Peroxynitrite Contributes to Behavioral Responses, Increased Trigeminal Excitability, and Changes in Mitochondrial Function in a Preclinical Model of Migraine

Administration of a nitric oxide (NO) donor triggers migraine attacks, but the mechanisms by which this occurs are unknown. Reactive nitroxidative species, including NO and peroxynitrite (PN), have been implicated in nociceptive sensitization, and neutralizing PN is antinociceptive. We determined whether PN contributes to nociceptive responses in two distinct models of migraine headache. Female and male mice were subjected to 3 consecutive days of restraint stress or to dural stimulation with the proinflammatory cytokine interleukin-6. Following resolution of the initial poststimulus behavioral responses, animals were tested for hyperalgesic priming using a normally non-noxious dose of the NO donor sodium nitroprusside (SNP) or dural pH 7.0, respectively. We measured periorbital von Frey and grimace responses in both models and measured stress-induced changes in 3-nitrotyrosine (3-NT) expression (a marker for PN activity) and trigeminal ganglia (TGs) mitochondrial function. Additionally, we recorded the neuronal activity of TGs in response to the PN generator SIN-1 [5-amino-3-(4-morpholinyl)-1,2,3-oxadiazolium chloride]. We then tested the effects of the PN decomposition catalysts Fe(III)5,10,15,20-tetrakis(N-methylpyridinium-4-yl) porphyrin (FeTMPyP) and FeTPPS [Fe(III)5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato chloride], or the PN scavenger MnTBAP [Mn(III)tetrakis(4-benzoic acid)porphyrin] against these behavioral, molecular, and neuronal changes. Neutralizing PN attenuated stress-induced periorbital hypersensitivity and priming to SNP, with no effect on priming to dural pH 7.0. These compounds also prevented stress-induced increases in 3-NT expression in both the TGs and dura mater, and attenuated TG neuronal hyperexcitability caused by SIN-1. Surprisingly, FeTMPyP attenuated changes in TG mitochondrial function caused by SNP in stressed males only. Together, these data strongly implicate PN in migraine mechanisms and highlight the therapeutic potential of targeting PN.SIGNIFICANCE STATEMENT Among the most reliable experimental triggers of migraine are nitric oxide donors. The mechanisms by which nitric oxide triggers attacks are unclear but may be because of reactive nitroxidative species such as peroxynitrite. Using mouse models of migraine headache, we show that peroxynitrite-modulating compounds attenuate behavioral, neuronal, and molecular changes caused by repeated stress and nitric oxide donors (two of the most common triggers of migraine in humans). Additionally, our results show a sex-specific regulation of mitochondrial function by peroxynitrite following stress, providing novel insight into the ways in which peroxynitrite may contribute to migraine-related mechanisms. Critically, our data underscore the potential in targeting peroxynitrite formation as a novel therapeutic for the treatment of migraine headache.

Convergence of peptidergic and non-peptidergic protein markers in the human dorsal root ganglion and spinal dorsal horn

Peripheral sensory neurons are characterized by their size, molecular profiles, and physiological responses to specific stimuli. In mouse, the peptidergic and non-peptidergic subsets of nociceptors are distinct and innervate different lamina of the spinal dorsal horn. The unique molecular signature and neuroanatomical organization of these neurons supports a labeled line theory for certain types of nociceptive stimuli. However, long-standing evidence supports the polymodal nature of nociceptors in many species. We have recently shown that the peptidergic marker, CGRP, and the non-peptidergic marker, P2X3R, show largely overlapping expression at the mRNA level in human dorsal root ganglion (DRG). Herein, our aim was to assess the protein distribution of nociceptor markers, including their central projections, in the human DRG and spinal cord. Using DRGs obtained from organ donors, we observed that CGRP and P2X3R were co-expressed by approximately 33% of human DRG neurons and TrpV1 was expressed in ~60% of human DRG neurons. In the dorsal spinal cord, CGRP, P2X3R, TrpV1, and Nav1.7 proteins stained the entirety of lamina 1-2, with only P2XR3 showing a gradient of expression. This was confirmed by measuring the size of the substantia gelatinosa using Hematoxylin and Eosin staining of adjacent sections. Our findings are consistent with the known polymodal nature of most primate nociceptors and indicate that the central projection patterns of nociceptors are different between mice and humans. Elucidating how human nociceptors connect to subsets of dorsal horn neurons will be important for understanding the physiological consequences of these species differences.

IL-6 induced upregulation of T-type Ca2+ currents and sensitization of DRG nociceptors is attenuated by MNK inhibition

