Top-down attention does not modulate mechanical hypersensitivity consecutive to central sensitization: insights from an experimental analysis.

ABSTRACT: According to the neurocognitive model of attention to pain, when the attentional resources invested in a task unrelated to pain are high, limited cognitive resources can be directed toward the pain. This is supported by experimental studies showing that diverting people's attention away from acute pain leads to experiencing less pain. Theoretical work has suggested that this phenomenon may present a top-down modulatory mechanism for persistent pain as well. However, conclusive empirical evidence is lacking. To fill this gap, we used a preregistered, double-blind, between-subject study design to investigate whether performing a tailored, demanding, and engaging working memory task unrelated to pain (difficult) vs a task that requires less mental effort to be performed (easy), could lead to lower development of secondary hypersensitivity-a hallmark of central sensitization. Eighty-five healthy volunteers, randomly assigned to one of the 2 conditions, performed a visual task with a different cognitive load (difficult vs easy), while secondary hypersensitivity was induced on their nondominant forearm using high-frequency stimulation. To assess the development of secondary hypersensitivity, sensitivity to mechanical stimuli was measured 3 times: T0, for baseline and 20 (T1) and 40 (T2) minutes after the procedure. We did not observe any significant difference in the development of secondary hypersensitivity between the 2 groups, neither in terms of the intensity of mechanical sensitivity nor its spatial extent. Our results suggest that a top-down modulation through attention might not be sufficient to affect pain sensitization and the development of secondary hypersensitivity.

High-speed imaging of evoked rodent mechanical behaviors yields variable results that are not predictive of inflammatory injury.

ABSTRACT: Few analgesics identified using preclinical models have successfully translated to clinical use. These translational limitations may be due to the unidimensional nature of behavioral response measures used to assess rodent nociception. Advances in high-speed videography for pain behavior allow for objective quantification of nuanced aspects of evoked paw withdrawal responses. However, whether videography-based assessments of mechanical hypersensitivity outperform traditional measurement reproducibility is unknown. First, we determined whether high-speed videography of paw withdrawal was reproducible across experimenters. Second, we examined whether this method distinguishes behavioral responses exhibited by naive mice and mice with complete Freund's adjuvant (CFA)-induced inflammation. Twelve experimenters stimulated naive C57BL/6 mice with varying mechanical stimuli. Paw withdrawal responses were recorded with high-speed videography and scored offline by one individual. Our group was unable to replicate the original findings produced by high-speed videography analysis. Surprisingly, ∼80% of variation was not accounted for by variables previously reported to distinguish between responses to innocuous and noxious stimuli (paw height, paw velocity, and pain score), or by additional variables (experimenter, time-of-day, and animal), but rather by unidentified factors. Similar high-speed videography assessments were performed in CFA- and vehicle-treated animals, and the cumulative data failed to reveal an effect of CFA injection on withdrawal as measured by high-speed videography. This study does not support using paw height, velocity, or pain score measurements from high-speed recordings to delineate behavioral responses to innocuous and noxious stimuli. Our group encourages the continued use of traditional mechanical withdrawal assessments until additional high-speed withdrawal measures are validated in established pain models.

Effects of Permethrin or Deltamethrin Exposure in Adult Sprague Dawley Rats on Acoustic and Light Prepulse Inhibition of Acoustic or Tactile Startle.

