Milena De Felice

Engagement of descending inhibition from the rostral ventromedial medulla protects against chronic neuropathic pain

Summary A rat population with variable responses to nerve injury indicated that activation of descending inhibition with a spinal noradrenergic component prevents development of neuropathic pain. ABSTRACT A puzzling observation is why peripheral nerve injury results in chronic pain in some, but not all, patients. We explored potential mechanisms that may prevent the expression of chronic pain. Sprague Dawley (SD) or Holtzman (HZ) rats showed no differences in baseline sensory thresholds or responses to inflammatory stimuli. However, spinal nerve ligation (SNL)‐induced tactile allodynia occurred in approximately 85% of SD and 50% of HZ rats, respectively. No apparent differences were observed in a survey of dorsal root ganglion or spinal neuropathic markers after SNL regardless of allodynic phenotype. SNL‐induced allodynia was reversed by administration of lidocaine within the rostral ventromedial medulla (RVM), a site that integrates descending pain modulation via pain inhibitory (ie, OFF) and excitatory (ie, ON) cells. However, in SD or HZ rats with SNL but without allodynia, RVM lidocaine precipitated allodynia. Additionally, RVM lidocaine produced conditioned place preference in allodynic SD or HZ rats but conditioned place aversion in nonallodynic HZ rats. Similarly, RVM U69,593 (kappa opioid agonist) or blockade of spinal α2 adrenergic receptors precipitated allodynia in previously nonallodynic HZ rats with SNL. All rats showed an equivalent first‐phase formalin responses. However, HZ rats had reduced second‐phase formalin behaviors along with fewer RVM OFF cell pauses and RVM ON cell bursts. Thus, expression of nerve injury‐induced pain may ultimately depend on descending modulation. Engagement of descending inhibition protects in the transition from acute to chronic pain. These unexpected findings might provide a mechanistic explanation for medications that engage descending inhibition or mimic its consequences.

Medullary pain facilitating neurons mediate allodynia in headache-related pain†

To develop and validate a model of cutaneous allodynia triggered by dural inflammation for pain associated with headaches. To explore neural mechanisms underlying cephalic and extracephalic allodynia.

Triptan-induced latent sensitization: a possible basis for medication overuse headache.

Identification of the neural mechanisms underlying medication overuse headache resulting from triptans.

Impaired Nociception and Inflammatory Pain Sensation in Mice Lacking the Prokineticin Receptor PKR1: Focus on Interaction between PKR1 and the Capsaicin Receptor TRPV1 in Pain Behavior

Bv8, prokineticin-1 or EG-VEGF (endocrine gland-derived vascular endothelial growth factor), and prokineticin-2, are naturally occurring peptide agonists of two G-protein-coupled receptors (GPCRs), prokineticin receptor 1 (PKR1) and PKR2. PKRs are expressed in neurons in the CNS and peripheral nervous system and many dorsal root ganglion (DRG) cells expressing PKRs also express transient receptor potential vanilloid receptor-1 (TRPV1). Mice lacking the pkr1 gene were generated to explore the role of the PKR1 receptor in nociceptive signaling and in nociceptor sensitization. When compared with wild-type littermates, mice lacking the pkr1 gene showed impaired responsiveness to noxious heat, mechanical stimuli, capsaicin, and protons. In wild-type mice, activation of PKRs by the PKR agonist Bv8 caused hyperalgesia and sensitized to the actions of capsaicin. pkr1-null mice exhibited impaired responses to Bv8 but showed normal hyperalgesic responses to bradykinin and PGE2 (prostaglandin E2). Conversely, trpv1-null mice showed a reduced pronociceptive response to Bv8. Additionally, pkr1-null mice showed diminished thermal hyperalgesia after acute inflammation elicited by mustard oil and reduced pain behavior after chronic inflammation produced by complete Freunds adjuvant. The number of neurons that responded with a [Ca2+]i increase to Bv8 exposure was five times lower in pkr1-null DRG cultures than in wild-type cultures. Furthermore, Bv8-responsive neurons from pkr1-null mice showed a significant reduction in the [Ca2+]i response to capsaicin. These findings indicate a modulatory role of PKR1 in acute nociception and inflammatory pain and disclose a pharmacological interaction between PKR1 and TRPV1 in nociceptor activation and sensitization.