Phosphorylation of the 5' cap-binding protein eIF4E by MAPK-interacting kinases (MNK1/2) is important for nociceptor sensitization and the development of chronic pain. IL-6-induced dorsal root ganglion (DRG) nociceptor excitability is attenuated in mice lacking eIF4E phosphorylation, in MNK1/2^-/- mice, and by the nonselective MNK1/2 inhibitor cercosporamide. Here, we sought to better understand the neurophysiological mechanisms underlying how IL-6 causes nociceptor excitability via MNK-eIF4E signaling using the highly selective MNK inhibitor eFT508. DRG neurons were cultured from male and female ICR mice, 4-7 wk old. DRG cultures were treated with vehicle, IL-6, eFT508 (pretreat) followed by IL-6, or eFT508 alone. Whole cell patch-clamp recordings were done on small-diameter neurons (20-30 pF) to measure membrane excitability in response to ramp depolarization. IL-6 treatment (1 h) resulted in increased action potential firing compared with vehicle at all ramp intensities, an effect that was blocked by pretreatment with eFT508. Basic membrane properties, including resting membrane potential, input resistance, and rheobase, were similar across groups. Latency to the first action potential in the ramp protocol was lower in the IL-6 group and rescued by eFT508 pretreatment. We also found that the amplitudes of T-type voltage-gated calcium channels (VGCCs) were increased in the DRG following IL-6 treatment, but not in the eFT508 cotreatment group. Our findings are consistent with a model wherein MNK-eIF4E signaling controls the translation of signaling factors that regulate T-type VGCCs in response to IL-6 treatment. Inhibition of MNK with eFT508 disrupts these events, thereby preventing nociceptor hyperexcitability.NEW & NOTEWORTHY In this study, we show that the MNK inhibitor and anti-tumor agent eFT508 (tomivosertib) is effective in attenuating IL-6 induced sensitization of dorsal root ganglion (DRG) nociceptors. Pretreatment with eFT508 in DRG cultures from mice helps mitigate the development of hyperexcitability in response to IL-6. Furthermore, our data reveal that the upregulation of T-type voltage-gated calcium channels following IL-6 application can be blocked by eFT508, implicating the MNK-eIF4E signaling pathway in membrane trafficking of ion channels.

Schizophrenia-Like Dopamine Release Abnormalities in a Mouse Model of NMDA Receptor Hypofunction

Amphetamine-induced augmentation of striatal dopamine and its blunted release in prefrontal cortex (PFC) is a hallmark of schizophrenia pathophysiology. Although N-methyl-D-aspartate receptor (NMDAR) hypofunction is also implicated in schizophrenia, it remains unclear whether NMDAR hypofunction leads to dopamine release abnormalities. We previously demonstrated schizophrenia-like phenotypes in GABAergic neuron-specific NMDAR hypofunctional mutant mice, in which Ppp1r2-Cre dependent deletion of indispensable NMDAR channel subunit Grin1 is induced in corticolimbic GABAergic neurons including parvalbumin (PV)-positive neurons, in postnatal development, but not in adulthood. Here, we report enhanced dopaminomimetic-induced locomotor activity in these mutants, along with bidirectional, site-specific changes in in vivo amphetamine-induced dopamine release: nucleus accumbens (NAc) dopamine release was enhanced by amphetamine in postnatal Ppp1r2-Cre/Grin1 knockout (KO) mice, whereas dopamine release was dramatically reduced in the medial PFC (mPFC) compared to controls. Basal tissue dopamine levels in both the NAc and mPFC were unaffected. Interestingly, the magnitude and distribution of amphetamine-induced c-Fos expression in dopamine neurons was comparable between genotypes across dopaminergic input subregions in the ventral tegmental area (VTA). These effects appear to be both developmentally and cell-type specifically modulated, since PV-specific Grin1 KO mice could induce the same effects as seen in postnatal-onset Ppp1r2-Cre/Grin1 KO mice, but no such abnormalities were observed in somatostatin-Cre/Grin1 KO mice or adult-onset Ppp1r2-Cre/Grin1 KO mice. These results suggest that PV GABAergic neuron-NMDAR hypofunction in postnatal development confers bidirectional NAc hyper- and mPFC hypo-sensitivity to amphetamine-induced dopamine release, similar to that classically observed in schizophrenia pathophysiology.

Spatial and temporal boundaries of NMDA receptor hypofunction leading to schizophrenia

The N-methyl-d-aspartate receptor hypofunction is one of the most prevalent models of schizophrenia. For example, healthy subjects treated with uncompetitive N-methyl-d-aspartate receptor antagonists elicit positive, negative, and cognitive-like symptoms of schizophrenia. Patients with anti-N-methyl-d-aspartate receptor encephalitis, which is likely caused by autoantibody-mediated down-regulation of cell surface N-methyl-d-aspartate receptors, often experience psychiatric symptoms similar to schizophrenia initially. However, where and when N-methyl-d-aspartate receptor hypofunction occurs in the brain of schizophrenic patients is poorly understood. Here we review the findings from N-methyl-d-aspartate receptor antagonist and autoantibody models, postmortem studies on N-methyl-d-aspartate receptor subunits, as well as the global and cell-type-specific knockout mouse models of subunit GluN1. We compare various conditional GluN1 knockout mouse strains, focusing on the onset of N-methyl-d-aspartate receptor deletion and on the cortical cell-types. Based on these results, we hypothesize that N-methyl-d-aspartate receptor hypofunction initially occurs in cortical GABAergic neurons during early postnatal development. The resulting GABA neuron maturation deficit may cause reduction of intrinsic excitability and GABA release, leading to disinhibition of pyramidal neurons. The cortical disinhibition in turn could elicit glutamate spillover and subsequent homeostatic down regulation of N-methyl-d-aspartate receptor function in pyramidal neurons in prodromal stage. These two temporally-distinct N-methyl-d-aspartate receptor hypofunctions may be complimentary, as neither alone may not be able to fully explain the entire schizophrenia pathophysiology. Potential underlying mechanisms for N-methyl-d-aspartate receptor hypofunction in cortical GABA neurons are also discussed, based on studies of naturally-occurring N-methyl-d-aspartate receptor antagonists, neuregulin/ErbB4 signaling pathway, and theoretical analysis of excitatory/inhibitory balance.