AAV-mediated gene transfer to colon-innervating primary afferent neurons

Investigation of neural circuits underlying visceral pain is hampered by the difficulty in achieving selective manipulations of individual circuit components. In this study, we adapted a dual AAV approach, used for projection-specific transgene expression in the CNS, to explore the potential for targeted delivery of transgenes to primary afferent neurons innervating visceral organs. Focusing on the extrinsic sensory innervation of the mouse colon, we first characterized the extent of dual transduction following intrathecal delivery of one AAV9 vector and intracolonic delivery of a second AAV9 vector. We found that if the two AAV9 vectors were delivered one week apart, dorsal root ganglion (DRG) neuron transduction by the second vector was greatly diminished. Following delivery of the two viruses on the same day, we observed colocalization of the transgenes in DRG neurons, indicating dual transduction. Next, we delivered intrathecally an AAV9 vector encoding the inhibitory chemogenetic actuator hM4D(Gi) in a Cre-recombinase dependent manner, and on the same day injected an AAV9 vector carrying Cre-recombinase in the colon. DRG expression of hM4D(Gi) was demonstrated at the mRNA and protein level. However, we were unable to demonstrate selective inhibition of visceral nociception following hM4D(Gi) activation. Taken together, these results establish a foundation for development of strategies for targeted transduction of primary afferent neurons for neuromodulation of peripheral neural circuits.

Long-term reversal of chronic pain behavior in rodents through elevation of spinal agmatine

Chronic pain remains a significant burden worldwide, and treatments are often limited by safety or efficacy. The decarboxylated form of L-arginine, agmatine, antagonizes N-methyl-d-aspartate receptors, inhibits nitric oxide synthase, and reverses behavioral neuroplasticity. We hypothesized that expressing the proposed synthetic enzyme for agmatine in the sensory pathway could reduce chronic pain without motor deficits. Intrathecal delivery of an adeno-associated viral (AAV) vector carrying the gene for arginine decarboxylase (ADC) prevented the development of chronic neuropathic pain as induced by spared nerve injury in mice and rats and persistently reversed established hypersensitivity 266 days post-injury. Spinal long-term potentiation was inhibited by both exogenous agmatine and AAV-human ADC (hADC) vector pre-treatment but was enhanced in rats treated with anti-agmatine immunoneutralizing antibodies. These data suggest that endogenous agmatine modulates the neuroplasticity associated with chronic pain. Development of approaches to access this inhibitory control of neuroplasticity associated with chronic pain may yield important non-opioid pain-relieving options.

Targeting the somatosensory system with AAV9 and AAV2retro viral vectors

Adeno-associated viral (AAV) vectors allow for site-specific and time-dependent genetic manipulation of neurons. However, for successful implementation of AAV vectors, major consideration must be given to the selection of viral serotype and route of delivery for efficient gene transfer into the cell type being investigated. Here we compare the transduction pattern of neurons in the somatosensory system following injection of AAV9 or AAV2retro in the parabrachial complex of the midbrain, the spinal cord dorsal horn, the intrathecal space, and the colon. Transduction was evaluated based on Cre-dependent expression of tdTomato in transgenic reporter mice, following delivery of AAV9 or AAV2retro carrying identical constructs that drive the expression of Cre/GFP. The pattern of distribution of tdTomato expression indicated notable differences in the access of the two AAV serotypes to primary afferent neurons via peripheral delivery in the colon and to spinal projections neurons via intracranial delivery within the parabrachial complex. Additionally, our results highlight the superior sensitivity of detection of neuronal transduction based on reporter expression relative to expression of viral products.

Biodistribution of Adeno-Associated Virus Serotype 5 Viral Vectors Following Intrathecal Injection

The pharmacokinetic profile of AAV particles following intrathecal delivery has not yet been clearly defined. The present study evaluated the distribution profile of adeno-associated virus serotype 5 (AAV5) viral vectors following lumbar intrathecal injection in mice. After a single bolus intrathecal injection, viral DNA concentrations in mouse whole blood, spinal cord, and peripheral tissues were determined using quantitative polymerase chain reaction (qPCR). The kinetics of AAV5 vector in whole blood and the concentration over time in spinal and peripheral tissues were analyzed. Distribution of the AAV5 vector to all levels of the spinal cord, dorsal root ganglia, and into systemic circulation occurred rapidly within 30 min following injection. Vector concentration in whole blood reached a maximum 6 h postinjection with a half-life of approximately 12 h. Area under the curve data revealed the highest concentration of vector distributed to dorsal root ganglia tissue. Immunohistochemical analysis revealed AAV5 particle colocalization with the pia mater at the spinal cord and macrophages in the dorsal root ganglia (DRG) 30 min after injection. These results demonstrate the widespread distribution of AAV5 particles through cerebrospinal fluid and preferential targeting of DRG tissue with possible clearance mechanisms via DRG macrophages.

