Kappa opioid receptor (KOR) and GAD67 immunoreactivity are found in OFF and NEUTRAL cells in the rostral ventromedial medulla

Opioid circuit opens path to pain relief

Manipulation of neural circuits targeted by morphine enables pain relief without opioids.

Morphine-responsive neurons that regulate mechanical antinociception

Opioids are widely used, effective analgesics to manage severe acute and chronic pain, although they have recently come under scrutiny because of epidemic levels of abuse. While these compounds act on numerous central and peripheral pain pathways, the neuroanatomical substrate for opioid analgesia is not fully understood. By means of single-cell transcriptomics and manipulation of morphine-responsive neurons, we have identified an ensemble of neurons in the rostral ventromedial medulla (RVM) that regulates mechanical nociception in mice. Among these, forced activation or silencing of excitatory RVMBDNF projection neurons mimicked or completely reversed morphine-induced mechanical antinociception, respectively, via a brain-derived neurotrophic factor (BDNF)/tropomyosin receptor kinase B (TrkB)-dependent mechanism and activation of inhibitory spinal galanin-positive neurons. Our results reveal a specific RVM-spinal circuit that scales mechanical nociception whose function confers the antinociceptive properties of morphine.

Local Synthesis of Estradiol in the Rostral Ventromedial Medulla Protects against Widespread Muscle Pain in Male Mice

Animal studies consistently demonstrate that testosterone is protective against pain in multiple models, including an animal model of activity-induced muscle pain. In this model, females develop widespread muscle hyperalgesia, and reducing testosterone levels in males results in widespread muscle hyperalgesia. Widespread pain is believed to be mediated by changes in the central nervous system, including the rostral ventromedial medulla (RVM). The enzyme that converts testosterone to estradiol, aromatase, is highly expressed in the RVM. Therefore, we hypothesized that testosterone is converted by aromatase to estradiol locally in the RVM to prevent development of widespread muscle hyperalgesia in male mice. This was tested through pharmacological inhibition of estrogen receptors (ERs), aromatase, or ER-α in the RVM which resulted in contralateral hyperalgesia in male mice (C57BL/6J). ER inhibition in the RVM had no effect on hyperalgesia in female mice. As prior studies show modulation of estradiol signaling alters GABA receptor and transporter expression, we examined if removal of testosterone in males would decrease mRNA expression of GABA receptor subunits and vesicular GABA transporter (VGAT). However, there were no differences in mRNA expression of GABA receptor subunits of VGAT between gonadectomized and sham control males. Lastly, we used RNAscope to determine expression of ER-α in the RVM and show expression in inhibitory (VGAT+), serotonergic (tryptophan hydroxylase 2+), and μ-opioid receptor expressing (MOR+) cells. In conclusion, testosterone protects males from development of widespread hyperalgesia through aromatization to estradiol and activation of ER-α which is widely expressed in multiple cell types in the RVM.

The 'in's and out's' of descending pain modulation from the rostral ventromedial medulla

The descending-pain modulating circuit controls the experience of pain by modulating transmission of sensory signals through the dorsal horn. This circuit's key output node, the rostral ventromedial medulla (RVM), integrates 'top-down' and 'bottom-up' inputs that regulate functionally defined RVM cell types, 'OFF-cells' and 'ON-cells', which respectively suppress or facilitate pain-related sensory processing. While recent advances have sought molecular definition of RVM cell types, conflicting behavioral findings highlight challenges involved in aligning functional and molecularly defined types. This review summarizes current understanding, derived primarily from rodent studies but with corroborating evidence from human imaging, of the role of RVM populations in pain modulation and persistent pain states and explores recent advances outlining inputs to, and outputs from, RVM pain-modulating neurons.

Dissecting neural circuits from rostral ventromedial medulla to spinal trigeminal nucleus bidirectionally modulating craniofacial mechanical sensitivity

Chronic craniofacial pain is intractable and its mechanisms remain unclarified. The rostral ventromedial medulla (RVM) plays a crucial role in descending pain facilitation and inhibition. It is unclear how the descending circuits from the RVM to spinal trigeminal nucleus (Sp5) are organized to bidirectionally modulate craniofacial nociception. We used viral tracing, in vivo optogenetics, calcium signaling recording, and chemogenetic manipulations to investigate the structure and function of RVM-Sp5 circuits. We found that most RVM neurons projecting to Sp5 were GABAergic or glutamatergic and facilitated or inhibited craniofacial nociception, respectively. Both GABAergic interneurons and glutamatergic projection neurons in Sp5 received RVM inputs: the former were antinociceptive, whereas the latter were pronociceptive. Furthermore, we demonstrated activation of both GABAergic and glutamatergic Sp5 neurons receiving RVM inputs in inflammation- or dysfunction-induced masseter hyperalgesia. Activating GABAergic Sp5 neurons or inhibiting glutamatergic Sp5 neurons that receive RVM projections reversed masseter hyperalgesia. Our study identifies specific cell types and projections of RVM-Sp5 circuits involved in facilitating or inhibiting craniofacial nociception respectively. Selective manipulation of RVM-Sp5 circuits can be used as potential treatment strategy to relieve chronic craniofacial muscle pain.

Optotagging and characterization of GABAergic rostral ventromedial medulla (RVM) neurons

The transmission of nociceptive and pruriceptive signals in the spinal cord is greatly influenced by descending modulation from brain areas such as the rostral ventromedial medulla (RVM). Within the RVM three classes of neurons have been discovered which are relevant to spinal pain modulation, the On, Off, and Neutral cells. These neurons were discovered due to their functional response to nociceptive stimulation. On cells are excited, Off cells are inhibited, and Neutral cells have no response to noxious stimulation. Since these neurons are identified by functional response characteristics it has been difficult to molecularly identify them. In the present study, we leverage our ability to perform optotagging within the RVM to determine whether RVM On, Off, and Neutral cells are GABAergic. We found that 27.27% of RVM On cells, 47.37% of RVM Off cells, and 42.6% of RVM Neutral cells were GABAergic. These results demonstrate that RVM On, Off, and Neutral cells represent a heterogeneous population of neurons and provide a reliable technique for the molecular identification of these neurons.

