In agnathans the existence of a hypothalamus-hypophysis-gonad axis is well documented. But at present the role of the steroid hormones remains to be clarified. Therefore the measurements of steroid hormones in plasma were combined with in vitro incubations of gonadal tissues with testosterone (T) and progesterone (P) as substrates. The animals were caught in the Netherlands in November at the beginning of their upstream migration and kept in tanks at 10-12°C. Blood samples were taken from December to April. Cortisol (C), P, T, and estradiol (E2) were measured by a commercially available EIA. The C levels in females rose until March (5.4 ng/ml), in males until April (4.6). P was only detectable in three of thirty-six specimens, T was not detected. The level of E2 increased parallel to the ovarian weight until March (2.3 ng/ml) and then declined rapidly (0.56) in April, whereas the gonadosomatic index (GSI) was twice the March figure (22.77 vs 13.05). In males the GSI increased until February (6.7) and fell towards April (3.15). In contrast the E2 level increased continously from December (0.1) to March and April (3.3 and 2.8 ng/ml). With P as substrate no T was produced in males or females, only traces of 11ßOH-T and 11keto-T were found. A large amount of T was converted into a steroid of the same quality as the one described by Kime (1): 15ßOH-T. Unfortunately, there is no assay available to measure this steroid for the clarification of its functional role.The synthesis of E2 in both sexes is very low (0.3-1.9%). The conclusion drawn is that in cyclostomes T is not a hormone but only a precursor and that E2 is a hormone for both sexes. It is doubtful whether the low production rate in the gonads is sufficient for the high plasma levels. There might well be another source for this hormone. 1. Kime DE, Rafter JJ; Gen Comp Endocrinol 44:69-76.
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Abt. 1