Immunocytochemical and Histological Differences in the Interrenal Axis of Feral Brown Trout, Salmo trutta, in Metal-Contaminated Waters David O. Norris, 1 Sarah B. Felt, John D. Woodling,* and Robert M. Dores² Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, Colorado 80309-0334; * Colorado Division of Wildlife, 6060 Broadway, Denver, Colorado 80216; and † Department of Biological Science, University of Denver, University Park, Denver, Colorado 80208-0178 Accepted August 18, 1997 There are more CRH-like immunoreactive neurons in the preoptic nucleus and nucleus lateralis tuberis in the brain of feral brown trout, Salmo trutta, living in cad- mium- and zinc-contaminated regions of the Eagle River than in fish from an uncontaminated control site. Histo- logical analyses revealed that interrenal cells are more stimulated (exhibiting both hypertrophy and hyperpla- sia) in fish living in contaminated sites than interrenal cells of fish at the control site. These results suggest that the hypothalamo-pituitary-interrenal (HPI) axis of fish living in the metal-contaminated water shows evidence of chronic stimulation. We suggest that assessment of these parameters of the HPI axis may be useful indices of chronic environmental stress in trout.  1997 Academic Press Numerous studies have examined the toxicity of heavy metals such as Cd, Zn, and many others, on freshwater fish, under both field and controlled labora- tory conditions (e.g., Benoit et al., 1976; McKim et al., 1970). Short-term laboratory exposure to low, nonle- thal levels of metals appears to have a protective effect on fish causing them to exhibit higher tolerance for acute toxic levels (Anadu et al., 1989; Hobson and Birge, 1989; Kito et al., 1982; Pascoe and Beattie, 1979; Sinley et al., 1974). It appears that fish acclimate by developing a detoxification mechanism to inactivate and/or excrete the toxic metals. Thus, low levels of exposure have a protective effect and make fish more tolerant to high levels of contamination. However, field studies of tolerance of brown trout (Salmo trutta) in two Colorado rivers chronically exposed to environ- mental contamination with Cd and Zn indicated that brown trout living in metal-contaminated water were more susceptible to a toxic dose of these metals than those living in areas of the same rivers with no or low metal contamination (Woodling, 1997). These results suggest that the adaptive mechanisms of the fish residing in metal-contaminated water were already operating at full or possibly diminished capacity and could not compensate for additional stress. The stress response system in fishes is mediated by the hypothalamo-pituitary-interrenal (HPI) axis which is homologous to the hypothalamo-pituitary-adreno- cortical (HPA) axis of mammals (Norris, 1997). Chronic stress or treatment with corticotropin (ACTH) causes hyperplasia and hypertrophy of interrenal tissue in teleosts (Ball and Olivereau, 1965; Barton and Iwama, 1 To whom correspondence should be addressed at Dept. E.P.O. Biology, University of Colorado, Boulder, CO 80309-0334. Fax: (303) 492-8699. E-mail: norrisd@spot.colorado.edu. General and Comparative Endocrinology 108, 343–351 (1997) Article No. GC977000 343 0016-6480/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.