Terminals Of Primary Sensory Neurons Research Articles

Peripheral Branch Injury Induces Oxytocin Receptor Expression at the Central Axon Terminals of Primary Sensory Neurons.

Considerable evidence suggests that oxytocin, as a regulatory nonapeptide, participates in modulatory mechanisms of nociception. Nonetheless, the role of this hypothalamic hormone and its receptor in the sensory pathway has yet to be fully explored. The present study performed immunohistochemistry, enzyme-linked immunosorbent assay, and RT-qPCR analysis to assess changes in the expression of the neuronal oxytocin receptor in female rats following tight ligation of the sciatic nerve after 1, 3, and 7 days of survival. Oxytocin receptor immunoreactivity was present in both dorsal root ganglia and lumbar spinal cord segments, but not accumulated at the site of the ligation of the peripheral nerve branch. We found a time-dependent change in the expression of oxytocin receptor mRNA in L5 dorsal root ganglion neurons, as well as an increase in the level of the receptor protein in the lumbar segment of the spinal cord. A peak in the expression was observed on day 3, which downturned slightly by day 7 after the nerve ligation. These results show that OTR expression is up-regulated in response to peripheral nerve lesions. We assume that the importance of OTR is to modify spinal presynaptic inputs of the sensory neurons upon injury-induced activation, thus to be targets of the descending oxytocinergic neurons from supraspinal levels. The findings of this study support the concept that oxytocin plays a role in somatosensory transmission.

Phenotyping Post-COVID Pain as a Nociceptive, Neuropathic, or Nociplastic Pain Condition.

Pain after an acute Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection and coronavirus disease 2019 (COVID-19) condition (post-COVID pain) is becoming a new healthcare emergency. Precision medicine refers to an evidence-based method of grouping patients based on their diagnostic/symptom presentation and then tailoring specific treatments accordingly. Evidence suggests that post-COVID pain can be categorized as nociceptive (i.e., pain attributable to the activation of the peripheral receptive terminals of primary afferent neurons in response to noxious chemical, mechanical, or thermal stimuli), neuropathic (i.e., pain associated with a lesion or disease of the somatosensory nervous system and limited to a “neuroanatomically plausible” distribution of the system), nociplastic (i.e., pain arising from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain), or mixed type (when two pain phenotypes co-exist). Each of these pain phenotypes may require a different treatment approach to maximize treatment effectiveness. Accordingly, the ability to classify post-COVID pain patients into one of these phenotypes would likely be critical for producing successful treatment outcomes. The 2021 International Association for the Study of Pain (IASP) clinical criteria and grading system provide a framework for classifying pain within a precision pain medicine approach. Here we present data supporting the possibility of grouping patients with post-COVID pain into pain phenotypes, using the 2021 IASP classification criteria, with a specific focus on nociplastic pain, which is probably the primary mechanism involved in post-COVID pain. Nociplastic pain, which is usually associated with comorbid symptomology (e.g., poor sleep quality, fatigue, cognitive–emotional disturbances, etc.) and is considered to be more difficult to treat than other pain types, may require a more nuanced multimodal treatment approach to achieve better treatment outcomes.

Thieno[2,3-d]pyrimidine-Based Positive Allosteric Modulators of Human Mas-Related G Protein-Coupled Receptor X1 (MRGPRX1).

Mas-related G protein-coupled receptor X1 (MRGPRX1) is a human sensory neuron-specific receptor and potential target for the treatment of pain. Positive allosteric modulators (PAMs) of MRGPRX1 have the potential to preferentially activate the receptors at the central terminals of primary sensory neurons and minimize itch side effects caused by peripheral activation. Using a high-throughput screening (HTS) hit, a series of thieno[2,3-d]pyrimidine-based molecules were synthesized and evaluated as human MRGPRX1 PAMs in HEK293 cells stably transfected with human MrgprX1 gene. An iterative process to improve potency and metabolic stability led to the discovery of orally available 6-(tert-butyl)-5-(3,4-dichlorophenyl)-4-(2-(trifluoromethoxy)phenoxy)thieno[2,3-d]pyrimidine (1t), which can be distributed to the spinal cord, the presumed site of action, following oral administration. In a neuropathic pain model induced by sciatic nerve chronic constriction injury (CCI), compound 1t (100 mg/kg, po) reduced behavioral heat hypersensitivity in humanized MRGPRX1 mice, demonstrating the therapeutic potential of MRGPRX1 PAMs in treating neuropathic pain.

