NWO research at Utrecht University has shown that when carp are subjected to stress, the development of their genital organs is delayed, so that they reach puberty later. It is likely that the stress hormone cortisol plays a major role in delaying puberty.
Changes in water temperature produce stress in fish. Dimitri Consten of Utrecht University subjected carp to a rapid reduction in water temperature three times a week from 25°C down to 14°C. This led to delayed development of their reproductive cells and the fish reached puberty later than normal. The researchers assumed that the hormone cortisol, which is released when an animal is under stress, plays a major role in this. This assumption was confirmed by means of two tests. In one of them, the biologists ‘switched off’ the cortisol in the stressed fish. These animals then developed to puberty in the normal manner. In a second test, cortisol was administered to fish which were not subjected to stress. In these fish, puberty was delayed.
Cortisol would seem to affect the testes. It directly delays the development of reproductive cells into sperm cells. This slows the growth of the sexual organs and also the supply of steroids to the blood. During puberty, steroids from the testes ensure that the brain, the pituitary gland and testes develop properly. Because the cortisol produced under stress reduces the supply of steroids, communication to the brain and the pituitary (a gland under the brain) is reduced. This means that, like the testes, these organs develop more slowly, thus slowing down overall development.
The whole complex of hormones involved in puberty is self-regulatory. The brain produces the gonadotropin-releasing hormone, which stimulates cells in the pituitary. On order, the pituitary then excretes the gonadotropins, the luteinising hormone and the hormone which stimulates the follicles. In the testes, the gonadotropins promote the production of reproductive cells and steroid hormones. The steroid hormones contribute to the production of the reproductive cells and ensure communication between the brain and the pituitary gland, thus completing the cycle.