The Role of The Rostral Ventromedial Medulla in Stress Responses

The rostral ventromedial medulla (RVM) is a brainstem structure critical for the descending pain modulation system involved in both pain facilitation and inhibition through its projection to the spinal cord. Since the RVM is well connected with pain- and stress-engaged brain structures, such as the anterior cingulate cortex, nucleus accumbens, and amygdala, its involvement in stress responses has become a matter of great interest. While chronic stress has been proposed as a trigger of pain chronification and related psychiatric comorbidities due to maladaptive stress responses, acute stress triggers analgesia and other adaptative responses. Here we reviewed and highlighted the critical role of the RVM in stress responses, mainly in acute stress-induced analgesia (SIA) and chronic stress-induced hyperalgesia (SIH), providing insights into pain chronification processes and comorbidity between chronic pain and psychiatric disorders.

Central circuit controlling thermoregulatory inversion and torpor-like state

To maintain core body temperature in mammals, the CNS thermoregulatory networks respond to cold exposure by increasing brown adipose tissue and shivering thermogenesis. However, in hibernation or torpor, this normal thermoregulatory response is supplanted by "thermoregulatory inversion", an altered homeostatic state in which cold exposure causes inhibition of thermogenesis and warm exposure stimulates thermogenesis. Here we demonstrate the existence of a novel, dynorphinergic thermoregulatory reflex pathway between the dorsolateral parabrachial nucleus and the dorsomedial hypothalamus that bypasses the normal thermoregulatory integrator in the hypothalamic preoptic area to play a critical role in mediating the inhibition of thermogenesis during thermoregulatory inversion. Our results indicate the existence of a neural circuit mechanism for thermoregulatory inversion within the CNS thermoregulatory pathways and support the potential for inducing a homeostatically-regulated, therapeutic hypothermia in non-hibernating species, including humans.

Kappa Opioid Receptors Reduce Serotonin Uptake and Escitalopram Efficacy in the Mouse Substantia Nigra Pars Reticulata

The serotonin and kappa opioid receptor (KOR) systems are strongly implicated in disorders of negative affect, such as anxiety and depression. KORs expressed on axon terminals inhibit the release of neurotransmitters, including serotonin. The substantia nigra pars reticulata (SNr) is involved in regulating affective behaviors. It receives the densest serotonergic innervation in the brain and has high KOR expression; however, the influence of KORs on serotonin transmission in this region is yet to be explored. Here, we used ex vivo fast-scan cyclic voltammetry (FSCV) to investigate the effects of a KOR agonist, U50, 488 (U50), and a selective serotonin reuptake inhibitor, escitalopram, on serotonin release and reuptake in the SNr. U50 alone reduced serotonin release and uptake, and escitalopram alone augmented serotonin release and slowed reuptake, while pretreatment with U50 blunted both the release and uptake effects of escitalopram. Here, we show that the KOR influences serotonin signaling in the SNr in multiple ways and short-term activation of the KOR alters serotonin responses to escitalopram. These interactions between KORs and serotonin may contribute to the complexity in the responses to treatments for disorders of negative affect. Ultimately, the KOR system may prove to be a promising pharmacological target, alongside traditional antidepressant treatments.

The chronification mechanism of orofacial inflammatory pain: Facilitation by GPER1 and microglia in the rostral ventral medulla

Background

Chronic orofacial pain is a common and incompletely defined clinical condition. The role of G protein-coupled estrogen receptor 1 (GPER1) as a new estrogen receptor in trunk and visceral pain regulation is well known. Here, we researched the role of GPER1 in the rostral ventral medulla (RVM) during chronic orofacial pain.

Methods and Results

A pain model was established where rats were injected in the temporomandibular joint with complete Freund's adjuvant (CFA) to simulate chronic orofacial pain. Following this a behavioral test was performed to establish pain threshold and results showed that the rats injected with CFA had abnormal pain in the orofacial regions. Additional Immunostaining and blot analysis indicated that microglia were activated in the RVM and GPER1 and c-Fos were significantly upregulated in the rats. Conversely, when the rats were injected with G15 (a GPER1 inhibitor) the abnormal pain the CFA rats were experiencing was alleviated and microglia activation was prevented. In addition, we found that G15 downregulated the expression of phospholipase C (PLC) and protein kinase C (PKC), inhibited the expression of GluA1, restores aberrant synaptic plasticity and reduces the overexpression of the synapse-associated proteins PSD-95 and syb-2 in the RVM of CFA rats.

Conclusion

The findings indicate that GPER1 mediates chronic orofacial pain through modulation of the PLC-PKC signal pathway, sensitization of the RVM region and enhancement of neural plasticity. These results of this study therefore suggest that GPER1 may serve as a potential therapeutic target for chronic orofacial pain.

