P75-expressing elements are necessary for anti-allodynic effects of spinal clonidine and neostigmine

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.

A tropomyosine receptor kinase inhibitor blocks spinal neuroplasticity essential for the anti-hypersensitivity effects of gabapentin and clonidine in rats with peripheral nerve injury

Spinally released brain-derived nerve growth factor (BDNF) after nerve injury is essential to anatomic and functional changes in spinal noradrenergic and cholinergic systems, which are engaged or targeted by commonly used treatments for neuropathic pain. Since BDNF signals via tropomyosine receptor kinases (trks), we tested whether trk blockade by repeated spinal injection of the trk inhibitor K252a would reduce anatomical (spinal noradrenergic and cholinergic fiber density), functional (α2-adrenoceptor-mediated direct stimulation of spinal cholinergic terminals), and behavioral (anti-hypersensitivity from systemic gabapentin and spinal clonidine) plasticity, which depends on BDNF. Spinal K252a treatment did not alter hypersensitivity from spinal nerve ligation (SNL), but blocked the SNL-associated increase in dopamine-β-hydroxylase (DβH) fiber density in the spinal cord dorsal horn while reducing spinal choline acetyltransferase (ChAT)-immunoreactivity. K252a treatment also abolished the facilitatory effect of dexmedetomidine on KCl-evoked acetylcholine release in spinal cord synaptosomes and reduced the anti-hypersensitivity effects of oral gabapentin and spinal clonidine. These results suggest that spinal trk signaling is essential for the anatomic and functional plasticity in noradrenergic and cholinergic systems after nerve injury and consequently for the analgesia from drugs that rely on these systems.

PERSPECTIVE

Many drugs approved for neuropathic pain engage spinal noradrenergic and cholinergic systems for analgesia. This study demonstrates that spinal trk signaling after nerve injury is important to neuroplasticity of these systems, which is critical for the analgesic action of common treatments for neuropathic pain.

Pharmacological profiles of alpha 2 adrenergic receptor agonists identified using genetically altered mice and isobolographic analysis

Endogenous, descending noradrenergic fibers impose analgesic control over spinal afferent circuitry mediating the rostrad transmission of pain signals. These fibers target alpha 2 adrenergic receptors (alpha(2)ARs) on both primary afferent terminals and secondary neurons, and their activation mediates substantial inhibitory control over this transmission, rivaling that of opioid receptors which share a similar pattern of distribution. The terminals of primary afferent nociceptive neurons and secondary spinal dorsal horn neurons express alpha(2A)AR and alpha(2C)AR subtypes, respectively. Spinal delivery of these agents serves to reduce their side effects, which are mediated largely at supraspinal sites, by concentrating the drugs at the spinal level. Targeting these spinal alpha(2)ARs with one of five selective therapeutic agonists, clonidine, dexmedetomidine, brimonidine, ST91 and moxonidine, produces significant antinociception that can work in concert with opioid agonists to yield synergistic antinociception. Application of several genetically altered mouse lines had facilitated identification of the primary receptor subtypes that likely mediate the antinociceptive effects of these agents. This review provides first an anatomical description of the localization of the three subtypes in the central nervous system, second a detailed account of the pharmacological history of each of the six primary agonists, and finally a comprehensive report of the specific interactions of other GPCR agonists with each of the six principal alpha(2)AR agonists featured.

Neuronal nitric oxide synthase is upregulated in a subset of primary sensory afferents after nerve injury which are necessary for analgesia from alpha2-adrenoceptor stimulation

