The potential role of chronic pain and the polytrauma clinical triad in predicting prodromal PD: A cross-sectional study of U.S. Veterans

Introduction

The research criteria for prodromal Parkinson disease (pPD) depends on prospectively validated clinical inputs with large effect sizes and/or high prevalence. Neither traumatic brain injury (TBI), post-traumatic stress disorder (PTSD), nor chronic pain are currently included in the calculator, despite recent evidence of association with pPD. These conditions are widely prevalent, co-occurring, and already known to confer risk of REM behavior disorder (RBD) and PD. Few studies have examined PD risk in the context of TBI and PTSD; none have examined chronic pain. This study aimed to measure the risk of pPD caused by TBI, PTSD, and chronic pain.

Methods

216 US Veterans were enrolled who had self-reported recurrent or persistent pain for at least three months. Of these, 44 met criteria for PTSD, 39 for TBI, and 41 for all three conditions. Several pain, sleep, affective, and trauma questionnaires were administered. Participants' history of RBD was determined via self-report, with a subset undergoing confirmatory video polysomnography.

Results

A greater proportion of Veterans with chronic pain met criteria for RBD (36 % vs. 10 %) and pPD (18.0 % vs. 8.3 %) compared to controls. Proportions were increased in RBD (70 %) and pPD (27 %) when chronic pain co-occurred with TBI and PTSD. Partial effects were seen with just TBI or PTSD alone. When analyzed as continuous variables, polytrauma symptom severity correlated with pPD probability (r = 0.28, P = 0.03).

Conclusion

These data demonstrate the potential utility of chronic pain, TBI, and PTSD in the prediction of pPD, and the importance of trauma-related factors in the pathogenesis of PD.

CFA-treated mice induce hyperalgesia in healthy mice via an olfactory mechanism

BACKGROUND

Social interactions with subjects experiencing pain can increase nociceptive sensitivity in observers, even without direct physical contact. In previous experiments, extended indirect exposure to soiled bedding from mice with alcohol withdrawal-related hyperalgesia enhanced nociception in their conspecifics. This finding suggested that olfactory cues could be sufficient for nociceptive hypersensitivity in otherwise untreated animals (also known as "bystanders").

AIM

The current study addressed this possibility using an inflammation-based hyperalgesia model and long- and short-term exposure paradigms in C57BL/6J mice.

MATERIALS & METHOD

Adult male and female mice received intraplantar injection of complete Freund's adjuvant (CFA) and were used as stimulus animals to otherwise naïve same-sex bystander mice (BS). Another group of untreated mice (OLF) was simultaneously exposed to the bedding of the stimulus mice.

RESULTS

In the long-term, 15-day exposure paradigm, the presence of CFA mice or their bedding resulted in reduced von Frey threshold but not Hargreaves paw withdrawal latency in BS or OLF mice. In the short-term paradigm, 1-hr interaction with CFA conspecifics or 1-hr exposure to their bedding induced mechanical hypersensitivity in BS and OLF mice lasting for 3 hrs. Chemical ablation of the main olfactory epithelium prevented bedding-induced and stimulus mice-induced mechanical hypersensitivity. Gas chromatography-mass spectrometry (GC-MS) analysis of the volatile compounds in the bedding of experimental mice revealed that CFA-treated mice released an increased number of compounds indicative of disease states.

DISCUSSION AND CONCLUSION

These results demonstrate that CFA-induced inflammatory pain can modulate nociception in bystander mice via an olfactory mechanism involving dynamic changes in volatile compounds detectable in the rodent bedding.

SIGNIFICANCE

Social context can influence nociceptive sensitivity. Recent studies suggested involvement of olfaction in this influence. In agreement with this idea, the present study shows that the presence of mice with inflammatory pain produces nociceptive hypersensitivity in nearby conspecifics. This enhanced nociception occurs via olfactory cues present in the mouse bedding. Analysis of the bedding from mice with inflammatory pain identifies a number of compounds indicative of disease states. These findings demonstrate the importance of olfactory system in influencing pain states.

Direct and Indirect Nociceptive Input from the Trigeminal Dorsal Horn to Pain-Modulating Neurons in the Rostral Ventromedial Medulla

The brain is able to amplify or suppress nociceptive signals by means of descending projections to the spinal and trigeminal dorsal horns from the rostral ventromedial medulla (RVM). Two physiologically defined cell classes within RVM, "ON-cells" and "OFF-cells," respectively facilitate and inhibit nociceptive transmission. However, sensory pathways through which nociceptive input drives changes in RVM cell activity are only now being defined. We recently showed that indirect inputs from the dorsal horn via the parabrachial complex (PB) convey nociceptive information to RVM. The purpose of the present study was to determine whether there are also direct dorsal horn inputs to RVM pain-modulating neurons. We focused on the trigeminal dorsal horn, which conveys sensory input from the face and head, and used a combination of single-cell recording with optogenetic activation and inhibition of projections to RVM and PB from the trigeminal interpolaris-caudalis transition zone (Vi/Vc) in male and female rats. We determined that a direct projection from ventral Vi/Vc to RVM carries nociceptive information to RVM pain-modulating neurons. This projection included a GABAergic component, which could contribute to nociceptive inhibition of OFF-cells. This approach also revealed a parallel, indirect, relay of trigeminal information to RVM via PB. Activation of the indirect pathway through PB produced a more sustained response in RVM compared with activation of the direct projection from Vi/Vc. These data demonstrate that a direct trigeminal output conveys nociceptive information to RVM pain-modulating neurons with a parallel indirect pathway through the parabrachial complex.SIGNIFICANCE STATEMENT Rostral ventromedial medulla (RVM) pain-modulating neurons respond to noxious stimulation, which implies that they receive input from pain-transmission circuits. However, the traditional view has been that there is no direct input to RVM pain-modulating neurons from the dorsal horn, and that nociceptive information is carried by indirect pathways. Indeed, we recently showed that noxious information can reach RVM pain-modulating neurons via the parabrachial complex (PB). Using in vivo electrophysiology and optogenetics, the present study identified a direct relay of nociceptive information from the trigeminal dorsal horn to physiologically identified pain-modulating neurons in RVM. Combined tracing and electrophysiology data revealed that the direct projection includes GABAergic neurons. Direct and indirect pathways may play distinct functional roles in recruiting pain-modulating neurons.

Brainstem pain-modulating neurons are sensitized to visual light in persistent inflammation

Many individuals with chronic pain report abnormal sensitivity to visual light, referred to as "photosensitivity" or "photophobia," yet how processing of light and nociceptive information come together remains a puzzle. Pain-modulating neurons in the rostral ventromedial medulla (RVM) have been shown to respond to bright visual light in male rats: activity of pain-enhancing ON-cells is increased, while that of pain-inhibiting OFF-cells is decreased. Since the RVM is the output node of a well-known pain modulation pathway, light-related input to these neurons could contribute to photosensitivity. The purpose of the present study was to fully characterize RVM ON- and OFF-cell responses to visual light by defining stimulus-response curves in male and female rats across a range of intensities (30 to 16,000 lx). We also determined if light-evoked responses are altered in animals subjected to persistent inflammation. We found that ON- and OFF-cells responded to relatively dim light (<1000 lx in naïve animals), with no difference between the sexes in threshold for light-evoked changes in firing or the percentage of responsive cells. Second, light-evoked suppression of OFF-cell firing was enhanced in persistent inflammation, with no change in light-evoked activation of ON-cells. These data indicate that pain-modulating neurons can be engaged by dim light, even under normal conditions. Further, they suggest that decreased descending inhibition during light exposure could contribute to reduced nociceptive thresholds in chronic pain states, resulting in light-induced somatic discomfort and aversion to light. Lastly, our findings argue for differences in how light and somatic stimuli engage RVM, and suggest that light-related input acts as a "top-down" regulatory input to RVM.