Comparative Effectiveness of Intracerebroventricular, Intrathecal, and Intranasal Routes of AAV9 Vector Administration for Genetic Therapy of Neurologic Disease in Murine Mucopolysaccharidosis Type I

Mucopolysaccharidosis type I (MPS I) is an inherited metabolic disorder caused by deficiency of the lysosomal enzyme alpha-L-iduronidase (IDUA). The two current treatments [hematopoietic stem cell transplantation (HSCT) and enzyme replacement therapy (ERT)], are insufficiently effective in addressing neurologic disease, in part due to the inability of lysosomal enzyme to cross the blood brain barrier. With a goal to more effectively treat neurologic disease, we have investigated the effectiveness of AAV-mediated IDUA gene delivery to the brain using several different routes of administration. Animals were treated by either direct intracerebroventricular (ICV) injection, by intrathecal (IT) infusion into the cerebrospinal fluid, or by intranasal (IN) instillation of AAV9-IDUA vector. AAV9-IDUA was administered to IDUA-deficient mice that were either immunosuppressed with cyclophosphamide (CP), or immunotolerized at birth by weekly injections of human iduronidase. In animals treated by ICV or IT administration, levels of IDUA enzyme ranged from 3- to 1000-fold that of wild type levels in all parts of the microdissected brain. In animals administered vector intranasally, enzyme levels were 100-fold that of wild type in the olfactory bulb, but enzyme expression was close to wild type levels in other parts of the brain. Glycosaminoglycan levels were reduced to normal in ICV and IT treated mice, and in IN treated mice they were normalized in the olfactory bulb, or reduced in other parts of the brain. Immunohistochemical analysis showed extensive IDUA expression in all parts of the brain of ICV treated mice, while IT treated animals showed transduction that was primarily restricted to the hind brain with some sporadic labeling seen in the mid- and fore brain. At 6 months of age, animals were tested for spatial navigation, memory, and neurocognitive function in the Barnes maze; all treated animals were indistinguishable from normal heterozygous control animals, while untreated IDUA deficient animals exhibited significant learning and spatial navigation deficits. We conclude that IT and IN routes are acceptable and alternate routes of administration, respectively, of AAV vector delivery to the brain with effective IDUA expression, while all three routes of administration prevent the emergence of neurocognitive deficiency in a mouse MPS I model.

Nociceptive Sensory Fibers Drive Interleukin-23 Production from CD301b+ Dermal Dendritic Cells and Drive Protective Cutaneous Immunity

Innate resistance to Candida albicans in mucosal tissues requires the production of interleukin-17A (IL-17A) by tissue-resident cells early during infection, but the mechanism of cytokine production has not been precisely defined. In the skin, we found that dermal γδ T cells were the dominant source of IL-17A during C. albicans infection and were required for pathogen resistance. Induction of IL-17A from dermal γδ T cells and resistance to C. albicans required IL-23 production from CD301b(+) dermal dendritic cells (dDCs). In addition, we found that sensory neurons were directly activated by C. albicans. Ablation of sensory neurons increased susceptibility to C. albicans infection, which could be rescued by exogenous addition of the neuropeptide CGRP. These data define a model in which nociceptive pathways in the skin drive production of IL-23 by CD301b(+) dDCs resulting in IL-17A production from γδ T cells and resistance to cutaneous candidiasis.