Bulbospinal nociceptive ON and OFF cells related neural circuits and transmitters

The rostral ventromedial medulla (RVM) is a bulbospinal nuclei in the descending pain modulation system, and directly affects spinal nociceptive transmission through pronociceptive ON cells and antinociceptive OFF cells in this area. The functional status of ON and OFF neurons play a pivotal role in pain chronification. As distinct pain modulative information converges in the RVM and affects ON and OFF cell excitability, neural circuits and transmitters correlated to RVM need to be defined for an in-depth understanding of central-mediated pain sensitivity. In this review, neural circuits including the role of the periaqueductal gray, locus coeruleus, parabrachial complex, hypothalamus, amygdala input to the RVM, and RVM output to the spinal dorsal horn are discussed. Meanwhile, the role of neurotransmitters is concluded, including serotonin, opioids, amino acids, cannabinoids, TRPV1, substance P and cholecystokinin, and their dynamic impact on both ON and OFF cell activities in modulating pain transmission. Via clarifying potential specific receptors of ON and OFF cells, more targeted therapies can be raised to generate pain relief for patients who suffer from chronic pain.

Kappa opioids inhibit the GABA/glycine terminals of rostral ventromedial medulla projections in the superficial dorsal horn of the spinal cord

Descending projections from neurons in the rostral ventromedial medulla (RVM) make synapses within the superficial dorsal horn (SDH) of the spinal cord that are involved in the modulation of nociception, the development of chronic pain and itch, and an important analgesic target for opioids. This projection is primarily inhibitory, but the relative contribution of GABAergic and glycinergic transmission is unknown and there is limited knowledge about the SDH neurons targeted. Additionally, the details of how spinal opioids mediate analgesia remain unclear, and no study has investigated the opioid modulation of this synapse. We address this using ex vivo optogenetic stimulation of RVM fibres in conjunction with whole-cell patch-clamp recordings from the SDH in spinal cord slices. We demonstrate that both GABAergic and glycinergic neurotransmission is employed and show that SDH target neurons have diverse morphological and electrical properties, consistent with both inhibitory and excitatory interneurons. Then, we describe a subtype of SDH neurons that have a glycine-dominant input, indicating that the quality of descending inhibition across cells is not uniform. Finally, we discovered that the kappa-opioid receptor agonist U69593 presynaptically suppressed most RVM-SDH synapses. By contrast, the mu-opioid receptor agonist DAMGO acted both pre- and post-synaptically at a subset of synapses, and the delta-opioid receptor agonist deltorphin II had little effect. These data provide important mechanistic information about a descending control pathway that regulates spinal circuits. This information is necessary to understand how sensory inputs are shaped and develop more reliable and effective alternatives to current opioid analgesics. Abstract figure legend We combined ex vivo optogenetic stimulation of RVM fibres with whole cell electrophysiology of SDH neurons to investigate the final synapse in a key descending pain modulatory pathway. We demonstrate that both glycine and GABA mediate signalling at the RVM-SDH synapse, that the SDH targets of RVM projections have diverse electrical and morphological characteristics, and that presynaptic inhibition is directly and consistently achieved by kappa opioid agonists. Opioid receptors shown are sized relative to the proportion of neurons that responded to its specific agonists (81 and 84percent of DF and non-DF neurons responded to kappa opioid receptor agonists, respectively. Responses that occurred in <255 percentage of neurons are not indicated here). This article is protected by copyright. All rights reserved.

Medullary kappa-opioid receptor neurons inhibit pain and itch through a descending circuit

In perilous and stressful situations, the ability to suppress pain can be critical for survival. The rostral ventromedial medulla contains neurons that robustly inhibit nocioception at the level of the spinal cord through a top-down modulatory pathway. Although much is known about the role of the rostral ventromedial medulla in the inhibition of pain, the precise ability to directly manipulate pain-inhibitory neurons in the rostral ventromedial medulla has never been achieved. We now expose a cellular circuit that inhibits nocioception and itch in mice. Through a combination of molecular, tracing and behavioural approaches, we found that rostral ventromedial medulla neurons containing the kappa-opioid receptor inhibit itch and nocioception. With chemogenetic inhibition, we uncovered that these neurons are required for stress-induced analgesia. Using intersectional chemogenetic and pharmacological approaches, we determined that rostral ventromedial medulla kappa-opioid receptor neurons inhibit nocioception and itch through a descending circuit. Lastly, we identified a dynorphinergic pathway arising from the periaqueductal grey that modulates nociception within the rostral ventromedial medulla. These discoveries highlight a distinct population of rostral ventromedial medulla neurons capable of broadly and robustly inhibiting itch and nocioception.

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.

G-Protein-Coupled Estrogen Receptor (GPER) in the Rostral Ventromedial Medulla Is Essential for Mobilizing Descending Inhibition of Itch

Chronic itch is a troublesome condition and often difficult to cure. Emerging evidence suggests that the periaqueductal gray (PAG)-rostral ventromedial medulla (RVM) pathway may play an important role in the regulation of itch, but the cellular organization and molecular mechanisms remain incompletely understood. Here, we report that a group of RVM neurons distinctively express the G-protein-coupled estrogen receptor (GPER), which mediates descending inhibition of itch. We found that GPER⁺ neurons in the RVM were activated in chronic itch conditions in rats and mice. Selective ablation or chemogenetic suppression of RVM GPER⁺ neurons resulted in mechanical alloknesis and increased scratching in response to pruritogens, whereas chemogenetic activation of GPER⁺ neurons abrogated itch responses, indicating that GPER⁺ neurons are antipruritic. Moreover, GPER-deficient mice and rats of either sex exhibited hypersensitivity to mechanical and chemical itch, a phenotype reversible by the µ type opioid receptor (MOR) antagonism. Additionally, significant MOR phosphorylation in the RVM was detected in chronic itch models in wild-type but not in GPER^-/- rats. Therefore, GPER not only identifies a population of medullary antipruritic neurons but may also determine the descending antipruritic tone through regulating µ opioid signaling.SIGNIFICANCE STATEMENT Therapeutic options for itch are limited because of an as yet incomplete understanding of the mechanisms of itch processing. Our data have provided novel insights into the cellular organization and molecular mechanisms of descending regulation of itch in normal and pathologic conditions. GPER⁺ neurons (largely GABAergic) in the RVM are antipruritic neurons under tonic opioidergic inhibition, activation of GPER promotes phosphorylation of MOR and disinhibition of the antipruritic GPER⁺ neurons from inhibitory opioidergic inputs, and failure to mobilize GPER⁺ neurons may result in the exacerbation of itch. Our data also illuminate on some of the outstanding questions in the field, such as the mechanisms underlying sex bias in itch, pain, and opioid analgesia and the paradoxical effects of morphine on pain and itch.

