ABSTRACT
To identify testicular steroids that may be involved in the feedback mechanisms of the hypothalamus-pituitary-gonad axis in African catfish, Clarias gariepinus, steroids, which are known to be produced by the testis in vitro, were identified and quantified in blood plasma before and after castration using gas chromatography followed by mass spectrometry. Before castration 20 testicular steroids were detected. Quantitatively dominant steroids were testosterone (16.9 +/- 4.3 ng/ml), androstenedione (12.0 +/- 3.9 ng/ml), and 11-ketotestosterone (6.7 +/- 1.8 ng/ml). After castration, only these steroids showed a strong decrease in plasma, indicating that they are of testicular origin. Assuming that steroids involved in the feedback to the pituitary are under gonadotropic control, fish were injected with two doses of a salmon gonadotropin releasing hormone analogue (sGnRHa). The lower sGnRHa dose (0.25 microgram/kg body weight) increased plasma GTH-II levels in the same range as those induced by castration, resulting in a significant increase in plasma levels of testosterone, androstenedione, and 11-ketotestosterone. After injection of a higher dose of sGnRHa (5.0 micrograms/kg body weight), there was a greater increase in plasma GTH-II level, as well as a significant increase in most of the other steroids studied. The decreased levels of testosterone, androstenedione, and 11-ketotestosterone after castration and the significant increase in these steroids following moderate GTH stimulation, suggest that one or more of these steroids plays a role in the feedback mechanism within the hypothalamus-pituitary-gonad axis.
