Specific receptor and fibre types of rainbow trout Oncorhynchus mykiss involved in the detection and discrimination of amino acids and a heterogeneous collection of compounds were investigated by recording the electrical activity of the maxillary branch of the facial nerve innervating taste buds inside the upper jaw. Proline (Pro), alanine (Ala), leucine (Leu), betaine (Bet) and 2-amino-3-guanidinopropionic acid (Agp) were the major amino acids detected by the gustatory system. The two experimental approaches, concentration-response curves and crossadaptations, showed that all amino acids were detected by three independent receptor types: Pro-, Agp/Bet-and Leu-receptors. Bile acids, the most potent stimulants recorded, were detected by a single receptor type independent of those for amino acids, with threshold concentrations of 10 À12 M. Strychnine, quinine and tetrodotoxin may have partially shared a single receptor mechanism. The gustatory sensibility narrowly tuned towards the amino acid spectrum compared to those for a diverse array of non-feeding stimulant chemicals, combined with feeding behaviour triggered primarily by vision and olfaction, suggest that in rainbow trout, and possibly other salmonid species, gustatory chemical cues, in addition to food finding and intake, play an important role in detecting poisonous prey and substances.

The developmental change in rat taste sensitivity during the period from the 2nd to the 12th postnatal week was studied by recording the responses of the chorda tympani nerve to lingual stimulation by NaCl, LiCl,KCl,NH4Cl,HCl, quinine hydrochloride(Q-HCl),and sucrose. The threshold for all stimulants except sucrose was lower in the 3-weekold rats than in the 12-week-old ones.The order of the magnitudes of the normalized neural responses to the 0.1 M monochloride salts of the gustatory nerves was NH4Cl>NaCl=LiCl>KCl in the 3-week-old rats, and NaCl=LiCl>NH4Cl>KCl in the 12-week-old rats.The magnitude of the normalized responses to NH4Cl, HC1, and Q-HCl at nearly all concentrations was greater at 3 weeks than at 12 weeks,while the magnitude of the responses to NaCl and LiCl at high concentrations was lesser at 3 weeks than at 12 weeks.Changes in the gustatory sensitivity to various taste stimuli during development can be classified into the following two types:(1)sensitivity-decreasing type the magnitude of the response to NH4Cl,Q-HCl,and HCl was greatest during the first few weeks and decreased thereafter,and(2)sensitivity-increasing type the magnitude of the response to NaCl, LiCl,and sucrose showed an increase accompanying the growth of rats from around the 2nd week after birth.Many psychological and behavioral studies on the development of the gustatory system have been performed on human fetuses (LILEY,1972),newborn babies (MALLER and DESOR,1974;),infants(DESOR et al.,1973,adults and elderly people (BYRD and GERTMAN,1959;COOPER et al.,1959),and rats (TEITELBAUM et al.,1969;JACOBS et al.,1977;OHI,1979). An electrophysiological study on the postnatal development of the gustatory system has also been thought necessary for full understanding of the ontogeny of gustatory function.However,neurophysiological studies on the development of taste receptors have been undertaken only by BRADLEY and MISTRETTA(1973),who

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