The physiology of supraspinal pain control centres: their postnatal maturation and role in injury induced long-term alterations in sensory processing

Young animals and humans respond to pain differently than adults. Their pain thresholds are lower, their responses to pain are exaggerated and uncoordinated and their sensitivities to analgesic drugs are different. Significantly, pain in early life can alter the way an individual responds to further pain after they mature. Both animals and humans exposed to pain in early life display altered sensory processing following maturation and have exaggerated responses to subsequent re-injury. Early life pain therefore has the potential to significantly affect the welfare of an individual throughout the rest of its life. One explanation for these differences is that within the spinal cord of young animals, the first part of the nervous system to process painful information, there is a lack of inhibitory neurotransmission. It is increasingly being recognised that supraspinal centres play a central role in pain processing, yet interestingly the contribution of these sites to neonatal nociception has been largely ignored. Instead the majority of research in this area has focussed upon developmental changes at the level of the dorsal horn of the spinal cord. In adults a major supraspinal source of inhibitory neurotransmission in the spinal cord are specialised areas in the brainstem. In healthy adults activity in one these areas, the rostral ventral medulla (RVM) is conveyed via descending spinal nerve tracts to the dorsal horn where they inhibit spinal excitability. However, in chronic pain states, the RVM increases pain by releasing neurotransmitters which excite spinal cord neurones. Recently I have shown that in young rats these bi-directional pathways do not exist, the RVM can only excite the spinal cord. I have shown that these pathways mature and become adult-like over a period around puberty. These studies have demonstrated the huge influence the RVM has in governing the excitability of nociceptive pathways in the spinal cord of young and adult animals. However we do not understand the processes taking place within the RVM that make it behave differently in young animals and we do not understand how this immaturity influences long-term responses to pain. I will perform a series of experiments that will understand the changes in the basic physiological properties of neurones in the RVM as an animal matures. Using electrophysiological techniques I will compare how individual RVM neurones respond to painful stimuli in anaesthetised rats of different postnatal ages (7, 21 and 40 (adult) days old), demonstrating how neurones which make up the RVM change as animals mature. I will study the processes that take place in the RVM that influence the magnitude of integrated pain behaviours and see how these change with increasing age. I will specifically activate pain detecting neurones (C-fibres) in the foot and measure the magnitude of integrated reflex behaviours whilst electrically or chemically manipulating the RVM. Crucially I will investigate what role the RVM plays in long-term alterations in sensory processing which are a consequence of early life pain by inducing unilateral hindpaw inflammation just after birth and studying RVM physiology and integrated pain responses in adulthood. I have preliminary data which demonstrate the key role that the RVM plays in these long-term alterations in sensory processing. This project is important because it will improve our understanding of how pain pathways form, how they behave differently in young animals, how important the RVM is in early life pain, and how pain in immaturity results in altered pain responses throughout life which has the potential to impact clinical and veterinary treatment of pain in early life.

Neonatal responses to pain are immature. Mechanical and thermal pain thresholds are lower; reflexes are exaggerated and uncoordinated reflecting a lack of inhibitory neurotransmission in the spinal dorsal horn. Normally these parameters mature over the first 4-6 weeks of life in the rodent. Considerable research performed in the spinal cord has elucidated some processes that differ between neonates and adults and the timecourse over which they change; however these spinal events cannot fully explain the differences between immature and mature reactions to painful stimuli. Supraspinal centres have become increasingly recognised as being central in pain processing in adults yet their role in the neonate has been largely ignored. I will study the role of brainstem (rostroventral medulla, RVM) nuclei in the short and long term modulation of spinal pain pathways over the postnatal period. I have previously demonstrated that the effects of RVM stimulation change significantly beginning in the 4th postnatal week in rats. Prior to this the RVM only facilitates spinal processing, after this age it both facilitates and inhibits responses. I hypothesise that the properties of RVM neurones are significantly different in neonates and adults, and that these differences are central to life-long alterations in sensory processing following early life pain. Using electrophysiological techniques I will perform the first study investigating changes in the properties of neurones in the RVM using rats at increasing postnatal ages. These studies will determine 1)single-unit activity in the RVM, 2) integrated responses of the RVM upon spinal excitability following C-fibre specific activation in the periphery and 3) the impact that early life pain has upon the maturation of the RVM. Comparisons will be made across age- and treatment groups and will describe the key physiological properties of neurones in the RVM which are essential for appropriate neonatal and adult responses to pain.

Who will benefit from this research and how? The studies proposed will significantly advance our understanding of the impact of pain in the neonatal period, how this effects life-long sensory processing and further our understanding of the contribution that supraspinal centres play in responses to pain. The benefits of funding this research will be felt by numerous different groups. 1) The scientific community, publication of data from these investigations will enhance our understanding of pain neurobiology. Academically this will provide the stimulus for future research and will aid the works of other interested groups. 2) Clinical and veterinary professionals involved in early life care will find be significantly informed from this work allowing them to be more aware of the long-term effects of early life pain and enabling the design of specialised analgesic regimes. 3) Patient support groups will be engaged with, allowing carers of animals and children to better understand pain in early life, its causes, consequences and treatment. 4) The pharmaceutical industry; increasingly the brainstem centres investigated in this project are the target of drug discovery, a more thorough understanding of how they mature will allow safe and efficacious drugs to be designed and their effects in neonates assessed prior to licensing. 5) Government and policy makers will be informed by these studies. Limiting pain in early life has long-term benefits in terms of health economics and animal welfare. As in adults, neonates who have well managed pain will have a reduced reliance on healthcare services compared to those patients whose pain is uncontrolled. As stated in Section 2 there is good evidence for neonates who are exposed to significant pain in early life having altered analgesic requirements later in life, limiting this pain may have the long term benefit of improving care in adulthood and reducing potential adverse events. Limiting pain experience in early life will promote the healthy development of individuals and allow them to thrive. This research could aid in the reallocation of resource from government to improving pain management in early life. Over what timescale will be the benefits be felt? The applicant and his collaborators are currently engaging with the groups identified above. The benefits of the proposed research will become apparent almost immediately with the applicant establishing a laboratory website linked to third party sites allowing interested parties (above) to establish strong communication links with him. The applicant and post-doc will disseminate the results of this research via publications, lectures, seminars, talks and interviews widely. This process will be on-going. What will be done to ensure that the beneficiaries are aware of this research? As already mentioned the applicant and his collaborators already engage with these groups. The applicant will ensure that this is maintained and improved. This work and the establishment of the applicant as an independent New Investigator will demonstrate to the beneficiaries the commitment of the UK to funding basic neurophysiological research that can significantly impact upon the care of young individuals. By funding this project this will help establish a wider base of pain research in the UK which will have added benefit to the beneficiaries identified above.