Variation in bitter taste perception may play a key role in diet selection, which has an uncontestable effect on human health. Although bitter compounds are often associated with toxic substances a large number of nutritionally significant food sources contain bitter phytochemicals (e.g., broccoli , spinach ) many of which can have beneficial value [1, 3] Understanding how bitter taste is perceived and detected is of great importance to understanding diet selection  and increasing the acceptability of pediatric medicines . Under normal feeding and drinking conditions taste compounds must inescapably mix with saliva before reaching their receptor targets but very little work has been done examining how salivary proteins modulate taste stimuli and alter bitter taste perception. Seventy percent of all salivary proteins make up a class of proteins referred to as proline-rich proteins (PRPs). PRP production is induced in rats by dietary exposure to tannic acid, a class of plant compounds that animals, including humans, regularly consume. It has been hypothesized that PRPs alter the acceptability of tannic acid by binding to tannins and thereby reduce the perceived intensity of the tannic acid solution [10, 11]. There is evidence to suggest that these proteins may have a similar relationship with other bitters. They are produced in close proximity to bitter taste receptors . Genes for PRPs and bitter taste receptors are interspersed on the same chromosome  and lastly, gene linkage studies have implicated a role for PRPs bitter acceptance in humans and mice [15-18].
Specific Aim 1 will establish if long term exposure to bitter compounds can increase the expression of PRPs and establish whether oral exposure is necessary and sufficient to cause induction. Saliva will be analyzed for PRP content before and after exposure to diets adulterated with bitter compounds, including quinine and tannic acid. To isolate the site of exposure necessary for PRP induction rats will also be given tannic acid via either oral or gastric exposure. The ability or inability to induce PRP production is not sufficient to predict interactios between the proteins and the bitter tastants. Therefore, Specific Aim 2 will establish if PRP induction can alter the detection threshold for and superthreshold responsiveness to multiple bitter stimuli. Rats will be used to derive psychophysical detection curves of several bitter tastants in the presence and absence of PRPs. Likewise, responsiveness in the suprathreshold range will be assessed in the presence and absence of PRPs with a series of brief-access tests. Collectively, these experiments will test the hypothesis that PRPs alter taste sensitivity for bittr taste stimuli and ultimately blunt unconditioned taste avoidance responses to these compounds. Together these aims could inform methods for increasing the palatability of healthy phytochemicals and pediatric medicines.
Understanding how bitter taste is perceived and detected is of great importance to understanding diet selection, which has an overwhelming effect on human health. A large number of nutritionally significant food sources contain bitter phytochemicals many of which can have beneficial value and many pediatric medicines are rejected due to the bitter side taste. Understanding variation in taste perception may assist in efforts to promote healthy eating in children and adults.
|Torregrossa, Ann-Marie; Nikonova, Larissa; Bales, Michelle B et al. (2014) Induction of salivary proteins modifies measures of both orosensory and postingestive feedback during exposure to a tannic acid diet. PLoS One 9:e105232|