Vagotomy-induced enhancement of mechanical hyperalgesia in the rat is sympathoadrenal-mediated

What is the Vagal-Adrenal Axis?

Some recent publications have used the term "vagal-adrenal axis" to account for mechanisms involved in the regulation of inflammation by electroacupuncture. This concept proposes that efferent parasympathetic nerve fibers in the vagus directly innervate the adrenal glands to influence catecholamine secretion. Here, we discuss evidence for anatomical and functional links between the vagi and adrenal glands that may be relevant in the context of inflammation and its neural control by factors, including acupuncture. First, we find that evidence for any direct vagal parasympathetic efferent innervation of the adrenal glands is weak and likely artifactual. Second, we find good evidence that vagal afferent fibers directly innervate the adrenal gland, although their function is uncertain. Third, we highlight a wealth of evidence for indirect pathways, whereby vagal afferent signals act via the central nervous system to modify adrenal-dependent anti-inflammatory responses. Vagal afferents, not efferents, are thus the likely key to these phenomena.

[Important role of relationship between brain and spleen in the mechanisms of chronic pain development and maintenance in fibromyalgia]

Fibromyalgia (FM) is characterized by chronic generalized pain accompanied by various symptoms, such as extreme fatigue, insomnia and depression. Clinical studies have indicated the presence of psychological stress, sympathetic nervous system hyperexcitation and immune system abnormalities, as a trigger for the onset of the disease, but the contribution to the pathogenesis of the disease remains unclear. Here, we employed the repeated acid saline-induced generalized pain (AcGP) model, as an experimental mouse model of FM. In this model, the unilateral repeated acid injection into gastrocnemius muscle induced transient and long-lasting mechanical hypersensitivity. We focused on the spleen, a secondary lymphoid organ, and found that the intravenous treatments of splenocytes derived from AcGP mice caused mechanical hypersensitivity in naїve mice. Since the spleen is directly innervated by sympathetic nerve, we examined whether adrenergic receptors are necessary for pain development or maintenance. The administration of butoxamine, a selective β2-blocker, prevented the development but did not reverse the maintenance of pain-like behavior in AcGP mice. Furthermore, β2-blockade in donor AcGP mice eliminated pain reproduction in recipient mice injected with AcGP splenocytes. We currently employed another model of FM, the intermittent psychological stress-induced generalized pain (IPGP) model and found that as in AcGP model, the sympathetic nervous system and the spleen play important roles. These results suggest that sympathetic β2 signaling is enhanced by physical/psychological stress, and that immune system cells in the spleen activated in response play an important role in the formation and maintenance of chronic pain.

Fibromyalgia Animal Models Using Intermittent Cold and Psychological Stress

Fibromyalgia (FM) is a chronic pain condition characterized by widespread musculoskeletal pain and other frequent symptoms such as fatigue, sleep disturbance, cognitive impairment, and mood disorder. Based on the view that intermittent stress would be the most probable etiology for FM, intermittent cold- and intermittent psychological stress-induced generalized pain (ICGP and IPGP) models in mice have been developed and validated as FM-like pain models in terms of the patho-physiological and pharmacotherapeutic features that are shared with clinical versions. Both models show long-lasting and generalized pain and female-predominant sex differences after gonadectomy. Like many other neuropathic pain models, ICGP and IPGP were abolished in lysophosphatidic acid receptor 1 (LPAR1) knock-out mice or by LPAR1 antagonist treatments, although deciding the clinical importance of this mechanism depends on waiting for the development of a clinically available LPAR1 antagonist. On the other hand, the nonsteroidal anti-inflammatory drug diclofenac with morphine did not suppress hyperalgesia in these models, and this is consistent with the clinical findings. Pharmacological studies suggest that the lack of morphine analgesia is associated with opioid tolerance upon the stress-induced release of endorphins and subsequent counterbalance through anti-opioid NMDA receptor mechanisms. Regarding pharmacotherapy, hyperalgesia in both models was suppressed by pregabalin and duloxetine, which have been approved for FM treatment in clinic. Notably, repeated treatments with mirtazapine, an α2 adrenergic receptor antagonist-type antidepressant, and donepezil, a drug for treating Alzheimer's disease, showed potent therapeutic actions in these models. However, the pharmacotherapeutic treatment should be carried out 3 months after stress, which is stated in the FM guideline, and many preclinical studies, such as those analyzing molecular and cellular mechanisms, as well as additional evidence using different animal models, are required. Thus, the ICGP and IPGP models have the potential to help discover and characterize new therapeutic medicines that might be used for the radical treatment of FM, although there are several limitations to be overcome.

Neurovascular Compression-Induced Intracranial Allodynia May Be the True Nature of Migraine Headache: an Interpretative Review

PURPOSE OF REVIEW

Surgical deactivation of migraine trigger sites by extracranial neurovascular decompression has produced encouraging results and challenged previous understanding of primary headaches. However, there is a lack of in-depth discussions on the pathophysiological basis of migraine surgery. This narrative review provides interpretation of relevant literature from the perspective of compressive neuropathic etiology, pathogenesis, and pathophysiology of migraine.

RECENT FINDINGS

Vasodilation, which can be asymptomatic in healthy subjects, may produce compression of cranial nerves in migraineurs at both extracranial and intracranial entrapment-prone sites. This may be predetermined by inherited and acquired anatomical factors and may include double crush-type lesions. Neurovascular compression can lead to sensitization of the trigeminal pathways and resultant cephalic hypersensitivity. While descending (central) trigeminal activation is possible, symptomatic intracranial sensitization can probably only occur in subjects who develop neurovascular entrapment of cranial nerves, which can explain why migraine does not invariably afflict everyone. Nerve compression-induced focal neuroinflammation and sensitization of any cranial nerve may neurogenically spread to other cranial nerves, which can explain the clinical complexity of migraine. Trigger dose-dependent alternating intensity of sensitization and its synchrony with cyclic central neural activities, including asymmetric nasal vasomotor oscillations, may explain the laterality and phasic nature of migraine pain. Intracranial allodynia, i.e., pain sensation upon non-painful stimulation, may better explain migraine pain than merely nociceptive mechanisms, because migraine cannot be associated with considerable intracranial structural changes and consequent painful stimuli. Understanding migraine as an intracranial allodynia could stimulate research aimed at elucidating the possible neuropathic compressive etiology of migraine and other primary headaches.

