Cell type and conduction velocity of rat primary sensory neurons with substance P-like immunoreactivity

Cryoneurolysis of the saphenous nerve in the pig: A proof-of-principle investigation

OBJECTIVE

To determine if in vivo cryoneurolysis inhibits ex vivo compound action potential (CAP) conduction in the porcine saphenous nerve and if this occurs rapidly enough to justify performing the technique before stifle surgery.

STUDY DESIGN

Blinded, controlled, randomized, preclinical study.

ANIMALS

A group of eight healthy, 8 weeks old, intact, female pigs anesthetized for an unrelated terminal study.

METHODS

Both saphenous nerves of each pig were exposed surgically, and 15 mm of a 20 gauge, closed-tip, commercial cryoneurolysis cannula were inserted cranial to each nerve within the neurovascular fascial sheath along its long axis. The cannula was only actuated on one limb, according to random allocation. Nerves were excised within 15 minutes of actuation and underwent testing in a nerve conduction chamber, where stimulus voltage was increased sequentially (from 0.1 to ≤ 1.9 V). An anesthesiologist blinded to treatment viewed recordings of time versus voltage for each nerve and answered 'yes' or 'no' when asked if an evoked CAP was observed. Fisher's exact test evaluated the incidence of CAP conduction between groups (p < 0.05 considered significant). Nerves were submitted for hematoxylin and eosin staining for blinded histopathological examination.

RESULTS

A CAP was conducted in 8/8 and 1/8 of the control and treated nerves, respectively (p = 0.001). Maximal responses in control nerves were 1.92 ± 0.19 mV (mean ± standard error). In the single treated nerve that conducted a CAP, the maximal CAP amplitude was 0.4 mV, lower than the lowest maximal CAP (1.19 mV) in the control nerves. All control nerves were histologically normal, and all treated nerves displayed mild perivascular and perineural inflammation (cuffs of lymphocytes, plasma cells and eosinophils, and edema).

CONCLUSIONS AND CLINICAL RELEVANCE

The rapid inhibition of CAP conduction warrants clinical investigation of saphenous cryoneurolysis for both intraoperative antinociception and postoperative analgesia in pigs undergoing experimental stifle surgery.

Neural Circuitry Polarization in the Spinal Dorsal Horn (SDH): A Novel Form of Dysregulated Circuitry Plasticity during Pain Pathogenesis

Pathological pain emerges from nociceptive system dysfunction, resulting in heightened pain circuit activity. Various forms of circuitry plasticity, such as central sensitization, synaptic plasticity, homeostatic plasticity, and excitation/inhibition balance, contribute to the malfunction of neural circuits during pain pathogenesis. Recently, a new form of plasticity in the spinal dorsal horn (SDH), named neural circuit polarization (NCP), was discovered in pain models induced by HIV-1 gp120 and chronic morphine administration. NCP manifests as an increase in excitatory postsynaptic currents (EPSCs) in excitatory neurons and a decrease in EPSCs in inhibitory neurons, presumably facilitating hyperactivation of pain circuits. The expression of NCP is associated with astrogliosis. Ablation of reactive astrocytes or suppression of astrogliosis blocks NCP and, concomitantly, the development of gp120- or morphine-induced pain. In this review, we aim to compare and integrate NCP with other forms of plasticity in pain circuits to improve the understanding of the pathogenic contribution of NCP and its cooperation with other forms of circuitry plasticity during the development of pathological pain.

Distribution and Chemistry of Phoenixin-14, a Newly Discovered Sensory Transmission Molecule in Porcine Afferent Neurons

Phoenixin-14 (PNX), initially discovered in the rat hypothalamus, was also detected in dorsal root ganglion (DRG) cells, where its involvement in the regulation of pain and/or itch sensation was suggested. However, there is a lack of data not only on its distribution in DRGs along individual segments of the spinal cord, but also on the pattern(s) of its co-occurrence with other sensory neurotransmitters. To fill the above-mentioned gap and expand our knowledge about the occurrence of PNX in mammalian species other than rodents, this study examined (i) the pattern(s) of PNX occurrence in DRG neurons of subsequent neuromeres along the porcine spinal cord, (ii) their intraganglionic distribution and (iii) the pattern(s) of PNX co-occurrence with other biologically active agents. PNX was found in approximately 20% of all nerve cells of each DRG examined; the largest subpopulation of PNX-positive (PNX+) cells were small-diameter neurons, accounting for 74% of all PNX-positive neurons found. PNX+ neurons also co-contained calcitonin gene-related peptide (CGRP; 96.1%), substance P (SP; 88.5%), nitric oxide synthase (nNOS; 52.1%), galanin (GAL; 20.7%), calretinin (CRT; 10%), pituitary adenylate cyclase-activating polypeptide (PACAP; 7.4%), cocaine and amphetamine related transcript (CART; 5.1%) or somatostatin (SOM; 4.7%). Although the exact function of PNX in DRGs is not yet known, the high degree of co-localization of this peptide with the main nociceptive transmitters SP and CGRP may suggests its function in modulation of pain transmission.

Mirogabalin Decreases Pain-like Behaviours and Improves Opioid and Ketamine Antinociception in a Mouse Model of Neuropathic Pain

Neuropathic pain remains a difficult clinical challenge due to its diverse aetiology and complex pathomechanisms, which are yet to be fully understood. Despite the variety of available therapies, many patients suffer from ineffective pain relief; hence, the search for more efficacious treatments continues. The new gabapentinoid, mirogabalin has recently been approved for clinical use. Although its main mechanism of action occurs at the α2σ-1 and α2σ-2 subunits of calcium channels and is well documented, how the drug affects the disturbed neuropathic interactions at the spinal cord level has not been clarified, which is crucial information from a clinical perspective. The findings of our study suggest that several indirect mechanisms may be responsible for the beneficial analgesic effect of mirogabalin. This is the first study to report that mirogabalin enhances the mRNA expression of spinal antinociceptive factors, such as IL-10 and IL-18BP, and reduces the concentration of the pronociceptive substance P. Importantly, mirogabalin improves the morphine-, buprenorphine-, oxycodone-, and ketamine-induced antinociceptive effects in a neuropathic pain model. Our findings support the hypothesis that enhancing opioid and ketamine analgesia by combining these drugs with mirogabalin may represent a new strategy for the effective pharmacotherapy of neuropathic pain.

