Abstract

Hypothalamic serotonin (5-HT; content and turnover) and monoamine oxidase (MAO) activity were measured in female catfish, Heteropneustes fossilis, after ovariectomy and supplementation with oestradiol-17β (OE₂) in the recrudescent and quiescent phases. These factors were correlated with changes in plasma levels of OE₂ and gonadotrophin. In the quiescent phase (December), neither ovariectomy nor OE₂ supplementation had any significant effect on 5-HT content and MAO activity. Plasma levels of OE₂ and gonadotrophin were undetectable in both control and treated fish, indicating that there was no feedback effect. In the recrudescent phase (prespawning, May), ovariectomy caused biphasic responses of MAO activity and 5-HT content. The enzyme activity decreased significantly after 2, 3, 4 and 5 weeks but increased significantly 6 weeks after ovariectomy. The 5-HT content varied in a biphasic manner with a significant increase at 2, 3 or 4 weeks and a significant decrease in week 6; there being no effect in week 5. 5-HT turnover was inhibited significantly only in week 4 after ovariectomy and did not show a biphasic pattern. In the ovariectomized groups, the OE₂ level decreased significantly in a progressive manner with a maximum reduction in week 6. The plasma level of gonadotrophin showed a significant bimodal pattern of increase with the peak in week 4 after ovariectomy, indicating a strong negative feedback effect of OE₂. The bimodal pattern of pituitary gonadotrophin release could be correlated with a similar pattern of increase in 5-HT content. OE₂ treatment of fish which had been ovariectomized 3 weeks previously had dose-dependent effects on the enzyme; the low dose (0·1 μg/g body weight) was stimulatory and the higher doses (0·5, 1·0 and 5·0 μg/g body weight) were inhibitory. The reverse was true for 5-HT content. Serotonergic turnover increased significantly only in the groups given high doses (1·0 and 5·0 μg/g body weight). The low dose of OE₂ (0·1 μg/g body weight) restored the gonadotrophin and OE₂ levels to those of the sham-ovariectomized vehicle-treated control group, whereas the high doses (0·5, 1·0 and 5·0 μg/g body weight) decreased the release of gonadotrophin in a dose-dependent manner. Our results suggest that OE₂ modulates MAO activity to alter hypothalamic 5-HT in a seasonally dependent manner. The ovariectomy-induced changes in plasma levels of gonadotrophin appear to be mediated, at least partly, by the feedback action of OE₂ on 5-HT metabolism.

Journal of Endocrinology (1994) 142, 193–203

An HPLC method was used to tentatively identify progesterone (P₄) and its metabolites (17-hydroxyprogesterone (17-P₄) and 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P)), corticosteroids (cortisol and corticosterone) and testosterone in ovary/follicular preparations of the catfish Heteropneustes fossilis associated with in vivo or in vitro oocyte maturation/ovulation. A single i.p. injection of human chorionic gonadotrophin (100 IU/fish, sampled at 0, 8 and 16 h) induced oocyte maturation and ovulation, which coincided with significant and progressive increases in 17,20β-P, and P₄ and 17-P₄, the precursors of the former. Both cortisol and corticosterone also increased significantly. Conversely, testosterone decreased significantly and progressively over time. Under in vitro conditions, incubation of post-vitellogenic (intact) follicles or follicular envelope (layer) with 2-hydroxyoestradiol (2-OHE₂, 5 μM for 0, 6 and 24 h) elicited a sharp significant increase in 17,20β-P, the increase being higher in the follicular envelope incubate. P₄ and 17-P₄ also registered significant increases over the time with the peak values at 24 h. Cortisol and corticosterone increased significantly in the intact follicle, but not in the follicular envelope incubate. Testosterone decreased significantly in the intact follicle, but increased significantly (24 h) in the follicular envelope incubate. Coincident with these changes, the percentage of germinal vesicle breakdown (GVBD) increased over the time in the intact follicle incubate (48.9% at 6 h and 79.8% at 24 h). Denuded oocytes on incubation with 2-OHE₂ (5 μM) did not produce any significant change in the percentage of GVBD or in the steroid profile. While corticosterone and 17,20β-P were undetected, P₄, 17-P₄, cortisol and testosterone were detected in low amounts. The results show that the 2-OHE₂-induced GVBD response seems to be mediated through the production of 17,20β-P and corticosteroids. It is suggested that hydroxyoestrogens seem to be a component in the gonadotrophin cascade of regulation of oocyte maturation/ovulation in the catfish.