Two-photon imaging reveals somatodendritic chloride gradient in retinal ON-type bipolar cells expressing the biosensor Clomeleon

The Gal4/UAS toolbox in zebrafish: new approaches for defining behavioral circuits

Over recent years, several groundbreaking techniques have been developed that allow for the anatomical description of neurons, and the observation and manipulation of their activity. Combined, these approaches should provide a great leap forward in our understanding of the structure and connectivity of the nervous system and how, as a network of individual neurons, it generates behavior. Zebrafish, given their external development and optical transparency, are an appealing system in which to employ these methods. These traits allow for direct observation of fluorescence in describing anatomy and observing neural activity, and for the manipulation of neurons using a host of light-triggered proteins. Gal4/Upstream Activating Sequence techniques, as they are based on a binary system, allow for the flexible deployment of a range of transgenes in expression patterns of interest. As such, they provide a promising approach for viewing neurons in a variety of ways, each of which can reveal something different about their structure, connectivity, or function. In this study, the author will review recent progress in the development of the Gal4/Upstream Activating Sequence system in zebrafish, feature examples of promising studies to date, and examine how various new technologies can be used in the future to untangle the complex mechanisms by which behavior is generated.

Cation-chloride cotransporters and neuronal function

Recent years have witnessed a steep increase in studies on the diverse roles of neuronal cation-chloride cotransporters (CCCs). The versatility of CCC gene transcription, posttranslational modification, and trafficking are on par with what is known about ion channels. The cell-specific and subcellular expression patterns of different CCC isoforms have a key role in modifying a neuron's electrophysiological phenotype during development, synaptic plasticity, and disease. While having a major role in controlling responses mediated by GABA(A) and glycine receptors, CCCs also show close interactions with glutamatergic signaling. A cross-talk among CCCs and trophic factors is important in short-term and long-term modification of neuronal properties. CCCs appear to be multifunctional proteins that are also involved in shaping neuronal structure at various stages of development, from stem cells to synaptogenesis. The rapidly expanding work on CCCs promotes our understanding of fundamental mechanisms that control brain development and functions under normal and pathophysiological conditions.

Expression analysis of green fluorescent protein in retinal neurons of four transgenic mouse lines

Transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of a cell-specific promoter have been used with great success to identify and label specific cell types of the retina. We studied the expression of EGFP in the retina of mice making use of four transgenic mouse lines. Expression of EGFP driven by the calretinin promoter was found in amacrine, displaced amacrine and ganglion cells. Comparison of the EGFP expression and calretinin immunolabeling showed that many but not all cells appear to be double labeled. Expression of EGFP under the control of the choline acetyltransferase promoter was found in amacrine cells; however, the cells did not correspond to the well known cholinergic (starburst) cells of the mouse retina. The expression of EGFP under the control of the parvalbumin promoter was restricted to amacrine cells of the inner nuclear layer and to cells of the ganglion cell layer (displaced amacrine cells and ganglion cells). Most of the cells were also immunoreactive for parvalbumin, however, differences in labeling intensity were observed. The expression of EGFP driven by the promoter for the 5-HT3 A receptor (5-HTR3A) was restricted to type 5 bipolar cells. In contrast, immunostaining for 5-HTR3A was found in synaptic hot spots in sublamina 1 of the inner plexiform layer and was not related to type 5 bipolar cells. The results show that these transgenic mice are very useful for future electrophysiological studies of specific types of amacrine and bipolar cells that express EGFP and thus permit directed microelectrode targeting under microscopic control.

Cone contacts, mosaics, and territories of bipolar cells in the mouse retina

We report a quantitative analysis of the different bipolar cell types of the mouse retina. They were identified in wild-type mice by specific antibodies or in transgenic mouse lines by specific expression of green fluorescent protein or Clomeleon. The bipolar cell densities, their cone contacts, their dendritic coverage, and their axonal tiling were measured in retinal whole mounts. The results show that each and all cones are contacted by at least one member of any given type of bipolar cell (not considering genuine blue cones). Consequently, each cone feeds its light signals into a minimum of 10 different bipolar cells. Parallel processing of an image projected onto the retina, therefore, starts at the first synapse of the retina, the cone pedicle. The quantitative analysis suggests that our proposed catalog of 11 cone bipolar cells and one rod bipolar cell is complete, and all major bipolar cell types of the mouse retina appear to have been discovered.