Photosensitivity Is Associated with Chronic Pain following Traumatic Brain Injury

Individuals with a history of traumatic brain injury (TBI) report increased rates of chronic pain. Photosensitivity is also a common chronic symptom following TBI and is prevalent among other types of chronic pain. The aim of this study was to better understand the relationship between chronic pain, pain-related disability, and photosensitivity in a TBI population. We quantified participants' visual photosensitivity thresholds (VPT) using an Ocular Photosensitivity Analyzer and measured pressure-pain sensitivity using pressure algometry. Participants also completed a battery of self-report measures related to chronic pain, TBI history, and mental health. A total of 395 participants completed testing, with 233 reporting a history of TBI. The TBI group was divided into 120 symptomatic TBI participants (s-TBI), and 113 asymptomatic TBI participants (a-TBI) based on their Neurobehavioral Symptom Inventory (NSI) scores. Participants in the s-TBI group scored significantly higher on self-reported chronic pain measures compared with a-TBI and no-TBI participants, including the Symptom Impact Questionnaire Revised (SIQR; p < 0.001) and the Michigan Body Map (MBM; p < 0.001). Despite differences in chronic pain complaints, groups displayed similar pressure-pain thresholds (p = 0.270). Additionally, s-TBI participants were more sensitive to light (lower VPT, p < 0.001), and VPT was correlated with SIQR scores across all participants (R = -0.452, p < 0.001). These data demonstrate that photosensitivity is associated with self-reported chronic pain and disability in individuals with chronic TBI symptomatology. Photosensitivity could therefore serve as a simple, more highly quantitative marker of high-impact chronic pain after TBI.

Shifting the Balance: How Top-Down and Bottom-Up Input Modulate Pain via the Rostral Ventromedial Medulla

The sensory experience of pain depends not only on the transmission of noxious information (nociception), but on the state of the body in a biological, psychological, and social milieu. A brainstem pain-modulating system with its output node in the rostral ventromedial medulla (RVM) can regulate the threshold and gain for nociceptive transmission. This review considers the current understanding of how RVM pain-modulating neurons, namely ON-cells and OFF-cells, are engaged by “top-down” cognitive and emotional factors, as well as by “bottom-up” sensory inputs, to enhance or suppress pain.

Physiological properties of pain-modulating neurons in rostral ventromedial medulla in female rats, and responses to opioid administration

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Physiological properties of RVM pain-modulating neurons were described in female rats.

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ON- and OFF-cells in females have fundamental properties comparable to those in males.

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As in males, RVM neuron output is altered in persistent inflammation and by morphine.

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This work provides a foundation for future studies of RVM in females.

Functional pain disorders disproportionately impact females, but most pain research in animals has been conducted in males. While there are anatomical and pharmacological sexual dimorphisms in brainstem pain-modulation circuits, the physiology of pain-modulating neurons that comprise a major functional output, the rostral ventromedial medulla (RVM), has not been explored in female animals. The goal of this study was to identify and characterize the activity of RVM cells in female, compared to male, rats. ON- and OFF-cells were identified within the RVM in females, with firing properties comparable to those described in males. In addition, both ON- and OFF-cells exhibited a sensitized response to somatic stimuli in females subjected to persistent inflammation, and both ON- and OFF-cells responded to systemically administered morphine at a dose sufficient to produce behavioral antinociception. These data demonstrate that the ON-/OFF-cell framework originally defined in males is also present in females, and that as in males, these neurons are recruited in females in persistent inflammation and by systemically administered morphine. Importantly, this work establishes a foundation for the use of female animals in studies of RVM and descending control.

Different methods for TBI diagnosis influence presence and symptoms of post-concussive syndrome in US Veterans

Common methods for evaluating history of traumatic brain injury (TBI) include self-report, electronic medical record review (EMR), and structured interviews such as the Head Trauma Events Characteristics (HTEC). Each has strengths and weaknesses, but little is known regarding how TBI diagnostic rates or the associated symptom profile differ among them. This study examined 200 Veterans recruited within the VA Portland Health Care System, each evaluated for TBI using self-report, EMR, and HTEC. Participants also completed validated questionnaires assessing chronic symptom severity in broad health-related domains (pain, sleep, quality of life, post-concussive symptoms). The HTEC was more sensitive (80% of participants in our cohort) than either self-report or EMR alone (40%). As expected from the high sensitivity, the HTEC+ group included many people with mild or no post-concussive symptoms. Participants were then grouped according to the degree of concordance across these three diagnostic methods: No-TBI, n=43; or TBI-positive in any one method (TBI-1dx, n=53), any two (TBI-2dx, n=45), or all three (TBI-3dx, n=59). The symptom profile of the TBI-1dx group was indistinguishable from the No TBI group. The TBI-3dx group carried the most severe symptom profile. These data show that understanding the method(s) used to ascertain TBI is essential when interpreting results from other studies, an issue that will be even more salient when interpreting data merged from multiple sources within centralized repositories (e.g., FITBIR). The development of a composite TBI assessment tool including self-report, medical record review, and neuropsychology outcomes is a crucial next step for the field.

From Pleasure to Pain, and Back Again: The Intricate Relationship Between Alcohol and Nociception

AIMS

A close and bidirectional relationship between alcohol consumption and pain has been previously reported and discussed in influential reviews. The goal of the present narrative review is to provide an update on the developments in this field in order to guide future research objectives.

METHODS

We evaluated both epidemiological and neurobiological literature interrogating the relationship between alcohol use and pain for the presence of significant effects. We outlined studies on interactions between alcohol use and pain using both self-reports and objective experimental measures and discussed potential underlying mechanisms of these interactions.

RESULTS

Epidemiological, preclinical and clinical literature point to three major interactions between alcohol use and pain: (a) alcohol use leading to hyperalgesia, (b) alcohol use moderating pain and hyperalgesia and (c) chronic pain as a risk factor predisposing to alcohol relapse. Neurobiological studies using animal models to assess these interactions have transitioned from mostly involuntary modes of experimenter-controlled alcohol administration to self-administration procedures, and increasingly indicate that neuronal circuits implicated in both withdrawal and anticipation stages of alcohol use disorder also have a role in chronic pain. Mechanistically, alterations in GABA, glutamate, the corticotropin-releasing factor system, endogenous opioids and protein kinase C appear to play crucial roles in this maladaptive overlap.

CONCLUSIONS

Many of the principles explaining the interactions between alcohol and pain remain on a strong foundation, but continuing progress in modeling these interactions and underlying systems will provide a clearer basis for understanding, and ultimately treating, the damaging aspects of this interaction.

Parabrachial Complex: A Hub for Pain and Aversion

The parabrachial nucleus (PBN) has long been recognized as a sensory relay receiving an array of interoceptive and exteroceptive inputs relevant to taste and ingestive behavior, pain, and multiple aspects of autonomic control, including respiration, blood pressure, water balance, and thermoregulation. Outputs are known to be similarly widespread and complex. How sensory information is handled in PBN and used to inform different outputs to maintain homeostasis and promote survival is only now being elucidated. With a focus on taste and ingestive behaviors, pain, and thermoregulation, this review is intended to provide a context for analysis of PBN circuits involved in aversion and avoidance, and consider how information of various modalities, interoceptive and exteroceptive, is processed within PBN and transmitted to distinct targets to signal challenge, and to engage appropriate behavioral and physiological responses to maintain homeostasis.