The effects of permethrin (PRM) and deltamethrin (DLM) on acoustic or light prepulse inhibition of the acoustic startle response (ASR) and tactile startle response (TSR) were studied in adult male Sprague Dawley rats. Preliminary studies were conducted to optimize the parameters of light and acoustic prepulse inhibition of ASR and TSR. Once these parameters were set, a new group of rats was administered PRM (0 or 90 mg/kg) or DLM (0 or 25 mg/kg) by gavage in 5 mL/kg corn oil. ASR and TSR were assessed using acoustic or light prepulses 6, 8, and 12 h after PRM and 2, 4, and 6 h after DLM exposure. PRM increased ASR 6 h post-treatment with no interaction with acoustic prepulse levels and with no effect on TSR. When light was used as the prepulse, PRM increased ASR and TSR at 6 h with no interaction with prepulse levels. DLM decreased ASR and TSR on trials without prepulses but not on trials with acoustic prepulses. DLM also decreased ASR when light prepulses were present 4 h post-treatment. A final experiment assessed whether the house light in the test cabinet affected ASR and TSR after PRM or DLM exposure. Rats had increased ASR and TSR when house lights were on compared with when they were off, but lighting did not differentially interact with PRM or DLM. Light and acoustic prepulses of ASR and TSR have different effects depending on the test agent and the test parameters.

Dependency on sex and stimulus quality of nociceptive behavior in a conscious visceral pain rat model.

Visceral pain may be influenced by many factors. The aim of this study was to analyze the impact of sex and quality of intracolonic mechanical stimulus on the behavioral manifestations of visceral pain in a preclinical model. Male and female young adult Wistar rats were sedated, and a 5 cm long latex balloon was inserted into the colon. Sedation was reverted and behavior was recorded. The pressure of the intracolonic balloon was gradually increased using a sphygmomanometer. Visceral sensitivity was measured as abdominal contractions in response to mechanical intracolonic stimulation. Two different types of stimulation were used: tonic and phasic. Phasic stimulation consisted of repeating several times (3x) the same short stimulus (20 s) within a 5 min interval allowing a 1 min break between individual stimuli. For tonic stimulation the stimulus was maintained throughout the whole 5 min interval. Both phasic and tonic stimulation produced a pressure-dependent increase of abdominal contractions. The abdominal response was more intense under phasic than under tonic stimulation, but with differences depending on the sex of the animals: females exhibited more contractions than males and of similar duration at all pressures, whereas duration of contractions pressure-dependently increased in males. The duration of tonically stimulated contractions was lower and not sex- or pressure-dependent. In the rat, responses to colonic distension depend on the quality of the stimulus, which also produces sex-dependent differences that must be taken into account in the development of models of pathology and visceral pain treatments.

Decoding Cellular Mechanisms for Mechanosensory Discrimination.

Single-cell RNA-sequencing and in vivo functional imaging provide expansive but disconnected views of neuronal diversity. Here, we developed a strategy for linking these modes of classification to explore molecular and cellular mechanisms responsible for detecting and encoding touch. By broadly mapping function to neuronal class, we uncovered a clear transcriptomic logic responsible for the sensitivity and selectivity of mammalian mechanosensory neurons. Notably, cell types with divergent gene-expression profiles often shared very similar properties, but we also discovered transcriptomically related neurons with specialized and divergent functions. Applying our approach to knockout mice revealed that Piezo2 differentially tunes all types of mechanosensory neurons with marked cell-class dependence. Together, our data demonstrate how mechanical stimuli recruit characteristic ensembles of transcriptomically defined neurons, providing rules to help explain the discriminatory power of touch. We anticipate a similar approach could expose fundamental principles governing representation of information throughout the nervous system.

A Functional Topographic Map for Spinal Sensorimotor Reflexes.

Cutaneous somatosensory modalities play pivotal roles in generating a wide range of sensorimotor behaviors, including protective and corrective reflexes that dynamically adapt ongoing movement and posture. How interneurons (INs) in the dorsal horn encode these modalities and transform them into stimulus-appropriate motor behaviors is not known. Here, we use an intersectional genetic approach to functionally assess the contribution that eight classes of dorsal excitatory INs make to sensorimotor reflex responses. We demonstrate that the dorsal horn is organized into spatially restricted excitatory modules composed of molecularly heterogeneous cell types. Laminae I/II INs drive chemical itch-induced scratching, laminae II/III INs generate paw withdrawal movements, and laminae III/IV INs modulate dynamic corrective reflexes. These data reveal a key principle in spinal somatosensory processing, namely, sensorimotor reflexes are driven by the differential spatial recruitment of excitatory neurons.

