Gustatory responses of the rainbow trout (Salmo gairdneri) palate to amino acids and derivatives

Summary

• 1. Gustatory responses to amino acids and derivatives obtained from the palatine nerve (VIIth cranial nerve) of rainbow trout (Salmogairdneri) were studied.

• 2. The response to an amino acid was characterized by its fast-adapting, phasic nature that returned to baseline within 5 s during continuous stimulation.

• 3. Complete recovery of the response took place 120 s after stimulation when tested with two identical stimuli given successively (Figs. 2, 3).

• 4. Among common amino acids tested, onlyL-isomers of proline, hydroxyproline, alanine, leucine, and phenylalanine were stimulatory (Table 1), demonstrating that the facial taste system is more narrowly-tuned to amino acid stimuli than the olfactory system. The arginine derivative,L-α-amino-β-guanidinopropionic acid (L-AGPA) andL-argininic acid, and betaine were also effective.

• 5. Concentration-response relations, when plotted semi-logarithmically, were generally sigmoidal, saturating at higher concentrations (>1 mM) (Fig. 5). The threshold concentration forL-proline, the most effective amino acid tested, was estimated to be 50 nM. The thresholds for other stimulatory chemicals ranged between 1 and 100 μM.

• 6. L-AGPA, though having higher threshold (10 μM), could induce responses three times the magnitude of that ofL-proline at 1 mM.

• 7. The stimulatory activity of heterocyclic-imino acids having 4-, 5-, and 6-membered rings and also of thioproline and hydroxyproline suggests that the receptor recognizes primarily the imino acid region (Fig. 6).

• 8. Taste responses to amino acids were independent of pH, except that arginine analogues were only active at basic pHs (Fig. 7 and Table 3). larginine was active only at pHs higher than 8.5. The palatal chemoreceptors were stimulated by waters with pHs below 7.0, suggesting the existence of receptors for pH and/or CO₂ (Fig. 7).