Neuronal expression of bitter taste receptors and downstream signaling molecules in the rat brainstem.

Previous studies have shown that molecules of the taste transduction pathway may serve as biochemical markers for chemoreceptive cells in respiratory and gastrointestinal tracts. In this study, we tested the hypothesis that brainstem neurons contain signaling molecules similar to those in taste buds which may sense the chemical composition of brain extracellular fluids. We used the reverse transcription polymerase chain reaction (RT-PCR), Western blot and immunohistochemical techniques to evaluate presence of different bitter-responsive type 2 taste receptors (T2Rs), their associated G-protein α-gustducin, the downstream signaling molecules phospholipase C isoform β2 (PLC-β2) and transient receptor potential melastatin 5 (TRPM5) in the brainstem of rats. RT-PCR confirmed the mRNA coding for α-gustducin, PLC-β2, TRPM5 and rT2R1 but not that of rT2R16, rT2R26 and rT2R38 in the medulla oblongata. Western blotting confirmed the presence of α-gustducin at the protein level in rat brainstem. Immunohistochemistry identified cells expressing α-gustducin and PLC-β2 at multiple cardiorespiratory and CO(2)/H(+) chemosensory sites, including rostral ventral medulla, facial, parapyramidal, solitary tract, hypoglossal and raphe nuclei. In the medullary raphe, α-gustducin and PLC-β2 were colocalized with a subpopulation of tryptophan hydroxylase (TPH)-immunoreactive serotonergic neurons, a subset of which has respiratory CO(2)/H(+) chemosensitivity. Presence of the T2R1 gene and other genes and proteins of the bitter taste transduction pathway in the brainstem implies additional functions for taste receptors and their effector molecules apart from their gustatory function.