In Vivo Documentation of Stimulus Velocity Tuning of Mechanically Induced Reflex Cough.

In order to clear airways and lungs defensive reflexes are provoked rather by the dynamic phase of mechanical stimulus. It is speculated that provocation of defensive response depends not only on stimulus duration but also on stimulus velocity. Fourteen adult rabbits were anaesthetized and tracheotomized. Mechanical stimulus was provoked by a mechanical probe introduced through the tracheotomy and rotated by a small electrical motor using a rotational velocity of 40 rpm/s and 20 rpm/s. Threshold, incidence and intensity of cough reflex (CR) were analyzed for each animal. Statistical comparisons between two velocities were performed using Friedman nonparametric test for repeated measurements. Results are median (25-75 %). The threshold of CR was significantly increased (p=0.005) from 350 ms (300-500 ms) to 550 ms (350-1150 ms) and the incidence of cough reflex was significantly reduced (p=0.002) from 50 % (19 50 %) to 0 % (0-25 %) when the rotational velocity of the mechanical probe was reduced by half. The findings of this study are of interest as they show that protective reflex cough, an important mechanism that allows clearing airways even during sleep or anesthesia, is tuned by mechanical stimulus velocity.

Acute chemotherapy-induced peripheral neuropathy due to oxaliplatin administration without cold stimulation.

PURPOSE: The incidence and time of onset of acute chemotherapy-induced peripheral neuropathy (ACIPN) caused by oxaliplatin remain unclarified. Hence, we investigated the prevalence, onset time, and location of ACIPN symptoms in patients with colorectal cancer (CRC) receiving oxaliplatin without cold stimulation. METHODS: The study cohort comprised patients receiving oxaliplatin for CRC at our hospital between April 2017 and August 2018. Patients were instructed not to touch and/or drink cold things and were monitored for ACIPN symptoms in the hospital for 24 h after chemotherapy. ACIPN symptoms that appeared > 24 h after chemotherapy were recorded at the next visit. Symptom appearance time was defined as the duration from the administration of chemotherapy until the appearance of paresthesia classified as grade 1 using the Common Terminology Criteria for Adverse Events. RESULTS: Forty-five patients received chemotherapy, comprising 23 men and 22 women, aged 67 years (29-88 years). The location of ACIPN was the fingers in 55.6% of cases, pharynx in 26.7%, perioral region in 24.4%, and feet in 6.7%. The average duration from oxaliplatin administration to symptom development was 182 min (range 62-443 min) for the fingers, 291 min (176-432 min) for the pharynx, 311 min (127-494 min) for the perioral region, and 297 min (234-355 min) for the feet. Pharyngeal symptoms were more common in patients older than 65 years than in those younger than 65 years. CONCLUSIONS: The incidence and time of the onset of ACIPN caused by oxaliplatin varies between the body and regions.

Aldosterone Synthase in Peripheral Sensory Neurons Contributes to Mechanical Hypersensitivity during Local Inflammation in Rats.

