Analgesic activities of the mixed opioid and NPFF receptors agonist DN-9 in a mouse model of formalin-induced orofacial inflammatory pain

Role of neuropeptides in orofacial pain: A literature review

BACKGROUND

Neuropeptides play a critical role in regulating pain and inflammation. Despite accumulating evidence has further uncovered the novel functions and mechanisms of different neuropeptides in orofacial pain sensation and transmission, there is deficient systematic description of neuropeptides' pain modulation in the orofacial region, especially in the trigeminal system.

OBJECTIVES

The present review aims to summarise several key neuropeptides and gain a better understanding of their major regulatory roles in orofacial inflammation and pain.

METHODS

We review and summarise current studies related to calcitonin gene-related peptide (CGRP), substance P (SP), opioid peptide (OP), galanin (GAL) and other neuropeptides' functions and mechanisms as well as promising targets for orofacial pain control.

RESULTS

A number of neuropeptides are clearly expressed in the trigeminal sensory system and have critical functions in the transduction and pathogenesis of orofacial pain. The functions, possible cellular and molecular mechanisms have been introduced and discussed. Neuropeptides and their agonists or antagonists which are widely studied to be potential treatment options of orofacial pain has been evaluated.

CONCLUSIONS

Various neuropeptides play important but distinct (pro-nociceptive or analgesic) roles in orofacial pain with different mechanisms. In summary, CGRP, SP, NPY, NKA, HK-1, VIP mainly play proinflammatory and pro-nociceptive effects while OP, GAL, OXT, OrxA mainly have inhibitory effects on orofacial pain.

NOP01, a NOP receptor agonist, produced potent and peripherally restricted antinociception in a formalin-induced mouse orofacial pain model

Orofacial pain is one of the most common medical challenges. A preliminary report indicates that the NOP receptor may act as a therapeutic target in orofacial pain. Previous studies have shown that [(pF)Phe⁴, Aib⁷, Aib¹¹, Arg¹⁴, Lys¹⁵]N/OFQ-NH₂ (NOP01) functions as a potent NOP receptor peptide agonist. This work aims to investigate the antinociception of NOP01 and its possible action mechanisms in a formalin-induced mouse orofacial pain model at different levels. Our results demonstrated that local, intraperitoneal (i.p.) or intrathecal (i.t.) injection of NOP01 produced dose-related antinociception in both phases of the formalin pain, which could be inhibited by the NOP receptor antagonist but not the classical opioid receptor antagonist. Furthermore, the antinociception induced by systemic NOP01 was blocked by local but not spinal pretreatment with the NOP receptor antagonist, suggesting the involvement of the peripheral NOP receptor in NOP01-induced systemic antinociception. Moreover, local injection of NOP01 markedly suppressed the expression of c-Fos protein induced by formalin in ipsilateral trigeminal ganglion (TG) neurons. In conclusion, this work suggests that NOP01 exerts significant antinociception on orofacial pain at both peripheral and spinal levels via the NOP receptor. Notably, NOP01 cannot readily penetrate the blood-brain barrier. Thus, NOP01 may behave as a potential compound for developing peripherally restricted analgesics.

Diurnal Variation in Trigeminal Pain Sensitivity in Mice

Management of time and circadian disruption is an extremely important factor in basic research on pain and analgesia. Although pain is known to vary throughout the day, the mechanism underlying this circadian variation remains largely unknown. In this study, we hypothesized that the process of pain transmission to the central nervous system (after receiving nociceptive stimuli from outside the body) would show day-night differences. Ten-week-old male mice were kept under a strict 12/12-h light/dark cycle for at least 10 days. Formalin was then injected into the second branch region of the trigeminal nerve and the duration of pain-related behaviors (PRBs) was assessed. Immunohistochemical staining was then performed, and the c-Fos-immunopositive cells in the trigeminal spinal tract subnucleus caudalis (Sp5C) were counted. The results showed that the duration of PRBs was longer and the number of c-Fos immunopositive cells in the Sp5C was higher at nighttime than during the day. In addition, the trigeminal ganglia (TG) were extracted from the mice and examined by quantitative real-time PCR to evaluate the daytime and nighttime expression of nociceptive receptors. The results showed that the mRNA expression of transient receptor potential ankyrin 1 in the TG was significantly higher at night than during the day. These results suggest that pain in the trigeminal nerve region is more intense at nighttime, when rodents are active, than during the daytime, partly due to differences in nociceptor expression.

