Sensitization of developing sensory neurons after incision

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ABSTRACT: Approximately 15-20% of children experience persistent or chronic pain. However, compared to adults, we know relatively little about the mechanisms of pediatric pain development. A basic understanding of nociceptive processing in the immature nervous system is therefore crucial in order to develop more appropriate treatments for pain in children. Patients with growth hormone deficiency (GHD) may provide insight into this clinical problem. Patients with GHD often display pain at rest in addition to deficits in growth. Moreover, treatment of certain pain patients with GH provides analgesia. We have found that cutaneous inflammation and muscle incision in mice reduces GH in the injured tissues. Observed changes in gene expression, afferent function and pain-related behaviors during neonatal injury are blocked by treating mice with exogenous GH. New pilot data suggests that macrophage dependent sequestering of GH at the site of peripheral injury, subsequently reduces inhibitory microRNA expression (e.g. miR-133a) within nociceptors to increase transcription factor (e.g. serum response factor (SRF)) dependent upregulation of various receptors and channels that modulate afferent function and pain-related beahviors. The main goal of this proposal is to determine the molecular mechanisms of how GH levels regulate sensory neuron sensitization during muscle incision and how this may underlie acute and persistent neonatal hypersensitivity. Specific Aim 1 will use a novel ex vivo somatosensory recording preparations to determine the effects of macrophage or sensory neuron specific knockout of the GH receptor on the sensitization of sensory neurons in uninjured neonatal mice or animals with muscle incision. Specific Aim 2 will test whether knockdown of a transcription factor (SRF) or overexpression of a microRNA (miR-133a), that is thought to regulate receptor expression in neurons (and thereby modulate peripheral sensitization), modifies these same changes in afferent function after muscle incision using in vivo siRNA-mediated knockdown or plasmid based overexpression strategies in single peripheral nerves in conjunction with ex vivo recording. Each of these two aims will be complemented by analysis of ongoing and evoked hypersensitivity. Finally, Specific Aim 3 will use behavioral analyses and/or ex vivo recording to determine the influence of localized GH treatments, GHr KO, SRF inhibition or miR-133a overexpression in neonatally incised mice on the prolonged effects to subsequent adolescent incision. These experiments will allow a better understanding of the unique mechanisms in primary sensory neurons by which peripheral GH levels regulate afferent sensitization and neonatal pain development. These studies will facilitate understanding of the transition from acute to chronic pediatric pain, and will allow us to determine the utility of GH as a pain therapy for children. This work may also lead to the design of more suitable treatments for pediatric pain that target specific pain receptor(s), signaling molecule(s) or afferent subtype(s).