but the detailed circuits made by long-range projections are unknown. We show that the light-gated channel channelrhodopsin-2 (ChR2) is delivered to axons in pyramidal neurons in vivo. In brain slices from ChR2-expressing mice, photostimulation of ChR2-positive axons can be transduced reliably into single action potentials. Combining photostimulation with whole-cell recordings of synaptic currents makes it possible to map circuits between presynaptic neurons, defined by ChR2 expression, and postsynaptic neurons, defined by targeted patching. We applied this technique, ChR2-assisted circuit mapping (CRACM), to map long-range callosal projections from layer (L) 2/3 of the somatosensory cortex. L2/3 axons connect with neurons in L5, L2/3 and L6, but not L4, in both ipsilateral and contralateral cortex. In both hemispheres the L2/3-to-L5 projection is stronger than the L2/3-to-L2/3 projection. Our results suggest that laminar specificity may be identical for local and long-range cortical projections.

Multiple-color optical activation, silencing, and desynchronization of neural activity, with single-spike temporal resolution

The quest to determine how precise neural activity patterns mediate computation, behavior, and pathology would be greatly aided by a set of tools for reliably activating and inactivating genetically targeted neurons, in a temporally precise and rapidly reversible fashion. Having earlier adapted a light-activated cation channel, channelrhodopsin-2 (ChR2), for allowing neurons to be stimulated by blue light, we searched for a complementary tool that would enable optical neuronal inhibition, driven by light of a second color. Here we report that targeting the codon-optimized form of the light-driven chloride pump halorhodopsin from the archaebacterium Natronomas pharaonis (hereafter abbreviated Halo) to genetically-specified neurons enables them to be silenced reliably, and reversibly, by millisecond-timescale pulses of yellow light. We show that trains of yellow and blue light pulses can drive high-fidelity sequences of hyperpolarizations and depolarizations in neurons simultaneously expressing yellow light-driven Halo and blue light-driven ChR2, allowing for the first time manipulations of neural synchrony without perturbation of other parameters such as spiking rates. The Halo/ChR2 system thus constitutes a powerful toolbox for multichannel photoinhibition and photostimulation of virally or transgenically targeted neural circuits without need for exogenous chemicals, enabling systematic analysis and engineering of the brain, and quantitative bioengineering of excitable cells.

Optimization of Somatic Inhibition at Critical Period Onset in Mouse Visual Cortex

Local GABAergic circuits trigger visual cortical plasticity in early postnatal life. How these diverse connections contribute to critical period onset was investigated by nonstationary fluctuation analysis following laser photo-uncaging of GABA onto discrete sites upon individual pyramidal cells in slices of mouse visual cortex. The GABA(A) receptor number decreased on the soma-proximal dendrite (SPD), but not at the axon initial segment, with age and sensory deprivation. Benzodiazepine sensitivity was also higher on the immature SPD. Too many or too few SPD receptors in immature or dark-reared mice, respectively, were adjusted to critical period levels by benzodiazepine treatment in vivo, which engages ocular dominance plasticity in these animal models. Combining GAD65 deletion with dark rearing from birth confirmed that an intermediate number of SPD receptors enable plasticity. Site-specific optimization of perisomatic GABA response may thus trigger experience-dependent development in visual cortex.

Differential wiring of local excitatory and inhibitory synaptic inputs to islet cells in rat spinal lamina II demonstrated by laser scanning photostimulation

The substantia gelatinosa (lamina II) of the spinal dorsal horn contains inhibitory and excitatory interneurons that are thought to play a critical role in the modulation of nociception. However, the organization of the intrinsic circuitry within lamina II remains poorly understood. We used glutamate uncaging by laser scanning photostimulation to map the location of neurons that give rise to local synaptic inputs to islet cells, a major class of inhibitory interneuron in lamina II. We also mapped the distribution of sites on the islet cells that exhibited direct (non-synaptic) responses to uncaging of excitatory and inhibitory transmitters. Local synaptic inputs to islet cells arose almost entirely from within lamina II, and these local inputs included both excitatory and inhibitory components. Furthermore, there was a striking segregation in the location of sites that evoked excitatory versus inhibitory synaptic inputs, such that inhibitory presynaptic neurons were distributed more proximal to the islet cell soma. This was paralleled in part by a differential distribution of transmitter receptor sites on the islet cell, in that inhibitory sites were confined to the peri-somatic region while excitatory sites were more widespread. This differential organization of excitatory and inhibitory inputs suggests a principle for the wiring of local circuitry within the substantia gelatinosa.

Light-induced depolarization of neurons using a modified Shaker K(+) channel and a molecular photoswitch

To trigger action potentials in neurons, most investigators use electrical or chemical stimulation. Here we describe an optical stimulation method based on semi-synthetic light-activated ion channels. These SPARK (synthetic photoisomerizable azobenzene-regulated K(+)) channels consist of a synthetic azobenzene-containing photoswitch and a genetically modified Shaker K(+) channel protein. SPARK channels with a wild-type selectivity filter elicit hyperpolarization and suppress action potential firing when activated by 390 nm light. A mutation in the pore converts the K(+)-selective Shaker channel into a nonselective cation channel. Activation of this modified channel with the same wavelength of light elicits depolarization of the membrane potential. Expression of these depolarizing SPARK channels in neurons allows light to rapidly and reversibly trigger action potential firing. Hence, hyper- and depolarizing SPARK channels provide a means for eliciting opposite effects on neurons in response to the same light stimulus.

