Hanna Viisanen

Inhibiting TRPA1 ion channel reduces loss of cutaneous nerve fiber function in diabetic animals: Sustained activation of the TRPA1 channel contributes to the pathogenesis of peripheral diabetic neuropathy

Peripheral diabetic neuropathy (PDN) is a devastating complication of diabetes mellitus (DM). Here we test the hypothesis that the transient receptor potential ankyrin 1 (TRPA1) ion channel on primary afferent nerve fibers is involved in the pathogenesis of PDN, due to sustained activation by reactive compounds generated in DM. DM was induced by streptozotocin in rats that were treated daily for 28 days with a TRPA1 channel antagonist (Chembridge-5861528) or vehicle. Laser Doppler flow method was used for assessing axon reflex induced by intraplantar injection of a TRPA1 channel agonist (cinnamaldehyde) and immunohistochemistry to assess substance P-like innervation of the skin. In vitro calcium imaging and patch clamp were used to assess whether endogenous TRPA1 agonists (4-hydroxynonenal and methylglyoxal) generated in DM induce sustained activation of the TRPA1 channel. Axon reflex induced by a TRPA1 channel agonist in the plantar skin was suppressed and the number of substance P-like immunoreactive nerve fibers was decreased 4 weeks after induction of DM. Prolonged treatment with Chembridge-5861528 reduced the DM-induced attenuation of the cutaneous axon reflex and loss of substance P-like immunoreactive nerve fibers. Moreover, in vitro calcium imaging and patch clamp results indicated that reactive compounds generated in DM (4-hydroxynonenal and methylglyoxal) produced sustained activations of the TRPA1 channel, a prerequisite for adverse long-term effects. The results indicate that the TRPA1 channel exerts an important role in the pathogenesis of PDN. Blocking the TRPA1 channel provides a selective disease-modifying treatment of PDN.

Tumor necrosis factor causes persistent sensitization of joint nociceptors to mechanical stimuli in rats

OBJECTIVE During inflammation in the joint, normal joint movements are usually painful. A neuronal mechanism for this form of mechanical hyperalgesia is the persistent sensitization of joint nociceptors to mechanical stimuli. Because tumor necrosis factor (TNF) is a major mediator of joint inflammation, we undertook the present study both to explore the potential of TNF to sensitize joint nociceptors to mechanical stimuli and to address the cellular mechanism involved. METHODS In anesthetized rats, action potentials (APs) were recorded from sensory nociceptive Aδ fibers and C fibers supplying the knee joint. We monitored responses to rotation of the knee joint at innocuous and noxious intensities. TNF, etanercept, and a p38 inhibitor were injected into the knee joint, and the cyclooxygenase (COX) inhibitor diclofenac was administered intraperitoneally. APs were also recorded in isolated cultured dorsal root ganglion (DRG) neurons in order to test for changes in neuronal excitability induced by TNF. RESULTS A single application of TNF into the normal knee joint caused a significant persistent sensitization of nociceptive sensory fibers to mechanical stimuli applied to the joint. This effect was dose dependent. It was prevented by coadministration of etanercept or by an inhibitor of p38, and it was attenuated by systemic application of a COX inhibitor. Patch clamp recordings from isolated DRG neurons showed a rapid increase in neuronal excitability induced by TNF. CONCLUSION TNF can induce a long-lasting sensitization of joint nociceptors to mechanical stimuli and thus can induce long-lasting mechanical hyperalgesia in joints. TNF can act directly on neurons, underscoring its role as a sensitizing pain mediator.