Morphine-Conditioned Placebo Analgesia in Female and Male Rats with Chronic Neuropathic Pain: c-Fos Expression in the Rostral Ventromedial Medulla

Placebo analgesia has great potential to overcome the inadequacies of current drug therapies to treat conditions of chronic pain. The rostral ventromedial medulla (RVM) has been implicated as a critical relay in the antinociceptive pathway underpinning placebo analgesia in humans. We developed a model of opiate-conditioned placebo analgesia in rats with neuropathic injury to identify medullary nuclei active during placebo analgesia. Using female and male rats the degree of thermal allodynia was first determined following nerve injury, and a pharmacological conditioning procedure, pairing contextual cues with the experience of morphine-induced analgesia, was used to elicit placebo analgesic reactions. This protocol revealed clear subpopulations of placebo reactors (36% of males, 25% of females) and non-reactors in proportions similar to those reported in human studies. We detected injury-specific c-Fos expression in the gracile nucleus and morphine-specific c-Fos expression in the serotonergic midline raphe nuclei and the caudal nuclei of the solitary tract. However, c-Fos expression did not differ between placebo reactors and non-reactors in either serotonergic or non-serotonergic neurons of the RVM. Despite a subpopulation of rats demonstrating placebo reactions, we found no evidence for enhanced activity in the nuclei from which the classical RVM → spinal cord descending analgesic pathways emerge.

Descending Control Mechanisms and Chronic Pain

PURPOSE OF REVIEW

The goal of the review was to highlight recent advances in our understanding of descending pain-modulating systems and how these contribute to persistent pain states, with an emphasis on the current state of knowledge around "bottom-up" (sensory) and "top-down" (higher structures mediating cognitive and emotional processing) influences on pain-modulating circuits.

RECENT FINDINGS

The connectivity, physiology, and function of these systems have been characterized extensively over the last 30 years. The field is now beginning to ask how and when these systems are engaged to modulate pain. A recent focus is on the parabrachial complex, now recognized as the major relay of nociceptive information to pain-modulating circuits, and plasticity in this circuit and its connections to the RVM is marked in persistent inflammatory pain. Top-down influences from higher structures, including hypothalamus, amygdala, and medial prefrontal areas, are also considered. The challenge will be to tease out mechanisms through which a particular behavioral context engages distinct circuits to enhance or suppress pain, and to understand how these mechanisms contribute to chronic pain.

Lateralized kappa opioid receptor signaling from the amygdala central nucleus promotes stress-induced functional pain

The response of diffuse noxious inhibitory controls (DNIC) is often decreased, or lost, in stress-related functional pain syndromes. Because the dynorphin/kappa opioid receptor (KOR) pathway is activated by stress, we determined its role in DNIC using a model of stress-induced functional pain. Male, Sprague-Dawley rats were primed for 7 days with systemic morphine resulting in opioid-induced hyperalgesia. Fourteen days after priming, when hyperalgesia was resolved, rats were exposed to environmental stress and DNIC was evaluated by measuring hind paw response threshold to noxious pressure (test stimulus) after capsaicin injection in the forepaw (conditioning stimulus). Morphine priming without stress did not alter DNIC. However, stress produced a loss of DNIC in morphine-primed rats in both hind paws that was abolished by systemic administration of the KOR antagonist, nor-binaltorphimine (nor-BNI). Microinjection of nor-BNI into the right, but not left, central nucleus of the amygdala (CeA) prevented the loss of DNIC in morphine-primed rats. Diffuse noxious inhibitory controls were not modulated by bilateral nor-BNI in the rostral ventromedial medulla. Stress increased dynorphin content in both the left and right CeA of primed rats, reaching significance only in the right CeA; no change was observed in the rostral ventromedial medulla or hypothalamus. Although morphine priming alone is not sufficient to influence DNIC, it establishes a state of latent sensitization that amplifies the consequences of stress. After priming, stress-induced dynorphin/KOR signaling from the right CeA inhibits DNIC in both hind paws, likely reflecting enhanced descending facilitation that masks descending inhibition. Kappa opioid receptor antagonists may provide a new therapeutic strategy for stress-related functional pain disorders.

Pain modulation from the brain during diabetic neuropathy: Uncovering the role of the rostroventromedial medulla

Diabetic neuropathy has a profound impact in the quality of life of patients who frequently complain of pain. The mechanisms underlying diabetic neuropathic pain (DNP) are no longer ascribed only to damage of peripheral nerves. The effects of diabetes at the central nervous system are currently considered causes of DPN. Management of DNP may be achieved by antidepressants that act on serotonin (5-HT) uptake, namely specific serotonin reuptake inhibitors. The rostroventromedial medulla (RVM) is a key pain control center involved in descending pain modulation at the spinal cord through local release of 5-HT and plays a peculiar role in the balance of bidirectional control (i.e. inhibitory and facilitatory) from the brain to the spinal cord. This review discusses recently uncovered neurobiological mechanisms that mediate nociceptive modulation from the RVM during diabetes installation. In early phases of the disease, facilitation of pain modulation from the RVM prevails through a triplet of mechanisms which include increase in serotonin expression at the RVM and consequent rise of serotonin levels at the spinal cord and upregulation of local facilitatory 5HT3 receptors, enhancement of spontaneous activity of facilitatory RVM neurons and up-regulation of the expression of transient receptor potential vanilloid type 1 (TRPV1) receptor. With the progression of diabetes the alterations in the RVM increase dramatically, with oxidative stress and neuronal death associated to microglia-mediated inflammation. In a manner similar to other central areas, like the thalamus, the RVM is likely to be a "pain generator/amplifier" during diabetes, accounting to increase DNP. Early interventions in DNP prevention using strategies that simultaneously tackle the exacerbation of 5-HT3 spinal receptors and of microglial RVM activity, namely those that increase the levels of anti-inflammatory cytokines, should be considered in the future of DNP treatment.