Plasticity in the Link between Pain-Transmitting and Pain-Modulating Systems in Acute and Persistent Inflammation

There is strong evidence that spinoparabrachial neurons in the superficial dorsal horn contribute to persistent pain states, and that the lateral parabrachial complex (PB) conveys relevant nociceptive information to higher structures. The role of PB itself in hyperalgesia and how it recruits descending facilitation has nevertheless received significantly less attention. The current study is a first step toward delineating the functional dynamics of PB and its link to descending control in acute and persistent inflammatory pain. In lightly anesthetized rats, we recorded behavioral withdrawal evoked by mechanical stimulation of the hindpaw and, simultaneously, the activity of identified pain-modulating neurons, "ON-cells" and "OFF-cells," in the rostral ventromedial medulla (RVM). This was done before and after the inactivation of PB, contralateral or ipsilateral to an inflamed paw [1 h, 1 d, or 5-6 d after intraplantar injection of Complete Freund's Adjuvant (CFA)]. The inactivation of contralateral, but not ipsilateral, PB interfered with nociceptive input to RVM under basal conditions, as well as in acute inflammation. By contrast, blocking ipsilateral, but not contralateral, PB in established inflammation interfered with behavioral hyperalgesia and ON-cell and OFF-cell responses. The lesioning of contralateral PB before CFA injection prevented this recruitment of ipsilateral PB in persistent inflammation. These experiments show that contralateral PB is required to initiate hyperalgesia, which is then maintained by ipsilateral PB, most likely in both cases via the engagement of pain-modulating neurons of the RVM.SIGNIFICANCE STATEMENT The lateral parabrachial complex (PB) relays nociceptive information to brain circuits that are important for the transmission and modulation of pain, but its specific role in persistent pain and engagement of descending control mechanisms has received relatively little attention. We show here that PB contralateral and ipsilateral to an inflammatory insult demonstrate different functions as inflammation persists, likely by engaging pain-facilitating neurons of the rostral ventromedial medulla. While the contralateral PB, the target of the major spinoparabrachial pathway, relays acute nociceptive information, the ipsilateral PB is recruited or unmasked in persistent inflammation to maintain hyperalgesia. These data point to plasticity in the PB itself or its direct and indirect connections with pain-modulating systems as central to the development and maintenance of persistent pain.

Increased Sleep Disturbances and Pain in Veterans With Comorbid Traumatic Brain Injury and Posttraumatic Stress Disorder

STUDY OBJECTIVES

Veterans are at an increased risk for traumatic brain injury (TBI) and posttraumatic stress disorder (PTSD), both of which are associated with sleep disturbances and increased pain. Furthermore, sleep disturbances and pain are reciprocally related such that each can exacerbate the other. Although both TBI and PTSD are independently linked to sleep disturbances and pain, it remains unclear whether Veterans with comorbid TBI+PTSD show worse sleep disturbances and pain compared to those with only TBI or PTSD. We hypothesized that sleep and pain would be worse in Veterans with comorbid TBI+PTSD compared to Veterans with only TBI or PTSD.

METHODS

Veterans (n = 639) from the VA Portland Health Care System completed overnight polysomnography and self-report questionnaires. Primary outcome variables were self-reported sleep disturbances and current pain intensity. Participants were categorized into four trauma-exposure groups: (1) neither: without TBI or PTSD (n = 383); (2) TBI: only TBI (n = 67); (3) PTSD: only PTSD (n = 126); and (4) TBI+PTSD: TBI and PTSD (n = 63).

RESULTS

The PTSD and TBI+PTSD groups reported worse sleep compared to the TBI and neither groups. The TBI+PTSD group reported the greatest pain intensity compared to the other groups.

CONCLUSIONS

These data suggest sleep and pain are worst in Veterans with TBI and PTSD, and that sleep is similarly impaired in Veterans with PTSD despite not having as much pain. Thus, although this is a complex relationship, these data suggest PTSD may be driving sleep disturbances, and the added effect of TBI in the comorbid group may be driving pain in this population.

Social transfer of pain in mice

A complex relationship exists between the psychosocial environment and the perception and experience of pain, and the mechanisms of the social communication of pain have yet to be elucidated. The present study examined the social communication of pain and demonstrates that "bystander" mice housed and tested in the same room as mice subjected to inflammatory pain or withdrawal from morphine or alcohol develop corresponding hyperalgesia. Olfactory cues mediate the transfer of hyperalgesia to the bystander mice, which can be measured using mechanical, thermal, and chemical tests. Hyperalgesia in bystanders does not co-occur with anxiety or changes in corticosterone and cannot be explained by visually dependent emotional contagion or stress-induced hyperalgesia. These experiments reveal the multifaceted relationship between the social environment and pain behavior and support the use of mice as a model system for investigating these factors. In addition, these experiments highlight the need for proper consideration of how experimental animals are housed and tested.

Distinct pathways for norepinephrine- and opioid-triggered antinociception from the amygdala

BACKGROUND

The amygdala has an important role in pain and pain modulation. We showed previously in animal studies that α2 -adrenoreceptor activation in the central nucleus of the amygdala (CeA) mediates hypoalgesia produced by restraint stress, and that direct application of an α2 -agonist in this region produces analgesia.

AIMS

In the present animal experiments, we investigated the pathways through which α2 -sensitive systems in the CeA produce behavioural analgesia. The CeA has dense connections to a descending pain modulatory network, centred in the midbrain periaqueductal grey (PAG) and the rostral ventromedial medulla (RVM), which is implicated in various forms of stress-related hypoalgesia and which mediates the antinociceptive effect of morphine applied in the basolateral amygdala. We investigated whether this circuit mediates the hypoalgesic effects of α2 -adrenergic agonist administration into the CeA as well as the contribution of endogenous opioids and cannabinoids. We also tested the possibility that activation of α2 -receptors in the CeA produces antinociception by recruitment of noradrenergic pathways projecting to the spinal cord.

RESULTS

Hypoalgesia resulting from bilateral application of the α2 -adrenergic agonist clonidine in the CeA was not reversed by chemical inactivation of the RVM or by systemic injections of naloxone (μ-opioid antagonist) or rimonabant (CB1 antagonist). By contrast, spinal α2 -receptor blockade (intrathecal idazoxan) completely prevented the hypoalgesic effect of clonidine in the CeA, and unmasked a small but significant hyperalgesia.

CONCLUSION

In rats, adrenergic actions in the CeA mediating hypoalgesia require spinal adrenergic neurotransmission but not the PAG-RVM pain modulatory network, or opiate or cannabinoid systems.

Pain Modulation and the Transition from Acute to Chronic Pain

There is now increasing evidence that pathological pain states are at least in part driven by changes in the brain itself. Descending modulatory pathways are known to mediate top-down regulation of nociceptive processing, transmitting cortical and limbic influences to the dorsal horn. However, these modulatory pathways are also intimately intertwined with ascending transmission pathways through positive and negative feedback loops. Models of persistent pain that fail to include descending modulatory pathways are thus incomplete. Although teasing out individual links in a recurrent network is never straightforward, it is imperative that understanding of pain modulation be fully integrated into how we think about pain.

Neuropeptide Y in the rostral ventromedial medulla reverses inflammatory and nerve injury hyperalgesia in rats via non-selective excitation of local neurons

Chronic pain reflects not only sensitization of the ascending nociceptive pathways, but also changes in descending modulation. The rostral ventromedial medulla (RVM) is a key structure in a well-studied descending pathway, and contains two classes of modulatory neurons, the ON-cells and the OFF-cells. Disinhibition of OFF-cells depresses nociception; increased ON-cell activity facilitates nociception. Multiple lines of evidence show that sensitization of ON-cells contributes to chronic pain, and reversing or blocking this sensitization is of interest as a treatment of persistent pain. Neuropeptide Y (NPY) acting via the Y1 receptor has been shown to attenuate hypersensitivity in nerve-injured animals without affecting normal nociception when microinjected into the RVM, but the neural basis for this effect was unknown. We hypothesized that behavioral anti-hyperalgesia was due to selective inhibition of ON-cells by NPY at the Y1 receptor. To explore the possibility of Y1 selectivity on ON-cells, we stained for the NPY-Y1 receptor in the RVM, and found it broadly expressed on both serotonergic and non-serotonergic neurons. In subsequent behavioral experiments, NPY microinjected into the RVM in lightly anesthetized animals reversed signs of mechanical hyperalgesia following either nerve injury or chronic hindpaw inflammation. Unexpectedly, rather than decreasing ON-cell activity, NPY increased spontaneous activity of both ON- and OFF-cells without altering noxious-evoked changes in firing. Based on these results, we conclude that the anti-hyperalgesic effects of NPY in the RVM are not explained by selective inhibition of ON-cells, but rather by increased spontaneous activity of OFF-cells. Although ON-cells undoubtedly facilitate nociception and contribute to hypersensitivity, the present results highlight the importance of parallel OFF-cell-mediated descending inhibition in limiting the expression of chronic pain.

