GABAA_A Receptors Containing the α\alpha2 Subunit Are Critical for Direction-Selective Inhibition in the Retina

Far from being a simple sensor, the retina actively participates in processing visual signals. One of the best understood aspects of this processing is the detection of motion direction. Direction-selective (DS) retinal circuits include several subtypes of ganglion cells (GCs) and inhibitory interneurons, such as starburst amacrine cells (SACs). Recent studies demonstrated a surprising complexity in the arrangement of synapses in the DS circuit, i.e. between SACs and DS ganglion cells. Thus, to fully understand retinal DS mechanisms, detailed knowledge of all synaptic elements involved, particularly the nature and localization of neurotransmitter receptors, is needed. Since inhibition from SACs onto DSGCs is crucial for generating retinal direction selectivity, we investigate here the nature of the GABA receptors mediating this interaction. We found that in the inner plexiform layer (IPL) of mouse and rabbit retina, GABA$_A$ receptor subunit $\alpha$2 (GABA$_A$R $\alpha$2) aggregated in synaptic clusters along two bands overlapping the dendritic plexuses of both ON and OFF SACs. On distal dendrites of individually labeled SACs in rabbit, GABA$_A$R $\alpha$2 was aligned with the majority of varicosities, the cell's output structures, and found postsynaptically on DSGC dendrites, both in the ON and OFF portion of the IPL. In GABA$_A$R $\alpha$2 knock-out (KO) mice, light responses of retinal GCs recorded with two-photon calcium imaging revealed a significant impairment of DS responses compared to their wild-type littermates. We observed a dramatic drop in the proportion of cells exhibiting DS phenotype in both the ON and ON-OFF populations, which strongly supports our anatomical findings that $\alpha$2-containing GABA$_A$Rs are critical for mediating retinal DS inhibition. Our study reveals for the first time, to the best of our knowledge, the precise functional localization of a specific receptor subunit in the retinal DS circuit

The dynamics of somatic exocytosis in monoaminergic neurons.

Some monoaminergic neurons can release neurotransmitters by exocytosis from their cell bodies. The amount of monoamine released by somatic exocytosis can be comparable to that released by synaptic exocytosis, though neither the underlying mechanisms nor the functional significance of somatic exocytosis are well understood. A detailed examination of these characteristics may provide new routes for therapeutic intervention in mood disorders, substance addiction, and neurodegenerative diseases. The relatively large size of the cell body provides a unique opportunity to understand the mechanism of this mode of neuronal exocytosis in microscopic detail. Here we used three photon and total internal reflection fluorescence microscopy to focus on the dynamics of the pre-exocytotic events and explore the nature of somatic vesicle storage, transport, and docking at the membrane of serotonergic neurons from raphe nuclei of the rat brain. We find that the vesicles (or unresolved vesicular clusters) are quiescent (mean square displacement, MSD ∼0.04 μm(2)/s) before depolarization, and they move minimally (<1 μm) from their locations over a time-scale of minutes. However, within minutes of depolarization, the vesicles become more dynamic (MSD ∼0.3 μm(2)/s), and display larger range (several μm) motions, though without any clear directionality. Docking and subsequent exocytosis at the membrane happen at a timescale (∼25 ms) that is slower than most synaptic exocytosis processes, but faster than almost all somatic exocytosis processes observed in endocrine cells. We conclude that, (A) depolarization causes de-tethering of the neurotransmitter vesicles from their storage locations, and this constitutes a critical event in somatic exocytosis; (B) their subsequent transport kinetics can be described by a process of constrained diffusion, and (C) the pre-exocytosis kinetics at the membrane is faster than most other somatic exocytosis processes reported so far

Fluorescence correlation microscopy with real-time alignment readout

In confocal fluorescence correlation microscopy (FCM) it is important to ensure that the correlation measurement is actually performed at the chosen location of the three-dimensional image of the specimen. We present a confocal FCM design that provides an automatic real-time readout of the location in the confocal microscopic image, which is aligned with the detector of the fluorescence correlation spectrometer. The design accomplishes this without using any special positioning device. The design is based on an apertured fluorescence detector placed close to the back aperture of the objective lens and can be easily incorporated into virtually any confocal microscope. We demonstrate the method by performing FCM measurements of a dye diffusing on a cell membrane