An in vitro study of horizontal connections in the intermediate layer of the superior colliculus

Some models propose that the spatial and temporal distributions of premotor activity in the intermediate layer of the superior colliculus are shaped by neuronal ensembles that give rise to local excitatory and distant inhibitory connections. One function proposed for these connections is to mediate a "winner-take-all" network; the short-range excitatory connections build up the activity of neighboring cells that command orienting movements in one direction, whereas the wide-ranging inhibitory projections attenuate the activity of remote cells that command incompatible movements. We used in vitro photostimulation and whole-cell patch-clamp recording to test these models by measuring the spatial extent of synaptic interactions within the rat intermediate layer. Uncaging glutamate over whole-cell patch-clamped cells in the intermediate layer elicited long-lasting inward currents, resulting from direct activation of glutamate receptors expressed by the cells, and brief synaptic currents evoked by activation of presynaptic neurons. The synaptic responses comprised clusters of excitatory and inhibitory currents. The size of these responses depended on the location of the stimulus with respect to the clamped cell. Large responses were commonly evoked by stimuli within 200 microm of the soma in the intermediate layer; smaller responses could occasionally be evoked from sites as distant as 500 microm. Responses evoked by stimulation beyond this distance were rare. Although the results demonstrated powerful local excitatory and inhibitory connections, they did not support the pattern of short-range excitation and widespread inhibition predicted by the winner-take-all hypothesis.

Enhanced excitatory synaptic connectivity in layer v pyramidal neurons of chronically injured epileptogenic neocortex in rats

Formation of new recurrent excitatory circuits after brain injuries has been hypothesized as a major factor contributing to epileptogenesis. Increases in total axonal length and the density of synaptic boutons are present in layer V pyramidal neurons of chronic partial isolations of rat neocortex, a model of posttraumatic epileptogenesis. To explore the functional consequences of these changes, we used laser-scanning photostimulation combined with whole-cell patch-clamp recording from neurons in layer V of somatosensory cortex to map changes in excitatory synaptic connectivity after injury. Coronal slices were submerged in artificial CSF (23 degrees C) containing 100 microM caged glutamate, APV (2-amino-5-phosphonovaleric acid), and high divalent cation concentration to block polysynaptic responses. Focal uncaging of glutamate, accomplished by switching a pulsed UV laser to give a 200-400 micros light stimulus, evoked single- or multiple-component composite EPSCs. In neurons of the partially isolated cortex, there were significant increases in the fraction of uncaging sites from which EPSCs could be evoked ("hot spots") and a decrease in the mean amplitude of individual elements in the composite EPSC. When plotted along the cortical depth, the changes in EPSCs took place mainly between 150 and 200 microm above and below the somata, suggesting a specific enhancement of recurrent excitatory connectivity among layer V pyramidal neurons of the undercut neocortex. These changes may shift the balance within cortical circuits toward increased synaptic excitation and contribute to epileptogenesis.

Laminar and columnar organization of ascending excitatory projections to layer 2/3 pyramidal neurons in rat barrel cortex

Excitatory synaptic projections to layer 2/3 (L2/3) pyramidal neurons in brain slices from the rat barrel cortex were measured using quantitative laser-scanning photostimulation (LSPS) mapping. In the barrel cortex, cytoarchitectonic "barrels" and "septa" in L4 define a stereotypical array of landmarks, allowing alignment and averaging of LSPS maps from multiple cells in different slices. We distinguished inputs to L2 and L3 neurons above barrels and septa. Average input maps revealed that barrel-related ascending projections (L4-->2/3barrel) interdigitated with a novel septum-related projection (L5A-->2septum). We also explored the functional organization of these projections by comparing the input maps of multiple cells in individual slices. L2/3 cells sharing the same barrel-related column showed strong correlations in their input maps, independent of their precise locations within the column; otherwise, correlations fell rapidly as a function of intersomatic separation. Our data indicate that barrel-related and septum-related columns are associated with distinct functional circuits. These projections are likely to mediate parallel processing of somatosensory signals within the barrel cortex, with L4-->2/3barrel and L5A-->2septum representing the intracortical continuations of, respectively, the subcortical lemniscal and paralemniscal systems conveying somatosensory information to the barrel cortex.

Local connections to specific types of layer 6 neurons in the rat visual cortex

Because layer 6 of the cerebral cortex receives direct thalamic input and provides projections back to the thalamus, it is in a unique position to influence thalamocortical interactions. Different types of layer 6 pyramidal neurons provide output to different thalamic nuclei, and it is therefore of interest to understand the sources of functional input to these neurons. We studied the morphologies and local excitatory input to individual layer 6 neurons in rat visual cortex by combining intracellular labeling and recording with laser-scanning photostimulation. As in previous photostimulation studies, we found significant differences in the sources of local excitatory input to different cell types. Most notably, there were differences in local input to neurons that, based on analogy to barrel cortex, are likely to project only to the lateral geniculate nucleus of the thalamus versus those that are likely to also project to the lateral posterior nucleus. The more striking finding, however, was the paucity of superficial layer input to layer 6 neurons in the rat visual cortex, contrasting sharply with layer 6 neurons in the primate visual cortex. These observations provide insight into differences in function between cortical projections to first-order versus higher-order thalamic nuclei and also show that these circuits can be organized differently in different species.

Photochemical tools for remote control of ion channels in excitable cells

Various strategies have been developed recently for imparting light sensitivity onto normally insensitive cells. These include expression of natural photosensitive proteins, photolysis of caged agonists of native cell surface receptors and photoswitching of isomerizable tethered ligands that act on specially engineered ion channels and receptor targets. The development of chemical tools for optically stimulating or inhibiting signaling proteins has particular relevance for the nervous system, where precise, noninvasive control is an experimental and medical necessity.

Millisecond-timescale, genetically targeted optical control of neural activity

Temporally precise, noninvasive control of activity in well-defined neuronal populations is a long-sought goal of systems neuroscience. We adapted for this purpose the naturally occurring algal protein Channelrhodopsin-2, a rapidly gated light-sensitive cation channel, by using lentiviral gene delivery in combination with high-speed optical switching to photostimulate mammalian neurons. We demonstrate reliable, millisecond-timescale control of neuronal spiking, as well as control of excitatory and inhibitory synaptic transmission. This technology allows the use of light to alter neural processing at the level of single spikes and synaptic events, yielding a widely applicable tool for neuroscientists and biomedical engineers.

Plasticity, synaptic strength, and epilepsy: what can we learn from ultrastructural data?