Peripheral Beta-2 Adrenergic Receptors Mediate the Sympathetic Efferent Activation from Central Nervous System to Splenocytes in a Mouse Model of Fibromyalgia

Abnormalities in the peripheral immune system are involved in the pathophysiology of fibromyalgia, although their contribution to the painful symptoms remains unknown. Our previous study reported the ability of splenocytes to develop pain-like behavior and an association between the central nervous system (CNS) and splenocytes. Since the spleen is directly innervated by sympathetic nerves, this study aimed to examine whether adrenergic receptors are necessary for pain development or maintenance using an acid saline-induced generalized pain (AcGP) model (an experimental model of fibromyalgia) and whether the activation of these receptors is also essential for pain reproduction by the adoptive transfer of AcGP splenocytes. The administration of selective β2-blockers, including one with only peripheral action, prevented the development but did not reverse the maintenance of pain-like behavior in acid saline-treated C57BL/6J mice. Neither a selective α1-blocker nor an anticholinergic drug affects the development of pain-like behavior. Furthermore, β2-blockade in donor AcGP mice eliminated pain reproduction in recipient mice injected with AcGP splenocytes. These results suggest that peripheral β2-adrenergic receptors play an important role in the efferent pathway from the CNS to splenocytes in pain development.

Lysophosphatidic acid receptor type-1 mediates brain activation in micro-Positron Emission Tomography analysis in a fibromyalgia-like mouse model

The intermittent cold stress-induced generalized pain response mimics the pathophysiological and pharmacotherapeutic features reported for fibromyalgia patients, including the presence of chronic generalized pain and female dominance. In addition, the intermittent cold stress-induced generalized pain is abolished in lysophosphatidic acid receptor type-1 knockout mice, as reported in many cases of neuropathic pain models. This study aimed to identify the brain loci involved in the intermittent cold stress generalized pain response and test their dependence on the lysophosphatidic acid receptor type-1. Positron emission tomography analyses using 2-deoxy-2-[¹⁸ F]fluoro-D-glucose in the presence of a pain stimulus showed that intermittent cold stress causes a significant increase in uptake in the ipsilateral regions, including the salience networking-related anterior cingulate cortex and insular cortex and the cognition-related hippocampus. A significant decrease was observed in the default mode network-related posterior cingulate cortex. Almost these intermittent cold stress-induced changes were abolished in lysophosphatidic acid receptor type-1 knockout mice. There results suggest that the intermittent cold stress-induced generalized pain response is mediated by the lysophosphatidic acid receptor type-1 in specific brain loci related to salience networking and cognition, which may lead to further developments in the treatment of fibromyalgia.

The Phantom Satiation Hypothesis of Bariatric Surgery

The excitation of vagal mechanoreceptors located in the stomach wall directly contributes to satiation. Thus, a loss of gastric innervation would normally be expected to result in abrogated satiation, hyperphagia, and unwanted weight gain. While Roux-en-Y-gastric bypass (RYGB) inevitably results in gastric denervation, paradoxically, bypassed subjects continue to experience satiation. Inspired by the literature in neurology on phantom limbs, I propose a new hypothesis in which damage to the stomach innervation during RYGB, including its vagal supply, leads to large-scale maladaptive changes in viscerosensory nerves and connected brain circuits. As a result, satiation may continue to arise, sometimes at exaggerated levels, even in subjects with a denervated or truncated stomach. The same maladaptive changes may also contribute to dysautonomia, unexplained pain, and new emotional responses to eating. I further revisit the metabolic benefits of bariatric surgery, with an emphasis on RYGB, in the light of this phantom satiation hypothesis.

Allodynia by Splenocytes From Mice With Acid-Induced Fibromyalgia-Like Generalized Pain and Its Sexual Dimorphic Regulation by Brain Microglia

Fibromyalgia (FM), a disease of unknown etiology characterized by chronic generalized pain, is partly recapitulated in an animal model induced by repeated acid saline injections into the gastrocnemius muscle. Here, we attempted to investigate the sex difference in pain hypersensitivity (mechanical allodynia and hypersensitivity to electrical stimulation) in the repeated acid saline-induced FM-like generalized pain (AcGP) model. The first unilateral acid injection into gastrocnemius muscle at day 0/D0 and second injection at D5 (post day 0, P0) induced transient and long-lasting mechanical allodynia, respectively, on both sides of male and female mice. The pretreatment with gonadectomy did not affect the first injection-induced allodynia in both sexes, but gradually reversed the second injection-induced allodynia in male but not female mice. Moreover, the AcGP in male mice was abolished by intracerebroventricular minocycline treatments during D4–P4 or P5–P11, but not by early treatments during D0–D5 in male but not female mice, suggesting that brain microglia are required for AcGP in late-onset and sex-dependent manners. We also found that the intravenous treatments of splenocytes derived from male but not female mice treated with AcGP caused allodynia in naive mice. In addition, the purified CD4⁺ T cells derived from splenocytes of acid-treated male mice retained the ability to cause allodynia in naive mice. These findings suggest that FM-like AcGP has multiple sexual dimorphic mechanisms.

Ketamine anesthesia enhances fear memory consolidation via noradrenergic activation in the basolateral amygdala

Trauma patients treated with ketamine during emergency care present aggravated early post- traumatic stress reaction which is highly predictive of post-traumatic stress disorder (PTSD) development and severity. The use of ketamine in the acute trauma phase may directly or indirectly interfere with neural processes of memory consolidation of the traumatic event, thus leading to the formation of maladaptive memories, a hallmark symptom of PTSD. We have recently shown that ketamine anesthesia, immediately after a traumatic event, enhances memory consolidation and leads to long-lasting alterations of social behavior in rats. Based on the evidence that ketamine induces a robust central and peripheral adrenergic/noradrenergic potentiation and that activation of this system is essential for the formation of memory for stressful events, we explored the possibility that the strong sympathomimetic action of ketamine might underlie its memory enhancing effects. We found that rats given immediate, but not delayed, post-training ketamine anesthesia (125 mg/kg) presented enhanced 48-h memory retention in an inhibitory avoidance task and that these effects were blocked by adrenal medullectomy, lesions of the locus coeruleus, systemic or intra-basolateral amygdala ß-adrenergic receptor antagonism. Thus, the memory enhancing effects of ketamine anesthesia are time-dependent and mediated by a combined peripheral-central sympathomimetic action. We elucidated a mechanism by which ketamine exacerbates acute post-traumatic reaction, possibly leading to development of PTSD symptomatology later in life. These findings will help guide for a better management of sedation/anesthesia in emergency care to promote the prophylaxis and reduce the risk of developing trauma-related disorders in trauma victims.

