CGRP monoclonal antibody prevents the loss of diffuse noxious inhibitory controls (DNIC) in a mouse model of post-traumatic headache.

AIM: Determine the role of calcitonin-gene related peptide in promoting post-traumatic headache and dysregulation of central pain modulation induced by mild traumatic brain injury in mice. METHODS: Mild traumatic brain injury was induced in lightly anesthetized male C57BL/6J mice by a weight drop onto a closed and unfixed skull, which allowed free head rotation after the impact. We first determined possible alterations in the diffuse noxious inhibitory controls, a measure of net descending pain inhibition called conditioned pain modulation in humans at day 2 following mild traumatic brain injury. Diffuse noxious inhibitory control was assessed as the latency to a thermally induced tail-flick that served as the test stimulus in the presence of right forepaw capsaicin injection that provided the conditioning stimulus. Post-traumatic headache-like behaviors were assessed by the development of cutaneous allodynia in the periorbital and hindpaw regions after mild traumatic brain injury. We then determined if intraperitoneal fremanezumab, an anti-calcitonin-gene related peptide monoclonal antibody or vehicle administered 2 h after sham or mild traumatic brain injury induction could alter cutaneous allodynia or diffuse noxious inhibitory control responses on day 2 post mild traumatic brain injury. RESULTS: In naïve and sham mice, capsaicin injection into the forepaw elevated the latency to tail-flick, reflecting the antinociceptive diffuse noxious inhibitory control response. Periorbital and hindpaw cutaneous allodynia, as well as a loss of diffuse noxious inhibitory control, was observed in mice 2 days after mild traumatic brain injury. Systemic treatment with fremanezumab blocked mild traumatic brain injury-induced cutaneous allodynia and prevented the loss of diffuse noxious inhibitory controls in mice subjected to a mild traumatic brain injury. INTERPRETATION: Sequestration of calcitonin-gene related peptide in the initial stages following mild traumatic brain injury blocked the acute allodynia that may reflect mild traumatic brain injury-related post-traumatic headache and, additionally, prevented the loss of net descending inhibition within central pain modulation pathways. As loss of conditioned pain modulation has been linked to multiple persistent pain conditions, dysregulation of descending modulatory pathways may contribute to the persistence of post-traumatic headache. Additionally, evaluation of the conditioned pain modulation/diffuse noxious inhibitory controls response may serve as a biomarker of vulnerability for chronic/persistent pain. These findings suggest that early anti-calcitonin-gene related peptide intervention has the potential to be effective both for the treatment of mild traumatic brain injury-induced post-traumatic headache, as well as inhibiting mechanisms that may promote post-traumatic headache persistence.

Loss of diffuse noxious inhibitory control after traumatic brain injury in rats: A chronic issue.

Chronic pain is one of the most challenging and debilitating symptoms to manage after traumatic brain injury (TBI), yet the underlying mechanisms remain elusive. The disruption of normal endogenous pain control mechanisms has been linked to several forms of chronic pain and may play a role in pain after TBI. We hypothesized therefore that dysfunctional descending noradrenergic and serotonergic pain control circuits may contribute to the loss of diffuse noxious inhibitory control (DNIC), a critical endogenous pain control mechanism, weeks to months after TBI. For these studies, the rat lateral fluid percussion model of mild TBI was used along with a DNIC paradigm involving a capsaicin-conditioning stimulus. We observed sustained failure of the DNIC response up to 180-days post injury. We confirmed, that descending α2 adrenoceptor-mediated noradrenergic signaling was critical for endogenous pain inhibition in uninjured rats. However, augmenting descending noradrenergic signaling using reboxetine, a selective noradrenaline reuptake inhibitor, failed to restore DNIC after TBI. Furthermore, blocking serotonin-mediated descending signaling using selective spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine was also unsuccessful at restoring endogenous pain modulation after TBI. Unexpectedly, increasing descending serotonergic signaling using the selective serotonin reuptake inhibitor escitalopram and the serotonin-norepinephrine reuptake inhibitor duloxetine restored the DNIC response in TBI rats at both 49- and 180- days post injury. Consistent with these observations, spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine eliminated the effects of escitalopram. Intact α2 adrenoceptor signaling, however, was not required for the serotonin-mediated restoration of DNIC after TBI. These results suggest that TBI causes maladaptation of descending nociceptive signaling mechanisms and changes in the function of both adrenergic and serotonergic circuits. Such changes could predispose those with TBI to chronic pain.

Duloxetine ameliorates the impairment of diffuse noxious inhibitory control in rat models of peripheral neuropathic pain and knee osteoarthritis pain.