Presynaptic NMDA receptors control nociceptive transmission at the spinal cord level in neuropathic pain.

Chronic neuropathic pain is a debilitating condition that remains challenging to treat. Glutamate N-methyl-D-aspartate receptor (NMDAR) antagonists have been used to treat neuropathic pain, but the exact sites of their actions have been unclear until recently. Although conventionally postsynaptic, NMDARs are also expressed presynaptically, particularly at the central terminals of primary sensory neurons, in the spinal dorsal horn. However, presynaptic NMDARs in the spinal cord are normally quiescent and are not actively involved in physiological nociceptive transmission. In this review, we describe the emerging role of presynaptic NMDARs at the spinal cord level in chronic neuropathic pain and the implications of molecular mechanisms for more effective treatment. Recent studies indicate that presynaptic NMDAR activity at the spinal cord level is increased in several neuropathic pain conditions but not in chronic inflammatory pain. Increased presynaptic NMDAR activity can potentiate glutamate release from primary afferent terminals to spinal dorsal horn neurons, which is crucial for the synaptic plasticity associated with neuropathic pain caused by traumatic nerve injury and chemotherapy-induced peripheral neuropathy. Furthermore, α2δ-1, previously considered a calcium channel subunit, can directly interact with NMDARs through its C-terminus to increase presynaptic NMDAR activity by facilitating synaptic trafficking of α2δ-1-NMDAR complexes in neuropathic pain caused by chemotherapeutic agents and peripheral nerve injury. Targeting α2δ-1-bound NMDARs with gabapentinoids or α2δ-1 C-terminus peptides can attenuate nociceptive drive form primary sensory nerves to dorsal horn neurons in neuropathic pain.

Local NGF and GDNF levels modulate morphology and function of porcine DRG neurites, In Vitro.

Nerve terminals of primary sensory neurons are influenced by their environment through target derived trophic factors, like nerve growth factor (NGF) or glial cell line-derived neurotrophic factor (GDNF). In mice, subpopulations of DRG neurons express receptors either for NGF or GDNF and therefore differentially respond to these neurotrophic factors. We probed neurite endings from porcine DRG neurons cultured in either NGF or GDNF and examined their shape, elongation and stimulus-evoked CGRP release. A compartmentalized culture system was employed allowing spatial separation of outgrown neurites from their somata and use of different growth factors in the compartments. We show that neurites of GDNF cultured somata extend into lateral compartments without added growth factor, unlike neurites of NGF cultured ones. Neurites of NGF cultured somata extend not only into NGF- but also into GDNF-containing compartments. GDNF at the site of terminals of NGF responsive somata led to a strong neurite arborization and formation of large growth cones, compared to neurites in medium with NGF. Functionally, we could detect evoked CGRP release from as few as 7 outgrown neurites per compartment and calculated release per mm neurite length. CGRP release was detected both in neurites from NGF and GDNF cultured somata, suggesting that also the latter ones are peptidergic in pig. When neurites of NGF cultured somata were grown in GDNF, capsaicin evoked a lower CGRP release than high potassium, compared to those grown in NGF. Our experiments demonstrate that the compartmented culture chamber can be a suitable model to assess neurite properties from trophic factor specific primary sensory neurons. With this model, insights into mechanisms of gain or loss of function of specific nociceptive neurites may be achieved.

Pathways of CGRP Release from Primary Sensory Neurons.

The benefit reported in a variety of clinical trials by a series of small molecule antagonists for the calcitonin gene-related peptide (CGRP) receptor, or four monoclonal antibodies against the neuropeptide or its receptor, has underscored the release of CGRP from terminals of primary sensory neurons, including trigeminal neurons, as one of the major mechanisms of migraine headaches. A large variety of excitatory ion channels and receptors have been reported to elicit CGRP release, thus proposing these agonists as migraine-provoking agents. On the other side, activators of inhibitory channels and receptors may be regarded as potential antimigraine agents. The knowledge of the intracellular pathways underlying the exocytotic process that results in CGRP secretion or its inhibition is, therefore, of importance for understanding how migraine pain originates and how to treat the disease.

