Exposureoftastebudstopotassiumpermanganate andformalinsuppressesthegustatoryneuralresponse intheNiletilapia Oreochromis niloticus (Linnaeus) Syed Yahiya Yacoob 1 , Kazuhiko Anraku 1 , Miguel Vazquez Archdale 1 , Tatsuro Matsuoka 1 & Sadao Kiyohara 2 1 Laboratory of Fishing Technology, Faculty of Fisheries, Kagoshima University, Kagoshima, Japan 2 Department of Chemistry and Bioscience, Faculty of Science, Kagoshima University, Kagoshima 890-0065, Japan Correspondence: Syed Yahiya Yacoob, Laboratory of Fishing Technology, Faculty of Fisheries, Kagoshima University, Shimoarata 4-50-20, Kagoshima 890-0056, Japan. E-mail: syahiya@hotmail.com Abstract The effects of exposing the taste receptors to two common aquaculture therapeutants, potassium per- manganate (KMnO 4 ) and formalin, on the gustatory responses in the Nile tilapia Oreochromis niloticus (Linnaeus) were studied using an electrophysiologi- cal approach. Electrical responses to the stimuli before and after perfusing the receptors with the toxicants were compared. Responses to 1.0mM l-methionine (l-met) were significantly reduced when the receptors were perfused with 2.0mgL 1 KMnO 4 or 200.0mgL 1 formalin. KMnO 4 sup- pressed the responses to 56%, whereas formalin reduced them to 83%. Both toxicants reduced the responses to l-arginine to a similar extent to that of l-met, indicating that the action of the toxicants is not specific for the receptor sites for amino acids. On the other hand, the toxicants suppressed the responses to quinine significantly more than those to amino acids, which indicates that the transduc- tion pathways for bitter substances are much more sensitive. The suppressions were reversible, suggest- ing that these toxicants do not cause any permanent damage to the physiological features of the taste receptors, but only interfere with their normal func- tioning. The possible mechanisms of suppression and their consequences on the feeding process are discussed. Keywords: gustatory, electrophysiology, suppres- sion, toxicants, potassium permanganate, formalin Introduction In aquaculture, many potential toxic compounds are used to treat fish, water and soil and subse- quently released into the aquatic environment. All the possible adverse effects that these compounds have on aquatic organisms must be elucidated before they are used for treatment purposes. Of many chemicals available for fish chemotherapy, potassium permanganate (KMnO 4 ) and formalin are the two toxic agents most commonly used to treat ectoparasitic and ectobacterial infections (Lay 1971; Stuart 1983). The negative effects of these toxicants on different aspects of fish physiology have been reported (Williams & Wootten 1981; Ghosh & Konar 1984; Cruz & Pitogo 1989; Sanchez, Speare, Sims & Johnson 1998; Schlenk, Colley, El-Alfy, Kirby & Griffin 2000). However, their effects on the chemical senses of fishes have never been studied. Chemical senses, mainly olfaction and gus- tation, mediate vital behaviours related to feeding and reproduction and are therefore critical for the survival of fish. Despite the physiological importance of chemical senses, the chemical reactivity, struc- tural complexity and exposed location of chemo- sensory membranes render them vulnerable to interaction with environmental toxic agents (Brown, Evans, Thompson & Hara 1982). The deleterious effects of some environmental toxicants, such as heavy metals, hydrocarbons and detergents, on fish chemoreceptors have already been proved (reviewed by Klaprat, Evans & Hara 1992). These principally include degeneration of Aquaculture Research, 2002, 33, 445±453 ß 2002 Blackwell Science Ltd 445