Supraspinal gene transfer by intrathecal adeno-associated virus serotype 5

We report the pattern of transgene expression across brain regions after intrathecal delivery of adeno-associated virus serotype 5 (AAV5). Labeling in hindbrain appeared to be primarily neuronal, and was detected in sensory nuclei of medulla, pontine nuclei, and all layers of cerebellar cortex. Expression in midbrain was minimal, and generally limited to isolated neurons and astrocytes in the cerebral peduncles. GFP immunoreactivity (-ir) in thalamus was most prominent in medial geniculate nucleus, and otherwise limited to posterior nuclei of the dorsal and lateral margins. Labeling was also observed in neurons and astrocytes of the hippocampal formation and amygdaloid complex. In the hippocampal formation, GFP-ir was found in neuronal cell bodies of the rostral ventral portion, but was largely restricted to fiber-like staining in the molecular layer of dentate gyrus and stratum lacunosum-moleculare of the rostral dorsal region. GFP-ir was seen in neurons and astroglia throughout caudal cortex, whereas in rostral regions of neocortex it was limited to isolated neurons and non-neuronal cells. Labeling was also present in olfactory bulb. These results demonstrate that intrathecal delivery of AAV5 vector leads to transgene expression in discrete CNS regions throughout the rostro-caudal extent of the neuraxis. A caudal-to-rostral gradient of decreasing GFP-ir was present in choroid plexus and Purkinje cells, suggesting that spread of virus through cerebrospinal fluid plays a role in the resulting transduction pattern. Other factors contributing to the observed expression pattern likely include variations in cell-surface receptors and inter-parenchymal space.

Biodistribution of adeno-associated virus serotype 9 (AAV9) vector after intrathecal and intravenous delivery in mouse

Adeno-associated virus serotype 9 (AAV9)-mediated gene transfer has been reported in central nervous system (CNS) and peripheral tissues. The current study compared the pattern of expression of Green Fluorescent Protein (GFP) across the mouse CNS and selected peripheral tissues after intrathecal (i.t.) or intravenous (i.v.) delivery of equivalent doses of single-stranded AAV9 vector. After i.t. delivery, GFP immunoreactivity (-ir) was observed in spinal neurons, primary afferent fibers and corresponding primary sensory neurons at all spinal levels. Robust transduction was seen in small and large dorsal root ganglion (DRG) neurons as well as trigeminal and vagal primary afferent neurons. Transduction efficiency in sensory ganglia was substantially lower in i.v. treated mice. In brain, i.v. delivery yielded GFP-immunoreactivity (-ir) primarily in spinal trigeminal tract, pituitary, and scattered isolated neurons and astrocytes. In contrast, after i.t. delivery, GFP-ir was widespread throughout CNS, with greater intensity and more abundant neuropil-like staining at 6 weeks compared to 3 weeks. Brain regions with prominent GFP-ir included cranial nerve nuclei, ventral pons, cerebellar cortex, hippocampus, pituitary, choroid plexus, and selected nuclei of midbrain, thalamus and hypothalamus. In cortex, GFP-ir was associated with blood vessels, and was seen in both neurons and astrocytes. In the periphery, GFP-ir in colon and ileum was present in the enteric nervous system in both i.v. and i.t. treated mice. Liver and adrenal cortex, but not adrenal medulla, also showed abundant GFP-ir after both routes of delivery. In summary, i.t. delivery yielded higher transduction efficiency in sensory neurons and the CNS. The observation of comparable gene transfer to peripheral tissues using the two routes indicates that a component of i.t. delivered vector is redistributed from the subarachnoid space to the systemic circulation.

The VGF-derived peptide TLQP-21 contributes to inflammatory and nerve injury-induced hypersensitivity

VGF (nonacronymic) is a granin-like protein that is packaged and proteolytically processed within the regulated secretory pathway. VGF and peptides derived from its processing have been implicated in neuroplasticity associated with learning, memory, depression, and chronic pain. In sensory neurons, VGF is rapidly increased following peripheral nerve injury and inflammation. Several bioactive peptides generated from the C-terminus of VGF have pronociceptive spinal effects. The goal of the present study was to examine the spinal effects of the peptide TLQP-21 and determine whether it participates in spinal mechanisms of persistent pain. Application of exogenous TLQP-21 induced dose-dependent thermal hyperalgesia in the warm-water immersion tail-withdrawal test. This hyperalgesia was inhibited by a p38 mitogen-activated protein kinase inhibitor, as well as inhibitors of cyclooxygenase and lipoxygenase. We used immunoneutralization of TLQP-21 to determine the function of the endogenous peptide in mechanisms underlying persistent pain. In mice injected intradermally with complete Freund adjuvant, intrathecal treatment with anti-TLQP-21 immediately prior to or 5hours after induction of inflammation dose-dependently inhibited tactile hypersensitivity and thermal hyperalgesia. Intrathecal anti-TL21 administration also attenuated the development and maintenance of tactile hypersensitivity in the spared nerve injury model of neuropathic pain. These results provide evidence that endogenous TLQP-21 peptide contributes to the mechanisms of spinal neuroplasticity after inflammation and nerve injury.