The anti-migraine component of butterbur extracts, isopetasin, desensitizes peptidergic nociceptors by acting on TRPA1 cation channel.

The mechanism of the anti-migraine action of extracts of butterbur [Petasites hybridus (L.) Gaertn.] is unknown. Here, we investigated the ability of isopetasin, a major constituent of these extracts, to specifically target TRPA1 channel and to affect functional responses relevant to migraine. Single-cell calcium imaging and patch-clamp recordings in human and rodent TRPA1-expressing cells, neurogenic motor responses in rodent isolated urinary bladder, release of CGRP from mouse spinal cord in vitro and facial rubbing in mice and meningeal blood flow in rats were examined. Isopetasin induced (i) calcium responses and currents in rat/mouse trigeminal ganglion (TG) neurons and in cells expressing the human TRPA1, (ii) substance P-mediated contractions of rat isolated urinary bladders and (iii) CGRP release from mouse dorsal spinal cord, responses that were selectively abolished by genetic deletion or pharmacological antagonism of TRPA1 channels. Pre-exposure to isopetasin produced marked desensitization of allyl isothiocyanate (AITC, TRPA1 channel agonist)- or capsaicin (TRPV1 channel agonist)-evoked currents in rat TG neurons, contractions of rat or mouse bladder and CGRP release from mouse central terminals of primary sensory neurons. Repeated intragastric administration of isopetasin attenuated mouse facial rubbing, evoked by local AITC or capsaicin, and dilation of rat meningeal arteries by acrolein or ethanol (TRPA1 and TRPV1 channel agonists respectively). Activation of TRPA1 channels by isopetasin results in excitation of neuropeptide-containing nociceptors, followed by marked heterologous neuronal desensitization. Such atten uation in pain and neurogenic inflammation may account for the anti-migraine action of butterbur.

A novel 3-(4,5-diphenyl-1,3-oxazol-2-yl)propanal oxime compound is a potent Transient Receptor Potential Ankyrin 1 and Vanilloid 1 (TRPA1 and V1) receptor antagonist

Transient Receptor Potential Ankyrin 1 and Vanilloid 1 (TRPA1, TRPV1) ion channels expressed on nociceptive primary sensory neurons are important regulators of pain and inflammation. TRPA1 is activated by several inflammatory mediators including formaldehyde and methylglyoxal that are products of the semicarbazide-sensitive amine-oxidase enzyme (SSAO). SZV-1287 is a new 3-(4,5-diphenyl-1,3-oxazol-2-yl)propanal oxime SSAO inhibitor, its chemical structure is similar to other oxime derivatives described as TRPA1 antagonists. Therefore, we investigated its effects on TRPA1 and TRPV1 receptor activation on the cell bodies and peripheral terminals of primary sensory neurons and TRPA1 or TRPV1 receptor-expressing cell lines.Calcium influx in response to the TRPA1 agonist allyl-isothiocyanate (AITC) (200μM) and the TRPV1 stimulator capsaicin (330nM) in rat trigeminal neurons or TRPA1 and TRPV1 receptor-expressing cell lines was measured by microfluorimetry or radioactive 45Ca2+ uptake experiments. Calcitonin gene-related peptide (CGRP) release as the indicator of 100μM AITC – or 100nM capsaicin-induced peripheral sensory nerve terminal activation was measured by radioimmunoassay.SZV-1287 (100, 500 and 1000nM) exerted a concentration-dependent significant inhibition on both AITC- and capsaicin-evoked calcium influx in trigeminal neurons and TRPA1 or TRPV1 receptor-expressing cell lines. It also significantly inhibited the TRPA1, but not the TRPV1 activation-induced CGRP release from the peripheral sensory nerve endings in a concentration-dependent manner. In contrast, the reference SSAO inhibitor LJP 1207 with a different structure had no effect on TRPA1 or TRPV1 activation in either model system.This is the first evidence that our novel oxime compound SZV-1287 originally developed as a SSAO inhibitor has a potent dual antagonistic action on TRPA1 and TRPV1 ion channels on primary sensory neurons.

