Activation of neurokinin-1 receptors increases the excitability of guinea pig dorsal root ganglion cells

Neurogenic substance P-influences on action potential production in afferent neurons of the kidney?

We recently showed that a substance P (SP)–dependent sympatho-inhibitory mechanism via afferent renal nerves is impaired in mesangioproliferative nephritis. Therefore, we tested the hypothesis that SP released from renal afferents inhibits the action potential (AP) production in their dorsal root ganglion (DRG) neurons. Cultured DRG neurons (Th11-L2) were investigated in current clamp mode to assess AP generation during both TRPV1 stimulation by protons (pH 6) and current injections with and without exposure to SP (0.5 µmol) or CGRP (0.5 µmol). Neurons were classified as tonic (sustained AP generation) or phasic (≤ 4 APs) upon current injection; voltage clamp experiments were performed for the investigation of TRPV1-mediated inward currents due to proton stimulation. Superfusion of renal neurons with protons and SP increased the number of action potentials in tonic neurons (9.6 ± 5 APs/10 s vs. 16.9 ± 6.1 APs/10 s, P < 0.05, mean ± SD, n = 7), while current injections with SP decreased it (15.2 ± 6 APs/600 ms vs. 10.2 ± 8 APs/600 ms, P < 0.05, mean ± SD, n = 29). Addition of SP significantly reduced acid-induced TRPV1-mediated currents in renal tonic neurons (− 518 ± 743 pA due to pH 6 superfusion vs. − 82 ± 50 pA due to pH 6 with SP superfusion). In conclusion, SP increased action potential production via a TRPV1-dependent mechanism in acid-sensitive renal neurons. On the other hand, current injection in the presence of SP led to decreased action potential production. Thus, the peptide SP modulates signaling pathways in renal neurons in an unexpected manner leading to both stimulation and inhibition of renal neuronal activity in different (e.g., acidic) environmental contexts.

Age-related reductions in the excitability of phasic dorsal root ganglion neurons innervating the urinary bladder in female rats

Previous studies have revealed an impairment in bladder sensory transduction in aged animals. To examine the contributions of electrical property changes of bladder primary afferents to this impairment, we compared the electrical properties of dorsal root ganglion (DRG) neurons innervating the bladder among young (3 months), middle-aged (12 months), and old (24 months) female rats. The DRG neurons were labeled using axonal tracing techniques. Whole-cell current-clamp recordings of small and medium-sized neurons were performed to assess their passive and active properties. Two patterns of firing were identified based on responses to super-threshold stimuli (1.5, 2.0, 2.5, and 3.0 × rheobase): tonic neurons fired more action potentials (APs), whereas phasic neurons fired only one AP at the onset of stimulus. Tonic neurons were smaller and had a slower rate of AP rise, longer AP duration, more depolarized voltage threshold, and greater rheobase than phasic neurons. In phasic neurons, there was an age-associated increase in voltage threshold and an increase of rheobase (P < 0.05), suggesting an age-related decrease in excitability. In addition, both middle-aged and old rats had longer AP durations and slower rates of AP rise than young rats (P < 0.05). In tonic neurons, old rats had a greater AP overshoot and greater rate of AP rise, but no age-associated changes were identified in any other electrical properties. Our results suggest that the electrical properties of tonic and phasic bladder afferents are differentially altered with aging. A decrease in excitability may contribute to age-related reductions in bladder sensory function.

Tachykinins modulate nociceptive responsiveness and sensitization: In vivo electrical characterization of primary sensory neurons in tachykinin knockout (Tac1 KO) mice

Since the failure of specific substance P antagonists to induce analgesia, the role of tachykinins in the development of neuropathic pain states has been discounted. This conclusion was reached without studies on the role of tachykinins in normal patterns of primary afferents response and sensitization or the consequences of their absence on the modulation of primary mechanonociceptive afferents after injury. Nociceptive afferents from animals lacking tachykinins (Tac1 knockout) showed a disrupted pattern of activation to tonic suprathreshold mechanical stimulation. These nociceptors failed to encode the duration and magnitude of natural pronociceptive stimuli or to develop mechanical sensitization as consequence of this stimulation. Moreover, paw edema, hypersensitivity, and weight bearing were also reduced in Tac1 knockout mice 24 h after paw incision surgery. At this time, nociceptive afferents from these animals did not show the normal sensitization to mechanical stimulation or altered membrane electrical hyperexcitability as observed in wild-type animals. These changes occurred despite a similar increase in calcitonin gene-related peptide immunoreactivity in sensory neurons in Tac1 knockout and normal mice. Based on these observations, we conclude that tachykinins are critical modulators of primary nociceptive afferents, with a preeminent role in the electrical control of their excitability with sustained activation or injury.

