Sweet, umami, and bitter tastants stimulate type II receptor cells to release ATP (Huang et al., 2005, 2009), thereby activating sensory afferents. (jneurosci.org)
Rather, sour tastants evoke release of serotonin (5 - hydroxytryptamine; 5 - HT) directly from type III cells (Huang et al., 2005, 2009). (jneurosci.org)
First author and graduate student Dhruv Zocchi measured the electrical responses from taste nerves in mice to various tastants as well as to water. (sciencedaily.com)