Glial-cytokine-neuronal Adaptations in the Ventral Hippocampus of Rats with Affective Behavioral Changes Following Peripheral Nerve Injury

Nerve damage leads to the development of disabling neuropathic pain in susceptible individuals, where patients present with pain as well as co-morbid behavioral changes, such as anhedonia, decreased motivation and depression. In this study we evaluated whether radial maze behavioral disruptions and glia-cytokine-neuronal adaptations in the hippocampus occurred in individual rats after nerve injury. Exploration behavior and spatial memory were quantified using a radial maze task, while mechanical allodynia was assessed using von Frey testing. Sciatic nerve chronic constriction injury (CCI) reduced withdrawal thresholds in all rats, while pellet-seeking behaviors were altered in some but not all rats. One group, termed 'No effect', had no behavioral changes compared to sham rats. Another group, termed 'Acute effect', had a temporary alteration to their exploration pattern, displaying more risk-assessment behavior in the early phase post-injury. In a third group, termed 'Lasting effect', exploratory behaviors were remarkably different for the entire post-injury period, showing a withdrawal from pellet-seeking. The withdrawal from pellet-seeking was found to be concomitant with distinct glial-cytokine-neuronal adaptations within the contralateral ventral hippocampus, including; increased expression of IL-1β and MCP-1; astrocyte atrophy and decreased area in the dentate gyrus; reactive microglia and increased FosB/ΔFosB expression in the cornu ammonis subfield. Therefore, glial-cytokine-neuronal adaptations in the ventral hippocampus may mediate individual differences in radial maze behavior following CCI. Our data suggest that individual neuroimmune signatures play a significant role in divergent behavioral trajectories following nerve injury, toward functional recovery and coping, or the emergence of ongoing affective state disturbances.