Optimized derivation and functional characterization of 5-HT neurons from human embryonic stem cells

The ability to study the characteristics of serotonin release from human serotonergic neurons is valuable both in terms of understanding disease pathology and in trying to understand how drugs that affect the serotonergic system alter neurotransmitter release. There is, however, no good in vitro system to model human serotonergic neurons. Although human embryonic stem (hES) cells offer an attractive model system, the derivation of serotonergic neurons from these cells has remained at a low efficiency. To address this problem, Nestin positive precursors from HUES7 hES cell line were first generated. These Nestin positive cells when terminally differentiated gave rise to 20% MAP-2 positive neurons. A high percentage (&#62;40%) of these neurons could be converted to serotonergic neurons. These serotonergic neurons expressed both serotonin and the neuron-specific tryptophan hydroxylase enzyme. In addition, they expressed several of the transcription factors that have been associated with serotonergic differentiation including Mash1 and Pet1. Finally, during the process of neuronal differentiation, the serotonin content, the localization of serotonin vesicles, and their ability to release serotonin following depolarization was characterized using a live cell serotonin imaging technique based on three-photon microscopy. Thus, for the first time, we demonstrate the feasibility of characterizing the development and function of human serotonergic neurons in vitro

Protein aggregation probed by two-photon fluorescence correlation spectroscopy of native tryptophan

Fluorescence correlation spectroscopy (FCS) has proven to be a powerful tool for the study of a range of biophysical problems including protein aggregation. However, the requirement of fluorescent labeling has been a major drawback of this approach. Here we show that the intrinsic tryptophan fluorescence, excited via a two-photon mechanism, can be effectively used to study the aggregation of tryptophan containing proteins by FCS. This method can also yield the tryptophan fluorescence lifetime in parallel, which provides a complementary parameter to understand the aggregation process. We demonstrate that the formation of soluble aggregates of barstar at pH 3.5 shows clear signatures both in the two-photon tryptophan FCS data and in the tryptophan lifetime analysis. The ability to probe the soluble aggregates of unmodified proteins is significant, given the major role played by this species in amyloid toxicity

Localization of candidate GABA_AR receptor subunits in relation to SAC dendrites and varicosities.

<p><b>A–B</b>. Immunolabeling pattern of the GABA_AR α2 subunit (<i>magenta</i>) in a vertical section of rabbit retina (single optical section; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer). Starburst amacrine cells (SACs) were labeled against choline acetyltransferase (ChAT, <i>green</i>). ON SACs have somata in the GCL and their dendrites form the inner ChAT band; OFF SACs are located in the INL and their dendrites form the outer band. Synaptic receptor clusters (<i>magenta</i>) are unevenly distributed across the neuropile, with GABA_AR α2 puncta concentrating in two bands along the SAC processes. <b>C</b>. Distal dendrites of a SAC injected with Neurobiotin (<i>green</i>) and co-stained with GABA_AR α2 antibody (<i>magenta</i>) (single optical section): the majority of SAC varicosities are associated with receptor staining. Examples for such association are illustrated at higher magnification: c1 and c2 show observed receptor distribution, c1′ and c2′ show randomized control (magenta channel rotated 90° clockwise). Significantly fewer varicosities are associated with receptor clusters in the rotated control. <b>D</b>. Distal dendrites (<i>green</i>) of a SAC injected as in C but co-stained with GABA_AR α1 (<i>magenta</i>) (single optical section): Some varicosities are associated with receptor clusters (see also magnifications in d1–d2). No obvious change in the degree of signal overlap is seen for the randomized control (d1′ and d2′). Scale bars: A, 10 µm (applies also to B); C, 10 µm (applies also to D); c1, 5 µm (applies to all insets).</p

Analysis of DS responses in GABA_AR α2 knock-out mice and wild-type animals.