Influence of peripheral nerve injury on response properties of locus coeruleus neurons and coeruleospinal antinociception in the rat

Noradrenergic locus coeruleus (LC) is involved in pain regulation. We studied whether response properties of LC neurons or coeruleospinal antinociception are changed 10-14 days following development of experimental neuropathy. Experiments were performed in spinal nerve-ligated, sham-operated and unoperated male rats under sodium pentobarbital anesthesia. Recordings of LC neurons indicated that responses evoked by noxious somatic stimulation were enhanced in nerve-injured animals, while the effects of nerve injury on spontaneous activity or the response to noxious visceral stimulation were not significant. Microinjection of glutamate into the central nucleus of the amygdala produced a dose-related inhibition of the discharge rate of LC neurons in nerve-injured animals but no significant effect on discharge rates in control groups. Assessment of the heat-induced hind limb withdrawal latency indicated that spinal antinociception induced by electrical stimulation of the LC was significantly weaker in nerve-injured than control animals. The results indicate that peripheral neuropathy induces bidirectional changes in coeruleospinal inhibition of pain. Increased responses of LC neurons to noxious somatic stimulation are likely to promote feedback inhibition of neuropathic hypersensitivity, while the enhanced inhibition of the LC from the amygdala is likely to suppress noradrenergic pain inhibition and promote neuropathic pain. It is proposed that the decreased spinal antinociception induced by direct stimulation of the LC may be explained by pronociceptive changes in the non-noradrenergic systems previously described in peripheral neuropathy. Furthermore, we propose the hypothesis that emotions processed by the amygdala enhance pain due to increased inhibition of the LC in peripheral neuropathy.

Descending modulation of neuropathic hypersensitivity by dopamine D2 receptors in or adjacent to the hypothalamic A11 cell group.

We determined the role of the dopamine D2 receptor in or adjacent to the dopaminergic A11 cell group in descending modulation of neuropathic hypersensitivity. Moreover, we determined the spinal neurotransmitter receptors mediating the modulatory effect. Neuropathy was produced by spinal nerve ligation in the rat that had a chronic cannula for drug delivery into A11 or a control site in the locus coeruleus, and a catheter for spinal drug delivery. Hypersensitivity was assessed by a withdrawal response to monofilaments. Quinpirole (a dopamine D2/D3 receptor agonist) in A11 attenuated hypersensitivity, without influencing thermal nociception in the uninjured tail. Quinpirole in the locus coeruleus failed to influence hypersensitivity. L-741,626 (a dopamine D2 receptor antagonist), raclopride (a dopamine D2/D3 receptor antagonist) and bicuculline (a GABA(A) receptor antagonist) in A11 reversed the antihypersensitivity effect of quinpirole. Raclopride or bicuculline alone in A11 had no effects, whereas muscimol (a GABA(A) receptor agonist) alone in A11 suppressed hypersensitivity. Spinal administration of atipamezole (an alpha(2)-adrenoceptor antagonist) or marginally also WAY-100635 (a 5-HT(1A) receptor antagonist), but not raclopride or bicuculline, reduced the antihypersensitivity effect induced by quinpirole in A11. Electrical stimulation of A11 produced thermal antinociception following intrathecal administration of saline but not raclopride. The results indicate that activation of the dopamine D2 receptor in A11 may selectively suppress neuropathic hypersensitivity, due to mechanisms that involve GABA(A) receptors in the hypothalamus and descending noradrenergic pathways acting on spinal alpha(2)-adrenoceptors, possibly together with a slight contribution of descending serotoninergic pathways acting on spinal 5-HT(1A) receptors.

Roles of the rostroventromedial medulla and the spinal 5-HT1A receptor in descending antinociception induced by motor cortex stimulation in the neuropathic rat

Electric stimulation of the primary motor cortex (M1) has been effective in suppressing pain-related responses in neuropathic as well as healthy control animals. We studied whether the rostroventromedial medulla (RVM) or the spinal 5-HT(1A) receptor contributes to antinociception induced by stimulation of M1 in neuropathic animals. Assessments of the noxious heat-evoked limb withdrawal reflecting spinal nociception was performed in rats with spinal nerve ligation-induced peripheral neuropathy under light pentobarbital anesthesia. Spinal antinociception induced by electric stimulation of M1 was reduced following block of the RVM with intramedullary injection of muscimol, a GABA(A) receptor agonist, or following intrathecal administration of WAY-100635, a 5-HT(1A) receptor antagonist. The results indicate that the RVM and the descending serotonergic pathway acting on the spinal 5-HT(1A) receptor contribute to spinal antinociception induced by M1 stimulation in neuropathic animals.