Passive membrane properties and electrotonic signal processing in retinal rod bipolar cells

Rod bipolar cells transmit visual signals from their dendrites, where they receive input from rod photoreceptors, to their axon terminals, where they synapse onto amacrine cells. Little is known, however, about the transmission and possible transformation of these signals. We have combined axon terminal recording in retinal slices, quantitative, light-microscopic morphological reconstruction and computer modelling to obtain detailed compartmental models of rat rod bipolar cells. Passive cable properties were estimated by directly fitting the current responses of the models evoked by voltage pulses to the physiologically recorded responses. At a holding potential of -60 mV, the average best-fit parameters were 1.1 microF cm(-2) for specific membrane capacitance (C(m)), 130 Omega cm for cytoplasmic resistivity (R(i)), and 24 kOmega cm(2) for specific membrane resistance (R(m)). The passive integration of excitatory and inhibitory synaptic inputs was examined by computer modelling with physiologically realistic synaptic conductance waveforms. For both transient and steady-state synaptic inhibition, the inhibitory effect was relatively insensitive to the location of the inhibition. For transient synaptic inhibition, the time window of effective inhibition depended critically on the relative timing of inhibition and excitation. The passive signal transmission between soma and axon terminal was examined by the electrotonic transform and quantified as the frequency-dependent voltage attenuation of sinusoidal voltage waveforms. For the range of parameters explored (axon diameter and length, R(i)), the lowest cutoff frequency observed was approximately 300 Hz, suggesting that realistic scotopic visual signals will be faithfully transmitted from soma to axon terminal, with minimal passive attenuation along the axon.

NKCC1 and AE3 appear to accumulate chloride in embryonic motoneurons

During early development, gamma-aminobutyric acid (GABA) depolarizes and excites neurons, contrary to its typical function in the mature nervous system. As a result, developing networks are hyperexcitable and experience a spontaneous network activity that is important for several aspects of development. GABA is depolarizing because chloride is accumulated beyond its passive distribution in these developing cells. Identifying all of the transporters that accumulate chloride in immature neurons has been elusive and it is unknown whether chloride levels are different at synaptic and extrasynaptic locations. We have therefore assessed intracellular chloride levels specifically at synaptic locations in embryonic motoneurons by measuring the GABAergic reversal potential (EGABA) for GABAA miniature postsynaptic currents. When whole cell patch solutions contained 17-52 mM chloride, we found that synaptic EGABA was around -30 mV. Because of the low HCO3- permeability of the GABAA receptor, this value of EGABA corresponds to approximately 50 mM intracellular chloride. It is likely that synaptic chloride is maintained at levels higher than the patch solution by chloride accumulators. We show that the Na+-K+-2Cl- cotransporter, NKCC1, is clearly involved in the accumulation of chloride in motoneurons because blocking this transporter hyperpolarized EGABA and reduced nerve potentials evoked by local application of a GABAA agonist. However, chloride accumulation following NKCC1 block was still clearly present. We find physiological evidence of chloride accumulation that is dependent on HCO3- and sensitive to an anion exchanger blocker. These results suggest that the anion exchanger, AE3, is also likely to contribute to chloride accumulation in embryonic motoneurons.

Monitoring of chloride and activity of glycine receptor channels using genetically encoded fluorescent sensors

Genetically encoded probes have become powerful tools for non-invasive monitoring of ions, distributions of proteins and the migration and formation of cellular components. We describe the functional expression of two molecular probes for non-invasive fluorescent monitoring of intracellular Cl ([Cl]i) and the functioning of glycine receptor (GlyR) channels. The first probe is a recently developed cyan fluorescent protein-yellow fluorescent protein-based construct, termed Cl-Sensor, with relatively high sensitivity to Cl (Kapp approximately 30 mM). In this study, we describe its expression in retina cells using in vivo electroporation and analyse changes in [Cl]i at depolarization and during the first three weeks of post-natal development. An application of 40 mM K+ causes an elevation in [Cl]i of approximately 40 mM. In photoreceptors from retina slices of a 6-day-old rat (P6 rat), the mean [Cl]i is approximately 50 mM, and for P16 and P21 rats it is approximately 30-35 mM. The second construct, termed BioSensor-GlyR, is a GlyR channel with Cl-Sensor incorporated into the cytoplasmic domain. This is the first molecular probe for spectroscopic monitoring of the functioning of receptor-operated channels. These types of probes offer a means of screening pharmacological agents and monitoring Cl under different physiological and pathological conditions and permit spectroscopic monitoring of the activity of GlyRs expressed in heterologous systems and neurons.

Axonal GABAA receptors

Type A GABA receptors (GABA(A)Rs) are well established as the main inhibitory receptors in the mature mammalian forebrain. In recent years, evidence has accumulated showing that GABA(A)Rs are prevalent not only in the somatodendritic compartment of CNS neurons, but also in their axonal compartment. Evidence for axonal GABA(A)Rs includes new immunohistochemical and immunogold data: direct recording from single axonal terminals; and effects of local applications of GABA(A)R modulators on action potential generation, on axonal calcium signalling, and on neurotransmitter release. Strikingly, whereas presynaptic GABA(A)Rs have long been considered inhibitory, the new studies in the mammalian brain mostly indicate an excitatory action. Depending on the neuron that is under study, axonal GABA(A)Rs can be activated by ambient GABA, by GABA spillover, or by an autocrine action, to increase either action potential firing and/or transmitter release. In certain neurons, the excitatory effects of axonal GABA(A)Rs persist into adulthood. Altogether, axonal GABA(A)Rs appear as potent neuronal modulators of the mammalian CNS.