BACKGROUND: Recent emerging evidence suggests that extra-adrenal synthesis of aldosterone occurs (e.g., within the failing heart and in certain brain areas). In this study, the authors investigated evidence for a local endogenous aldosterone production through its key processing enzyme aldosterone synthase within peripheral nociceptive neurons. METHODS: In male Wistar rats (n = 5 to 8 per group) with Freund's complete adjuvant hind paw inflammation, the authors examined aldosterone, aldosterone synthase, and mineralocorticoid receptor expression in peripheral sensory neurons using quantitative reverse transcriptase-polymerase chain reaction, Western blot, immunohistochemistry, and immunoprecipitation. Moreover, the authors explored the nociceptive behavioral changes after selective mineralocorticoid receptor antagonist, canrenoate-K, or specific aldosterone synthase inhibitor application. RESULTS: In rats with Freund's complete adjuvant-induced hind paw inflammation subcutaneous and intrathecal application of mineralocorticoid receptor antagonist, canrenoate-K, rapidly and dose-dependently attenuated nociceptive behavior (94 and 48% reduction in mean paw pressure thresholds, respectively), suggesting a tonic activation of neuronal mineralocorticoid receptors by an endogenous ligand. Indeed, aldosterone immunoreactivity was abundant in peptidergic nociceptive neurons of dorsal root ganglia and colocalized predominantly with its processing enzyme aldosterone synthase and mineralocorticoid receptors. Moreover, aldosterone and its synthesizing enzyme were significantly upregulated in peripheral sensory neurons under inflammatory conditions. The membrane mineralocorticoid receptor consistently coimmunoprecipitated with endogenous aldosterone, confirming a functional link between mineralocorticoid receptors and its endogenous ligand. Importantly, inhibition of endogenous aldosterone production in peripheral sensory neurons by a specific aldosterone synthase inhibitor attenuated nociceptive behavior after hind paw inflammation (a 32% reduction in paw pressure thresholds; inflammation, 47 ± 2 [mean ± SD] vs. inflammation + aldosterone synthase inhibitor, 62 ± 2). CONCLUSIONS: Local production of aldosterone by its processing enzyme aldosterone synthase within peripheral sensory neurons contributes to ongoing mechanical hypersensitivity during local inflammation via intrinsic activation of neuronal mineralocorticoid receptors.

Spinal caspase-6 contributes to remifentanil-induced hyperalgesia via regulating CCL21/CXCR3 pathway in rats.

BACKGROUND: Neuroinflammation in the spinal cord is a pathological event in remifentanil-induced hyperalgesia (RIH), but its underlying molecular mechanisms remain unclear. Recent studies recapitulate the significance of the intracellular protease caspase-6 in the release of inflammatory mediators and synaptic plasticity in pathologic pain. Also, chemokine CCL21 is involved in microglia activation and nociceptive transduction. This study examined whether spinal caspase-6 is associated with RIH via CCL21 and its receptor CXCR3. METHODS: The acute exposure to remifentanil (1 µg kg-1 min-1for 60 min) was used to establish RIH, verified by assessment of mechanical paw withdrawal threshold and thermal paw withdrawal latency. The caspase-6 inhibitor, a neutralizing antibody against CCL21 (anti-CCL21), a selective CXCR3 antagonist NBI-74330, recombinant caspase-6 and CCL21 were used for the investigation of pathogenesis as well as the prevention of hyperalgesia. The expression of caspase-6, CCL21 and CXCR3 was also evaluated by RT-qPCR and Western blot. RESULTS: This study discovered mechanical allodynia and thermal hyperalgesia along with the increase in the expression of spinal caspase-6 and CCL21/CXCR3 after remifentanil exposure. Central caspase-6 inhibition prevented behavioral RIH and spinal up-regulation of CCL21/CXCR3 level. Intrathecal anti-CCL21 injection reduced RIH and spinal expression of CXCR3. The delivery of recombinant caspase-6 facilitated acute nociceptive hypersensitivity and increased spinal CXCR3 release in naïve rats, reversing by co-application of anti-CCL21. Also, NBI-74330 attenuated RIH and exogenous CCL21-caused acute pain behaviors. CONCLUSION: This study highlighted that spinal caspase-6-mediated up-regulation of CCL21/CXCR3 is vital in the pathogenesis of RIH in rats.

Antiallodynic and anti-inflammatory effects of intrathecal R-PIA in a rat model of vincristine-induced peripheral neuropathy.