Adaptations in responsiveness of brainstem pain-modulating neurons in acute compared with chronic inflammation

Despite similar behavioral hypersensitivity, acute and chronic pain have distinct neural bases. We used intraplantar injection of complete Freund's adjuvant to directly compare activity of pain-modulating neurons in the rostral ventromedial medulla (RVM) in acute vs chronic inflammation. Heat-evoked and von Frey-evoked withdrawal reflexes and corresponding RVM neuronal activity were recorded in lightly anesthetized animals either during the first hour after complete Freund's adjuvant injection (acute) or 3 to 10 days later (chronic). Thermal and modest mechanical hyperalgesia during acute inflammation were associated with increases in the spontaneous activity of pain-facilitating ON-cells and suppression of pain-inhibiting OFF-cells. Acute hyperalgesia was reversed by RVM block, showing that the increased activity of RVM ON-cells is necessary for acute behavioral hypersensitivity. In chronic inflammation, thermal hyperalgesia had resolved but mechanical hyperalgesia had become pronounced. The spontaneous discharges of ON- and OFF-cells were not different from those in control subjects, but the mechanical response thresholds for both cell classes were reduced into the innocuous range. RVM block in the chronic condition worsened mechanical hyperalgesia. These studies identify distinct contributions of RVM ON- and OFF-cells to acute and chronic inflammatory hyperalgesia. During early immune-mediated inflammation, ON-cell spontaneous activity promotes hyperalgesia. After inflammation is established, the antinociceptive influence of OFF-cells is dominant, yet the lowered threshold for the OFF-cell pause allows behavioral responses to stimuli that would normally be considered innocuous. The efficacy of OFF-cells in counteracting sensitization of ascending transmission pathways could therefore be an important determining factor in development of chronic inflammatory pain.

The dorsomedial hypothalamus mediates stress-induced hyperalgesia and is the source of the pronociceptive peptide cholecystokinin in the rostral ventromedial medulla

While intense or highly arousing stressors have long been known to suppress pain, relatively mild or chronic stress can enhance pain. The mechanisms underlying stress-induced hyperalgesia (SIH) are only now being defined. The physiological and neuroendocrine effects of mild stress are mediated by the dorsomedial hypothalamus (DMH), which has documented connections with the rostral ventromedial medulla (RVM), a brainstem region capable of facilitating nociception. We hypothesized that stress engages both the DMH and the RVM to produce hyperalgesia. Direct pharmacological activation of the DMH increased sensitivity to mechanical stimulation in awake animals, confirming that the DMH can mediate behavioral hyperalgesia. A behavioral model of mild stress also produced mechanical hyperalgesia, which was blocked by inactivation of either the DMH or the RVM. The neuropeptide cholecystokinin (CCK) acts in the RVM to enhance nociception and is abundant in the DMH. Using a retrograde tracer and immunohistochemical labeling, we determined that CCK-expressing neurons in the DMH are the only significant supraspinal source of CCK in the RVM. However, not all neurons projecting from the DMH to the RVM contained CCK, and microinjection of the CCK2 receptor antagonist YM022 in the RVM did not interfere with SIH, suggesting that transmitters in addition to CCK play a significant role in this connection during acute stress. While the RVM has a well-established role in facilitation of nociception, the DMH, with its well-documented role in stress, may also be engaged in a number of chronic or abnormal pain states. Taken as a whole, these findings establish an anatomical and functional connection between the DMH and RVM by which stress can facilitate pain.

Deep brain stimulation entrains local neuronal firing in human globus pallidus internus

Deep brain stimulation (DBS) in the internal segment of the globus pallidus (GPi) relieves the motor symptoms of Parkinson's disease, yet the mechanism of action remains uncertain. To address the question of how therapeutic stimulation changes neuronal firing in the human brain, we studied the effects of GPi stimulation on local neurons in unanesthetized patients. Eleven patients with idiopathic Parkinson's disease consented to participate in neuronal recordings during stimulator implantation surgery. A recording microelectrode and a DBS macroelectrode were advanced through the GPi in parallel until a single neuron was isolated. After a baseline period, stimulation was initiated with varying voltages and different stimulation sites. The intra-operative stimulation parameters (1-8 V, 88-180 Hz, 0.1-ms pulses) were comparable with the postoperative DBS settings. Stimulation in the GPi did not silence local neuronal activity uniformly, but instead loosely entrained firing and decreased net activity in a voltage-dependent fashion. Most neurons had decreased activity during stimulation, although some increased or did not change firing rate. Thirty-three of 45 neurons displayed complex patterns of entrainment during stimulation, and burst-firing was decreased consistently after stimulation. Recorded spike trains from patients were used as input into a model of a thalamocortical relay neuron. Only spike trains that occurred during therapeutically relevant voltages significantly reduced transmission error, an effect attributable to changes in firing patterns. These data indicate that DBS in the human GPi does not silence neuronal activity, but instead disrupts the pathological firing patterns through loose entrainment of neuronal activity.

Pain-facilitating medullary neurons contribute to opioid-induced respiratory depression

Respiratory depression is a therapy-limiting side effect of opioid analgesics, yet our understanding of the brain circuits mediating this potentially lethal outcome remains incomplete. Here we studied the contribution of the rostral ventromedial medulla (RVM), a region long implicated in pain modulation and homeostatic regulation, to opioid-induced respiratory depression. Microinjection of the μ-opioid agonist DAMGO in the RVM of lightly anesthetized rats produced both analgesia and respiratory depression, showing that neurons in this region can modulate breathing. Blocking opioid action in the RVM by microinjecting the opioid antagonist naltrexone reversed the analgesic and respiratory effects of systemically administered morphine, showing that this region plays a role in both the analgesic and respiratory-depressant properties of systemically administered morphine. The distribution of neurons directly inhibited by RVM opioid microinjection was determined with a fluorescent opioid peptide, dermorphin-Alexa 594, and found to be concentrated in and around the RVM. The non-opioid analgesic improgan, like DAMGO, produced antinociception but, unlike DAMGO, stimulated breathing when microinjected into the RVM. Concurrent recording of RVM neurons during improgan microinjection showed that this agent activated RVM ON-cells, OFF-cells, and NEUTRAL-cells. Since opioids are known to activate OFF-cells but suppress ON-cell firing, the differential respiratory response to these two analgesic drugs is best explained by their opposing effects on the activity of RVM ON-cells. These findings show that pain relief can be separated pharmacologically from respiratory depression and identify RVM OFF-cells as important central targets for continued development of potent analgesics with fewer side effects.

A novel, non-invasive method of respiratory monitoring for use with stereotactic procedures

Accurate monitoring of respiration is often needed for neurophysiological studies, as either a dependent experimental variable or an indicator of physiological state. Current options for respiratory monitoring of animals held in a stereotaxic frame include EMG recordings, pneumotachograph measurements, inductance-plethysmography, whole-body plethysmography (WBP), and visual monitoring. While powerful, many of these methods prevent access to the animal's body, interfere with experimental manipulations, or require deep anesthesia and additional surgery. For experiments where these issues may be problematic, we developed a non-invasive method of recording respiratory parameters specifically for use with animals held in a stereotaxic frame. This system, ventilation pressure transduction (VPT), measures variations in pressure at the animal's nostril from inward and outward airflow during breathing. These pressure changes are detected by a sensitive pressure transducer, then filtered and amplified. The output is an analog signal representing each breath. VPT was validated against WBP using 10% carbon dioxide and systemic morphine (4mg/kg) challenges in lightly anesthetized animals. VPT accurately represented breathing rate and tidal volume changes under both baseline and challenge conditions. This novel technique can therefore be used to measure respiratory rate and relative tidal volume when stereotaxic procedures are needed for neuronal manipulations and recording.

