Changes in pituitary, ovarian, and testicular activity in harbour seals (Phoca vitulina) in relation to season and sexual maturity

This study examined pituitary sensitivity to exogenous gonadotrophin-releas ing hormone (GnRH) in relation to time of year and changes in plasma proges terone, testosterone, luteinising hormone (LH), and follicle-stimulating ho rmone (FSH) concentrations during the annual cycle of adult and juvenile ha rbour seals (Phoca vitulina vitulina). There was no significant difference in seasonal changes in reproductive hormone levels between six captive indi viduals and seals caught from the wild (n = 80). Based on results from both the captive and the wild individuals, the plasma progesterone concentratio n was elevated in late gestation, whereas it declined at the equivalent tim e in nonpregnant females. The progesterone concentration was also elevated during the phase of delayed implantation, but there was no difference betwe en pregnant and nonpregnant seals. In captive seals given exogenous GnRH, t he LH concentration peaked 10-20 min after injection of GnRH, and the magni tude of the LH response depended upon the dose up to similar to 120 ng.kg(- 1). FSH did not respond to GnRH in the time course of the experiment. Pitui tary sensitivity to GnRH was greatest in the summer and autumn and lowest i n the winter and spring. Seasonal changes in pituitary sensitivity were app arent in both adults and juveniles. There was evidence that three of the fo ur juvenile seals attained puberty during the study. We conclude that the s easonal patterns of pituitary sensitivity and plasma hormone concentration in harbour seals are consistent with a reproductive cycle under photoperiod ic control and with the general mechanisms underlying photoperiodic control of seasonal reproduction in higher vertebrates.