Imaging synaptic inhibition throughout the brain via genetically targeted Clomeleon

Here we survey a molecular genetic approach for imaging synaptic inhibition. This approach is based on measuring intracellular chloride concentration ([Cl(-)](i)) with the fluorescent chloride indicator protein, Clomeleon. We first describe several different ways to express Clomeleon in selected populations of neurons in the mouse brain. These methods include targeted viral gene transfer, conditional expression controlled by Cre recombination, and transgenesis based on the neuron-specific promoter, thy1. Next, we evaluate the feasibility of using different lines of thy1::Clomeleon transgenic mice to image synaptic inhibition in several different brain regions: the hippocampus, the deep cerebellar nuclei (DCN), the basolateral nucleus of the amygdala, and the superior colliculus (SC). Activation of hippocampal interneurons caused [Cl(-)](i) to rise transiently in individual postsynaptic CA1 pyramidal neurons. [Cl(-)](i) increased linearly with the number of electrical stimuli in a train, with peak changes as large as 4 mM. These responses were largely mediated by GABA receptors because they were blocked by antagonists of GABA receptors, such as GABAzine and bicuculline. Similar responses to synaptic activity were observed in DCN neurons, amygdalar principal cells, and collicular premotor neurons. However, in contrast to the hippocampus, the responses in these three regions were largely insensitive to antagonists of inhibitory neurotransmitter receptors. This indicates that synaptic activity can also cause Cl(-) influx through alternate pathways that remain to be identified. We conclude that Clomeleon imaging permits non-invasive, spatiotemporally precise recordings of [Cl(-)](i) in a large variety of neurons, and provides new opportunities for imaging synaptic inhibition and other forms of neuronal chloride signaling.

Localization and possible function of P2Y(4) receptors in the rodent retina

Extracellular ATP acts as a neurotransmitter in the retina, via the activation of ionotropic P2X receptors and metabotropic P2Y receptors. The expression of various P2X and P2Y receptor subtypes has been demonstrated in the retina, but the localization of P2Y receptors and their role in retinal signaling remains ill defined. In this study, we were interested in determining the localization of the P2Y(4) receptor subtype in the rat retina, and using the electroretinogram (ERG) to assess whether activation of these receptors modulated visual transmission. Using light and electron microscopy, we demonstrated that P2Y(4) receptors were expressed pre-synaptically in rod bipolar cells and in processes postsynaptic to cone bipolar cells. Furthermore, we show that the expression of P2Y(4) receptors on rod bipolar cell axon terminals is reduced following dark adaptation, suggesting receptor expression may be dependent on retinal activity. Finally, using the electroretinogram, we show that intravitreal injection of uridine triphosphate, a P2Y receptor agonist, decreases the amplitude of the rod PII, supporting a role for P2Y receptors in altering inner retinal function. Taken together, these results suggest a role for P2Y(4) receptors in the modulation of inner retinal signaling.

Glycine input induces the synaptic facilitation in salamander rod photoreceptors

Glycinergic synapses in photoreceptors are made by centrifugal feedback neurons in the network, but the function of the synapses is largely unknown. Here we report that glycinergic input enhances photoreceptor synapses in amphibian retinas. Using specific antibodies against a glycine transporter (GlyT2) and glycine receptor beta subunit, we identified the morphology of glycinergic input in photoreceptor terminals. Electrophysiological recordings indicated that 10 muM glycine depolarized rods and activated voltage-gated Ca(2+) channels in the neurons. The effects facilitated glutamate vesicle release in photoreceptors, meanwhile increased the spontaneous excitatory postsynaptic currents in Off-bipolar cells. Endogenous glycine feedback also enhanced glutamate transmission in photoreceptors. Additionally, inhibition of a Cl(-) uptake transporter NKCC1 with bumetanid effectively eliminated glycine-evoked a weak depolarization in rods, suggesting that NKCC1 maintains a high Cl(-) level in rods, which causes to depolarize in responding to glycine input. This study reveals a new function of glycine in retinal synaptic transmission.

Treatment of epilepsy: the GABA-transaminase inhibitor, vigabatrin, induces neuronal plasticity in the mouse retina

Vigabatrin was a major drug in the treatment of epilepsy until the discovery that it was associated with an irreversible constriction of the visual field. Nevertheless, the drug is still prescribed for infantile spasms and refractory epilepsy. Disorganization of the photoreceptor nuclear layer and cone photoreceptor damage have been described in albino rats. To investigate the vigabatrin-elicited retinal toxicity further, we examined the retinal tissue of albino mice treated with two vigabatrin doses. The higher dose did not always cause the photoreceptor layer disorganization after 1 month of treatment. However, it triggered a massive synaptic plasticity in retinal areas showing a normal layering of the retina. This plasticity was shown by the withdrawal of rod but not cone photoreceptor terminals from the outer plexiform layers towards their cell bodies. Furthermore, both rod bipolar cells and horizontal cells exhibited dendritic sprouting into the photoreceptor nuclear layer. Withdrawing rod photoreceptors appeared to form ectopic contacts with growing postsynaptic dendrites. Indeed, contacts between rods and bipolar cells, and between bipolar cells and horizontal cells were observed deep inside the outer nuclear layer. This neuronal plasticity is highly suggestive of an impaired glutamate release by photoreceptors because similar observations have been reported in different genetically modified mice with deficient synaptic transmission. Such a synaptic deficit is consistent with the decrease in glutamate concentration induced by vigabatrin. This description of the neuronal plasticity associated with vigabatrin provides new insights into its retinal toxicity in epileptic patients.

