Effects of PGE(2) on neurons from rat dorsal root ganglia in intact and adjuvant-inflamed rats: role of NGF on PGE(2)-induced depolarization

The cannabinoid agonist CB-13 produces peripherally mediated analgesia in mice but elicits tolerance and signs of central nervous system activity with repeated dosing

ABSTRACT

Activation of cannabinoid receptor type 1 (CB 1 ) produces analgesia in a variety of preclinical models of pain; however, engagement of central CB 1 receptors is accompanied by unwanted side effects, such as psychoactivity, tolerance, and dependence. Therefore, some efforts to develop novel analgesics have focused on targeting peripheral CB 1 receptors to circumvent central CB 1 -related side effects. In the present study, we evaluated the effects of acute and repeated dosing with the peripherally selective CB 1 -preferring agonist CB-13 on nociception and central CB 1 -related phenotypes in a model of inflammatory pain in mice. We also evaluated cellular mechanisms underlying CB-13-induced antinociception in vitro using cultured mouse dorsal root ganglion neurons. CB-13 reduced inflammation-induced mechanical allodynia in male and female mice in a peripheral CB 1 -receptor-dependent manner and relieved inflammatory thermal hyperalgesia. In cultured mouse dorsal root ganglion neurons, CB-13 reduced TRPV1 sensitization and neuronal hyperexcitability induced by the inflammatory mediator prostaglandin E 2 , providing potential mechanistic explanations for the analgesic actions of peripheral CB 1 receptor activation. With acute dosing, phenotypes associated with central CB 1 receptor activation occurred only at a dose of CB-13 approximately 10-fold the ED 50 for reducing allodynia. Strikingly, repeated dosing resulted in both analgesic tolerance and CB 1 receptor dependence, even at a dose that did not produce central CB 1 -receptor-mediated phenotypes on acute dosing. This suggests that repeated CB-13 dosing leads to increased CNS exposure and unwanted engagement of central CB 1 receptors. Thus, caution is warranted regarding therapeutic use of CB-13 with the goal of avoiding CNS side effects. Nonetheless, the clear analgesic effect of acute peripheral CB 1 receptor activation suggests that peripherally restricted cannabinoids are a viable target for novel analgesic development.

Molecular mechanisms underlying the actions of arachidonic acid-derived prostaglandins on peripheral nociception

Arachidonic acid-derived prostaglandins not only contribute to the development of inflammation as intercellular pro-inflammatory mediators, but also promote the excitability of the peripheral somatosensory system, contributing to pain exacerbation. Peripheral tissues undergo many forms of diseases that are frequently accompanied by inflammation. The somatosensory nerves innervating the inflamed areas experience heightened excitability and generate and transmit pain signals. Extensive studies have been carried out to elucidate how prostaglandins play their roles for such signaling at the cellular and molecular levels. Here, we briefly summarize the roles of arachidonic acid-derived prostaglandins, focusing on four prostaglandins and one thromboxane, particularly in terms of their actions on afferent nociceptors. We discuss the biosynthesis of the prostaglandins, their specific action sites, the pathological alteration of the expression levels of related proteins, the neuronal outcomes of receptor stimulation, their correlation with behavioral nociception, and the pharmacological efficacy of their regulators. This overview will help to a better understanding of the pathological roles that prostaglandins play in the somatosensory system and to a finding of critical molecular contributors to normalizing pain.

Fluorescent Visualisation of Oxytocin in the Hypothalamo-neurohypophysial/-spinal Pathways After Chronic Inflammation in Oxytocin-Monomeric Red Fluorescent Protein 1 Transgenic Rats

