JOURNAL OF EXPERIMENTAL ZOOLOGY 286:405–413 (2000) © 2000 WILEY-LISS, INC. JEZ 0785D Changes Along the Pituitary-Gonadal Axis During Maturation of the Black Carp, Mylopharyngodon piceus GAL GUR, 1 * PHILIPPA MELAMED, 1 ACHIKAM GISSIS, 3 AND ZVI YARON 1,2 1 Department of Zoology, Tel-Aviv University, Tel Aviv 69978, Israel 2 The Norman and Rose Lederer Chair of Experimental Biology, Tel Aviv University, Tel Aviv 69978, Israel 3 Fish Hatchery, Kibbutz Ha’Maapil, Mobile Post Hepher 38945, Israel ABSTRACT The black carp, Mylopharyngodon piceus, is a late-maturing cyprinid reaching sexual maturity at the age of 6–7 years. The present work attempted to define nonfunctional sites along the pituitary–gonadal axis in immature fish utilizing in vivo and in vitro challenge experi- ments. Two- and 3-year old fish injected with salmon gonadotropin-releasing hormone analog (sGnRHa; 10 μg/kg) and metoclopramide (20 mg/kg) did not reveal any increase in circulating gonadotropin (cGtH) or estradiol (E 2 ) level. Furthermore, cGtH release from cultured pituitary cells of fish at these ages did not increase in response to sGnRH (0.1 nM – 1 μM) but was aug- mented when exposed to TPA (12.5 nM). However, 4-year old female fish did respond to the above treatments both in vivo and in vitro. These results suggest the existence of nonfunctional site(s) proximal to the activation of PKC in the immature black carp gonadotrophs, probably at the level of GnRH receptors. These site(s) start to become functional in 4-year old fish. Two- and 3-year old fish injected with common carp pituitary extract (CPE) containing 350 μg cGtH/kg did not show any increase in circulating E 2 . In addition, the estrogen secretion from fragments of the rudimen- tary gonads did not increase after exposure to CPE containing cGtH (0.5–4 μg/ml) but was el- evated dose-dependently by exposure to dbcAMP (0.3–3 mM). However, the ovaries of 4-year old fish did respond to the gonadotropic stimulation, both in vivo and in vitro. These results suggest the existence of other non-functional site(s) in the immature black carp, proximal to the formation of cAMP in the gonads, probably at the level of GtH receptors. These site(s) start to become func- tional in 4-year old females. Another source of E 2 was discovered in the immature black carp: namely, the fat pad adjacent to the gonads. In contrast to the visceral adipose tissue, the fat pad secretes estrogen in response to cAMP elevation in 2- and 3-year old fish while in 4-year old fish it also responds to gonadotropic stimulation. Due to its large mass and high steroidogenic potency, it is assumed that the gonadal fat pad is involved in the process of puberty in the black carp. J. Exp. Zool. 286:405–413, 2000. © 2000 Wiley-Liss, Inc. It has been hypothesized that ovarian develop- ment is correlated with feeding and somatic growth and that environmental factors may af- fect maturation through their effect on growth rate. Precocious maturation of male Atlantic salmon parr can be delayed by imposing feed re- striction at specific times of the year (Rowe and Thorpe, ’90; Thorpe et al., ’90). In field experi- ments, advanced maturation in common carp was found to be dependent on low maintenance den- sity and adequate food supply (Sehgal and Toor, ’95). In contrast, catfish reared at 30°C and poorly fed matured at lower body weight and had higher relative fertility than well-fed fish, but absolute fecundity was higher in the well-fed fish. It was Parts of these results were reported in the Fifth International Sym- posium on Fish Physiology of Reproduction, Austin 1995. Grant sponsor: The United States—Israel Binational Agricultural Research and Development Fund (BARD); Grant numbers: IS-2149- 92 and IS-2634-95. *Correspondence to: Ms. Gal Gur, Department of Zoology, Tel-Aviv University, Tel Aviv 69978, Israel. E-mail: galgur@post.tau.ac.il Received 13 April 1999; Accepted 16 August 1999 suggested, therefore, that lower feeding regimes lead to investment of body resources in ovarian rather than somatic growth (Richter et al., ’82). Similar results were obtain in the Atlantic cod in which reduced growth and liver size in feed-re- stricted groups did not result in lower proportions of maturing fish (Karlsen et al., ’95).