Transporter-mediated GABA responses in horizontal and bipolar cells of zebrafish retina

GABA-mediated interactions between horizontal cells (HCs) and bipolar cells (BCs) transform signals within the image-processing circuitry of distal retina. To further understand this process, we have studied the GABA-driven membrane responses from isolated retinal neurons. Papain-dissociated retinal cells from adult zebrafish were exposed to GABAergic ligands while transmembrane potentials were monitored with a fluorescent voltage-sensitive dye (oxonol, DiBaC4(5)). In HCs hyperpolarizing, ionotropic GABA responses were almost never seen, nor were responses to baclofen or glycine. A GABA-induced depolarization followed by after hyperpolarization (dep/AHP) occurred in 38% of HCs. The median fluorescence increase (dep component) was 0.17 log units, about 22 mV. HC dep/AHP was not blocked by bicuculline or picrotoxin. Muscimol rarely evoked dep/AHP responses. In BCs picrotoxin sensitive, hyperpolarizing, ionotropic GABA and muscimol responses occurred in most cells. A picrotoxin insensitive dep/AHP response was seen in about 5% of BCs. The median fluorescence increase (dep component) was 0.18 log units, about 23 mV. Some BCs expressed both muscimol-induced hyperpolarizations and GABA-induced dep/AHP responses. For all cells, the pooled Hill fit to median dep amplitudes, in response to treatments with a GABA concentration series, gave an apparent k of 0.61 muM and an n of 1.1. The dep/AHP responses of all cells required both extracellular Na+ and Cl(-), as dep/AHP was blocked reversibly by Li+ substituted for Na+ and irreversibly by isethionate substituted for Cl(-). All cells with dep/AHP responses in zebrafish have the membrane physiology of neurons expressing GABA transporters. These cells likely accumulate GABA, a characteristic of GABAergic neurons. We suggest Na+ drives GABA into these cells, depolarizing the plasma membrane and triggering Na+, K+-dependent ATPase. The ATPase activity generates AHP. In addition to a GABA clearance function, these large-amplitude transporter responses may provide an outer plexiform layer GABA sensor mechanism.

Spatial and temporal distribution patterns of Na-K-2Cl cotransporter in adult and developing mouse retinas

The Na-K-2Cl cotransporter (NKCC) is a Cl(-) uptake transporter that is responsible for maintaining a Cl(-) equilibrium potential positive to the resting potential in neurons. If NKCC is active, GABA and glycine can depolarize neurons. In view of the abundance of GABAergic and glycinergic synapses in retina, we undertook a series of studies using immunocytochemical techniques to determine the distribution of NKCC in retinas of both developing and adult mice. We found NKCC antibody (T4) labeling present in retinas from wild-type mice, but not in NKCC1-deficient mice, suggesting that the NKCC1 subtype is a major Cl(-) uptake transporter in mouse retina. Strong labeling of NKCC1 was present in horizontal cells and rod-bipolar dendrites in adult mice. Interestingly, we also found that a diffuse labeling pattern was present in photoreceptor terminals. However, NKCC1 was barely detectable in the inner retina of adult mice. Using an antibody against K-Cl cotransporter 2 (KCC2), we found that KCC2, a transporter that extrudes Cl(-), was primarily expressed in the inner retina. The expression of NKCC1 in developing mouse retinas was studied from postnatal day (P) 1 to P21, NKCC1 labeling first appeared in the dendrites of horizontal and rod-bipolar cells as early as P7, followed by photoreceptor terminals between P10-P14; with expression gradually increasing concomitantly with the growth of synaptic terminals and dendrites throughout retinal development. In the inner retina, NKCC1 labeling was initially observed in the inner plexiform layer at P1, but labeling diminished after P5. The developmental increase in NKCC expression only occurred in the outer retina. Our results suggest that the distal synapses and synaptogenesis in mouse retinas undergo a unique process with a high intracellular Cl(-) presence due to NKCC1 expression.

