DISTRIBUTION PATTERNS OF PALATAL TASTE-BUDS AND THEIR RESPONSES TO AMINO-ACIDS IN SALMONIDS

The distribution pattern of the palatal taste buds and their electrica l response characteristics to amino acids were studied in five salmoni d species: Arctic grayling (Thymallus arcticus), lake whitefish (Coreg onus clupeaformis), Atlantic salmon (Salmo salar), brown trout (Salmo trutta), and kokanee (Oncorhynchus nerka). In these fish the most dens e taste bud populations were found on ridges in the area around the pa latine teeth. With the exception of lake whitefish, the ridges adjacen t to the teeth on the head and shaft of the prevomer were also dense i n taste buds, which appear to be largely responsible for aminco acid d etection. All species, except for lake whitefish, responded well to L- proline (L-Pro) and L-alpha-amino-beta-guanidinopropionic acid (L-AGPA ), with thresholds at 10(-8)-10(-7) and 10(-5)-10(-4)M, respectively. These species also responded to varying degrees to hydroxy-L-proline ( L-Hpr) and L-alanine (L-Ala), believed to be agonists for L-Pro recept ors. In addition, kokanee detected L-phenylalanine, L-leucine and beta ine. Of six lake whitefish examined, only one specimen responded to L- Pro, L-AGPA and L-arginine. We concluded that in these five salmonid s pecies L-Pro and L-AGPA receptors play a major role in amino acid gust ation, and further proposed a hypothesis that salmonids may have initi ally evolved these two gustatory receptors. With phylogenetic advancem ent, they gained greater response capabilities by (1) acquiring new re ceptor types, and (2) by losing the specificity of existing receptors.