BACKGROUND: Studies investigating the correlation between spinal adenosine A1 receptors and vincristine-induced peripheral neuropathy (VIPN) are limited. This study explored the role of intrathecal N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) in the rat model of VIPN. METHODS: Vincristine (100 µg/kg) was intraperitoneally administered for 10 days (two 5-day cycles with a 2-day pause) and VIPN was induced in rats. Pain was assessed by evaluating mechanical hyperalgesia, mechanical dynamic allodynia, thermal hyperalgesia, cold allodynia, and mechanical static allodynia. Biochemically, tumor necrosis factor-alpha (TNF-α) level and myeloperoxidase (MPO) activity were measured in the tissue from beneath the sciatic nerve. RESULTS: Vincristine administration resulted in the development of cold allodynia, mechanical hyperalgesia, thermal hyperalgesia, mechanical dynamic allodynia, and mechanical static allodynia. Intrathecally administered R-PIA (1.0 and 3.0 µg/10 µl) reversed vincristine-induced neuropathic pain (cold and mechanical static allodynia). The attenuating effect peaked 15 min after intrathecal administration of R-PIA after which it decreased until 180 min. However, pretreatment with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 µg/10 µl) 15 min before intrathecal R-PIA administration significantly attenuated the antiallodynic effect of R-PIA. This antiallodynic effect of intrathecal R-PIA may be mediated through adenosine A1 receptors in the spinal cord. Intrathecally administered R-PIA also attenuated vincristine-induced increases in TNF-α level and MPO activity. However, pretreatment with intrathecal DPCPX significantly reversed this attenuation. CONCLUSIONS: These results suggest that intrathecally administered R-PIA attenuates cold and mechanical static allodynia in a rat model of VIPN, partially due to its anti-inflammatory actions.

Hypothalamic activation discriminates painful and non-painful initiation of the trigeminal autonomic reflex - an fMRI study.

BACKGROUND: The role of the trigeminal autonomic reflex in headache syndromes, such as cluster headache, is undisputed but sparsely investigated. The aim of the present study was therefore, to identify neural correlates that play a role in the initiation of the trigeminal autonomic reflex. We further aimed to discriminate between components of the reflex that are involved in nociceptive compared to non-nociceptive processing. METHODS: Kinetic Oscillation Stimulation (KOS) in the left nostril was applied in order to provoke autonomic symptoms (e.g. lacrimation) via the trigeminal autonomic reflex in 26 healthy participants using functional magnetic resonance imaging. Unpleasantness and painfulness were assessed on a visual analog scale (VAS), in order to assess the quality of the stimulus (e.g. pain or no pain). RESULTS: During non-painful activation, specific regions involved in the trigeminal autonomic reflex became activated, including several brainstem nuclei but also cerebellar and bilateral insular regions. However, when the input leading to activation of the trigeminal autonomic reflex was perceived as painful, activation of the anterior hypothalamus, the locus coeruleus (LC), the ventral posteriomedial nucleus of the thalamus (VPM), as well as an activation of ipsilateral insular regions, was observed. CONCLUSION: Our results suggest the anterior hypothalamus, besides the thalamus and specific brain stem regions, play a significant role in networks that mediate autonomic output (e.g. lacrimation) following trigeminal input, but only if the trigeminal system is activated by a stimulus comprising a painful component.

Offset analgesia: somatotopic endogenous pain modulation in migraine.

The complex mechanisms underlying migraine are not entirely understood. It has been suggested that descending endogenous pain modulation is an important contributing factor, although research is controversial. A frequently used method to quantify the inhibitory pain modulation system is offset analgesia (OA), defined as a disproportionally large decrease in pain perception in response to a small decrease of painful stimulation. The aim of this study is to evaluate the OA response in patients with migraine and healthy controls, measured at the forehead (trigeminal, V1) and forearm (extratrigeminal). Patients with episodic migraine during the headache-free interval (n = 26) and age- and sex-matched headache-free controls (n = 26) were included in this cross-sectional study. All participants underwent an individualized OA paradigm consisting of 3 stimulus offset trials and 3 constant temperature trials examined at both, a trigeminal and an extratrigeminal test site. Items from the quantitative sensory testing protocol were additionally included. In contrast to the extratrigeminal area, a reduced OA response was shown in the trigeminal area in patients with migraine compared with healthy controls (P < 0.01, mean difference: 13.7, 95% confidence interval: 3.8-23.6). Statistically significant differences between the trigeminal area and the extratrigeminal area were neither observed in healthy controls nor in patients with migraine (P > 0.05). Mechanical detection, mechanical pain threshold, warm detection, and heat pain threshold showed no significant differences between groups or test sites (P > 0.05). In summary, patients with episodic migraine in the headache-free interval exhibited somatotopically specific differences in endogenous pain modulation.

