Thomas Euler

Directionally selective calcium signals in dendrites of starburst amacrine cells

The detection of image motion is fundamental to vision. In many species, unique classes of retinal ganglion cells selectively respond to visual stimuli that move in specific directions. It is not known which retinal cell first performs the neural computations that give rise to directional selectivity in the ganglion cell. A prominent candidate has been an interneuron called the ‘starburst amacrine cell’. Using two-photon optical recordings of intracellular calcium concentration, here we find that individual dendritic branches of starburst cells act as independent computation modules. Dendritic calcium signals, but not somatic membrane voltage, are directionally selective for stimuli that move centrifugally from the cell soma. This demonstrates that direction selectivity is computed locally in dendritic branches at a stage before ganglion cells.

Functional fluorescent Ca2+ indicator proteins in transgenic mice under TET control.

Genetically encoded fluorescent calcium indicator proteins (FCIPs) are promising tools to study calcium dynamics in many activity-dependent molecular and cellular processes. Great hopes—for the measurement of population activity, in particular—have therefore been placed on calcium indicators derived from the green fluorescent protein and their expression in (selected) neuronal populations. Calcium transients can rise within milliseconds, making them suitable as reporters of fast neuronal activity. We here report the production of stable transgenic mouse lines with two different functional calcium indicators, inverse pericam and camgaroo-2, under the control of the tetracycline-inducible promoter. Using a variety of in vitro and in vivo assays, we find that stimuli known to increase intracellular calcium concentration (somatically triggered action potentials (APs) and synaptic and sensory stimulation) can cause substantial and rapid changes in FCIP fluorescence of inverse pericam and camgaroo-2.

The functional diversity of retinal ganglion cells in the mouse

In the vertebrate visual system, all output of the retina is carried by retinal ganglion cells. Each type encodes distinct visual features in parallel for transmission to the brain. How many such ‘output channels’ exist and what each encodes are areas of intense debate. In the mouse, anatomical estimates range from 15 to 20 channels, and only a handful are functionally understood. By combining two-photon calcium imaging to obtain dense retinal recordings and unsupervised clustering of the resulting sample of more than 11,000 cells, here we show that the mouse retina harbours substantially more than 30 functional output channels. These include all known and several new ganglion cell types, as verified by genetic and anatomical criteria. Therefore, information channels from the mouse eye to the mouse brain are considerably more diverse than shown thus far by anatomical studies, suggesting an encoding strategy resembling that used in state-of-the-art artificial vision systems.

The Primordial, Blue-Cone Color System of the Mouse Retina

Humans and old world primates have trichromatic color vision based on three spectral types of cone [long-wavelength (L-), middle-wavelength (M-), and short-wavelength (S-) cones]. All other placental mammals are dichromats, and their color vision depends on the comparison of L- and S-cone signals; however, their cone-selective retinal circuitry is still unknown. Here, we identified the S-cone-selective (blue cone) bipolar cells of the mouse retina. They were labeled in a transgenic mouse expressing Clomeleon, a chloride-sensitive fluorescent protein, under the control of the thy1 promoter. Blue-cone bipolar cells comprise only 1-2% of the bipolar cell population, and their dendrites selectively contact S-opsin-expressing cones. In the dorsal half of the mouse retina, only 3-5% of the cones express S-opsin, and they are all contacted by blue-cone bipolar cells, whereas all L-opsin-expressing cones (∼95%) are avoided. In the ventral mouse retina, the great majority of cones express both S- and L-opsin. They are not contacted by blue-cone bipolar cells. A minority of ventral cones express S-opsin only, and they are selectively contacted by blue-cone bipolar cells. We suggest that these are genuine S-cones. In contrast to the other cones, their pedicles contain only low amounts of cone arrestin. The blue-cone bipolar cells of the mouse retina and their cone selectivity are closely similar to primate blue-cone bipolars, and we suggest that they both represent the phylogenetically ancient color system of the mammalian retina.

Seeing Things in Motion: Models, Circuits, and Mechanisms

Motion vision provides essential cues for navigation and course control as well as for mate, prey, or predator detection. Consequently, neurons responding to visual motion in a direction-selective way are found in almost all species that see. However, directional information is not explicitly encoded at the level of a single photoreceptor. Rather, it has to be computed from the spatio-temporal excitation level of at least two photoreceptors. How this computation is done and how this computation is implemented in terms of neural circuitry and membrane biophysics have remained the focus of intense research over many decades. Here, we review recent progress made in this area with an emphasis on insects and the vertebrate retina.