Diffuse noxious inhibitory control (DNIC) is a phenomenon to reflect descending pain modulation in animals. Conditioned pain modulation (CPM) is the human counterpart of DNIC and is reduced in patients with several chronic pain conditions. Duloxetine is a serotonin and noradrenaline reuptake inhibitor that ameliorates CPM impairment in patients with diabetic neuropathy. Although some studies have reported the effects of different pharmacological agents on CPM, few studies have compared the effects of some analgesics in both humans and rodents. Therefore, we established a stable evaluation method for DNIC in rats and determined whether duloxetine and other specific analgesics affect DNIC impairment in rat models of peripheral neuropathic pain and osteoarthritis pain, two types of chronic pain. As a conditioning stimulus, capsaicin was injected into the forepaw of rats. The paw withdrawal threshold (PWT) in response to mechanical pressure was measured for the hindpaw. Peripheral neuropathic pain and osteoarthritis pain models were developed by partial sciatic nerve ligation (PSNL) and the intra-articular injection of 2 mg monoiodoacetate (MIA), respectively. Capsaicin (30-100 µg/site) increased the PWT, in a dose-dependent manner, in naive rats. The threshold significantly increased at 30 µg and reached its maximal level at 100 µg. The change in PWT following capsaicin injection was significantly reduced in PSNL-treated rats, but the threshold was increased by the subcutaneous administration of duloxetine (10 mg/kg). The oral administrations of pregabalin (10 mg/kg) and celecoxib (3 mg/kg) did not affect the PWT in PSNL-treated rats. Similarly, MIA-injected rats also showed a reduced change in PWT following capsaicin injection. Duloxetine, but not pregabalin and celecoxib, significantly increased the PWT in MIA-injected rats. These results suggested that duloxetine can directly ameliorate DNIC impairment in rat models of chronic pain. Duloxetine may be useful for modulating chronic pain by restoring function to the endogenous, descending, inhibitory pathway.

Kappa opioid signaling in the right central amygdala causes hind paw specific loss of diffuse noxious inhibitory controls in experimental neuropathic pain.

Diffuse noxious inhibitory controls (DNICs) is a pain-inhibits-pain phenomenon demonstrated in humans and animals. Diffuse noxious inhibitory control is diminished in many chronic pain states, including neuropathic pain. The efficiency of DNIC has been suggested to prospectively predict both the likelihood of pain chronification and treatment response. Little is known as to why DNIC is dysfunctional in neuropathic pain. Here, we evaluated DNIC in the rat L5/L6 spinal nerve ligation (SNL) model of chronic pain using both behavioral and electrophysiological outcomes. For behavior, nociceptive thresholds were determined using response to noxious paw pressure on both hind paws as the test stimulus before, and after, injection of a conditioning stimulus of capsaicin into the left forepaw. Functionally, the spike firing of spinal wide-dynamic-range neuronal activity was evaluated before and during noxious ear pinch, while stimulating the ipsilateral paw with von Frey hairs of increased bending force. In both assays, the DNIC response was significantly diminished in the ipsilateral (ie, injured) paw of SNL animals. However, behavioral loss of DNIC was not observed on the contralateral (ie, uninjured) paw. Systemic application of nor-binaltorphimine, a kappa opioid antagonist, did not ameliorate SNL-induced hyperalgesia but reversed loss of the behavioral DNIC response. Microinjection of nor-binaltorphimine into the right central amygdala (RCeA) of SNL rats did not affect baseline thresholds but restored DNIC both behaviorally and electrophysiologically. Cumulatively, these data suggest that net enhanced descending facilitations may be mediated by kappa opioid receptor signaling from the right central amygdala to promote diminished DNIC after neuropathy.

An investigation into the noradrenergic and serotonergic contributions of diffuse noxious inhibitory controls in a monoiodoacetate model of osteoarthritis.

Osteoarthritis (OA) is a debilitating conditioning with pain as the major clinical symptom. Understanding the mechanisms that drive OA-associated chronic pain is crucial for developing the most effective analgesics. Although the degradation of the joint is the initial trigger for the development of chronic pain, the discordance between radiographic joint damage and the reported pain experience in patients, coupled with clinical features that cannot be explained by purely peripheral mechanisms, suggest there are often other factors at play. Therefore, this study considers the central contributions of chronic pain, using a monoiodoacetate (MIA) model of OA. Particularly, this study explores the functionality of descending controls over the course of the model by assessing diffuse noxious inhibitory controls (DNIC). Early-phase MIA animals have a functional DNIC system, whereas DNIC are abolished in late-phase MIA animals, indicating a dysregulation in descending modulation over the course of the model. In early-phase animals, blocking the actions of spinal α2-adrenergic receptors completely abolishes DNIC, whereas blocking the actions of spinal 5-HT7 receptors only partially decreases the magnitude of DNIC. However, activating the spinal α2-adrenergic or 5-HT7 receptors in late-phase MIA animals restored DNIC-induced neuronal inhibition. This study confirms that descending noradrenergic signaling is crucial for DNIC expression. Furthermore, we suggest a compensatory increase in descending serotonergic inhibition acting at 5-HT7 receptors as the model progresses such that receptor activation is sufficient to override the imbalance in descending controls and mediate neuronal inhibition. NEW & NOTEWORTHY This study showed that there are both noradrenergic and serotonergic components contributing to the expression of diffuse noxious inhibitory controls (DNIC). Furthermore, although a tonic descending noradrenergic tone is always crucial for the expression of DNIC, variations in descending serotonergic signaling over the course of the model mean this component plays a more vital role in states of sensitization.

