Department of Physiology and Biophysics
University of Colorado School of Medicine
RC1 North Tower, P18-7101
Mail Stop 8307
Aurora, CO 80045
Our brains process a wide variety of sensory and internally-generated signals with marvelous spatial and energetic efficiency. In the lab, we seek to describe the mechanisms that enable neural circuits to efficiently detect, amplify and transmit relevant information under diverse physiological conditions. This knowledge is required to understand how the healthy brain works and to develop strategies to treat abnormal neurological conditions. To achieve this goal, we combine advanced experimental techniques, computer modeling and statistical analyses to study signal processing in single cells and neural networks in the retina, where neuronal and network dynamics can be studied in the well-defined context of visual processing.
The retina converts light into visual signals transmitted to the rest of the brain. Neurons in the retina process visual information to extract specific features (such as color, brightness, object movement or presence of edges) from the visual scene. These computations are confined within the circuitry of the retina, and can be preserved in-vitro. This fact significantly reduces the complexity of recording physiologically-relevant signals from the retina and increases the experimental toolset. We also take advantage of the wealth of knowledge in the fields of retinal anatomy, neuronal connectivity, and physiology, to construct and examine realistic hypotheses in well-constrained models and targeted experiments.
In our experimental studies we will take a multidisciplinary approach, combining electrophysiology, multi-photon imaging, genetic manipulations and modeling techniques. We specifically focus on neuronal dendrites, where a significant part of neuronal processing occurs.
Our first goal is to understand how NMDA receptors, a type of glutamatergic receptor, contributes to dendritic function. NMDA receptors are present in many brain regions and are a crucial component in neuronal excitability, synaptic plasticity and learning and memory. NMDA receptor dysfunction is a significant factor in the progression of many pathological conditions. In a way, NMDA receptors are the wild card of synaptic function – their computational role changes dramatically based on synaptic activation parameters and network architecture. For this reason, it is hard to prescribe a single operation or contribution to computations that NMDA receptors perform in-vivo. In the retina, NMDA receptors are expressed in many cell types; each of these cells is embedded in a different network infrastructure that facilitates extraction of specific features out of the visual scene. Thus, the retina presents an opportunity to study dendritic information processing in diverse computational tasks and provides a convenient system for theoretical comparison between different algorithms of neurocomputation. We combine use electrophysiology and two-photon imaging in genetically labeled cells to characterize cellular responses to the visual input and test the participation of NMDA receptors using pharmacological and genetic manipulations.
NMDARs Perform Multiplicative Scaling of Synaptic Inputs in DSGCs
(A) Retinal DS network schematic. For simplicity, only the ON pathway is shown. Visual information is conveyed from photoreceptors through bipolar cells to DSGCs by glutamatergic inputs that are not DS. DS is computed first in SACs, which provide inhibitory GABAergic and excitatory cholinergic drive to DSGCs. (B) Impact of additive (purple, +) versus multiplicative (yellow, ×) excitatory scaling of baseline responses (gray). (i) Additive scaling increases the responses by a constant in all directions of stimulation, whereas multiplication scales the responses in different directions proportionally. (ii) Multiplicative, but not additive, scaling preserves DS. (iii) On a PD versus ND plot, additive scaling follows a 45° angle line (right), whereas multiplicative scaling follows a line that connects the unscaled responses to the origin.(C) Polar plot of subthreshold responses in a DSGC to a bright bar moving across the retina in eight directions. (D) PD and ND responses from the same cell before (blue) and after NMDAR blockade by AP5 (black). (E) Postsynaptic reponses in control (blue) and AP5 (black; n = 19). Squares, mean (± SD) of the dataset. Yellow (×) and purple (+) dashed lines indicate theoretical multiplicative and additive scaling of responses in AP5. Adapted from Poleg-Polsky and Diamond, 2016a.
Our second goal is to describe the mechanistic principles of direction selectivity (DS) in the retina. The first step of DS is computed in the dendrites of starburst amacrine cells (SACs). A long-standing question in the field is how SACs detect and transmit directional information. Using detailed models and recordings from SAC dendrites, we expand and refine the range and properties of possible mechanisms by which this computation takes place. For example, by recording from individual synaptic sites, we were able to reveal how communication in the DS circuit can be reliable despite of a significant biological noise.
