The research focus of the Cellular Neuroscience Section is to elucidate the mechanisms of pain and nerve injury especially at the level of the dorsal root ganglia and spinal cord. The spinal neuronal circuitry that subserves nociception and nerve injury is composed of three parts: intrinsic spinal neurons that are either local circuit neurons or projection neurons, the terminals of primary afferent neurons that carry inputs from the periphery, and axon terminals that descend to the spinal cord from the brain. Activity in these circuits forms the basis for modulation of the neuronal output from the spinal cord. This project uses a variety of cellular and molecular techniques to study pain pathways in animal models of pain and nerve injury. Our current research efforts focus on the development of pain pathways and the effect of persistent pain and tissue injury during the early neonatal time. Nociceptive neuronal circuits develop during both embryonic and postnatal times when painful stimuli are normally absent or limited. Pain thus represents a unique sensory stimulus, where the pathways are programmed to develop without significant natural sensory stimulation. We have used an animal model of hind paw peripheral inflammation to investigate development of spinal cord circuitry in animals that experienced persistent inflammatory pain as neonates. To produce persistent inflammatory stimulation in newborn rat pups, complete Freund?s adjuvant (CFA) was injected into the left hind paw. Edema and erythema occurred shortly after CFA injection and persisted for 5-7 days. This neonatal experience resulted in altered primary afferent neuronal circuits that persisted into adulthood. There was an increased density in small diameter nociceptive primary afferents in the segmental distribution of the sciatic nerve (the sciatic nerve innervates the hind paw territory that had experienced the peripheral inflammation), especially at caudal levels. In addition, the sciatic nerve primary afferents innervated new caudal spinal areas that typically do not receive inputs from the sciatic nerve. The behavioral response to noxious stimulation was also altered in the adult neonatal treated animals. There appeared to be greater sensitivity to pain in the neonatal treated animals. To examine functional changes in spinal cord activity at the cellular and molecular level, two studies were performed. Adult neonatal treated animals received a bilateral injection of CFA into their hind paws to produce persistent painful inflammation. After 24 hrs, the animals were sacrificed and the tissue harvested. In one group of animals, Fos-like immunoreactivity was used as a marker for neuronal activity in dorsal horn nociceptive neuronal pathways. A significant increase in the number of Fos labeled neurons was found on the side of the spinal cord that matched the neonatal inflamed hind paw. These data identify an increase in the number of spinal neurons responding to painful stimulation after the neonatal experience of pain. A second series of studies examined changes in dynorphin gene expression using the same experimental paradigm. Dynorphin mRNA has a low constitutive expression in the spinal cord and exhibits a dramatic induction following painful stimulation. In this study, the increase in dynorphin mRNA expression was greater on the side of the spinal cord that had experienced the neonatal inflammation. These observations identify altered cellular and molecular responses to painful stimulation in adults that had experienced pain as neonates. The physiological responsiveness of single neurons in the dorsal horn to noxious and non-noxious stimuli was further examined. Adult neonatal treated rats exhibited higher evoked firing rates in response to brush noxious heat and noxious pinch relative to the same intensities of stimulation applied to untreated rats. Spontaneous activity was also higher in adult neonatal treated rats. Large increases in receptive field size were also observed. These changes were present in nociceptive specific as well as the wide dynamic range class of dorsal horn neurons. These data demonstrate a dramatic alteration in spinal cord neuronal circuitry after neonatal persistent inflammatory pain. This plasticity in nociceptive neuronal circuits results in altered responses to painful stimuli in the adult and may explain unique developmental and behavioral differences in neonates who experience persistent pain. These observations draw attention to the need for adequate pain management in the neonate.
