Neuromedin U Inhibits T-type Ca²⁺ Channel Currents And Decreases Membrane Excitability In Small Dorsal Root Ganglia Neurons In Mice

Objective: To investigate the mechanisms of neuromedin U on T-type calcium channel currents in small dorsal root ganglia neurons in mice.Methods: Molecular Biology methods was used to identificate the type of neuromedin U receptors of the cultural dorsal root ganglia neurons .The whole-cell patch clamp recording was used to investigate the effect of neuromedin U on T-type Ca2+ currents (T-currents) in small DRG neurons and pharmacological methods was used to clarify its signaling pathways.Results: In the present study, we investigated the expression profile of NMU receptors in mouse dorsal root ganglia (DRG) and identified a novel functional role of NMU in modulating T-type Ca2+ channel currents (T-currents) as well as membrane excitability in small DRG neurons. We found that NMU inhibited T-currents in a dose-dependent manner in mouse small DRG neurons that endogenously expressed NMU type 1 (NMUR1), but not NMUR2 receptors. NMU (1μM) reversibly inhibited T-currents by ²7.4%. This inhibitory effect was blocked by GDP-β-S or pertussis toxin (PTX), indicating the involvement of a Gi/oα-protein. Using depolarizing prepulse or intracellular application of QEHA, a synthetic peptide which competitively blocks G-proteinβγsubunit (Gβγ) mediated signaling, we found the absence of functional coupling between Gβγand T-type Ca2+ channels. Pretreatment of the cells with H89, a protein kinase A (PKA), or intracellular application of PKI 5-24, blocked NMU-induced T-current inhibition, whereas inhibition of phospholipase C(U73122)or protein kinase C(GF109203X or Chelerythrine chloride) elicited no such effects. In addition, we observed a significant decreased firing frequency of action potentials of small DRG neurons induced by NMU, which could be abrogated by pretreatment of the cells with NiCl₂ (100μM).NMU (1μM) reversibly inhibited firing frequency of action potentials by 51.56±6.53%.Conclusions: NMU inhibits T-currents via PTX-sensitive PKA pathway, which might contribute to its physiological functions including neuronal hypoexcitability in small DRG neurons in mice.