2D <em>vs</em> 3D morphological analysis of dorsal root ganglia in health and painful neuropathy

Dorsal root ganglia (DRGs) are clusters of sensory neurons that transmit the sensory information from the periphery to the central nervous system, and satellite glial cells (SGCs), their supporting trophic cells. Sensory neurons are pseudounipolar neurons with a heterogeneous neurochemistry reflecting their functional features. DRGs, not protected by the blood brain barrier, are vulnerable to stress and damage of different origin (i.e., toxic, mechanical, metabolic, genetic) that can involve sensory neurons, SGCs or, considering their intimate intercommunication, both cell populations. DRG damage, primary or secondary to nerve damage, produces a sensory peripheral neuropathy, characterized by neurophysiological abnormalities, numbness, paraesthesia and dysesthesia, tingling and burning sensations and neuropathic pain. DRG stress can be morphologically detected by light and electron microscope analysis with alterations in cell size (swelling/atrophy) and in different subcellular compartments (i.e., mitochondria, endoplasmic reticulum, and nucleus) of neurons and/or SGCs. In addition, neurochemical changes can be used to portray abnormalities of neurons and SGC. Conventional immunostaining, i.e., immunohistochemical detection of specific molecules in tissue slices, can be employed to detect, localize and quantify particular markers of damage in neurons (i.e., nuclear expression of ATF3) or SGCs (i.e., increased expression of GFAP), markers of apoptosis (i.e., caspases), markers of mitochondrial suffering and oxidative stress (i.e., 8-OHdG), markers of tissue inflammation (i.e., CD68 for macrophage infiltration) etc. However classical (2D) methods of immunostaining disrupt the overall organization of the DRG, thus resulting in the loss of some crucial information. Whole-mount (3D) methods have been recently developed to investigate DRG morphology and neurochemistry without tissue slicing, giving the opportunity to study the intimate relationship between SGCs and sensory neurons in health and disease. Here, we aim to compare classical (2D) vs whole-mount (3D) approaches to highlight “pros” and “cons” of the two methodologies when analysing neuropathy-induced alterations in DRGs.

The role of substance P in acupuncture signal transduction and effects

BACKGROUND

Acupuncture has been used to treat a wide variety of diseases, disorders, and conditions for more than 2500 years. While the anatomical structures of acupuncture points (or acupoints) are largely unknown, our previous studies have suggested that many acupoints can be identified as cutaneous neurogenic inflammatory spots (neurogenic spots or Neuro-Sps), arising from the release of neuropeptides from activated small diameter sensory afferents at topographically distinct body surfaces due to the convergence of visceral and somatic afferents. In turn, the neuropeptides released during neurogenic inflammation may play important roles in the effects of acupuncture as well as the formation of active acupoints. Thus, the present study has focused on the role of substance P (SP) in acupuncture signal transduction and effects.

METHODS

Neuro-Sps were detected by using in vivo fluorescence imaging after intravenous injection of Evans blue dye (EBD) and compared with traditional acupoints. Stimulatory effects of the Neuro-Sps were examined in a rat model of immobilization-induced hypertension (IMH). The roles of increased SP in Neuro-Sps were also investigated by using immunohistochemistry, in vivo single-fiber peripheral nerve recordings, and in vivo midbrain extracellular recordings.

RESULTS

Neurogenic inflammation quickly appeared at acupoints on the wrist and was fully developed within 15 min in IMH model. The Neuro-Sps showed an increased release of SP from afferent nerve terminals. Mechanical stimulation of these Neuro-Sps increased cell excitability in the midbrain (rostral ventrolateral medulla) and alleviated the development of hypertension, which was blocked by the local injection of the SP receptor antagonist CP-99994 into Neuro-Sps prior to acupuncture and mimicked by the local injection of capsaicin. Single fiber recordings of peripheral nerves showed that increased SP into the Neuro-Sps elevated the sensitivity of A- and C-fibers in response to acupuncture stimulation. In addition, the discharge rates of spinal wide dynamic response (WDR) neurons significantly increased following SP or acupuncture treatment in Neuro-Sps in normal rats, but decreased following the injection of CP-99994 into Neuro-Sps in IMH rats.

CONCLUSIONS

Our findings suggest that SP released during neurogenic inflammation enhances the responses of sensory afferents to the needling of acupoints and triggers acupuncture signaling to generate acupuncture effects.

Calcium Increase and Substance P Release Induced by the Neurotoxin Brevetoxin-1 in Sensory Neurons: Involvement of PAR2 Activation through Both Cathepsin S and Canonical Signaling

Red tides involving Karenia brevis expose humans to brevetoxins (PbTxs). Oral exposition triggers neurotoxic shellfish poisoning, whereas inhalation induces a respiratory syndrome and sensory disturbances. No curative treatment is available and the pathophysiology is not fully elucidated. Protease-activated receptor 2 (PAR2), cathepsin S (Cat-S) and substance P (SP) release are crucial mediators of the sensory effects of ciguatoxins (CTXs) which are PbTx analogs. This work explored the role of PAR2 and Cat-S in PbTx-1-induced sensory effects and deciphered the signaling pathway involved. We performed calcium imaging, PAR2 immunolocalization and SP release experiments in monocultured sensory neurons or co-cultured with keratinocytes treated with PbTx-1 or P-CTX-2. We demonstrated that PbTx-1-induced calcium increase and SP release involved Cat-S, PAR2 and transient receptor potential vanilloid 4 (TRPV4). The PbTx-1-induced signaling pathway included protein kinase A (PKA) and TRPV4, which are compatible with the PAR2 biased signaling induced by Cat-S. Internalization of PAR2 and protein kinase C (PKC), inositol triphosphate receptor and TRPV4 activation evoked by PbTx-1 are compatible with the PAR2 canonical signaling. Our results suggest that PbTx-1-induced sensory disturbances involve the PAR2-TRPV4 pathway. We identified PAR2, Cat-S, PKA, and PKC that are involved in TRPV4 sensitization induced by PbTx-1 in sensory neurons.

A test-retest reliability study of assessing small cutaneous fibers by measuring current perception threshold with pin electrodes

Background

The quantitative measurement of current perception threshold (CPT) has been used as a method to assess the function of nerve fibers in neuropathy diseases. The aim of this study was to assess the test-retest reliability measuring CPT using the circular pin electrodes for assessing the function of cutaneous thin nerve fibers.

