G-Protein Activation by Neurokinin-1 Receptors Is Dynamically Regulated during Persistent Nociception

Spinal neuropeptide Y Y1 receptor-expressing neurons are a pharmacotherapeutic target for the alleviation of neuropathic pain

Peripheral nerve injury sensitizes a complex network of spinal cord dorsal horn (DH) neurons to produce allodynia and neuropathic pain. The identification of a druggable target within this network has remained elusive, but a promising candidate is the neuropeptide Y (NPY) Y1 receptor-expressing interneuron (Y1-IN) population. We report that spared nerve injury (SNI) enhanced the excitability of Y1-INs and elicited allodynia (mechanical and cold hypersensitivity) and affective pain. Similarly, chemogenetic or optogenetic activation of Y1-INs in uninjured mice elicited behavioral signs of spontaneous, allodynic, and affective pain. SNI-induced allodynia was reduced by chemogenetic inhibition of Y1-INs, or intrathecal administration of a Y1-selective agonist. Conditional deletion of Npy1r in DH neurons, but not peripheral afferent neurons prevented the anti-hyperalgesic effects of the intrathecal Y1 agonist. We conclude that spinal Y1-INs are necessary and sufficient for the behavioral symptoms of neuropathic pain and represent a promising target for future pharmacotherapeutic development of Y1 agonists.

Strategies for Behaviorally Phenotyping the Transgenic Mouse

The techniques and protocols to modify the mouse genome described in this volume allow researchers to produce genetic models of a remarkable number and breadth of human disease. The generation of gene-modified mice offers profoundly powerful approaches for bringing known or purported human gene disruptions into mouse models, but the degree to which the resultant mutant mouse recapitulates the complex physiological and behavioral features of the human disease state is a key variable in the ultimate usefulness of the mouse model organism. Accordingly, the behavioral characterization of mice with novel targeted gene mutations is an important initial step in determining the potential impact of a novel mouse model. This chapter addresses strategies useful in the initial observations of the animal that assist in directing the choice of secondary tests to assess more detailed aspects of potentially disrupted behaviors that may be relevant to the disease being modeled. An initial standardized, comprehensive screen that assesses general health, reflexes, and sensorimotor functions is the first step in characterizing behavioral phenotype, and results often suggest areas where more complex complementary behavioral assays may reveal more detailed disruption of normal behavior. This sequential, standardized approach reduces variability between subjects; this chapter also addresses approaches to reducing experimental artifacts due to handling, test order, testing facility environment, and other sources. This brief overview of behavioral phenotyping approaches is intended to provide practical information to streamline initial characterization of new mouse models and maximize the usefulness of efforts to use these models to study human health and disease.

Assessing complex movement behaviors in rodent models of neurological disorders

Behavioral phenotyping is a crucial step in validating animal models of human disease. Most traditional behavioral analyses rely on investigator observation of animal subjects, which can be confounded by inter-observer variability, scoring consistency, and the ability to observe extremely rapid, small, or repetitive movements. Force-Plate Actimeter (FPA)-based assessments can quantify locomotor activity and detailed motor activity with an incredibly rich data stream that can reveal details of movement unobservable by the naked eye. This report describes four specific examples of FPA analysis of behavior that have been useful in specific rat or mouse models of human neurological disease, which show how FPA analysis can be used to capture and quantify specific features of the complex behavioral phenotypes of these animal models. The first example quantifies nociceptive behavior of the rat following injection of formalin into the footpad as a common model of persistent inflammatory pain. The second uses actimetry to quantify intense, rapid circling behaviors in a transgenic mouse that overexpresses human laminin α5, a basement membrane protein. The third example assesses place preference behaviors in a rat model of migraine headache modeling phonophobia and photophobia. In the fourth example, FPA analysis revealed a unique movement signature emerged with age in a digenic mutant mouse model of Tourette Syndrome. Taken together, these approaches demonstrate the power and usefulness of the FPA in the examination and quantification of minute details of motor behaviors, greatly expanding the scope and detail of behavioral phenotyping of preclinical models of human disease.

Rats bred for low and high running capacity display alterations in peripheral tissues and nerves relevant to neuropathy and pain

INTRODUCTION

Diet and activity are recognized as modulators of nervous system disease, including pain. Studies of exercise consistently reveal a benefit on pain. This study focused on female rats to understand differences related to metabolic status and peripheral nerve function in females.

METHODS

Here, we investigated parameters of peripheral nerve function relevant to pain in rats selectively bred for high (high-capacity runners; HCR) or low endurance exercise capacity (low-capacity runners; LCR) resulting in divergent intrinsic aerobic capacities and susceptibility for metabolic conditions.

