Neuropeptides Involved in Facial Nerve Regeneration

Expression Patterns of SMAD1-8 in the Peripheral Facial Nerve Following Compressive Nerve Injury or Axotomy

Facial nerve injury can lead to significant functional impairment, emotional impacts, and difficulties in social and economic activities. Although peripheral nerves have the potential for recovery, incomplete regeneration can pose challenges. Suppressor of Mothers Against Decapentaplegic Homolog (SMAD) proteins are crucial in the nerve-regeneration process. The study aimed to investigate the changes in SMAD protein expression involved in peripheral nerve regeneration following facial nerve injury induced by compression or axotomy in a pre-clinical study conducted on Sprague Dawley rats. Facial nerve recovery was assessed at 1, 2, 3, 4, 8, and 12 weeks post-facial nerve compression and axotomy using behavioral tests, including whisker movement and eyelid blink-reflex tests. Additionally, the role of SMAD proteins in the nerve regeneration process was evaluated by analyzing the expression of SMAD1-8 proteins at 2 and 12 weeks post-injury. Behavioral tests revealed significant impairment in facial nerve function in both the Compression and Axotomy groups compared with the Sham group at early time points. Recovery was observed in the Compression group by 2 weeks, whereas the Axotomy group exhibited prolonged impairment through 12 weeks. SMAD protein analyses showed increased expression of SMAD2, SMAD7, and SMAD8 following compression injury, whereas axotomy led to more extensive increases in expression that included SMAD1, SMAD2, SMAD3, SMAD4, SMAD6, SMAD7, and SMAD8. These findings suggest that SMAD proteins play differential roles in nerve regeneration following facial nerve injuries caused by compression versus axotomy. The distinct expression patterns of SMAD proteins highlight their potential as therapeutic targets for enhancing nerve regeneration and functional recovery in peripheral nerve injuries.

Neuronal TRPV1-CGRP axis regulates peripheral nerve regeneration through ERK/HIF-1 signaling pathway

Severe trauma frequently leads to nerve damage. Peripheral nerves possess a degree of regenerative ability, and actively promoting their recovery can help restore the sensory and functional capacities of tissues. The neuropeptide calcitonin gene-related peptide (CGRP) is believed to regulate the repair of injured peripheral nerves, with neuronal transient receptor potential vanilloid type 1 (TRPV1) potentially serving as a crucial upstream factor. In this study, we established a mouse model of sciatic nerve (SN) crush injury and found that intrathecal injection of capsaicin (Cap) activated the neuronal TRPV1-CGRP axis, thereby promoting SN repair. Conversely, the application of capsazepine (Cpz), which inhibits the neuronal TRPV1-CGRP axis, delayed SN repair. Local restoration of CGRP expression at the injury site enhanced the repair process. In vitro experiments, we employed the rat Schwann cell (SC) line RSC96 to establish an indirect co-culture model of neurons and SCs. We observed that the proliferation, migration, expression of myelination-associated proteins, and neurotrophic secretion functions of RSC96 cells are positively correlated with the degree of activation of neuronal TRPV1. Inhibition of neuronal TRPV1, followed by the restoration of CGRP levels, improved these functions in RSC96 cells. Furthermore, activation of the neuronal TRPV1-CGRP axis resulted in an upregulation of extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation levels and an increase in hypoxia-inducible factor 1α (HIF-1α) accumulation in RSC96 cells, thereby promoting their proliferation and migration. In summary, this study demonstrates that neuronal TRPV1-CGRP axis can regulate biological behavior of SCs and axon regeneration by activating the ERK/HIF-1 signaling pathway following peripheral nerve injury. This finding clarifies the role of CGRP in neuroregulatory networks and provides a novel reference point for the development of drugs and biomaterials for treating nerve damage.

Reduced Gene Expression of KCC2 Accelerates Axonal Regeneration and Reduces Motor Dysfunctions after Tibial Nerve Severance and Suturing

Gamma-aminobutyric acid and glycine (GABA/Gly) are predominantly inhibitory neurotransmitters in the mature central nervous system; however, they mediate membrane potential depolarization during development. These differences in actions depend on intracellular Cl- concentrations ([Cl-]i), which are primarily regulated by potassium chloride cotransporter 2 (KCC2). After nerve injury, KCC2 expression markedly decreases and GABA/Gly mediate depolarization. Following nerve regeneration, KCC2 expression recovers and GABA/Gly become inhibitory, suggesting that KCC2 reduction and GABA/Gly excitation may be crucial for axonal regeneration. To directly clarify their involvement in regeneration, we analyzed recovery processes after tibial nerve severance and suturing between heterozygous KCC2 knockout mice (HT), whose KCC2 levels are halved, and their wild-type littermates (WT). Compared with WT mice, the sciatic functional index-indicating lower limb motor function-was significantly higher until 28 days after operation (D28) in HT mice. Furthermore, at D7, many neurofilament-positive fibers were elongated into the distal part of the sutured nerve in HT mice only, and myelinated axonal density was significantly higher at D21 and D28 in HT animals. Electron microscopy and galanin immunohistochemistry indicated a shorter nerve degeneration period in HT mice. Moreover, a less severe decrease in choline acetyltransferase was observed in HT mice. These results suggest that nerve degeneration and regeneration proceed more rapidly in HT mice, resulting in milder motor dysfunction. Via similar microglial activation, nerve surgery may reduce KCC2 levels more rapidly in HT mice, followed by earlier increased [Cl-]i and longer-lasting GABA/Gly excitation. Taken together, reduced KCC2 may accelerate nerve regeneration via GABA/Gly excitation.

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.

Central Facial Nervous System Biomolecules Involved in Peripheral Facial Nerve Injury Responses and Potential Therapeutic Strategies

Peripheral facial nerve injury leads to changes in the expression of various neuroactive substances that affect nerve cell damage, survival, growth, and regeneration. In the case of peripheral facial nerve damage, the injury directly affects the peripheral nerves and induces changes in the central nervous system (CNS) through various factors, but the substances involved in these changes in the CNS are not well understood. The objective of this review is to investigate the biomolecules involved in peripheral facial nerve damage so as to gain insight into the mechanisms and limitations of targeting the CNS after such damage and identify potential facial nerve treatment strategies. To this end, we searched PubMed using keywords and exclusion criteria and selected 29 eligible experimental studies. Our analysis summarizes basic experimental studies on changes in the CNS following peripheral facial nerve damage, focusing on biomolecules that increase or decrease in the CNS and/or those involved in the damage, and reviews various approaches for treating facial nerve injury. By establishing the biomolecules in the CNS that change after peripheral nerve damage, we can expect to identify factors that play an important role in functional recovery from facial nerve damage. Accordingly, this review could represent a significant step toward developing treatment strategies for peripheral facial palsy.