In recent years, it has become apparent that the synaptic output from SACs provides qualitatively different information, depending on the type of the postsynaptic cell and transmitter type. SAC release both GABA and acetylcholine to the output (ganglion) neurons and to neighboring SACs. GABAergic signals carry significant DS information, while the cholinergic drive is either non-directional or can even have a reversed directional preference. How can these different release patterns occur in the same dendrite? Are the signals transmitted to other SACs different from those delivered to the ganglion cells? Answering these questions would constitute a major victory for theoretical neuroscience and enhance our understanding of visual signal processing and dendritic computational capabilities.
Based on detailed numerical simulations of synaptic integration in SACs dendrites we hypothesize that distinct classes of voltage-gated channels, known to be present in SACs, can form dendritic micro-domains with different rules for synaptic release. We aim to show that these dendritic channels can enhance the computation of DS, and even generate DS signals de-novo.
Membrane potential and dendritic calcium recording from SAC reveal different contrast dependence. (A) Schematic of experimental design. SACs were patched and filled with a calcium indicator. Pink shading indicates approximately the area of the dendrites from which GABA and ACh release occurs. Visual stimulation consisted of a bar moving in the centrifugal direction (ie, from the soma to the recorded dendritic location). (B) Representative somatic PSPs (left) and corresponding dendritic calcium transients 1 from the rectangular ROI in A (right). The dotted line and filled triangle mark the beginning of 2P laser scanning. The open triangle and the shaded area indicate the approximate entrance and the time interval over which the stimulus occupied the cell's receptive field. Asterisks indicate comparison between responses and baseline (**p < 0.01, ***p < 0.001, t test). Inset, Heat map showing the spatial distribution of dendritic calcium signals from the dotted region in A for two stimulus contrast levels, color-coded by the peak dF/F values. Adapted from Poleg-Polsky and Diamond, 2016b.
In addition to the physiological function of the retina, we are focused on revealing changes to neuronal and circuit function that occur in disease states. Circuits in the early visual system can provide a unique opportunity to study disease-related aberrations in neural function. Our current focus is on Multiple Sclerosis (MS), a common disease of the brain.
MS is a severe debilitating disease that affects millions of individuals worldwide. Gray matter demyelination and inflammation in MS lead to neuronal energy depletion, synaptic loss, and eventually to cell death. It is currently unclear, however, which part of the neuron fails first in MS. Axonal damage most likely starts a vicious cycle within a cell, where energy misbalance triggers hyper activation of synaptic inputs, accelerating action potential firing and inflicting a further energetic burden on the traumatized axon. Remarkably, recent studies found that synaptic loss occurs independently of demyelination in MS brains, suggesting that neurological disability may progress due to processes that are intrinsic to the gray matter. We utilize mouse models of MS to study how optic nerve demyelination affects neuronal function and processing of synaptic inputs. We can measure changes in membrane potential, calcium influx, and signalling in retinal ganglion cells undergoing axonal damage. We will examine whether MS affects NMDA receptors and other voltage-gated channel expression and activity. Identifying these mechanisms is an essential step in understanding neurodegenerative diseases and devising treatment options for malfunctioning neural circuits.
Alon did his graduate work with Prof. Jackie Schiller (Technion, Israel) where he studied the contribution of dendritic NMDA receptors to the function of cortical pyramidal neurons. Alon showed that dendritic spikes form independent computational units within individual cells, enrich neuronal information processing and plasticity, create a unique communication code between cells and establish new rules for interactions between excitatory and inhibitory inputs in spiking dendrites.
He moved to Bethesda MD for his postdoctoral training in the lab of Jeffrey Diamond (NIH) to study visual processing in the retina. Alon helped to define novel network and synaptic mechanisms that ensure reliable responsivity in the direction selectivity (DS) circuit. He found that synaptic inputs, instead of summating together, interact multiplicatively, aided by activation of NMDA receptors. This action of NMDA receptors increases the gain of light-evoked responses, but unlike most electric amplifiers, which typically distort the signal (all too often a musical performance is ruined by a faulty amplifier), the unique mechanism of NMDA-mediated multiplication was shown to improve the fidelity of DSGC performance.
John studied molecular mechanisms of retinal ganglion cell (RGC) axon guidance in the zebrafish retinotectal system as a graduate student in Chi-Bin Chien’s lab at the University of Utah. His research demonstrated that the zebrafish ß-actin 3’ untranslated region (3’UTR) is sufficient to target heterologous mRNA for local translation in RGC growth cones during pathfinding in the optic tract, and that insulin-like growth factor II mRNA-binding protein 1 (Igf2bp1), which is known to bind the ß-actin 3’UTR zipcode, is required for RGC axon outgrowth in vivo. He also showed that Cyfip2 functions cell autonomously during dorsonasal RGC axon sorting in the zebrafish retinotectal system.