Pathogenic mechanisms of lipid mediator lysophosphatidic acid in chronic pain

A number of membrane lipid-derived mediators play pivotal roles in the initiation, maintenance, and regulation of various types of acute and chronic pain. Acute pain, comprising nociceptive and inflammatory pain warns us about the presence of damage or harmful stimuli. However, it can be efficiently reversed by opioid analgesics and anti-inflammatory drugs. Prostaglandin E₂ and I₂, the representative lipid mediators, are well-known causes of acute pain. However, some lipid mediators such as lipoxins, resolvins or endocannabinoids suppress acute pain. Various types of peripheral and central neuropathic pain (NeuP) as well as fibromyalgia (FM) are representatives of chronic pain and refractory owing to abnormal pain processing distinct from acute pain. Accumulating evidence demonstrated that lipid mediators represented by lysophosphatidic acid (LPA) are involved in the initiation and maintenance of both NeuP and FM in experimental animal models. The LPAR₁-mediated peripheral mechanisms including dorsal root demyelination, Ca_vα2δ1 expression in dorsal root ganglion, and LPAR₃-mediated amplification of central LPA production via glial cells are involved in the series of molecular mechanisms underlying NeuP. This review also discusses the involvement of lipid mediators in emerging research directives, including itch-sensing, sexual dimorphism, and the peripheral immune system.

The Adrenal Medulla Modulates Mechanical Allodynia in a Rat Model of Neuropathic Pain

We have investigated whether the stress response mediated by the adrenal medulla in rats subjected to chronic constriction injury of the sciatic nerve (CCI) modulates their nocifensive behavior. Treatment with SK29661 (300 mg/kg; intraperitoneal (I.P.)), a selective inhibitor of phenylethanolamine N-methyltransferase (PNMT) that converts noradrenaline (NA) into adrenaline (A), fully reverted mechanical allodynia in the injured hind paw without affecting mechanical sensitivity in the contralateral paw. The effect was fast and reversible and was associated with a decrease in the A to NA ratio (A/NA) in the adrenal gland and circulating blood, an A/NA that was elevated by CCI. 1,2,3,4-tetrahydroisoquinoline-7-sulfonamide (SKF29661) did not affect exocytosis evoked by Ca²⁺ entry as well as major ionic conductances (voltage-gated Na⁺, Ca²⁺, and K⁺ channels, nicotinic acetylcholine receptors) involved in stimulus-secretion coupling in chromaffin cells, suggesting that it acted by changing the relative content of the two adrenal catecholamines. Denervation of the adrenal medulla by surgical splanchnectomy attenuated mechanical allodynia in neuropathic animals, hence confirming the involvement of the adrenal medulla in the pathophysiology of the CCI model. Inhibition of PNMT appears to be an effective and probably safe way to modulate adrenal medulla activity and, in turn, to alleviate pain secondary to the injury of a peripheral nerve.

Mirtazapine, an α2 Antagonist-Type Antidepressant, Reverses Pain and Lack of Morphine Analgesia in Fibromyalgia-Like Mouse Models

Treatment of fibromyalgia is an unmet medical need; however, its pathogenesis is still poorly understood. In a series of studies, we have demonstrated that some pharmacological treatments reverse generalized chronic pain but do not affect the lack of morphine analgesia in the intermittent cold stress (ICS)-induced fibromyalgia-like pain model in mice. Here we report that repeated intraperitoneal treatments with mirtazapine, which is presumed to disinhibit 5-hydroxytriptamine (5-HT) release and activate 5-HT1 receptor through mechanisms of blocking presynaptic adrenergic α2 and postsynaptic 5-HT2 and 5-HT3 receptors, completely reversed the chronic pain for more than 4 to 5 days after the cessation of treatments. The repeated mirtazapine treatments also recovered the morphine analgesia after the return of nociceptive threshold to the normal level. The microinjection of small interfering RNA (siRNA) adrenergic α2a receptor (ADRA2A) into the habenula, which showed a selective upregulation of α2 receptor gene expression after ICS, reversed the hyperalgesia but did not recover the morphine analgesia. However, both reversal of hyperalgesia and recovery of morphine analgesia were observed when siRNA ADRA2A was administered intracerebroventricularly. As the habenular is reported to be involved in the emotion/reward-related pain and hypoalgesia, these results suggest that mirtazapine could attenuate pain and/or augment hypoalgesia by blocking the habenular α2 receptor after ICS. The recovery of morphine analgesia in the ICS model, on the other hand, seems to be mediated through a blockade of α2 receptor in unidentified brain regions. SIGNIFICANCE STATEMENT: This study reports possible mechanisms underlying the complete reversal of hyperalgesia and recovery of morphine analgesia by mirtazapine, a unique antidepressant with adrenergic α2 and serotonergic receptor antagonist properties, in a type of intermittently repeated stress (ICS)-induced fibromyalgia-like pain model. Habenula, a brain region which is related to the control of emotional pain, was found to play key roles in the antihyperalgesia, whereas other brain regions appeared to be involved in the recovery of morphine analgesia in the ICS model.

LPA receptor signaling as a therapeutic target for radical treatment of neuropathic pain and fibromyalgia

Since the first discovery that the bioactive lipid, lysophosphatidic acid (LPA) and LPA₁ receptor signaling play a role in the initiation of neuropathic pain (NeuP), accumulated reports have supported the original findings and extended the study toward possible therapeutic applications. The present review describes beneficial roles of LPA receptor signaling in a variety of chronic pain, such as peripheral NeuP induced by nerve injury, chemotherapy and diabetes, central NeuP induced by cerebral ischemia with hemorrhage and spinal cord injury, and fibromyalgia-like wide spread pain induced by repeated cold, psychological and muscular acidic stress. Emerging mechanistic findings are the feed-forward amplification of LPA production through LPA₁, LPA₃ and microglia and the evidence for maintenance of chronic pain by LPA receptor signaling.

Effect of distal esophageal irritation on the changes of cystometry parameters to esophagus and colon distentions in rats

The coexistence of different visceral pathologies in patients suffering from irritable bowel syndrome, interstitial cystitis, and other pathologies, necessitates the study of these pathologies under complicated conditions. In the present study, cystometry recordings were used to investigate the effect of distal esophageal chemical irritation on the urinary bladder interaction with distal colon distention, distal esophageal distention, and electrical stimulation of abdominal branches of vagus nerve. Distal esophageal chemical irritation significantly decreased the intercontraction time via decreasing the voiding time. Also, distal esophageal chemical irritation significantly decreased the pressure amplitude by decreasing the maximum pressure. Following distal esophageal chemical irritation, distal esophageal distention was able to significantly decrease the intercontraction time by decreasing the storage time. However, 3 mL distal colon distention significantly increased the intercontraction time by increasing the storage time. On the other hand, following distal esophageal chemical irritation, electrical stimulation of abdominal branches of vagus nerve did not have any significant effect on intercontraction time. However, electrical stimulation of abdominal branches of vagus nerve significantly increased the pressure amplitude by increasing the maximum pressure. The results of this study demonstrate that urinary bladder function and interaction of bladder with other viscera can be affected by chemical irritation of distal esophagus.