An investigation into the inhibitory function of serotonin in diffuse noxious inhibitory controls in the neuropathic rat.

BACKGROUND: Following neuropathy α2-adrenoceptor-mediated diffuse noxious inhibitory controls (DNIC), whereby a noxious conditioning stimulus inhibits the activity of spinal wide dynamic range (WDR) neurons, are abolished, and spinal 5-HT7 receptor densities are increased. Here, we manipulate spinal 5-HT content in spinal nerve ligated (SNL) animals and investigate which 5-HT receptor mediated actions predominate. METHODS: Using in vivo electrophysiology we recorded WDR neuronal responses to von frey filaments applied to the hind paw before, and concurrent to, a noxious ear pinch (the conditioning stimulus) in isoflurane-anaesthetised rats. The expression of DNIC was quantified as a reduction in WDR neuronal firing in the presence of conditioning stimulus and was investigated in SNL rats following spinal application of (1) selective serotonin reuptake inhibitors (SSRIs) citalopram or fluoxetine, or dual application of (2) SSRI plus 5-HT7 receptor antagonist SB269970, or (3) SSRI plus α2 adrenoceptor antagonist atipamezole. RESULTS: DNIC were revealed in SNL animals following spinal application of SSRI, but this effect was abolished upon joint application of SSRI plus SB269970 or atipamezole. CONCLUSIONS: We propose that in SNL animals the inhibitory actions (quantified as the presence of DNIC) of excess spinal 5-HT (presumed present following application of SSRI) were mediated via 5-HT7 receptors. The anti-nociception depends upon an underlying tonic noradrenergic inhibitory tone via the α2-adrenoceptor. SIGNIFICANCE: Following neuropathy enhanced spinal serotonin availability switches the predominant spinal 5-HT receptor-mediated actions but also alters noradrenergic signalling. We highlight the therapeutic complexity of SSRIs and monoamine modulators for the treatment of neuropathic pain.

Impaired diffuse noxious inhibitory controls in specific alternation of rhythm in temperature-stressed rats.

Fibromyalgia is characterized by chronic widespread musculoskeletal pain. A hypofunction in descending pain inhibitory systems is considered to be involved in the chronic pain of fibromyalgia. We examined functional changes in descending pain inhibitory systems in rats with specific alternation of rhythm in temperature (SART) stress, by measuring the strength of diffuse noxious inhibitory controls (DNIC). Hindpaw withdrawal thresholds to mechanical von Frey filament or fiber-specific electrical stimuli by the Neurometer system were used to measure the pain response. To induce DNIC, capsaicin was injected into the intraplantar of the forepaw. SART-stressed rats were established by exposure to repeated cold stress for 4 days. In the control rats, heterotopic intraplantar capsaicin injection increased withdrawal threshold, indicative of analgesia by DNIC. The strength of DNIC was reduced by naloxone (µ-opioid receptor antagonist, intraperitoneally and intracerebroventricularly), yohimbine (α2-adrenoceptor antagonist, intrathecally), and WAY-100635 (5-HT1A receptor antagonist, intrathecally) in the von Frey test. In SART-stressed rats, capsaicin injection did not increase withdrawal threshold in the von Frey test, indicating deficits in DNIC. In the Neurometer test, deficient DNIC in SART-stressed rats were observed only for Aδ- and C-fibers, but not Aß-fibers stimulation. Analgesic effect of intracerebroventricular morphine was markedly reduced in SART-stressed rats compared with the control rats. Taken together, in SART-stressed rats, capsaicin-induced DNIC were deficient, and a hypofunction of opioid-mediated central pain modulation system may cause the DNIC deficit.

Reduction of conditioned pain modulation in humans by naltrexone: an exploratory study of the effects of pain catastrophizing.

The current study tested the hypothesis that conditioned pain modulation is mediated by the release of endogenous opioids with a placebo-controlled (sugar pill) study of naltrexone (50 mg) in 33 healthy volunteers over two counter-balanced sessions. Pain modulation consisted of rating of heat pain (palm) during concurrent cold water immersion (foot). Compared to baseline heat pain ratings, concurrent foot immersion lowered pain intensity ratings, which suggests an inhibitory effect, was reduced with naltrexone, suggesting at least partial dependence of inhibition on endogenous opioids. An exploratory analysis revealed that individual differences in catastrophizing moderated the effects of naltrexone; endogenous opioid blockade abolished modulation in subjects lower in catastrophizing while modulation was unaffected by naltrexone among high catastrophizers. The results suggest a role of endogenous opioids in endogenous analgesia, but hint that multiple systems might contribute to conditioned pain modulation, and that these systems might be differentially activated as a function of individual differences in responses to pain.