Dissociated gender-specific effects of recurrent seizures on GABA signaling in CA1 pyramidal neurons: role of GABA(A) receptors

Early in development, the depolarizing GABA(A)ergic signaling is needed for normal neuronal differentiation. It is shown here that hyperpolarizing reversal potentials of GABA(A)ergic postsynaptic currents (E(GABA)) appear earlier in female than in male rat CA1 pyramidal neurons because of increased potassium chloride cotransporter 2 (KCC2) expression and decreased bumetanide-sensitive chloride transport in females. Three episodes of neonatal kainic acid-induced status epilepticus (3KA-SE), each elicited at postnatal days 4 (P4)-P6, reverse the direction of GABA(A)ergic responses in both sexes. In males, 3KA-SE trigger a premature appearance of hyperpolarizing GABA(A)ergic signaling at P9, instead of P14. This is driven by an increase in KCC2 expression and decrease in bumetanide-sensitive chloride cotransport. In 3KA-SE females, E(GABA) transiently becomes depolarizing at P8-P13 because of increase in the activity of a bumetanide-sensitive NKCC1 (sodium potassium chloride cotransporter 1)-like chloride cotransporter. However, females regain their hyperpolarizing GABA(A)ergic signaling at P14 and do not manifest spontaneous seizures in adulthood. In maternally separated stressed controls, a hyperpolarizing shift in E(GABA) was observed in both sexes, associated with decreased bumetanide-sensitive chloride cotransport, whereas KCC2 immunoreactivity was increased in males only. GABA(A) receptor blockade at the time of 3KA-SE or maternal separation reversed their effects on E(GABA). These data suggest that the direction of GABA(A)-receptor signaling may be a determining factor for the age and sex-specific effects of prolonged seizures in the hippocampus, because they relate to normal brain development and possibly epileptogenesis. These effects differ from the consequences of severe stress.

Genetically encoded chloride indicator with improved sensitivity

Chloride (Cl) is the most abundant physiological anion. Abnormalities in Cl regulation are instrumental in the development of several important diseases including motor disorders and epilepsy. Because of difficulties in the spectroscopic measurement of Cl in live tissues there is little knowledge available regarding the mechanisms of regulation of intracellular Cl concentration. Several years ago, a CFP-YFP based ratiometric Cl indicator (Clomeleon) was introduced [Kuner, T., Augustine, G.J. A genetically encoded ratiometric indicator for chloride: capturing chloride transients in cultured hippocampal neurons. Neuron 2000; 27: 447-59]. This construct with relatively low sensitivity to Cl (K(app) approximately 160 mM) allows ratiometric monitoring of Cl using fluorescence emission ratio. Here, we propose a new CFP-YFP-based construct (Cl-sensor) with relatively high sensitivity to Cl (K(app) approximately 30 mM) due to triple YFP mutant. The construct also exhibits good pH sensitivity with pK(alpha) ranging from 7.1 to 8.0 pH units at different Cl concentrations. Using Cl-sensor we determined non-invasively the distribution of [Cl](i) in cultured CHO cells, in neurons of primary hippocampal cultures and in photoreceptors of rat retina. This genetically encoded indicator offers a means for monitoring Cl and pH under different physiological conditions and high-throughput screening of pharmacological agents.

GABAergic control of CA3-driven network events in the developing hippocampus

Endogenous activity is a characteristic feature of developing neuronal networks. In the neonatal rat hippocampus, spontaneously occurring network events known as "Giant Depolarizing Potentials" (GDPs) are seen in vitro at a stage when GABAergic transmission is depolarizing. GDPs are triggered by the CA3 region and they are seen as brief recurrent events in field-potential recordings, paralleled by depolarization and spiking of pyramidal neurons. In the light of current data, GDPs are triggered by the glutamatergic pyramidal neurons which act as conditional pacemakers, while the depolarizing action of GABA plays a permissive role for the generation of these events in in vitro preparations. From an in vivo perspective, GDPs appear to be an immature form of hippocampal sharp waves.

Synaptic connections between layer 5B pyramidal neurons in mouse somatosensory cortex are independent of apical dendrite bundling

Rodent somatosensory barrel cortex is organized both physiologically and anatomically in columns with a cross-sectional diameter of 100-400 microm. The underlying anatomical correlate of physiologically defined, much narrower minicolumns (20-60 microm in diameter) remains unclear. The minicolumn has been proposed to be a fundamental functional unit in the cortex, and one anatomical component of a minicolumn is thought to be a cluster of pyramidal cells in layer 5B (L5B) that contribute their apical dendrite to distinct bundles. In transgenic mice with fluorescently labeled L5B pyramidal cells, which project to the pons and thalamus, we investigated whether the pyramidal cells of a cluster also share functional properties. We found that apical dendrite bundles in the transgenic mice were anatomically similar to apical dendrite bundles previously proposed to be part of minicolumns. We made targeted whole-cell recordings in acute brain slices from pairs of fluorescently labeled L5B pyramidal cells that were located either in the same cluster or in adjacent clusters and subsequently reconstructed their dendritic arbors. Pyramids within the same cluster had larger common dendritic domains compared with pyramids in adjacent clusters but did not receive more correlated synaptic inputs. L5B pyramids within and between clusters have similar connection probabilities and unitary EPSP amplitudes. Furthermore, intrinsically bursting and regular spiking pyramidal cells were both present within the same cluster. In conclusion, intrinsic electrical excitability and the properties of synaptic connections between this subtype of L5B pyramidal cells are independent of the cell clusters defined by bundling of their apical dendrites.