Depolarizing, inhibitory GABA type A receptor activity regulates GABAergic synapse plasticity via ERK and BDNF signaling

γ-aminobutyric acid (GABA) begins as the key excitatory neurotransmitter in newly forming circuits, with chloride efflux from GABA type A receptors (GABA_ARs) producing membrane depolarization, which promotes calcium entry, dendritic outgrowth and synaptogenesis. As development proceeds, GABAergic signaling switches to inhibitory hyperpolarizing neurotransmission. Despite the evidence of impaired GABAergic neurotransmission in neurodevelopmental disorders, little is understood on how agonist-dependent GABA_AR activation controls the formation and plasticity of GABAergic synapses. We have identified a weakly depolarizing and inhibitory GABA_AR response in cortical neurons that occurs during the transition period from GABA_AR depolarizing excitation to hyperpolarizing inhibitory activity. We show here that treatment with the GABA_AR agonist muscimol mediates structural changes that diminish GABAergic synapse strength through postsynaptic and presynaptic plasticity via intracellular Ca²⁺ stores, ERK and BDNF/TrkB signaling. Muscimol decreases synaptic localization of surface γ2 GABA_ARs and gephyrin postsynaptic scaffold while β2/3 non-γ2 GABA_ARs accumulate in the synapse. Concurrent with this structural plasticity, muscimol treatment decreases synaptic currents while enhancing the γ2 containing benzodiazepine sensitive GABA_AR tonic current in an ERK dependent manner. We further demonstrate that GABA_AR activation leads to a decrease in presynaptic GAD65 levels via BDNF/TrkB signaling. Together these data reveal a novel mechanism for agonist induced GABAergic synapse plasticity that can occur on the timescale of minutes, contributing to rapid modification of synaptic and circuit function.

Netupitant, a Potent and Highly Selective NK1 Receptor Antagonist, Alleviates Acetic Acid-Induced Bladder Overactivity in Anesthetized Guinea-Pigs

Introduction. Tachykinins potently contract the isolated urinary bladder from a number of animal species and play an important role in the regulation of the micturition reflex. On the guinea-pig isolated urinary bladder we examined the effects of a new potent and selective NK1 receptor antagonist (netupitant) on the contractions induced by a selective NK1 receptor agonist, SP-methylester (SP-OMe). Moreover, the effects of netupitant and another selective NK1 antagonist (L-733,060) were studied in anesthetized guinea-pigs using two experimental models, the isovolumetric bladder contractions and a model of bladder overactivity induced by intravesical administration of acetic acid (AA). Methods and Results. Detrusor muscle strips were mounted in 5 mL organ baths and isometric contractions to cumulative concentrations of SP-OME were recorded before and after incubation with increasing concentrations of netupitant. In anesthetized female guinea-pigs, reflex bladder activity was examined under isovolumetric conditions with the bladder distended with saline or during cystometry using intravesical infusion of AA. After a 30 min stabilization period, netupitant (0.1-3 mg/kg, i.v.) or L-733,060 (3-10 mg/kg, i.v.) were administered. In the detrusor muscle, netupitant produced a concentration-dependent inhibition (mean pKB = 9.24) of the responses to SP-OMe. Under isovolumetric conditions, netupitant or L-733,060 reduced bladder contraction frequency in a dose-dependent manner, but neither drug changed bladder contraction amplitude. In the AA model, netupitant dose-dependently increased intercontraction interval (ICI) but had no effect on the amplitude of micturition (AM). L-733,060 dose-dependently increased ICI also but this effect was paralleled by a significant reduction of AM. Conclusion. Netupitant decreases the frequency of reflex bladder contractions without altering their amplitude, suggesting that this drug targets the afferent limb of the micturition reflex circuit and therefore may be useful clinically in treating bladder overactivity symptoms.

Redox-Dependent Modulation of T-Type Ca(2+) Channels in Sensory Neurons Contributes to Acute Anti-Nociceptive Effect of Substance P

AIMS

Neuropeptide substance P (SP) is produced and released by a subset of peripheral sensory neurons that respond to tissue damage (nociceptors). SP exerts excitatory effects in the central nervous system, but peripheral SP actions are still poorly understood; therefore, here, we aimed at investigating these peripheral mechanisms.

