The Effects Of Monocrotophos On The Expression Of GH,GHR1 And IGF-I Of Early Developmental Stages Of Zebrafish

Monocrotophos (MCP) is one of the organophosphorous pesticides with high water solubility and shows potential impact on human and aquatic organism. It has been proved that exposure to MCP can cause a descending of the egg quality and an increase of the embryo malformation, however, the exact mechanism explaining the effects of MCP on growth and development of fish remains unknown. The growth of fish is under the control of the GH/IGF-I axis, meanwhile, growth hormone (GH), growth hormone receptor (GHR) and Insulin-like growth factor-I (IGF-I) play important roles in this regulation. GH has a wide range of physiological effects, including growth promotion, energy mobilization, reproduction, appetite and osmoregulation, by interacting with GHR on target tissues to stimulate the synthesis and secretion of IGF-I. The GH/IGF-I axis is affected by multiple factors: the secretion and synthesis of GH is regulated by neuroendocrine factors from brain and peripheral gland; sex steroid hormones can regulate the synthesis and secretion of GH by affecting the expression and functions of factors from hypothalamus and pituitary gland. Zebrafish is a model vertebrate species in developmental biology researches since it has the characteristics such as short life cycle, available reproductive performance, easily rearing, etc. All these make it a widely used organism in researches on effects of endocrine disrupting chemicals. In order to confirm the effects of MCP on GH/IGF-I axis and explore the potential mechanisms, zebrafish were exposured to 0.001, 0.010 and 0.100 mg/L MCP during the early developmental stages, the expression patterns of GH, GHR and IGF-I were examined by semi-quantitative PCR and real-time PCR.The results showed that:(1) After a 21 dph exposure to graded concentrations of MCP (0.001, 0.010, 0.100mg/L), GH expression of all exposure groups were stimulated, with a significant increase in the 0.010mg/L dose group (P<0.05). At the same time, GHR1 transcription were reduced in all the MCP-exposure groups, and remarkable decreases were observed in both the 0.001 and 0.010mg/L groups (P<0.05, respectively). The expressions of IGF-I were inhibited by MCP at some extent, and the 0.001,0.010mg/L MCP-exposure groups showed significant reduction (P<0.05).(2) After a 30 dph exposure to graded concentrations of MCP (0.001, 0.010, 0.100mg/L), the expression of GH were raised gradually with the increase of MCP concentrations, and the GH expression of 0.010 and 0.100mg/L groups were increased remarkably (P<0.05 and P<0.01, respectively). However, treating with MCP had no obvious effects on the expression of GHR1 except for a significant increase in 0.001mg/L MCP group (P<0.05). At the same time After exposed to MCP for 30 dph, the expression of IGF-I in all treatments were inhibited somewhat, with a significant decrease by exposure to MCP of 0.010mg/L concentration (P<0.05).(3) After a 42 dph exposure to graded concentrations of MCP (0.001, 0.010, 0.100mg/L), the GH expressions were decreased in a dose-dependent manner, with a significant decrease in the 0.100mg/L MCP-exposure group (P<0.01). However, the MCP had no obvious effects on the expressions of GHR1. The IGF-I transcriptions were significantly reduced by exposure to MCP of 0.001 , 0.10, 1.00mg/L concentrations (P<0.01).In conclusion, the effects of MCP on expressions of GH, GHR1 and IGF-I were variable during different developmental stages of zebrafish. MCP stimulated the expressions of GH during the period of sex differentiation, but it inhibited the GH expressions when the process completed. The effects of MCP on GHR1 expressions were various in different developmental stages, showing no dose-effect relationship. Meanwhile, the expressions of IGF-I were reduced throughout the whole exposure periods, with the most remarkable decrease on 42dph. It is concluded that although MCP affected the expressions of GH and GHR1, the effects of MCP on growth of fish were not mediated by GH or GHR1 but IGF-I. We also confirmed that the effects of MCP on GH/IGF-I axis were probably related to the disturbing effects of MCP on sex steroids levels.