Sensitivities to Thermal and Mechanical Stimuli: Adults With Sickle Cell Disease Compared to Healthy, Pain-Free African American Controls.

Evidence supports, but is inconclusive that sensitization contributes to chronic pain in some adults with sickle cell disease (SCD). We determined the prevalence of pain sensitization among adults with SCD pain compared with pain-free healthy adults. In a cross sectional, single session study of 186 African American outpatients with SCD pain (age 18-74 years, 59% female) and 124 healthy age, gender, and race matched control subjects (age 18-69 years, 49% female), we compared responses to standard thermal (Medoc TSA II) and mechanical stimuli (von Frey filaments). Although we observed no significant differences in thermal thresholds between controls and patients, patients with SCD had lower pain thresholds to mechanical stimuli and reported higher pain intensity scores to all thermal and mechanical stimuli at a non-painful body site. Compared with controls, about twice as many patients with SCD showed sensitization: 12% versus 23% at the anterior forearm site (P = .02), and 16% versus 32% across 3 tested sites (P = .004). Among patients with SCD, 18% exhibited some element of central sensitization. Findings indicate that persistent allodynia and hyperalgesia can be part of the SCD pain experience and should be considered when selecting therapies for SCD pain. PERSPECTIVE: Compared with matched healthy controls, quantitative sensory testing in adults with pain and sickle cell disease (SCD) demonstrates higher prevalence of sensitization, including central sensitization. The findings of allodynia and hyperalgesia may indicate neuropathic pain and could contribute to a paradigm shift in assessment and treatment of SCD pain.

Current understanding of premonitory networks in migraine: A window to attack generation.

AIM: To describe neuronal networks underlying commonly reported migraine premonitory symptoms and to discuss how these might precipitate migraine pain. BACKGROUND: Migraine headache is frequently preceded by a distinct and well characterized premonitory phase including symptoms like yawning, sleep disturbances, alterations in appetite and food intake and hypersensitivity to certain external stimuli. Recent neuroimaging studies strongly suggest the hypothalamus as the key mediator of the premonitory phase and also suggested alterations in hypothalamic networks as a mechanism of migraine attack generation. When looking at the vast evidence from basic research within the last decades, hypothalamic and thalamic networks are most likely to integrate peripheral influences with central mechanisms, facilitating the precipitation of migraine headaches. These networks include sleep, feeding and stress modulating centers within the hypothalamus, thalamic pathways and brainstem centers closely involved in trigeminal pain processing such as the spinal trigeminal nucleus and the rostral ventromedial medulla, all of which are closely interconnected. CONCLUSION: Taken together, these networks represent the pathophysiological basis for migraine premonitory symptoms as well as a possible integration site of peripheral so-called "triggers" with central attack facilitating processes.

Growth Factor Signaling Regulates Mechanical Nociception in Flies and Vertebrates.