Resveratrol engages AMPK to attenuate ERK and mTOR signaling in sensory neurons and inhibits incision-induced acute and chronic pain

BackgroundDespite advances in our understanding of basic mechanisms driving post-surgical pain, treating incision-induced pain remains a major clinical challenge. Moreover, surgery has been implicated as a major cause of chronic pain conditions. Hence, more efficacious treatments are needed to inhibit incision-induced pain and prevent the transition to chronic pain following surgery. We reasoned that activators of AMP-activated protein kinase (AMPK) may represent a novel treatment avenue for the local treatment of incision-induced pain because AMPK activators inhibit ERK and mTOR signaling, two important pathways involved in the sensitization of peripheral nociceptors.ResultsTo test this hypothesis we used a potent and efficacious activator of AMPK, resveratrol. Our results demonstrate that resveratrol profoundly inhibits ERK and mTOR signaling in sensory neurons in a time- and concentration-dependent fashion and that these effects are mediated by AMPK activation and independent of sirtuin activity. Interleukin-6 (IL-6) is thought to play an important role in incision-induced pain and resveratrol potently inhibited IL-6-mediated signaling to ERK in sensory neurons and blocked IL-6-mediated allodynia in vivo through a local mechanism of action. Using a model of incision-induced allodynia in mice, we further demonstrate that local injection of resveratrol around the surgical wound strongly attenuates incision-induced allodynia. Intraplantar IL-6 injection and plantar incision induces persistent nociceptive sensitization to PGE2 injection into the affected paw after the resolution of allodynia to the initial stimulus. We further show that resveratrol treatment at the time of IL-6 injection or plantar incision completely blocks the development of persistent nociceptive sensitization consistent with the blockade of a transition to a chronic pain state by resveratrol treatment.ConclusionsThese results highlight the importance of signaling to translation control in peripheral sensitization of nociceptors and provide further evidence for activation of AMPK as a novel treatment avenue for acute and chronic pain states.

Dural afferents express acid-sensing ion channels: a role for decreased meningeal pH in migraine headache

&NA; Migraine headache is one of the most common neurological disorders. The pathological conditions that directly initiate afferent pain signaling are poorly understood. In trigeminal neurons retrogradely labeled from the cranial meninges, we have recorded pH‐evoked currents using whole‐cell patch‐clamp electrophysiology. Approximately 80% of dural‐afferent neurons responded to a pH 6.0 application with a rapidly activating and rapidly desensitizing ASIC‐like current that often exceeded 20 nA in amplitude. Inward currents were observed in response to a wide range of pH values and 30% of the neurons exhibited inward currents at pH 7.1. These currents led to action potentials in 53%, 30% and 7% of the dural afferents at pH 6.8, 6.9 and 7.0, respectively. Small decreases in extracellular pH were also able to generate sustained window currents and sustained membrane depolarizations. Amiloride, a non‐specific blocker of ASIC channels, inhibited the peak currents evoked upon application of decreased pH while no inhibition was observed upon application of TRPV1 antagonists. The desensitization time constant of pH 6.0‐evoked currents in the majority of dural afferents was less than 500 ms which is consistent with that reported for ASIC3 homomeric or heteromeric channels. Finally, application of pH 5.0 synthetic‐interstitial fluid to the dura produced significant decreases in facial and hind‐paw withdrawal threshold, an effect blocked by amiloride but not TRPV1 antagonists, suggesting that ASIC activation produces migraine‐related behavior in vivo. These data provide a cellular mechanism by which decreased pH in the meninges following ischemic or inflammatory events directly excites afferent pain‐sensing neurons potentially contributing to migraine headache.

Constitutive activity at the cannabinoid CB1 receptor is required for behavioral response to noxious chemical stimulation of TRPV1: Antinociceptive actions of CB1 inverse agonists

