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Rewards, Perils and Pitfalls of Untangling Spinal Pain Circuits

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Rewards, Perils and Pitfalls of Untangling Spinal Pain Circuits Available online at www.sciencedirect.com ScienceDirect Rewards, perils and pitfalls of untangling spinal pain circuits 1,2 3 Brett A Graham and David I Hughes Pain is a complex perception that is fundamental to our daily change in either the peripheral and/or central nervous survival. Under normal circumstances, it serves an important system and is defined as a pain state that has persisted for protective function to guard against tissue damage or alert the over three months. Since this form of pain can arise either body to dangerous environments. Under pathological states, spontaneously or persist beyond injury, it “lacks the acute however, the perception of pain can become chronic, warning function of physical nociception” [6], represent- maladaptive, resistant to treatment, and presents a serious ing a maladaptive state that can be debilitating, and is clinical and societal problem. A wealth of literature suggests often resistant to treatment. There is a broad agreement that disruption of sensory processing within the spinal cord in the field that this burden can only be better addressed contributes to chronic pain, but our limited understanding of by an improved understanding of how the body perceives spinal circuitry in health and disease remains a barrier to the pain under normal circumstances, and how this compares development of new therapeutic strategies. The aim of this brief to chronic pain states, requiring detailed analysis of pain review is to outline current thinking about how individual signalling mechanisms from the periphery to the brain. components of functionally distinct spinal microcircuits can be Given the importance of the spinal cord to the transmis- identified and manipulated to determine their role in influencing sion of pain this view has driven efforts to define our perception of pain in acute and chronic states. the spinal circuits that process nociceptive information, identify individual neuronal components of these circuits, Addresses and determine the functional significance these cells 1 School of Biomedical Sciences & Pharmacy, Faculty of Health, (Figure 1). Here, we aim to provide a brief overview of University of Newcastle, Callaghan, Australia 2 our current understanding on spinal circuits in the super- Hunter Medical Research Institute (HMRI), New Lambton Heights, NSW, Australia ficial dorsal horn, laminae I to III. The review will then 3 Institute of Neuroscience Psychology, College of Medical, Veterinary & focus on technological advances that help us identify Life Sciences, University of Glasgow, Glasgow, UK individual elements of pain circuits, and on experimental approaches that can be used to manipulate the activity of Corresponding author: Hughes, David I ([email protected]) these cells to determine their functional significance in pain perception in health and disease. Current Opinion in Physiology 2019, 11:xx–yy This review comes from a themed issue on Physiology of Pain Defining spinal interneuron populations The spinal dorsal horn is the principle termination site of Edited by Cheryl Stucky and Lucy Donaldson primary afferent fibres serving a range of sensory modali- ties, and is the first site where this information may be modulated by discrete spinal circuits before it is relayed to higher brain centres for conscious perception. https://doi.org/10.1016/j.cophys.2019.04.015 The predominant neuronal populations in this region are ã 2468-8673/ 2019 The Authors. Published by Elsevier Ltd. This is an interneurons, whose role is to process and modulate open access article under the CC BY license (http://creativecommons. afferent input before it is relayed to supraspinal sites org/licenses/by/4.0/). via projections neurons. Spinal interneurons account for the majority of neurons in lamina I, and virtually all in lamina II [4 ]. These cells may be subdivided broadly into Background two functional populations based on their neurotransmit- The somatosensory system plays a fundamental role in ter content: inhibitory interneurons, which use GABA our ability to perceive, and ultimately react to, our envi- and/or glycine; and excitatory interneurons, which use ronment. While there is debate about how this sensory glutamate. Other properties of dorsal horn neurons have information from the periphery is encoded and conveyed also been studied widely using anatomical, electrophysi- to the brain for ultimate perception [1,2], it is well ological and pharmacological approaches in animal mod- established that the spinal dorsal horn plays a key role els [7–12], and these data have provided important insight in the gating, modulation and relay of nociceptive sensory into the extraordinary complexity