Central nervous system synapses have an intrinsic plastic capacity to adapt to new conditions with rapid changes in their structure. Such activity-dependent refinement occurs during development and learning, and shares features with diseases such as epilepsy. Quantitative ultrastructural studies based on serial sectioning and reconstructions have shown various structural changes associated with synaptic strength involving both dendritic spines and postsynaptic densities (PSDs) during long-term potentiation (LTP). In this review, we focus on experimental studies that have analyzed at the ultrastructural level the consequences of LTP in rodents, and plastic changes in the hippocampus of experimental models of epilepsy and human tissue obtained during surgeries for intractable temporal lobe epilepsy (TLE). Modifications in spine morphology, increases in the proportion of synapses with perforated PSDs, and formation of multiple spine boutons arising from the same dendrite are the possible sequence of events that accompany hippocampal LTP. Structural remodeling of mossy fiber synapses and formation of aberrant synaptic contacts in the dentate gyrus are common features in experimental models of epilepsy and in human TLE. Combined electrophysiological and ultrastructural studies in kindled rats and chronic epileptic animals have indicated the occurrence of seizure- and neuron loss-induced changes in the hippocampal network. In these experiments, the synaptic contacts on granule cells are similar to those described for LTP. Such changes could be associated with enhancement of synaptic efficiency and may be important in epileptogenesis.

Controlling synaptic input patterns in vitro by dynamic photo stimulation

Recent experimental and theoretical work indicates that both the intensity and the temporal structure of synaptic activity strongly modulate the integrative properties of single neurons in the intact brain. However, studying these effects experimentally is complicated by the fact that, in experimental systems, network activity is either absent, as in the acute slice preparation, or difficult to monitor and to control, as in in vivo recordings. Here, we present a new implementation of neurotransmitter uncaging in acute brain slices that uses functional projections to generate tightly controlled, spatio-temporally structured synaptic input patterns in individual neurons. For that, a set of presynaptic neurons is activated in a precisely timed sequence through focal photolytic release of caged glutamate with the help of a fast laser scanning system. Integration of synaptic inputs can be studied in postsynaptic neurons that are not directly stimulated with the laser, but receive input from the targeted neurons through intact axonal projections. Our new approach of dynamic photo stimulation employs functional synapses, accounts for their spatial distribution on the dendrites, and thus allows study of the integrative properties of single neurons with physiologically realistic input. Data obtained with our new technique suggest that, not only the neuronal spike generator, but also synaptic transmission and dendritic integration in neocortical pyramidal cells, can be highly reliable.

Optical release of caged glutamate for stimulation of neurons in the in vitro slice preparation

Optical stimulation techniques prove useful to map functional inputs in the in vitro brain slice preparation: Glutamate released by a focused beam of UV light induces action potentials, which can be detected in postsynaptic neurons. The direct activation effect is influenced by factors such as compound concentration, focus depth, light absorption in the tissue, and sensitivity of different neuronal domains. We analyze information derived from direct stimulation experiments in slices from rat barrel cortex and construct a computational model of a layer V pyramidal neuron that reproduces the experimental findings. The model predictions concerning the influence of focus depth on input maps and action potential generation are investigated further in subsequent experiments where the focus depth of a high-numerical-aperture lens is systematically varied. With our setup flashes from a xenon light source can activate neuronal compartments to a depth of 200 mum below the surface of the slice. The response amplitude is influenced both by tissue depth and focus plane. Specific somatodendritic structures can be targeted as the probability of action potential induction falls off exponentially with distance. Somata and primary apical dendrites are most sensitive to uncaged glutamate with locally increased sensitivity on proximal apical dendrites. We conclude that optical stimulation can be targeted with high precision.

Modulation of synaptic transmission in the rat nucleus of the solitary tract by endomorphin-1

Activation of opioid receptors in the periphery and centrally in the brain results in inhibition of gastric and other vagally mediated functions. The aim of this study was to examine the role of the endogenous opioid agonist endomorphin 1 (EM-1) in regulating synaptic transmission within the nucleus tractus solitarius (NTS), an integration site for autonomic functions. We performed whole cell patch-clamp recordings from coronal brain slices of the rat medulla. A subset of the neurons studied was prelabeled with a stomach injection of the transsynaptic retrograde virus expressing EGFP, PRV-152. Solitary tract stimulation resulted in constant latency excitatory postsynaptic currents (EPSCs) that were decreased in amplitude by EM-1 (0.01-10 microM). The paired-pulse ratio was increased with little change in input resistance, suggesting a presynaptic mechanism. Spontaneous EPSCs were decreased in both frequency and amplitude by EM-1, and miniature EPSCs were reduced in frequency but not amplitude, suggesting a presynaptic mechanism for the effect. Spontaneous inhibitory postsynaptic currents (IPSCs) were also reduced in frequency by EM-1, but the effect was blocked by TTX, suggesting activity at receptors on the somata of local inhibitory neurons. Synaptic input arising from local NTS neurons, which were activated by focal photolysis of caged glutamate, was inhibited by EM-1. The actions of EM-1 were similar to those of D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) and were blocked by naltrexone, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), or D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP). These results suggest that EM-1 acts at mu-opioid receptors to modulate viscerosensory input and specific components of local synaptic circuitry in the NTS.

VSDI: a new era in functional imaging of cortical dynamics

During the last few decades, neuroscientists have benefited from the emergence of many powerful functional imaging techniques that cover broad spatial and temporal scales. We can now image single molecules controlling cell differentiation, growth and death; single cells and their neurites processing electrical inputs and sending outputs; neuronal circuits performing neural computations in vitro; and the intact brain. At present, imaging based on voltage-sensitive dyes (VSDI) offers the highest spatial and temporal resolution for imaging neocortical functions in the living brain, and has paved the way for a new era in the functional imaging of cortical dynamics. It has facilitated the exploration of fundamental mechanisms that underlie neocortical development, function and plasticity at the fundamental level of the cortical column.

