Lysophosphatidic acid provides a missing link between osteoarthritis and joint neuropathic pain.

OBJECTIVE: Emerging evidence suggests that osteoarthritis (OA) has a neuropathic component; however, the identity of the molecules responsible for this peripheral neuropathy is unknown. The aim of this study was to determine the contribution of the bioactive lipid lysophosphatidic acid (LPA) to joint neuropathy and pain. DESIGN: Male Lewis rats received an intra-articular injection of 50 µg of LPA into the knee and allowed to recover for up to 21 days. Saphenous nerve myelination was assessed by g-ratio calculation from electron micrographs and afferent nerve damage visualised by activation transcription factor-3 (ATF-3) expression. Nerve conduction velocity was measured electrophysiologically and joint pain was determined by hindlimb incapacitance. The effect of the LPA antagonist Ki-16425 was also evaluated. Experiments were repeated in the sodium monoiodoacetate (MIA) model of OA. RESULTS: LPA caused joint nerve demyelination which resulted in a drop in nerve conduction velocity. Sensory neurones were ATF-3 positive and animals exhibited joint pain and knee joint damage. MIA-treated rats also showed signs of demyelination and joint neuropathy with concomitant pain. Nerve damage and pain could be ameliorated by Ki-16425 pre-treatment. CONCLUSION: Intra-articular injection of LPA caused knee joint neuropathy, joint damage and pain. Pharmacological blockade of LPA receptors inhibited joint nerve damage and hindlimb incapacitance. Thus, LPA is a candidate molecule for the development of OA nerve damage and the origin of joint neuropathic pain.

Electrophysiological characterization of spinal neurons in different models of diabetes type 1- and type 2-induced neuropathy in rats.

Diabetic polyneuropathy (DPN) is a devastating complication of diabetes. The underlying pathogenesis of DPN is still elusive and an effective treatment devoid of side effects presents a challenge. There is evidence that in type-1 and -2 diabetes, metabolic and morphological changes lead to peripheral nerve damage and altered central nociceptive transmission, which may contribute to neuropathic pain symptoms. We characterized the electrophysiological response properties of spinal wide dynamic range (WDR) neurons in three diabetic models. The streptozotocin (STZ) model was used as a drug-induced model of type-1 diabetes, and the BioBreeding/Worcester (BB/Wor) and Zucker diabetic fatty (ZDF) rat models were used for genetic DPN models. Data were compared to the respective control group (BB/Wor diabetic-resistant, Zucker lean (ZL) and saline-injected Wistar rat). Response properties of WDR neurons to mechanical stimulation and spontaneous activity were assessed. We found abnormal response properties of spinal WDR neurons in all diabetic rats but not controls. Profound differences between models were observed. In BB/Wor diabetic rats evoked responses were increased, while in ZDF rats spontaneous activity was increased and in STZ rats mainly after discharges were increased. The abnormal response properties of neurons might indicate differential pathological, diabetes-induced, changes in spinal neuronal transmission. This study shows for the first time that specific electrophysiological response properties are characteristic for certain models of DPN and that these might reflect the diverse and complex symptomatology of DPN in the clinic.

The bradykinin B1 receptor antagonist BI113823 reverses inflammatory hyperalgesia by desensitization of peripheral and spinal neurons.