Differential Regulation of Gonadotropins as Revealed by Transcriptomes of Distinct LH and FSH Cells of Fish Pituitary

From mammals to fish, reproduction is driven by luteinizing hormone (LH) and follicle-stimulating hormone (FSH) temporally secreted from the pituitary gland. Teleost fish are an excellent model for addressing the unique regulation and function of each gonadotropin cell since, unlike mammals, they synthesize and secrete LH and FSH from distinct cells. Only very distant vertebrate classes (such as fish and birds) demonstrate the mono-hormonal strategy, suggesting a potential convergent evolution. Cell-specific transcriptome analysis of double-labeled transgenic tilapia expressing GFP and RFP in LH or FSH cells, respectively, yielded genes specifically enriched in each cell type, revealing differences in hormone regulation, receptor expression, cell signaling, and electrical properties. Each cell type expresses a unique GPCR signature that reveals the direct regulation of metabolic and homeostatic hormones. Comparing these novel transcriptomes to that of rat gonadotrophs revealed conserved genes that might specifically contribute to each gonadotropin activity in mammals, suggesting conserved mechanisms controlling the differential regulation of gonadotropins in vertebrates.

Peripherally Acting Opioids in Orofacial Pain

The activation of opioid receptors by exogenous or endogenous opioids can produce significant analgesic effects in peripheral tissues. Numerous researchers have demonstrated the expression of peripheral opioid receptors (PORs) and endogenous opioid peptides (EOPs) in the orofacial region. Growing evidence has shown the involvement of PORs and immune cell-derived EOPs in the modulation of orofacial pain. In this review, we discuss the role of PORs and EOPs in orofacial pain and the possible cellular mechanisms involved. Furthermore, the potential development of therapeutic strategies for orofacial pain is also summarized.

NFAT5 Deficiency Alleviates Formalin-Induced Inflammatory Pain Through mTOR

Nuclear factor of activated T cells (NFAT5) is a well-known transcription factor that regulates the expression of genes involved in osmotic stress. However, the role of NFAT5 in inflammatory pain remains unknown. Here, we studied the function of NFAT5 in inflammatory pain using NFAT5-heterozygous (Het) mice. To study inflammatory pain, we injected 10 µL of 2% formalin into the right hind paws of mice and monitored pain behaviors, such as licking, lifting, and flinching, for 60 min. After the first 15 min (phase I), there were no significant differences in pain behaviors between wild-type (WT) and NFAT5-Het mice. However, from 15–60 min (phase II), NFAT5-Het mice displayed significantly fewer pain behaviors compared to WT mice. Further, the expression levels of inflammatory-pain-related factors, including c-Fos, phosphorylated extracellular signal-regulated kinase (p-ERK), and phosphorylated n-methyl-D-aspartate receptor subunit 2B (p-NR2B), were significantly elevated in the spinal dorsal neurons of formalin-treated WT mice but was not elevated in NFAT5-Het mice. Similarly, c-Fos, p-ERK, and p-NR2B levels were significantly higher in glutamate-treated PC12 neuronal cells but were not affected by Nfat5 silencing in glutamate-treated PC12 cells. Altogether, our findings suggest that NFAT5 deficiency may mitigate formalin-induced inflammatory pain by upregulating mammalian target of rapamycin (mTOR) expression and downregulating its downstream factors in spinal dorsal neurons. Therefore, NFAT5 is a potential therapeutic target for the treatment of inflammatory pain.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

The role of peripheral opioid receptors in orofacial pain

Opioid receptors are widely distributed in the central and peripheral nervous systems and non-neuronal tissues. Numerous researchers have noted the pivotal role of peripheral opioid receptors (PORs) in analgesia. Accumulating evidence has shown the existence of PORs in the trigeminal nerve system, indicating that PORs may be involved in the modulation of orofacial pain. In this review, we summarise the recent evidence for the role of PORs in orofacial pain and discuss the possible cellular mechanisms.