Abstract
This study provides direct evidence for a robust effect of saliva proteins on ingestive responses to tannic acid. Proline-rich proteins (PRPs) were elevated in the saliva of mice, via chronic treatments with the β-adrenergic agonist isoproterenol (IPR) and the effects of this manipulation on intake of tannic acid were examined. Because salivary PRPs from rodents bind readily to tannic acid, it was hypothesized that elevated salivary PRPs would lower the free concentration of ingested tannic acid. In experiment 1, the ingestive sensitivity of IPR- and saline-injected mice of four strains (SW, BALB, C3H and B6) to 0.5 mM tannic acid was compared. IPR treatment significantly reduced the tannic acid sensitivity of the BALBs, but not the SWs, C3Hs or B6s, as measured by a two-choice test. Furthermore, whole-mouth saliva of mice from the four strains was compared in terms of (i) flow rate, (ii) relative PRP concentration and (iii) tannin binding capacity. As compared to the other mouse strains, the saliva of IPR-injected BALBs appeared to contain PRPs that had a higher tannin binding capacity, and that occurred at higher concentrations, with the exception of the C3Hs. Salivary flow rate did not differ among mouse strains. In experiment 2, the effect of IPR treatment on ingestive responses of BALBs to two concentrations of tannic acid (0.5 and 1.0 mM) was examined using a lickometer device. Intake measures (lick rate, burst duration, number of bursts and overall lick rate) indicated that the IPR-injected BALBs drank the 0.5 mM tannic acid solution as if it was water. Saline-injected BALBs rejected the 0.5 mM tannic acid solution almost immediately. Whereas both the IPR- and saline-injected BALBs rejected the 1.0 mM tannic acid solution, the latter group rejected it more strongly. These results suggest that salivary PRPs in the IPR-treated BALBs bound to a significant portion of the ingested tannic acid. In so doing, the PRPs dminished the free concentration and, hence, aversive taste quality of the tannic acid.

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