Neuromedin U Type 1 Receptor Stimulation of A-type K+ Current Requires the βγ Subunits of Go Protein, Protein Kinase A, and Extracellular Signal-regulated Kinase 1/2 (ERK1/2) in Sensory Neurons

Although neuromedin U (NMU) has been implicated in analgesia, the detailed mechanisms still remain unclear. In this study, we identify a novel functional role of NMU type 1 receptor (NMUR1) in regulating the transient outward K+ currents (I-A) in small dorsal root ganglion (DRG) neurons. We found that NMU reversibly increased I-A in a dose-dependent manner, instead the sustained delayed rectifier K+ current (I-DR) was not affected. This NMU-induced I-A increase was pertussis toxin-sensitive and was totally reversed by NMUR1 knockdown. Intracellular application of GDP beta S (guanosine 5'-O-(2-thiodiphosphate)), QEHA peptide, or a selective antibody raised against the G alpha(o) or G beta blocked the stimulatory effects of NMU. Pretreatment of the cells with the protein kinase A (PKA) inhibitor or ERK inhibitor abolished the NMU-induced I-A response, whereas inhibition of phosphatidylinositol 3-kinase or PKC had no such effects. Exposure of DRG neurons to NMU markedly induced the phosphorylation of ERK (p-ERK), whereas p-JNK or p-p38 was not affected. Moreover, the NMU-induced p-ERK increase was attenuated by PKA inhibition and activation of PKA by foskolin would mimic the NMU-induced I-A increase. Functionally, we observed a significant decrease of the firing rate of neuronal action potential induced by NMU and pretreatment of DRG neurons with 4-AP could abolish this effect. In summary, these results suggested that NMU increases I-A via activation of NMUR1 that couples sequentially to the down-stream activities of G beta gamma of the G(o) protein, PKA, and ERK, which could contribute to its physiological functions including neuronal hypoexcitability in DRG neurons.