Overview: Vision, Rhythms and Blues
Broadly stated, our lab is interested in understanding how the brain works. Of course, the answer to that broad question can take many forms and encompass many facets of neuroscience, but the aspect of brain function which we find most compelling is neural plasticity - the relatively slow and sustained changes in neural function that result from the interplay of ionic, chemical and genetic signaling in neural circuits, which take place in the brain when we adapt to new levels of stimuli, when we express daily biological rhythms, or when we learn.
These mechanisms mediate many basic mechanisms of the mind, regulating sensation, motivation, mood and sleep. They prepare your eyes to face the light of dawn each day (through action on retinal circuits), initiate the motor movements to get you out of bed (in the brain's basal ganglia), register the first sip of coffee (in the reward circuitry), elevate your mood as you and the day get brighter, and, finally, send you off to sleep at night (through the brain's daily biological clock).
Our research concentrates on the mechanisms of plasticity as they are expressed in three linked subsystems of the central nervous system - the visual, circadian and serotonergic systems that mediate our sense of sight, drive our daily rhythms and influence our mood. Specifically, our research targets key populations of neurons that regulate plasticity through release of the modulatory biogenic amine transmitters, dopamine and serotonin, or that generate endogenous daily rhythms through gene-driven processes within neurons. Mechanistically, we focus on synaptic ion channel signaling and how it is influenced by inter-neuronal modulation, the regulation of autonomous neuronal activity for neurosecretion, and circadian rhythms and gene expression dynamics as a functional measure of neural activity in living neuronal ensembles.
In addressing the cellular, molecular and genetic mechanisms of dopaminergic, serotonergic and biological clock neurons, our research provides basic knowledge relevant to a wide range of neurological disorders including dopaminergic disorders, such as Parkinsonism,schizophrenia and addiction; circadian disorders, such as winter depression and sleep phase syndromes; serotonergic disorders, such as major depression and bipolar disorders; and ocular disorders, such as photoreceptor degeneration and myopia.
There are currently four primary projects in the lab:
Vision: Neural Network Adaptation in the Retina
Rhythms: Molecular Physiology of Circadian Pacemaking and Circadian Organization of the Retina
Blues: Serotonin and Circadian Interactions
Our laboratory takes a multidisciplinary approach, combining experiments at the behavioral, cellular, molecular, and genetic levels to uncover novel neural mechanisms that govern vision, biological rhythms, and mood. Specialized methods used and developed in our lab are described below and their use in specific projects is described in those corresponding sections. As our primary research organisms, we use two vertebrate model systems, in which the genome is readily manipulated, the mouse and the zebrafish, to facilitate the use of reporter gene animals, as well as gene knock-outs, knock-ins and mutants.
An overarching theme in our lab is the use of reporter animals, which harbor fluorescent or luminescent reporter transgenes that identify specific neuron populations or report the dynamics of gene expression in living tissue. Our lab has been at the forefront of developing both reporter mouse models and the imaging and physiological techniques for their use. We have undertaken the development of reporter transgene mice to address two significant experimental limitations in neuroscience research. First, many populations of neurons of critical interest, including dopaminergic neurons and biological clock neurons, are difficult to identify in living tissue based on anatomical parameters alone. Thus, methods to mark and visualize specific classes of neurons greatly facilitate electrophysiological and molecular analysis of these neuronal populations. Second, neuronal gene expression dynamics are fundamental to biological rhythms and to other neural plasticity processes, yet could not be monitored in real-time in living neurons and ensembles. Thus, dynamic reporters of neuronal gene expression provide a window to the genetic signaling in neural ensembles.The approach we have taken is to make mice, in which populations of neurons are marked genetically using transgene constructs, in which specific gene promoters drive fluorescent reporter proteins.
We have developed two reporter mouse preparations in our own laboratory. For the study of dopaminergic neurons we developed TH ::RFP mice, in which the promoter from the tyrosine hydroxylase gene, part of the dopamine synthetic pathway, drives a marker gene that synthesizes a red fluorescent protein, originally from coral. In these mice, dopamine neurons glow red, and thus we can readily locate and target these sparse neurons in living retinas and brain tissue. For the study of biological clock neurons we developed Per1::GFP mice, which harbor a transgene , in which the promoter from the circadian clock gene Per1 drives a short half-life version of green fluorescent protein, originally from jellyfish. In these mice, the neurons of the biological clock glow green, and the intensity of the glow is a real-time read-out of the rhythmic Per1 transcription activity. Thus, in these mice we can not only localize and target clock neurons, we can read the time on their internal molecular clocks. In additions to these models developed in our own laboratory, we have incorporated additional reporter animals developed in other labs, including PER2::LUC mice for luminescence circadian gene expression (J. Takahashi), ePET1::YFP mice for the study of serotonergic neurons (E. Deneris), and TH::GFP zebra fish for the study of dopaminergic neurons (W. Driever).
Our unique Per1::GFPcircadian reporter mouse, which provides a real-time fluorescent gene expression signal that can be readily resolved at the single neuron level, has led us to develop novel imaging methods unique to our laboratory. First, to enable the use of single cell gene expression dynamics as an assay for circadian clock function and organization, we developed a system for confocal real-time gene expression imaging. Using this system, we can simultaneously assay the circadian gene expression rhythms of dozens of individual biological clock neurons in a living brain slice from a behaviorally characterized adult animal and address how the biological clock network is organized and how it encodes information. In practice, this system is comprised of a custom, multi-well imaging chamber, Zeiss PASCAL confocal microscope and attendant image analysis software. Also developed in our laboratory, is the ability to combine electrophysiology with single-cell gene expression imaging to identify the phase of neurons being recorded, correlate ionic and genetic signaling on a neuron-to-neuron basis and select neurons for recording bases on gene expression state. Additional specific methods developed in our laboratory, including dual wheel/IR behavioral monitoring and single cell RT PCR for clock gene ensembles are described in the project sections.
Our laboratory has extensive collaborations with faculty within the Department of Biological Sciences and the faculty in the departments of Pharmacology, Ophthalmology. Cell and Developmental Biology, and Neurology. Prof. McMahon is a member of the Vanderbilt Kennedy Center, Vanderbilt Vision Research Center, Vanderbilt Center for Molecular Neuroscience, Vanderbilt Silvio O. Conte Center for Neuroscience Research. He is also the preceptor for the Vanderbilt Vision Research Training Grant, the Vanderbilt Neurogenomics Training Grant and the Vanderbilt Ion Channel Training Grant.