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The hypothalamic-pituitary-adrenal axis (HPA) is a complex system regulated by multiple factors. Sexual dimorphism of this axis has been described in different species under physiological conditions and it has been proposed that sexual hormones could have an effect on it. There are only a few reports about sex-linked variations in HPA axis hormones in the dog. Thus, studying the impact of sexual hormones on the HPA axis would broaden the knowledge about its function in this species. Therefore, the objective of this study was to determine whether there are variations in HPA plasma hormones (ACTH, alfa-melanocyte-stimulating hormone (α-MSH) and cortisol) according to the sex and photoperiod (positive or negative photoperiod were considered when the duration of the light hours of the day was more than 12 or less than 12, respectively) under basal conditions (like anoestrus) and throughout the oestrous cycle in the female dog. The population under study consisted of 11 intact female and 14 intact male dogs. Under basal conditions neither ACTH nor α-MSH concentrations showed differ - ences between sexes and different photoperiods. Cortisol showed greater values in the negative photoperiod than in the positive, both in females and males ( P = 0.03 and P = 0.015, respectively). Throughout the oestrous cycle, all the studied hormones showed variations ( P < 0.0001). The greatest concentrations of ACTH were observed at proestrus, while α-MSH and cortisol showed their greatest concentrations at oestrus. The three hormones decreased in diestrus . ACTH and cortisol concentrations were higher in the negative photoperiod ( P = 0.04 and P < 0.0001, respectively), while α-MSH concentrations were higher in the positive photoperiod ( P = 0.012). In the group of females oestradiol and progesterone correlated with ACTH ( r = 0.75, P < 0.0001; r = 0.34, P < 0.01, respectively), α-MSH (r = 0.49, P < 0.0001; r = 0.52, P < 0.0001, respectively) and cortisol ( r = 0.33, P < 0.01; r = 0.5, P < 0.0001, respectively). These results show that in females, HPA axis hormones vary during the oestrous cycle in relation to oestradiol and progesterone fluctuations. The ACTH, α-MSH and cortisol concentrations also showed differences between photoperiods in females, but only cortisol did so in males. These findings suggest that sexual hormones could have an effect on the HPA axis. Further research needs to be done to fully understand this interaction and the mechanisms involved.

Plasma ACTH, α-MSH and cortisol variations in the dog during the oestrous cycle in different photoperiods