Reactive oxygen species are second messengers of neurokinin signaling in peripheral sensory neurons.

This paper describes a newly recognized signaling mechanism in which neurokinins (NKs) activate mitochondrial release of reactive oxygen species (ROS) to increase M-type K⁺ currents in a select subpopulation of small size dorsal root ganglion (DRG) neurons from neonatal rats that fire single action potentials (APs) in response to prolonged depolarizing pulses. APs in these neurons are relatively rapidly repolarizing and have a pronounced and rapid hyperpolarizing afterpotential. Stimulation of M-currents by NKs seems to be mediated by activation of NK₁ receptors despite expression of other subtypes of NK receptors (NK₂, NK₃) in this subpopulation of neurons. This enhancement of M-type channels, which may rely on a phospholipase pathway that does not stimulate IP₃-dependent intracellular Ca²⁺ release, has a slow onset (>15 min.) and is resistant to washout of NK.

In some neurons NKs also inhibit M-currents, an effect presumably due to depletion of PIP₂. Stimulation of M-type currents is consistent with the transient hyperpolarization we reported previously (1). The lack of effect of NKs on Ca²⁺ release was also found in other studies (Zhang et al., submitted). The current study is an impressive and thorough one that sets forth yet another signaling mechanism that controls nociceptive responses in the presence of NKs.

Despite the novelty of the findings there are discrepancies with published literature that need to be explained; among them is the lack of an excitatory effect of NKs vs. previous reports (1, 2). Are these differences due to the age of the animals—neonate in this study vs. adult in (1); acute dissociation in this study vs. long-term cultures in (1); or selection of neurons (small <20 mm) in this study vs. medium size (>20 mm) in (1)?

Inhibition of A-type K⁺ channels has been reported to be responsible in part for the increased excitability in adult rat DRG neurons and is mediated by activation of NK₂ receptors (3, 4). In addition, activation of NK₂ receptors in adult rat neurons, and NK₁ in guinea pig DRG neurons (attached figure, unpublished data), may increase excitability and lower the firing threshold by activating another signaling mechanism, presumably the ERK/MAP pathway, which enhances TTX-resistant Na⁺ currents. 

It is also unclear whether ROS act directly on M-type channels or if some other molecule downstream of ROS is the signaling chemical. For example, it has been reported that NO interacts chemically with mono- and poly-unsaturated fatty acids to form electrophilic fatty acids that then react with cysteine residues of TRP channels to activate them but in large concentrations also inhibit Na⁺ channels (5).

Figure Legend: 

Time course and effects of various drugs on Na⁺currents in guinea pig DRG neurons. A. Effects of various agents on Na⁺currents activated at -23 (left) and 0 (right) mV in adult guinea pig DRG neurons. Cells were held at -80 mV and test pulses at two voltages repeated every 10 s. B-E. Current traces for steady-state effects of drugs at two voltages as shown in A. B. Tetrodotoxin (TTX) blocked most of the current activated at -23 mV with little effect on Na⁺currents activated at more depolarized potentials. C. The selective NK₁agonist SarMet SP (0.5 μM) partially increased the TTX-resistant (TTXr) I_Naat negative voltages. D. MEN 10376, an NK₂antagonist, had no effect on the SarMetSP-activated Na⁺current that was blocked by netupitant (NTP), an NK₁selective antagonist. E. The remaining current was blocked by A803567, a TTXr Nav 1.8 blocker. This was in contrast to rat DRG neurons in which the effect of SP on TTXr Na⁺currents was mediated by NK₂receptors. F. Voltage stimulation protocol used to generate simultaneous current-voltage relationship (G) and steady-state inactivation curve (H).

References:

1. Sculptoreanu A, de Groat WC. Neurokinins enhance excitability in capsaicin-responsive DRG neurons. Exp Neurol. 2007 May;205(1):92-100.

2. Abdulla FA, Stebbing MJ, Smith PA. Effects of substance P on excitability and ionic currents of normal and axotomized rat dorsal root ganglion neurons. Eur J Neurosci. 2001 Feb;13(3):545-52.

3. de Groat WC, Sculptoreanu A. Substance P and an NK2-selective agonist inhibit heteropodatoxin-sensitive low threshold inactivating K+-currents in DRG neurons. 2008. Society for Neuroscience (abstract).

4. Sculptoreanu A., Artim DE, de Groat WC. Neurokinins inhibit low threshold inactivating K+ currents in capsaicin responsive DRG neurons. Exp. Neurol. 2009 Oct;219(2):562–73.

5. Sculptoreanu A., Kullmann FA, Artim DE., Bazley FA., Schopfer F., Woodcock S., Freeman BA., de Groat WC. Nitro-oleic acid inhibits firing and activates TRPV1- and TRPA1-mediated inward currents in DRG neurons from adult male rats. J Pharmacol Exp Ther.  2010 Jun;333(3):883-95.