Visualization of spinal afferent innervation in the mouse colon by AAV8-mediated GFP expression

BACKGROUND

Primary afferent neurons whose cell bodies reside in thoracolumbar and lumbosacral dorsal root ganglia (DRG) innervate colon and transmit sensory signals from colon to spinal cord under normal conditions and conditions of visceral hypersensitivity. Histologically, these extrinsic afferents cannot be differentiated from intrinsic fibers of enteric neurons because all known markers label neurons of both populations. Adeno-associated virus (AAV) vectors are capable of transducing DRG neurons after intrathecal administration. We hypothesized that AAV-driven overexpression of green fluorescent protein (GFP) in DRG would enable visualization of extrinsic spinal afferents in colon separately from enteric neurons.

METHODS

Recombinant AAV serotype 8 (rAAV8) vector carrying the GFP gene was delivered via direct lumbar puncture. Green fluorescent protein labeling in DRG and colon was examined using immunohistochemistry.

KEY RESULTS

Analysis of colon from rAAV8-GFP-treated mice demonstrated GFP-immunoreactivity (GFP-ir) within mesenteric nerves, smooth muscle layers, myenteric plexus, submucosa, and mucosa, but not in cell bodies of enteric neurons. Notably, GFP-ir colocalized with CGRP and TRPV1 in mucosa, myenteric plexus, and globular-like clusters surrounding nuclei within myenteric ganglia. In addition, GFP-positive fibers were observed in close association with blood vessels of mucosa and submucosa. Analysis of GFP-ir in thoracolumbar and lumbosacral DRG revealed that levels of expression in colon and L6 DRG appeared to be related.

CONCLUSIONS & INFERENCES

These results demonstrate the feasibility of gene transfer to mouse colonic spinal sensory neurons using intrathecal delivery of AAV vectors and the utility of this approach for histological analysis of spinal afferent nerve fibers within colon.

Differential adeno-associated virus mediated gene transfer to sensory neurons following intrathecal delivery by direct lumbar puncture

Background

Neuronal transduction by adeno-associated viral (AAV) vectors has been demonstrated in cortex, brainstem, cerebellum, and sensory ganglia. Intrathecal delivery of AAV serotypes that transduce neurons in dorsal root ganglia (DRG) and spinal cord offers substantial opportunities to 1) further study mechanisms underlying chronic pain, and 2) develop novel gene-based therapies for the treatment and management of chronic pain using a non-invasive delivery route with established safety margins. In this study we have compared expression patterns of AAV serotype 5 (AAV5)- and AAV serotype 8 (AAV8)-mediated gene transfer to sensory neurons following intrathecal delivery by direct lumbar puncture.

Results

Intravenous mannitol pre-treatment significantly enhanced transduction of primary sensory neurons after direct lumbar puncture injection of AAV5 (rAAV5-GFP) or AAV8 (rAAV8-GFP) carrying the green fluorescent protein (GFP) gene. The presence of GFP in DRG neurons was consistent with the following evidence for primary afferent origin of the majority of GFP-positive fibers in spinal cord: 1) GFP-positive axons were evident in both dorsal roots and dorsal columns; and 2) dorsal rhizotomy, which severs the primary afferent input to spinal cord, abolished the majority of GFP labeling in dorsal horn. We found that both rAAV5-GFP and rAAV8-GFP appear to preferentially target large-diameter DRG neurons, while excluding the isolectin-B4 (IB4) -binding population of small diameter neurons. In addition, a larger proportion of CGRP-positive cells was transduced by rAAV5-GFP, compared to rAAV8-GFP.

Conclusions

The present study demonstrates the feasibility of minimally invasive gene transfer to sensory neurons using direct lumbar puncture and provides evidence for differential targeting of subtypes of DRG neurons by AAV vectors.