BACKGROUND: Bradykinin is a neuropeptide released after tissue damage which plays an important role in inflammatory pain. The up-regulation of the bradykinin B1 receptor in response to inflammation makes it an attractive target for drug development. Aim was to investigate if the selective B1 receptor antagonist BI113823 reduces inflammation-induced mechanical hyperalgesia and if the effect is mediated via peripheral and/or spinal B1 receptor antagonism. METHODS: Electrophysiological recordings of peripheral afferents and spinal neurons were combined with behavioural experiments to better understand the underlying mechanisms of B1 receptor antagonism. Experiments were performed 24 h after injection of complete Freund's adjuvant (CFA) or saline into the paw of Wistar rats. A gene expression analysis for the B1 receptor was performed in different tissues. BI113823 was administered orally or intrathecally to assess effects on CFA-induced hyperalgesia. Peripheral afferents of the saphenous nerve as well as spinal wide dynamic range (WDR) and nociceptive-specific (NS) neurons were recorded, and mechanosensitivity was measured before and after BI113823 administration. RESULTS: BI113823 reduced CFA-induced mechanical hyperalgesia when administered orally or intrathecally. An increased B1 receptor gene expression was found in peripheral and spinal neural tissue. BI113823 significantly reduced mechanosensitivity of peripheral afferents and spinal NS neurons, but had no effect on WDR neurons. CONCLUSION: The selective bradykinin B1 receptor antagonist BI113823 reduces CFA-induced mechanical hyperalgesia which is mediated via antagonism of peripheral as well as spinal bradykinin B1 receptors. The selective modulation of CFA-sensitized spinal NS neurons by BI113823 could be a promising property for the treatment of inflammatory pain.

Activation of PAR(2) receptors sensitizes primary afferents and causes leukocyte rolling and adherence in the rat knee joint.

BACKGROUND AND PURPOSE: The PAR(2) receptors are involved in chronic arthritis by mechanisms that are as yet unclear. Here, we examined PAR(2) activation in the rat knee joint. EXPERIMENTAL APPROACH: PAR(2) in rat knee joint dorsal root ganglia (DRG) cells at L3-L5, retrogradely labelled with Fluoro-gold (FG) were demonstrated immunohistochemically. Electrophysiological recordings from knee joint nerve fibres in urethane anaesthetized Wistar rats assessed the effects of stimulating joint PAR(2) with its activating peptide, 2-furoyl-LIGRLO-NH(2) (1-100 nmol·100 µL(-1) , via close intra-arterial injection). Fibre firing rate was recorded during joint rotations before and 15 min after administration of PAR(2) activating peptide or control peptide. Leukocyte kinetics in the synovial vasculature upon PAR(2) activation were followed by intravital microscopy for 60 min after perfusion of 2-furoyl-LIGRLO-NH(2) or control peptide. Roles for transient receptor potential vanilloid-1 (TRPV1) or neurokinin-1 (NK(1) ) receptors in the PAR(2) responses were assessed using the selective antagonists, SB366791 and RP67580 respectively. KEY RESULTS: PAR(2) were expressed in 59 ± 5% of FG-positive DRG cells; 100 nmol 2-furoyl-LIGRLO-NH(2) increased joint fibre firing rate during normal and noxious rotation, maximal at 3 min (normal; 110 ± 43%, noxious; 90 ± 31%). 2-Furoyl-LIGRLO-NH(2) also significantly increased leukocyte rolling and adhesion over 60 min. All these effects were blocked by pre-treatment with SB366791 and RP67580 (P < 0.05 compared with 2-furoyl-LIGRLO-NH(2) alone). CONCLUSIONS AND IMPLICATIONS: PAR(2) receptors play an acute inflammatory role in the knee joint via TRPV1- and NK(1) -dependent mechanisms involving both PAR(2) -mediated neuronal sensitization and leukocyte trafficking.

Paradoxical effects of the cannabinoid CB2 receptor agonist GW405833 on rat osteoarthritic knee joint pain.