Chronic Diffuse Pain and Functional Gastrointestinal Disorders After Traumatic Stress: Pathophysiology Through a Polyvagal Perspective

Chronic diffuse pain disorders, such as fibromyalgia, and functional gastrointestinal disorders (FGIDs), such as irritable bowel syndrome, place substantial burden on those affected and on the medical system. Despite their sizable impact, their pathophysiology is poorly understood. In contrast to an approach that focuses on the correlation between heart rate variability (HRV) and a specific organ or symptom, we propose that a bio-evolutionary threat-related autonomic response—as outlined in the Polyvagal Theory—may serve as a plausible explanation of how HRV, particularly respiratory sinus arrhythmia (RSA), would index the pathophysiology of these disorders. Evidence comes from: (1) the well-documented atypical autonomic regulation of the heart common to fibromyalgia and irritable bowel syndrome reflected in dampened RSA, (2) the neural architecture that integrates the heart, pain pathways, and the gastrointestinal tract, (3) the common physical co-morbidities shared by chronic diffuse pain and FGIDs, many of which are functionally regulated by the autonomic nervous system, (4) the elevated risk of chronic diffuse pain and FGIDs following traumatic stress or abuse, (5) and the elevated risk of chronic diffuse pain and FGIDs in individuals with anxiety and panic disorders. This novel conceptualization points to a pathogenesis rooted in changes to brain-body autonomic feedback loops in response to evolutionarily-salient threat cues, providing an integrated biopsychosocial model of chronic diffuse pain and FGIDs and suggesting new, non-pharmacological treatment strategies.

LPA1 receptor involvement in fibromyalgia-like pain induced by intermittent psychological stress, empathy

Treatment for fibromyalgia is an unmet medical need and its pathogenesis is still poorly understood. The present study demonstrated that intermittent psychological stress (IPS), or empathy causes generalized chronic abnormal pain with female predominance. The persistence of the pain phenotype was dependent on the unpredictability of the stressor. The pain was reversed by pregabalin (PGB), duloxetine (DLX) or mirtazapine (Mir), but not by diclofenac or morphine. Differential administration of these existing medicines revealed that the sites of PGB and Mir actions exist in the brain, but not in the spinal cord, while that of DLX is preferentially in the spinal cord. It is interesting to note that the intracerebroventricular injection of PGB or Mir showed potent analgesia for 24 h or longer, though systemic injection of these medicines shows anti-hyperalgesia just for several hours. These results indicate that initial intense actions in the target brain may prevent the forthcoming development of pain memory. IPS-induced abnormal pain was prevented in mice deficient of lysophosphatidic acid receptor 1 (LPA1) gene, and completely cured by the repeated intrathecal treatments with LPA1 antagonist, AM966, which did not show acute action. All these results suggest that IPS model is an experimental animal model, which mimics the pathophysiology and pharmacotherapy in fibromyalgia in clinic, and LPA1 signaling plays crucial roles in the IPS-induced fibromyalgia-like abnormal pain.

Prostaglandin levels, vaginal innervation, and cyst innervation as peripheral contributors to endometriosis-associated vaginal hyperalgesia in rodents

Endometriosis is a painful condition characterized by growth of endometrial cysts outside the uterus. Here, we tested the hypothesis that peripheral innervation and prostaglandin levels contribute to endometriosis-associated pain. Female Sprague-Dawley rats (n = 16) were surgically instrumented by transplanting uterine tissue onto mesenteric arteries within the peritoneal cavity to create a model of endometriosis which forms extra-uterine endometrial cysts and vaginal hyperalgesia. Our results describe a significant positive correlation between endometriosis-induced vaginal hyperalgesia and cyst innervation density (sensory, r = 0.70, p = 0.003; sympathetic, r = 0.55, p = 0.03), vaginal canal sympathetic innervation density (r = 0.80, p = 0.003), and peritoneal fluid levels of the prostaglandins PGE2 (r = 0.65, p = 0.01) and PGF2α (r = 0.63, p = 0.02). These results support the involvement of cyst innervation and prostaglandins in endometriosis-associated pain. We also describe how sympathetic innervation density of the vaginal canal is an important predictor of vaginal hyperalgesia.

The influence of distal colon irritation on the changes of cystometry parameters to esophagus and colon distentions

The co-occurrence of multiple pathologies in the pelvic viscera in the same patient, such as, irritable bowel syndrome and interstitial cystitis, indicates the complexity of viscero-visceral interactions and the necessity to study these interactions under multiple pathological conditions. In the present study, the effect of distal colon irritation (DCI) on the urinary bladder interaction with distal esophagus distention (DED), distal colon distention (DCD), and electrical stimulation of the abdominal branches of vagus nerve (abd-vagus) were investigated using cystometry parameters. The DCI significantly decreased the intercontraction time (ICT) by decreasing the storage time (ST); nonetheless, DED and Abd-vagus were still able to significantly decrease the ICT and ST following DCI. However, DCD had no effect on ICT following the DCI. The DCI, also, significantly decreased the Intravesical pressure amplitude (P-amplitude) by increasing the resting pressure (RP). Although DED has no effect on the P-amplitude, both in the intact and the irritated animals, the abd-vagus significantly increased the P-amplitude following DCI by increasing the maximum pressure (MP). In the contrary, 3mL DCD significantly increased the P-amplitude by increasing the MP and lost that effect following the DCI. Concerning the pressure threshold (PT), none of the stimuli had any significant changes in the intact animals. However, DCI significantly decreased the PT, also, the abd-vagus and 3mL DCD significantly decreased the PT. The results of this study indicate that chemical irritation of colon complicates the effects of mechanical irritation of esophagus and colon on urinary bladder function.

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 Aβ-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.

The Relationships Between Parasympathetic Function and Pain Perception: The Role of Anxiety

BACKGROUND

Previous studies have identified relationships between autonomic function and pain perception. Anxiety was found to influence both autonomic and pain responses. We examined the effect of anxiety level on parasympathetic function and pain perception as well as on the relationships between these 2 systems.

METHODS

Thirty healthy females were divided into high- and low-anxiety groups according to their trait anxiety levels. Parasympathetic function was obtained using heart rate variability, deep breathing, and Valsalva ratios. Pain perception parameters of heat pain thresholds, pain rating of supra-thresholds stimulus, mechanical temporal summation, and conditioned pain modulation response were examined.

RESULTS

The low-anxiety and high-anxiety groups exhibited no significant differences in the parasympathetic function and pain perception parameters. Assessment of the associations revealed that in the high-anxiety group, higher mean ratings of the tonic heat pain stimulus were significantly correlated with higher rMSSD (r² = 0.358, P = 0.019), but this was not found for the low-anxiety group (P = 0.282). In addition, in the high-anxiety group, efficient conditioned pain modulation response was correlated with higher deep breathing ratio (r² = 0.363, P = 0.023); however, in the low-anxiety group, the correlation did not reach significance (P = 0.109).