Coexpression of alpha 2A-adrenergic and delta-opioid receptors in substance P-containing terminals in rat dorsal horn

Agonists acting at alpha(2)-adrenergic and opioid receptors (alpha(2)ARs and ORs, respectively) inhibit pain transmission in the spinal cord. When coadministered, agonists activating these receptors interact in a synergistic manner. Although the existence of alpha(2)AR/OR synergy has been well characterized, its mechanism remains poorly understood. The formation of heterooligomers has been proposed as a molecular basis for interactions between neuronal G-protein-coupled receptors. The relevance of heterooligomer formation to spinal analgesic synergy requires demonstration of the expression of both receptors within the same neuron as well as the localization of both receptors in the same neuronal compartment. We used immunohistochemistry to investigate the spatial relationship between alpha(2)ARs and ORs in the rat spinal cord to determine whether coexpression could be demonstrated between these receptors. We observed extensive colocalization between alpha(2A)-adrenergic and delta-opioid receptors (DOP) on substance P (SP)-immunoreactive (-ir) varicosities in the superficial dorsal horn of the spinal cord and in peripheral nerve terminals in the skin. alpha(2A)AR- and DOP-ir elements were colocalized in subcellular structures of 0.5 mum or less in diameter in isolated nerve terminals. Furthermore, coincubation of isolated synaptosomes with alpha(2)AR and DOP agonists resulted in a greater-than-additive increase in the inhibition of K(+)-stimulated neuropeptide release. These findings suggest that coexpression of the synergistic receptor pair alpha(2A)AR-DOP on primary afferent nociceptive fibers may represent an anatomical substrate for analgesic synergy, perhaps as a result of protein-protein interactions such as heterooligomerization.

Potassium- and capsaicin-induced release of agmatine from spinal nerve terminals

Agmatine (decarboxylated arginine) was originally identified in the CNS as an imidazoline receptor ligand. Further studies demonstrated that agmatine antagonizes NMDA receptors and inhibits nitric oxide synthase. Intrathecally administered agmatine inhibits opioid tolerance and hyperalgesia evoked by inflammation, nerve injury, and intrathecally administered NMDA. These actions suggest an anti-glutamatergic role for agmatine in the spinal cord. We have previously reported that radiolabeled agmatine is transported into spinal synaptosomes in an energy- and temperature-dependent manner. In the present study, we demonstrate that agmatine is releasable from purified spinal nerve terminals upon depolarization. When exposed to either elevated potassium or capsaicin, tritiated agmatine (but not its precursor L-arginine or its metabolite putrescine) is released in a calcium-dependent manner. Control experiments confirmed that the observed release was specific to depolarization and not due to permeabilization of or degradation of synaptosomes. That capsaicin-evoked stimulation results in agmatine release implicates the participation of primary afferent nerve terminals. Radiolabeled agmatine also accumulates in purified spinal synaptosomal vesicles in a temperature-dependent manner, suggesting that the source of releasable agmatine may be vesicular in origin. These results support the proposal that agmatine may serve as a spinal neuromodulator involved in pain processing.

Identification of some lectin IB4 binding proteins in rat dorsal root ganglia

Lectins are proteins that bind to glycoproteins and glycolipids. The isolectin Griffonia simplicifolia I-B4 (IB4) recognizes terminal alpha-galactose and binds to a subset of small and medium-sized neurons in the dorsal root ganglia (DRG). Using one and two-dimensional gel electrophoresis, we have identified several proteins that bind IB4 in sciatic nerve, dorsal horn, and DRG. Treatment with the enzyme alpha-galactosidase reduces IB4 binding, strongly suggesting the binding is specific for the IB4 epitope. Mass spectrometric analysis of tryptic digests of alpha-galactosidase sensitive bands identified three proteins that bind IB4: the laminin beta 2 chain and the light and medium subunits of neurofilaments.

Effects of peripheral nerve injury on delta opioid receptor (DOR) immunoreactivity in the rat spinal cord

Morphine and other opioids have direct analgesic actions in the spinal cord and chronic spinal administration of opioid agonists is used clinically in patients suffering from severe, chronic pain. Neuropathic pain resulting from peripheral nerve injury is often less sensitive to opioid therapy than other forms of chronic pain in both humans and animal models. Changes in spinal mu-opioid receptor (MOR) expression have been demonstrated in animal models of neuropathic pain. However, these changes alone fail to account for the attenuation of opioid activity. Reduced expression of delta-opioid receptors (DOR) following peripheral nerve injury has been reported but most of these reports are limited to subjective observation. The magnitude and consistency of these changes is therefore unclear. In addition, previous studies did not evaluate the effects of nerve injury on behavioral measures to confirm induction of aberrant pain symptoms. We therefore performed quantitative image analysis to evaluate the effect of peripheral nerve injury on DOR-immunoreactivity in spinal cord sections from rats previously characterized for sensory responsiveness. We observed statistically significant decreases ipsilateral to nerve injury in all three models tested: sciatic nerve transection, chronic constriction injury of the sciatic nerve and L5/L6 spinal nerve ligation. These results suggest that decreases in the expression of DOR are a common feature of peripheral nerve injury.