RESULTS

SP acutely inhibited T-type voltage-gated Ca(2+) channels in nociceptors. The effect was mediated by neurokinin 1 (NK1) receptor-induced stimulation of intracellular release of reactive oxygen species (ROS), as it can be prevented or reversed by the reducing agent dithiothreitol and mimicked by exogenous or endogenous ROS. This redox-mediated T-type Ca(2+) channel inhibition operated through the modulation of CaV3.2 channel sensitivity to ambient zinc, as it can be prevented or reversed by zinc chelation and mimicked by exogenous zinc. Elimination of the zinc-binding site in CaV3.2 rendered the channel insensitive to SP-mediated inhibition. Importantly, peripherally applied SP significantly reduced bradykinin-induced nociception in rats in vivo; knock-down of CaV3.2 significantly reduced this anti-nociceptive effect. This atypical signaling cascade shared the initial steps with the SP-mediated augmentation of M-type K(+) channels described earlier.

INNOVATION

Our study established a mechanism underlying the peripheral anti-nociceptive effect of SP whereby this neuropeptide produces ROS-dependent inhibition of pro-algesic T-type Ca(2+) current and concurrent enhancement of anti-algesic M-type K(+) current. These findings will lead to a better understanding of mechanisms of endogenous analgesia.

CONCLUSION

SP modulates T-type channel activity in nociceptors by a redox-dependent tuning of channel sensitivity to zinc; this novel modulatory pathway contributes to the peripheral anti-nociceptive effect of SP. Antioxid. Redox Signal. 25, 233-251.

Accelerated onset of the vesicovesical reflex in postnatal NGF-OE mice and the role of neuropeptides

The mechanisms underlying the postnatal maturation of micturition from a somatovesical to a vesicovesical reflex are not known but may involve neuropeptides in the lower urinary tract. A transgenic mouse model with chronic urothelial overexpression (OE) of NGF exhibited increased voiding frequency, increased number of non-voiding contractions, altered morphology and hyperinnervation of the urinary bladder by peptidergic (e.g., Sub P and CGRP) nerve fibers in the adult. In early postnatal and adult NGF-OE mice we have now examined: (1) micturition onset using filter paper void assays and open-outlet, continuous fill, conscious cystometry; (2) innervation and neurochemical coding of the suburothelial plexus of the urinary bladder using immunohistochemistry and semi-quantitative image analyses; (3) neuropeptide protein and transcript expression in urinary bladder of postnatal and adult NGF-OE mice using Q-PCR and ELISAs and (4) the effects of intravesical instillation of a neurokinin (NK)-1 receptor antagonist on bladder function in postnatal and adult NGF-OE mice using conscious cystometry. Postnatal NGF-OE mice exhibit age-dependent (R²=0.996-0.998; p≤0.01) increases in Sub and CGRP expression in the urothelium and significantly (p≤0.01) increased peptidergic hyperinnervation of the suburothelial nerve plexus. By as early as P7, NGF-OE mice exhibit a vesicovesical reflex in response to intravesical instillation of saline whereas littermate WT mice require perigenital stimulation to elicit a micturition reflex until P13 when vesicovesical reflexes are first observed. Intravesical instillation of a NK-1 receptor antagonist, netupitant (0.1μg/ml), significantly (p≤0.01) increased void volume and the interval between micturition events with no effects on bladder pressure (baseline, threshold, peak) in postnatal NGF-OE mice; effects on WT mice were few. NGF-induced pleiotropic effects on neuropeptide (e.g., Sub P) expression in the urinary bladder contribute to the maturation of the micturition reflex and are excitatory to the micturition reflex in postnatal NGF-OE mice. These studies provide insight into the mechanisms that contribute to the postnatal development of the micturition reflex.