Visualizing circuits and systems using transgenic reporters of neural activity

Genetically encoded sensors of neural activity enable visualization of circuit-level function in the central nervous system. Although our understanding of the molecular events that regulate neuronal firing, synaptic function, and plasticity has expanded rapidly over the past 15 years, an appreciation for how cellular changes are functionally integrated at the circuit level has lagged. A new generation of tools that employ fluorescent sensors of neural activity promises unique opportunities to bridge the gap between cellular level and system level analysis. This review will focus on genetically encoded sensors. A primary advantage of these indicators is that they can be nonselectively introduced to large populations of cells using either transgenic-mediated or viral-mediated approaches. This ability removes the nontrivial obstacles of how to get chemical indicators into cells of interest, a problem that has dogged investigators who have been interested in mapping neural function in the intact CNS. Five different types of approaches and their relative utility will be reviewed here: first, reporters of immediate-early gene (IEG) activation using promoters such as c-fos and arc; second, voltage-based sensors, such as GFP-coupled Na+ and K+ channels; third, Cl*-based sensors; fourth, Ca2+-based sensors, such as Camgaroo and the troponin-based TN-L15; and fifth, pH-based sensors, which have been particularly useful for examining synaptic activity of highly convergent afferents in sensory systems in vivo. Particular attention will be paid to reporters of IEG expression, because these tools employ the built-in threshold function that occurs with activation of gene expression, provoking new experimental questions by expanding the timescale of analysis for circuit-level and system-level functional mapping.

Transgenic expression of fluorescent proteins in respiratory neurons

We screened transgenic mouse lines with Thy1.2 promoter-induced expression of fluorescent proteins (FPs) for targeting of respiratory neuronal populations in the medulla oblongata. Respiratory neurons were found to be tagged by FPs within the ventral respiratory column (VRC), the pre-Bötzinger complex (preBötC) and the rostral ventral respiratory group (rVRG) interneurons. A subset of neurons in the preBötC, labeled with the enhanced yellow fluorescent protein (EYFP), showed inspiratory activity during whole cell recordings from rhythmic slice preparations. Additionally, a subpopulation of EYFP-labeled preBötC neurons expressed both NK1- and mu-opioid receptors. Furthermore, the spinal trigeminal nucleus, the lateral reticular nucleus (LRT) and the hypoglossal nucleus demonstrated intense EYFP expression whereas other regions of the medulla were devoid of neuronal EYFP labeling (e.g. the nucleus ambiguous). In conclusion, Thy1.2-FP transgenic mice will facilitate the functional analysis of respiratory-related neurons in the medulla and improve the three dimensional analysis of cells contributing to this important neuronal circuit.

Two-photon microscopy: shedding light on the chemistry of vision

Two-photon microscopy (TPM) has come to occupy a prominent place in modern biological research with its ability to resolve the three-dimensional distribution of molecules deep inside living tissue. TPM can employ two different types of signals, fluorescence and second harmonic generation, to image biological structures with subcellular resolution. Two-photon excited fluorescence imaging is a powerful technique with which to monitor the dynamic behavior of the chemical components of tissues, whereas second harmonic imaging provides novel ways to study their spatial organization. Using TPM, great strides have been made toward understanding the metabolism, structure, signal transduction, and signal transmission in the eye. These include the characterization of the spatial distribution, transport, and metabolism of the endogenous retinoids, molecules essential for the detection of light, as well as the elucidation of the architecture of the living cornea. In this review, we present and discuss the current applications of TPM for the chemical and structural imaging of the eye. In addition, we address what we see as the future potential of TPM for eye research. This relatively new method of microscopy has been the subject of numerous technical improvements in terms of the optics and indicators used, improvements that should lead to more detailed biochemical characterizations of the eyes of live animals and even to imaging of the human eye in vivo.