Cytotoxic targeting of isolectin IB4-binding sensory neurons

The isolectin I-B4 (IB4) binds specifically to a subset of small sensory neurons. We used a conjugate of IB4 and the toxin saporin to examine in vivo the contribution of IB4-binding sensory neurons to nociception. A single dose of the conjugate was injected unilaterally into the sciatic nerve of rats. The treatment resulted in a permanent selective loss of IB4-binding neurons as indicated by histological analysis of dorsal root ganglia, spinal cord, and skin from treated animals. Behavioral measurements showed that 7-10 days after the injection, conjugate-treated rats had elevated thermal and mechanical nociceptive thresholds. However, 21 days post-treatment the nociceptive thresholds returned to baseline levels. These results demonstrate the utility of the IB4-saporin conjugate as a tool for selective cytotoxic targeting and provide behavioral evidence for the role of IB4-binding neurons in nociception. The decreased sensitivity to noxious stimuli associated with the loss of IB4-binding neurons indicates that these sensory neurons are essential for the signaling of acute pain. Furthermore, the unexpected recovery of nociceptive thresholds suggests that the loss of IB4-binding neurons triggers changes in the processing of nociceptive information, which may represent a compensatory mechanism for the decreased sensitivity to acute pain.

Distribution and chemical coding of orphanin FQ/nociceptin-immunoreactive neurons in the myenteric plexus of guinea pig intestines and sphincter of Oddi

Longitudinal muscle-myenteric plexus preparations of guinea pig intestines and sphincter of Oddi (SO) were immunostained for orphanin FQ/nociceptin. Orphanin FQ-immunoreactive (OFQ-IR) neurons and nerve fibers were relatively abundant in the SO, duodenum, ileum, cecum, and distal colon, with fewer neurons and nerve fibers observed in the proximal colon. Double staining with antibodies directed against the neuron-specific RNA binding protein Hu revealed that while the numbers of OFQ-IR neurons per ganglion decreased along the gut tube, similar proportions (7-9%) of neurons in these regions were OFQ-IR, whereas <1% of the neurons in the proximal colon were OFQ positive. In the ileum, where 8% of the myenteric neurons were OFQ-IR, all OFQ-IR neurons expressed choline acetyltransferase. In addition, multiple-label immunohistochemistry demonstrated that 58% of the OFQ-IR neurons were calretinin-IR, 52% were substance P-IR, and 28% were enkephalin-IR. Nitric oxide synthase immunoreactivity was observed in about 5% of OFQ-IR neurons, or 0.4% of the total population, and a similar proportion of the OFQ-IR neurons was positive for vasoactive intestinal peptide. No OFQ-IR neurons were immunoreactive for calbindin, somatostatin, or serotonin. These results, combined with previous studies of chemical coding and projection patterns in the guinea pig myenteric plexus, indicate that OFQ-IR is expressed preferentially in excitatory motor neurons projecting to the longitudinal and circular muscle layers, as well as a small subgroup of descending interneurons. Because OFQ is expressed by excitatory motor neurons, and because this peptide inhibits excitatory neurotransmission in the guinea pig ileum, it is likely that OFQ acts through a feedback autoinhibitory mechanism.

