romp. Biochem. Physiol. Vol. 1oOC. NO.l/2. PP. X3-257, 1991 Printed in Great Britain 0306~4492/91 $3.00 + 0.00 0 1991 Pergamon Press plc zyxwvuts MINI-REVIEW STRUCTURAL AND FUNCTIONAL EFFECTS OF HEAVY METALS ON THE NERVOUS SYSTEM, INCLUDING SENSE ORGANS, OF FISH ERIK BAATRUP Institute of Zoology and Zoophysiology, University of Aarhus, Bygn. 140,DK-8000 Aarhus C., Denmark (Received 1 October 1990) Abstract-l. Today, fish in the environment are inevitably exposed to chemical pollution. Although most hazardous substances are present at concentrations far below the lethal level, they may still cause serious damage to the life processes of these animals. 2. Fish depend on an intact nervous system, including their sense organs, for mediating relevant behaviour such as food search, predator recognition, communication and orientation. 3. Unfortunately, the nervous system is most vulnerable and injuries to its elements may dramatically change the behaviour and consequently the survival of fish. 4. Heavy metals are well known pollutants in the aquatic environment. Their interaction with relevant chemical stimuli may interfere with the communication between fish and environment. 5. The affinity for a number of ligands and macromolecules makes heavy metals most potent neurotoxins. 6. The present Mini-Review highlights some aspects of how trace concentrations of mercury, copper and lead affect the integrity of the fish nervous system; structurally, physiologically and biochemically. INTRODUCTION Increasing discharge of wastes from industrial and agricultural activities has dramatically altered the conditions of aquatic life. The resultant dissolved toxicants act directly and continuously on aquatic animals, including fish. This means that these animals are exposed to chemical pollution, and even low sublethal doses of hazardous substances may cause serious damage to their life processes. Fish depend on their external and internal senses for mediating behaviour such as food search, predator recognition, communication orientation and migration. Stimuli are perceived by specialized sensory structures and converted into electrical signals, which are conducted to the central nervous system. Here, neural infor- mation is integrated and appropriate behavioural responses are generated. Unfortunately, the nervous system is one of the most vulnerable parts of the animal body, and injuries to its elements may strongly influence the behaviour (Smith, 1984) and survival of the organism. In fish, both acute and chronic injuries to the nervous system can occur. Dissolved toxicants act directly on superficial sense organs, including olfactory and taste organs, which are not protected by “external barriers” or internal detoxifying systems (Sutterlin, 1974). Pollutants may disrupt normal chemosensory function by masking or counteracting biologically relevant chemical signals, or they may cause direct morphological and physio- logical damage to the receptors. In addition, xeno- biotics absorbed across the skin or the gills may enter the blood stream and thereby reach internal sense organs and other nervous tissues, including the brain. Heavy metals are well-known pollutants in the aquatic environment (Fijrstner and Wittmann, 1979; Bryan, 1984). Their tendency to accumulate in fish tissues (Bryan, 1984) and their strong affinity and interaction with a number of ligands and macro- molecules (Vallee and Ulmer, 1972; Clarkson, 1986) make heavy metals most potent neurotoxins. Accord- ingly, heavy metals occupy a central position in fish neurotoxicology (reviewed by Sutterlin, 1974; Hara Ed al., 1983). The present Mini-Review will focus on the various effects of mercury, copper, and lead upon the nervous system of fish, including some of their special sense organs. It should be emphasized that the limited space rules out an exhaustive interpretation of the subject. Only a few examples will be used by way of illustration. Since the toxic effects of metals on the nervous system have been studied much more exten- sively in mammals (Chang, 1985; Lai et al., 1985; Sandstead, 1986), results from mammalian species are included, where it is essential. MERCURY Mercury can exist in three oxidation states in natural waters: Hg’, Hg’+, and Hz+. The distribution of these forms depends upon the pH, redox potential and availability of anions which form stable com- plexes with mercury. Inorganic mercury occurs, in addition to metallic mercury, in compounds such as mercurous mercury (Hg,Cl,) or mercuric mercury (e.g. HgCl,). In the environment, inorganic mercury may be transformed into organic mercury com- pounds. Most important is the microbial methylation 253