Mechanism insight into the in situ reactions of repeated intramuscular progesterone injections

Intramuscular injections of progesterone (P4) are common during assisted reproduction, which can cause painful injection area reactions, and the current study was therefore initiated to determine whether P4 was involved in these adverse local effects. Female Sprague-Dawley rats were given daily intramuscular injection of vehicle oil or P4-in-oil with or without dermal administration of ketoprofen (Ket) gel at right biceps femoris muscle of hindlimb for 5 weeks. It was found that rats receiving repeated vehicle oil injections developed nociception-related behaviours together with induration formation and dorsal root ganglion (DRG) damage, indicating that the vehicle oil contributed to the side-effect reactions. Interestingly, P4 injections caused more nociception-related behaviours than those of vehicle oil as reflected by both nociception score and muscle withdrawal threshold evaluations, which were impressively relieved by Ket. In fact, P4 induced higher induration occurrence rate with larger volume that was alleviated by Ket. Further ELISA assays supported that P4 rather than vehicle oil profoundly elevated inflammatory factor levels. Moreover, an extensive upregulation of Na_v 1.8 was observed at L2, L3, and L5 of DRG in response to P4, indicating a sole role of P4 in Na_v 1.8 upregulation. Collectively, P4 may contribute to the painful injection area reactions via stimulating inflammation and DRG Na_v 1.8 upregulation in rats.