alpha2-Adrenoceptor (AR) agonists increase in analgesic potency and efficacy after peripheral nerve injury, and their effects are blocked by neuronal nitric oxide synthase (nNOS) inhibitors and M4 muscarinic receptor antagonists only after injury. We tested whether nNOS and M4 muscarinic receptors are co-expressed in the spinal cord, and whether destruction of a subset of sensory afferents which are essential to alpha2-AR analgesia would also destroy nNOS and M4 receptor expression. Male Sprague-Dawley rats underwent left L5 and L6 spinal nerve ligation. Lumbar spinal cord was removed and immunostained for M4 muscarinic receptors and nNOS alone and for co-expression. Others received intrathecal injection of saporin linked to an antibody to the neurotrophin receptor p75(NTR), which eliminates cells expressing this receptor and the analgesic effects of alpha2-AR agonists. nNOS staining of fibers in the superficial dorsal horn was dramatically increased after spinal nerve ligation, and this was abolished by saporin linked anti-p75(NTR) treatment. In contrast, nNOS staining in dorsal horn neurons was unaltered by these manipulations. M4 receptors were present on neurons in the dorsal horn, some of which co-expressed nNOS, but their pattern of expression was not altered by these manipulations. Peripheral nerve injury increases nNOS expression in fibers in the superficial dorsal horn, some of which likely express p75(NTR), and alpha2-AR agonists may reduce injury-induced sensitization by activation of nNOS in these fibers In contrast, changes in nNOS and M4 receptor location on spinal cord neurons are not responsible for increased analgesic potency of alpha2-AR agonists after nerve injury.

Acetaminophen recruits spinal p42/p44 MAPKs and GH/IGF-1 receptors to produce analgesia via the serotonergic system

The mechanism of action of acetaminophen is currently widely discussed. Direct inhibition of cyclooxygenase isoforms remains the commonly advanced hypothesis. We combined behavioral studies with molecular techniques to investigate the mechanism of action of acetaminophen in a model of tonic pain in rats. We show that acetaminophen indirectly stimulates spinal 5-hydroxytryptamine (5-HT)1A receptors in the formalin test, thereby increasing transcript and protein levels of low-affinity neurotrophin receptor, insulin-like growth factor-1 (IGF-1) receptor alpha subunit, and growth hormone receptor and reducing the amount of somatostatin 3 receptor (sst3R) mRNA. Those cellular events seem to be important for the antinociceptive activity of acetaminophen. Indeed, down-regulation of sst3R mRNA depends on acetaminophen-elicited, 5-HT1A receptor-dependent increase in neuronal extracellular signal-regulated kinase 1/2 (ERK1/2) activities that mediate antinociception. In addition, spinal growth hormone (GH) and IGF-1 receptors would also be involved in the antinociceptive activity of the analgesic at different degrees. Our results show the involvement of specific 5-HT1A receptor-dependent cellular events in acetaminophen-produced antinociception and consequently indicate that inhibition of cyclooxygenase activities is not the exclusive mechanism involved. Furthermore, we propose that the mechanisms of 5-HT1A receptor-elicited antinociception and the role of the spinal ERK1/2 pathway in nociception are more intricate than suspected so far and that the GH/IGF-1 axis is an interesting new player in the regulation of spinal nociception.

Targeted toxins in pain

Although only recently applied to the study of nociception, 'molecular neurosurgery', producing highly selective neural lesions using targeted cytotoxins, has proven a valuable tool for analysis of nociceptive systems and promises to yield much more information on the role of specific types of neurons in pain perception and possibly new pain therapies. Neuropeptide-toxin conjugates, particularly, substance P-saporin, have proven useful research tools and may find clinical applications. Targeting non-lethal moieties (enzymes, genes, viruses) also may prove useful for research and therapeutic purposes.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Alpha2 receptors and agonists in pain management

Alpha(2) agonists have been in clinical use for decades, primarily in the treatment of hypertension. In recent years, alpha(2) agonists have found wider application, particularly in the fields of anesthesia and pain management. It has been noted that these agents can enhance analgesia provided by traditional analgesics, such as opiates, and may result in opiate-sparing effects. This has important implications for the management of acute postoperative pain and chronic pain states, including disorders involving spasticity or myofascial pain, neuropathic pain, and chronic daily headaches. The clinical utility of these agents is ever expanding, as they are gaining broader use in neuraxial analgesia, and new applications are continuously under investigation. The alpha(2) agonists that are currently employed in anesthesia and pain management include clonidine, tizanidine, and dexmedetomidine. Moxonidine and radolmidine, which are not currently in clinical use in humans, may offer favorable side-effect profiles when compared with traditional alpha(2) agonists, and may thereby allow for more widespread pain management applications.