In vitro and in vivo pharmacological characterization of Pronetupitant, a prodrug of the neurokinin 1 receptor antagonist Netupitant

The aim of the present study was to investigate the pharmacological activity of Pronetupitant, a novel compound designed to act as prodrug of the NK1 antagonist Netupitant. In receptor binding experiments Pronetupitant displayed high selectivity for the NK1 receptor. In a calcium mobilization assay performed on CHONK1 cells Pronetupitant (100 nM, 15 min preincubation) behaved as an NK1 antagonist more potent than Netupitant (pK(B) 8.72 and 7.54, respectively). In the guinea pig ileum bioassay Pronetupitant antagonized the contractile effect of SP showing a similar potency as Netupitant (pK(B)≈9). Similar results were obtained with 5 min preincubation time while at 2 min only Pronetupitant produced significant effects. In vivo in mice the intrathecal injection of 0.1 nmol SP elicited the typical scratching, biting and licking (SBL) nociceptive response. This effect of SP was dose dependently (0.1-10 mg/kg) antagonized by Pronetupitant given intravenously 2 h before the peptide. Superimposable results were obtained using Netupitant. Pharmacokinetic studies performed in rats demonstrate that Pronetupitant, after i.v. administration, is quickly (few minutes) and completely converted to Netupitant. Collectively the present results indicated that Pronetupitant acts in vitro as selective NK1 antagonist more potent than Netupitant. However based on the short half-life measured for Pronetupitant in rats, the in vivo action of Pronetupitant can be entirely interpreted as due to its conversion to Netupitant.

Anoctamin 1 contributes to inflammatory and nerve-injury induced hypersensitivity

BACKGROUND

Various pathological conditions such as inflammation or injury can evoke pain hypersensitivity. That represents the response to innocuous stimuli or exaggerated response to noxious stimuli. The molecular mechanism based on the pain hypersensitivity is associated with changes in many of ion channels in dorsal-root ganglion (DRG) neurons. Anoctamin 1 (ANO1/TMEM16A), a Ca2+ activated chloride channel is highly visible in small DRG neurons and responds to heat. Mice with an abolished function of ANO1 in DRG neurons demonstrated attenuated pain-like behaviors when exposed to noxious heat, suggesting a role in acute thermal nociception. In this study, we further examined the function of ANO1 in mediating inflammation- or injury-induced hyperalgesia or allodynia.

RESULTS

Using Advillin/Ano1fl/fl (Adv/Ano1fl/fl) mice that have a functional ablation of Ano1 mainly in DRG neurons, we were able to determine its role in mediating thermal hyperalgesia and mechanical allodynia induced by inflammation or nerve injury. The thermal hyperalgesia and mechanical allodynia induced by carrageenan injection and spared-nerve injury were significantly reduced in Adv/Ano1fl/fl mice. In addition, flinching or licking behavior after bradykinin or formalin injection was also significantly reduced in Adv/Ano1fl/fl mice. Since pathological conditions augment nociceptive behaviors, we expected ANO1's contribution to the excitability of DRG neurons. Indeed, the application of inflammatory mediators reduced the threshold for action potential (rheobase) or time for induction of the first action potential in DRG neurons isolated from control (Ano1fl/fl) mice. These parameters for neuronal excitability induced by inflammatory mediators were not changed in Adv/Ano1fl/fl mice, suggesting an active contribution of ANO1 in augmenting the neuronal excitability.

CONCLUSIONS

In addition to ANO1's role in mediating acute thermal pain as a heat sensor, ANO1 is also capable of augmenting the excitability of DRG neurons under inflammatory or neuropathic conditions and thereby aggravates inflammation- or tissue injury-induced pathological pain.

Botulinum toxin B in the sensory afferent: transmitter release, spinal activation, and pain behavior

We addressed the hypothesis that intraplantar botulinum toxin B (rimabotulinumtoxin B: BoNT-B) has an early local effect upon peripheral afferent terminal releasing function and, over time, will be transported to the central terminals of the primary afferent. Once in the terminals it will cleave synaptic protein, block spinal afferent transmitter release, and thereby prevent spinal nociceptive excitation and behavior. In mice, C57Bl/6 males, intraplantar BoNT-B (1 U) given unilaterally into the hind paw had no effect upon survival or motor function, but ipsilaterally decreased: (1) intraplantar formalin-evoked flinching; (2) intraplantar capsaicin-evoked plasma extravasation in the hind paw measured by Evans blue in the paw; (3) intraplantar formalin-evoked dorsal horn substance P (SP) release (neurokinin 1 [NK1] receptor internalization); (4) intraplantar formalin-evoked dorsal horn neuronal activation (c-fos); (5) ipsilateral dorsal root ganglion (DRG) vesicle-associated membrane protein (VAMP); (6) ipsilateral SP release otherwise evoked bilaterally by intrathecal capsaicin; (7) ipsilateral activation of c-fos otherwise evoked bilaterally by intrathecal SP. These results indicate that BoNT-B, after unilateral intraplantar delivery, is taken up by the peripheral terminal, is locally active (blocking plasma extravasation), is transported to the ipsilateral DRG to cleave VAMP, and is acting presynaptically to block release from the spinal peptidergic terminal. The observations following intrathecal SP offer evidence for a possible transsynaptic effect of intraplantar BoNT. These results provide robust evidence that peripheral BoNT-B can alter peripheral and central terminal release from a nociceptor and attenuate downstream nociceptive processing via a presynaptic effect, with further evidence suggesting a possible postsynaptic effect.

