I received my PhD in Neuroscience from M.I.T. and received postdoctoral training at the University of Aarhus, Denmark; University of Oslo, Norway and University of Lausanne, Switzerland. I was Assistant Professor at Northwestern University Medical School from 1978-82 when I moved to the University of Cincinnati College of Medicine where I rose to the rank of Professor and Vice Chair of the Department of Cell Biology and Anatomy. In 1994, I moved to the University of Maryland School of Medicine as Professor and Chair of the Department of Anatomy and Neurobiology. In 1996 I became founding Director of the University of Maryland, Baltimore Program in Neuroscience. In 2006 I was named Donald E. Wilson Distinguished Professor.
All biological systems share common mechanisms at the cellular and molecular levels, yet they exhibit tremendous diversity of organization and function. This diversity arises at the level of cell-cell communication. The most complex degree of cell-cell communication is in the nervous system. In the brain, this complexity is expressed at the level of neural networks. Neural networks, thus, are the unique, defining characteristic of the nervous system. Our research centers on understanding the organization, function and development of neural networks using the mammalian olfactory bulb as a model cortical network.
From Molecules to Networks: The Neurobiology of Olfactory Bulb Glomeruli
Odorant molecules are transduced by olfactory receptor neurons (ORNs) in the nose. ORNs express a single odorant receptor (OR) from a total of ~1000 ORs. ORNs with the same OR project a few fixed glomeruli in the olfactory bulb. Glomeruli, thus, comprise a spatial map that reflects the moment-to-moment activity of ORNs. The brain ‘computes’ the identity, concentration and location of odors from these patterns of glomerular activity.
Patterns of glomerular input are transformed into output signals that are transmitted to olfactory cortex in the form of action potentials from mitral and tufted (MT) cells. Inhibitory circuits shape this transformation. We have identified four distinct glomerular inhibitory networks. Two of these are intraglomerular, operating at the level of a single glomerulus. Another, the interglomerular circuit, links hundreds of glomeruli in a network that uses lateral inhibition to normalize inputs to MT cells. A multiglomerular circuit links smaller numbers of neighboring glomeruli that are roughly the size of ‘clusters’ of glomeruli responsive to structurally similar or environmentally salient odors. Our central hypothesis is that OB inhibitory circuits are functionally organized to incorporate both spatial and temporal information to shape OB output. We are also testing the hypothesis that some of these inhibitory circuits are strongly modified by odor experience whereas others are not.
Olfaction is temporally dynamic. Odors are sampled by sniffing. This imposes a strong temporal structure on the patterns of input to olfactory glomeruli. Thus, in addition to spatial organization, the temporal structure of ON input to glomeruli contains information about odorant identity and concentration. Therefore, temporal sensory input patterns are likely to shape the postsynaptic processing of olfactory information. We have discovered that one type of glomerular neuron – the external tufted (ET) cell – links ON input to all these inhibitory circuits. ET cells intrinsically burst in the range of sniffing frequencies and are entrained by repetitive ON input. Thus, they are exquisitely well suited to endow glomerular inhibitory circuits with dynamic characteristics that can utilize information inherent in the temporal structure of olfactory input.
Our research aims to elucidate the neural machinery of olfactory bulb glomeruli. We use electrophysiological, optical imaging, optogenetics, computational, neuroanatomical, genetic, and molecular approaches to investigate the intrinsic characteristics of glomerular neurons, to elucidate the organization and dynamic properties of glomerular circuits, and to explore their contributions to the OB input-output function.
Central Modulation of Glomerular Function
Olfactory bulb output via mitral/tufted (MT) cells shows remarkable modulation of sensory responses in the awake animal. Glomerular circuits are targeted by centrifugal inputs from cortical and neuromodulatory centers in higher regions of the brain. As glomeruli are the first sites of synaptic integration in the sense of smell, modulation of these initial stages of odor coding impacts information processing at all subsequent levels of the brain. We are investigating how serotonin (5HT) and acetylcholine (ACh) from central neuromodulatory centers affects sensory processing in glomerular circuits. The research builds on recent advances in our understanding of these circuits and their importance in shaping OB output.
This research integrates information from bulb slices, anesthetized animals, and awake, head-fixed and freely moving mice. Our multidimensional research aims to link modulation of glomerular circuits to the activation of specific neuromodulatory systems – cholinergic inputs from the diagonal band and serotonergic inputs from the raphe nuclei. Our working hypothesis is that ACh and 5HT differentially modulate glomerular processing to shape both pre- and postsynaptic inhibition as well as MT cell excitability. We further hypothesize that ACh modulation reduces the impact of sensory input while at the same time increasing the excitability MT cells. This could reduce odor detection but increase discrimination. The research investigates modulation of early olfactory processing at levels ranging from cells and circuits in brain slices to single neurons and networks in anesthetized and head-fixed unanesthetized animals.
Lab Techniques and Equipment:
Brain slices: patch clamping recording, voltage sensitive dyes, Ca2+ imaging, gene-targeted GFP mice to record identified neurons, optogenetic activation of identified inputs, computational modeling of intrinsic cellular activity
Reconstructions of dye-filled, physiologically-characterized neurons; tract tracing, immunohistochemistry, image analysis
In situ hybridization, PCR, gene chips, proteomics, transgenic mice, optogenetics
Dr. Adam Puche, Associate Professor: Neuroanatomy, electrophysiology, molecular biology.
Positions available for postdoctoral fellows (July 2010): We are seeking talented postdoctoral fellows to participate in a new project to investigate cholinergic and serotonergic modulation of olfactory bulb circuits and odor processing. The experiments involve interdisciplinary approaches ranging from single cell patch clamping/imaging in slices to unit recording recordings from anesthetized and awake, head-fixed mice. The project takes advantage of transgenic mice with GFP-labeling of specific neurons to target recordings to specific microcircuits, coupled with the use of optogenetic approaches to selectively activate cholinergic and serotonergic inputs in slices and whole animals. This is an opportunity to become involved in exciting research that is tightly coordinated between in vitro and in vivo experimental approaches. Contact Michael T. Shipley for more information.