Light-activated ion channels for remote control of neuronal firing

Neurons have ion channels that are directly gated by voltage, ligands and temperature but not by light. Using structure-based design, we have developed a new chemical gate that confers light sensitivity to an ion channel. The gate includes a functional group for selective conjugation to an engineered K(+) channel, a pore blocker and a photoisomerizable azobenzene. Long-wavelength light drives the azobenzene moiety into its extended trans configuration, allowing the blocker to reach the pore. Short-wavelength light generates the shorter cis configuration, retracting the blocker and allowing conduction. Exogenous expression of these channels in rat hippocampal neurons, followed by chemical modification with the photoswitchable gate, enables different wavelengths of light to switch action potential firing on and off. These synthetic photoisomerizable azobenzene-regulated K(+) (SPARK) channels allow rapid, precise and reversible control over neuronal firing, with potential applications for dissecting neural circuits and controlling activity downstream from sites of neural damage or degeneration.

Spatial distribution of inhibitory synaptic connections during development of ferret primary visual cortex

Intracortical inhibition in the primary visual cortex plays an important role in creating properties like orientation and direction selectivity. However, the development of the spatial pattern of inhibitory connections is largely unexplored. This was investigated in the present study. Tangential slices of layers 2/3 of ferret striate cortex were prepared for whole-cell patch clamp recordings, and presynaptic inhibitory inputs to pyramidal neurons were scanned by local photolysis of Nmoc-caged glutamate. Inhibitory synaptic currents (IPSCs) were first detected around postnatal day (P) 17. They originated locally around the recorded cells. Both the number and the total areas supplying the inhibitory inputs increased thereafter and peaked at the time around and shortly after eye opening (P29-37). A refinement period then followed in which the areas providing the majority of inhibitory inputs shrank from 600 microm around the recorded neurons to 200-300 microm in more mature animals (>/=P38). The amplitude of IPSCs increased progressively with increasing age. Long-range inhibitory inputs (>600 microm) were present around eye opening and they often developed into a clustered patchy pattern in more mature animals (>/=P38). In summary, our results show a refinement and clustering in the spatial pattern of inhibitory connections during postnatal development of ferret visual cortex.

Effects of sensory deprivation on columnar organization of neuronal circuits in the rat barrel cortex

We examined whether sensory deprivation during formation of the cortical circuitry influences the pattern of intracortical single-cell connections in rat barrel cortex. Excitatory postsynaptic potentials (EPSPs) from layer 2/3 (L2/3) pyramidal neurons were recorded in vitro using patch-clamp techniques. In order to evoke EPSPs, presynaptic neurons were stimulated by photolytically applied glutamate, thus generating action potentials. Synaptic connections between the stimulated and the recorded neuron were identified by the occurrence of PSPs following photostimulation. Sensory deprivation changed the pattern of projections from L4 and L2/3 neurons to L2/3 pyramidal cells. In slices of non-deprived rats 86% of the total presynaptic neurons were located in the first and only 10% in the second barrel column. Deprivation changed these values to 67% and 26%, respectively. Therefore, the probability of presynaptic cells projecting to L2/3 neurons was shifted from adjacent to more remote barrel columns. These results indicate that deprivation of sensory input influences the pattern of intracortical connections.

Light-directed electrical stimulation of neurons cultured on silicon wafers

Dissociated neurons cultured in vitro can serve as a model system for studying the dynamics of neural networks. Such studies depend on techniques for stimulating patterns of neural activity. We show a technique for extracellular stimulation of dissociated neurons cultured on silicon wafers. When the silicon surface is reverse biased, electrical current can be generated near any neuron by pulsing a laser. Complex spatiotemporal stimulation patterns can be produced by directing a single beam with an acousto-optic deflector. The technique can generate a stimulating current at any location in the culture. This contrasts with multielectrode arrays (MEAs), which can stimulate only at fixed electrode locations. To characterize reliability and spatial selectivity of stimulation, we used intracellular (patch-clamp) recordings to monitor the effect of targeted laser pulses on cultured hippocampal neurons. Action potentials could be stimulated with submillisecond precision and 100-micron spatial resolution at rates exceeding 100 Hz. Optimal control parameters for stimulation are discussed.

Laminar patterns of local excitatory input to layer 5 neurons in macaque primary visual cortex

Layer 5 neurons in primary visual cortex make putative reciprocal feedback connections to the superficial layers. To test this hypothesis, we employed scanning laser photostimulation combined with intracellular dye injection to examine local functional excitatory inputs to and axonal projections from individual layer 5 neurons in brain slices from monkey V1. In contrast with previous studies of other V1 neurons, layer 5 neurons received significant input from nearly all of the cortical layers, suggesting individual layer 5 cells integrate information from a broad range of input sources. Nevertheless relative strengths of laminar inputs varied across neurons. Cluster analysis of relative strength of laminar inputs to individual layer 5 neurons revealed four discrete clusters representing recurring input patterns; each cluster included both excitatory and inhibitory neurons. Twenty-five of 40 layer 5 neurons fell into two clusters, both characterized by very strong input from superficial layers. These input patterns are consistent with layer 5 neurons providing feedback to superficial layers. The remaining 15 neurons received stronger input from deep layers. Differences in input from layer 4Calpha versus 4Cbeta also suggest specific associations of the magnocellular and parvocellular visual pathways, with populations receiving stronger input from deep versus superficial cortical layers.

Topographic specificity of functional connections from hippocampal CA3 to CA1

The hippocampus is a cortical region thought to play an important role in learning and memory. Most of our knowledge about the detailed organization of hippocampal circuitry responsible for these functions is derived from anatomical studies. These studies present an incomplete picture, however, because the functional character and importance of connections are often not revealed by anatomy. Here, we used a physiological method (photostimulation with caged glutamate) to probe the fine pattern of functional connectivity between the CA3 and CA1 subfields in the mouse hippocampal slice preparation. We recorded intracellularly from CA1 and CA3 pyramidal neurons while scanning with photostimulation across the entire CA3 subfield with high spatial resolution. Our results show that, at a given septotemporal level, nearby CA1 neurons receive synaptic inputs from neighboring CA3 neurons. Thus, the CA3 to CA1 mapping preserves neighbor relations.