Abstract 109: Soluble Epoxide Hydrolase Gene Deletion is Protective Against Delayed Microvascular Vasospasm in a Mouse Model of Subarachnoid Hemorrhage

Accumulating evidence supports a role for microvascular dysfunction in the development of delayed cerebral ischemia following subarachnoid hemorrhage (SAH). The mechanism underlying delayed microvascular vasospasm (dMVS) is unknown. Using 3-dimensional, quantitative optical microangiography (OMAG), we assessed early and delayed changes in perfusion and dimensions of cerebrocortical microvessels in a mouse model of SAH. We tested the hypothesis that the development of dMVS after SAH is linked to changes in expression of soluble epoxide hydrolase (sEH), a key enzyme in the metabolism of a group of vasodilator eicosanoids called epoxyeicosatrienoic acids (EETs). Wild-type (WT, n=6) mice and mice lacking the sEH gene (sEHKO, n=4) were subjected to endovascular perforation to induce SAH, and followed for 72 hours to assess dMVS using OMAG. At 24 hours after SAH, no apparent change in cortical perfusion was observed in either WT or sEHKO mice compared to baseline (p>0.05), although both strains experienced constriction of individual cortical microvessels (-7.4 +/- 1.1% and -6.2 +/- 1.2% change from baseline (p<0.05) in WT and sEHKO mice respectively). At 72 hours after SAH, WT mice developed further microvascular vasoconstriction (-9.6 +/- 1% change from baseline (p<0.05 compared to baseline and 24h) ) and sustained a significant decrease (14.5 +/- 4.5% ) in cortical perfusion compared to baseline perfusion (p<0.05), whereas sEHKO mice did not show further changes in microvascular diameters, and were protected from the delayed drop in cortical perfusion. We conclude that genetic deletion of sEH can protect against dMVS in a mouse model of SAH. Our results suggest that EETs pathway may be involved in the pathogenesis of dMVS following SAH.

Physiological basis for inhibition of morphine and improgan antinociception by CC12, a P450 epoxygenase inhibitor

Many analgesic drugs, including μ-opioids, cannabinoids, and the novel nonopioid analgesic improgan, produce antinociception by actions in the rostral ventromedial medulla (RVM). There they activate pain-inhibiting neurons, termed "OFF-cells," defined by a nociceptive reflex-related pause in activity. Based on recent functional evidence that neuronal P450 epoxygenases are important for the central antinociceptive actions of morphine and improgan, we explored the convergence of opioid and nonopioid analgesic drug actions in RVM by studying the effects of the P450 epoxygenase inhibitor CC12 on the analgesic drug-induced activation of these OFF-cells and on behavioral antinociception. In rats lightly anesthetized with isoflurane, we recorded the effects of intraventricular morphine and improgan, with and without CC12 pretreatment, on tail flick latency and activity of identified RVM neurons: OFF-cells, ON-cells (pronociceptive neurons), and neutral cells (unresponsive to analgesic drugs). CC12 pretreatment preserved reflex-related changes in OFF-cell firing and blocked the analgesic actions of both drugs, without interfering with the increase in spontaneous firing induced by improgan or morphine. CC12 blocked suppression of evoked ON-cell firing by improgan, but not morphine. CC12 pretreatment had no effect by itself on RVM neurons or behavior. These data show that the epoxygenase inhibitor CC12 works downstream from receptors for both μ-opioid and improgan, at the inhibitory input mediating the OFF-cell pause. This circuit-level analysis thus provides a cellular basis for the convergence of opioid and nonopioid analgesic actions in the RVM. A presynaptic P450 epoxygenase may therefore be an important target for development of clinically useful nonopioid analgesic drugs.

Neural basis for improgan antinociception

Improgan, the prototype compound of a novel class of non-opioid analgesic drugs derived from histamine antagonists, attenuates thermal and mechanical nociception in rodents following intracerebroventricular (i.c.v.) administration. Improgan does not bind to known opioid, histamine or cannabinoid receptors, and its molecular target has not been identified. It is known however, that improgan acts directly in the periaqueductal gray and the rostral ventromedial medulla to produce its antinociceptive effects, and that inactivation of the rostral ventromedial medulla prevents the antinociceptive effect of improgan given i.c.v. Here we used in vivo single-cell recording in lightly anesthetized rats to show that improgan engages pain-modulating neurons in the medulla to produce antinociception. Following improgan administration, OFF-cells, which inhibit nociception, became continuously active and no longer paused during noxious stimulation. The increase in OFF-cell firing does not represent a non-specific neuroexcitant effect of this drug, since ON-cell discharge, associated with net nociceptive facilitation, was depressed. NEUTRAL-cell firing was unaffected by improgan. The net response of rostral ventromedial medulla (RVM) neurons to improgan is thus comparable to that evoked by mu-opioids and cannabinoids, well known RVM-active analgesic drugs. This common basis for improgan, opioid, and cannabinoid antinociception in the RVM supports the idea that improgan functions as a specific analgesic agent.

Deep brain stimulation does not silence neurons in subthalamic nucleus in Parkinson's patients

Two broad hypotheses have been advanced to explain the clinical efficacy of deep brain stimulation (DBS) in the subthalamic nucleus (STN) for treatment of Parkinson's disease. One is that stimulation inactivates STN neurons, producing a functional lesion. The other is that electrical stimulation activates the STN output, thus "jamming" pathological activity in basal ganglia-corticothalamic circuits. Evidence consistent with both concepts has been adduced from modeling and animal studies, as well as from recordings in patients. However, the stimulation parameters used in many recording studies have not been well matched to those used clinically. In this study, we recorded STN activity in patients with Parkinson's disease during stimulation delivered through a clinical DBS electrode using standard therapeutic stimulus parameters. A microelectrode was used to record the firing of a single STN neuron during DBS (3-5 V, 80-200 Hz, 90- to 200-micros pulses; 33 neurons/11 patients). Firing rate was unchanged during the stimulus trains, and the recorded neurons did not show prolonged (s) changes in firing rate on termination of the stimulation. However, a brief (approximately 1 ms), short-latency (6 ms) postpulse inhibition was seen in 10 of 14 neurons analyzed. A subset of neurons displayed altered firing patterns, with a predominant shift toward random firing. These data do not support the idea that DBS inactivates the STN and are instead more consistent with the hypothesis that this stimulation provides a null signal to basal ganglia-corticothalamic circuitry that has been altered as part of Parkinson's disease.

Brainstem control of cerebral blood flow and application to acute vasospasm following experimental subarachnoid hemorrhage

Symptomatic ischemia following aneurysmal subarachnoid hemorrhage (SAH) is common but poorly understood and inadequately treated. Severe constriction of the major arteries at the base of the brain, termed vasospasm, traditionally has been thought to be a proximal event underlying these ischemias, although microvascular changes also have been described. The vast majority of studies aimed at understanding the pathogenesis of ischemic deficits, and vasospasm have focused on the interaction of the "spasmogen" of the extravasated blood with the smooth muscle and endothelium of the arteries. This has led to a comparative neglect of the contribution of the CNS to the maintenance of cerebral perfusion. In the present study, we focused on the role of the rostral ventromedial medulla (RVM) in modulating cerebral perfusion at rest and following an experimental SAH in the rat. Changes in cerebral blood flow (CBF) were measured using laser-Doppler flowmetry and three-dimensional optical microangiography. Focal application of a GABA(A) receptor agonist and antagonist was used to respectively inactivate and activate the RVM. We show here that the RVM modulates cerebral blood flow under resting conditions, and further, contributes to restoration of cerebral perfusion following a high-grade SAH. Failure of this brainstem compensatory mechanism could be significant for acute perfusion deficits seen in patients following subarachnoid hemorrhage.

Medullary pain facilitating neurons mediate allodynia in headache-related pain

OBJECTIVE

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

METHODS

Inflammatory mediators (IM) were applied to the dura of unanesthetized rats via previously implanted cannulas, and sensory thresholds of the face and hind-paws were characterized.