Mechanical sensitization is one of the most difficult clinical pain problems to treat. However, the molecular and genetic bases of mechanical nociception are unclear. Here we develop a Drosophila model of mechanical nociception to investigate the ion channels and signaling pathways that regulate mechanical nociception. We fabricated von Frey filaments that span the subthreshold to high noxious range for Drosophila larvae. Using these, we discovered that pressure (force/area), rather than force per se, is the main determinant of aversive rolling responses to noxious mechanical stimuli. We demonstrated that the RTK PDGF/VEGF receptor (Pvr) and its ligands (Pvfs 2 and 3) are required for mechanical nociception and normal dendritic branching. Pvr is expressed and functions in class IV sensory neurons, whereas Pvf2 and Pvf3 are produced by multiple tissues. Constitutive overexpression of Pvr and its ligands or inducible overexpression of Pvr led to mechanical hypersensitivity that could be partially separated from morphological effects. Genetic analyses revealed that the Piezo and Pain ion channels are required for mechanical hypersensitivity observed upon ectopic activation of Pvr signaling. PDGF, but not VEGF, peptides caused mechanical hypersensitivity in rats. Pharmacological inhibition of VEGF receptor Type 2 (VEGFR-2) signaling attenuated mechanical nociception in rats, suggesting a conserved role for PDGF and VEGFR-2 signaling in regulating mechanical nociception. VEGFR-2 inhibition also attenuated morphine analgesic tolerance in rats. Our results reveal that a conserved RTK signaling pathway regulates baseline mechanical nociception in flies and rats.SIGNIFICANCE STATEMENT Hypersensitivity to touch is poorly understood and extremely difficult to treat. Using a refined Drosophila model of mechanical nociception, we discovered a conserved VEGF-related receptor tyrosine kinase signaling pathway that regulates mechanical nociception in flies. Importantly, pharmacological inhibition of VEGF receptor Type 2 signaling in rats causes analgesia and blocks opioid tolerance. We have thus established a robust, genetically tractable system for the rapid identification and functional analysis of conserved genes underlying mechanical pain sensitivity.

Cutaneous nociceptive sensitization affects the directional discrimination - but not the 2-point discrimination.

Background and aims Several pain conditions have been shown to reduce the discriminative abilities of external stimuli. The aim of this study was to investigate how cutaneous sensitization affects the tempo-spatial discrimination for both painful laser stimulation and mechanical stimulation. Methods Fifteen healthy subjects were presented with two different stimulation paradigms, a continuous line stimulation and a 2-point stimulation. Line stimulations were delivered in two different directions in lengths of 25, 50, 75, and 100 mm. Two-point distances from 0 to 100 mm were tested. The subjects reported the perceived intensity, and either direction (line stimulations) or number of perceived points (2-point stimulations). All stimuli were tested both before and after topical capsaicin (8% concentration) sensitization (30 min). Results All mechanical line stimulations were reported correctly before capsaicin and 3 stimulations (out of 240) were reported incorrectly after capsaicin. For the laser line stimulation, the directional discrimination threshold (DDT) was 69.5 mm before capsaicin and 76.3 mm after capsaicin. The 2-point discrimination threshold for laser stimulation was 70.3 mm before capsaicin and 68.0 mm after, for the mechanical stimuli it was 31.5 mm before capsaicin and 31.0 mm after capsaicin. The perceived intensities were increased for the laser line stimulations after capsaicin (linear mixed model (LMM), p < 0.001) and increased with stimulation length (LMM, p < 0.001). For mechanical stimuli, NRS was increased following capsaicin (LMM, p < 0.001). The intensities for both mechanical and laser 2-point stimuli increased after capsaicin and increased with distance between points (LMM, p < 0.01). Conclusions The findings show how cutaneous sensitization appears to affect directional discrimination to a larger extent than the 2-point discrimination. Implications This study is the first to investigate how directional discrimination is altered during sensitization. If such measures can be optimized they may provide a new method to probe the neural mechanisms in pain patients.

Cooling the skin for assessing small-fibre function.

