Antinociceptive effects of novel epibatidine analogs through activation of α4β2 nicotinic receptors

Exploring Cholinergic Compounds for Peripheral Neuropathic Pain Management: A Comprehensive Scoping Review of Rodent Model Studies

Neuropathic pain affects about 7-8% of the population, and its management still poses challenges with unmet needs. Over the past decades, researchers have explored the cholinergic system (muscarinic and nicotinic acetylcholine receptors: mAChR and nAChR) and compounds targeting these receptors as potential analgesics for neuropathic pain management. This scoping review aims to provide an overview of studies on peripheral neuropathic pain (PNP) in rodent models, exploring compounds targeting cholinergic neurotransmission. The inclusion criteria were original articles on PNP in rodent models that explored the use of compounds directly targeting cholinergic neurotransmission and reported results of nociceptive behavioral assays. The literature search was performed in the PubMed and Web of Science databases (1 January 2000-22 April 2023). The selection process yielded 82 publications, encompassing 62 compounds. The most studied compounds were agonists of α4β2 nAChR and α7 nAChR, and antagonists of α9/α10 nAChR, along with those increasing acetylcholine and targeting mAChRs. Studies mainly reported antinociceptive effects in traumatic PNP models, and to a lesser extent, chemotherapy-induced neuropathy or diabetic models. These preclinical studies underscore the considerable potential of cholinergic compounds in the management of PNP, warranting the initiation of clinical trials.

Synthesis of substituted pyridines with diverse functional groups via the remodeling of (Aza)indole/Benzofuran skeletons

Substituted pyridines with diverse functional groups are important structural motifs found in numerous bioactive molecules. Several methodologies for the introduction of various bio-relevant functional groups to pyridine have been reported, but there is still a need for a single robust method allowing the selective introduction of multiple functional groups. This study reports a ring cleavage methodology reaction for the synthesis of 2-alkyl/aryl 3-electron-withdrawing groups (esters, sulfones, and phosphonates) 5-aminoaryl/phenol pyridines via the remodeling of 3-formyl (aza)indoles/benzofurans. Totally ninety-three 5-aminoaryl pyridines and thirty-three 5-phenol pyridines were synthesized showing the robustness of the developed methodology. The application of this methodology further provided a privileged pyridine scaffold containing biologically relevant molecules and direct drug/natural product conjugation with ethyl 2-methyl nicotinate.

BDNF-TrkB signaling pathway-mediated microglial activation induces neuronal KCC2 downregulation contributing to dynamic allodynia following spared nerve injury

Mechanical allodynia can be evoked by punctate pressure contact with the skin (punctate mechanical allodynia) and dynamic contact stimulation induced by gentle touching of the skin (dynamic mechanical allodynia). Dynamic allodynia is insensitive to morphine treatment and is transmitted through the spinal dorsal horn by a specific neuronal pathway, which is different from that for punctate allodynia, leading to difficulties in clinical treatment. K+-Cl- cotransporter-2 (KCC2) is one of the major determinants of inhibitory efficiency, and the inhibitory system in the spinal cord is important in the regulation of neuropathic pain. The aim of the current study was to determine whether neuronal KCC2 is involved in the induction of dynamic allodynia and to identify underlying spinal mechanisms involved in this process. Dynamic and punctate allodynia were assessed using either von Frey filaments or a paint brush in a spared nerve injury (SNI) mouse model. Our study discovered that the downregulated neuronal membrane KCC2 (mKCC2) in the spinal dorsal horn of SNI mice is closely associated with SNI-induced dynamic allodynia, as the prevention of KCC2 downregulation significantly suppressed the induction of dynamic allodynia. The over activation of microglia in the spinal dorsal horn after SNI was at least one of the triggers in SNI-induced mKCC2 reduction and dynamic allodynia, as these effects were blocked by the inhibition of microglial activation. Finally, the BDNF-TrkB pathway mediated by activated microglial affected SNI-induced dynamic allodynia through neuronal KCC2 downregulation. Overall, our findings revealed that activation of microglia through the BDNF-TrkB pathway affected neuronal KCC2 downregulation, contributing to dynamic allodynia induction in an SNI mouse model.

Pentameric Ligand-Gated Ion Channels as Pharmacological Targets Against Chronic Pain

Chronic pain is a common detrimental condition that affects around 20% of the world population. The current drugs to treat chronic pain states, especially neuropathic pain, have a limited clinical efficiency and present significant adverse effects that complicates their regular use. Recent studies have proposed new therapeutic strategies focused on the pharmacological modulation of G-protein-coupled receptors, transporters, enzymes, and ion channels expressed on the nociceptive pathways. The present work intends to summarize recent advances on the pharmacological modulation of pentameric ligand-gated ion channels, which plays a key role in pain processing. Experimental data have shown that novel allosteric modulators targeting the excitatory nicotinic acetylcholine receptor, as well as the inhibitory GABA_A and glycine receptors, reverse chronic pain-related behaviors in preclinical assays. Collectively, these evidences strongly suggest the pharmacological modulation of pentameric ligand-gated ion channels is a promising strategy towards the development of novel therapeutics to treat chronic pain states in humans.

Memantine selectively prevented the induction of dynamic allodynia by blocking Kir2.1 channel and inhibiting the activation of microglia in spinal dorsal horn of mice in spared nerve injury model

Background

Memantine is one of the important clinical medications in treating moderate to severe Alzheimer disease. The effect of memantine on preventing or treating punctate allodynia has been thoroughly studied but not on the induction of dynamic allodynia. The aim of this study is to investigate whether memantine could prevent the induction of dynamic allodynia and its underlying spinal mechanisms.

Results

(1) In in vivo spared nerve injury pain model, pretreatment with memantine at a lower dose (10 nmol, intrathecal; memantine-10) selectively prevented the induction of dynamic allodynia but not the punctate allodynia. (2) Pretreatment with either MK801-10 (MK801-10 nmol, intrathecal) or higher dose of memantine (30 nmol, intrathecal; memantine-30) prevented the induction of both dynamic and punctate allodynia. (3) Memantine-10 showed significant effect on the inhibition of the spared nerve injury-induced overactivation of microglia in spinal dorsal horn. (4) In contrast, in complete freund′s adjuvant (CFA) model, memantine-10 neither affected the CFA injection-induced activation of microglia in spinal dorsal horn nor the induction of dynamic allodynia. (5) Immunohistological studies showed Kir2.1 channel distributed widely and co-localized with microglia in the spinal dorsal horn of mice. (6) Pretreatment with either minocycline, a microglia inhibitor, or ML133, a Kir2.1 inhibitor, both selectively prevented the overactivation of microglia in spinal dorsal horn and the induction of dynamic allodynia following spared nerve injury.

Conclusion

The selective inhibitory effect on the induction of dynamic allodynia in spared nerve injury model by low dose of the memantine (memantine-10) was tightly correlated with the blockade of microglia Kir2.1 channel to suppress the microglia activation.