RESULTS

IM elicited robust facial and hind-paw allodynia, which peaked within 3 hours. These effects were reminiscent of cutaneous allodynia seen in patients with migraine or other primary headache conditions, and were reversed by agents used clinically in the treatment of migraine, including sumatriptan, naproxen, and a calcitonin gene-related peptide antagonist. Consistent with clinical observations, the allodynia was unaffected by a neurokinin-1 antagonist. Having established facial and hind-paw allodynia as a useful animal surrogate of headache-associated allodynia, we next showed that blocking pain-facilitating processes in the rostral ventromedial medulla (RVM) interfered with its expression. Bupivacaine, destruction of putative pain-facilitating neurons, or block of cholecystokinin receptors prevented or significantly attenuated IM-induced allodynia. Electrophysiological studies confirmed activation of pain-facilitating RVM "on" cells and transient suppression of RVM "off" cells after IM.

INTERPRETATION

Facial and hind-paw allodynia associated with dural stimulation is a useful surrogate of pain associated with primary headache including migraine and may be exploited mechanistically for development of novel therapeutic strategies for headache pain. The data also demonstrate the requirement for activation of descending facilitation from the RVM for the expression of cranial and extracranial cutaneous allodynia, and are consistent with a brainstem generator of allodynia associated with headache disorders.

Descending control of nociception: Specificity, recruitment and plasticity

The dorsal horn of the spinal cord is the location of the first synapse in pain pathways, and as such, offers a very powerful target for regulation of nociceptive transmission by both local segmental and supraspinal mechanisms. Descending control of spinal nociception originates from many brain regions and plays a critical role in determining the experience of both acute and chronic pain. The earlier concept of descending control as an "analgesia system" is now being replaced with a more nuanced model in which pain input is prioritized relative to other competing behavioral needs and homeostatic demands. Descending control arises from a number of supraspinal sites, including the midline periaqueductal gray-rostral ventromedial medulla (PAG-RVM) system, and the more lateral and caudal dorsal reticular nucleus (DRt) and ventrolateral medulla (VLM). Inhibitory control from the PAG-RVM system preferentially suppresses nociceptive inputs mediated by C-fibers, preserving sensory-discriminative information conveyed by more rapidly conducting A-fibers. Analysis of the circuitry within the RVM reveals that the neural basis for bidirectional control from the midline system is two populations of neurons, ON-cells and OFF-cells, that are differentially recruited by higher structures important in fear, illness and psychological stress to enhance or inhibit pain. Dynamic shifts in the balance between pain inhibiting and facilitating outflows from the brainstem play a role in setting the gain of nociceptive processing as dictated by behavioral priorities, but are also likely to contribute to pathological pain states.

A possible neural basis for stress-induced hyperalgesia

Intense stress and fear have long been known to give rise to a suppression of pain termed "stress-induced analgesia", mediated by brainstem pain-modulating circuitry, including pain-inhibiting neurons of the rostral ventromedial medulla. However, stress does not invariably suppress pain, and indeed, may exacerbate it. Although there is a growing support for the idea of "stress-induced hyperalgesia", the neurobiological basis for this effect remains almost entirely unknown. Using simultaneous single-cell recording and functional analysis, we show here that stimulation of the dorsomedial nucleus of the hypothalamus, known to be a critical component of central mechanisms mediating neuroendocrine, cardiovascular and thermogenic responses to mild or "emotional" stressors such as air puff, also triggers thermal hyperalgesia by recruiting pain-facilitating neurons, "ON-cells", in the rostral ventromedial medulla. Activity of identified RVM ON-cells, OFF-cells and NEUTRAL cells, nociceptive withdrawal thresholds, rectal temperature, and heart rate were recorded in lightly anesthetized rats. In addition to the expected increases in body temperature and heart rate, disinhibition of the DMH induced a robust activation of ON-cells, suppression of OFF-cell firing and behavioral hyperalgesia. Blocking ON-cell activation prevented hyperalgesia, but did not interfere with DMH-induced thermogenesis or tachycardia, pointing to differentiation of neural substrates for autonomic and nociceptive modulation within the RVM. These data demonstrate a top-down activation of brainstem pain-facilitating neurons, and suggest a possible neural circuit for stress-induced hyperalgesia.

Alpha(2)-noradrenergic antagonist administration into the central nucleus of the amygdala blocks stress-induced hypoalgesia in awake behaving rats

Stress-induced hypoalgesia (SIH) is an adaptive behavioral phenomenon mediated in part by the amygdala. Acute stress increases amygdalar noradrenaline levels and focal application of alpha(2)-adrenoceptor agonists in the central nucleus of the amygdala (CeA) is antinociceptive. We hypothesized that alpha(2)-adrenoceptor antagonist administration into the CeA may block SIH. Bilateral microinjections of drug or saline via chronically implanted CeA cannulae were followed by either a period of restraint stress or rest. The nocifensive paw-withdrawal latency (PWL) to a focused beam of light was measured. PWLs were longer in restrained rats, constituting SIH. Microinjection of the alpha(2)-adrenoceptor antagonist idazoxan into the CeA prior to restraint blocked SIH. Idazoxan administration in unrestrained rats had no effect. Microinjection of the alpha(2)-adrenoceptor agonist clonidine in unrestrained rats caused dose dependent hypoalgesia, mimicking the effects of environmental stress. alpha(2)-Adrenoceptor function in the CeA is necessary for restraint-induced SIH.

Purinergic receptor immunoreactivity in the rostral ventromedial medulla

The rostral ventromedial medulla (RVM) has long been recognized to play a pivotal role in nociceptive modulation. Pro-nociception within the RVM is associated with a distinct functional class of neurons, ON-cells that begin to discharge immediately before nocifensive reflexes. Anti-nociceptive function within the RVM, including the analgesic response to opiates, is associated with another distinct class, OFF-cells, which pause immediately prior to nocifensive reflexes. A third class of RVM neurons, NEUTRAL-cells, does not alter firing in association with nocifensive reflexes. ON-, OFF- and NEUTRAL-cells show differential responsiveness to various behaviorally relevant neuromodulators, including purinergic ligands. Iontophoresis of semi-selective P2X ligands, which are associated with nociceptive transmission in the spinal cord and dorsal root ganglia, preferentially activate ON-cells. By contrast, P2Y ligands activate OFF-cells and P1 ligands suppress the firing of NEUTRAL cells. The current study investigates the distribution of P2X, P2Y and P1 receptor immunoreactivity in RVM neurons of Sprague-Dawley rats. Co-localization with tryptophan hydroxylase (TPH), a well-established marker for serotonergic neurons was also studied. Immunoreactivity for the four purinergic receptor subtypes examined was abundant in all anatomical subdivisions of the RVM. By contrast, TPH-immunoreactivity was restricted to a relatively small subset of RVM neurons concentrated in the nucleus raphe magnus and pallidus, as expected. There was a significant degree of co-localization of each purinergic receptor subtype with TPH-immunoreactivity. This co-localization was most pronounced for P2Y1 receptor immunoreactivity, although this was the least abundant among the different purinergic receptor subtypes examined. Immunoreactivity for multiple purinergic receptor subtypes was often co-localized in single neurons. These results confirm the physiological finding that purinergic receptors are widely expressed in the RVM. Purinergic neurotransmission in this region may play an important role in nociception and/or nociceptive modulation, as at other levels of the neuraxis.

Noradrenergic agonist administration into the central nucleus of the amygdala increases the tail-flick latency in lightly anesthetized rats

The amygdala is a medial forebrain structure with an established role in nociceptive modulation, including the expression of stress-induced hypoalgesia (SIH). Projections from the locus coeruleus increase levels of noradrenaline in the amygdala during acute stress. alpha(2)-Noradrenergic receptor agonists have significant clinical utility as analgesic agents. We therefore hypothesized that alpha(2)-noradrenergic activation of the amygdala may result in behaviorally measurable hypoalgesia. Lightly anesthetized rats underwent microinjection of the alpha(2)-noradrenergic agonist clonidine into the amygdala and intermittent measurement of thermal nociception using the tail-flick latency (TFL). Bilateral microinjection of clonidine into the central nucleus of the amygdala (CeA) resulted in a significant, dose-dependent increase in TFL. This effect was blocked by systemic pre-treatment with the alpha(2)-antagonist yohimbine or by local pre-injection of the alpha(2)-antagonist idazoxan but not by local pre-injection of the alpha(1)-antagonist WB-4101. When injected alone, no antagonist resulted in a significant change in TFL compared with baseline. Clonidine injection into the amygdala but outside the CeA, including the basolateral nucleus of the amygdala, did not significantly alter TFL. These results demonstrate that anatomically and pharmacologically specific activation of alpha(2)-receptors in the CeA in lightly anesthetized rats results in behaviorally measurable antinociception.