OBJECTIVE: The present study examined whether local administration of the cannabinoid-2 (CB(2)) receptor agonist GW405833 could modulate joint nociception in control rat knee joints and in an animal model of osteoarthritis (OA). METHOD: OA was induced in male Wistar rats by intra-articular injection of sodium monoiodo-acetate with a recovery period of 14 days. Immunohistochemistry was used to evaluate the expression of CB(2) and transient receptor potential vanilloid channel-1 (TRPV1) receptors in the dorsal root ganglion (DRG) and synovial membrane of sham- and sodium mono-iodoacetate (MIA)-treated animals. Electrophysiological recordings were made from knee joint primary afferents in response to rotation of the joint both before and following close intra-arterial injection of different doses of GW405833. The effect of intra-articular GW405833 on joint pain perception was determined by hindlimb incapacitance. An in vitro neuronal release assay was used to see if GW405833 caused release of an inflammatory neuropeptide (calcitonin gene-related peptide - CGRP). RESULTS: CB(2) and TRPV1 receptors were co-localized in DRG neurons and synoviocytes in both sham- and MIA-treated animals. Local application of the GW405833 significantly reduced joint afferent firing rate by up to 31% in control knees. In OA knee joints, however, GW405833 had a pronounced sensitising effect on joint mechanoreceptors. Co-administration of GW405833 with the CB(2) receptor antagonist AM630 or pre-administration of the TRPV1 ion channel antagonist SB366791 attenuated the sensitising effect of GW405833. In the pain studies, intra-articular injection of GW405833 into OA knees augmented hindlimb incapacitance, but had no effect on pain behaviour in saline-injected control joints. GW405833 evoked increased CGRP release via a TRPV1 channel-dependent mechanism. CONCLUSION: These data indicate that GW405833 reduces the mechanosensitivity of afferent nerve fibres in control joints but causes nociceptive responses in OA joints. The observed pro-nociceptive effect of GW405833 appears to involve TRPV1 receptors.

Cathepsin K inhibition reduces CTXII levels and joint pain in the guinea pig model of spontaneous osteoarthritis.

Cathepsin K is a cysteine proteinase which is believed to contribute to osteoarthritis (OA) pathogenesis. This brief report evaluates the effect of the novel selective cathepsin K inhibitor AZ12606133 on cartilage metabolism in the Dunkin-Hartley guinea pig model of spontaneous OA. In parallel, electrophysiological studies were performed to determine whether acute and chronic treatment with the cathepsin K inhibitor could alter joint nociception. Acute treatment of OA knees with AZ12606133 had no effect on joint afferent nerve activity; however, prolonged (1 month) administration of the cathepsin K inhibitor delivered via a chronically implanted osmotic pump significantly reduced mechanosensitivity in response to both non-noxious and noxious joint movements. Urinal concentrations of the cartilage breakdown products cross-linked C-telopeptides of type II collagen (CTXII) were also reduced by chronic cathepsin K inhibition. These data suggest that prolonged AZ12606133 administration can reduce cartilage turnover and joint nociception in the Dunkin-Hartley guinea pig model of spontaneous OA.

Central neural activation of hyperdynamic circulation in portal hypertensive rats depends on vagal afferent nerves.

BACKGROUND: Hyperdynamic circulation in portal hypertensive rats depends on central neural c-fos gene expression, but how the portal hypertensive signal activates central c-fos expression remains obscure. AIM: To elucidate the afferent pathway from the gut to the central cardiovascular regulatory nuclei in portal hypertensive rats. METHODS: Cervical vagus nerves and the coeliac ganglion were treated with topical capsaicin to denervate the sensory afferents. Surgical portal vein stenosis (PVS) was performed 3 weeks after denervation. Effectiveness of vagal afferent denervation was confirmed by absent oesophagus to brainstem transfer of the neural tracer cholera toxin-conjugated horseradish peroxidase. Fos, the protein product of the c-fos gene, was detected by immunohistochemistry in central cardiovascular regulatory nuclei. Vagal nerve activity was measured after acute portal hypertension induced by an inflatable cuff around the portal vein. Cardiac output and mean arterial pressure were recorded. RESULTS: In vagal capsaicin-treated rats, no horseradish peroxidase was detected in the brainstem after oesophageal injection. In vagal capsaicin-treated rats subjected to PVS, Fos expression was significantly decreased in both the solitary tract nucleus and paraventricular nucleus compared with untreated PVS rats. Acute portal hypertension stimulated vagal nerve electrical activity. The hyperdynamic circulation was completely abrogated in vagal capsaicin-treated PVS rats. Capsaicin induced no neural or haemodynamic changes in either sham-operated controls or coeliac ganglion-treated PVS rats. CONCLUSION: In portal hypertensive rats, central cardiovascular regulatory nuclei initiate hyperdynamic circulation in response to a gut signal associated with portal hypertension. Portal hypertension signals to the central nuclei via capsaicin-sensitive vagal afferent nerves.

