Types of fish assays used most often include:
1. Rapid developmental toxicity assays (utilizing fathead minnow, medaka, and zebrafish) are now available. As a complement to the targeted in vitro assays included in the High Throughput and Whole Cell Activity screens, in vivo rapid developmental toxicity screens provide a quick and inexpensive method to detect adverse interactions between test chemicals and a vertebrate whole animal system.
The primary advantage of assays using lower vertebrates is that the embryos develop rapidly and exercise their complete repertoire of gene expression and molecular signaling during the short transition from fertilization to organogenesis. During this window of development, there is a high probability of detecting an adverse interaction between an EDC and its molecular target that manifests as developmental delays or discrete morphological abnormalities including pericardial and yolk sac edemas, curved body axis, and eye, jaw, craniofacial, fin, and/or pigmentation defects[33-38]. The developing embryo can also be monitored for a series of cardiovascular[39] and behavioral [40] endpoints.
Factors for Consideration in Fish EDC Studies
2. Reproduction assays (using medaka, or fathead minnow). Reproduction tests are well established using this model system, and have been used to assess a number of chemicals suspected of having endocrine activity. Partial life-cycle assays employ short-term exposure during critical windows of sensitivity (i.e. sexual differentiation, gonadal development, active reproduction), whereas full life-cycle assays initiate chronic exposure with newly fertilized eggs.
In adult fish, active reproduction represents a period of sensitivity to chemicals that target the hypothalamo-pituitary-gonadal (HPG) axis. Assays designed to exploit this window of susceptibility assess apical (whole organism) endpoints following short-term (typically 21 days) exposure to a chemical.
Several transgenic zebrafish lines have been engineered to detect direct transcriptional activation of specific endocrine signaling pathways in “reporter gene assays”. These assays specifically and rapidly detect aspects of endocrine disruption. Researchers are limited, however, by the types of reporter line available and should understand that a variety of disruptions to the system may be missed because transcriptional reporter-based models are not capable of detecting non-genomic signaling.[44, 45]