Distribution and chemical coding of orphanin FQ/nociceptin-immunoreactive neurons in the myenteric plexus of guinea pig intestines and sphincter of Oddi

Longitudinal muscle-myenteric plexus preparations of guinea pig intestines and sphincter of Oddi (SO) were immunostained for orphanin FQ/nociceptin. Orphanin FQ-immunoreactive (OFQ-IR) neurons and nerve fibers were relatively abundant in the SO, duodenum, ileum, cecum, and distal colon, with fewer neurons and nerve fibers observed in the proximal colon. Double staining with antibodies directed against the neuron-specific RNA binding protein Hu revealed that while the numbers of OFQ-IR neurons per ganglion decreased along the gut tube, similar proportions (7-9%) of neurons in these regions were OFQ-IR, whereas <1% of the neurons in the proximal colon were OFQ positive. In the ileum, where 8% of the myenteric neurons were OFQ-IR, all OFQ-IR neurons expressed choline acetyltransferase. In addition, multiple-label immunohistochemistry demonstrated that 58% of the OFQ-IR neurons were calretinin-IR, 52% were substance P-IR, and 28% were enkephalin-IR. Nitric oxide synthase immunoreactivity was observed in about 5% of OFQ-IR neurons, or 0.4% of the total population, and a similar proportion of the OFQ-IR neurons was positive for vasoactive intestinal peptide. No OFQ-IR neurons were immunoreactive for calbindin, somatostatin, or serotonin. These results, combined with previous studies of chemical coding and projection patterns in the guinea pig myenteric plexus, indicate that OFQ-IR is expressed preferentially in excitatory motor neurons projecting to the longitudinal and circular muscle layers, as well as a small subgroup of descending interneurons. Because OFQ is expressed by excitatory motor neurons, and because this peptide inhibits excitatory neurotransmission in the guinea pig ileum, it is likely that OFQ acts through a feedback autoinhibitory mechanism.

Effects of peripheral nerve injury on alpha-2A and alpha-2C adrenergic receptor immunoreactivity in the rat spinal cord

Neuropathic pain resulting from peripheral nerve injury can often be relieved by administration of alpha-adrenergic receptor antagonists. Tonic activation of alpha-adrenergic receptors may therefore facilitate the hyperalgesia and allodynia associated with neuropathic pain. It is currently unclear whether alpha2A- or alpha2c-adrenergic receptor subtypes are involved in the pro-nociceptive actions of alpha-adrenergic receptors under neuropathic conditions. We therefore investigated the effects of peripheral nerve injury on the expression of these subtypes in rat spinal cord using immunohistochemical techniques. In addition, neuropeptide Y immunoreactivity was examined as an internal control because it has previously been shown to be up-regulated following nerve injury. We observed a decrease in alpha2A-adrenergic receptor immunoreactivity in the spinal cord ipsilateral to three models of neuropathic pain: complete sciatic nerve transection, chronic constriction injury of the sciatic nerve and L5/L6 spinal nerve ligation. The extent of this down-regulation was significantly correlated with the magnitude of injury-induced changes in mechanical sensitivity. In contrast, alpha2c-adrenergic receptor immunoreactivity was only increased in the spinal nerve ligation model; these increases did not correlate with changes in mechanical sensitivity. Neuropeptide Y immunoreactivity was up-regulated in all models examined. Increased expression of neuropeptide Y correlated with changes in mechanical sensitivity. The decrease in alpha2A-adrenergic receptor immunoreactivity and the lack of consistent changes in alpha2C-adrenergic receptor immunoreactivity suggest that neither of these receptor subtypes is likely to be responsible for the abnormal adrenergic sensitivity observed following nerve injury. On the contrary, the decrease in alpha2A-adrenergic receptor immunoreactivity following nerve injury may result in an attenuation of the influence of descending inhibitory noradrenergic input into the spinal cord resulting in increased excitatory transmitter release following peripheral stimuli.

P2X3 is expressed by DRG neurons that terminate in inner lamina II

The P2X3 receptor subunit, a member of the P2X family of ATP-gated ion channels, is almost exclusively localized in sensory neurons. In the present study, we sought to gain insight into the role of P2X3 and P2X3-containing neurons in sensory transmission, using immunohistochemical approaches. In rat dorsal root ganglia (DRG), P2X3-immunoreactivity (-ir) was observed in small- and medium-sized neurons. Approximately 40% of DRG neuronal profiles in normal rats contained P2X3-ir. In rats that had received neonatal capsaicin treatment, the number of P2X3-positive neurons was decreased by approximately 70%. Analysis of the colocalization of P2X3-ir with cytochemical markers of DRG neurons indicated that approximately 94% of the P2X3-positive neuronal profiles were labelled by isolectin B4 from Bandeiraea simplicifolia, while only 3% contained substance P-ir, and 7% contained somatostatin-ir. In dorsal horn of rat spinal cord, P2X3-ir was observed in the inner portion of lamina II and was reduced subsequent to dorsal rhizotomy, as well as subsequent to neonatal capsaicin treatment. Finally, P2X3-ir accumulated proximal to the site of sciatic nerve ligation, and was seen in nerve fibres in skin and corneal epithelium. In summary, our results suggest that P2X3 is expressed by a functionally heterogeneous population of BSI-B4-binding sensory neurons, and is transported into both central and peripheral processes of these neurons.