Methods

CPT measurement was repeated on two separate days with at least one-week interval in 55 volunteers. Superficial blood flow (SBF) and skin temperature (ST) were measured on the skin in an around area concentric to the circular pin electrodes after the process of finding CPTs. The coefficient of variation (CV) and intra-class correlation coefficient (ICC) were calculated. The correlation between each two of CPT, SBF increment and ST increment was analyzed.

Results

No significant differences were found for CPT, SBF and ST between two sessions. SBF was found to be significantly increased after the process of finding CPT. CPT values of males were found to be higher than females. SBF increment was found to be positively correlated with ST increment. The ICC values for CPT, SBF and ST were 0.595, 0.852 and 0.728, respectively. The CV values for CPT, SBF and ST were 25.53%, 12.59% and 1.94%, respectively.

Conclusions

The reliability of CPT measurement using circular pin electrodes is fair, and need consistence of measurements in longitudinal studies.

Contribution of GABAergic modulation in DRGs to electroacupuncture analgesia in incisional neck pain rats

Purpose

Acupuncture therapy is effective for relieving postoperative pain. Our previous study showed that electroacupuncture (EA) at Futu (LI18) and Hegu (LI4)–Neiguan (PC6) could alleviate incisional neck pain, which was related with its effect in upregulating γ-aminobutyric acid (GABA) expression in cervical (C3–6) dorsal root ganglions (DRGs); but whether its receptor subsets GABA_Aα2R and GABA_BR1 in C3–6 DRGs are involved in EA analgesia or not, it remains unknown.

Materials and methods

Seventy-five male Sprague Dawley rats were randomized to normal control, model, LI18, LI4–PC6, and Zusanli (ST36)–Yanglingquan (GB34) groups. The incisional neck pain model was established by making a longitudinal incision along the midline of the rats’ neck, followed by repeated mechanical stimulation. EA was applied to bilateral LI18, LI4–PC6, or ST36–GB34 for 30 minutes at 4, 24, and 48 hours after operation. The thermal pain threshold of the neck was detected by a tail-flick unit, and the C3–6 DRGs were removed for assaying the immunoactivity of substance P (SP), GABA_Aα2R, glial fibrillary acidic protein (GFAP; a marker of satellite glial cells [SGCs]), and GABA_BR1 and the expression of GABA_Aα2R and GABA_BR1 mRNA and proteins using immunofluorescence, real-time PCR, and Western blotting, respectively.

Results

The cervical thermal pain threshold was significantly lower in the model group than the normal group (P<0.001), indicating hyperalgesia after neck incision, and was considerably increased in both EA-LI18 and LI4–PC6 groups (P<0.001), but not in ST36–GB34 group compared with model group (P>0.05). Immunofluorescence staining showed that GABA_Aα2 R expressed on SP⁺ neurons, and GABA_BR1 on SGCs. EA of LI18 and LI4–PC6 markedly suppressed the modeling-induced upregulation of the immunoactivity of SP (P<0.001 and P<0.01, respectively) and GFAP (P<0.01 and P<0.001, respectively) and significantly reversed neck incision–induced downregulation of the expression of GABA_Aα2R and GABA_BR1 mRNAs and proteins (P<0.05).

Conclusion

EA of LI18 and LI4–PC6 has an analgesic effect in incisional neck pain rats, which is related to its effects in upregulating GABAergic inhibitory modulation on nociceptive peptidergic neurons and SGCs in cervical DRGs.

Non-myelinated C-fibers, but not myelinated A-fibers, elongate into the epidermis in dry skin with itch

Chronic skin diseases with itch and dry skin show increased peripheral nerve fiber elongation into the epidermis. However, the characteristics of the elongated nerve fibers remain unclear. Therefore, we investigated the characteristics of the elongated nerve fibers using a dry skin mouse model with itch. In this mouse model, prepared via repetitive treatments with an acetone/ether mixture and water, the stratum corneum water content was decreased, whereas spontaneous scratching and epidermal hyperplasia were increased. In addition, the number of substance P (SP)- and calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers (C-fibers) was increased in the epidermis of treated mice compared to that in non-treated control mice. However, neurofilament 200-immunoreactive nerve fibers (A-fibers) were not detected in the epidermis of treated mice. These results suggest that the elongated epidermal peripheral nerve fibers comprise SP/CGRP-containing C-fibers but not A-fibers. Thus, these fibers may be involved in the induction of dry skin pruritus.

Aberrant distribution of the trochlear nerve: A cadaveric study supported by immunohistochemistry

The trochlear nerve is generally considered to be a purely motor nerve supplying one extraocular muscle, the superior oblique. In the current study, 28 orbits were dissected and in one orbit (3.6%), the trochlear nerve divided into two main branches. The medial branch followed the classical course, entered the superior oblique muscle and was presumed to be motor in function. However, before entering the muscle, it partially fused with the frontal nerve, and gave a bundle of nerve fibres to the frontal nerve. The lateral branch gave a communication to the frontal nerve, travelled along the lacrimal nerve, received a branch from the lacrimal nerve then penetrated the lacrimal gland. The lateral branch was presumed to be sensory. Paraffin sections from the two branches were stained using immunohistochemistry. The two branches had different nerve fibre populations and showed distinct differences in neurofilament proteins (NFP) immuno-labelling. While both branches showed intense labelling for NFP-H, the lateral branch showed no staining or faint staining for NFP-M and NFP-L respectively, but the medial branch showed moderate labelling for both the NFP-M and NFP-L. Staining for substance P, a marker for nociceptive fibres, showed intense staining in a subset of fibres in the lateral branch, but no staining in the medial branch. Calcitonin gene-related peptide labelling was evident in some axons and some Schwann cells in the medial branch but widespread, weak and fine granular in the lateral branch. These findings indicate that, in some individuals (3.6%), the trochlear nerve may contain motor and sensory fibres, suggesting inter-nuclear communication within the brainstem during embryogenesis or mixing of nerve fibres in their extra-axial pathways.

Distribution pattern of dorsal root ganglion neurons synthesizing nitric oxide synthase in different animal species

The main aim of the present review is to provide at first a short survey of the basic anatomical description of sensory ganglion neurons in relation to cell size, conduction velocity, thickness of myelin sheath, and functional classification of their processes. In addition, we have focused on discussing current knowledge about the distribution pattern of neuronal nitric oxide synthase containing sensory neurons especially in the dorsal root ganglia in different animal species; hence, there is a large controversy in relation to interpretation of the results dealing with this interesting field of research.

