Mark Baccei - US grants

Description (provided by applicant): The overall goal of the proposed research is to better comprehend the maturation of spinal pain networks in the neonate. It is now clear that knowledge of the environmental and genetic factors which influence the formation of nociceptive circuits in the central nervous system (CNS) is essential to improving the treatment of pain in children. Recent behavioral evidence suggests that sensory experience during the neonatal period regulates the maturation of central pain networks and can have long-term effects on pain processing. An understanding of how emergent central pain circuits are modulated by sensory input at the cellular level will yield crucial insights into the mechanisms underlying these persistent changes in pain sensitivity. Unfortunately, nothing is currently known about how the level of activity in primary sensory afferent pathways influences synaptic function in developing nociceptive circuits. Since the synaptic integration of nociceptive signals within the CNS begins in the superficial dorsal horn (SDH) of the spinal cord, a detailed characterization of how synaptic networks in the SDH are shaped by primary afferent input during the early postnatal period represents a logical and important first step towards addressing this issue. We hypothesize that excitatory synaptic function in the SDH is modulated by sensory input from the periphery during a critical period of postnatal development and that the effects of tissue injury on SDH synaptic networks are therefore highly age-dependent. This proposal will test this hypothesis by manipulating the level of activity in primary afferent pathways during the early postnatal period and subsequently characterizing the properties of glutamatergic synaptic signaling in the rat dorsal horn using electrophysiological, immunohistochemical and biochemical approaches. This will involve two specific aims: (1) To determine if the efficacy of glutamatergic synapses in the SDH of the rat spinal cord depends on the level of primary afferent input during a critical postnatal period;and (2) To determine whether the consequences of peripheral inflammation for synaptic function in the developing SDH depend on postnatal age. Since the intensive care treatment of infants often requires multiple invasive procedures which alter the normal pattern of somatosensory input to the spinal cord at a time when the immature nervous system is capable of significant plasticity, understanding how sensory experience affects synaptic development in the dorsal horn is necessary to fully realize the short and long-term consequences of these procedures for pain sensation. PUBLIC HEALTH RELEVANCE: It is now universally recognized that human infants can experience considerable pain as a result of disease, surgery or intensive care therapies. However, the clinical treatment of pain in adults cannot easily be translated to children, as it is clear that the immature nervous system is not merely a simplified version of the mature form but rather organized in a fundamentally different manner. Increasing our understanding of the maturation of pain systems at the cellular level will not only provide insight into the mechanisms of action of those analgesics already prescribed to children, but also aid in the design of novel analgesic strategies which are developmentally appropriate.

DESCRIPTION (provided by applicant): Although it is clear that even the youngest infants can experience significant pain as the result of injury, disease, surgery or intensive care therapy, little is known about how tissue damage affects the intrinsic excitability of neurons within developing pain circuits in the central nervous system (CNS). Since the superficial dorsal horn of the spinal cord (SDH) functions as a critical relay station in the pain pathway, a better understanding of how the intrinsic membrane properties of SDH neurons are influenced by tissue injury at different ages represents an important and logical first step towards addressing this issue. The long-term goal is to improve the clinical treatment of pain in infants and children by identifying novel analgesic strategies which are more developmentally appropriate. The overall objective of this application, which is the next step in pursuit of that goal, is to identify the key ionic conductances which regulate the intrinsic excitability of developing neurons within the SDH under normal and pathological conditions. The central hypothesis is that excitatory interneurons within lamina I exhibit intrinsic, pacemaker-type oscillations during a critical period of early postnatal development, which are driven by persistent Na+ and Ca2+ currents and facilitated by neonatal tissue damage. The rationale of the proposed research is that understanding how intrinsic neuronal excitability is specifically regulated within immature nociceptive circuits will reveal new ways to modulate their output which would not be evident from studies in the adult. Guided by strong preliminary data, the central hypothesis will be tested and the overall objective of this application achieved by pursuing the following specific aims: 1) Identify the SDH neurons which are spontaneously active during early postnatal development;(2) Elucidate the ionic mechanisms which drive the intrinsic firing of neonatal SDH neurons;and (3) Detect cell-type-specific changes in ion channel expression and intrinsic excitability within the developing SDH network following early tissue damage. These aims will be accomplished by using in vitro electrophysiological, immunohistochemical and biochemical techniques to characterize the ionic mechanisms regulating neuronal excitability in identified subtypes of developing SDH neurons and determine the extent to which these intrinsic membrane properties are modulated by tissue injury in an age-dependent manner. The outcome of these investigations will be new insight into how activity within immature spinal pain networks is controlled at the cellular level. As a result, the proposed research is significant because it will begin to provide the knowledge needed to develop evidence- based treatments for chronic pediatric pain. PUBLIC HEALTH RELEVANCE: The expected outcomes of the proposed research will have a positive impact on public health by illustrating that the underlying causes of pain hypersensitivity under pathological conditions are highly developmentally regulated. By elucidating the mechanisms by which early tissue damage modulates intrinsic neuronal excitability within central nociceptive networks during postnatal development, this research also brings the promise of identifying novel approaches to regulate pain pathways in an age-specific manner, which would greatly aid efforts to improve the clinical management of pediatric pain.