Robust syntaxin-4 immunoreactivity in mammalian horizontal cell processes

Horizontal cells mediate inhibitory feed-forward and feedback communication in the outer retina; however, mechanisms that underlie transmitter release from mammalian horizontal cells are poorly understood. Toward determining whether the molecular machinery for exocytosis is present in horizontal cells, we investigated the localization of syntaxin-4, a SNARE protein involved in targeting vesicles to the plasma membrane, in mouse, rat, and rabbit retinae using immunocytochemistry. We report robust expression of syntaxin-4 in the outer plexiform layer of all three species. Syntaxin-4 occurred in processes and tips of horizontal cells, with regularly spaced, thicker sandwich-like structures along the processes. Double labeling with syntaxin-4 and calbindin antibodies, a horizontal cell marker, demonstrated syntaxin-4 localization to horizontal cell processes; whereas, double labeling with PKC antibodies, a rod bipolar cell (RBC) marker, showed a lack of co-localization, with syntaxin-4 immunolabeling occurring just distal to RBC dendritic tips. Syntaxin-4 immunolabeling occurred within VGLUT-1-immunoreactive photoreceptor terminals and underneath synaptic ribbons, labeled by CtBP2/RIBEYE antibodies, consistent with localization in invaginating horizontal cell tips at photoreceptor triad synapses. Vertical sections of retina immunostained for syntaxin-4 and peanut agglutinin (PNA) established that the prominent patches of syntaxin-4 immunoreactivity were adjacent to the base of cone pedicles. Horizontal sections through the OPL indicate a one-to-one co-localization of syntaxin-4 densities at likely all cone pedicles, with syntaxin-4 immunoreactivity interdigitating with PNA labeling. Pre-embedding immuno-electron microscopy confirmed the subcellular localization of syntaxin-4 labeling to lateral elements at both rod and cone triad synapses. Finally, co-localization with SNAP-25, a possible binding partner of syntaxin-4, indicated co-expression of these SNARE proteins in the same subcellular compartment of the horizontal cell. Taken together, the strong expression of these two SNARE proteins in the processes and endings of horizontal cells at rod and cone terminals suggests that horizontal cell axons and dendrites are likely sites of exocytotic activity.

Cation Cl- cotransporters in the dendrites of goldfish bipolar cells

Bipolar cells receive an input from gamma-aminobutyric acid ergic horizontal cells in distal retinas. gamma-Aminobutyric acid can induce either an inhibitory or an excitatory Cl(-) response based on the intracellular Cl(-) concentration in the bipolar dendrites. Two cation Cl(-) cotransporters, Na-K-2Cl(-) and K-2Cl(-), differently control local Cl(-) levels in neurons by uptaking or extracting Cl(-), respectively. By using immunocytochemical techniques, we found that Na-K-2Cl(-) and K-2Cl(-) were preferentially distributed in On-bipolar and Off-bipolar dendrites in goldfish retinas. Na-K-2Cl(-) was colocalized with protein kinase C in On-bipolar dendrites, whereas K-2Cl(-) was located in Off-bipolar dendrites, fully overlapping with ionotropic glutamate receptor GluR4. Possibly, an internal Cl(-) level is higher in the On-bipolar dendrites than in the Off-bipolar dendrites. gamma-Aminobutyric acid inputs in the distal retina might excite On-bipolar cells, but inhibit Off-bipolar cells.

NKCC1 does not accumulate chloride in developing retinal neurons

GABA excites immature neurons due to their relatively high intracellular chloride concentration. This initial high concentration is commonly attributed to the ubiquitous chloride cotransporter NKCC1, which uses a sodium gradient to accumulate chloride. Here we tested this hypothesis in immature retinal amacrine and ganglion cells. Western blotting detected NKCC1 at birth and its expression first increased, then decreased to the adult level. Immunocytochemistry confirmed this early expression of NKCC1 and localized it to all nuclear layers. In the ganglion cell layer, staining peaked at P4 and then decreased with age, becoming undetectable in adult. In comparison, KCC2, the chloride extruder, steadily increased with age localizing primarily to the synaptic layers. For functional tests, we used calcium imaging with fura-2 and chloride imaging with 6-methoxy-N-ethylquinolinium iodide. If NKCC1 accumulates chloride in ganglion and amacrine cells, deleting or blocking it should abolish the GABA-evoked calcium rise. However, at P0-5 GABA consistently evoked a calcium rise that was not abolished in the NKCC1-null retinas, nor by applying high concentrations of bumetanide (NKCC blocker) for long periods. Furthermore, intracellular chloride concentration in amacrine and ganglion cells of the NKCC1-null retinas was approximately 30 mM, same as in wild type at this age. This concentration was not lowered by applying bumetanide or by decreasing extracellular sodium concentration. Costaining for NKCC1 and cellular markers suggested that at P3, NKCC1 is restricted to Müller cells. We conclude that NKCC1 does not serve to accumulate chloride in immature retinal neurons, but it may enable Müller cells to buffer extracellular chloride.

Functional circuitry for peripheral suppression in Mammalian Y-type retinal ganglion cells

A retinal ganglion cell receptive field is made up of an excitatory center and an inhibitory surround. The surround has two components: one driven by horizontal cells at the first synaptic layer and one driven by amacrine cells at the second synaptic layer. Here we characterized how amacrine cells inhibit the center response of on- and off-center Y-type ganglion cells in the in vitro guinea pig retina. A high spatial frequency grating (4-5 cyc/mm), beyond the spatial resolution of horizontal cells, drifted in the ganglion cell receptive field periphery to stimulate amacrine cells. The peripheral grating suppressed the ganglion cell spiking response to a central spot. Suppression of spiking was strongest and observed most consistently in off cells. In intracellular recordings, the grating suppressed the subthreshold membrane potential in two ways: a reduced slope (gain) of the stimulus-response curve by approximately 20-30% and, in off cells, a tonic approximately 1-mV hyperpolarization. In voltage clamp, the grating increased an inhibitory conductance in all cells and simultaneously decreased an excitatory conductance in off cells. To determine whether center response inhibition was presynaptic or postsynaptic (shunting), we measured center response gain under voltage-clamp and current-clamp conditions. Under both conditions, the peripheral grating reduced center response gain similarly. This result suggests that reduced gain in the ganglion cell subthreshold center response reflects inhibition of presynaptic bipolar terminals. Thus amacrine cells suppressed ganglion cell center response gain primarily by inhibiting bipolar cell glutamate release.