Efficacy of Kilohertz-Frequency and Conventional Spinal Cord Stimulation in Rat Models of Different Pain Conditions

The aim was to compare the effects of high‐frequency spinal cord stimulation (HF‐SCS) at subparesthetic intensity with conventional SCS in rat models of different types of pain. In addition, microrecordings of afferent activity in the dorsal columns during both types of SCS were performed to elucidate their mode of action.

The role of the dopamine D2 receptor in descending control of pain induced by motor cortex stimulation in the neuropathic rat.

We studied in rats with a spinal nerve ligation-induced neuropathy whether dopamine D2 receptors (D2Rs) play a role in descending control of pain induced by stimulation of the primary motor cortex (M1). Noxious heat-evoked responses were determined in spinal dorsal horn wide-dynamic range (WDR) and nociceptive-specific (NS) neurons, with and without electrical M1 stimulation. A D2R antagonist, raclopride, was administered into the dorsal striatum or spinally in attempts to reverse spinal antinociception induced by M1 stimulation. Moreover, influence of M1 stimulation on the noxious heat-induced limb withdrawal reflex was determined following block of spinal D2Rs with raclopride or a lidocaine-induced block of the hypothalamic A11 cell group, the main source of spinal dopamine. Striatal administration of raclopride enhanced the heat-evoked baseline responses of WDR but not NS neurons and reversed the M1 stimulation-induced suppression of the heat response in WDR neurons. Following spinal administration of raclopride, M1 stimulation failed to suppress the heat response of WDR neurons, whereas the heat response of NS neurons was enhanced by M1-stimulation. After blocking the A11 with lidocaine or spinal D2Rs with raclopride, M1 stimulation failed to suppress the noxious heat-evoked withdrawal reflex. The results indicate that descending pain control induced by stimulation of the M1 cortex in neuropathic animals involves supraspinal (presumably striatal) and, through A11, spinal D2Rs. Supraspinal and spinal D2Rs have partly dissociative effects on spinal dorsal horn WDR and NS neurons, possibly reflecting differential roles and wirings that these sensory neurons have in pain-processing circuitries.

Histamine in the locus coeruleus promotes descending noradrenergic inhibition of neuropathic hypersensitivity.

Among brain structures receiving efferent projections from the histaminergic tuberomammillary nucleus is the pontine locus coeruleus (LC) involved in descending noradrenergic control of pain. Here we studied whether histamine in the LC is involved in descending regulation of neuropathic hypersensitivity. Peripheral neuropathy was induced by unilateral spinal nerve ligation in the rat with a chronic intracerebral and intrathecal catheter for drug administrations. Mechanical hypersensitivity in the injured limb was assessed by monofilaments. Heat nociception was assessed by determining radiant heat-induced paw flick. Histamine in the LC produced a dose-related (1-10μg) mechanical antihypersensitivity effect (maximum effect at 15min and duration of effect 30min), without influence on heat nociception. Pretreatment of LC with zolantidine (histamine H2 receptor antagonist), but not with pyrilamine (histamine H1 receptor antagonist), and spinal administration of atipamezole (an α2-adrenoceptor antagonist), prazosine (an α1-adrenoceptor antagonist) or bicuculline (a GABAA receptor antagonist) attenuated the antihypersensitivity effect of histamine. The histamine-induced antihypersensitivity effect was also reduced by pretreatment of LC with fadolmidine, an α2-adrenoceptor agonist inducing autoinhibition of noradrenergic cell bodies. Zolantidine or pyrilamine alone in the LC failed to influence pain behavior, while A-960656 (histamine H3 receptor antagonist) suppressed hypersensitivity. A plausible explanation for these findings is that histamine, due to excitatory action mediated by the histamine H2 receptor on noradrenergic cell bodies, promotes descending spinal α1/2-adrenoceptor-mediated inhibition of neuropathic hypersensitivity. Blocking the autoinhibitory histamine H3 receptor on histaminergic nerve terminals in the LC facilitates release of histamine and thereby, increases descending noradrenergic pain inhibition.