Oxytocin (OXT) is a well-known neurohypophysial hormone that is synthesised in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus. The projection of magnocellular neurosecretory cells, which synthesise OXT and arginine vasopressin in the PVN and SON, to the posterior pituitary plays an essential role in mammalian labour and lactation through its peripheral action. However, previous studies have shown that parvocellular OXTergic cells in the PVN, which project to the medulla and spinal cord, are involved in various physiological functions (e.g. sensory modulation and autonomic). In the present study, we examined OXT expression in the PVN, SON and spinal cord after chronic inflammation from adjuvant arthritis (AA). We used transgenic rats that express OXT and the monomeric red fluorescent protein 1 (mRFP1) fusion gene to visualise both the magnocellular and parvocellular OXTergic pathways. OXT-mRFP1 fluorescence intensity was significantly increased in the PVN, SON, dorsal horn of the spinal cord and posterior pituitary in AA rats. The levels of OXT-mRFP1 mRNA were significantly increased in the PVN and SON of AA rats. These results suggested that OXT was up-regulated in both hypothalamic magnocellular neurosecretory cells and parvocellular cells by chronic inflammation, and also that OXT in the PVN-spinal pathway may be involved in sensory modulation. OXT-mRFP1 transgenic rats are a very useful model for visualising the OXTergic pathways from vesicles in a single cell to terminals in in vitro preparations.

Using gait analysis to assess weight bearing in rats with Freund׳s complete adjuvant-induced monoarthritis to improve predictivity: Interfering with the cyclooxygenase and nerve growth factor pathways

Lack of predictive power for drug effects has been a major criticism against animal pain models. It is therefore important to define the utility and limitations of different models. The aim of this study was to extend previous work on gait analysis as a tool to investigate pharmacological effects in monoarthritic rats, specifically to test the hypothesis that monoarthritis induced by Freund׳s complete adjuvant (FCA) provides a better estimate of overall analgesic efficacy of established, and novel, clinically effective and ineffective therapeutic approaches. Male rats injected intra-articularly into one ankle joint with FCA (1.0mg/ml) were treated with the monoclonal antibody to nerve growth factor (NGF), MEDI-578, the inhibitors of tropomyosin receptor kinases A, B and C (pan-Trk) AZ6623 or AZ7092, the transient receptor potential vanilloid 1 (TRPV1) inhibitor AZD1386, or the cyclooxygenase (COX) inhibitors naproxen, ibuprofen, valdecoxib or rofecoxib. Effects on weight bearing during locomotion were tested using video capture of print images. The apparent efficacy in this model was Trk inhibitors≥anti-NGF antibody>COX inhibitors. The TRPV1 inhibitor was ineffective. Together with previous data, the results support using gait-related parameters in the monoarthritis model. FCA as induction agent seems to provide a good overall prediction of analgesic efficacy in disorders with inflammatory joint pain.

The N-methyl-D-aspartate-evoked cytoplasmic calcium increase in adult rat dorsal root ganglion neuronal somata was potentiated by substance P pretreatment in a protein kinase C-dependent manner

The involvement of substance P (SP) in neuronal sensitization through the activation of the neurokinin-1-receptor (NK1r) in postsynaptic dorsal horn neurons has been well established. In contrast, the role of SP and NK1r in primary sensory dorsal root ganglion (DRG) neurons, in particular in the soma, is not well understood. In this study, we evaluated whether SP modulated the NMDA-evoked transient increase in cytoplasmic Ca2+ ([Ca2+]cyt) in the soma of dissociated adult DRG neurons. Cultures were treated with nerve growth factor (NGF), prostaglandin E2 (PGE2) or both NGF+PGE2. Treatment with NGF+PGE2 increased the percentage of N-methyl-D-aspartate (NMDA) responsive neurons. There was no correlation between the percentage of NMDA responsive neurons and the level of expression of the NR1 and NR2B subunits of the NMDA receptor or of the NK1r. Pretreatment with SP did not alter the percentage of NMDA responsive neurons; while it potentiated the NMDA-evoked [Ca2+]cyt transient by increasing its magnitude and by prolonging the period during which small- and some medium-sized neurons remained NMDA responsive. The SP-mediated potentiation was blocked by the SP-antagonist ([D-Pro4, D-Trp7,9]-SP (4-11)) and by the protein kinase C (PKC) blocker bisindolylmaleimide I (BIM); and correlated with the phosphorylation of PKCε. The Nk1r agonist [Sar9, Met(O2)11]-SP (SarMet-SP) also potentiated the NMDA-evoked [Ca2+]cyt transient. Exposure to SP or SarMet-SP produced a rapid increase in the labeling of phosphorylated-PKCε. In none of the conditions we detected phosphorylation of the NR2B subunit at Ser-1303. Phosphorylation of the NR2B subunit at Tyr1472 was enhanced to a similar extent in cells exposed to NMDA, SP or NMDA+SP, and that enhancement was blocked by BIM. Our findings suggest that NGF and PGE2 may contribute to the injury-evoked sensitization of DRG neurons in part by enhancing their NMDA-evoked [Ca2+]cyt transient in all sized DRG neurons; and that SP may further contribute to the DRG sensitization by enhancing and prolonging the NMDA-evoked increase in [Ca2+]cyt in small- and medium-sized DRG neurons.