CONCLUSIONS

This study demonstrates the role of anxiety level on the relationships between parasympathetic function and pain perception. We suggest that a situation of high anxiety leads to higher norepinephrine levels that can influence both parasympathetic function and pain perception, thus explaining the significant relationships found between these 2 systems only in subjects with high anxiety.

ATP release mechanisms of endothelial cell-mediated stimulus-dependent hyperalgesia

Endothelin-1 (ET-1) acts on endothelial cells to enhance mechanical stimulation-induced release of adenosine triphosphate (ATP), which in turn can act on sensory neurons innervating blood vessels to contribute to vascular pain, a phenomenon we have referred to as stimulus-dependent hyperalgesia (SDH). In the present study, we evaluated the role of the major classes of ATP release mechanisms to SDH: vesicular exocytosis, plasma membrane-associated ATP synthase, ATP-binding cassette transporters, and ion channels. Inhibitors of vesicular exocytosis (ie, monensin, brefeldin A, and bafilomycin), plasma membrane-associated ATPase (ie, oligomycin and pigment epithelium-derived factor peptide 34-mer), and connexin ion channels (carbenoxolone and flufenamic acid) but not ATP-binding cassette transporter (ie, dipyridamole, nicardipine, or CFTRinh-172) attenuated SDH. This study reports a role of ATP in SDH and suggests novel targets for the treatment of vascular pain syndromes.

PERSPECTIVE

ET-1 acts on endothelial cells to produce mechanical stimulation-induced hyperalgesia. Inhibitors of 3 different ATP release mechanisms attenuated this SDH. This study provides support for a role of ATP in SDH and suggests novel targets for the treatment of vascular pain syndromes.

Sympathetic nervous system and inflammation: a conceptual view

The peripheral sympathetic nervous system is organized into function-specific pathways that transmit the activity from the central nervous system to its target tissues. The transmission of the impulse activity in the sympathetic ganglia and to the effector tissues is target cell specific and guarantees that the centrally generated command is faithfully transmitted. This is the neurobiological basis of autonomic regulations in which the sympathetic nervous system is involved. Each sympathetic pathway is connected to distinct central circuits in the spinal cord, lower and upper brain stem and hypothalamus. In addition to its conventional functions, the sympathetic nervous system is involved in protection of body tissues against challenges arising from the environment as well as from within the body. This function includes the modulation of inflammation, nociceptors and above all the immune system. Primary and secondary lymphoid organs are innervated by sympathetic postganglionic neurons and processes in the immune tissue are modulated by activity in these sympathetic neurons via adrenoceptors in the membranes of the immune cells (see Bellinger and Lorton, 2014). Are the primary and secondary lymphoid organs innervated by a functionally specific sympathetic pathway that is responsible for the modulation of the functioning of the immune tissue by the brain? Or is this modulation of immune functions a general function of the sympathetic nervous system independent of its specific functions? Which central circuits are involved in the neural regulation of the immune system in the context of neural regulation of body protection? What is the function of the sympatho-adrenal system, involving epinephrine, in the modulation of immune functions?

Acute inflammation in the joint: its control by the sympathetic nervous system and by neuroendocrine systems

Inflammation of tissues is under neural control involving neuroendocrine, sympathetic and central nervous systems. Here we used the acute experimental inflammatory model of bradykinin-induced plasma extravasation (BK-induced PE) of the rat knee joint to investigate the neural and neuroendocrine components controlling this inflammation. 1. BK-induced PE is largely dependent on the sympathetic innervation of the synovium, but not on activity in these neurons and not on release of norepinephrine. 2. BK-induced PE is under the control of the hypothalamo-pituitary-adrenal (HPA) system and the sympatho-adrenal (SA) system, activation of both leading to depression of BK-induced PE. The inhibitory effect of the HPA system is mediated by corticosterone and dependent on the sympathetic innervation of the synovium. The inhibitory effect of the SA system is mediated by epinephrine and β2-adrenoceptors. 3. BK-induced PE is inhibited during noxious stimulation of somatic or visceral tissues and is mediated by the neuroendocrine systems. The nociceptive-neuroendocrine reflex circuits are (for the SA system) spinal and spino-bulbo-spinal. 4. The nociceptive-neuroendocrine reflex circuits controlling BK-induced PE are under powerful inhibitory control of vagal afferent neurons innervating the defense line (connected to the gut-associated lymphoid tissue) of the gastrointestinal tract. This inhibitory link between the visceral defense line and the central mechanisms controlling inflammatory mechanisms in body tissues serves to co-ordinate protective defensive mechanisms of the body. 5. The circuits of the nociceptive-neuroendocrine reflexes are under control of the forebrain. In this way, the defensive mechanisms of inflammation in the body are co-ordinated, optimized, terminated as appropriate, and adapted to the behavior of the organism.

Stress in the adult rat exacerbates muscle pain induced by early-life stress

BACKGROUND

Early-life stress and exposure to stressful stimuli play a major role in the development of chronic widespread pain in adults. However, how they interact in chronic pain syndromes remains unclear.

METHODS

Dams and neonatal litters were submitted to a restriction of nesting material (neonatal limited bedding [NLB]) for 1 week. As adults, these rats were exposed to a painless sound stress protocol. The involvement of sympathoadrenal catecholamines interleukin 6 (IL-6) and tumor necrosis factor alpha (TNFα) in nociception was evaluated through behavioral and enzyme-linked immunosorbent assays, surgical interventions, and intrathecal antisense treatments.

RESULTS

Adult NLB rats exhibited mild muscle hyperalgesia, which was markedly aggravated by sound stress (peaking 15 days after exposure). Adrenal medullectomy did not modify hyperalgesia in NLB rats but prevented its aggravation by sound stress. Sustained administration of epinephrine to NLB rats mimicked sound stress effect. Intrathecal treatment with antisense directed to IL-6 receptor subunit gp130 (gp130), but not to tumor necrosis factor receptor type 1 (TNFR1), inhibited hyperalgesia in NLB rats. However, antisense against either gp130 or TNFR1 inhibited sound stress-induced enhancement of hyperalgesia. Compared with control rats, NLB rats exhibit increased plasma levels of IL-6 but decreased levels of TNFα, whereas sound stress increases IL-6 plasma levels in control rats but not in NLB rats.

CONCLUSIONS

Early-life stress induces a persistent elevation of IL-6, hyperalgesia, and susceptibility to chronic muscle pain, which is unveiled by exposure to stress in adults. This probably depends on an interaction between adrenal catecholamines and proinflammatory cytokines acting at muscle nociceptor level.