Dissociated modulation of conditioned place-preference and mechanical hypersensitivity by a TRPA1 channel antagonist in peripheral neuropathy

Transient receptor potential ankyrin 1 (TRPA1) channel antagonists have suppressed mechanical hypersensitivity in peripheral neuropathy, while their effect on ongoing neuropathic pain is not yet known. Here, we assessed whether blocking the TRPA1 channel induces place-preference, an index for the relief of ongoing pain, in two experimental rat models of peripheral neuropathy. Diabetic neuropathy was induced by streptozotocin and spared nerve injury (SNI) model of neuropathy by ligation of two sciatic nerve branches. Conditioned place-preference (CPP) paradigm involved pairing of the drug treatment with one of the chambers of a CPP device once or four times, and the time spent in each chamber was recorded after conditioning sessions to reveal place-preference. The mechanical antihypersensitivity effect was assessed by the monofilament test immediately after the conditioning sessions. Intraperitoneally (30mg/kg; diabetic and SNI model) or intrathecally (10μg; diabetic model) administered Chembridge-5861528 (CHEM) was used as a selective TRPA1 channel antagonist. In diabetic and SNI models of neuropathy, CHEM failed to induce CPP at a dose that significantly attenuated mechanical hypersensitivity, independent of the route of drug administration or number of successive conditioning sessions. Intrathecal clonidine (an α2-adrenoceptor agonist; 10μg), in contrast, induced CPP in SNI but not control animals. The results indicate that ongoing pain, as revealed by CPP, is less sensitive to treatment by the TRPA1 channel antagonist than mechanical hypersensitivity in peripheral neuropathy.

Galanin-Mediated Behavioural Hyperalgesia from the Dorsomedial Nucleus of the Hypothalamus Involves Two Independent Descending Pronociceptive Pathways

Activation of the dorsomedial nucleus of the hypothalamus (DMH) by galanin (GAL) induces behavioural hyperalgesia. Since DMH neurones do not project directly to the spinal cord, we hypothesized that the medullary dorsal reticular nucleus (DRt), a pronociceptive region projecting to the spinal dorsal horn (SDH) and/or the serotoninergic raphe-spinal pathway acting on the spinal 5-HT3 receptor (5HT3R) could relay descending nociceptive facilitation induced by GAL in the DMH. Heat-evoked paw-withdrawal latency (PWL) and activity of SDH neurones were assessed in monoarthritic (ARTH) and control (SHAM) animals after pharmacological manipulations of the DMH, DRt and spinal cord. The results showed that GAL in the DMH and glutamate in the DRt lead to behavioural hyperalgesia in both SHAM and ARTH animals, which is accompanied particularly by an increase in heat-evoked responses of wide-dynamic range neurons, a group of nociceptive SDH neurones. Facilitation of pain behaviour induced by GAL in the DMH was reversed by lidocaine in the DRt and by ondansetron, a 5HT3R antagonist, in the spinal cord. However, the hyperalgesia induced by glutamate in the DRt was not blocked by spinal ondansetron. In addition, in ARTH but not SHAM animals PWL was increased after lidocaine in the DRt and ondansetron in the spinal cord. Our data demonstrate that GAL in the DMH activates two independent descending facilitatory pathways: (i) one relays in the DRt and (ii) the other one involves 5-HT neurones acting on spinal 5HT3Rs. In experimental ARTH, the tonic pain-facilitatory action is increased in both of these descending pathways.