Myelinated and unmyelinated nerve fibers reinnervate tissue-engineered dermo-epidermal human skin analogs in an in vivo model

PURPOSE

The clinical application of autologous tissue-engineered skin analogs is an important strategy to cover large skin defects. Investigating biological dynamics, such as reinnervation after transplantation, is essential to improve the quality of such skin analogs. Previously, we have examined that our skin substitutes are reinnervated by host peripheral nerve fibers as early as 8 weeks after transplantation. Here, we wanted to investigate the presence and possible differences regarding myelinated and unmyelinated host nerve fibers 15 weeks after the transplantation of light and dark human tissue-engineered skin analogs.

METHODS

Human epidermal keratinocytes, melanocytes, and dermal fibroblasts were isolated from human light and dark skin biopsies. Keratinocytes and melanocytes were seeded on fibroblast-containing collagen type I hydrogels after expansion in culture. After additional culturing, the tissue-engineered dermo-epidermal skin analogs were transplanted onto full-thickness skin wounds created on the back of immuno-incompetent rats. Skin substitutes were excised and analyzed 15 weeks after transplantation. Histological sections were examined with regard to the ingrowth pattern of myelinated and unmyelinated nerve fibers into the skin analogs using markers, such as Substance P, NF200, and S100-Beta.

RESULTS

We found myelinated and unmyelinated peripheral host nerve fibers 15 weeks after transplantation in the dermal part of our human skin substitutes. In particular, we identified large-diameter-myelinated Aβ- and Aδ-fibers, and small-diameter C-fibers. Furthermore, we observed myelinated nerves in close proximity to CD31-positive blood capillaries. In the long run, both types of ingrown host fibers showed an identical pattern in both light and dark skin analogs.

CONCLUSION

Our data suggest that myelinated and unmyelinated peripheral nerves reinnervate human skin substitutes in a long-term in vivo transplantation assay. Our tissue-engineered skin analogs attract A- and C-fibers to supply both light and dark skin analogs. Potentially, this process restores skin sensitivity and has, therefore, a significant relevance with regard to future application of autologous pigmented dermo-epidermal skin substitutes onto patients.

Role of neurokinin type 1 receptor in nociception at the periphery and the spinal level in the rat

OBJECTIVES

Noxious stimuli activate small to medium-sized dorsal root ganglion (DRG) neurons. Intense noxious stimuli result in the release of substance P (SP) from the central terminals of these neurons. It binds to the neurokinin type 1 receptor (NK1r) and sensitises the dorsal horn neurons. SP is also released from the peripheral terminals leading to neurogenic inflammation. However, their individual contribution at spinal and peripheral levels to postincisional nociception has not been delineated as yet.

METHODS

Sprague-Dawley rats were administered different doses (3-100 μg) of an NK1r antagonist (L760735) by intrathecal (i.t.) route before hind paw incision. On the basis of its antinociceptive effect on guarding behaviour, the 30 μg dose was selected for further study. In different sets of animals, this was administered i.t. (postemptive) and intrawound (i.w.). Finally, in another group, drug (30 μg) was administered through both i.t and i.w. routes. The antinociceptive effect was assessed and compared. Expression of SP was examined in the spinal cord. Intrawound concentration of SP and inflammatory mediators was also evaluated.

RESULTS

Postemptive i.t. administration significantly attenuated guarding and allodynia. Guarding was alone decreased after i.w. drug treatment. Combined drug administration further attenuated all nociceptive parameters, more so after postemptive treatment. Expression of SP in the spinal cord decreased post incision but increased in the paw tissue. Inflammatory mediators like the nerve growth factor also increased after incision.

CONCLUSION

In conclusion, SP acting through the NK1r appears to be an important mediator of nociception, more so at the spinal level. These findings could have clinical relevance.

Primary afferent neurons containing calcitonin gene-related peptide but not substance P in forepaw skin, dorsal root ganglia, and spinal cord of mice

In mice dorsal root ganglia (DRG), some neurons express calcitonin gene-related peptide (CGRP) without substance P (SP; CGRP(+) SP(-) ). The projections and functions of these neurons are unknown. Therefore, we combined in vitro axonal tracing with multiple-labeling immunohistochemistry to neurochemically define these neurons and characterize their peripheral and central projections. Cervical spinal cord, DRG, and forepaw skin were removed from C57Bl/6 mice and multiple-labeled for CGRP, SP, and either marker for the sensory neuron subpopulations transient receptor potential vanilloid type 1 (TRPV1), neurofilament 200 (NF200), or vesicular glutamate transporter 2 (VGluT1). To determine central projections of CGRP(+) SP(-) neurons, Neurobiotin (NB) was applied to the C7 ventral ramus and visualized in DRG and spinal cord sections colabeled for CGRP and SP. Half (50%) of the CGRP-immunoreactive DRG neurons lacked detectable SP and had a mean soma size of 473 ± 14 μm(2) (n = 5); 89% of the CGRP(+) SP(-) neurons expressed NF200 (n = 5), but only 32% expressed TRPV1 (n = 5). Cutaneous CGRP(+) SP(-) fibers were numerous within dermal papillae and around hair shafts (n = 4). CGRP(+) SP(-) boutons were prevalent in lateral lamina I and in lamina IV/V of the dorsal horn (n = 5). NB predominantly labeled fibers penetrating lamina IV/V, 6 ± 3% contained CGRP (n = 5), and 21 ± 2% contained VGluT1 (n = 3). CGRP(+) SP(-) afferent neurons are likely to be non-nociceptive. Their soma size, neurochemical profile, and peripheral and central targets suggest that CGRP(+) SP(-) neurons are polymodal mechanoceptors.

Targeting Opioid-Induced Hyperalgesia in Clinical Treatment: Neurobiological Considerations

Opioid analgesics have become a cornerstone in the treatment of moderate to severe pain, resulting in a steady rise of opioid prescriptions. Subsequently, there has been a striking increase in the number of opioid-dependent individuals, opioid-related overdoses, and fatalities. Clinical use of opioids is further complicated by an increasingly deleterious profile of side effects beyond addiction, including tolerance and opioid-induced hyperalgesia (OIH), where OIH is defined as an increased sensitivity to already painful stimuli. This paradoxical state of increased nociception results from acute and long-term exposure to opioids, and appears to develop in a substantial subset of patients using opioids. Recently, there has been considerable interest in developing an efficacious treatment regimen for acute and chronic pain. However, there are currently no well-established treatments for OIH. Several substrates have emerged as potential modulators of OIH, including the N-methyl-D-aspartate and γ-aminobutyric acid receptors, and most notably, the innate neuroimmune system. This review summarizes the neurobiology of OIH in the context of clinical treatment; specifically, we review evidence for several pathways that show promise for the treatment of pain going forward, as prospective adjuvants to opioid analgesics. Overall, we suggest that this paradoxical state be considered an additional target of clinical treatment for chronic pain.