Postsynaptic modulation of AMPA- and NMDA-receptor currents by Group III metabotropic glutamate receptors in rat nucleus accumbens

Whole cell patch clamp recordings from rat nucleus accumbens neurons were made in order to study the effect of metabotropic glutamate receptors and dopamine on postsynaptic glutamate receptor mediated currents. AMPA- and NMDA-R currents were evoked by flash photolysis of caged glutamate, while spike-dependent release of neurotransmitters was prevented by adding tetrodotoxin and bicuculline to the bath solution. Spontaneous potentiation of NMDA- but not AMPA-R current was observed in the early phase of stimulation, followed by depotentiation and subsequent stabilization. The Group III metabotropic glutamate receptor antagonist MAP4 induced a transient potentiation of both AMPA- and NMDA-R current amplitudes, without affecting rise times and decay time constants. In contrast, the Group I-II metabotropic glutamate receptor antagonist MCPG and the neurotransmitter dopamine did not exert significant effects on either AMPA- or NMDA-R currents. These data suggest that at least one of the Group III subtypes is located postsynaptically in the nucleus accumbens and is able to dampen the activity of ionotropic glutamatergic receptors. In contrast, our results do not support a modulation of postsynaptic AMPA- and NMDA-R currents by Group I/II metabotropic glutamate receptors or dopamine. Modulation of both AMPA- and NMDA-R currents in the nucleus accumbens is likely to play a major role in setting the cellular excitability in response to behaviourally relevant limbic inputs, and in regulating the plasticity of these responses.

Circuit analysis of experience-dependent plasticity in the developing rat barrel cortex

Sensory deprivation during a critical period reduces spine motility and disrupts receptive field structure of layer 2/3 neurons in rat barrel cortex. To determine the locus of plasticity, we used laser scanning photostimulation, allowing us to rapidly map intracortical synaptic connectivity in brain slices. Layer 2/3 neurons differed in their spatial distributions of presynaptic partners: neurons directly above barrels received, on average, significantly more layer 4 input than those above the septa separating barrels. Complementary connectivity was found in deprived cortex: neurons above septa were now strongly coupled to septal regions, while connectivity between barrel regions and layer 2/3 was reduced. These results reveal competitive interactions between barrel and septal circuits in the establishment of precise intracortical circuits.

Stimulating neurons with light

Recent technological advances have enabled the use of different optical methods to activate neurons, including 'caged' glutamate, photoactivation of genetically engineered cascades, and direct two-photon excitation. The ability to use light as a stimulation tool provides, in principle, a non-invasive method for the temporally and spatially precise activation of any neuron or any part of a neuron. When combined with two-photon excitation, excellent spatial control can be achieved even in complex and highly scattering preparations, such as living nervous tissue. Different methods that have been developed in the last several decades have been used to probe neuronal sensitivity, mimic synaptic input, and elucidate patterns of neural connectivity.

Reassessment of the effects of cycloheximide on mossy fiber sprouting and epileptogenesis in the pilocarpine model of temporal lobe epilepsy

A feature of animal models of temporal lobe epilepsy and the human disorder is hippocampal sclerosis and Timm stain in the inner molecular layer (IML) of the dentate gyrus, which represents synaptic reorganization and may be important in epileptogenesis. We reassessed the hypothesis that pre-treatment with cycloheximide (CHX) prevents Timm staining in the IML following pilocarpine (PILO)-induced status epilepticus (a multifocal model of temporal lobe epilepsy), but allows epileptogenesis (i.e., chronic spontaneous seizures) after a latent period. Hippocampal slices from PILO-treated rats without Timm stain in the IML after CHX treatment were hypothesized to lack the electrophysiological abnormalities suggestive of recurrent excitation. The primary experimental groups were as follows: 1) CHX (1 mg/kg) 30-45 min prior to administration of PILO (320 mg/kg ip, 2) only PILO, and 3) only saline (0.5 ml, IP). The CHX pre-treatment significantly decreased the number of rats that responded to PILO with status epilepticus compared to rats that received only PILO. Pre-treatment with CHX did not significantly alter the spontaneous motor seizure rate post-treatment compared to treatment with PILO alone in those animals from each group that developed status epilepticus during PILO treatment. Timm stain in the IML was not significantly different between the PILO- and PILO+CHX-treated rats. Using quantitative methods, CHX did not prevent hilar, CA1, or CA3 neuronal loss compared to the PILO-treated rats. Extracellular responses to hilar stimulation in 30 microM bicuculline and 6 mM [K(+)](o) demonstrated all-or-none bursting in both the CHX+PILO- and PILO-treated rats but not in control rats. Whole cell recordings from granule cells, using glutamate flash photolysis to activate other granule cells, showed that both the CHX+PILO- and PILO-treated rats had excitatory synaptic interactions in the granule cell layer, which were not found after saline treatment. Some rats responded to PILO (with or without CHX pre-treatment) with only one or a few seizures at treatment, and some of these animals (n = 4) demonstrated spontaneous motor seizures within 2 mo after treatment. Timm staining and neuron loss in this group were not clearly different from saline-treated rats. These results suggest that in the PILO model, pre-treatment with CHX does not affect mossy fiber sprouting in the IML of epileptic rats and does not prevent the formation of recurrent excitatory circuits. However, the develoment of spontaneous motor seizures, in a small number of rats, could occur without detectable hippocampal neuron loss or mossy fiber sprouting, as assessed by the Timm stain method.