Differential distribution of alpha2A and alpha2C adrenergic receptor immunoreactivity in the rat spinal cord

alpha2-Adrenergic receptors (alpha2-ARs) mediate a number of physiological phenomena, including spinal analgesia. We have developed subtype-selective antisera against the C termini of the alpha2A-AR and alpha2C-AR to investigate the relative distribution and cellular source or sources of these receptor subtypes in the rat spinal cord. Immunoreactivity (IR) for both receptor subtypes was observed in the superficial layers of the dorsal horn of the spinal cord. Our results suggest that the primary localization of the alpha2A-AR in the rat spinal cord is on the terminals of capsaicin-sensitive, substance P (SP)-containing primary afferent fibers. In contrast, the majority of alpha2C-AR-IR was not of primary afferent origin, not strongly colocalized with SP-IR, and not sensitive to neonatal capsaicin treatment. Spinal alpha2C-AR-IR does not appear to colocalize with the neurokinin-1 receptor, nor is it localized on astrocytes, as evidenced by a lack of costaining with the glial marker GFAP. However, some colocalization was observed between alpha2C-AR-IR and enkephalin-IR, suggesting that the alpha2C-AR may be expressed by a subset of spinal interneurons. Interestingly, neither subtype was detected on descending noradrenergic terminals. These results indicate that the alpha2-AR subtypes investigated are likely expressed by different subpopulations of neurons and may therefore subserve different physiological functions in the spinal cord, with the alpha2A-AR being more likely to play a role in the modulation of nociceptive information.

An acid sensing ion channel (ASIC) localizes to small primary afferent neurons in rats

The acid sensing ion channel (ASIC) identified in rat brain and spinal cord is potentially involved in the transmission of acid-induced nociception. We have developed polyclonal antisera against ASIC, and used them to screen rat brain and spinal cord using immunocytochemistry. ASIC-immunoreactivity (-ir) is present in but not limited to the superficial dorsal horn, the dorsal root ganglia (DRG) and the spinal trigeminal nucleus, as well as peripheral nerve fibers. These observations, combined with the disappearance of ASIC-ir following dorsal rhizotomy, suggest localization of ASIC to primary afferents. DRG ASIC-ir co-localizes with substance P (SP) and calcitonin gene-related peptide (CGRP)-ir in small capsaicin-sensitive cell bodies, suggesting that ASIC is poised to play a role in the transduction of noxious stimuli.

Immunohistochemical study of the P2X2 and P2X3 receptor subunits in rat and monkey sensory neurons and their central terminals

Of the cloned P2X receptor subunits, six are expressed in sensory neurons, suggesting that the native channels may be heteromultimers with diverse composition. It has been proposed that P2X2 and P2X3 form heteromultimers in sensory neurons. We further tested this hypothesis by examining the relationship of P2X2 and P2X3 immunocytochemically. In rat dorsal root and nodose ganglia, P2X2- and P2X3-immunoreactivity (-ir) were highly colocalized, although single-labeled cells were also present. In dorsal root ganglia (DRG), in some cases P2X2-ir appeared to be present in satellite cells. In dorsal horn of spinal cord, at low magnification the laminar localization of P2X2- and P2X3-ir overlapped, but at high magnification colocalization was rarely observed. In contrast, in the solitary tract and its nucleus (NTS), colocalization of P2X2- and P2X3-ir was seen at low and high magnification. These results suggest that the relationship of P2X2- and P2X3-ir is different in nodose and dorsal root ganglia and might reflect differences in the targeting of P2X receptors in different sensory neurons. In monkey, P2X2-ir was observed in DRG neurons and satellite cells and in dorsal horn of spinal cord. P2X3-ir was also seen in DRG neurons. However, the presence of P2X2-ir in NTS as well as the presence of P2X3-ir in spinal cord and NTS could not be established definitively. These results suggest species differences, although a more extensive study of primate sensory systems is necessary.