11.2 Effects of gonadectomy and sex steroid therapy on the hypothalamus and pituitary
11.3 Feedback actions of sex steroids on gonadotropin secretion
The information thus far discussed in sections 7, 8 and 10 indicates that the gonadotropic hormones (section 5) regulated the activity of the gonads through the production of steroid hormones (sections 6 and 9). The sex steroids not only regulate reproduction, sexual behaviour and secondary sexual characters but also gonadotropin secretion (de Vlaming, 1974). These effects of sex steroids are mediated in part by their actions on the central nervous system. Information is now available concerning the specific brain areas involved in the regulation of gonadal functions (section 3). The sex steroids act on their target sites in the brain and through the neurosecretory principles regulate the release of gonadotropin (GtH) from the pituitary. This mode of control of GtH release by blood sex steroid levels is known as feedback mechanism. There are examples of positive and negative feedback actions (Peter, 1973; Peter and Crim, 1979).
Castration induces hypertrophy of gonadotropic cells and hypersecretion of gonadotropin; the effects can be reversed by administration of estradiol or testosterone. Robertson (1958) reported de-granulation of gonadotropic cells in the pituitary and accelerated development of tastes in the rainbow trout after unilateral castration. Similarly, unilateral ovariectomy in the catfish, Heteropneustes fossilis, results in compensatory hypertrophy of the remaining ovary during all periods except the postspawning period; the time interval required for the remaining ovary to achieve full compensatory hypertrophy decreases with the proximity of the surgery to the spawning season (Goswami and Sundararaj, 1968a). Administration of estradiol benzoate blocks ovarian compensatory hypertrophy (Goswami and Sundararaj, 1968b) as well as vitellogenesis in intact catfish (Sundararaj and Goswami, 1968). In the same species, bilateral gonadectomy results in hyperactivity of the gonadotropic cells in both sexes (Sundararaj and Nayyar, 1969a; Anand and Sundararaj, 1975) and the inhibitory effect of sex steroids operates via. hypothalamic neurosecretory cells and pituitary (Viswanathan and Sundararaj, 1974b). Similarly, ovariectomy in another catfish, Clarias batrachus, increases neurosecretory material in the nucleus preopticus (NPO) which decreases after estrogen treatment (Dixit, 1970).
In rainbow trout, bilateral castration at any stage of the testicular cycle, including the, period when the testes are inactive, increases plasma GtH levels (Billard, Richard and Breton, 1.976, 1977; Billard, 1978a). These results indicate that the negative feedback action of testicular steroids on GtH secretion occurs at all stages in the breeding cycle.
Sex steroids are taken up and sequestered in the pituitary, the NPO, the nucleus lateralis tuberis (NLT), the nucleus recessus lateralis and other locations in the brain in many species of fishes, suggesting that a number of sites in the brain can serve as loci for feedback action of sex steroids (Kim. et al., 1978; Peter and Crim, 1979). In the sexually mature female goldfish, serum GtH levels increase markedly following implantation of antiestrogens in the pituitary, whereas similar implants in the NLT area result only in a modest GtH increase and have no effect when implanted elsewhere (Billard and Peter, 1977). Thus the NLT and pituitary appear to be the principal sites for feedback action of estrogens in the female goldfish. In rainbow trout, the postcastration rise in plasma GtH can be partially blocked by implantation of 11-ketotestosterone in the pituitary (Billard, 1978a). In the Atlantic salmon parr, testosterone has a positive feedback action on GtH secretion, since testosterone implants in brain (NLT) or pituitary result in a rapid increase in GtH levels in the pituitary (Crim and Peter, 1978; Crim and Evans, 1979, 1980).
In the catfish, Heteropneustes fossilis, the transition from the vitellogenic phase to the maintenance phase is accompanied by a drop in plasma levels of estradiol and an increase in the levels of testosterone (Sundararaj, Lamba and Goswami, 1980). Since testosterone. is reported to have a positive feedback effect on GtH in other fishes (see Crim and Peter, 1978; Crim and Evans, 1.980), it is surmised that in catfish also, high levels of testosterone during the maintenance phase may help in storage of GtH for eventual release as a surge to induce maturation and ovulation.