Blockade of Adrenal Medulla-Derived Epinephrine Potentiates Bee Venom-Induced Antinociception in the Mouse Formalin Test: Involvement of Peripheral β -Adrenoceptors

The injection of diluted bee venom (DBV) into an acupoint has been used traditionally in eastern medicine to treat a variety of inflammatory chronic pain conditions. We have previously shown that DBV had a potent antinociceptive efficacy in several rodent pain models. However, the peripheral mechanisms underlying DBV-induced antinociception remain unclear. The present study was designed to investigate the role of peripheral epinephrine on the DBV-induced antinociceptive effect in the mouse formalin assay. Adrenalectomy significantly enhanced the antinociceptive effect of DBV during the late phase of the formalin test, while chemical sympathectomy had no effect. Intraperitoneal injection of epinephrine blocked this adrenalectomy-induced enhancement of the DBV-induced antinociceptive effect. Moreover, injection of a phenylethanolamine N-methyltransferase (PNMT) inhibitor enhanced the DBV-induced antinociceptive effect. Administration of nonselective β-adrenergic antagonists also significantly potentiated this DBV-induced antinociception, in a manner similar to adrenalectomy. These results demonstrate that the antinociceptive effect of DBV treatment can be significantly enhanced by modulation of adrenal medulla-derived epinephrine and this effect is mediated by peripheral β-adrenoceptors. Thus, DBV acupoint stimulation in combination with inhibition of peripheral β-adrenoceptors could be a potentially novel strategy for the management of inflammatory pain.

Role of endothelial cells in antihyperalgesia induced by a triptan and β-blocker

While blood vessels have long been implicated in diverse pain syndromes (e.g., migraine headache, angina pectoris, vasculitis, and Raynaud's syndrome), underlying mechanisms remain to be elucidated. Recent evidence supports a contribution of the vascular endothelium in endothelin-1-induced hyperalgesia, and its enhancement by repeated mechanical stimulation; a phenomenon referred to as stimulus-induced enhancement of (endothelin) hyperalgesia (SIEH). SIEH is thought to be mediated by release of ATP from endothelial cells, to act on P2X3 receptors on nociceptors. In the present study we evaluated the ability of another vasoactive hyperalgesic agent, epinephrine, to induce endothelial cell-dependent hyperalgesia and SIEH. We found that epinephrine also produces hyperalgesia and SIEH. Both P2X3 receptor antagonists, A317491 and octoxynol-9, which attenuate endothelial cell function, eliminated SIEH without affecting epinephrine hyperalgesia. We further evaluated the hypothesis that members of two important classes of drugs used to treat migraine headache, whose receptors are present in endothelial cells - the triptans and β blockers - have a vascular component to their anti-hyperalgesic action. For this, we tested the effect of ICI-118,551, a β₂-adrenergic receptor antagonist and sumatriptan, an agonist at 5-HT1B and 5-HT₁D receptors, on nociceptive effects of endothelin and epinephrine. ICI-118,551 inhibited endothelin SIEH, and attenuated epinephrine hyperalgesia and SIEH. Sumatriptan inhibited epinephrine SIEH and inhibited endothelin hyperalgesia and SIEH, while having no effect on epinephrine hyperalgesia or the hyperalgesia induced by a prototypical direct-acting inflammatory mediator, prostaglandin E₂. These results support the suggestion that triptans and β-blockers interact with the endothelial cell component of the blood vessel to produce anti-hyperalgesia.

Independent contributions of alcohol and stress axis hormones to painful peripheral neuropathy

Painful small-fiber peripheral neuropathy is a debilitating complication of chronic alcohol abuse. Evidence from previous studies suggests that neuroendocrine mechanisms, in combination with other, as yet unidentified actions of alcohol, are required to produce this neuropathic pain syndrome. In addition to neurotoxic effects of alcohol, in the setting of alcohol abuse neuroendocrine stress axes release glucocorticoids and catecholamines. Since receptors for these stress hormones are located on nociceptors, at which they can act to cause neuronal dysfunction, we tested the hypothesis that alcohol and stress hormones act on the nociceptor, independently, to produce neuropathic pain. We used a rat model, which allows the distinction of the effects of alcohol from those produced by neuroendocrine stress axis mediators. We now demonstrate that topical application of alcohol and exposure to unpredictable sound stress, each alone, has no effect on the nociceptive threshold. However, when animals that had previous exposure to alcohol were subsequently exposed to stress, they rapidly developed mechanical hyperalgesia. Conversely, sound stress followed by topical alcohol exposure also produced mechanical hyperalgesia. The contribution of stress hormones was prevented by spinal intrathecal administration of oligodeoxynucleotides antisense to β(2)-adrenergic or glucocorticoid receptor mRNA, which attenuates receptor level in nociceptors, as well as by adrenal medullectomy. These experiments establish an independent role of alcohol and stress hormones on the primary afferent nociceptor in the induction of painful peripheral neuropathy.

Potential autonomic risk factors for chronic TMD: descriptive data and empirically identified domains from the OPPERA case-control study

Several case-control studies have been conducted that examine the association between autonomic variables and persistent pain conditions; however, there is a surprising dearth of published studies in this area that have focused on temporomandibular disorders (TMD). The current study presents autonomic findings from the baseline case-control study of the OPPERA (Orofacial Pain: Prospective Evaluation and Risk Assessment) cooperative agreement. Measures of arterial blood pressure, heart rate, heart rate variability, and indirect measures of baroreflex sensitivity were assessed at rest and in response to a physical (orthostatic) and psychological (Stroop) stressors in 1,633 TMD-free controls and 185 TMD cases. In bivariate and demographically adjusted analyses, greater odds of TMD case status were associated with elevated heart rates, reduced heart rate variability, and reduced surrogate measures of baroreflex sensitivity across all experimental procedures. Principal component analysis was undertaken to identify latent constructs revealing 5 components. These findings provide evidence of associations between autonomic factors and TMD. Future prospective analyses in the OPPERA cohort will determine if the presence of these autonomic factors predicts increased risk for developing new onset TMD.

PERSPECTIVE

This article reports autonomic findings from the OPPERA Study, a large prospective cohort study designed to discover causal determinants of TMD pain. Findings indicate statistically significant differences between TMD cases and controls across multiple autonomic constructs at rest and during both physical and psychologically challenging conditions. Future analyses will determine whether these autonomic factors increase risk for new onset TMD.

