Changes of circulating β-endorphin, adrenocorticotrophin and cortisol concentrations during growth and rearing in Thoroughbred foals

The aim of this study was to investigate the effects of growing and weaning on circulating levels of β-endorphin, adrenocorticotrophin (ACTH) and cortisol in foals during their first 13 months of life. We also investigated circulating concentrations of these hormones changed during the first three days of weaning. Blood samples were collected from 13 clinically healthy Thoroughbred foals monthly. All foals were weaned at 4 months and additional blood samples were taken 1, 2 and 3 days after weaning. The results obtained showed that growth affected β-endorphin, ACTH and cortisol values (P < .0001). Circulating β-endorphin concentrations averaged 21.00 ± 18.69 at 11 months of age and 233.69 ± 41.72 pg/ml at 7 months. Serum ACTH concentrations averaged 3.04 ± 0.89 at 4 months and 9.48 ± 0.07 pmol/l at 8 months. Serum cortisol concentrations averaged 51.95 ± 22.43 nmol/l at 1 month and 227.19 ± 40.25 nmol/l at 13 months. Positive correlations were found between ACTH and β-endorphin (r = 0.76; P < .05) and between ACTH and cortisol levels (r = 0.54; P < .05). A negative correlation was observed between β-endorphin and cortisol levels (r = − 0.66; P < .05) over 13 months of growth. Positive correlations between body weight and cortisol levels in both colts (r = 0.80; P < .001) and fillies (r = 0.82; P < .001) were observed during growth. Weaning effects were shown for β-endorphin, cortisol (P < .0001) and ACTH (P < .01) levels. Higher β-endorphin concentrations at 3 days (P < .001) and higher ACTH and cortisol concentrations at 2 and 3 days (P < .05) of weaning compared to 1 day values were found. Gender effects (P < .0001) were shown for ACTH and cortisol levels, with the lowest ACTH levels at 1 and at 13 months in fillies (P < .0001). In addition, fillies showed the highest cortisol levels at 1 month (P < .05). No significant differences between colts and fillies were observed for β-endorphin changes. The results obtained provide additional evidence of the recognized effects of growth and weaning in modulating the opioid system and pituitary–adrenocortical axis of Thoroughbred foals.