Electrophysiological evidence that the vasoactive intestinal peptide receptor antagonist VIP6-28 reduces nociception in an animal model of osteoarthritis.

OBJECTIVE: The present study examined whether local administration of the neuropeptide vasoactive intestinal polypeptide (VIP) could modulate joint nociception in normal rat knee joints and if the VIP antagonist VIP(6-28) could ameliorate joint mechanosensitivity in an animal model of osteoarthritis (OA). METHODS: OA was induced in male Wistar rats by intra-articular injection of 3mg sodium monoiodo-acetate with a recovery period of 14 days. Electrophysiological recordings were made from knee joint primary afferents in response to normal rotation and noxious hyper-rotation of the joint both before and following close intra-arterial injection of different doses of VIP and VIP(6-28). RESULTS: Local application of VIP to normal knees caused afferent firing rate to be significantly enhanced during normal rotation (up to 180% P<0.01; n=17) and during hyper-rotation (up to 37% P<0.01; n=17) of the knee. VIP-induced sensitization was blocked by pre-administration of the VIP receptor antagonist VIP(6-28). In the OA group, application of VIP(6-28) caused afferent firing rate to be significantly reduced during normal rotation (up to 45% P<0.05; n=17) and during hyper-rotation (up to 34% P<0.01; n=15) of the knee joint. CONCLUSION: These findings indicate that VIP is involved in peripheral sensitization of knee joint afferents especially in response to normal joint movements. OA-induced sensitization of knee joint afferents was inhibited by local administration of VIP(6-28), indicating that VIP is released into OA knee joints, potentially contributing to joint pain. As such, VIP(6-28) may prove to be a beneficial agent for the treatment of arthritis pain.

Dynamic response properties of visual neurons and context-dependent surround effects on receptive fields in the tectum of the salamander Plethodon shermani.

Neuronal responses to complex prey-like stimuli and rectangles were investigated in the tectum of the salamander Plethodon shermani using extracellular single-cell recording. Cricket dummies differing in size, contrast or movement pattern or a rectangle were moved singly through the excitatory receptive field of a neuron. Paired presentations were performed, in which a reference stimulus was moved inside and the different cricket dummies or the rectangle outside the excitatory receptive field. Visual object recognition involves much more complex spatial and temporal processing than previously assumed in amphibians. This concerns significant changes in absolute number of spikes, temporal discharge pattern, and receptive field size. At single presentation of stimuli, the number of discharges was significantly changed compared with the reference stimulus, and in the majority of neurons the temporal pattern of discharges was changed in addition. At paired presentation of stimuli, neurons mainly revealed a significant decrease in average spike number and a reduction of excitatory receptive field size to presentation of the reference stimulus inside the excitatory receptive field, when a large-sized cricket stimulus or the rectangle was located outside the excitatory receptive field. This inhibition was significantly greater for the large-sized cricket stimulus than for the rectangle, and indicates the biological relevance of the prey-like stimulus in object selection. The response properties of tectal neurons at single or paired presentation of stimuli indicate that tectal neurons integrate information across a much larger part of visual space than covered by the excitatory receptive field. The spike number of a tectal neuron and the spatio-temporal extent of its excitatory receptive field are not fixed but depend on the context, i.e. the stimulus type and combination. This dynamic processing corresponds with the selection of the stimuli in the visual orienting behavior of Plethodon investigated in a previous study, and we assume that tectal processing is modulated by top down processes as well as feedback circuitries.