In this clinical and neurophysiological study using a novel cold stimulator, we aim at investigating whether cold-evoked potentials (CEPs) may prove to be a reliable diagnostic tool to assess trigeminal small-fibre function. Using a novel device consisting of micro-Peltier elements, we recorded CEPs after stimulating the supraorbital and perioral regions and the hand dorsum in 15 healthy participants and in 2 patients with exemplary facial neuropathic pain conditions. We measured peripheral conduction velocity at the upper arm and studied the brain generators using source analysis. In healthy participants and patients, we also compared CEPs with laser-evoked potentials. In the healthy participants, cold stimulation evoked reproducible scalp potentials, similar to those elicited by laser pulses, although with a latency of about 30 ms longer. The mean peripheral conduction velocity, estimated at the upper arm, was 12.7 m/seconds. The main waves of the scalp potentials originated from the anterior cingulate gyrus and were preceded by activity in the bilateral opercular regions and bilateral dorsolateral frontal regions. Unlike laser stimulation, cold stimulation evoked scalp potential of similar amplitude across perioral, supraorbital, and hand dorsum stimulation. In patients with facial neuropathic pain, CEP recording showed the selective damage of cold pathways providing complementary information to laser-evoked potential recording. Our clinical and neurophysiological study shows that this new device provides reliable information on trigeminal small fibres mediating cold sensation and might be useful for investigating patients with facial neuropathic pain associated with a distinct damage of cold-mediating fibres.

Haploinsufficiency of the brain-derived neurotrophic factor gene is associated with reduced pain sensitivity.

Rare pain-insensitive individuals offer unique insights into how pain circuits function and have led to the development of new strategies for pain control. We investigated pain sensitivity in humans with WAGR (Wilms tumor, aniridia, genitourinary anomaly, and range of intellectual disabilities) syndrome, who have variably sized heterozygous deletion of the 11p13 region. The deletion region can be inclusive or exclusive of the brain-derived neurotrophic factor (BDNF) gene, a crucial trophic factor for nociceptive afferents. Nociceptive responses assessed by quantitative sensory testing demonstrated reduced pain sensitivity only in the WAGR subjects whose deletion boundaries included the BDNF gene. Corresponding behavioral assessments were made in heterozygous Bdnf knockout rats to examine the specific role of Bdnf. These analogous experiments revealed impairment of Aδ- and C-fiber-mediated heat nociception, determined by acute nociceptive thermal stimuli, and in aversive behaviors evoked when the rats were placed on a hot plate. Similar results were obtained for C-fiber-mediated cold responses and cold avoidance on a cold-plate device. Together, these results suggested a blunted responsiveness to aversive stimuli. Our parallel observations in humans and rats show that hemizygous deletion of the BDNF gene reduces pain sensitivity and establishes BDNF as a determinant of nociceptive sensitivity.

Neural Mechanisms Involved in the Noxious Physical Stress-Induced Inhibition of Ovarian Estradiol Secretion.

Stress is known to change the secretion of ovarian steroid hormones via the hypothalamic-pituitary-ovarian (HPO) axis. Noxious physical stress can cause reflex responses in visceral function via autonomic nerves. This article reviews our recent animal studies on neural mechanisms involved in ovarian estradiol secretion induced by noxious physical stress stimulation. In anesthetized rats, noxious physical stress (pinching the hindpaw or electrical stimulation of the tibial nerve) decreased ovarian estradiol secretion. These noxious stress-induced ovarian hormonal responses were observed after decerebration but were abolished after spinal transection. Electrical stimulation of the ovarian sympathetic nerves (superior ovarian nerves: SON) decreased ovarian estradiol secretion. The reduced secretion of ovarian estradiol induced by hindpaw pinching was abolished by bilateral severance of the SON. Efferent activity of the SON was increased following hindpaw pinching. Thus, the inhibition of ovarian estradiol secretion during noxious physical stress was mainly integrated in the brainstem, and this inhibitory response was due to reflex activation of sympathetic nerves to the ovary. In rats, the sympathetic inhibitory regulation of ovarian estradiol secretion was pronounced when the HPO axis was inhibited by chronic estradiol treatment. Considering the female life cycle, extensive physical stress may inhibit ovarian function, especially before puberty and during old ages when the HPO axis is inactive. Anat Rec, 302:904-911, 2019. © 2019 Wiley Periodicals, Inc.