Project Summary/Abstract Although tissue damage commonly occurs during neonatal intensive care treatment and can alter pain sensitivity throughout life, whether such early injuries can evoke long-term changes in synaptic function within mature nociceptive pathways remains unknown. As a result, the cellular and molecular mechanisms which contribute to the persistent alterations in pain sensitivity following neonatal injury are still unclear. The long- term goal is to improve the clinical treatment of pain by determining how neonatal tissue injury influences nociceptive processing throughout development. The overall objective of this application is to identify changes within the mature rodent superficial dorsal horn (SDH) network following early tissue damage that facilitate activity-dependent plasticity at nociceptive synapses onto ascending projection neurons, which constitute the output of the spinal pain network. The central hypothesis is that neonatal tissue damage evokes persistent deficits in the function of spinal inhibitory circuits which result in decreased feed-forward inhibition of adult lamina I projection neurons, leading to an enhancement of long-term potentiation (LTP) at nociceptive synapses onto these cells. The rationale of the proposed research is that by elucidating how early tissue damage modulates the future plasticity of synapses onto adult projection neurons, these experiments will reveal potential mechanisms by which developing spinal pain circuits can be primed to produce a greater degree of hyperexcitability following injuries at later ages. Guided by strong preliminary data, the central hypothesis will be tested and the overall objective of this application achieved by pursuing the following specific aims: (1) Identify the prolonged effects of neonatal tissue injury on the efficacy of GABAergic and glycinergic signaling onto mature lamina I projection neurons; (2) Elucidate how early tissue damage modulates the integration of sensory input within spinal lamina I projection neurons during adulthood; and (3) Determine the extent to which neonatal injury alters synaptic plasticity in mature spinal projection neurons. These aims will be accomplished by using in vitro electrophysiological, immunohistochemical, and tract-tracing techniques to characterize the effects of neonatal tissue damage on synaptic signaling within the adult SDH and determine the overall consequences of early injury for signal processing within ascending projection neurons. The outcome of these investigations will be the identification of permanent alterations in the synaptic organization of spinal pain networks following early tissue damage which promote the amplification of ascending pain signals in the CNS following subsequent noxious stimulation. As a result, the proposed research is significant because it will enhance our understanding of how nociceptive synaptic plasticity in central pain pathways is modulated by painful experience during the neonatal period and thus provide mechanistic insight into the emerging link between pediatric and adult chronic pain conditions.

Project Summary/Abstract While chronic pain represents a massive public health problem with a staggering economic cost of $560-$635 billion each year in the U.S. alone, the molecular mechanisms driving neuronal hyperexcitability within nociceptive pathways remain incompletely understood. Significant progress has been made towards elucidating the genetic heterogeneity of primary sensory neurons and their plasticity in the aftermath of nerve or tissue damage. However, much less is known about the comprehensive molecular profile of those neurons that convey nociceptive information from the spinal cord to the brain, despite their clear importance for pain perception. A better understanding of the complete pattern of gene expression within spinal projection neurons could reveal new evidence-based strategies to selectively dampen the output of the spinal nociceptive network as a means to alleviate chronic pain. The long-term goal is to better understand how nerve and tissue damage alter the function of nociceptive circuits in the CNS. The objective of this application is to identify injury-evoked changes in gene expression within spinal projection neurons that enhance their firing under chronic pain conditions. The central hypothesis is that ascending spinal projection neurons exhibit a unique molecular phenotype that is significantly modulated by peripheral injury to promote membrane hyperexcitability. The rationale for the proposed work is that the identification of genes that are preferentially expressed in spinal projection neurons will yield new pharmacological approaches to suppress the ascending flow of nociceptive information to the brain, while minimizing unwanted disruptions to global sensorimotor processing within the spinal cord. The central hypothesis will be tested by pursuing the following specific aims: (1) Identify genes that are enriched in ascending projection neurons within the adult spinal cord; and (2) Elucidate changes in gene expression in projection neurons under chronic pain conditions that increase membrane excitability. These aims will be accomplished by using translating ribosome affinity purification (TRAP) and next generation RNA sequencing techniques in combination with bioinformatics, electrophysiological, immunohistochemical and in situ hybridization approaches. The proposed work is innovative because it will reveal, for the first time, the genetic phenotype of those neurons connecting the spinal nociceptive circuit to the mouse brain that are critically involved in the generation of neuropathic and inflammatory pain, as well as elucidate how the molecular signature of this population changes during the chronic pain state. The outcome of these investigations will be the discovery of new, cell type-specific markers of spinal projection neurons and the identification of potential genetic mechanisms by which peripheral injuries can amplify the ?gain? of nociceptive transmission in the spinal cord. As a result, the proposed research is significant because it will reveal novel molecular targets which could be manipulated to selectively silence ascending spinal projection neurons after injury, in order to evoke safe and effective analgesia while minimizing undesirable side effects.