Inhibition of backpropagating action potentials in mitral cell secondary dendrites

The mammalian olfactory bulb is a geometrically organized signal-processing array that utilizes lateral inhibitory circuits to transform spatially patterned inputs. A major part of the lateral circuitry consists of extensively radiating secondary dendrites of mitral cells. These dendrites are bidirectional cables: they convey granule cell inhibitory input to the mitral soma, and they conduct backpropagating action potentials that trigger glutamate release at dendrodendritic synapses. This study examined how mitral cell firing is affected by inhibitory inputs at different distances along the secondary dendrite and what happens to backpropagating action potentials when they encounter inhibition. These are key questions for understanding the range and spatial dependence of lateral signaling between mitral cells. Backpropagating action potentials were monitored in vitro by simultaneous somatic and dendritic whole cell recording from individual mitral cells in rat olfactory bulb slices, and inhibition was applied focally to dendrites by laser flash photolysis of caged GABA (2.5-microm spot). Photolysis was calibrated to activate conductances similar in magnitude to GABA(A)-mediated inhibition from granule cell spines. Under somatic voltage-clamp with CsCl dialysis, uncaging GABA onto the soma, axon initial segment, primary and secondary dendrites evoked bicuculline-sensitive currents (up to -1.4 nA at -60 mV; reversal at approximatety 0 mV). The currents exhibited a patchy distribution along the axon and dendrites. In current-clamp recordings, repetitive firing driven by somatic current injection was blocked by uncaging GABA on the secondary dendrite approximately 140 microm from the soma, and the blocking distance decreased with increasing current. In the secondary dendrites, backpropagated action potentials were measured 93-152 microm from the soma, where they were attenuated by a factor of 0.75 +/- 0.07 (mean +/- SD) and slightly broadened (1.19 +/- 0.10), independent of activity (35-107 Hz). Uncaging GABA on the distal dendrite had little effect on somatic spikes but attenuated backpropagating action potentials by a factor of 0.68 +/- 0.15 (0.45-0.60 microJ flash with 1-mM caged GABA); attenuation was localized to a zone of width 16.3 +/- 4.2 microm around the point of GABA release. These results reveal the contrasting actions of inhibition at different locations along the dendrite: proximal inhibition blocks firing by shunting somatic current, whereas distal inhibition can impose spatial patterns of dendrodendritic transmission by locally attenuating backpropagating action potentials. The secondary dendrites are designed with a high safety factor for backpropagation, to facilitate reliable transmission of the outgoing spike-coded data stream, in parallel with the integration of inhibitory inputs.

Multiphoton stimulation of neurons

We pulsed the activation of neurons using a femtosecond laser. Pyramidal neurons are depolarized and fire action potentials when high intensity mode-locked infrared light irradiates somatic membranes and axon initial segments. This depolarization is reversible, does not occur with CW laser light, and appears to be due to multiphoton excitation. We describe two regimes of multiphoton optical stimulation. Low intensity, long duration laser irradiation produces a sustained depolarization, insensitive to sodium channel blockers yet sensitive to antioxidants. On the other hand, high intensity, short duration irradiation can induce fast depolarizations, which appear due to different mechanism. The combination of multiphoton stimulation and optical probing could enable systematic analysis of circuits.

Infrared-guided laser stimulation of neurons in brain slices

Infrared-guided laser stimulation is a new technique that allows precise and rapid stimulation of visualized neurons in brain slices. Infrared imaging of neurons with a new contrast system is combined with the photolytic release of caged neurotransmitters by an ultraviolet (UV) laser. Addition of caged neurotransmitters to the superfusion medium of neurons in brain slices allows local excitation in the micrometer range with a focused spot of UV light. In this way, the distribution of glutamate or gamma-aminobutyric acid (GABA) receptors on neuronal dendrites can be mapped. Furthermore, this technique can be used to map the connectivity of neuronal networks through the controlled stimulation of neighboring neurons. Because the laser stimulation can be performed much faster than can paired recording, it is also possible to search for synaptic connections between distant neurons that have a low probability of connectivity.

Relationships of local inhibitory and excitatory circuits to orientation preference maps in ferret visual cortex

The contribution and precise role of intracortical circuits in generating orientation tuned responses in visual cortical neurons is still controversial. To address this question, the relationship between excitatory and inhibitory synaptic connections and orientation maps in ferret striate cortex was investigated by combining in vivo optical imaging and in vitro scanning laser photostimulation. Excitatory and inhibitory inputs to pyramidal cells originated preferentially from regions with similar orientation preference. Prominent cross-orientation inhibition was not observed, arguing against cross-orientation models of orientation selectivity. The tuning of inhibitory inputs was significantly broader in both layer 2/3 and layer 5/6 pyramidal neurons compared to the tuning of excitatory inputs. Local excitatory inputs were more prominent in the 0-20 degrees tuning difference range between pre- and postsynaptic cells than inhibitory inputs, whereas inhibition dominated in the 20-40 degrees tuning difference range. These differences in tuning of excitatory and inhibitory inputs onto individual cells are consistent with the predictions of recurrent models of orientation selectivity.

Response correlation maps of neurons in the mammalian olfactory bulb

To define the relationship between glomerular activation patterns and neuronal olfactory responses in the main olfactory bulb, intracellular recordings were combined with optical imaging of intrinsic signals. Response correlation maps (RCMs) were constructed by correlating the fluctuations in membrane potential and firing rate during odorant presentations with patterns of glomerular activation. The RCMs indicated that mitral/tufted cells were excited by activation of a focal region surrounding their principal glomerulus and generally inhibited by activation of more distant regions. However, the structure of the RCMs and the relative contribution of excitatory and inhibitory glomerular input evolved and even changed sign during and after odorant application. These data suggest a dynamic center-surround organization of mitral/tufted cell receptive fields.

Glutamate receptors form hot spots on apical dendrites of neocortical pyramidal neurons

Apical dendrites of layer V cortical pyramidal neurons are a major target for glutamatergic synaptic inputs from cortical and subcortical brain regions. Because innervation from these regions is somewhat laminar along the dendrites, knowing the distribution of glutamate receptors on the apical dendrites is of prime importance for understanding the function of neural circuits in the neocortex. To examine this issue, we used infrared-guided laser stimulation combined with whole cell recordings to quantify the spatial distribution of glutamate receptors along the apical dendrites of layer V pyramidal neurons. Focally applied (<10 microm) flash photolysis of caged glutamate on the soma and along the apical dendrite revealed a highly nonuniform distribution of glutamate responsivity. Up to four membrane areas (extent 22 microm) of enhanced glutamate responsivity (hot spots) were detected on the dendrites with the amplitude and integral of glutamate-evoked responses at hot spots being three times larger than responses evoked at neighboring sites. We found no association of these physiological hot spots with dendritic branch points. It appeared that the larger responses evoked at hot spots resulted from an increase in activation of both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors and not a recruitment of voltage-activated sodium or calcium conductances. Stimulation of hot spots did, however, facilitate the triggering of both Na+ spikes and Ca(2+) spikes, suggesting that hot spots may serve as dendritic initiation zones for regenerative spikes.