The action of prostaglandins on ion channels

Prostaglandins, in particular PGE(2) and prostacyclin PGI(2) have diverse biological effects. Most importantly, they are involved in inflammation and pain. Prostaglandins in nano- and micromolar concentrations sensitize nerve cells, i.e. make them more sensitive to electrical or chemical stimuli. Sensitization arises from the effect of prostaglandins on ion channels and occurs both at the peripheral terminal of nociceptors at the site of tissue injury (peripheral sensitization) and at the synapses in the spinal cord (central sensitization). The first step is the binding of prostaglandins to receptors in the cell membrane, mainly EP and IP receptors. The receptors couple via G proteins to enzymes such as adenylate cyclase and phospholipase C (PLC). Activation of adenylate cyclase leads to increase of cAMP and subsequent activation of protein kinase A (PKA) or PKA-independent effects of cAMP, e.g. mediated by Epac (=exchange protein activated by cAMP). Activation of PLC causes increase of inositol phosphates and increase of cytosolic calcium. This article summarizes the effects of PGE(2), PGE(1), PGI2 and its stable analogues on non-selective cation channels and sodium, potassium, calcium and chloride channels. It describes the mechanism responsible for the facilitatory or inhibitory prostaglandin effects on ion channels. Understanding these mechanisms is essential for the development of useful new analgesics.

New tricks by an old dogma: mechanisms of the Organizational/Activational Hypothesis of steroid-mediated sexual differentiation of brain and behavior

The hormonal regulation of sexual behavior has been the topic of study for over 50 years and yet controversies persist regarding the importance of early versus late events and the identity of the critical neural and cellular substrates. We have taken a mechanistic approach toward the masculinizing actions of the gonadal steroid estradiol, as a means to understand how organization of the neuroarchitechture during a perinatal sensitive period exerts enduring influences on adult behavior. We have identified important roles for prostaglandins, FAK and paxillin, PI3 kinase and glutamate, and determined that cell-to-cell signaling is a critical component of the early organizational events. We have further determined that the mechanisms mediating different components of sexual behavior are distinct and regionally specific. The multitude of mechanisms by which the steroid estradiol, exerts divergent effects on the developing nervous system provides for a multitude of phenotypes which can vary significantly both within and between the sexes.

Nerve growth factor acts with the beta2-adrenoceptor to induce spontaneous nociceptive behavior during temporomandibular joint inflammatory hyperalgesia

AIMS

The aim of this study was to investigate whether the injection of nerve growth factor induces spontaneous nociceptive behavior in the intact or sensitized temporomandibular joint (TMJ) of rats.

MAIN METHODS

NGF was injected into the TMJ 1 h after the TMJ injection of saline or carrageenan and the spontaneous nociceptive behavior was quantified. The mechanism involved in this phenomenon was investigated by the injection of NGF into the carrageenan-sensitized TMJ in the presence of indomethacin or of beta-adrenergic antagonists.

KEY FINDINGS

NGF injected into the TMJ sensitized by a prior TMJ injection of carrageenan but not into the intact TMJ induced a significant nociceptive behavior. Co-injection of the non-specific Trk receptor antagonist k252A with NGF 1 h after the TMJ injection of carrageenan significantly reduced NGF-induced spontaneous nociception supporting the Trk receptor activation in this nociceptive effect. Blockade of prostaglandin synthesis by indomethacin before the TMJ injection of carrageenan did not reduce NGF-induced nociception. Co-administration of carrageenan with the beta2-adrenoceptor antagonist ICI 118.55 but not with the beta1-adrenoceptor antagonist atenolol significantly reduced NGF-induced nociception. The injection of NGF the TMJ sensitized by a previous TMJ injection of epinephrine also induced nociceptive behavior.