<p><b>A</b>. Two-photon micrograph showing an optical section (110×110 µm², at the level of the GCL) of mouse retina, with 56 ganglion cells (and displaced amacrine cells) stained by the calcium indicator dye OGB-1 via electroporation. <b>B</b>. Calcium responses (ΔF/F) evoked by a bar stimulus moving in 8 directions measured in four exemplary GCs (trace color matches color of ROIs [regions of interest] in A): an ON (<i>blue</i>), an ON-DS (<i>green</i>), an ON-OFF (<i>yellow</i>) and an ON-OFF-DS (<i>red</i>) ganglion cell. Polar plots of the response amplitudes, with the preferred direction (<i>black line</i>) indicated, are shown in the center of the traces and reflect the different DS tuning strength of the cells (see also direction selectivity index, <i>DSi</i>; for definition see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035109#s4" target="_blank"><i>Methods</i></a>). <b>C</b>. Histogram (<i>top</i>) showing the <i>DSi</i> distribution across all recorded GCL cells in GABA_AR α2 knock-out mice (<i>gray bars</i>, n=2553 cells in 2 mice) and wild-type controls (<i>black bars</i>, n=1002 cells in 4 mice). <i>Bottom</i>: Difference between histograms (from top), illustrating the drop in the number of cells with higher DS-indices. <b>D</b>. Percentage of ON, OFF, ON-OFF, as well as non-responsive (NR) GCL cells in control (<i>black bars</i>) and knock-out animals (<i>gray bars</i>). <b>E</b>. ON-OFF and ON cells with a <i>DSi</i>>0.4 in the two groups of animals (cells were included or rejected after manual inspection of responses; see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035109#s2" target="_blank">Results</a> for complete criteria). (For E and D, relative cell type numbers were determined for each of the recorded GCL field –18 fields in wild-type, 30 in knock-out mice; with approx. 50–60 cells each– and then averaged; error bars indicate S.E.M.).</p

Localization of candidate GABA_AR receptor subunits in relation to DSGC dendrites.

<p><b>A–D</b>. Collapsed confocal stacks of two morphologically identified Neurobiotin-injected DSGCs (ON and OFF arbors shown separately). <b>A′</b>. Magnification of dendrite (ON layer) of the cell shown in A–B (<i>green</i>), co-stained with antibodies against GABA_AR α2 (<i>magenta</i>). Dendrites are covered with receptor puncta, as evident at higher magnification in examples from the ON (a) and the OFF layer (b). <b>C′</b>. Magnification of dendrite (ON layer) of the cell in C–D (<i>green</i>), co-stained with GABA_AR α1 antibodies (<i>magenta</i>). Only occasional receptor staining is found along the dendrites, as shown at higher magnification in examples from the ON (c) and the OFF arbor (d). Scale bars: A, 100 µm (applies also to B–D); A′, 20 µm (applies also to C′); a, 5 µm (applies also to b–d).</p

Label-Free Dopamine Imaging in Live Rat Brain Slices

Dopaminergic neurotransmission has been investigated extensively, yet direct optical probing of dopamine has not been possible in live cells. Here we image intracellular dopamine with sub-micrometer three-dimensional resolution by harnessing its intrinsic mid-ultraviolet (UV) autofluorescence. Two-photon excitation with visible light (540 nm) in conjunction with a non-epifluorescent detection scheme is used to circumvent the UV toxicity and the UV transmission problems. The method is established by imaging dopamine in a dopaminergic cell line and in control cells (glia), and is validated by mass spectrometry. We further show that individual dopamine vesicles/vesicular clusters can be imaged in cultured rat brain slices, thereby providing a direct visualization of the intracellular events preceding dopamine release induced by depolarization or amphetamine exposure. Our technique opens up a previously inaccessible mid-ultraviolet spectral regime (excitation ∼ 270 nm, emission < 320 nm) for label-free imaging of native molecules in live tissue