Mechanical Hypersensitivity Occurs in a Mouse Model of Sickle Cell Disease

The acute painful event is the hallmark of sickle cell disease (SCD) in humans, accounting for the majority of hospitalizations. In addition, pain managed at home is underreported in both children and adults with SCD. Vaso-occlusion is intricately linked to the development of sickle cell pain, initiating tissue damage and numerous biochemical, electrochemical, and neurologic cascades which affect nociception (the perception of pain). It has been hypothesized that repeated painful events may lead to sensitization, which can manifest as allodynia (painful response to a usually non-painful stimulus) and chronic pain. However, the mechanisms of pain in SCD remain poorly understood. The objective of this study was to evaluate nociception in a mouse model of SCD both at baseline and after acute vaso-occlusion. The Berkeley mouse model of SCD exhibits many characteristics of human SCD, including vaso-occlusion. Acute vaso-occlusive events in the Berkeley SCD mouse can be induced by exposure to short periods of hypoxia followed by reoxygenation. We compared pain behavior responses to heat and mechanical force in female Berkeley SCD (SS) mice before and after exposure to 1 hour of hypoxia (FiO₂ 10%) followed by 1 hour of reoxygenation in room air. Pain behavior in SS mice was also compared to that in control female HbA (AA) mice, which solely express normal human hemoglobin and are maintained on the same mixed genetic background as the SS mice. Female mice expressing both normal and sickle human hemoglobin (AS mice) comprised an additional control group. Pain behavior was assessed by two methods: the Hargreaves test of thermal (heat) nociception and the von Frey test of mechanical allodynia. The Hargreaves test is conducted by applying a radiant heat stimulus to the plantar surface of the hind paw (Hargreaves, et al, 1988). The time to withdrawal of the paw from the heat stimulus is recorded as the paw withdrawal latency. Baseline data from the Hargreaves test of 12 AA, 12 AS, and 14 SS mice showed a subtle decrease in paw withdrawal latency in SS as compared to AA or AS mice; however, the differences in paw withdrawal latency between groups were not statistically significant. Exposure to hypoxia-reoxygenation did not affect paw withdrawal latency in response to heat stimulation in any of the three groups of mice. The second test for pain behavior used was the von Frey test for mechanical hypersensitivity (i.e., allodynia). Mice are tested for response to the application of increasing and decreasing force strengths of nylon monofilaments to the plantar surface of the hind paws. The paw withdrawal threshold was determined using the up-down method of Dixon (1980). Baseline data from 12 AA, 12 AS and 12 SS mice showed a significant difference in paw withdrawal threshold between groups (p=0.05); SS mice exhibited the lowest paw withdrawal threshold, reflecting increased sensitivity to mechanical stimuli. Exposure to hypoxia-reoxygenation did not alter the paw withdrawal threshold of AA and AS mice and tended to decrease paw withdrawal threshold of SS mice, consistent with vaso-occlusion causing pain in SS mice exposed to hypoxia-reoxygenation. These results suggest that Berkeley SS mice exhibit mechanical hypersensitivity at baseline compared to AA and AS mice, perhaps reflective of increased baseline vaso-occlusion in SS as compared to control mice. Exposure to hypoxia-reoxygenation, which induces RBC sickling, acute vaso-occlusion and tissue damage in SS mice, accentuates the sensitivity of the SS mice to mechanical stimuli. The results of these ongoing studies will expand our understanding of mechanisms underlying sickle cell-associated pain behavior in mice with SCD. Translationally, this may lead to improved pain management strategies for both acute and chronic pain experienced by individuals affected by this chronic debilitating disease.