Functional bitter taste signaling and stimulus intensity influence diet-induced salivary protein upregulation.

Exposure to tannic acid- and quinine-containing diets upregulates subsets of salivary proteins (SPs), which in turn, results in increased acceptance of the "bitter" diet. However, it is unclear what aspects of the diet consumption influence SP upregulation. It has been thought that taste receptor activation is necessary for upregulation of SPs since sensory stimulation precedes changes in SPs, but no studies have confirmed the contribution of taste to SP upregulation. To test this hypothesis, we used TRPM5 KO and TRPM4/5 DKO mice that have impaired bitter taste signaling. These mice are able to upregulate SPs in response to isoproterenol injections but not when fed a tannic acid diet, which is both bitter and astringent. WT mice upregulate SPs after a tannic acid diet exposure while KO models do not. These data suggest that the mice are able to alter their SP expression but taste signaling is needed to upregulate SPs to diet treatments. The astringency alone from the tannic acid was not able to upregulate SPs. We then asked whether SP upregulation varies with stimulus intensity. Rats were fed quinine or sucrose octaacetate (SOA) at three different concentrations. All rats decrease intake on first day of bitter diet exposure at all three concentrations but increase acceptance of the diet by the fifth day of the quinine. In response to quinine diet, animals show concentration-dependent SP upregulation at the 14 kDa band, other bands were upregulated by diet but were not concentration-dependent. SOA did not alter protein expression at any concentration.