SIGNIFICANCE

Taken together, these results indicate that NGF can induce TMJ nociception during TMJ inflammation. Moreover, the expression of this nociceptive response seems to depend on the synergic activity of NGF and sympathetic amines released during TMJ inflammation acting on beta2-adrenergic receptors.

Dermatomal laser-evoked potentials: a diagnostic approach to the dorsal root. Norm data in healthy volunteers and changes in patients with radiculopathy

We conducted a cross-sectional study of 40 radiculopathy patients in comparison with norm data from healthy subjects using a new electrophysiological method. Early manifestations of dorsal root impairment escape objective diagnosis by conventional somatosensory-evoked potentials due to the overlapping innervation of the affected dermatome by thickly myelinated mechanoreceptive afferents projecting to adjacent intact roots. Evidence suggested less intersegmental overlap for thermonociceptive afferents rendering laser-evoked potentials (LEP) sensitive to monosegmental dorsal root damage. Therefore we used this new method to study acute manifestations of monosegmental dorsal root pathology. Dorsal root function was tested in 12 healthy subjects and 40 sciatica patients by intraindividual interside comparison. Mechanosensibility and thermosensibility were clinically investigated. LEP were induced by moderately painful laser stimuli. The LEP were evaluated by amplitude and latency of the averaged electroencephalogram. Normal interside differences of LEP for amplitude were +/-22% (lower limb) and +/-35% (upper limb) and +/-15 to +/-16% for latency. Twenty-six patients (65%) showed significant LEP changes, mainly amplitude decreases. Six of these patients exhibited latency prolongations. Clinical testing yielded more frequent pathological results for pain compared to mechanosensibility. The study confirmed our preliminary evidence of LEP sensitivity to objectively document dorsal root impairment in patients suffering from acute monosegmental radiculopathy. This result opens the perspective of electrophysiologically differentiating the presence or absence of dorsal root pathology in patients with similar clinical symptoms but possibly different prognoses, which require different therapies.

Adenosine 5'-triphosphate and its relationship with other mediators that activate pelvic nerve afferent neurons in the rat colorectum

Evidence of a role for purinergic signalling in visceral afferents involving P2X₂, P2X₃ and P2Y₁ receptors exists, which appears to be important during inflammation. This study aimed to evaluate the degree of interaction between adenosine 5′-triphosphate (ATP) and other mediators that activate sensory nerves in the colorectum. Recordings from pelvic nerve afferents were made during application of agents to the in-vitro colorectal preparation. Analysis allowed calculation of single unit activity. When applied individually, bradykinin (78%) and 5-hydoxytryptamine (77%) activated the greatest number of neurons, followed by substance P, protons, ATP and capsaicin. Prostaglandin E₂ stimulated the least number (54%) and had a longer latency. Seventy-seven percent of all units studied either responded to both ATP and capsaicin or to neither, giving the greatest degree of activity correlation. Five percent of units were activated by all seven agents and no units were activated by a single agent alone. 5-hydroxytryptamine, capsaicin and protons, when co-applied with ATP, increased pelvic nerve activity to a greater degree than the sum of the individual responses. It is concluded that ATP activates pelvic nerve afferents and acts synergistically with protons, capsaicin and 5-hydroxytryptamine. The pattern of neuronal activation suggests that visceral afferents are polymodal but the receptor expression on their terminals is variable.

