Serial implantation of testosterone pellets, at 30-day intervals to day 75 or at 15-day intervals to day 105, stimulated the pituitary content of gonadotropin (GtH) to increase approximately 24 fold; however, no significant changes in serum GtH levels were detected at the sampling times used. Vitellogenesis was stimulated in the ovary of all testosterone-implanted animals; the ovary of at least one eel appeared to be at a preovulatory state (gonadosomatic INDEX = 39.8%), as demonstrated by the presence of large yolky oocytes. Serial implantation of LHRH-A or domperidone along with testosterone did not further stimulate pituitary GtH content or ovarian development. The results demonstrate that chronic treatment with testosterone alone can stimulate the brain-pituitary-ovary axis of the Japanese silver eel to induce ovarian development to, or nearly to, the prespawning stage.

In the present study, three preprosomatostatin (PSS) cDNAs were characterized from hypothalamus of orange-spotted grouper Epinephelus coioides. The first cDNA encodes a 123-amino acid protein (PSSI) that contains the SS14 sequence at its C-terminal extremity and that is identical to that of PSSI of human and other vertebrates. The second cDNA encodes a 127-amino acid protein (PSSII) that contains the SS28 sequence with [Tyr7, Gly10]-SS14 at its C-terminus. The third cDNA encodes a 110-amino acid protein (PSSIII) that contains the somatostatin variant [Pro2]-SS14 at its C-terminal extremity. All these three PSS mRNAs were expressed in brain and pituitary with different mRNA levels. In peripheral tissues, PSSII was more widely distributed than PSSI and PSSIII. High mRNA levels of PSS were found in stomach, intestine and ovary. PSS mRNAs were detected throughout embryogeny and early larval development. Its levels increased with the embryonic development and maintained a higher level during larva developing. The mRNA distribution suggests that the three grouper PSS products play important physiological functions in adult fish as well as in cell growth and organ differentiation in embryo and larva development.

Ghrelin was recently demonstrated as an endogenous ligand of the growth hormone (GH) secretagogue receptor (GHS-R), which could promote the release of GH in mammal significantly. The present study conducted to determine whether ghrelin stimulate the release and synthesis of GH in orange-spotted grouper (Epinephelus coioides). Rat ghrelin was incubated with the pituitary fragments of grouper in static culture system. The culture medium was collected at 1, 6, 12, 18 and 24h after incubation to detect the contents ofGH by homologous radioimmunoassay. The level of GH mRNA in the pituitary fragments was measured by a sensitive chemiluminescent ribonuclease protection assay. The results showed that rat ghrelin not only stimulated the release of GH but also augmented the GH mRNA level in grouper. It suggested that the ghrelin-like peptide and the GHS-R involved in the regulation of GH synthesis and release in grouper. The present study would provide a better understanding of the regulatory mechanism ofGHrelease in marine fish.

Effects of cysteamine hydrochloride (CSH)-a somatostatin-inhibiting agent on growth hormone (GH) secretion from pituitary fragments (PF) or hypothalamus plus pituitary fragments (HPF) under static incubation conditions, serum GH, 3,5,30-triiodothyronine (T3) and thyroxine (T4) levels, and growth in juvenile grass carp (Ctenopharyngodon idellus) were investigated. CSH (0.1, 1, and 10mM) had no influences on GH release from PF after 1 and 6h incubation, but was effective in stimulating GH release from HPF in a dose-dependent manner after 1 and 6h incubation. Moreover, prolonged treatment of HPF with CSH decreased the magnitude of enhancement of GH levels in culture medium. CSH and neuropeptides [e.g., human GH-releasing hormone (hGHRH, 100nM), luteinizing hormone-releasing hormone analog (LHRH-A, [D-Trp6, Pro9]LHRH, 100 nM)], or salmon gonadotropin-releasing hormone analogue (sGnRH-A, [D-Ala6, Pro9]LHRH, 100 nM), alone and in combination during static incubation stimulated GH release from HPF after 1h incubation; in addition, there was an additive, not a synergistic effect of CSH and neuropeptides on stimulation of GH release. Administration of CSH (2.5mg/g diet) in combination with LHRH-A (5lg/g diet) in diet twice daily for 8 weeks resulted in higher serum GH, T3, and T4 levels, ratio of RNA/DNA in muscle, food conversion efficiency, and growth rate than CSH or LHRH-A alone. At trial termination, significant decreases in condition factors and body lipid levels were observed in fish fed with CSH and/or LHRH-A. No significant differences were recorded for viscero-somatic index, hepato-somatic index, and percent body moisture and protein in muscle. These findings, taken as a whole, strongly suggest that the action of CSH stimulating GH release in vitro appears to be mediated through hypothalamic pathways and dietary delivery of CSH directly or indirectly stimulates endogenous GH, T3, and T4 secretion, and subsequently leads to a increase in growth rate in grass carp.

Effects of cysteamine hydrochloride (CSH)-a somatostatin-inhibiting agent-on serum growth hormone (GH) levels and growth in juvenile grass carp (Ctenopharyngodon idellus) were studied. Intraperitoneal (i.p.) injection and single or 10-day feeding of different doses of CSH significantly increased serum GH levels. CSH and luteinizing hormonereleasing hormone analog (LHRH-A, D-Ala6,Pro9-Net-LHRH), alone and in combination i.p. injection, and single or 10-day administration in diet resulted in an enhancement of serum GH contents; in addition, there was an additive, not synergistic effect of CSH and LHRH-A on elevation of serum GH levels. Ten day feeding of CSH, or CSH and LHRHA, alone and in combination caused a significant increase in muscle RNAyDNA ratio. These results provide evidence that CSH significantly increases serum GH levels and promotes short-term growth in juvenile grass carp.