Subdiaphragmatic vagotomy increases the sensitivity of lumbar Aδ primary afferent neurons along with voltage-dependent potassium channels in rats

Subdiaphragmatic vagal dysfunction causes chronic pain. To verify whether this chronic pain is accompanied by enhanced peripheral nociceptive sensitivity, we evaluated primary afferent neuronal excitability in subdiaphragmatic vagotomized (SDV) rats. SDV rats showed a decrease in the electrical stimuli-induced hind limb-flexion threshold at 250 Hz, but showed no similar effect at 5 or 2000 Hz, which indicated that lumbar primary afferent Aδ sensitivity was enhanced in SDV rats. The whole-cell patch-clamp technique also revealed the hyper-excitability of acutely dissociated medium-sized lumbar dorsal root ganglion (DRG) neurons isolated from SDV rats. The contribution of changes in voltage-dependent potassium (Kv) channels was assessed, and transient A-type K(+) (I(A) ) current density was apparently decreased. Moreover, Kv4.3 immunoreactivity in medium-sized DRG neurons was significantly reduced in SDV rats compared to sham. These results indicate that SDV causes hyper-excitability of lumbar primary Aδ afferent neurons, which may be induced along with suppressing I(A) currents via the decreased expression of Kv4.3. Thus, peripheral Aδ neuroplasticity may contribute to the chronic lower limb pain caused by SDV.

Effect of esophagus distention on urinary bladder function in rats

AIMS

Micturition process is a spinobulbospinal reflex that is affected by the viscero-visceral interactions due to convergent inputs into spinal and/or supraspinal centers controlling that reflex. Although interaction between bladder and other pelvic organs, such as colon, are well studied, the viscero-visceral interaction between urinary bladder and internal organs in other regions are rarely studied.

METHODS

In the present study, continuous filling cystometry recordings, in male rats, were used to investigate the effects of mechanical stimulation of distal-esophagus (distention), as well as, electrical stimulation of abdominal branches of the vagus nerve on urinary bladder micturition cycles.

RESULTS

Distal esophagus distention and electrical stimulation of the vagus nerve significantly increased the micturition frequency through decreasing the time of the storage phase of the micturition cycle. However, bilateral cervical vagotomy eliminated the effects of distal esophagus distention and electrical stimulation of vagus nerve on micturition cycles.

CONCLUSIONS

The results of this study indicate that there is a viscero-visceral interaction between esophagus and urinary bladder, which is mediated through vagal afferents. Understanding the properties of the viscero-visceral interactions affecting the urinary bladder will help in the diagnosis and management of micturition problems.

Randall-Selitto test: a new approach for the detection of neuropathic pain after spinal cord injury

In this work we assess the usefulness of the Randall-Selitto test as a method to detect and quantify neuropathic pain responses in rats subjected to different spinal cord injuries. The mechanical nociceptive thresholds were significantly reduced during follow-up after spinal cord contusion or transection. Our results demonstrate that the Randall-Selitto test allows the detection of neuropathic pain both in forepaws and hindpaws, as well as in dorsal and plantar surfaces. Moreover, it does not require weight support capacity, so it can be used at early time points after the injury. This is the first time that this method has been used to describe the changes in nociceptive thresholds that take place after spinal cord injuries of different severities over time.

Psychological stress induces temporary masticatory muscle mechanical sensitivity in rats

To explore the relationship between psychological stress and masticatory muscle pain, we created a communication stress animal model to determine whether psychological stress could induce increased mechanical sensitivity in masticatory muscles and to study the changes of mechanical nociceptive thresholds after stress removal. Forty-eight male Sprague-Dawley rats were divided into a control group (CON), a foot-shocked group (FS, including 3 subgroups recorded as FS-1, FS-2, and FS-3), a psychological stress group (PS), and a drug treatment group (DT). PS and DT rats were confined in a communication box for one hour a day to observe the psychological responses of neighboring FS rats.Measurements of the mechanical nociceptive thresholds of the bilateral temporal and masseter muscles showed a stimulus-response relationship between psychological stress and muscle mechanical sensitivity. The DT rats, who received a diazepam injection, showed almost the same mechanical sensitivity of the masticatory muscles to that of the control in response to psychological stress. Fourteen days after the psychological stressor was removed, the mechanical nociceptive thresholds returned to normal. These findings suggest that psychological stress is directly related to masticatory muscle pain. Removal of the stressor could be a useful method for relieving mechanical sensitivity increase induced by psychological stress.

Subdiaphragmatic vagotomy promotes nociceptive sensitivity of deep tissue in rats

To verify whether vagal dysfunction is associated with chronic pain, we evaluated the effects of subdiaphragmatic vagotomy (vgx) on the sensitivity toward noxious stimuli in rats. Vgx rats showed sustained hyperalgesia in the gastrocnemius muscle without tissue damage (no increase in vgx-induced plasma creatine phosphokinase or lactose dehydrogenase levels) accompanied by hypersensitivity to colonic distension. We found a dramatic increase in the levels of metabotropic glutamate receptor 5, protein kinase C (PKC) gamma and phosphorylated-PKCgamma within the spinal cord dorsal horn in vgx rats, which suggests that vgx may evoke sensory nerve plasticity. Morphine produced a dose-dependent increase in the withdrawal threshold in both vgx and sham-operated rats, but the effect of a lower dose in vgx rats was weaker than that in sham-operated rats. Muscle hyperalgesia in vgx rats was also attenuated by gabapentin and amitriptyline, but was not affected by diclofenac, dexamethasone or diazepam. These findings indicate that subdiaphragmatic vagal dysfunction caused chronic muscle hyperalgesia accompanied by visceral pain and both gabapentin and amitriptyline were effective for subdiaphragmatic vagotomy-induced pain, which are partially similar to fibromyalgia syndrome. Furthermore, this chronic muscle pain may result from nociceptive neuroplasticity of the spinal cord dorsal horn.

Effects of 17beta-estradiol on responses of viscerosomatic convergent thalamic neurons in the ovariectomized female rat

Ovarian hormones have been shown to exert multiple effects on CNS function and viscerosomatic convergent activity. Ovariectomized (OVX) female rats were used in the present study to examine the long-term effects of proestrus levels of 17beta-estradiol (EB) delivered by a 60-day time-released subcutaneous pellet on the response properties of viscerosomatic convergent thalamic neurons. In addition, avoidance thresholds to mechanical stimulation for one of the convergent somatic territories, the trunk, was assessed using an electro-von Frey anesthesiometer before and at the end of the 6-wk post-OVX/implant period prior to the terminal electrophysiological experiments, which were done under urethane anesthesia. Rats implanted with an EB-containing pellet, relative to placebo controls, demonstrated 1) altered thalamic response frequencies and thresholds for cervix and vaginal but not colon stimulation; 2) some response variations for just the lateral group of thalamic subnuclei; and 3) altered thalamic response frequencies and thresholds for trunk stimulation. Thalamic response thresholds for trunk pressure in EB versus placebo rats were consistent with the avoidance thresholds obtained from the same groups. In addition, EB replacement affected visceral and somatic thresholds in opposite ways (i.e., reproductive-related structures were less sensitive to pressure, whereas somatic regions showed increased sensitivity). These results have obvious reproductive advantages (i.e., decreased reproductive organ sensitivity for copulation and increased trunk sensitivity for lordosis posturing), as well as possible clinical implications in women suffering from chronic pelvic pain syndromes and/or neuropathic pain.