RESULTS

LCR female rats have reduced mechanical sensitivity, higher intraepidermal nerve fiber density and TrkA-positive epidermal axons, increased numbers of Langerhans and mast cells in cutaneous tissues, and a higher fat content despite similar overall body weights compared to female HCR rats. Sensory and motor nerve conduction velocities, thermal sensitivity, and mRNA expression of selected genes relevant to peripheral sensation were not different.

CONCLUSIONS

These results suggest that aerobic capacity and metabolic status influence sensory sensitivity and aspects of inflammation in peripheral tissues that could lead to poor responses to tissue damage and painful stimuli. The LCR and HCR rats should prove useful as models to assess how the metabolic status impacts pain.

The ethyl acetate fraction from Physalis alkekengi inhibits LPS-induced pro-inflammatory mediators in BV2 cells and inflammatory pain in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Physalis alkekengi is an edible herb whose fruit and calyx are traditionally used to treat a wide range of diseases including inflammation, toothache, and rheumatism. However, the effects of Physalis alkekengi fruit along with its calyx (PAF) on neuroinflammation and inflammatory pain behavior have not been reported yet.

AIM OF THE STUDY

This study evaluated the anti-inflammatory effect of PAF on lipopolysaccharide (LPS)-induced neuroinflammation and several in vivo model of inflammatory pain in mice.

MATERIALS AND METHODS

Here, first we studied the effects of PAF fractions on the production of pro-inflammatory mediators in LPS-treated BV2 microglial cells using enzyme-linked immunosorbent assay. The translocation of nuclear factor-kappa B (NF-κB) and the involvements of Akt and mitogen-activated protein (MAP) kinases in ethyl acetate fraction of PAF (PAF-EA)-mediated anti-inflammatory effect were measured using Western blotting. In in vivo experiments, the efficacy of PAF-EA was evaluated at the doses of 100 and 200mg/kg using several chemical-induced models of inflammatory pain such as acetic acid-induced writhing, formalin-induced paw licking and edema.

RESULTS

We found that compared to other fractions, the PAF-EA more potently inhibited the LPS-induced generation of nitric oxide, tumor necrosis factor-α, interleukin-6 and reactive oxygen species. It also inhibited LPS-induced nuclear translocation of NF-κB. These actions of EA fraction were found to be associated with a disruption of Akt and MAP kinases signaling pathways. The EA fraction also significantly inhibited acetic acid-induced writhing, formalin-induced licking time and edema in mice.

CONCLUSIONS

Our findings support the ethnopharmacological use of P. alkekengi fruit along with its calyx as an anti-inflammatory agent and suggest that the EA fraction of PAF may serve as a potential candidate to treat different neurological disorders and pain associated with inflammation.

Sustained pain-related depression of behavior: effects of intraplantar formalin and complete freund's adjuvant on intracranial self-stimulation (ICSS) and endogenous kappa opioid biomarkers in rats

BACKGROUND

Intraplantar administration of complete Freund's adjuvant (CFA) and formalin are two noxious stimuli commonly used to produce sustained pain-related behaviors in rodents for research on neurobiology and treatment of pain. One clinically relevant manifestation of pain is depression of behavior and mood. This study compared effects of intraplantar CFA and formalin on depression of positively reinforced operant behavior in an assay of intracranial self-stimulation (ICSS) in rats. Effects of CFA and formalin on other physiological and behavioral measures, and opioid effects on formalin-induced depression of ICSS, were also examined.

RESULTS

There were four main findings. First, consistent with previous studies, both CFA and formalin produced similar paw swelling and mechanical hypersensitivity. Second, CFA produced weak and transient depression of ICSS, whereas formalin produced a more robust and sustained depression of ICSS that lasted at least 14 days. Third, formalin-induced depression of ICSS was reversed by morphine doses that did not significantly alter ICSS in saline-treated rats, suggesting that formalin effects on ICSS can be interpreted as an example of pain-related and analgesic-reversible depression of behavior. Finally, formalin-induced depression of ICSS was not associated with changes in central biomarkers for activation of endogenous kappa opioid systems, which have been implicated in depressive-like states in rodents, nor was it blocked by the kappa antagonist norbinaltorphimine.

CONCLUSIONS

These results suggest differential efficacy of sustained pain stimuli to depress brain reward function in rats as assessed with ICSS. Formalin-induced depression of ICSS does not appear to engage brain kappa opioid systems.