Improvements in impaired GABA and GAD65/67 production in the spinal dorsal horn contribute to exercise-induced hypoalgesia in a mouse model of neuropathic pain

Background

Physical exercise effectively attenuates neuropathic pain, and multiple events including the inhibition of activated glial cells in the spinal dorsal horn, activation of the descending pain inhibitory system, and reductions in pro-inflammatory cytokines in injured peripheral nerves may contribute to exercise-induced hypoalgesia. Since fewer GABAergic hypoalgesic interneurons exist in the dorsal horn in neuropathic pain model animals, the recovery of impaired GABAergic inhibition in the dorsal horn may improve pain behavior. We herein determined whether the production of gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD) in the dorsal horn is restored by treadmill running and contributes to exercise-induced hypoalgesia in neuropathic pain model mice. C57BL/6 J mice underwent partial sciatic nerve ligation (PSL). PSL-Runner mice ran on a treadmill at 7 m/min for 60 min/day, 5 days/week, from two days after PSL.

Results

Mechanical allodynia and heat hyperalgesia developed in PSL-Sedentary mice but were significantly attenuated in PSL-Runner mice. PSL markedly decreased GABA and GAD65/67 levels in neuropils in the ipsilateral dorsal horn, while treadmill running inhibited these reductions. GABA+ neuronal nuclei+ interneuron numbers in the ipsilateral dorsal horn were significantly decreased in PSL-Sedentary mice but not in PSL-Runner mice. Pain behavior thresholds positively correlated with GABA and GAD65/67 levels and GABAergic interneuron numbers in the ipsilateral dorsal horns of PSL-Sedentary and -Runner mice.

Conclusions

Treadmill running prevented PSL-induced reductions in GAD65/67 production, and, thus, GABA levels may be retained in interneurons and neuropils in the superficial dorsal horn. Therefore, improvements in impaired GABAergic inhibition may be involved in exercise-induced hypoalgesia.

Dynorphin A analogs for the treatment of chronic neuropathic pain

Chronic pain is one of the most ubiquitous diseases in the world, but treatment is difficult with conventional methods, due to undesirable side effects of treatments and unknown mechanisms of pathological pain states. The endogenous peptide, dynorphin A has long been established as a target for the treatment of pain. Interestingly, this unique peptide has both inhibitory (opioid in nature) and excitatory activities (nonopioid) in the CNS. Both of these effects have been found to play a role in pain and much work has been done to develop therapeutics to enhance the inhibitory effects. Here we will review the dynorphin A compounds that have been designed for the modulation of pain and will discuss where the field stands today.

The activation of supraspinal GPR40/FFA1 receptor signalling regulates the descending pain control system

BACKGROUND AND PURPOSE

The ω-3 polyunsaturated fatty acids exert antinociceptive effects in inflammatory and neuropathic pain; however, the underlying mechanisms remain unclear. Docosahexaenoic acid-induced antinociception may be mediated by the orphan GPR40, now identified as the free fatty acid receptor 1 (FFA1 receptor). Here, we examined the involvement of supraspinal FFA1 receptor signalling in the regulation of inhibitory pain control systems consisting of serotonergic and noradrenergic neurons.

EXPERIMENTAL APPROACH

Formalin-induced pain behaviours were measured in mice. Antinociception induced by FFA1 receptor agonists was examined by intrathecal injections of a catecholaminergic toxin, 5-HT lowering drug or these antagonists. The expression of FFA1 receptor protein and c-Fos was estimated by immunohistochemistry, and the levels of noradrenaline and 5-HT in the spinal cord were measured by LC-MS/MS.

KEY RESULTS

FFA1 receptors colocalized with NeuN (a neuron marker) in the medulla oblongata and with tryptophan hydroxylase (TPH; a serotonergic neuron marker) and dopamine β-hydroxylase (DBH; a noradrenergic neuron marker). A single i.c.v. injection of GW9508, a FFA1 receptor agonist, increased the number of c-Fos-positive cells and the number of neurons double-labelled for c-Fos and TPH and/or DBH. It decreased formalin-induced pain behaviour. This effect was inhibited by pretreatment with 6-hydroxydopamine, DL-p-chlorophenylalanine, yohimbine or WAY100635. Furthermore, GW9508 facilitated the release of noradrenaline and 5-HT in the spinal cord. In addition, GW1100, a FFA1 receptor antagonist, significantly increased formalin-induced pain-related behaviour.

CONCLUSION AND IMPLICATIONS

Activation of the FFA1 receptor signalling pathway may play an important role in the regulation of the descending pain control system.

