Pharmacological modulation of neutrophils, in contrast to that of macrophages/microglia, is sex independent and delays the development of morphine tolerance in a mouse model of neuropathic pain

Identifying sex-specific mechanisms underlying neuropathic pain, as well as its therapy, remains a challenge. Recent studies suggest important roles for neutrophils, macrophages and microglia in the development of hypersensitivity and the morphine effectiveness. Therefore, the aim of this study was to test whether substances that inhibit the activation/influx of neutrophils (4-aminobenzoic hydrazide) and microglia/macrophages (minocycline) can help achieve pain relief in male/female mice and improve the analgesic effectiveness of morphine in neuropathy in both sexes. Our behavioral studies performed using chronic constriction injury of the sciatic nerve indicate that repeated twice-daily administrations of 4-aminobenzoic hydrazide (in both sexes) and minocycline (only in males) cause analgesia and delay morphine tolerance development. Observations in female include the absence of alleviation of tactile hypersensitivity (von Frey test) following minocycline, and even an increase in thermal hypersensitivity (cold plate test), which may be of clinical importance. This may be explained by a lack of impact on IBA-1 protein level after repeated administration of minocycline in females, along with increased levels of pronociceptive factors such as CCL2, CXCL2, and TNFα. Notably, the repeated twice-daily administration of 4-aminobenzoic hydrazide has beneficial analgesic effects and delays morphine tolerance development in both sexes. Its influence on male mice is likely caused by its impact on spinal neutrophil activation/influx and on the level of anti-(IL-4)/pro-(CCL2) nociceptive cytokines. Our studies provide the first evidence that the inhibition of neutrophil activation/influx during long-term morphine treatment can improve its efficacy and delay the development of opioid tolerance in neuropathy in both sexes.