Environmental pollutants are hypothesized to be one of the causes of recent declines in wild populations of Atlantic salmon (Salmo salar) across Eastern Canada and the United States. Some of these pollutants, such as polychlorinated biphenyls and dioxins, are known to induce expression of the CYP1A subfamily of genes. We applied a highly sensitive technique, quantitative reverse transcription-polymerase chain reaction (RT-PCR), for measuring the levels of CYP1A induction in Atlantic salmon. This assay was used to detect patterns of CYP1A mRNA levels, a direct measure of CYP1A expression, in Atlantic salmon exposed to pollutants under both laboratory and field conditions. Two groups of salmon were acclimated to 11 and 17 °C, respectively. Each subject then received an intraperitoneal injection (50 mg kg-1) of either b-naphthoflavone (BNF) in corn oil (10 mg BNF ml-1 corn oil) or corn oil alone. After 48 h, salmon gill, kidney, liver, and brain were collected for RNA isolation and analysis. All tissues showed induction of CYP1A by BNF. The highest base level of CYP1A expression (2.56 × 1010 molecules/μg RNA) was found in gill tissue. Kidney had the highest mean induction at five orders of magnitude while gill tissue showed the lowest mean induction at two orders of magnitude. The quantitative RT-PCR was also applied to salmon sampled from two streams in Massachusetts, USA. Salmon liver and gill tissue sampled from Millers River (South Royalston, Worcester County), known to contain polychlorinated biphenyls (PCBs), showed on average a two orders of magnitude induction over those collected from a stream with no known contamination (Fourmile Brook, Northfield, Franklin County). Overall, the data show CYP1A exists and is inducible in Atlantic salmon gill, brain, kidney, and liver tissue. In addition, the results obtained demonstrate that quantitative PCR analysis of CYP1A expression is useful in studying ecotoxicity in populations of Atlantic salmon in the wild.

Introduction

Atlantic salmon populations across Eastern Canada and the United States have suffered a steady decline for the past 30 years (Anderson et al., 2000). This decline has resulted in the listing of Atlantic salmon as an endangered species in the state of Maine as of November, 2000 (US Depart- ment of Interior, 2000). In some cases, this decline has been attributed to the sublethal effects of pesticides. Fairchild et al. (1999) suggested that endocrine disrupting chemicals have caused de- clines in salmon populations by altering or inhibit- ing the parr-smolt transformation. Other compounds such as polyaromatic hydrocarbons, polychlorinated biphenyls, dioxins, and furans can also have physiological and pathological effects on fish populations at sublethal concentrations (Gok- søyr and Husøy, 1998). These compounds are known to stimulate expression of various members of the cytochrome P450 family of genes, particu- larly those of the CYP1 family (Hahn and Stege- man, 1994; Hahn et al., 1998; Stegeman et al., 2001).

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