of this region, suggest- input to brain higher centres [3,4 ,5 ]. The perception of ing much heterogeneity still exists within the inhibitory acute ‘day-to-day’ pain is fundamental to our wellbeing, and excitatory interneuron populations. For example, serving a protective role by raising our awareness to studies combining both in vitro electrophysiological threatening environments and to actual tissue damage. experiments in spinal cord slices with subsequent ana- Chronic (persistent) pain, which affects approximately tomical approaches helped characterise the biophysical 20% of the global population, results from a pathological properties of both lamina I projection neurons [13,14] and www.sciencedirect.com Current Opinion in Physiology 2019, 11:35–41 36 Physiology of Pain Figure 1 exhibiting the same dendritic orientation but the extent of this branching distinguishing them, may not faithfully NeuN translate to mouse. Several anatomical and electrophysi- Pax2 ological studies have established similar caveats in com- parable populations of neurons elsewhere in the central nervous system between rats and mice, and this may also apply to spinal interneurons [27,28]. Nonetheless, these multidisciplinary studies identify a number of anatomical and electrophysiological features that collectively, repre- sent characteristic signatures to help define distinct popu- μ lations of dorsal horn interneurons. Current Opinion in Physiology Targeted approaches to studying circuitry The advent of transgenic mouse lines in which fluores- Neuroanatomical organisation of the mouse spinal dorsal horn. cent proteins, driven by a genetic promoter, label discrete The neuronal architecture of the spinal dorsal horn is highly complex. populations of neurons, has been instrumental in further- It is made up of three principal types of neurons (blue): projection neurons, excitatory interneurons, and inhibitory interneurons ing our understanding of interneurons and the circuits (magenta). Interneurons modulate and gate sensory information they form by allowing us to target these cells specifically entering the dorsal horn, whereas projection neurons relay this in electrophysiological experiments [29]. These information on to higher supraspinal centres. approaches, initially developed to study circuits in the hippocampus, were soon adopted to target inhibitory interneurons in the spinal dorsal horn. Mice where green interneurons in lamina II [15–19], but rarely addressed fluorescent protein (GFP) was expressed under the con- questions of connectivity, apart from a few notable excep- trol of either the prion promotor or that for the GABA- tions [20–25]. Nonetheless, these studies provide a synthesising enzyme GAD67 allowed the study of wealth of information on the heterogeneity of neuronal GABAergic interneurons [30,31], whereas the glycine populations in this region, using untargeted sampling transporter 2 gene was used to label glycinergic inter- method as all recordings were carried out ‘blind’ to cell neurons [32]. Similar approaches were later employed to identity. While this is an ideal approach for sampling study excitatory interneurons in the spinal cord, using the across the population, it is not an efficient approach to vesicular glutamate transporter type 2 gene as the pro- resolve discrete synaptic circuits. motor [33]. Collectively, these studies represent some of the most significant advances in efforts to characterise A number of studies have attempted to better resolve spinal cord circuitry. Further refinements targeting even dorsal horn heterogeneity by correlating across multiple more discrete populations of interneurons have followed, classification properties. For example, distinct physiolog- driven in part by our understanding of the neurochemical ical properties have been identified in spinal interneurons signatures of subpopulations of dorsal horn interneurons based on action potential discharge patterns in response [34,35,36 ]. By mapping the laminar distribution of these to current injections, subthreshold currents and respon- cells in the dorsal horn, and defining their responsiveness siveness to drug application [10,26], and these appear to to various peripheral stimuli [37,38], it has been possible correlate with particular classes of interneurons defined to implicate-specific subpopulations in the processing of by their morphology [16,18]. These studies, carried out in nociceptive afferent input. For example, in the spinal hamsters and mice, identified five classes of morphologi- dorsal horn, the calcium binding protein calretinin (CR) is cally distinct interneuron in lamina II, comprising of islet expressed primarily in excitatory interneurons and these cells, radial cells, central cells, vertical cells and unclassi- have been implicated in circuits underlying both acute fied cells, and each of these classes
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