CO2-inhibited neurons in the medullary raphé are GABAergic

Previous studies have reported subsets of medullary raphé neurons that are either stimulated or inhibited by CO2/pH in vitro, in situ, and in vivo. We tested the hypothesis that medullary raphé CO2-inhibited neurons are GABAergic. Extracellular recordings in unanesthetized juvenile in situ rat preparations showed reversible hypercapnia-induced suppression of 19% (63/323) of medullary raphé neurons, and this suppression persisted after antagonism of NMDA, AMPA/kainate, and GABAA receptors. We stained a subset of CO2-inhibited cells and found that most (11/12) had glutamic acid decarboxylase 67 immunoreactivity (GAD67-ir). These data indicate that the majority of acidosis-inhibited medullary raphé neurons are GABAergic, and that their chemosensitivity is independent of major fast synaptic inputs. Thus, CO2-sensitive GABAergic neurons may play a role in central CO2/pH chemoreception.

"On-" and "off-" cells in the rostral ventromedial medulla of rats held in thermoneutral conditions: are they involved in thermoregulation?

In thermal neutral condition, rats display cyclic variations of the vasomotion of the tail and paws, synchronized with fluctuations of blood pressure, heart rate, and core body temperature. "On-" and "off-" cells located in the rostral ventromedial medulla, a cerebral structure implicated in somatic sympathetic drive, 1) exhibit similar spontaneous cyclic activities in antiphase and 2) are activated and inhibited by thermal nociceptive stimuli, respectively. We aimed at evaluating the implication of such neurons in autonomic regulation by establishing correlations between their firing and blood pressure, heart rate, and skin and core body temperature variations. When, during a cycle, a relative high core body temperature was reached, the on-cells were activated and within half a minute, the off-cells and blood pressure were depressed, followed by heart rate depression within a further minute; vasodilatation of the tail followed invariably within ∼3 min, often completed with vasodilatation of hind paws. The outcome was an increased heat loss that lessened the core body temperature. When the decrease of core body temperature achieved a few tenths of degrees, sympathetic activation switches off and converse variations occurred, providing cycles of three to seven periods/h. On- and off-cell activities were correlated with inhibition and activation of the sympathetic system, respectively. The temporal sequence of events was as follows: core body temperature → on-cell → off-cell ∼ blood pressure → heart rate → skin temperature → core body temperature. The function of on- and off-cells in nociception should be reexamined, taking into account their correlation with autonomic regulations.

Capsaicin-responsive corneal afferents do not contain TRPV1 at their central terminals in trigeminal nucleus caudalis in rats

We examined the substrates for ocular nociception in adult male Sprague-Dawley rats. Capsaicin application to the ocular surface in awake rats evoked nocifensive responses and suppressed spontaneous grooming responses. Thus, peripheral capsaicin was able to activate the central pathways encoding ocular nociception. Our capsaicin stimulus evoked c-Fos expression in a select population of neurons within rostral trigeminal nucleus caudalis in anesthetized rats. These activated neurons also received direct contacts from corneal afferent fibers traced with cholera toxin B from the corneal surface. However, the central terminals of the corneal afferents that contacted capsaicin-activated trigeminal neurons did not contain TRPV1. To determine if TRPV1 expression had been altered by capsaicin stimulation, we examined TRPV1 content of corneal afferents in animals that did not receive capsaicin stimulation. These studies confirmed that while TRPV1 was present in 30% of CTb-labeled corneal afferent neurons within the trigeminal ganglion, TRPV1 was only detected in 2% of the central terminals of these corneal afferents within the trigeminal nucleus caudalis. Other TRP channels were also present in low proportions of central corneal afferent terminals in unstimulated animals (TRPM8, 2%; TRPA1, 10%). These findings indicate that a pathway from the cornea to rostral trigeminal nucleus caudalis is involved in corneal nociceptive transmission, but that central TRP channel expression is unrelated to the type of stimulus transduced by the peripheral nociceptive endings.

Descending pain modulation and chronification of pain

PURPOSE OF REVIEW

Chronic pain is an important public health problem that negatively impacts quality of life of affected individuals and exacts an enormous socio-economic cost. Currently available therapeutics provide inadequate management of pain in many patients. Acute pain states generally resolve in most patients. However, for reasons that are poorly understood, in some individuals, acute pain can transform to a chronic state. Our understanding of the risk factors that underlie the development of chronic pain is limited. Recent studies have suggested an important contribution of dysfunction in descending pain modulatory circuits to pain 'chronification'. Human studies provide insights into possible endogenous and exogenous factors that may promote the conversion of pain into a chronic condition.

RECENT FINDINGS

Descending pain modulatory systems have been studied and characterized in animal models. Human brain imaging techniques, deep brain stimulation and the mechanisms of action of drugs that are effective in the treatment of pain confirm the clinical relevance of top-down pain modulatory circuits. Growing evidence supports the concept that chronic pain is associated with a dysregulation in descending pain modulation. Disruption of the balance of descending modulatory circuits to favour facilitation may promote and maintain chronic pain. Recent findings suggest that diminished descending inhibition is likely to be an important element in determining whether pain may become chronic. This view is consistent with the clinical success of drugs that enhance spinal noradrenergic activity, such as serotonin/norepinephrine reuptake inhibitors (SNRIs), in the treatment of chronic pain states. Consistent with this concept, a robust descending inhibitory system may be normally engaged to protect against the development of chronic pain. Imaging studies show that higher cortical and subcortical centres that govern emotional, motivational and cognitive processes communicate directly with descending pain modulatory circuits providing a mechanistic basis to explain how exogenous factors can influence the expression of chronic pain in a susceptible individual.

SUMMARY

Preclinical studies coupled with clinical pharmacologic and neuroimaging investigations have advanced our understanding of brain circuits that modulate pain. Descending pain facilitatory and inhibitory circuits arising ultimately in the brainstem provide mechanisms that can be engaged to promote or protect against pain 'chronification'. These systems interact with higher centres, thus providing a means through which exogenous factors can influence the risk of pain chronification. A greater understanding of the role of descending pain modulation can lead to novel therapeutic directions aimed at normalizing aberrant processes that can lead to chronic pain.

