Possible mechanisms underlying the involvement of microglial P2X4 receptors in the comorbidity of pain and depression

Microglia’s P2X4 receptor may be a key target involved in the occurrence and development of pain–depression comorbidity. The objective of this study is to explore the possible mechanisms underlying the involvement of microglial P2X4 receptors in pain–depression comorbidity. Sixty male SD rats were randomly divided into six groups: normal control group (10), sham operation group (10), chronic constriction injury of sciatic nerve group (CCI group, 10), CCI + antagonist group (10), chronic unpredicted mild stress model group (CUMS group, 10), CUMS + antagonist group (10). The normal group received no treatment, the sham group was injected with normal saline through stereotaxic localisation, the CCI group was ligated with sciatic nerve of left leg, the CUMS group was stimulated by stress for 30 days. The rats in the CCI group and CUMS group were injected with P2X4 receptor antagonist 30 days later. The mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were measured in each group on different days, followed by forced swimming tests in all groups to measure floating immobility time in water. Finally, P2X4’s expression in rostral ventromedial medulla (RVM) and spinal lumbosacral swelling were detected by Western Blotting and immunofluorescence, respectively, whilst expression of 5-HT in the spinal dorsal horn was measured via immunohistochemistry. Compared with the sham group, both MWT and TWL in the CCI group exhibited a decreasing trend (lowest level at approximately 2 weeks before remaining stable). Floating state time in the forced swimming experiment was significantly prolonged. Simultaneously, P2X4’s expression increased in RVM, as did that of 5-HT in spinal cord, but the expression of 5-HT in spinal cord significantly decreased after injection of P2X4 receptor antagonist in RVM. Compared with the normal group, MWT and TWL in the CUMS group decreased gradually, and floating state time reached the longest. P2X4 receptor expression in the RVM increased more significantly than that in the CCI group, and that of 5-HT in the spinal cord increased. Post injection of the P2X4 receptor antagonist, the MWT and TWL were higher than those before injection, and forced swimming time decreased. Moreover, P2X4 receptor expression in RVM increased, and expression of 5-HT in the spinal cord decreased. P2X4 receptor in RVM may be involved in the pathogenesis of pain–depression comorbidity.