Efficacy and safety of repeated dosing of netupitant, a neurokinin-1 receptor antagonist, in treating overactive bladder

AIM

NK-1 receptors in sensory nerves, the spinal cord and bladder smooth muscle participate in complex sensory mechanisms that regulate bladder activity. This study was designed to assess the efficacy and safety of a new NK-1 receptor antagonist, netupitant, in patients with OAB.

METHODS

This was a phase II, multicenter, double-blind study in which adults with OAB symptoms >6 months were randomized to receive 1 of 3 doses of netupitant (50, 100, 200 mg) or placebo once daily for 8 weeks. The primary efficacy endpoint was percentage change from baseline in average number of daily micturitions at week 8. Urinary incontinence, urge urinary incontinence (UUI), and urgency episodes were also assessed.

RESULTS

The primary efficacy endpoint was similar in the treatment groups (-13.85 for placebo to -16.17 in the netupitant 200 mg group) with no statistically significant differences between netupitant and placebo. The same was true for most secondary endpoints although a significant difference for improvement in UUI episodes and a trend for the greatest decrease in urgency episodes were seen in the netupitant 100 mg group. Netupitant was well tolerated with most treatment emergent adverse events (AEs) being mild. While the overall incidence of AEs increased with netupitant dose, there was no evidence for this dose dependency based on relationship to treatment, intensity, or time to onset.

CONCLUSIONS

The study failed to demonstrate superiority of netupitant versus placebo in decreasing OAB symptoms, despite a trend favoring netupitant 100 mg. There were no safety concerns with daily administration of netupitant over 8 weeks.

Mechanisms involved in the nociception triggered by the venom of the armed spider Phoneutria nigriventer

BACKGROUND

The frequency of accidental spider bites in Brazil is growing, and poisoning due to bites from the spider genus Phoneutria nigriventer is the second most frequent source of such accidents. Intense local pain is the major symptom reported after bites of P. nigriventer, although the mechanisms involved are still poorly understood. Therefore, the aim of this study was to identify the mechanisms involved in nociception triggered by the venom of Phoneutria nigriventer (PNV).

METHODOLOGY/PRINCIPAL FINDINGS

Twenty microliters of PNV or PBS was injected into the mouse paw (intraplantar, i.pl.). The time spent licking the injected paw was considered indicative of the level of nociception. I.pl. injection of PNV produced spontaneous nociception, which was reduced by arachnid antivenin (ArAv), local anaesthetics, opioids, acetaminophen and dipyrone, but not indomethacin. Boiling or dialysing the venom reduced the nociception induced by the venom. PNV-induced nociception is not dependent on glutamate or histamine receptors or on mast cell degranulation, but it is mediated by the stimulation of sensory fibres that contain serotonin 4 (5-HT4) and vanilloid receptors (TRPV1). We detected a kallikrein-like kinin-generating enzyme activity in tissue treated with PNV, which also contributes to nociception. Inhibition of enzymatic activity or administration of a receptor antagonist for kinin B2 was able to inhibit the nociception induced by PNV. PNV nociception was also reduced by the blockade of tetrodotoxin-sensitive Na(+) channels, acid-sensitive ion channels (ASIC) and TRPV1 receptors.

CONCLUSION/SIGNIFICANCE

Results suggest that both low- and high-molecular-weight toxins of PNV produce spontaneous nociception through direct or indirect action of kinin B2, TRPV1, 5-HT4 or ASIC receptors and voltage-dependent sodium channels present in sensory neurons but not in mast cells. Understanding the mechanisms involved in nociception caused by PNV are of interest not only for better treating poisoning by P. nigriventer but also appreciating the diversity of targets triggered by PNV toxins.