The role of metabotropic glutamate receptors in gastrointestinal chemosensation

In the oral cavity, L-glutamate confers umami taste via transduction pathways of metabotropic glutamate and T1R receptors in taste cells. This taste quality induces cephalic phase responses, reflex-like consummatory behavior and conveys nutrient information to the brain. Recent work has shown that chemosensation is not restricted to taste tissue. The gastrointestinal (GI) tract also responds to chemical signals in the lumen. We reported that L-glutamate in the stomach lumen activates gastric afferent fibers of the vagus nerve. And this response is specific to L-glutamate. Other amino acids do not stimulate vagal afferent fibers from the gastric branch. However, the precise molecular processes for this activation are still unknown. Taste related proteins such as the a-subunit of gustducin, the transient receptor potential TRPM5 or T2R bitter taste receptors have been already described in the GI tract. Likewise, several umami receptors have the potential to mediate this stimulation of the vagus nerve. Yet, among them, mGluR1 is the only excitatory glutamate receptor with a high specificity for L-glutamate in the rat. We investigated in the present study the expression and distribution of mGluR1 in the gastric mucosa, and tested the effect of L-glutamate in stomach function. [J Physiol Sci. 2007;57 Suppl:S111]