PKCɛ mediates substance P inhibition of GABAA receptors-mediated current in rat dorsal root ganglion

The mechanism underlying the modulatory effect of substance P (SP) on GABA-activated response in rat dorsal root ganglion (DRG) neurons was investigated. In freshly dissociated rat DRG neurons, whole-cell patch-clamp technique was used to record GABA-activated current and sharp electrode intracellular recording technique was used to record GABA-induced membrane depolarization. Application of GABA (1-1000 μmol/L) induced an inward current in a concentration-dependent manner in 114 out of 127 DRG neurons (89.8 %) examined with whole-cell patch-clamp recordings. Bath application of GABA (1-1000 μmol/L) evoked a depolarizing response in 236 out of 257 (91.8%) DRG neurons examined with intracellular recordings. Application of SP (0.001-1 μmol/L) suppressed the GABA-activated inward current and membrane depolarization. The inhibitory effects were concentration-dependent and could be blocked by the selective neurokinin 1 (NK1) receptors antagonist spantide but not by L659187 and SR142801 (1 μmol/L, n=7), selective antagonists of NK2 and NK3. The inhibitory effect of SP was significantly reduced by the calcium chelator BAPTA-AM, phospholipase C (PLC) inhibitor U73122, and PKC inhibitor chelerythrine, respectively. The PKA inhibitor H-89 did not affect the SP effect. Remarkably, the inhibitory effect of SP on GABA-activated current was nearly completely removed by a selective PKCε inhibitor epilon-V1-2 but not by safingol and LY333531, selective inhibitors of PKCα and PKCβ. Our results suggest that NK1 receptor mediates SP-induced inhibition of GABA-activated current and membrane depolarization by activating intracellular PLC-Ca²⁺-PKCε cascade. SP might regulate the excitability of peripheral nociceptors through inhibition of the "pre-synaptic inhibition" evoked by GABA, which may explain its role in pain and neurogenic inflammation.

The chemerin receptor 23 agonist, chemerin, attenuates monosynaptic C-fibre input to lamina I neurokinin 1 receptor expressing rat spinal cord neurons in inflammatory pain

Background

Recent evidence has shown that the chemerin receptor 23 (ChemR23) represents a novel inflammatory pain target, whereby the ChemR23 agonists, resolvin E1 and chemerin, can inhibit inflammatory pain hypersensitivity, by a mechanism that involves normalisation of potentiated spinal cord responses. This study has examined the ability of the ChemR23 agonist, chemerin, to modulate synaptic input to lamina I neurokinin 1 receptor expressing (NK1R+) dorsal horn neurons, which are known to be crucial for the manifestation of inflammatory pain.

Results

Whole-cell patch-clamp recordings from pre-identified lamina I NK1R+ neurons, in rat spinal cord slices, revealed that chemerin significantly attenuates capsaicin potentiation of miniature excitatory postsynaptic current (mEPSC) frequency, but is without effect in non-potentiated conditions. In tissue isolated from complete Freund’s adjuvant (CFA) treated rats, chemerin significantly reduced the peak amplitude of monosynaptic C-fibre evoked excitatory postsynaptic currents (eEPSCs) in a subset of lamina I NK1R+ neurons, termed chemerin responders. However, chemerin did not alter the peak amplitude of monosynaptic C-fibre eEPSCs in control tissue. Furthermore, paired-pulse recordings in CFA tissue demonstrated that chemerin significantly reduced paired-pulse depression in the subset of neurons classified as chemerin responders, but was without effect in non-responders, indicating that chemerin acts presynaptically to attenuate monosynaptic C-fibre input to a subset of lamina I NK1R+ neurons.

Conclusions

These results suggest that the reported ability of ChemR23 agonists to attenuate inflammatory pain hypersensitivity may in part be due to a presynaptic inhibition of monosynaptic C-fibre input to lamina I NK1R+ neurons and provides further evidence that ChemR23 represents a promising inflammatory pain target.

NOP receptor mediates anti-analgesia induced by agonist-antagonist opioids

Clinical studies have shown that agonist-antagonist opioid analgesics that produce their analgesic effect via action on the kappa-opioid receptor, produce a delayed-onset anti-analgesia in men but not women, an effect blocked by co-administration of a low dose of naloxone. We now report the same time-dependent anti-analgesia and its underlying mechanism in an animal model. Using the Randall-Selitto paw-withdrawal assay in male rats, we found that nalbuphine, pentazocine, and butorphanol each produced analgesia during the first hour followed by anti-analgesia starting at ∼90min after administration in males but not females, closely mimicking its clinical effects. As observed in humans, co-administration of nalbuphine with naloxone in a dose ratio of 12.5:1 blocked anti-analgesia but not analgesia. Administration of the highly selective kappa-opioid receptor agonist U69593 produced analgesia without subsequent anti-analgesia, and confirmed by the failure of the selective kappa antagonist nor-binaltorphimine to block nalbuphine-induced anti-analgesia, indicating that anti-analgesia is not mediated by kappa-opioid receptors. We therefore tested the role of other receptors in nalbuphine anti-analgesia. Nociceptin/orphanin FQ (NOP) and sigma-1 and sigma-2 receptors were chosen on the basis of their known anti-analgesic effects and receptor binding studies. The selective NOP receptor antagonists, JTC801, and J-113397, but not the sigma receptor antagonist, BD 1047, antagonized nalbuphine anti-analgesia. Furthermore, the NOP receptor agonist NNC 63-0532 produced anti-analgesia with the same delay in onset observed with the three agonist-antagonists, but without producing preceding analgesia and this anti-analgesia was also blocked by naloxone. These results strongly support the suggestion that clinically used agonist-antagonists act at the NOP receptor to produce anti-analgesia.

