Rat Drg Neurons Nk-1 Receptor Activation In The Modulation Of Ttx Is Not Sensitive Sodium Channels And Trpv1 Receptors

Substance P(SP),a member of tachykinin family,is a well-known pain-related neuropeptide in the spinal cord which is released by unmyelinated primary afferent fiber terminals of small dorsal root ganglion(DRG) neurons and participates in the spinal transmission of nociceptive signals.It is well documented that the SP receptor neurokinin-1(NK-1) is densely distributed in the superficial dorsal horn and involved in the development of chronic pain and central sensitization after intense noxious stimulation and tissue/nerve injury.In addition to the spinal expression of the NK-1,increasing evidence strongly suggests the presynaptic expression of NK-1 in DRG neurons.The immunohistochemical evidence revealed that the NK-1 was expressed in the unmyelinated axons of the glabrous skin,and the DRG neuron soma in rats.By means of intracellular or whole-cell patch clamp recordings,SP was shown to be able to induce the depolarization of DRG or trigeminal ganglion neurons in the different species and potentiate the TRPV1 currents.However,the function of DRG-expressed NK-1 receptor needs to be further understood.In DRG neurons,there are two types of TTX-resistant sodium channels,Na_v1.8 and Na_v1.9.They are mainly expressed in primary nociceptive neurons and play an important role in pain signal processing.TRPV1,a non-selective cation channel,is another pain-related molecular specifically expressed in nociceptors.It is activated by noxious heat,acid or capsaicin and participates in thermal pain sensation and thermal hyperalgesia.In the present study,we investigated the modulation effects of the NK-1 agonist on Na_v1.8,Navl.9 and TRPV1 in isolated small-sized DRG neurons. 1.Na_v1.8NK-1 agonist[Sar⁹,Met(O₂)¹¹]-substance P(Sar-SP) significantly enhanced Na_v1.8 currents in a subgroup(43.3%) of DRG neurons,and the enhancement was blocked by NK-1 antagonist Win51708 and protein kinase C(PKC) inhibitor BIM,but not protein kinase A(PKA) inhibitor H-89.In particular,the inhibitor for PKCε,a PKC isoform,completely blocked this effect.Under current clamp model,Sar-SP reduced the amount of current required to evoke action potentials and increased the firing rate in a subgroup of DRG neurons.2.Na_v1.9Sar-SP significantly enhanced Na_v1.9 currents in 36.0%DRG neurons,and the enhancement was blocked by NK-1 antagonist Win51708 and PKC inhibitor BIM, but not protein PKA inhibitor H-89.Under current clamp model,Sar-SP significantly potentiated the spontaneous discharge of DRG neurons.3.TRPV1In 47.8%(11/23) TRPV1 positive DRG neurons,Sar-SP significantly enhanced capsaicin induced currents and increase in[Ca²⁺]_i.Sar-SP also enhanced heat induced currents and reduced thermal responsive threshold.Intraplantar injection of Sar-SP caused a thermal hyperalgesia,which could be blocked by TRPV1 antagonist capsazepine.These data suggest that activation of NK-1 receptor potentiates Na_v1.8,Na_v1.9 and TRPV1,probably participating in the generation of inflammatory hyperalgesia.