Of the handful of neurotransmitters modulating taste response, norepinephrine (NE) may be the least understood. Although the physiological role of NE has been tested, it is still unclear whether the rat taste bud may serve as an endogenous source of neurotransmitter or if it must be taken up from outside the taste bud. Immunocytochemical analysis of DBH has shown that DBH-LIR is present in taste cells of the rat circumvallate papillae. Specifically, DBH-LIR is present in a subset of Type II taste cells. Nearly all (98%) DBH-LIR cells express PLC 2-LIR, and about half (41%) of PLC 2-LIR cells express DBH-LIR. DBH-LIR does not co-localize with NCAM, and thus, is not present in Type III cells. NET-LIR is present in the taste cells of the rat circumvallate papillae. Double labeling of NET-LIR and IP3R3-LIR indicates that NET is present in Type II taste cells. A large proportion (77%) of NET-LIR cells express IP3R3-LIR and all (100%) IP3R3-LIR cells express NET-LIR. A small proportion (7.8%) of NET-LIR cells also express syntaxin-1-LIR in Type III cells. About a third (32.8%) of syntaxin-1-LIR cells express NET-LIR."