Evidence for A1 and A 3 receptors mediating adenosine-induced intracellular calcium release in the dorsal root ganglion neurons by using confocal microscopy imaging

Adenosine exerts a key role in analgesia. In the present study, adenosine-induced Ca(2+) responses were revealed by using confocal microscopy imaging in the rat dorsal root ganglia (DRG) neurons in vitro. Our results showed that adenosine could evoke increases in the intracellular Ca(2+) concentration in the DRG neurons. In addition, by application of selective receptor antagonists, two types of receptors, A1R and A3R, were identified to be involved in the adenosine-induced Ca(2+) release from intracellular stores in neurons. Altogether, these results suggest that confocal microscopy imaging combined with fluorescent dyes could help to detect the analgesic-induced ion signaling in single cell.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Endocannabinoid-dependent plasticity at spinal nociceptor synapses

Neuroplastic changes at the spinal synapses between primary nociceptors and second order dorsal horn neurons play key roles in pain and analgesia. NMDA receptor-dependent forms of long-term plasticity have been studied extensively at these synapses, but little is known about possible contributions of the endocannabinoid system. Here, we addressed the role of cannabinoid (CB)1 receptors in activity-dependent plasticity at these synapses. We report that conditional low-frequency stimulation of high-threshold primary sensory nerve fibres paired with depolarisation of the postsynaptic neuron evoked robust long-term depression (LTD)of excitatory synaptic transmission by about 40% in the vast majority (90%) of recordings made in wild-type mice. When recordings were made from global or nociceptor-specific CB(1) receptor-deficient mice (CB(1) (−/− ) mice and sns-CB(1)(−/−) mice), the portion of neurons exhibiting LTD was strongly reduced to about 25%. Accordingly, LTD was prevented to a similar extent by the CB1 receptor antagonist AM251 and mimicked by pharmacological activation of CB1 receptors. In a subset of neurons with EPSCs of particularly high stimulation thresholds, we furthermore found that the absence of CB(1) receptors in CB(1)(−/−) and sns-CB(1)(−/−) mice converted the response to the paired conditioning stimulation protocol from LTD to long-term potentiation (LTP). Our results identify CB1 receptor-dependent LTD as a form of synaptic plasticity previously unknown in spinal nociceptors. They furthermore suggest that prevention of LTP may be a second hither to unknown function of CB1 receptors in primary nociceptors. Both findings may have important implications for our understanding of endogenous pain control mechanisms and of analgesia evoked by cannabinoid receptor agonists.

Topical FK962 facilitates axonal regeneration and recovery of corneal sensitivity after flap surgery in rabbits

PURPOSE

To test if the drug FK962 (N-(1-acetylpiperidin-4-yl)-4-fluorobenzamide) facilitates axonal elongation and recovery of corneal sensitivity after creation of a corneal flap in rabbits.

DESIGN

Animal study.

METHODS

Primary cultures of rabbit trigeminal ganglion cells were used to test if FK962 promoted nerve elongation in vitro. A 130 μm-thick×8.6 mm-diameter flap was created in rabbit corneas where topical 10(-6) M FK962 was administered 4 times daily. After treatment of 7 days, corneal mechanical sensitivity was measured using a Cochet-Bonnet esthesiometer. Whole-mount corneal sections were prepared, sensory nerve axons were stained with antibody for neurofilament, and axonal elongation from transected nerve termini were scored using standardized criteria. Ocular pharmacokinetics modeling was used to predict permeation of topical FK962 into cornea.

RESULTS

FK962 accelerated sprouting and elongation of neurites in cultured neuronal cells from rabbit trigeminal ganglia. In the in vivo rabbit model, distal axons from transected nerve termini in corneas disappeared soon after flap surgery; but with time, axons regenerated and elongated. Topical application of 10(-6) M FK962 for 7 days significantly enhanced axonal elongation and increased corneal sensitivity. Increased corneal sensitivity was directly and significantly correlated with axonal elongation, suggesting functional enhancement of re-innervation by FK962.

CONCLUSIONS

Results from a rabbit model of laser in situ keratomileusis (LASIK) surgery showed that topical FK962 facilitated corneal re-innervation leading to recovery of sensitivity. Results suggested that topical application of FK962 might decrease complications in patients after LASIK surgery.

B-Afferents: A fundamental division of the nervous system mediating homeostasis?

The peripheral nervous system (PNS) has classically been separated into a somatic division composed of both afferent and efferent pathways and an autonomic division containing only efferents. J. N. Langley, who codified this asymmetrical plan at the beginning of the twentieth century, considered different afferents, including visceral ones, as candidates for inclusion in his concept of the “autonomic nervous system” (ANS), but he finally excluded all candidates for lack of any distinguishing histological markers. Langley's classification has been enormously influential in shaping modern ideas about both the structure and the function of the PNS. We survey recent information about the PNS and argue that many of the sensory neurons designated as “visceral” and “somatic” are in fact part of a histologically distinct group of afferents concerned primarily autonomic function. These afferents have traditionally been known as “small dark” neurons or B-neurons. In this target article we outline an association between autonomic and B-neurons based on ontogeny, cell phenotype, and functional relations, grouping them together as part of a common reflex system involved in homeostasis. This more parsimonious classification of the PNS, made possible by the identification of a group of afferents associated primarily with the ANS, avoids a number of confusions produced by the classical orientation. It may also have practical implications for an understanding of nociception, homeostatic reflexes, and the evolution of the nervous system.

Differential expression of ATP7A, ATP7B and CTR1 in adult rat dorsal root ganglion tissue

BACKGROUND

ATP7A, ATP7B and CTR1 are metal transporting proteins that control the cellular disposition of copper and platinum drugs, but their expression in dorsal root ganglion (DRG) tissue and their role in platinum-induced neurotoxicity are unknown. To investigate the DRG expression of ATP7A, ATP7B and CTR1, lumbar DRG and reference tissues were collected for real time quantitative PCR, RT-PCR, immunohistochemistry and Western blot analysis from healthy control adult rats or from animals treated with intraperitoneal oxaliplatin (1.85 mg/kg) or drug vehicle twice weekly for 8 weeks.

RESULTS

In DRG tissue from healthy control animals, ATP7A mRNA was clearly detectable at levels similar to those found in the brain and spinal cord, and intense ATP7A immunoreactivity was localised to the cytoplasm of cell bodies of smaller DRG neurons without staining of satellite cells, nerve fibres or co-localisation with phosphorylated heavy neurofilament subunit (pNF-H). High levels of CTR1 mRNA were detected in all tissues from healthy control animals, and strong CTR1 immunoreactivity was associated with plasma membranes and vesicular cytoplasmic structures of the cell bodies of larger-sized DRG neurons without co-localization with ATP7A. DRG neurons with strong expression of ATP7A or CTR1 had distinct cell body size profiles with minimal overlap between them. Oxaliplatin treatment did not alter the size profile of strongly ATP7A-immunoreactive neurons but significantly reduced the size profile of strongly CTR1-immunoreactive neurons. ATP7B mRNA was barely detectable, and no specific immunoreactivity for ATP7B was found, in DRG tissue from healthy control animals.