Purinergic actions on neurons that modulate nociception in the rostral ventromedial medulla

The rostral ventromedial medulla (RVM) serves as a critical link in bulbo-spinal nociceptive modulation. Within the RVM, 'off-cells' pause and 'on-cells' discharge immediately prior to a nocifensive reflex. These neurons are also activated and inactivated, respectively, by local or systemic application of opioids. Off-cell activation leads to behavioral anti-nociception and on-cell activation to hyperalgesia. Thus, on- and off-cell populations allow bi-directional modulation of nociception by the RVM. A third neuronal population, neutral cells, shows no reflex-related change in discharge. The role of neutral cells in nociception, if any, is unknown. We investigated the responses of on-, off- and neutral cells to the iontophoretic application of purinergic ligands in lightly anesthetized rats. On-cell firing increased rapidly in response to application of ATP and to the P2X-receptor agonist, alpha,beta-methylene ATP. Off-cell firing increased gradually in response to ATP and to the P2Y-receptor agonist, 2-methylthio-ATP. All of these responses were attenuated or reversed by the non-specific P2-receptor antagonists, suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). Activation of off-cells was preferentially antagonized by the relatively selective P2Y antagonist, MRS2179. By contrast with activation of on- and off-cells by ATP, neutral cell firing was depressed by ATP, adenosine and the P1-receptor agonist, 5'-(N-ethylcarboxamido) adenosine (NECA). Neutral cell responses to these agonists were at least partially reversed by the adenosine-receptor antagonist, 8-phenyltheophylline (8PT). These data imply that on-cells preferentially express P2X-receptors, off-cells P2Y-receptors and neutral cells P1-receptors. Immunohistochemical localization of purinergic receptors confirms the presence of some subtypes of P2X, P2Y and A1 receptors on neuronal cell bodies and fibers within the RVM. The differential responses of on-, off- and neutral-cells to purinergic ligands highlight the value of pharmacological signatures in further delineation of the anatomy, connectivity and function of this therapeutically important system.

NMDA receptor-mediated activation of medullary pro-nociceptive neurons is required for secondary thermal hyperalgesia

There is now direct evidence that a class of neurons in the rostral ventromedial medulla (RVM) exerts a net facilitatory influence on spinal nociception. The present experiments were designed to test whether activation of these neurons, referred to as "on-cells", is required as part of a positive feedback loop leading to secondary hyperalgesia in acute inflammation produced by topical application of mustard oil. Activity of a characterized RVM neuron and paw withdrawals to heat (plantar surface) were recorded in barbiturate-anesthetized rats. Following three baseline trials, mustard oil was applied to the skin above the knee. Cell activity and paw withdrawal latencies were monitored for an additional 45min. Application of mustard oil produced an increase in on-cell discharge that was associated with a substantial decrease in withdrawal latency of the ipsilateral paw. Blocking on-cell activation using local infusion of the NMDA-receptor antagonist AP5 into the RVM prevented hyperalgesia. Secondary thermal hyperalgesia following mustard oil was also associated with a significant decrease in the firing of "off-cells", a cell population thought to exert a net inhibitory influence on nociception. Depression of off-cell firing was unaffected by AP5 microinjection. The firing of "neutral cells", which have no documented role in nociceptive modulation, was unchanged following mustard oil and also unaffected by AP5 infusion in the RVM. Brainstem descending controls are receiving increasing attention in efforts to understand hyperalgesia and persistent pain states. The present experiments demonstrate that a novel, NMDA-mediated activation of on-cells is required for secondary thermal hyperalgesia in acute inflammation.

Role for Medullary Pain Facilitating Neurons in Secondary Thermal Hyperalgesia

The rostral ventromedial medulla (RVM) has recently received considerable attention in efforts to understand mechanisms of hyperalgesia and persistent pain states. Three classes of neurons can be identified in the RVM based on responses associated with nocifensive reflexes: on cells, off cells, and neutral cells. There is now direct evidence that on cells exert a net facilitating effect on spinal nociception and that off cells depress nociception. These experiments tested whether the secondary hyperalgesia produced by topical application of mustard oil involves an activation of on cells in RVM. Firing of a characterized RVM neuron and the latencies of withdrawal reflexes evoked by noxious heat were recorded in lightly anesthetized rats before and after application of mustard oil to the shaved skin of the leg above the knee. Mineral oil was applied as a control. Mustard oil produced a significant increase in ongoing and reflex-related discharge of on cells, as well as a decrease in the activity of off cells. neutral cell firing was uniformly unchanged after application of mustard oil. The alterations in on and off cell firing were associated with a significant decrease in the latency to withdraw the paw of the treated limb from the heat stimulus, and this hyperalgesia was blocked by microinjection of lidocaine within the RVM. Withdrawals evoked by heating the contralateral hindpaw, forepaw, and tail were unchanged after mustard oil application. These experiments support a pronociceptive role for on cells and suggest that these neurons contribute to secondary hyperalgesia in inflammation.

Nociceptin/orphanin FQ: pain, stress and neural circuits

First isolated some 10 years ago as the endogenous ligand for the "orphan opioid receptor" (ORL-1, now designated NOP), nociceptin/orphanin FQ (N/OFQ) has proved to be a potent inhibitory neuropeptide found across the neuraxis. Because of the homologies between opioids and N/OFQ, functional studies of this peptide have focused most heavily on pain and analgesia. This behavioral literature has been marked by a lack of consistency across laboratories, but much of the data can be explained by considering the potent inhibitory actions of N/OFQ in well-defined modulatory circuits. Presently, the most closely studied such circuit is the rostral ventromedial medulla (RVM), where administration of N/OFQ can block opioid analgesia (by inhibiting opioid-activated pain-inhibiting neurons), but under other conditions produces apparent hypoalgesia (by inhibiting pain-facilitating neurons). The net behavioral effect of N/OFQ in the RVM thus depends on whether experimental conditions are such that the pain-facilitating or pain-inhibiting neurons are active at the time the peptide is given. An important recent finding is that N/OFQ antagonists have antinociceptive properties when given supra-spinally. Although the likelihood of interactions between stress and analgesia systems must be considered in interpreting these data, they suggest that N/OFQ antagonists have potential as clinically useful analgesic drugs.

Prostaglandin E2 in the medial preoptic area produces hyperalgesia and activates pain-modulating circuitry in the rostral ventromedial medulla

Prostaglandin E2 (PGE2) produced in the medial preoptic region (MPO) in response to immune signals is generally accepted to play a major role in triggering the illness response, a complex of physiological and behavioral changes induced by infection or injury. Hyperalgesia is now thought to be an important component of the illness response, yet the specific mechanisms through which the MPO acts to facilitate nociception have not been established. However, the MPO does project to the rostral ventromedial medulla (RVM), a region with a well-documented role in pain modulation, both directly and indirectly via the periaqueductal gray. To test whether PGE2 in the MPO produces thermal hyperalgesia by recruiting nociceptive modulating neurons in the RVM, we recorded the effects of focal application of PGE2 in the MPO on paw withdrawal latency and activity of identified nociceptive modulating neurons in the RVM of lightly anesthetized rats. Microinjection of a sub-pyrogenic dose of PGE2 (50 fg in 200 nl) into the MPO produced thermal hyperalgesia, as measured by a significant decrease in paw withdrawal latency. In animals displaying behavioral hyperalgesia, the PGE2 microinjection activated on-cells, RVM neurons thought to facilitate nociception, and suppressed the firing of off-cells, RVM neurons believed to have an inhibitory effect on nociception. A large body of evidence has implicated prostaglandins in the MPO in generation of the illness response, especially fever. The present study indicates that the MPO also contributes to the hyperalgesic component of the illness response, most likely by recruiting the nociceptive modulating circuitry of the RVM.