Subcortical barrelette-like and barreloid-like structures in the prosimian galago (Otolemur garnetti)

Galagos are prosimian primates that resemble ancestral primates more than most other extant primates. As in many other mammals, the facial vibrissae of galagos are distributed across the upper and lower jaws and above the eye. In rats and mice, the mystacial macrovibrissae are represented throughout the ascending trigeminal pathways as arrays of cytoarchitecturally distinct modules, with each module having a nearly one-to-one relationship with a specific facial whisker. The macrovibrissal representations are termed barrelettes in the trigeminal somatosensory brainstem, barreloids in the ventroposterior medial subnucleus of the thalamus, and barrels in primary somatosensory cortex. Despite the presence of facial whiskers in all nonhuman primates, barrel-like structures have not been reported in primates. By staining for cytochrome oxidase, Nissl, and vesicular glutamate transporter proteins, we show a distinct array of barrelette-like and barreloid-like modules in the principal sensory nucleus, the spinal trigeminal nucleus, and the ventroposterior medial subnucleus of the galago, Otolemur garnetti. Labeled terminals of primary sensory neurons in the brainstem and cell bodies of thalamocortically projecting neurons demonstrate that barrelette-like and barreloid-like modules are located in areas of these somatosensory nuclei that are topographically consistent with their role in facial touch. Serendipitously, the plane of section that best displays the barreloid-like modules reveals a remarkably distinct homunculus-like patterning which, we believe, is one of the clearest somatotopic maps of an entire body surface yet found.

Noradrenergic modulation of itch transmission in the spinal cord.

Inhibition of both itching and scratching is important in the treatment of chronic pruritic diseases, because itching has a negative impact on quality of life and vigorous scratching worsens skin conditions. Pharmacological modulation of itch transmission in the dorsal horn is an effective way to inhibit both itching and scratching in pruritic diseases. Pruriceptive transmission in the spinal dorsal horn undergoes inhibitory modulation by the descending noradrenergic system. The noradrenergic inhibition is mediated by excitatory α₁-adrenoceptors located on inhibitory interneurons and inhibitory α₂-adrenoceptors located on central terminals of primary sensory neurons. The descending noradrenergic system and α-adrenoceptors in the dorsal horn are potential targets for antipruritic drugs.

MrgC agonism at central terminals of primary sensory neurons inhibits neuropathic pain

Chronic neuropathic pain is often refractory to current pharmacotherapies. The rodent Mas-related G-protein-coupled receptor subtype C (MrgC) shares substantial homogeneity with its human homologue, MrgX1, and is located specifically in small-diameter dorsal root ganglion neurons. However, evidence regarding the role of MrgC in chronic pain conditions has been disparate and inconsistent. Accordingly, the therapeutic value of MrgX1 as a target for pain treatment in humans remains uncertain. Here, we found that intrathecal injection of BAM8-22 (a 15-amino acid peptide MrgC agonist) and JHU58 (a novel dipeptide MrgC agonist) inhibited both mechanical and heat hypersensitivity in rats after an L5 spinal nerve ligation (SNL). Intrathecal JHU58-induced pain inhibition was dose dependent in SNL rats. Importantly, drug efficacy was lost in Mrg-cluster gene knockout (Mrg KO) mice and was blocked by gene silencing with intrathecal MrgC siRNA and by a selective MrgC receptor antagonist in SNL rats, suggesting that the drug action is MrgC dependent. Further, in a mouse model of trigeminal neuropathic pain, microinjection of JHU58 into ipsilateral subnucleus caudalis inhibited mechanical hypersensitivity in wild-type but not Mrg KO mice. Finally, JHU58 attenuated the miniature excitatory postsynaptic currents frequency both in medullary dorsal horn neurons of mice after trigeminal nerve injury and in lumbar spinal dorsal horn neurons of mice after SNL. We provide multiple lines of evidence that MrgC agonism at spinal but not peripheral sites may constitute a novel pain inhibitory mechanism that involves inhibition of peripheral excitatory inputs onto postsynaptic dorsal horn neurons in different rodent models of neuropathic pain.