Corneal pain activates a trigemino-parabrachial pathway in rats

Corneal pain is mediated by primary afferent fibers projecting to the dorsal horn of the medulla, specifically the trigeminal nucleus caudalis. In contrast to reflex responses, the conscious perception of pain requires transmission of neural activity to higher brain centers. Ascending pain transmission is mediated primarily by pathways to either the thalamus or parabrachial nuclei. We previously showed that some corneal afferent fibers preferentially contact parabrachial-projecting neurons in the rostral pole of the trigeminal nucleus caudalis, but the role of these projection neurons in transmitting noxious information from the cornea has not been established. In the present study, we show that noxious stimulation of the corneal surface activates neurons in the rostral pole of the nucleus caudalis, including parabrachially projecting neurons that receive direct input from corneal afferent fibers. We used immunocytochemical detection of c-Fos protein as an index of neuronal activation after noxious ocular stimulation. Animals had previously received injections of a retrograde tracer into either thalamic or parabrachial nuclei to identify projection neurons in the trigeminal dorsal horn. Noxious stimulation of the cornea induced c-Fos in neurons sending projections to parabrachial nuclei, but not thalamic nuclei. We also confirmed that corneal afferent fibers identified with cholera toxin B preferentially target trigeminal dorsal horn neurons projecting to the parabrachial nucleus. The parabrachial region sends ascending projections to brain regions involved in emotional and homeostatic responses. Activation of the ascending parabrachial system may explain the extraordinary salience of stimulation of corneal nociceptors.

A group of non-serotonergic cells is CO2-stimulated in the medullary raphé

Serotonin/substance P synthesizing cells in the raphé nuclei of the brain are candidates for designation as central chemoreceptors that are stimulated by CO2/pH. We have previously demonstrated that these neurons are CO2-stimulated in situ. Evidence also suggests that CO2-inhibited raphé neurons recorded in vitro and in situ synthesize GABA. Unknown is whether there are other types of chemosensitive cells in the raphé. Here, we showed that a previously unrecognized pool of raphé neurons also exhibit chemosensitivity, and that they are not serotonergic. We used extracellular recording of individual raphé neurons in the unanesthetized juvenile rat in situ perfused decerebrate brainstem preparation to assess chemosensitivity of raphé neurons. Subsequent juxtacellular labeling of individually recorded cells, and immunohistochemistry for the serotonin synthesizing enzyme tryptophan hydroxylase and for neurokinin-1 receptor (NK1R; the receptor for substance P) indicated a group of CO2-stimulated cells that are not serotonergic, but express NK1R and are closely apposed to surrounding serotonergic cells. CO2-stimulated 5-HT and non-5-HT cells constitute distinct groups that have different firing characteristics and hypercapnic sensitivities. Non-5-HT cells fire faster and are more robustly stimulated by CO2 than are 5-HT cells. Thus, we have characterized a previously unrecognized type of CO2-stimulated medullary raphé neuron that is not serotonergic, but may receive input from neighboring serotonin/substance P synthesizing chemosensitive neurons. The potential network properties of the three types of chemosensitive raphé neurons (the present non-5-HT cells, serotonergic cells, and CO2-inhibited cells) remain to be elucidated.

Medullary serotonin neurons are CO2 sensitive in situ

Brainstem central chemoreceptors are critical to the hypercapnic ventilatory response, but their location and identity are poorly understood. When studied in vitro, serotonin-synthesizing (5-HT) neurons within the rat medullary raphé are intrinsically stimulated by CO2/acidosis. The contributions of these neurons to central chemosensitivity in vivo, however, are controversial. Lacking is documentation of CO2-sensitive 5-HT neurons in intact experimental preparations and understanding of their spatial and proportional distribution. Here we test the hypothesis that 5-HT neurons in the rat medullary raphé are sensitive to arterial hypercapnia. We use extracellular recording and hypercapnic challenge of spontaneously active medullary raphé neurons in the unanesthetized in situ perfused decerebrate brainstem preparation to assess chemosensitivity of individual cells. Juxtacellular labeling of a subset of recorded neurons and subsequent immunohistochemistry for the 5-HT-synthesizing enzyme tryptophan hydroxylase (TPH) identify or exclude this neurotransmitter phenotype in electrophysiologically characterized chemosensitive and insensitive cells. We show that the medullary raphé houses a heterogeneous population, including chemosensitive and insensitive 5-HT neurons. Of 124 recorded cells, 16 cells were juxtacellularly filled, visualized, and immunohistochemically identified as 5-HT synthesizing, based on TPH-immunoreactivity. Forty-four percent of 5-HT cells were CO2 stimulated (increased firing rate with hypercapnia), while 56% were unstimulated. Our results demonstrate that medullary raphé neurons are heterogeneous and clearly include a subset of 5-HT neurons that are excited by arterial hypercapnia. Together with data identifying intrinsically CO2-sensitive 5-HT neurons in vitro, these results support a role for such cells as central chemoreceptors in the intact system.