The potential modulation of TRPV1 nociceptive activity by the CB1 receptor was investigated here using CB1 wild-type (WT) and knock-out (KO) mice as well as selective CB1 inverse agonists. No significant differences were detected in baseline thermal thresholds of ICR, CB1WT or CB1KO mice. Intraplantar capsaicin produced dose- and time-related paw flinch responses in ICR and CB1WT mice and induced plasma extravasation yet minimal responses were seen in CB1KO animals with no apparent differences in TRPV1 channel expression. Capsaicin-evoked CGRP release from spinal cord tissue and capsaicin-evoked action potentials on isolated skin-nerve preparation were significantly decreased in CB1KO mice. Pretreatment with intraplantar galanin and bradykinin, compounds known to sensitize TRPV1 receptors, restored capsaicin-induced flinching in CB1KO mice. The possibility that constitutive activity at the CB1 receptor is required to maintain the TRPV1 receptor in a “sensitized” state was tested using CB1 inverse agonists. The CB1 inverse agonists elicited concentration-related inhibition of capsaicin-induced calcium influx in F-11 cells and produced dose-related inhibition of capsaicin-induced flinching in ICR mice. These data suggest that constitutive activity at the CB1 receptor maintains the TRPV1 channel in a sensitized state responsive to noxious chemical stimuli. Treatment with CB1 inverse agonists may promote desensitization of the channel resulting in antinociceptive actions against chemical stimulus modalities. These studies propose possible therapeutic exploitation of a novel mechanism providing pain relief by CB1 inverse agonists.

Capturing the aversive state of cephalic pain preclinically

Preclinical evaluation of headache by behavioral assessment of reward from pain relief.

Spinal NK‐1 receptor‐expressing neurons and descending pathways support fentanyl‐induced pain hypersensitivity in a rat model of postoperative pain

The clinically important opioid fentanyl, administered acutely, enhances mechanical hypersensitivity in a model of surgical pain induced by plantar incision. Activity of neurokinin‐1 (NK‐1) receptor‐expressing ascending spinal neurons, descending pathways originating in the rostral ventromedial medulla (RVM), and spinal dynorphin are necessary for the development and maintenance of hyperalgesia during sustained morphine exposure, suggesting that these mechanisms may also be important in opioid enhancement of surgical pain. Therefore, we examined the roles of these mechanisms in sensory hypersensitivity produced by acute fentanyl administration in rats not undergoing surgical incision and in rats undergoing plantar incision. In non‐operated rats, fentanyl induced analgesia followed by immediate and long‐lasting sensory hypersensitivity, as previously described. Fentanyl also enhanced pain sensitivity induced by plantar incision. Ablation of NK‐1‐expressing spinal neurons by pre‐treatment with substance P–Saporin reduced sensory hypersensitivity in fentanyl‐treated rats and, to a lesser extent, in fentanyl‐treated rats with a surgical incision. Microinjection of lidocaine into the RVM completely reversed fentanyl‐induced sensory hypersensitivity and fentanyl enhancement of incision‐induced sensory hypersensitivity. RVM lidocaine injection resulted in a slight reduction of incision‐induced sensory hypersensitivity in the absence of fentanyl pre‐treatment. Spinal dynorphin content increased by 30 ± 7% and 66 ± 17% in fentanyl‐ and fentanyl/incision‐treated rats. Spinal administration of antiserum to dynorphin attenuated sensory hypersensitivity in fentanyl‐treated rats. These data support a partial role of NK‐1 receptor‐containing ascending pathways and a crucial role of descending facilitatory pathways in fentanyl‐induced hyperalgesia and in the enhanced hyperalgesia produced by fentanyl treatment following surgical incision.

Descending Facilitation Maintains Long-Term Spontaneous Neuropathic Pain

UNLABELLED Neuropathic pain is frequently characterized by spontaneous pain (ie, pain at rest) and, in some cases, by cold- and touch-induced allodynia. Mechanisms underlying the chronicity of neuropathic pain are not well understood. Rats received spinal nerve ligation (SNL) and were monitored for tactile and thermal thresholds. While heat hypersensitivity returned to baseline levels within approximately 35 to 40 days, tactile hypersensitivity was still present at 580 days after SNL. Tactile hypersensitivity at post-SNL day 60 (D60) was reversed by microinjection of 1) lidocaine; 2) a cholecystokinin 2 receptor antagonist into the rostral ventromedial medulla; or 3) dorsolateral funiculus lesion. Rostral ventromedial medulla lidocaine at D60 or spinal ondansetron, a 5-hydroxytryptamine 3 antagonist, at post-SNL D42 produced conditioned place preference selectively in SNL-treated rats, suggesting long-lasting spontaneous pain. Touch-induced FOS was increased in the spinal dorsal horn of SNL rats at D60 and prevented by prior dorsolateral funiculus lesion, suggesting that long-lasting tactile hypersensitivity depends upon spinal sensitization, which is mediated in part by descending facilitation, in spite of resolution of heat hypersensitivity. PERSPECTIVE These data suggest that spontaneous pain is present for an extended period of time and, consistent with likely actions of clinically effective drugs, is maintained by descending facilitation.