The cellular, molecular and ionic basis of GABA(A) receptor signalling

GABA(A) receptors mediate fast synaptic inhibition in the CNS. Whilst this is undoubtedly true, it is a gross oversimplification of their actions. The receptors themselves are diverse, being formed from a variety of subunits, each with a different temporal and spatial pattern of expression. This diversity is reflected in differences in subcellular targetting and in the subtleties of their response to GABA. While activation of the receptors leads to an inevitable increase in membrane conductance, the voltage response is dictated by the distribution of the permeant Cl(-) and HCO(3)(-) ions, which is established by anion transporters. Similar to GABA(A) receptors, the expression of these transporters is not only developmentally regulated but shows cell-specific and subcellular variation. Untangling all these complexities allows us to appreciate the variety of GABA-mediated signalling, a diverse set of phenomena encompassing both synaptic and non-synaptic functions that can be overtly excitatory as well as inhibitory.

Imaging synaptic inhibition in transgenic mice expressing the chloride indicator, Clomeleon

We describe here a molecular genetic approach for imaging synaptic inhibition. The thy-1 promoter was used to express high levels of Clomeleon, a ratiometric fluorescent indicator for chloride ions, in discrete populations of neurons in the brains of transgenic mice. Clomeleon was functional after chronic expression and provided non-invasive readouts of intracellular chloride concentration ([Cl(-)](i)) in brain slices, allowing us to quantify age-dependent declines in resting [Cl(-)](i) during neuronal development. Activation of hippocampal interneurons caused [Cl(-)](i) to rise transiently in individual postsynaptic pyramidal neurons. [Cl(-)](i) increased in direct proportion to the amount of inhibitory transmission, with peak changes as large as 4 mM. Integrating responses over populations of pyramidal neurons allowed sensitive detection of synaptic inhibition. Thus, Clomeleon imaging permits non-invasive, spatiotemporally resolved recordings of [Cl(-)](i) in a large variety of neurons, opening up new opportunities for imaging synaptic inhibition and other forms of chloride signaling.

Estimate of the chloride concentration in a central glutamatergic terminal: a gramicidin perforated-patch study on the calyx of Held

The function of presynaptic terminals is regulated by intracellular Cl-, the levels of which modify vesicular endocytosis and transmitter refilling and mediate the effects of presynaptic ligand-gated Cl- channels. Nevertheless, the concentration of Cl- in a central nerve terminal is unknown, and it is unclear whether terminals can regulate Cl- independently of the soma. Using perforated-patch recording in a mammalian synapse, we found that terminals accumulate Cl- up to 21 mm, between four and five times higher than in their parent cell bodies. Changing [Cl-] did not alter vesicular glutamate content in intact terminals, unlike in vitro experiments. Thus, glutamatergic terminals maintain an elevated Cl- concentration without compromising synaptic transmission.

Activation of a presynaptic glutamate transporter regulates synaptic transmission through electrical signaling

Whereas glutamate transporters in glial cells and postsynaptic neurons contribute significantly to re-uptake of synaptically released transmitter, the functional role of presynaptic glutamate transporters is poorly understood. Here, we used electrophysiological recording to examine the functional properties of a presynaptic glutamate transporter in rat retinal rod bipolar cells and its role in regulating glutamatergic synaptic transmission between rod bipolar cells and amacrine cells. Release of glutamate activated the presynaptic transporter with a time course that suggested a perisynaptic localization. The transporter was also activated by spillover of glutamate from neighboring rod bipolar cells. By recording from pairs of rod bipolar cells and AII amacrine cells, we demonstrate that activation of the transporter-associated anion current hyperpolarizes the presynaptic terminal and thereby inhibits synaptic transmission by suppressing transmitter release. Given the evidence for presynaptic glutamate transporters, similar mechanisms could be of general importance for transmission in the nervous system.

In vivo imaging of the diseased nervous system

In vivo microscopy is an exciting tool for neurological research because it can reveal how single cells respond to damage of the nervous system. This helps us to understand how diseases unfold and how therapies work. Here, we review the optical imaging techniques used to visualize the different parts of the nervous system, and how they have provided fresh insights into the aetiology and therapeutics of neurological diseases. We focus our discussion on five areas of neuropathology (trauma, degeneration, ischaemia, inflammation and seizures) in which in vivo microscopy has had the greatest impact. We discuss the challenging issues in the field, and argue that the convergence of new optical and non-optical methods will be necessary to overcome these challenges.