PGE2 increases the tetrodotoxin-resistant Nav1.9 sodium current in mouse DRG neurons via G-proteins

Inflammation caused by tissue damage results in pain, reflecting an increase in excitability of the primary afferent neurons innervating the area. There is some evidence to suggest that altered function of voltage-gated sodium channels is responsible for the hyperexcitability produced by inflammatory agents, possibly acting through G-proteins, but the role of different channel subtypes has not been fully explored. The tetrodotoxin-resistant (TTX-R) sodium channel Na(v)1.9 is expressed selectively in C- and A-fibre nociceptive-type units and is upregulated by G-protein activation. In this study, we examined the effects of the inflammatory agent prostaglandin-E(2) (PGE(2)) on Na(v)1.9 current in both Na(v)1.8-null and wild-type (WT) mice and explored the role of specific G-proteins in modulation. PGE(2) caused a twofold increase in Na(v)1.9 current (p<0.05) in both systems. Steady-state activation was shifted in a hyperpolarizing direction by 6-8 mV and availability of channels by 12 mV. No differences in the activation and inactivation kinetics could be detected. The increase in current was blocked by pertussis toxin (PTX) but not cholera toxin (CTX), showing involvement of G(i/o) but not G(s) subunits. Our data indicate that Na(v)1.9 current can be increased during inflammation via a G-protein dependent mechanism and suggest that this could contribute to the regulation of electrogenesis in dorsal root ganglia (DRG) neurons.

Effects of free fatty acids on sodium currents in rat dorsal root ganglion neurons

Free fatty acids (FFAs), especially polyunsaturated fatty acids (PUFAs), are potent modulators of muscle-type sodium channels. It is not known if they also modulate sodium channels of sensory neurons. In this study, we investigated the effects of FFAs on the fast tetrodotoxin-sensitive (fTTX-S) and the slow tetrodotoxin-resistant (sTTX-R) sodium currents in rat dorsal root ganglion neurons. At a holding potential of -80 mV, PUFAs potently inhibited fTTX-S current, but monounsaturated fatty acids (MUFAs) and saturated fatty acids (SFAs) to a lesser extent. All FFAs initially increased sTTX-R current, and then decreased it slightly. PUFAs and MUFAs produced a hyperpolarizing shift of the steady-state inactivation voltage for both types of sodium currents. The shift generally increased with the number of unsaturated bonds. FFAs did not change the maximum amplitude of fTTX-S current, but increased that of sTTX-R current. Most FFAs shifted the activation voltage for fTTX-S current in the hyperpolarizing direction, which was not dependent on the degree of unsaturation. MUFAs and SFAs shifted the activation voltage for sTTX-R current in the hyperpolarizing direction, but PUFAs were without effect. The modulation of sodium currents by FFAs, especially PUFAs, may have considerable impact on the excitability of sensory neurons.

Diclofenac inhibition of sodium currents in rat dorsal root ganglion neurons

The effects of diclofenac, a nonsteroidal anti-inflammatory drug (NSAID), on the fast tetrodotoxin-sensitive (TTX-S) and the slow tetrodotoxin-resistant (TTX-R) sodium currents in rat dorsal root ganglion neurons were investigated using the whole-cell patch-clamp method. Diclofenac suppressed both sodium currents in a dose-dependent manner. The apparent dissociation constants for the diclofenac suppression of TTX-S and TTX-R sodium currents were estimated to be 14 and 97 microM, respectively, at a holding potential of -80 mV. Diclofenac had no effect on the kinetic parameters of the activation process in either type of sodium current. However, diclofenac produced shifts of the steady-state inactivation curves in the hyperpolarizing direction in both types of sodium currents in a dose-dependent manner. At sufficiently negative holding potentials, the inhibitory effects of diclofenac on both types of sodium currents were minimal. The results suggested that diclofenac might bind to sodium channels with a greater affinity when they are in the inactivated state than when they are in the resting state. Effects of other NSAIDs (acetylsalicylic acid, antipyrin, indomethacin and flufenamic acid) on sodium currents were tested. Among these, only flufenamic acid suppressed the sodium currents to a considerable extent. Thus, the chemical structure of each NSAID, not the inhibition of cyclooxygenase, seems to be an important determinant in the sodium current inhibition. The suppression of sodium currents in sensory neurons by diclofenac and flufenamic acid would contribute to their analgesic activity in addition to their inhibition of cyclooxygenase.