Serum prolactin concentrations in the captive female African elephant (Loxodonta africana): potential effects of season and steroid hormone interactions.

Research was conducted to determine whether seasonal changes in prolactin secretion occur in nonpregnant female African elephants and to examine potential functional interrelationships between secretion of prolactin, cortisol, and progesterone. Weekly blood samples were taken for 18 months from four female African elephants and the sera were analyzed by RIA for progesterone, cortisol, and prolactin concentrations. There was no significant effect of season on serum concentrations of prolactin. Estrous cycles averaged 14 weeks in length and were composed of a 9-week luteal phase and a 5-week follicular phase (based on progesterone concentrations consistently >200 and <200 pg/ml, respectively). Estrous cycle synchronicity was evident between pairs of elephants. Serum concentrations of prolactin (3.91 +/- 0.69 ng/ml; range: 0.84-15.8 ng/ml) were significantly lower during the luteal, compared with the follicular, phase (P < 0.0001; t test) and were positively correlated with serum concentrations of cortisol (r = 0.14; P < 0.05). Mean (+/-SE) serum concentration of cortisol was 5.7 +/- 1.3 ng/ml (range: 1.4-19.3 ng/ml), and concentrations of this adrenal steroid were negatively correlated with progesterone concentrations (r = -0.15; P < 0.01). Increased serum concentrations of prolactin detected during the follicular phase suggest that this hormone may be regulated by ovarian estrogens and may play a role in modulating ovarian function in the elephant. Copyright 1999 Academic Press.