Mechanisms of pain modulation by sex hormones in migraine

A number of pain conditions, acute as well as chronic, are much more prevalent in women, such as temporomandibular disorder (TMD), irritable bowel syndrome, fibromyalgia, and migraine. The association of female sex steroids with these nociceptive conditions is well known, but the mechanisms of their effects on pain signaling are yet to be deciphered. We reviewed the mechanisms through which female sex steroids might influence the trigeminal nociceptive pathways with a focus on migraine. Sex steroid receptors are located in trigeminal circuits, providing the molecular substrate for direct effects. In addition to classical genomic effects, sex steroids exert rapid nongenomic actions to modulate nociceptive signaling. Although there are only a handful of studies that have directly addressed the effect of sex hormones in animal models of migraine, the putative mechanisms can be extrapolated from observations in animal models of other trigeminal pain disorders, like TMD. Sex hormones may regulate sensitization of trigeminal neurons by modulating expression of nociceptive mediator such as calcitonin gene-related peptide. Its expression is mostly positively regulated by estrogen, although a few studies also report an inverse relationship. Serotonin (5-Hydroxytryptamine [5-HT]) is a neurotransmitter implicated in migraine; its synthesis is enhanced in most parts of brain by estrogen, which increases expression of the rate-limiting enzyme tryptophan hydroxylase and decreases expression of the serotonin re-uptake transporter. Downstream signaling, including extracellular signal-regulated kinase activation, calcium-dependent mechanisms, and cAMP response element-binding activation, are thought to be the major signaling events affected by sex hormones. These findings need to be confirmed in migraine-specific animal models that may also provide clues to additional ion channels, neuropeptides, and intracellular signaling cascades that contribute to the increased prevalence of migraine in women.

Diminished neurokinin-1 receptor availability in patients with two forms of chronic visceral pain

Central sensitization and dysregulation of peripheral substance P and neurokinin-1 receptor (NK-1R) signaling are associated with chronic abdominal pain in inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). Although positron emission tomography (PET) has demonstrated that patients with injury-related chronic pain have diminished NK-1R availability in the brain, it is unknown whether these deficits are present in IBD and IBS patients, who have etiologically distinct forms of non-injury-related chronic pain. This study's aim was to determine if patients with IBD or IBS exhibit deficits in brain expression of NK-1Rs relative to healthy controls (HCs), the extent to which expression patterns differ across patient populations, and if these patterns differentially relate to clinical parameters. PET with [(18)F]SPA-RQ was used to measure NK-1R availability by quantifying binding potential (BP) in the 3 groups. Exploratory correlation analyses were performed to detect associations between NK-1R BP and physical symptoms. Compared to HCs, IBD patients had NK-1R BP deficits across a widespread network of cortical and subcortical regions. IBS patients had similar, but less pronounced deficits. BP in a subset of these regions was robustly related to discrete clinical parameters in each patient population. Widespread deficits in NK-1R BP occur in IBD and, to a lesser extent, IBS; however, discrete clinical parameters relate to NK-1R BP in each patient population. This suggests that potential pharmacological interventions that target NK-1R signaling may be most effective for treating distinct symptoms in IBD and IBS.

De novo expression of the neurokinin 1 receptor in spinal lamina I pyramidal neurons in polyarthritis

Spinal lamina I (LI) neurons play a major role in the transmission and integration of pain-related information that is relayed to higher centers. Alterations in the excitability of these neurons influence chronic pain development, and expression of the neurokinin 1 receptor (NK-1r) is thought to play a major role in such changes. Novel expression of NK-1r may underlie hyperexcitability in new populations of LI neurons. LI projection neurons can be classified morphologically into fusiform, pyramidal, and multipolar cells, differing in their functional properties, with the pyramidal type being nonnociceptive. In agreement with this, we have shown that spinoparabrachial pyramidal neurons seldom express NK-1r, in contrast with the other two cell types. In this study we investigated in the rat the long-term changes in NK-1r expression by spinoparabrachial LI neurons following the unilateral injection in the hindpaw plantar surface of complete Freund's adjuvant (CFA). Cholera toxin subunit B (CTb) was injected unilaterally into the parabrachial nucleus. Our results revealed that, ipsilaterally, pyramidal neurons were seldom immunoreactive for NK-1r both in saline-injected and in CFA-injected rats, up to 10 days post-CFA. However, a considerable number of pyramidal cells were immunoreactive for NK-1r at 15, 21, and 30 days post-CFA. Our data raise the possibility -- which needs to be confirmed by electrophysiology -- that most LI projection neurons of the pyramidal type are likely nonnociceptive in naive animals but might become nociceptive following the development of arthritis.

Dissociation of spinal microglia morphological activation and peripheral inflammation in inflammatory pain models

We compared the effects of peripheral Freund's Complete Adjuvant (CFA) and formalin injection on spinal microglia activation. Both qualitative and quantitative analyses showed signs of microglia activation on the ipsilateral side of the lumbar dorsal horn on day 3, day 7 and day 14 after formalin injection. However, significant microglia morphological alteration was not found in the CFA model. At the injection site in the paw, CFA injection induced considerably more inflammation than formalin injection. Although spinal microglia might be activated in inflammatory pain models, morphologically, spinal microglia activation was not closely correlated with peripheral inflammation.