ABSTRACT
Fish, unlike terrestrial vertebrates, detect chemical compounds dissolved in the surrounding water and the entire process of olfaction takes place in water. Of the four main classes of chemical compounds (amino acids, bile acids, prostaglandins, and sex steroids) identified as specific olfactory stimuli (odorants) for fish to date, amino acids are by far the most widely studied chemicals for fish olfaction. The purpose of this study was to re-examine data on electro-olfactogram (EOO) experiments in rainbow trout in an attempt to determine receptor types through which naturally occurring amino acids might be detected and thereby information translated into fish behaviours. Three lines of experimental evidence-concentration-response relationship, crossadaptation, and binary mixture-indicate that at least three receptors for
32 Fish Chemosenses
Olfaction begins with the binding of an odorant molecule to a receptor on the olfactory neuron (or olfactory sensory neuron or receptor neuron) surface, initiating a cascade of enzymatic reactions that results in the production of a second messenger and the eventual depolarization of the neuronal membrane, which leads to triggering of action potential (Shepherd, 1994; Buck, 1996). Olfactory receptors belonging to Q, protein,coupled receptor superfamily encoded by a large multigene family were first identified in rat (Buck and Axel, 1991) and then in other species including fishes (Ngai et al., 1993b; Cao et al., 1998; Freitag et al., 1998; Naito et al., 1998). The size of the receptor repertoire is estimated to be 50, 100 in fishes (Ngai et al., 1993a; Barth et al., 1996; Weth et al., 1996) and as many as 1000 in higher vertebrates (Buck and Axel, 1991; Parmentier et al., 1992; Raming et al., 1993). In the channel catfish (Ictalurus punctatus), each receptor gene expresses in approximately 1% of olfactory neurons (Ngai et al., 1993a), suggesting that each neuron may express only a single receptor gene. To understand how olfactory neurons transduce_ the information represented by the molecular structure of odorants, it is essential to identify definite pairing of receptors with odorants. To date, however, functional evidence that cloned olfactory receptors indeed mediate specific odorants has been obtained only in a few species (Zhao et al., 1998; Speca et al., 1999; Touhara et al., 1999; Gaillard et al., 2002).