CONCLUSIONS

In conclusion, adult rat DRG tissue exhibits a specific pattern of expression of copper transporters with distinct subsets of peripheral sensory neurons intensely expressing either ATP7A or CTR1, but not both or ATP7B. The neuron subtype-specific and largely non-overlapping distribution of ATP7A and CTR1 within rat DRG tissue may be required to support the potentially differing cuproenzyme requirements of distinct subsets of sensory neurons, and could influence the transport and neurotoxicity of oxaliplatin.

Responsiveness of C neurons in rat dorsal root ganglion to 5-hydroxytryptamine-induced pruritic stimuli in vivo

Itching is a common symptom in dermatologic diseases and causes restless scratching of the skin, which aggravates the condition. The mechanism of the itch sensation, however, is enigmatic. The present study included behavioral tests and electrophysiological recordings from rat dorsal root ganglion (DRG) neurons in vivo to analyze the response to pruritic stimuli induced by topical application of 5-hydroxytryptamine (5-HT) to the skin. Topically applied 5-HT to the rostral back evoked scratching, whereas application of the vehicle did not. Following subcutaneous injection of the opioid receptor antagonist naloxone, the number of scratches decreased, suggesting that the scratching was preferentially mediated by itch but not pain sensation. To elucidate the firing properties of DRG neurons in response to topically applied 5-HT, intracellular recordings were made from DRG neurons in vivo. None of the Abeta and Adelta neurons responded to 5-HT; in contrast, 25 of 91 C neurons (27%) exhibited repetitive firing in response to 5-HT, which could be classified into two firing patterns: one was a transient type, characterized by low firing frequency that decreased within 5 min; the other was a long-lasting type, having high firing frequency that continued increasing after 5 min. The time course of the firing pattern of long-lasting C neurons was comparable to the scratching behavior. Intriguingly, the long-lasting-type neurons had a significantly smaller fast afterhyperpolarization than that of the 5-HT-insensitive neurons. These observations suggest that the long-lasting-firing C neurons in rat DRG sensitive to 5-HT are responsible for conveying pruritic information to the spinal cord.

Immunohistochemical characterization of TH13-L2 spinal ganglia neurons in sheep (Ovis aries)

Spinal ganglia (SG) neurons are commonly classified according to various specific features. The most widespread classification based on morphological and ultrastructural features subdivides SG neurons into light and small dark neurons. Using immunohistochemical, histochemical and lectin methods, it is possible to further subdivide the small dark neurons into two subpopulations: peptidergic and nonpeptidergic neurons. The majority of studies on SG neurons were carried out on mice and rats; there are few or no studies on large mammals. In this study, some of the widely used neuronal markers, neurofilament 200 kDa (NF200), substance P (SP), calcitonin gene-related peptide (CGRP) and isolectin B4 (IB4), were employed to characterize neuronal nitric oxide synthase (nNOS)-immunoreactivity (-IR) in sheep (Ovis aries) SG (Th13-L2) neurons. The majority of the SG neurons were IB4-labeled (79 +/- 10%), followed by NF200- (45 +/- 4%), NOS- (44 +/- 10%), SP- (42 +/- 5%) and CGRP-IR (35 +/- 7%) neurons. The triple staining experiments showed that a higher percentage (75 +/- 16%) of NOS-IR neurons bound both IB4 and CGRP, or both IB4 and SP (49 +/- 6%). The IB4 marker showed an unexpected staining pattern; in fact, IB4-labeled neurons largely colocalized with NF200, usually considered a marker of light SG neurons, and with CGRP and SP. For this reason, IB4 cannot be employed in sheep to differentiate between light and dark neurons, or between peptidergic and nonpeptidergic neurons. These results suggest the importance of being cautious when comparing data among different species.

A phenotypically restricted set of primary afferent nerve fibers innervate the bone versus skin: therapeutic opportunity for treating skeletal pain

Although musculoskeletal pain is one of the most common causes of chronic pain and physical disability in both developing and developed countries, relatively little is known about the nerve fibers and mechanisms that drive skeletal pain. Small diameter sensory nerve fibers, most of which are C-fiber nociceptors, can be separated into two broad populations: the peptide-rich and peptide-poor nerve fibers. Peptide-rich nerve fibers express substance P (SP) and calcitonin gene-related peptide (CGRP). In contrast, the peptide-poor nerve fibers bind to isolectin B4 (IB(4)) and express the purinergic receptor P(2)X(3) and Mas-related G protein-coupled receptor member d (Mrgprd). In the present report, we used mice in which the Mrgprd(+) nerve fibers express genetically encoded axonal tracers to determine the peptide-rich and peptide-poor sensory nerve fibers that innervate the glabrous skin of the hindpaw as compared to the bone marrow, mineralized bone and periosteum of the femur. Whereas the skin is richly innervated by CGRP(+), SP(+), P(2)X(3)(+) and Mrgprd(+) sensory nerve fibers, the bone marrow, mineralized bone and periosteum receive a significant innervation by SP(+) and CGRP(+), but not Mrgprd(+) and P(2)X(3)(+) nerve fibers. This lack of redundancy in the populations of C-fibers that innervate the bone may present a unique therapeutic opportunity for targeting skeletal pain as the peptide-rich and peptide-poor sensory nerve fibers generally express a different repertoire of receptors and channels to detect noxious stimuli. Thus, therapies that target the specific types of C-nerve fibers that innervate the bone may be uniquely effective in attenuating skeletal pain as compared to skin pain.

