Current research in the Koerber laboratory includes several projects designed to investigate the processing of somatosensory information and alteration in this process following injury. These investigations focus on several levels of the somatosensory system including primary sensory neurons, the tissues they innervate and the structure and function of spinal networks involved the perception of touch, pain and itch. In order to carry out these studies we employ an array of electrophysiological, anatomical and molecular biological approaches including state of the art optogenetic, pharmacogenetic and single cell qPCR techniques.
SPECIFIC ONGOING RESEARCH PROJECTS INCLUDE:
Peripheral nerve regeneration and sensory neuron plasticity
Nociception is a complex process by which an organism detects potentially damaging stimuli, processes this information, and then executes an appropriate behavioral response. Chronic pain following acute peripheral nerve injury is a major public health issue. Our primary goal in these studies is to understand the cellular processes that underlie plasticity in cutaneous and muscle sensory neurons following injury and regeneration. Specifically, we are examining injury-induced plasticity of primary sensory neurons reinnervating either skin or striated muscle. We use our ex vivo skin or muscle/nerve/DRG/spinal cord preparations and our novel method for recovering individual functionally characterized sensory neurons to determine the comprehensive phenotypes of cutaneous C-fibers and Group IV muscle afferents in naïve mice. This includes single cell qPCR of individual functionally characterized sensory neurons. In our preliminary studies we have identified several unique functional classes of these fibers and have documented novel dynamic changes in gene expression. The determination of the specific receptors/channels responsible for sensitization of these fibers will provide new insights to these processes and more importantly could provide potential targets for the development of pharmaceutical therapies. These new therapies could provide for improved treatments for the alleviation of the adverse symptoms of chronic neuropathic pain.
Characterization of keratinocyte-neural communication
(Collaboration with Drs. Kathryn Albers and Brian Davis)
Stimulus detection by the skin has previously been thought to be strictly sensory afferent dependent. It is now recognized that production of growth factors, neuromodulators (e.g., ATP, ACh, CGRP) and activation of ion channels (e.g., TRP, Ca2+, Na+) by epidermal keratinocytes can have a profound effect on sensory signaling. To unravel these complex interactions and the mechanisms of neural-keratinocyte communication we developed an optogenetic mouse model in which light activated channelrhodopsin (ChR2) is targeted to basel keratinocytes using the K14 promoter driving Cre recombinase. Blue light stimulation of the skin of K14-ChR2 mice was found to elicit nocifensive behavioral responses. Electrophysiological analysis of this activation using a skin-nerve-ganglia and spinal cord ex vivo preparation showed activation of both tactile afferent fibers and C-fiber nociceptors. We also found that different subtypes of cutaneous afferents are activated at different levels suggesting heterogeneity in skin-neural communication. Using these new genetic models we propose three specific aims to advance these initial findings. Aim 1 experiments will use binding assays to determine if light stimulation of ChR2 keratinocytes elicits release of selected neuromodulators. Aim 2 will determine how light activation of keratinocytes or sensory afferents affects response properties of functionally defined subsets of cutaneous sensory neurons. We will also determine how this activation compares to mechanical and/or thermal stimulation of the skin. Aim 3 experiments will determine the contribution of changes in keratinocytes and sensory neurons to thermal and mechanical hyperalgesia in a model of inflammatory pain examine. These studies will define if hyperalgesia is caused by changes in primary afferents, skin keratinocytes or both. These studies will significantly advance our understanding of how the skin and sensory nervous system communicate under normal and inflamed conditions.
Indentifying specific excitatory and inhibitory spinal circuits involved in chronic pain
(Collaboration with Dr. Sarah Ross)
Mechanical allodynia and related hypersensitivities are frequent symptoms found in patients with chronic neuropathic pain. There is a diverse population of cells in the superficial spinal dorsal horn laminae I-III. Recent eloquent anatomical studies have begun to map the circuitry involving these different cell types. Numerous electrophysiological studies employing spinal cord slices have also added to our understanding of these networks. Together these studies have lead to several hypotheses about the organization of this circuitry and how sensory inputs mediating touch, pain and itch may be processed in the superficial dorsal horn. In this project we are using the combination of multiple optogenetic and pharmacogenetic approaches and our ex vivo skin/nerve/DRG/spinal cord preparation to examine spinal circuits integrating cutaneous sensory inputs in the superficial dorsal horn. We have recently acquired several transgenic cre-lines that allow us to optically activate specific types of spinal interneurons or specific types of cutaneous afferent fibers. The overall goal of the project is to determine the distribution of the identified inputs to identified spinal cord tract neurons (spinothalamic, spinal-parabrachial, i.e. pain). Focusing initially on the presence or absence of inputs from A-beta and/or A-delta LTMRs to these tract cells and how this connectivity may change in nerve injury models that are well known to result in mechanical allodynia. There are other projects including examining the inputs to and excitability of identified populations of inhibitory neurons and how the inputs and excitability changes in the same pain models.