Modulating cutaneous afferent input: Identifying a source of presynaptic (axo-axonic) inputs inthe mouse spinal dorsal horn

Each time we touch an object, feel the cold winter wind on our hands or the warmth from a cup of tea through our fingers, specific sensory nerve endings in the skin responding to mechanical and thermal stimuli have been activated. This sensory information is transmitted along specific types of nerve fibres that project from these peripheral endings and into our central nervous system where it can then be perceived as touch, cold or warmth sensations. Although our central nervous system receives a constant barrage of sensory information such as the pressure of clothes on our skin or the warmth of our feet in our shoes, we are able to subconsciously filter or prioritise this information, enabling us to mount contextually relevant responses to our environment and carry on with our daily business without overloading our sensory systems. However, in cases where people have damaged peripheral nerves in accidents or as a result of surgical treatment, the efficiency of this filtering system sometimes changes and the individual may as a consequence develop an altered state of sensory processing, where previously innocuous stimuli are now perceived as being painful (tactile allodynia). The aim of this project is to identify a population of cells that are responsible for controlling the flow sensory input entering the spinal cord directly, thereby allowing the central nervous system to filter incoming sensory information without the need for more complex processing. I propose that spinal dorsal horn cells that express the calcium-binding protein parvalbumin are a population likely to carry out this function and therefore constitute a very important (and easily identifiable) class of interneuron. I will aim to characterise these nerve cells in the rat and mouse using a series of immunohistological experiments to determine the morphological features of these cells and establish which class of sensory fibres (and consequently sensory modalities) they are likely to control. I will then determine the electrophysiological properties of these cells by carrying out targeted recording experiments in a transgenic mouse model where all cells containing parvalbumin have been genetically altered to emit an intrinsic green fluorescence. Having established both the electrophysiological and neuroanatomical characteristics of these cells, I then intend to see which, if any, of these properties change in mice that have developed tactile allodynia following surgically-induced peripheral nerve injury. I believe that this project is critical in helping us gain a better understanding of the basic circuitry of the spinal cord, how these cells filter sensory information entering the central nervous system and also what changes occur that contribute to the development of tactile allodynia. By determining these basic properties, it is hoped that specific pharmacological therapies can then be developed to manage or alleviate chronic pain conditions more effectively.

The spinal dorsal horn (SDH) is the principal termination site of afferent fibres serving a wide range sensory modalities including tactile sensation, thermo reception and nociception. In addition to projection neurons that relay this information to the brain, the SDH also contains interneurons which play a critical role in processing information at a segmental level. Attempts to classify these interneurons have been based on the cells' morphological, neurochemical and electrophysiological features however relatively little is known about the precise functional roles of either these cells or the circuits in which they are involved. My preliminary studies have identified parvalbumin (PV)-expressing cells in the rat and mouse SDH as a source of inhibitory presynaptic inputs on to neurochemically-defined central terminals of A-beta afferents and suggest that a discrete, neurochemically-distinct population contributes to the modulation of low threshold mechanoreceptor information specifically. These preliminary studies also found that the morphological properties of these cells and the electrophysiological properties of PV cells recorded from spinal cord slices of a transgenic mouse strain in which enhanced green fluorescent protein is expressed under the PV promoter, are consistent with those of islet cells. As the loss of presynaptic inhibition has been implicated in the development of chronic pain, it is plausible that a selective reduction of PV-cell mediated inhibition could contribute to the development of tactile allodynia. This project will use a combination of neuroanatomical and electrophysiological approaches challenge the hypothesis that PV-expressing dorsal horn interneurons are a source of presynaptic inputs on to cutaneous afferent fibre terminals and changes in their anatomical, neurochemical or physiological properties contribute to the development of tactile allodynia.

In a recent survey conducted by the independent research company Ipsos MORI, in collaboration with the European Federation of IASP Chapters, the World Institute of Pain and OPEN Minds, Pain STORY reported that 7.8 million people in the UK live with chronic pain. Similarly, in a report by the Chief Medical Officer, Sir Liam Donaldson, published in March, 2009(1), it was estimated that of the 5 million people in the UK that develop chronic pain annually, only two-thirds of these will recover having undergone current treatment methods. It is therefore evident that new and more effective approaches to manage chronic pain must still be developed and to facilitate this process our basic understanding of the mechanisms involved in these conditions must first improve. Who will benefit? The findings of this project will be of significant interest to basic scientists studying the circuitry of the spinal dorsal horn and also those studying the development of neuropathic pain states. The ultimate beneficiaries of this project would be chronic pain patients and would, in turn, impact greatly on the nation's health (and consequently wealth and culture), however much depends on how the results of this study can be used to develop better treatments. How will they benefit? The structural complexity of the central nervous system reflects its adaptability and underlies its functional significance, however this makes it very difficult to identify, define and study discrete neuronal circuits. The pilot studies I have conducted to support this project have identified a neurochemically-defined population of interneurons that appear to be important in modulating sensory information passing into the spinal cord. The aims of the proposed experiments are to determine the functional significance of these cells and further our understanding of how their morphological, physiological or neurochemical features change following peripheral nerve injury. The findings of this project will improve our knowledge of the complex circuitry involved in modulating sensory input into the spinal cord and also further our understanding of changes to these circuits that may underlie the development of tactile allodynia. This novel information may then lead to the development of more effective treatments for patients suffering from chronic pain. This research is therefore highly relevant to the BBSRC's policy priority areas in both the advancement of fundamental understanding of complex biological processes and increasing international collaboration. (1) 150 year of the Annual Report of the Chief Medical Officer: On the state of public health 2008. Department of Health. Published 16 March, 2009.