In recent years there has been a surge of interest in identifying membrane receptors responsible for detecting the taste of foods and other ingestible substances. Knowledge about these receptors may be very important for dietary regulation of people in general and may be especially important for people with health issues that lead to dietary challenges, e.g., diabetics, cardiovascular disease, elderly, mentally challenged.
My research is focused on taste transduction (receptor mechanisms), afferent signaling (CNS neuronal coding), and perception (behavioral) of sweet, umami and amino acid substances. Currently I have NIH and NSF support for several ongoing behavioral and molecular studies addressing questions about L-amino acid and umami tastes. Some of these studies are examining the similarities and differences in taste qualities of monosodium glutamate (MSG) and other L-amino acids. These studies will provide new insights into whether one, two, or maybe more receptors are activated by umami stimuli and which receptors are responsible for umami taste perception.
To facilitate this research I am beginning behavioral assays of T1R3 transgenic mice provided by Dr. Bob Margolskee at Mt. Sinai School of Medicine. By evaluating the perceptual capacity of these transgenic mice for a variety of taste stimuli, these experiments should help in assessing the potential contributions of each receptor to taste perception. Because the taste of MSG is complex, other dimensions of glutamate taste perception need to be explored as well. For example, one of the defining characteristics of umami taste is a synergistic interaction between 5' ribonucleotide monophosphates and umami substances, but curiously, little is know about the taste qualities of these monophosphates.
I am presently studying the taste qualities of the two most important monophosphates with rats to determine whether these nucleotides activate the same receptor(s) as other umami substances, or if there are separate receptors for these substances. Another intriguing (and one of the more puzzling) characteristic of MSG taste is its apparent "sweet" quality. At first glance, one might assume that MSG and sucrose elicit quite different tastes, especially if each substance represents a different primary taste. However, my work and the research of others have found that if the taste of sodium is minimized, rats perceive the taste of MSG as quite similar to the taste of sucrose, suggesting that these substances share taste qualities. How this might happen is still a mystery.
It is possible that there may be some overlap in the taste receptors that MSG and sucrose activate, but given the current evidence in the research literature, it seems more likely that there may be convergence of afferent taste signals at some point in the nervous system. Converging signals may result to some kind of interaction between the neurological mechanisms responsible for umami and sweet tastes. I am conducting a number of experiments to study this interaction.
In addition to behavioral studies, I am combining immunocytochemical and in situ methods to "double-label" neurons in the brain stem that have responded to MSG and/or to sucrose presented to the animal. If the taste signals for MSG and sucrose are conducted along different fibers in the nervous system, then we anticipate that there will be few double-label cells (cells that responded to both stimuli) but if there is some type of convergence of these signals, then we expect to see more double labeled cells. We will be applying these methods to our studies of amino acid taste in the near future.