Short-term profiles of plasma gonadotropin and estradiol-17β levels in the female rainbow trout, from early ovarian recrudescence and throughout vitellogenesis

At various stages throughout the annual reproductive cycle, female rainbow trout (Salmo gairdneri) were fitted with a catheter in the dorsal aorta. They were bled via the catheter at frequencies of once every 30, 60, or 240 min over periods of 5 to 24 hr. Gonadotropin (GtH), estradiol-17β (E217β), and estrone levels were measured in the plasma samples. At early ovarian recrudescence (March), short-term (1–2 hr), high-amplitude (ΔGtH = up to 100 ng/ml), episodic pulses of GtH were recorded in the plasma of 12 of the 26 studied fish. In the others, GtH levels remained low and constant. No synchronization was found among the individual GtH profiles. E217β levels in the same fish were low and constant while estrone was not detectable. At early stages of exogenous vitellogenesis (June), plasma GtH (1–3 ng/ml) and E217β (0.5–1.5 ng/ml) levels were low and constant. At advanced stages of exogenous vitellogenesis (September–October), fluctuating GtH levels were found again in most of the females; basal GtH concentrations were only slightly higher than those recorded in June. The fluctuations consisted of short-term (1–3 hr) random GtH pulses of moderate amplitude (ΔGtH = up to 5 ng/ml), occurring at a relatively high frequency (up to 5 per 12 hr). Although no regular synchronous daily pattern of GtH was noted, most of the GtH pulses were observed during the photophase and early scotophase. The appearance of GtH pulsatility during exogenous vitellogenesis was accompanied by a large increase in plasma E217β up to levels ranging from 6 to 30 ng/ml. In contrast to the GtH profiles, the E217β profiles showed continuous and progressive variations, superimposed on the abrupt GtH pulses, and a high degree of synchronization. E217β levels increased during the photophase and reached maxima toward and during early scotophase