Endocannabinoid System Regulation in Pyometra-Affected and Healthy Canine Uteri

Pyometra is a frequent and life-threatening reproductive disorder in bitches, characterized by profound immune and inflammatory responses within the uterus. The endocannabinoid system (eCS) is a key modulator of immune regulation, tissue homeostasis, and inflammation; however, its role in canine uterine physiology and pathology remains unexplored. This study aimed to characterize the presence and regulation of eCS components in the uterus of healthy and pyometra-affected dogs. Twenty-eight bitches were categorized into four groups: closed-cervix pyometra (CP; <i>n</i> = 7), open-cervix pyometra (OP; <i>n</i> = 7), diestrus (DE; <i>n</i> = 7), and anestrus (AE; <i>n</i> = 7). Uterine tissues were obtained by ovariohysterectomy. Serum progesterone, anandamide (AEA), and 2-arachidonylglycerol (2-AG) concentrations were quantified, while the uterine expression of cannabinoid receptors (CB1 and CB2) was assessed using real-time PCR and localized by immunohistochemistry (IHC). Serum AEA levels were significantly reduced in CP compared with AE (<i>p</i> = 0.017), whereas 2-AG differences did not reach significance (<i>p</i> = 0.072). Both CB1 and CB2 were consistently expressed across all groups, with IHC revealing receptor-specific patterns within uterine compartments. Collectively, these findings demonstrate for the first time that the canine uterus possesses a functional eCS, and that its modulation is linked to reproductive physiology and pyometra-associated inflammatory processes.