Activity-dependent silencing reveals functionally distinct itch-generating sensory neurons

The peripheral terminals of primary sensory neurons detect histamine and non-histamine itch-provoking ligands through molecularly distinct transduction mechanisms. It remains unclear, however, whether these distinct pruritogens activate the same or different afferent fibers. We utilized a strategy of reversibly silencing specific subsets of murine pruritogen-sensitive sensory axons by targeted delivery of a charged sodium-channel blocker and found that functional blockade of histamine itch did not affect the itch evoked by chloroquine or SLIGRL-NH2, and vice versa. Notably, blocking itch-generating fibers did not reduce pain-associated behavior. However, silencing TRPV1+ or TRPA1+ neurons allowed AITC or capsaicin respectively to evoke itch, implying that certain peripheral afferents may normally indirectly inhibit algogens from eliciting itch. These findings support the presence of functionally distinct sets of itch-generating neurons and suggest that targeted silencing of activated sensory fibers may represent a clinically useful anti-pruritic therapeutic approach for histaminergic and non-histaminergic pruritus.

Cyclin-Dependent Kinase 5 Controls TRPV1 Membrane Trafficking and the Heat Sensitivity of Nociceptors through KIF13B

The number of functional transient receptor potential vanilloid 1 (TRPV1) channels at the surface, especially at the peripheral terminals of primary sensory neurons, regulates heat sensitivity, and increased surface localization of TRPV1s contributes to heat hyperalgesia. However, the mechanisms for regulating TRPV1 surface localization are essentially unknown. Here, we show that cyclin-dependent kinase 5 (Cdk5), a new player in thermal pain sensation, positively regulates TRPV1 surface localization. Active Cdk5 was found to promote TRPV1 anterograde transport in vivo, suggesting a regulatory role of Cdk5 in TRPV1 membrane trafficking. TRPV1-containing vesicles bind to the forkhead-associated (FHA) domain of the KIF13B (kinesin-3 family member 13B) and are thus delivered to the cell surface. Overexpression of Cdk5 or its activator p35 promoted and inhibition of Cdk5 activity prevented the KIF13B-TRPV1 association, indicating that Cdk5 promotes TRPV1 anterograde transport by mediating the motor-cargo association. Cdk5 phosphorylates KIF13B at Thr-506, a residue located in the FHA domain. T506A mutation reduced the motor-cargo interaction and the cell-permeable TAT-T506 peptide, targeting to the Thr-506, decreased TRPV1 surface localization, demonstrating the essential role of Thr-506 phosphorylation in TRPV1 transport. Moreover, complete Freund's adjuvant (CFA) injection-induced activation of Cdk5 increased the anterograde transport of TRPV1s, contributing to the development and possibly the maintenance of heat hyperalgesia, whereas intrathecal delivery of the TAT-T506 peptide alleviated CFA-induced heat hyperalgesia in rats. Thus, Cdk5 regulation of TRPV1 membrane trafficking is a fundamental mechanism controlling the heat sensitivity of nociceptors, and moderate inhibition of Thr-506 phosphorylation during inflammation might be helpful for the treatment of inflammatory thermal pain.

Chronic Opioid Potentiates Presynaptic but Impairs Postsynaptic N-Methyl-d-aspartic Acid Receptor Activity in Spinal Cords: IMPLICATIONS FOR OPIOID HYPERALGESIA AND TOLERANCE

Chronic Opioid Potentiates Presynaptic but Impairs Postsynaptic N-Methyl-d-aspartic Acid Receptor Activity in Spinal Cords: IMPLICATIONS FOR OPIOID HYPERALGESIA AND TOLERANCE

Mechanisms-based classifications of musculoskeletal pain: Part 3 of 3: Symptoms and signs of nociceptive pain in patients with low back (±leg) pain

