Current Research

Visual information processing begins in the retina. Stimulus features, such as spatial extent, intensity, color, edges, motion direction etc., are extracted for each retinal location, involving circuits built from various types (~70)of neurons. This information is coded for transmission via the optic nerve to higher visual centers. The retina is a part of the brain that can be easily isolated and stays fully functional for hours. The tissue is highly transparent and, thus, ideal for optical recordings using multi-photon microscopy (Denk & Detwiler, 1999,PNAS 96:7035-40). Furthermore, the retina is amenable to genetic manipulations, for example by viral gene transfer via intraocular injection, which, unlike for other part or the brain, does not require surgery and stereotactical targeting.

Computation of image motion direction in the retina

One type of interneuron we study (in close collaboration with P. Detwiler, University of Washington, Seattle, and W. Denk) is the so-called ‘starburst’ amacrine cell (SAC,Fig. 1). SACs are presynaptic to direction-selective (DS)ganglion cells, which fire strongly when a visual stimulus moves in a certain direction but remain silent when the stimulus moves into the opposite direction (Barlow et al.,1964, J Physiol 173:377-407). Earlier studies have shown that DS ganglion cells receive directionally tuned synapticin put (reviewed in Demb, 2007, Neuron 55:179-186). Withmulti-photon Ca2+ imaging of SAC dendrites we have shown that light stimuli moving from the soma to the dendritic tips (centrifugal, CF) elicit larger Ca2+ signals than motion in the opposite direction (centripetal, CP). Thus, SAC dendrites provide a directionally tuned output signal. The SAC’s dendritic sectors operate largely independently and can be viewed as processing units that signal CF motion.We studied the mechanisms underlying the computation of DS signals in SAC dendrites  by measuring the electrical responses to CF and CP moving stimuli using whole-cell patch-clamp recordings. Spectral analysis revealed larger non- linearities in the electrical response to CF motion than to CP motion. Dendritic DS persists in the absence of inhibitory transmission, which indicates that inhibitory network interactions are not essential for DS in SACs. The relationship between non-linearity and holding potential indicates motion direction-dependent activation of voltage gated channels, pointing at a dendritic DS mechanism that relies on intrinsic properties. Calcium changes in SAC dendrites evoked by voltage steps and monitored by multi-photon imaging (Fig. 1) indicate that Ca2+ channels are active at rest. The data suggests that, in part due to glutamatergic input, the distal dendriteis tonically depolarized relative to the soma. The resultingsomato-dendritic voltage gradient is likely a key element in “dendrite-autonomous” DS computation, because it allows distal Ca2+ channels to operate at a point in their activation range where the gain is maximal and small voltage deflections result in large currents.

Euler Fig1
Fig. 1 Starburst amacrine cell (a) in the rabbit retina filled with fluorescent Ca2+ indicator via a patch-clamp electrode(from left) in voltage-clamp configuration. Right: [Ca2+]-dependent fluorescence (in arbitrary units), recorded by multi-photon microscopy, increased in a distal dendritic branch when stepping the somatic holding voltage (VCOM)from -75 (upper panel) to -2 mV (lower panel; averages of15 frames). Changes in somatic current (Im) and dendritic[Ca2+] (b) evoked by somatic voltage steps (different cell than in a). The hyperpolarizing step induces a decrease in [Ca2+], indicating that voltage-gated Ca2+ channels in the distal dendrites are open at the somatic resting potential. SACs contain only high-voltage activated (HVA)Ca2+ channels, therefore the dendrites must be tonically depolarized relative to the soma to allow HVA channels to be activated. Ca2+ responses as a function of VCOM (c) before(black) during (red) and after (blue) bath application of CNQX (n=3 cells; lines: fitted activation curves). Blocking CNQX-sensitive input shifts the [Ca2+] vs. voltage curve towards depolarized potentials, indicating that a tonicglut amatergic inward current is, at least in part, responsible for depolarizing the dendrites.(b, c from [3]; b: traces are single trials)

 

In related experiments we map light-stimulus evoked Ca2+signals along SAC dendrites and apply pharmacology. Here we aim to determine the contribution of different types of Ca2+ channels to the SAC response. Further we want to find out if intracellular Ca2+ signaling, which is known to play a role in transmitter release from amacrine cells, contributes to the SACs’ output signals.

Bipolar cells are parallel information channels in the retina

Another focus of the group is on signal processing in retinal bipolar cells. For example, we studied the role of intracellular[Cl-] gradients and their dynamics in neuronal processing(in collaboration with T. Kuner). ON bipolar cells, which are depolarized by light, have long been proposed to sustain an axo-dendritic [Cl-] gradient that allows these cells to process GABAergic input differentially at their two ends, the dendrites and the axon terminal.

Euler Fig2
Fig. 2 Clomeleon-expressing bipolar cells (a) injected with fluorescent dye (red) and imaged with multi-photon microscopy (fixed section; Clomeleon labeling in green).Several retinal layers can be distinguished: OPL: outer plexiform layer; INL: inner nuclear layer; IPL: inner plexiformlayer; double-labeled bipolar cell in yellow. Fixed flatmounted retina (b) co-labeled with antibodies againstGluR5 (red) revealing cone pedicles. A subset of Clomeleon bipolar cells (*) contacts only few pedicles (examplesencircled), which belong to blue-sensitive cones [5]. Bluecone-selective bipolar cells (c) marked by asterisks. Left: YFP fluorescence; right: map of intracellular Cl- concentration([Cl-]). [Cl-] is high in the varicosities (arrowheads), where cone pedicles are contacted. Traces showing dendritic (red)and somatic (blue) [Cl-] (d) in a blue cone-selective bipolar cell during GABA application and during co-application of GABA antagonists. The transient decrease in dendritic[Cl-] suggests that GABA mediates depolarization in blue cone-selective bipolar cells. (Scale bars: 20??; a-c: collapsed image stacks; d: traces are averages of 3 trials; all panels from [4], adapted)

 

To test this hypothesis we used ratiometric multi-photon microscopy and mapped the local [Cl-] in bipolar cells expressing the genetically-encoded Cl- indicator Clomeleon (Kuner & Augustine, 2000, Neuron 27, 447-459). We found that ON bipolar cells generate [Cl-] gradients with high [Cl-]in their dendrites (Fig. 2), allowing these cells to receive via the same neurotransmitter, GABA, excitation at their dendrites and inhibition at their axon terminal. Dendritic[Cl-] is particularly high in so-called blue-cone bipolar cells, which receive input exclusively from short-wavelength (‘blue’)sensitive cones. To find out, if high dendritic [Cl-] in these bipolar cells is playing a particular role in chromatic processing, we now study the blue-cone circuit in mouse retina with both anatomical (in collaboration with Silke Haverkamp, MPIH, Frankfurt) and physiological techniques.

Editor: Email
Latest Revision: 2012-08-24
zum Seitenanfang/up