Neurotoxic catecholamine metabolite in nociceptors contributes to painful peripheral neuropathy

The neurotoxic effects of catecholamine metabolites have been implicated in neurodegenerative diseases. As some sensory neurons express tyrosine hydroxylase and monoamine oxidase (MAO), we investigated the potential contribution of catecholamine metabolites to neuropathic pain in a model of alcoholic neuropathy. The presence of catecholamines in sensory neurons is supported by capsaicin-stimulated epinephrine release, an effect enhanced in ethanol-fed rats. mRNA for enzymes in dorsal root ganglia involved in catecholamine uptake and metabolism, dopamine beta-hydroxylase and MAO-A, were decreased by neonatal administration of capsaicin. Ethanol-induced hyperalgesia was attenuated by systemic and local peripheral administration of inhibitors of MAO-A, reduction of norepinephrine transporter (NET) in sensory neurons and a NET inhibitor. Finally, intradermal injection of 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL), a neurotoxic MAO-A catecholamine metabolite, produced robust mechanical hyperalgesia. These observations suggest that catecholamines in nociceptors are metabolized to neurotoxic products by MAO-A, which can cause neuronal dysfunction underlying neuropathic pain.

Alcohol-induced stress in painful alcoholic neuropathy

Chronic alcohol consumption induces a painful small-fiber peripheral neuropathy, the severity of which increases during alcohol withdrawal. Chronic alcohol consumption also produces a sustained increase in stress hormones, epinephrine and corticosterone, that is exacerbated during alcohol withdrawal. We report that adrenal medullectomy and administration of a glucocorticoid receptor antagonist, mifepristone (RU 38486), both prevented and reversed a model of painful peripheral neuropathy in alcohol binge-drinking rats. Chronic administration of stress levels of epinephrine to rats that had undergone adrenal medullectomy and were being fed the alcohol diet reconstituted this phenotype. Intrathecal administration of oligodeoxynucleotides antisense to the beta(2)-adrenergic- or glucocorticoid-receptor also prevented and reversed the pro-nociceptive effects of ethanol. Our results suggest a convergence of the effects of mediators of the hypothalamic-pituitary- and sympathoadrenal-stress axes on sensory neurons in the induction and maintenance of alcohol-induced painful peripheral neuropathy.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Changes in rat brainstem responsiveness to somatovisceral inputs following acute bladder irritation

A number of clinical studies indicate the coexistence of multiple chronic pelvic diseases and pain syndromes. An association between various conditions related to the pelvic viscera may relate to a high degree of central visceral convergence, which is a requisite for the cross-organ coordination that is necessary for their normal functions. In the present study, a population of neurons receiving a high degree of somatovisceral convergence (those in the medullary reticular formation--MRF) was targeted in order to examine the effect of infusing a chemical irritant into one organ on the responsiveness of convergent inputs from various visceral and somatic regions of the body, using electrophysiological techniques. Acute irritation of the urinary bladder (UB) with 2% acetic acid significantly decreased the percentage of convergent MRF neuronal responses to UB distention and urethral infusion and significantly increased the percentage responding to whole body, mainly due to stimulation of the face. Irritation also produced a significant increase in the response duration of MRF neurons to distention of colon as well as the bladder (for those few UB responses that still remained). These results indicate that a pelvic/visceral pathology confined to one organ can affect at least some of the convergent responses from other regions of the body. The findings suggest that MRF neurons contribute to the cross-talk between different regions of the body under both normal and pathological conditions.

Changes of pain perception, autonomic function, and endocrine parameters during treatment of anorectic adolescents

OBJECTIVES

The underlying mechanisms of reduced pain perception in anorexia nervosa (AN) are unknown. To gain more insight into the pathology, the authors investigated pain perception, autonomic function, and endocrine parameters before and during successful treatment of adolescent AN patients.

METHOD

Heat pain perception was assessed in 15 female adolescent AN patients and matched controls. Results were correlated with autonomic and endocrine parameters (free triiodothyronine, free cortisol). Autonomic function was studied using heart rate variability and pupillary light reflex assessment. To investigate the influence of therapy on these parameters, data were obtained at three different time points.

RESULTS

Heat pain thresholds were significantly increased in the acute state and decreased after weight had been regained for 6 months. Similarly, an increased parasympathetic tone was present in the acute state only. The relative amplitude of the pupillary light reflex showed a positive correlation to pain thresholds over time and predicted disease progression. In addition, the authors found a negative correlation between increased pain thresholds and low free cortisol.

CONCLUSION

Increased pain thresholds are associated with increased parasympathetic tone and a hypothyroid state in AN. This may either indicate common central mechanisms or suggest a causative interaction.

Neurogenic inflammation and arthritis

Inflammation and inflammatory diseases are sexually dimorphic, but the underlying causes for this observed sexual dimorphism are poorly understood. We discuss neural-immune mechanisms that underlie sexual dimorphism in three critical aspects of the inflammatory process-plasma extravasation, neutrophil function, and inflammatory hyperalgesia. Plasma extravasation and accumulation/activation of leukocytes into tissues are critical components in inflammation and are required for several other aspects of the inflammatory response. Pain (hyperalgesia) also markedly influences the magnitude of other components of the inflammatory response and induces a feedback control of plasma extravasation and neutrophil function. More important, this feedback control itself is powerfully modulated by vagal afferent activity and both the function of the primary afferent nociceptor and the modulation of inflammatory hyperalgesia by vagal afferent activity are highly sexually dimorphic.

Estradiol-associated variation in responses of rostral medullary neurons to somatovisceral stimulation

The lordosis posture and cervix stimulation during copulation are important reproductive events involving complex neural circuitries that are under hormonal influence. An important component of this circuitry, neurons within the medullary reticular formation (MRF), was examined in the present study using electrophysiological techniques. Single unit extracellular recordings were performed in the MRF of 27 urethane-anesthetized female rats. Using bilateral electrical stimulation of the dorsal nerve of the clitoris as the search stimulus, a detailed examination of the somatovisceral convergent responses of 585 individual MRF neurons was made. A total of 7 different groups of cycling and ovariectomized/hormone-supplemented rats were examined and their neuronal response properties to mechanical stimulation of various pelvic organs (cervix pressure, vaginal distension, colon distension) compared. The results indicate the existence of complex response properties as well as several variations in MRF response characteristics that are hormone-dependent. Specifically, estradiol is associated with hyposensitivity to cervix pressure and hypersensitivity to stroking the face. These opposing effects of estradiol in the same subset of neurons likely relate to lordosis behavior which can be either disrupted or elicited, depending on the area being stimulated (upper versus lower parts of the body, respectively).