As a mechanisms-based classification of pain ‘nociceptive pain’ (NP) refers to pain attributable to the activation of the peripheral receptive terminals of primary afferent neurones in response to noxious chemical, mechanical or thermal stimuli. The symptoms and signs associated with clinical classifications of NP have not been extensively studied. The purpose of this study was to identify symptoms and signs associated with a clinical classification of NP in patients with low back (±leg) pain.Using a cross-sectional, between-subjects design; four hundred and sixty-four patients with low back (±leg) pain were assessed using a standardised assessment protocol after which their pain was assigned a mechanisms-based classification based on experienced clinical judgement. Clinicians then completed a clinical criteria checklist indicating the presence/absence of various symptoms and signs.A regression analysis identified a cluster of seven clinical criteria predictive of NP, including: ‘Pain localised to the area of injury/dysfunction’, ‘Clear, proportionate mechanical/anatomical nature to aggravating and easing factors’, ‘Usually intermittent and sharp with movement/mechanical provocation; may be a more constant dull ache or throb at rest’, and the absence of ‘Pain in association with other dysesthesias’, ‘Night pain/disturbed sleep’, ‘Antalgic postures/movement patterns’ and ‘Pain variously described as burning, shooting, sharp or electric-shock-like’. This cluster was found to have high levels of classification accuracy (sensitivity 90.9%, 95% CI: 86.6–94.1; specificity 91.0%, 95% CI: 86.1–94.6).Pattern recognition of this empirically-derived cluster of symptoms and signs may help clinicians identify an assumed dominance of NP mechanisms in patients with low back pain disorders.

Roles of Glia, Immune Cells and the Thermo-TRP Channels, TRPV1, TRPA1 and TRPM8, in Pathological Pain

Studies into the interactions between glia/immune cells and neurons have focused on the induction of patho-logical (neuropathic or inflammatory) pain. Growing evidence of close relationships between peripheral and central glia and pathological pain has emerged during the last 2 decades. Numerous experimental studies have showed the release of cytokines and inflammatory neuropeptides from peripheral and central terminals of primary sensory neurons and from ac-tivated peripheral and central glia after nerve injury (crush, ligation or transection), which in turn act in a paracrine or autocrine manner. Cytokines induce the synthesis of algogens (pain-inducing substances such as prostaglandin) which leads to the primary (peripheral) or secondary (central) sensitization responsible for hyperalgesia or allodynia under in-flammatory conditions. The review has also highlighted the role of thermo transient receptor potential (TRP) channels TRPV1, TRPA1 and TRPM8 in the induction of pathological pain. The noxious heat sensor TRPV1 has an overt role in noxious heat hyperalgesia or allodynia, whereas TRPA1 and TRPM8 seem to have roles in noxious cold or mechanical al-lodynia, although results are inconsistent. Close mutual interrelationships between immune and glial cells and thermoTRP channels via cytokines or pro-inflammatory neuropeptides cannot be ignored when attempting to explain the induction and continuation of pathological pain. Investigations on the initial signals sent to the central area (superficial dorsal horn) remote from injured (or infectious) sites are a key point to clarify the mechanisms of pathological pain.

Presynaptic α2-GABAAReceptors in Primary Afferent Depolarization and Spinal Pain Control

Spinal dorsal horn GABA(A) receptors are found both postsynaptically on central neurons and presynaptically on axons and/or terminals of primary sensory neurons, where they mediate primary afferent depolarization (PAD) and presynaptic inhibition. Both phenomena have been studied extensively on a cellular level, but their role in sensory processing in vivo has remained elusive, due to inherent difficulties to selectively interfere with presynaptic receptors. Here, we address the contribution of a major subpopulation of GABA(A) receptors (those containing the α2 subunit) to spinal pain control in mice lacking α2-GABA(A) receptors specifically in primary nociceptors (sns-α2(-/-) mice). sns-α2(-/-) mice exhibited GABA(A) receptor currents and dorsal root potentials of normal amplitude in vitro, and normal response thresholds to thermal and mechanical stimulation in vivo, and developed normal inflammatory and neuropathic pain sensitization. However, the positive allosteric GABA(A) receptor modulator diazepam (DZP) had almost completely lost its potentiating effect on PAD and presynaptic inhibition in vitro and a major part of its spinal antihyperalgesic action against inflammatory hyperalgesia in vivo. Our results thus show that part of the antihyperalgesic action of spinally applied DZP occurs through facilitated activation of GABA(A) receptors residing on primary nociceptors.