John performed 2 postdoctoral research projects at the University of Utah, where he investigated the function of Lhx2 and Shh signaling during neurogenesis in the mouse retina, and then established mRNA expression profiles of several genes in the hypothalamus of Lef1 conditional knockout (CKO) mice. He then joined K.C. Brennan’s lab for further postdoctoral training at the University of Utah, and focused on the pathophysiology of traumatic brain injury (TBI) and post-traumatic headache (PTH). For this research, John characterized in vivo physiological alterations that occur in pyramidal neurons as a result of TBI in the mouse cortex during spontaneous and evoked sensory activity, as well as cortical spreading depression (CSD). John moved to Denver and joined the lab in February 2019 to investigate mechanisms of direction selectivity in the mouse retina and how these mechanisms are affected by disease and injury.
Josh graduated from Kansas State University with his BS in psychological sciences after which he completed a 2-year post-baccalaureate fellowship at the National Institute on Aging in the Laboratory of Behavioral Neuroscience. He joined the graduate neuroscience program at the University of Colorado, Anschutz Medical Campus in the Fall of 2018 and in the following year, he became a member of the laboratories of both Dr. Alon Poleg-Polsky and Dr. Gidon Felsen. Josh is mapping cell-type specific retino-collicular connectivity using a combination of retrograde viral tracing of superior colliculus-projecting retinal ganglion cells (RGCs) and two-photon imaging of RGCs in live whole-mount retina. The goal of this study is to identify what types of RGCs target which of the 4-5 functionally distinct classes of visually responsive neurons in the superficial layers of the superior colliculus (SC). In addition, Josh is investigating how saccadic eye movements, or simulated saccades suppress visual sensitivity focusing on visual responses in the superficial layers of the SC and the RGCs which project to the superficial SC. The goal of this project is to separate out the distinct contributions of motor activity and visual processing in saccadic suppression.
Poleg-Polsky A. Dendritic spikes expand the range of well-tolerated population noise structures. Manuscript submitted to J Neuroscience.
Kumar A, Schiff O, Barkai E, Mel BW, Poleg-Polsky A*, Schiller J. NMDA spikes mediate amplification of inputs in the rat piriform cortex. Elife, 2018 Dec 21;7. pii: e38446. doi: 10.7554/eLife.38446.
Poleg-Polsky A, Ding H, Diamond JS. Functional Compartmentalization within Starburst Amacrine Cell Dendrites in the Retina. Cell Rep, 2018 Mar 13;22(11):2898-2908. doi: 10.1016/j.celrep.2018.02.064.
Ding H, Smith RG, Poleg-Polsky A, Diamond JS, Briggman KL. Species-specific wiring for direction selectivity in the mammalian retina. Nature, 2016, doi:10.1038/nature18609
Poleg-Polsky A, Diamond JS. Retinal circuitry balances contrast tuning of excitation and inhibition to enable reliable computation of direction selectivity. J Neuroscience, 2016, 36(21), 5861-76.
Poleg-Polsky A, Diamond JS. NMDA receptors multiplicatively scale visual signals in direction selective ganglion cells. Neuron, 2016, 89(6), 1277-90.
Poleg-Polsky A. Effects of Neural Morphology and Input Distribution on Synaptic Processing by Global and Focal NMDA-Spikes. PLoS One. 2015, 10(10), e0140254.
Lavzin M, Rapoport S, Polsky A, Garion L, Schiller J. Nonlinear dendritic processing determines angular tuning of barrel cortex neurons in vivo. Nature, 2012, 490(7420), 397-401.
Polsky A, Mel B, Schiller J. Encoding and decoding bursts by NMDA spikes in basal dendrites of layer 5 pyramidal neurons. J Neuroscience, 2009, 29(38), 11891-903.
Gordon U*, Polsky A*, Schiller J. Plasticity compartments in basal dendrites of neocortical pyramidal neurons. J Neuroscience, 2006, 26(49), 12717-26. *Co-First Author
Polsky A, Mel BW, Schiller J. Computational subunits in thin dendrites of pyramidal cells. Nature Neuroscience, 2004, 7(6), 621-7.
We are looking for enthusiastic Postdoctoral Fellows with an interest in retinal function, neuronal signaling and/or neurological disorders. We welcome inquiries from graduate students regarding lab rotations. Short-term modeling and experimental projects are available and previous experience is not a requirement.