Early life injury alters spinal astrocyte development.

Neonatal injury alters synaptic transmission in the spinal superficial dorsal horn (SDH), resulting in aberrant amplification of ascending nociceptive transmission. Astrocytes orchestrate synapse development and function across the CNS and play a critical role in the emergence and maintenance of persistent pain. However, little is currently known about the postnatal development of spinal astrocytes, nor about how the maturation of SDH astrocytes is impacted by early life injury. Here, we used a hindpaw incision model of postsurgical pain in postnatal day (P)3 mice of both sexes to elucidate the effects of neonatal injury on the maturation of SDH astrocytes. Three-dimensional morphological analysis of individual astrocytes revealed that incision elicits age-dependent changes to astrocyte structure. At P4, spinal astrocytes in incised mice show increased size and complexity compared to naïve controls. This is reversed at P10 and P24, as astrocytes from incised mice are smaller and less ramified compared to their naïve counterparts. Transcriptomic analysis of spinal astrocytes revealed acute changes to gene expression after neonatal injury, as 76 differentially expressed genes (DEGs) were identified at P4 (such as Thbs1, Efemp1, Acta1, Acta2, Tpm2 and Fgf14), which included genes related to cell motility and cytoskeletal organization, but very few DEGs were detected at P10 and P24. Lastly, we identified that microglial engulfment of astrocyte material occurs in the developing dorsal horn, and that this process is altered by neonatal incision in a sex-dependent manner. These data illustrate, for the first time, that neonatal injury alters the postnatal development of spinal astrocytes.Significance Statement Neonatal tissue damage persistently remodels synaptic circuits in the spinal superficial dorsal horn (SDH), which has been implicated in the ability of early life injury to "prime" developing nociceptive pathways. While astrocytes clearly regulate synapse formation, pruning and function across the CNS, nothing is known about the degree to which neonatal injury modulates the properties of astrocytes within the developing SDH. The present study demonstrates that neonatal hindpaw incision evokes age-dependent transcriptional and morphological plasticity in spinal astrocytes, highlighted by a prolonged reduction in the size and complexity of astrocytes following early life injury. These findings yield new insight into the cellular mechanisms by which neonatal tissue damage can exert long-term effects on spinal nociceptive processing.