Enhanced nociceptive behavior and expansion of associated primary afferents in a rabbit model of cerebral palsy

Spastic cerebral palsy (CP) is a movement disorder marked by hypertonia and hyperreflexia, and the most prevalent comorbidity is pain. Since spinal nociceptive afferents contribute to both the sensation of painful stimuli as well as reflex circuits involved in movement, we investigated the relationship between prenatal hypoxia-ischemia (HI) injury which can cause CP, and possible changes in spinal nociceptive circuitry. To do this, we examined nociceptive afferents and mechanical and thermal sensitivity of New Zealand White rabbit kits after prenatal HI or a sham surgical procedure. As described previously, a range of motor deficits similar to spastic CP was observed in kits born naturally after HI (40 minutes at ∼70-80% gestation). We found that HI caused an expansion of peptidergic afferents (marked by expression of calcitonin gene-related peptide; CGRP) in both the superficial and deep dorsal horn at postnatal day (P)5. Non-peptidergic nociceptive afferent arborization (labeled by isolectin B4; IB4) was unaltered in HI kits but overlap of the two populations (peptidergic and non-peptidergic nociceptors) was increased by HI. Density of glial fibrillary acidic protein (GFAP) was unchanged within spinal white matter regions important in nociceptive transmission at P5. We found that mechanical and thermal nociception was enhanced in HI kits even in the absence of motor deficits. These findings suggest that prenatal HI injury impacts spinal sensory pathways in addition to the more well-established disruptions to descending motor circuits. In conclusion, changes to spinal nociceptive circuitry could disrupt spinalreflexes and contribute to pain experienced by individuals with CP. Significance Statement Perinatal injuries that cause cerebral palsy (CP) typically involve global insults to the central nervous system and are capable of modulating development of both motor and sensory systems. Most individuals with CP experience pain, yet whether nociception is enhanced in this disorder is unexplored. Here, we demonstrate altered topographic distribution of nociceptive afferents in the spinal cord dorsal horn of neonatal rabbits that experienced hypoxic-ischemic (HI) injury in utero; these anatomical changes were associated with nocifensive behavior indicative of pain-like behaviors. Our findings suggest that CP-causative injuries alter spinal sensory pathways (not only descending motor circuits), contributing to increased pain in CP. ### Competing Interest Statement The authors have declared no competing interest. * (5HT) : Serotonin (CGRP) : calcitonin gene-related peptide (CNS) : central nervous system (CP) : cerebral palsy (DAPI) : Diamidino-2-Phenylindole (DC) : dorsal column (DRG) : dorsal root ganglion (GFAP) : glial fibrillary acidic protein (GMFCS) : Gross Motor Function Classification System (HI) : hypoxia-ischemia (IB4) : isolectin B4 (IHC) : immunohistochemistry (LST) : lateral spinothalamic tract (NHS) : normal horse serum (NSAIDs) : nonsteroidal anti-inflammatory drugs (P) : postnatal day (PBS) : phosphate buffered saline (PFA) : paraformaldehyde (ROIs) : regions of interest (SDGM) : superficial dorsal gray matter (VST) : ventral spinothalamic tract