Spinal neuroplasticity in chronic pain

Abstract Neuroplastic changes play an important role in the generation and maintenance of chronic pain syndromes. Such changes occur at all levels of the neuraxis, from the peripheral terminals of primary sensory neurons to the cerebral cortex. Changes observed in the spinal dorsal horn in particular provide a mechanistic basis for many of the characteristics of chronic pain syndromes. While facilitated synaptic transmission between nociceptive fibers and spinal projection neurons contributes to enhanced perception of noxious stim­uli (hyperalgesia), diminished function of GA­BA-ergic and glycinergic interneurons not only induces hyperalgesia, but also triggers no­ciceptive reactions on exposure to innocuous stimuli and spontaneous pain behavior in the absence of any sensory stimulation. Spinal disinhibition thus recapitulates typical symptoms of chronic pathological pain syndromes. Studies performed by various groups over the last 10 years demonstrate that such spinal disinhibition occurs naturally in response to peripheral inflammation and nerve dam­age. The present article summarizes current status of this research.

Peripheral opioid receptor agonists for analgesia: A comprehensive review

It is established that opioid receptors are present in the dorsal root ganglia and the central as well as peripheral terminals ofprimary afferent neurons. Now, it has been shown that peripheral terminals of afferent nerves can be the sites of the intrinsic modulation of nociception and that opioid analgesia can be mediated by peripheral opioid receptors as well. This review focuses on two areas: the first on describing the peripheral opioidergic system, and the second on the review of the current state of development of peripherally active opioid receptor agonists with theirpotential clinical applications. Online and manual search using key words such as peripheral opioid receptors, peripheral (or peripherally restricted) opioid agonists, and peripheral mu-, kappa-, and delta-opioid receptor agonists, followed by full-text access and further cross-referencing. The obvious theoretical advantage of using these molecules is that analgesia is achieved while avoiding the bothersome-to-dangerous centrally mediated adverse effects of centrally acting opioids. Molecules knownum for their central action (e.g.,, morphine) have been used in peripheral tissues (joints, bone, teeth) with reasonable but varied success. Over the last 10-15 years, several molecules with peripherally restricted opioid agonist activity have been developed and several more are in the "clinical pipeline. "Although none is available as an approved medication till date, a few (e.g.; the peripherally restricted kappa-agonist FE200665, also known as CR665) have completed phase I clinical trials and currently in phase II. Others such as loperamide, which is approved for use as an antidiarrheal drug, have been found to be variably useful as a peripherally acting opioid analgesic. Substantive research is currently underway and this is an exciting research area for both basic and applied clinical fields. Various ways to enhance peripheral opioid analgesia are suggested.

Temporal Neuropathologic and Behavioral Phenotype of 6neo/6neoPompe Disease Mice

Pompe disease (glycogen storage disease II) is caused by mutations in the acid alpha-glucosidase gene. The most common form is rapidly progressive with glycogen storage, particularly in muscle, which leads to profound weakness, cardiac failure, and death by the age of 2 years. Although usually considered a muscle disease, glycogen storage also occurs in the CNS. We evaluated the progression of neuropathologic and behavioral abnormalities in a Pompe disease mouse model (6neo/6neo) that displays many features of the human disease. Homozygous mutant mice store excess glycogen within large neurons of hindbrain, spinal cord, and sensory ganglia by the age of 1 month; accumulations then spread progressively within many CNS cell types. "Silver degeneration" and Fluoro-Jade C stains revealed severe degeneration in axon terminals of primary sensory neurons at 3 to 9 months. These abnormalities were accompanied by progressive behavioral impairment on rotorod, wire hanging, and foot fault tests. The extensive neuropathologic alterations in this model suggest that therapy of skeletal and cardiac muscle disorders by systemic enzyme replacement therapy may not be sufficient to reverse functional deficits due to CNS glycogen storage, particularly early-onset, rapidly progressive disease. A better understanding of the basis for clinical manifestations is needed to correlate CNS pathology with Pompe disease manifestations.