Layer-specific intracolumnar and transcolumnar functional connectivity of layer V pyramidal cells in rat barrel cortex

Layer V pyramidal cells in rat barrel cortex are considered to play an important role in intracolumnar and transcolumnar signal processing. However, the precise circuitry mediating this processing is still incompletely understood. Here we obtained detailed maps of excitatory and inhibitory synaptic inputs onto the two major layer V pyramidal cell subtypes, intrinsically burst spiking (IB) and regular spiking (RS) cells, using a combination of caged glutamate photolysis, whole-cell patch-clamp recording, and three-dimensional reconstruction of biocytin-labeled cells. To excite presynaptic neurons with laminar specificity, the release of caged glutamate was calibrated and restricted to small areas of 50 x 50 microm in all cortical layers and in at least two neighboring barrel-related columns. IB cells received intracolumnar excitatory input from all layers, with the largest EPSP amplitudes originating from neurons in layers IV and VI. Prominent transcolumnar excitatory inputs were provided by presynaptic neurons also located in layers IV, V, and VI of neighboring columns. Inhibitory inputs were rare. In contrast, RS cells received distinct intracolumnar inhibitory inputs, especially from layers II/III and V. Intracolumnar excitatory inputs to RS cells were prominent from layers II-V, but relatively weak from layer VI. Conspicuous transcolumnar excitatory inputs could be evoked solely in layers IV and V. Our results show that layer V pyramidal cells are synaptically driven by presynaptic neurons located in every layer of the barrel cortex. RS cells seem to be preferentially involved in intracolumnar signal processing, whereas IB cells effectively integrate excitatory inputs across several columns.

Layer-specific input to distinct cell types in layer 6 of monkey primary visual cortex

Layer 6 of monkey V1 contains a physiologically and anatomically diverse population of excitatory pyramidal neurons. Distinctive arborization patterns of axons and dendrites within the functionally specialized cortical layers define eight types of layer 6 pyramidal neurons and suggest unique information processing roles for each cell type. To address how input sources contribute to cellular function, we examined the laminar sources of functional excitatory input onto individual layer 6 pyramidal neurons using scanning laser photostimulation. We find that excitatory input sources correlate with cell type. Class I neurons with axonal arbors selectively targeting magnocellular (M) recipient layer 4Calpha receive input from M-dominated layer 4B, whereas class I neurons whose axonal arbors target parvocellular (P) recipient layer 4Cbeta receive input from P-dominated layer 2/3. Surprisingly, these neuronal types do not differ significantly in the inputs they receive directly from layers 4Calpha or 4Cbeta. Class II cells, which lack dense axonal arbors within layer 4C, receive excitatory input from layers targeted by their local axons. Specifically, type IIA cells project axons to and receive input from the deep but not superficial layers. Type IIB neurons project to and receive input from the deepest and most superficial, but not middle layers. Type IIC neurons arborize throughout the cortical layers and tend to receive inputs from all cortical layers. These observations have implications for the functional roles of different layer 6 cell types in visual information processing.

Recording and analyzing synaptic currents and synaptic potentials

Intracellular recording of synaptic currents (PSCs) under voltage clamp conditions provides the most accurate and direct means for measuring the earliest effects of neurotransmitters. With this tool, combined with pharmacological or ionic manipulations, one can obtain information about the type of transmitter used at a synapse, the dynamics of transmitter-receptor interactions, the types and numbers of receptors activated, the effects of drugs on transmission, functional neural circuitry, and indications about the mechanisms of synaptic plasticity. Each synaptic current or potential is a reflection of many experimental variables: the ionic composition of the solutions, the temperature, the presence of pharmacological agents, the rate of synaptic stimulation, the history of stimulation, the variables of the recording system, as well as other factors unique to each preparation. Correct analysis of data requires all these parameters be considered. Both stimulus-evoked and spontaneous synaptic events are covered in this unit since conclusions about synaptic and drug mechanisms are strongest when based upon recording of both types of activity. This unit outlines basic considerations for recording PSCs and PSPs in addition to guidelines for data analysis.

Two functional channels from primary visual cortex to dorsal visual cortical areas

Relationships between the M and P retino-geniculo-cortical visual pathways and "dorsal" visual areas were investigated by measuring the sources of local excitatory input to individual neurons in layer 4B of primary visual cortex. We found that contributions of the M and P pathways to layer 4B neurons are dependent on cell type. Spiny stellate neurons receive strong M input through layer 4Calpha and no significant P input through layer 4Cbeta. In contrast, pyramidal neurons in layer 4B receive strong input from both layers 4Calpha and 4Cbeta. These observations, along with evidence that direct input from layer 4B to area MT arises predominantly from spiny stellates, suggest that these different cell types constitute two functionally specialized subsystems.

GABA(A) and GABA(B) receptors on neocortical neurons are differentially distributed

The distribution of functional neurotransmitter receptors on the surface of neurons is highly relevant for synaptic transmission and signal processing. To map functional GABA(A) and GABA(B) receptors on the somadendritic membrane of rat neocortical layer V pyramidal neurons we used patch-clamp recording in combination with infrared-guided laser stimulation to release gamma-aminobutyric acid (GABA) photolytically. The data strongly suggest that relatively more GABA(A) receptors are located at the apical dendrite and relatively more GABA(B) receptors near the soma. Such a specific distribution of GABA(A) and GABA(B) receptors may serve to compensate for differences in electrotonic voltage attenuation between GABA(A) and GABA(B) receptor-mediated inhibitory postsynaptic potentials (IPSPs).