Prostaglandin E2 in the midbrain periaqueductal gray produces hyperalgesia and activates pain-modulating circuitry in the rostral ventromedial medulla

Recent years have seen significant advances in our understanding of the peripheral and spinal mechanisms through which prostaglandins contribute to nociceptive sensitization. By contrast, the possibility of a supraspinal contribution of these compounds to facilitated pain states has received relatively little attention. One possible mechanism through which prostaglandins could act supraspinally to facilitate nociception would be by recruitment of descending facilitation from brainstem pain-modulating systems. The rostral ventromedial medulla (RVM) is now known to contribute to enhanced responding in a variety of inflammatory and nerve injury models. Its major supraspinal input, the midbrain periaqueductal gray (PAG), expresses prostanoid receptors and synthetic enzymes. The aim of the present study was to determine whether direct application of prostaglandin E(2) (PGE(2)) within the ventrolateral PAG is sufficient to produce hyperalgesia, and whether any hyperalgesia could be mediated by recruiting nociceptive modulating neurons in the RVM. We determined the effects of focal application of PGE(2) in the PAG on paw withdrawal latency and activity of identified nociceptive modulating neurons in the RVM of lightly anesthetized rats. Microinjection of PGE(2) (50 fg in 200 nl) into the PAG produced a significant decrease in paw withdrawal latency. The PGE(2) microinjection activated on-cells, RVM neurons thought to facilitate nociception, and suppressed the firing of off-cells, RVM neurons believed to have an inhibitory effect on nociception. These data demonstrate a prostaglandin-sensitive descending facilitation from the PAG, and suggest that this is mediated by on- and off-cells in the RVM.

Dissociation of hyperalgesia from fever following intracerebroventricular administration of interleukin-1β in the rat

Interleukin-1beta (IL-1beta) is a cytokine that contributes to the hyperalgesia, inactivity, and fever associated with illness. These three components of the illness response occur simultaneously following peripheral administration of IL-1beta. The objective of the present study was to determine whether hyperalgesia, inactivity, and fever correspond following central administration. Rats were injected with IL-1beta (0.05 pg-50 ng/10 microl) into the lateral ventricle and core body temperature and activity were assessed for 5.5 h using radio telemetry while rats remained in their home cage. Rats were removed from the cage periodically to assess nociception by measuring the latency for hindpaw withdrawal to radiant heat. The two highest doses of IL-1beta (5 and 50 ng) caused an increase in core body temperature and a decrease in activity beginning 105 min following administration. No change in nociception was evident at any time after administration of IL-1beta regardless of dose. These data indicate that the hyperalgesia associated with fever is triggered by a peripheral, not a central action of IL-1beta, presumably by activation of vagal afferents.

Neural Basis for the Hyperalgesic Action of Cholecystokinin in the Rostral Ventromedial Medulla

The analgesic actions of opioids can be modified by endogenous “anti-opioid” peptides, among them cholecystokinin (CCK). CCK is now thought to have a broader, pronociceptive role, and contributes to hyperalgesia in inflammatory and neuropathic pain states. The aim of this study was to determine whether anti-opioid and pronociceptive actions of CCK have a common underlying mechanism. We showed previously that a low dose of CCK microinjected into the rostral ventromedial medulla (RVM) blocked the analgesic effect of systemically administered morphine by preventing activation of off-cells, which are the antinociceptive output of this well characterized pain-modulating region. At this anti-opioid dose, CCK had no effect on the spontaneous activity of these neurons or on the activity of on-cells (hypothesized to facilitate nociception) or “neutral cells” (which have no known role in pain modulation). In this study, we used microinjection of a higher dose of CCK into the RVM to test whether activation of on-cells could explain the pronociceptive action of this peptide. Paw withdrawal latencies to noxious heat and the activity of a characterized RVM neuron were recorded in rats lightly anesthetized with methohexital. CCK (30 ng/200 nl) activated on-cells selectively and produced behavioral hyperalgesia. Firing of off-cells and neutral cells was unaffected. These data show that direct, selective activation of RVM on-cells by CCK is sufficient to produce thermal hyperalgesia and indicate that the anti-opioid and pronociceptive effects of this peptide are mediated by actions on different RVM cell classes.

Nociceptive facilitating neurons in the rostral ventromedial medulla

The role of the periaqueductal gray-rostral ventromedial medulla (RVM) system in descending inhibition of nociception has been studied for over 30 years. The neural basis for this antinociceptive action is reasonably well understood, with strong evidence that activation of a class of RVM neurons termed 'off-cells' exerts a net inhibitory effect on nociception. However, it has recently become clear that this system can facilitate, as well as inhibit pain. Although the mechanisms underlying the facilitation of nociception have not been conclusively identified, indirect evidence points to activation of a class of neurons termed 'on-cells' as mediating descending facilitation. Here we used focal infusion of the tridecapeptide neurotensin within the RVM in lightly anesthetized rats to activate on-cells selectively. Neurotensin has been shown in awake animals to produce a dose-related, bi-directional effect on nociception when applied within the RVM, with hyperalgesia at low doses, and analgesia at higher doses. Using a combination of single cell recording and behavioral testing, we now show that on-cells are activated selectively by low-dose neurotensin, and that the activation of on-cells by neurotensin results in enhanced nociceptive responding, as measured by the paw withdrawal reflex. Furthermore, higher neurotensin doses recruit off-cells in addition to on-cells, producing behavioral antinociception. Selective activation of on-cells is thus sufficient to produce hyperalgesia, confirming the role of these neurons in facilitating nociception. Activation of on-cells likely contributes to enhanced sensitivity to noxious stimulation or reduced sensitivity to analgesic drugs in a variety of conditions.

Lesions of the periaqueductal gray disrupt input to the rostral ventromedial medulla following microinjections of morphine into the medial or basolateral nuclei of the amygdala

Microinjections of morphine into the basolateral (BLa) and medial (MEa) nuclei of the amygdala differentially affect rostral ventromedial medulla (RVM) neuronal activity and nocifensive behaviors. PAG lesions attenuated or blocked the effects of both BLa and MEa morphine on RVM cell activity, and interfered with the behavioral antinociception produced by BLa infusions. These results demonstrate that the influences from both the BLa and MEa to the RVM are relayed via the PAG.

Simultaneous analysis of the time course for changes in core body temperature, activity, and nociception following systemic administration of interleukin-1β in the rat

The aches and pains that accompany fever appear to be mediated, at least in part, by the peripheral release of cytokines such as interleukin-1beta (IL-1beta). The objective of this study was to determine, whether changes in nociceptive sensitivity produced by IL-1beta administration are temporally linked to changes in core body temperature. Experiment 1 examined nociceptive responsiveness for a period of 3 h following systemic administration of IL-1beta (1, 3, 10 and 20 microg/kg). The two highest doses of IL-1beta produced a drop in temperature beginning approximately 60 min after cytokine administration. This hypothermia lasted 90 min and was associated with hyperalgesia. Experiment 2 examined changes in temperature and nociception for 12 h following administration of IL-1beta (10 microg/kg). An early, short-lived hypothermia was followed by a significant hyperthermia from 3.25 to 6.5 h following IL-1beta administration. This late-occurring fever was accompanied by hyperalgesia. Both the hypo- and hyperthermia phases were associated with a reduction in locomotor activity. Given that repeated nociceptive testing may confound assessment of temperature and activity, Experiment 3 examined the effects of IL-1beta (10 microg/kg) administration on temperature and activity in rats that remained in their home cages. The biphasic change in temperature and the reduction in activity were nearly identical to that reported in Experiment 2, indicating that repeated nociceptive testing did not confound these data. The results of this study demonstrate that, two phases of hyperalgesia occur and coincide with the periods of altered thermoregulation produced by systemic administration of IL-1beta.