Three kinds of current in response to substance P in bullfrog DRG neurons

Chloride - The Underrated Ion in Nociceptors

In contrast to pain processing neurons in the spinal cord, where the importance of chloride conductances is already well established, chloride homeostasis in primary afferent neurons has received less attention. Sensory neurons maintain high intracellular chloride concentrations through balanced activity of Na⁺-K⁺-2Cl^– cotransporter 1 (NKCC1) and K⁺-Cl^– cotransporter 2 (KCC2). Whereas in other cell types activation of chloride conductances causes hyperpolarization, activation of the same conductances in primary afferent neurons may lead to inhibitory or excitatory depolarization depending on the actual chloride reversal potential and the total amount of chloride efflux during channel or transporter activation. Dorsal root ganglion (DRG) neurons express a multitude of chloride channel types belonging to different channel families, such as ligand-gated, ionotropic γ-aminobutyric acid (GABA) or glycine receptors, Ca²⁺-activated chloride channels of the anoctamin/TMEM16, bestrophin or tweety-homolog family, CLC chloride channels and transporters, cystic fibrosis transmembrane conductance regulator (CFTR) as well as volume-regulated anion channels (VRACs). Specific chloride conductances are involved in signal transduction and amplification at the peripheral nerve terminal, contribute to excitability and action potential generation of sensory neurons, or crucially shape synaptic transmission in the spinal dorsal horn. In addition, chloride channels can be modified by a plethora of inflammatory mediators affecting them directly, via protein-protein interaction, or through signaling cascades. Since chloride channels as well as mediators that modulate chloride fluxes are regulated in pain disorders and contribute to nociceptor excitation and sensitization it is timely and important to emphasize their critical role in nociceptive primary afferents in this review.

Central mechanisms I: plasticity of central pathways

Cough is the most common symptom for which individuals seek medical attention and spend health-care dollars. Despite the burden induced by cough, the current treatments for cough are only partially effective. Delineating the sites and mechanisms in the cough central network for changes in the cough reflex could lead to new therapeutic strategies and drug target sites for more effective treatments. The first synaptic target in the CNS for the cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS); these neurons reorganize the primary sensory information into a coherent output. The NTS neurons have been shown to undergo neuroplasticity under a variety of conditions, such as respiratory disorders, stress, and exposures to environmental pollutants. The NTS contains a rich innervation of substance P immunoreactive nerve terminals, suggesting that substance P might be important in altered cough reflex response. This chapter summarizes our current findings on the role of substance P in enhanced cough reflex as well as the potential NTS targets for the action of substance P.

Sustained depolarization-induced propagation of [Ca2+]i oscillations in cultured DRG neurons: the involvement of extracellular ATP and P2Y receptor activation

Recently emerging evidence implicates a number of neuroactive substances and their receptors in mediating complex cell-to-cell communications in the ganglia. In the present study, we characterized the nonsynaptic chemical coupling mediated by extracellular ATP in dorsal root ganglia (DRG) neuron cultures by using the real time imaging of ATP, whole-cell patch clamping, in conjunction with confocal calcium imaging. Sustained depolarization by electrical stimulation evoked intracellular Ca2+ concentrations ([Ca2+]i) oscillations in individual DRG neurons, and subsequent ATP-dependent propagation [Ca2+]i oscillations to surrounding non-stimulated neighbors. [Ca2+]i oscillations were suppressed by inositol-1,4,5-trisphosphate (IP3) receptor antagonist 2-APB, but not ryanodine. The propagation of [Ca2+]i oscillations was prevented by the presence of the ATP-degrading enzyme, apyrase, and completely abolished by the blockase of G protein-coupled purinergic receptors-PLC-IP3 pathway with suramin, U73122 or 2-APB. In parallel, sustained depolarization elicited robust ATP release and diffusion from the stimulation site. Moreover, exogenous application of ATP to DRG cultures in large concentration elicits the [Ca2+]i oscillations in most neurons. Taken together, this data demonstrates that sustained membrane depolarization elicited ATP release, acting through a highly sensitive P2Y receptors/IP3-mediated signaling pathway to mediate the propagation of intercellular Ca2+ signaling, which suggest a novel signaling pathway for neuronal communication in DRG.

Sustained potentiation by substance P of NMDA-activated current in rat primary sensory neurons

This study aimed to explore the modulatory effect of substance P (SP) on the current response mediated by N-methyl-D-aspartate (NMDA) receptor in rat primary sensory neurons and its time course using whole-cell patch clamp technique. The majority of neurons (179/213, 84.0%) examined were sensitive to NMDA (0.1-1000 microM) with an inward current, and a proportion of the NMDA-sensitive neurons also responded to SP (78/98, 80.0%) with an inward current. Pretreatment with SP potentiated the NMDA-activated current (INMDA) in a non-competitive manner, which is shown in that SP shifted the concentration-response curve for NMDA upwards compared with the control; the maximal value of INMDA increased fourfold, while the EC50 values for both curves were very close (28 vs. 30 microM). Furthermore, this potentiating effect was time-dependent: the amplitude of INMDA reached its maximum 20 min after SP preapplication, and thereafter maintained a steady level of about 2-3 times its control for 2 or even 3 h. This sustained potentiation by SP of INMDA could be blocked by extracellular application of WIN51708, a selective non-peptide antagonist of NK-1 receptor; and abolished by intracellular application of either BAPTA, or H-7, or KN-93. Though NMDA applied alone also induced a short-term (less than 20 min) self-potentiation of INMDA, it could be abolished by intracellular dialysis of BAPTA or KN-93 completely. As is known, the cell body of dorsal root ganglion (DRG) neurons is generally used as an accessible model for studying the characteristics of the membrane of primary afferent terminals in the dorsal horn of spinal cord. Therefore, these results may offer a clue to the explanation of the symptoms of chronic pain.