Excitatory synaptic input to granule cells increases with time after kainate treatment

Temporal lobe epilepsy is usually associated with a latent period and an increased seizure frequency following a precipitating insult. After kainate treatment, the mossy fibers of the dentate gyrus are hypothesized to form recurrent excitatory circuits between granule cells, thus leading to a progressive increase in the excitatory input to granule cells. Three groups of animals were studied as a function of time after kainate treatment: 1-2 wk, 2-4 wk, and 10-51 wk. All the animals studied 10-51 wk after kainate treatment were observed to have repetitive spontaneous seizures. Whole cell patch-clamp recordings in hippocampal slices showed that the amplitude and frequency of spontaneous excitatory postsynaptic currents (EPSCs) in granule cells increased with time after kainate treatment. This increased excitatory synaptic input was correlated with the intensity of the Timm stain in the inner molecular layer (IML). Flash photolysis of caged glutamate applied in the granule cell layer evoked repetitive EPSCs in 10, 32, and 66% of the granule cells at the different times after kainate treatment. When inhibition was reduced with bicuculline, photostimulation of the granule cell layer evoked epileptiform bursts of action potentials only in granule cells from rats 10-51 wk after kainate treatment. These data support the hypothesis that kainate-induced mossy fiber sprouting in the IML results in the progressive formation of aberrant excitatory connections between granule cells. They also suggest that the probability of occurrence of electrographic seizures in the dentate gyrus increases with time after kainate treatment.

Laminar sources of synaptic input to cortical inhibitory interneurons and pyramidal neurons

The functional role of an individual neuron within a cortical circuit is largely determined by that neuron's synaptic input. We examined the laminar sources of local input to subtypes of cortical neurons in layer 2/3 of rat visual cortex using laser scanning photostimulation. We identified three distinct laminar patterns of excitatory input that correspond to physiological and morphological subtypes of neurons. Fast-spiking inhibitory basket cells and excitatory pyramidal neurons received strong excitatory input from middle cortical layers. In contrast, adapting inhibitory interneurons received their strongest excitatory input either from deep layers or laterally from within layer 2/3. Thus, differential laminar sources of excitatory inputs contribute to the functional diversity of cortical inhibitory interneurons.

Fourier transform infrared spectroscopic characterization of a photolabile precursor of glutamate

Recently, it has been demonstrated that Fourier transform infrared spectroscopy (FTIR) detects conformational changes in the glutamate receptor ligand-binding domain that are associated with agonist binding. Combined with flash photolysis, this observation offers the prospect of following conformational changes at individual protein and agonist moieties in parallel and with high temporal resolution. Here, we demonstrate that gamma(alpha-carboxy-2-nitrobenzyl) glutamate (caged glutamate) does not interact with the protein, and that following photolysis with UV light the FTIR difference spectrum indicated changes in the protein tertiary and secondary interactions. These changes were similar to those observed for the protein upon addition of free glutamate. Thus, caged glutamate and its photolysis by-products are inert in this system, whereas the released glutamate exhibits full activity. Difference spectra of caged glutamate and of reaction analogs permitted identification of and correction for FTIR signals arising from the photolytic reaction and confirmed that its products are indeed glutamate and 2-nitrosophenyl glyoxalic acid.

Laminar characteristics of functional connectivity in rat barrel cortex revealed by stimulation with caged-glutamate

In rodent somatosensory (barrel) cortex input is processed by whisker-related columns before the integrated output is fed into behaviorally-relevant circuits. The layer-specific activation patterns of the rat barrel cortex were examined with a set-up for scanning functional connectivity in brain slices. Flash-induced release of caged-glutamate at a large number of stimulation sites was used in combination with simultaneous field potential recordings from layers II to VI with five electrodes. The field potentials revealed striking differences between the cortical layers. Glutamate-release in layer IV and lower layer III was most effective in evoking excitation in all other cortical layers, whereas field potentials recorded from layer IV itself were caused by stimulation of a very restricted columnar zone only. Field potentials in layers II and III were strongly driven by stimulation in layer IV and less consistently and much weaker by layer V. Layer V was the only lamina capable of responding to stimulation of all other cortical layers, thus displaying the largest input maps. Layer VI possessed functional connectivity intrinsically and with layer V. These data lead us to suggest that thalamic input may be boosted by its main target layer IV to start a sequence of excitation in layer IV, passing to the supragranular layers and finally reaching the infragranular layers. This sequence is likely to be backed-up by other simultaneous steps of transmission including a layer IV-to-V interaction. We proposed that the increasing size of the receptive fields when sampling granular, supragranular and infragranular layers in vivo, might have its functional basis in the laminar interactions described here in an in vitro preparation.

gamma-Aminobutyrate, alpha-carboxy-2-nitrobenzyl ester selectively blocks inhibitory synaptic transmission in rat dentate gyrus

gamma-Aminobutyrate, alpha-carboxy-2-nitrobenzyl ester (cGABA) is a stable photoactivatable probe used to study gamma-aminobutyrate (GABA) receptors. GABA is released from this compound when it is exposed to ultraviolet light, but little is known about the electrophysiological effects of the compound itself. Whole cell patch clamp recordings on rat hippocampal slices demonstrated that cGABA blocked polysynaptic inhibitory postsynaptic currents (IPSCs) evoked in dentate granule cells by antidromic stimulation of the mossy fibers. It also reduced monosynaptically evoked IPSCs with an IC(50) of 28 microM. In contrast, cGABA had no effect on excitatory postsynaptic currents (EPSCs) evoked by perforant path stimulation. The effect of cGABA was not mediated by depression of GABA release through activation of presynaptic GABA(B) receptors. cGABA inhibited muscimol-evoked currents by only 15% at a concentration of 40 microM. At this same concentration, it reduced the mean frequency of miniature inhibitory postsynaptic potentials by 71%, their mean peak amplitude by 44%, their mean decay time constant by 26% and the mean charge transfer per event by 52%. These effects may be explained by a phenothiazine-like modification of GABA(A) receptor kinetics and/or a selective block of somatic GABA synapses.