Role of neurotrophin signalling in the differentiation of neurons from dorsal root ganglia and sympathetic ganglia

Manipulation of neurotrophin (NT) signalling by administration or depletion of NTs, by transgenic overexpression or by deletion of genes coding for NTs and their receptors has demonstrated the importance of NT signalling for the survival and differentiation of neurons in sympathetic and dorsal root ganglia (DRG). Combination with mutation of the proapoptotic Bax gene allows the separation of survival and differentiation effects. These studies together with cell culture analysis suggest that NT signalling directly regulates the differentiation of neuron subpopulations and their integration into neural networks. The high-affinity NT receptors trkA, trkB and trkC are restricted to subpopulations of mature neurons, whereas their expression at early developmental stages largely overlaps. trkC is expressed throughout sympathetic ganglia and DRG early after ganglion formation but becomes restricted to small neuron subpopulations during embryogenesis when trkA is turned on. The temporal relationship between trkA and trkC expression is conserved between sympathetic ganglia and DRG. In DRG, NGF signalling is required not only for survival, but also for the differentiation of nociceptors. Expression of neuropeptides calcitonin gene-related peptide and substance P, which specify peptidergic nociceptors, depends on nerve growth factor (NGF) signalling. ret expression indicative of non-peptidergic nociceptors is also promoted by the NGF-signalling pathway. Regulation of TRP channels by NGF signalling might specify the temperature sensitivity of afferent neurons embryonically. The manipulation of NGF levels "tunes" heat sensitivity in nociceptors at postnatal and adult stages. Brain-derived neurotrophic factor signalling is required for subpopulations of DRG neurons that are not fully characterized; it affects mechanical sensitivity in slowly adapting, low-threshold mechanoreceptors and might involve the regulation of DEG/ENaC ion channels. NT3 signalling is required for the generation and survival of various DRG neuron classes, in particular proprioceptors. Its importance for peripheral projections and central connectivity of proprioceptors demonstrates the significance of NT signalling for integrating responsive neurons in neural networks. The molecular targets of NT3 signalling in proprioceptor differentiation remain to be characterized. In sympathetic ganglia, NGF signalling regulates dendritic development and axonal projections. Its role in the specification of other neuronal properties is less well analysed. In vitro analysis suggests the involvement of NT signalling in the choice between the noradrenergic and cholinergic transmitter phenotype, in the expression of various classes of ion channels and for target connectivity. In vivo analysis is required to show the degree to which NT signalling regulates these sympathetic neuron properties in developing embryos and postnatally.

Substance P release and neurokinin 1 receptor activation in the rat spinal cord increase with the firing frequency of C-fibers

Both the firing frequency of primary afferents and neurokinin 1 receptor (NK1R) internalization in dorsal horn neurons increase with the intensity of noxious stimulus. Accordingly, we studied how the pattern of firing of primary afferent influences NK1R internalization. In rat spinal cord slices, electrical stimulation of the dorsal root evoked NK1R internalization in lamina I neurons by inducing substance P release from primary afferents. The stimulation frequency had pronounced effects on NK1R internalization, which increased up to 100 Hz and then diminished abruptly at 200 Hz. Peptidase inhibitors increased NK1R internalization at frequencies below 30 Hz, indicating that peptidases limit the access of substance P to the receptor at moderate firing rates. NK1R internalization increased with number of pulses at all frequencies, but maximal internalization was substantially lower at 1-10 Hz than at 30 Hz. Pulses organized into bursts produced the same NK1R internalization as sustained 30 Hz stimulation. To determine whether substance P release induced at high stimulation frequencies was from C-fibers, we recorded compound action potentials in the sciatic nerve of anesthetized rats. We observed substantial NK1R internalization when stimulating at intensities evoking a C-elevation, but not at intensities evoking only an Adelta-elevation. Each pulse in trains at frequencies up to 100 Hz evoked a C-elevation, demonstrating that C-fibers can follow these high frequencies. C-elevation amplitudes declined progressively with increasing stimulation frequency, which was likely caused by a combination of factors including temporal dispersion. In conclusion, the instantaneous firing frequency in C-fibers determines the amount of substance P released by noxious stimuli.

The role of peptides in central sensitization

Peptides released in the spinal cord from the central terminals of nociceptors contribute to the persistent hyperalgesia that defines the clinical experience of chronic pain. Using substance P (SP) and calcitonin gene-related peptide (CGRP) as examples, this review addresses the multiple mechanisms through which peptidergic neurotransmission contributes to the development and maintenance of chronic pain. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release and receptors on spinal neurons increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent mechanisms. Substance P activates neurokinin receptors (3 subtypes) which couple to phospholipase C and the generation of the intracellular messengers whose downstream effects include depolarizing the membrane and facilitating the function of AMPA and NMDA receptors. Activation of neurokinin-1 receptors also increases the synthesis of prostaglandins whereas activation of neurokinin-3 receptors increases the synthesis of nitric oxide. Both products act as retrograde messengers across synapses and facilitate nociceptive signaling in the spinal cord. Whereas these cellular effects of CGRP and SP at the level of the spinal cord contribute to the development of increased synaptic strength between nociceptors and spinal neurons in the pathway for pain, the different intracellular signaling pathways also activate different transcription factors. The activated transcription factors initiate changes in the expression of genes that contribute to long-term changes in the excitability of spinal and maintain hyperalgesia.

Identification and immunohistochemical characterization of colospinal afferent neurons in the rat

The classification, morphology and function of enteric neurons have been extensively studied in the small and large intestine. However, little is known about enteric neurons that directly project to the CNS. Previous studies have identified these unique neurons in the rectum, rectospinal neurons, but little was done to characterize them. Therefore, the aim of this study was to identify and characterize enteric neurons in the rat colon that directly project to the CNS by using retrograde neuronal tracing and immunohistochemistry. By applying the retrograde tracers 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) and Fluorogold (FG) to the L6/S1 segments of the spinal cord, we identified these neurons in both the myenteric and submucosal plexuses of the colon. These neurons were immunoreactive for neurofilament (NF) a marker for Adelta-fibers and isolectin-B4 (IB(4)) a marker for C-fibers. These neurons expressed the enzyme neuronal nitric oxide synthase (nNOS) as well as peptides associated with sensory neurons such as substance P (SP) and vasoactive intestinal polypeptide (VIP) but did not express calcitonin gene-related peptide (CGRP). The N-methyl-D-aspartate (NMDA) receptor subunits NR1 and NR2D and proteinase-activated receptor-2 (PAR2) were also found in these neurons. However they did not express the transient receptor potential receptor V1 (TRPV1) or neurokinin 1 receptor (NK1). The expression of the peptides and receptors suggests that there are at least two separate populations of neurons projecting from the colon to the CNS. The data suggest that these colospinal afferent neurons (CANs) might be involved in nociception. Whether sensory information from CANs is perceived by the animal or is part of the parasympathetic reflex is currently not known.