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The toxicity of pulp and paper mill effluents (PPMEs) has been greatly decreased, yet some continue to negatively affect fish reproduction. We hypothesized that PPMEs are affecting the brain resulting in decreased reproductive performance. Our goal was to use gene expression profiling to test the hypothesis that PPMEs are having an effect on neural systems in the fathead minnow (FHM; Pimephales promelas) in vivo. Sexually mature male and female FHM were exposed to 100% final biotreated PPMEs from 5 different sources for 5 days. Using an oligo-array (15K genes) we examined the effect of PPMEs on gene expression in the hypothalamus of female fish. We validated selected genes (cholecystokinin, RevErbbeta2, and urotensin I) that were identified by microarray analysis using real-time reverse-transcriptase polymerase chain reaction (RT-PCR). We compared the FHM microarray dataset to multiple microarray datasets from experiments conducted with goldfish injected with different dopaminergic pharmaceuticals to examine whether PPMEs could be affecting the dopamine system. Exposure of FHM to PPMEs resulted in varying degrees of spawning inhibition. Microarray analysis revealed surprisingly few genes in the brain that were commonly affected by the different PPMEs. Real-time RT-PCR confirmed the changes in expression for cholecystokinin, RevErbbeta2, and urotensin I. Comparison of the FHM and goldfish microarray datasets suggest that some PPMEs may be acting on the dopamine system. We show that PPMEs are neuroactive in fish and may be acting through some of the pathways in a manner similar to dopamine., (Copyright 2010 Elsevier B.V. All rights reserved.)