Functional Stability of Retinal Ganglion Cells after Degeneration-induced Changes in Synaptic Input

Glutamate released from photoreceptors controls the activity and output of parallel pathways in the retina. When photoreceptors die because of degenerative diseases, surviving retinal networks are left without their major source of input, but little is known about how photoreceptor loss affects ongoing synaptic activity and retinal output. Here, we use patch-clamp recording and two-photon microscopy to investigate morphological and physiological properties of identified types of ON and OFF retinal ganglion cells (RGCs) in the adult (36–210 d old) retinal degeneration rd-1/rd-1 mouse. We find that strong rhythmic synaptic input drives ongoing oscillatory spike activity in both ON and OFF RGCs at a fundamental “beating” frequency of ∼10 Hz. Despite this aberrant activity, ON and OFF cells maintain their characteristic dendritic stratification, intrinsic firing properties, including rebound firing in OFF cells, balance of synaptic excitation and inhibition, and dendritic calcium signaling. Thus, RGCs are inherently stable during degeneration-induced retinal activity.

Direction-selective dendritic action potentials in rabbit retina.

Dendritic spikes that propagate toward the soma are well documented, but their physiological role remains uncertain. Our in vitro patch-clamp recordings and two-photon calcium imaging show that direction-selective retinal ganglion cells (DSGCs) utilize orthograde dendritic spikes during physiological activity. DSGCs signal the direction of image motion. Excitatory subthreshold postsynaptic potentials are observed in DSGCs for motion in all directions and provide a weakly tuned directional signal. However, spikes are generated over only a narrow range of motion angles, indicating that spike generation greatly enhances directional tuning. Our results indicate that spikes are initiated at multiple sites within the dendritic arbors of DSGCs and that each dendritic spike initiates a somatic spike. We propose that dendritic spike failure, produced by local inhibitory inputs, might be a critical factor that enhances directional tuning of somatic spikes.

A dendrite-autonomous mechanism for direction selectivity in retinal starburst amacrine cells

Detection of image motion direction begins in the retina, with starburst amacrine cells (SACs) playing a major role. SACs generate larger dendritic Ca2+ signals when motion is from their somata towards their dendritic tips than for motion in the opposite direction. To study the mechanisms underlying the computation of direction selectivity (DS) in SAC dendrites, electrical responses to expanding and contracting circular wave visual stimuli were measured via somatic whole-cell recordings and quantified using Fourier analysis. Fundamental and, especially, harmonic frequency components were larger for expanding stimuli. This DS persists in the presence of GABA and glycine receptor antagonists, suggesting that inhibitory network interactions are not essential. The presence of harmonics indicates nonlinearity, which, as the relationship between harmonic amplitudes and holding potential indicates, is likely due to the activation of voltage-gated channels. [Ca2+] changes in SAC dendrites evoked by voltage steps and monitored by two-photon microscopy suggest that the distal dendrite is tonically depolarized relative to the soma, due in part to resting currents mediated by tonic glutamatergic synaptic input, and that high-voltage–activated Ca2+ channels are active at rest. Supported by compartmental modeling, we conclude that dendritic DS in SACs can be computed by the dendrites themselves, relying on voltage-gated channels and a dendritic voltage gradient, which provides the spatial asymmetry necessary for direction discrimination.

Open Labware: 3-D Printing Your Own Lab Equipment

The introduction of affordable, consumer-oriented 3-D printers is a milestone in the current “maker movement,” which has been heralded as the next industrial revolution. Combined with free and open sharing of detailed design blueprints and accessible development tools, rapid prototypes of complex products can now be assembled in one’s own garage—a game-changer reminiscent of the early days of personal computing. At the same time, 3-D printing has also allowed the scientific and engineering community to build the “little things” that help a lab get up and running much faster and easier than ever before.

G protein subunit Gγ13 is coexpressed with Gαo, Gβ3, and Gβ4 in retinal ON bipolar cells

We investigated the expression of Gγ13, a recently discovered G protein subunit, and a selection of Gβ subunits in retinal bipolar cells, by using a transgenic mouse strain in which green fluorescent protein is strongly expressed in a single type of cone bipolar cell. The cells have ON morphology, and patch‐clamp recordings in slices confirmed that they are of the physiological ON type. Immunohistochemistry showed that Gγ13 is expressed in rod bipolar cells and ON cone bipolar cells, where it is colocalized in the dendrites with Gαo. ON and OFF cone bipolar cells and rod bipolar cells were identified among dissociated cells by their green fluorescence and/or distinct morphology. Hybridization of single‐cell polymerase chain reaction products with cDNA probes for G protein subunits Gβ1 to 5 showed that Gβ3, Gβ4, and Gγ13 are coexpressed in ON bipolar cells but not present in OFF bipolar cells. Gβ1, 2, and 5 are expressed in partially overlapping subpopulations of cone bipolar cells. Gγ13 and Gβ3 and/or Gβ4, thus, seem selectively to participate in signal transduction by ON bipolar cells. J. Comp. Neurol. 455:1–10, 2003.