Pronociceptive changes in the activity of rostroventromedial medulla (RVM) pain modulatory cells in the streptozotocin-diabetic rat

Neuropathic pain is one of the most frequent complications of diabetes. The increased neuronal activity of primary afferents and spinal cord neurons in streptozotocin (STZ)-diabetic rats increases the recruitment of the nociceptive ascending pathways, which may affect the activity of pain control circuits in the brain. This study aimed to characterize the electrophysiological responses of neurons of the rostroventromedial medulla (RVM), a key brainstem area involved in descending modulation of nociceptive neurotransmission at the spinal cord, in STZ-diabetic rats. Spontaneous and noxious-evoked activity of ON-like cells (pain facilitatory cells) and OFF-like cells (pain inhibitory cells) in the RVM were analyzed by single cell extracellular electrophysiological recordings in STZ-diabetic rats with behavioral signs of diabetic neuropathic pain 4 weeks after diabetes induction and in age-matched non-diabetic controls (CTRL). The electrophysiological analysis revealed an increase in the spontaneous activity of RVM pronociceptive ON-like cells in STZ-diabetic rats when compared to CTRL. On the contrary, the number of active antinociceptive OFF-like cells was significantly lower in the STZ-diabetic rats and their spontaneous activity was decreased when compared with CTRL. Overall, the changes in the activity of RVM pain modulatory cells in STZ-diabetic rats point to enhancement of descending pain facilitation. Based on similar results obtained at the RVM in traumatic neuropathic pain models, the changes in the electrophysiological responses of RVM in STZ-diabetic rats may account for exacerbated pain-like behaviors in diabetic neuropathy.

Opioids disrupt pro-nociceptive modulation mediated by raphe magnus

In anesthetized rats, opioid analgesia is accompanied by a specific pattern of tonic activity in two neuronal populations within the medullary raphe magnus (RM): opioids silence pain-facilitatory ON cells and produce sustained discharge in pain-inhibitory OFF cells. These tonic activity patterns, hypothesized to generate a tonic analgesic state, have not been observed in recordings made without anesthesia. Therefore, we recorded ON and OFF cell activity before and after an analgesic dose of morphine in unanesthetized mice. The tonic activity of ON and OFF cells was unchanged by morphine. Rather, morphine suppressed the phasic ON cell excitation and OFF cell inhibition evoked by noxious stimulation. Before morphine, the magnitude of the noxious stimulus-evoked burst in ON cells correlated with motor withdrawal magnitude, suggesting that ON cells facilitate nocifensive motor reactions. Contrary to model prediction, OFF cell activity was greater before stimulus trials that evoked withdrawals than those without withdrawals. Since withdrawals only occurred when OFF cell activity was suppressed, a decrease in OFF cell activity appears to serve as a pro-nociceptive signal that synchronizes and therefore strengthens the ensuing motor reaction. We further propose that morphine acts in RM to suppress ON and OFF cell phasic responses and thereby disable RM's pro-nociceptive output. Thus, RM cells produce antinociception by failing to exert the pro-nociceptive effects normally engaged by noxious stimulation. These findings revise the conventional understanding of supraspinal opioid analgesia and demonstrate that RM produces on demand rather than state modulation, allowing RM cells to serve other functions during pain-free periods.

The perception and endogenous modulation of pain

Pain is often perceived an unpleasant experience that includes sensory and emotional/motivational responses. Accordingly, pain serves as a powerful teaching signal enabling an organism to avoid injury, and is critical to survival. However, maladaptive pain, such as neuropathic or idiopathic pain, serves no survival function. Genomic studies of individuals with congenital insensitivity to pain or paroxysmal pain syndromes considerable increased our understanding of the function of peripheral nociceptors, and especially of the roles of voltage-gated sodium channels and of nerve growth factor (NGF)/TrkA receptors in nociceptive transduction and transmission. Brain imaging studies revealed a "pain matrix," consisting of cortical and subcortical regions that respond to noxious inputs and can positively or negatively modulate pain through activation of descending pain modulatory systems. Projections from the periaqueductal grey (PAG) and the rostroventromedial medulla (RVM) to the trigeminal and spinal dorsal horns can inhibit or promote further nociceptive inputs. The "pain matrix" can explain such varied phenomena as stress-induced analgesia, placebo effect and the role of expectation on pain perception. Disruptions in these systems may account for the existence idiopathic pan states such as fibromyalgia. Increased understanding of pain modulatory systems will lead to development of more effective therapeutics for chronic pain.

Corneal afferents differentially target thalamic- and parabrachial-projecting neurons in spinal trigeminal nucleus caudalis

Dorsal horn neurons send ascending projections to both thalamic nuclei and parabrachial nuclei; these pathways are thought to be critical pathways for central processing of nociceptive information. Afferents from the corneal surface of the eye mediate nociception from this tissue which is susceptible to clinically important pain syndromes. This study examined corneal afferents to the trigeminal dorsal horn and compared inputs to thalamic- and parabrachial-projecting neurons. We used anterograde tracing with cholera toxin B subunit to identify corneal afferent projections to trigeminal dorsal horn, and the retrograde tracer FluoroGold to identify projection neurons. Studies were conducted in adult male Sprague-Dawley rats. Our analysis was conducted at two distinct levels of the trigeminal nucleus caudalis (Vc) which receive corneal afferent projections. We found that corneal afferents project more densely to the rostral pole of Vc than the caudal pole. We also quantified the number of thalamic- and parabrachial-projecting neurons in the regions of Vc that receive corneal afferents. Corneal afferent inputs to both groups of projection neurons were also more abundant in the rostral pole of Vc. Finally, by comparing the frequency of corneal afferent appositions to thalamic- versus parabrachial-projecting neurons, we found that corneal afferents preferentially target parabrachial-projecting neurons in trigeminal dorsal horn. These results suggest that nociceptive pain from the cornea may be primarily mediated by a non-thalamic ascending pathway.

Noxious stimulation excites serotonergic neurons: a comparison between the lateral paragigantocellular reticular and the raphe magnus nuclei

The present study was designed to record electrophysiological responses to graded noxious thermal stimuli of serotonergic and nonserotonergic neurons in the lateral paragigantocellular reticular (LPGi) and the raphe magnus (RMg) nuclei in rats. All of the neurons recorded were juxtacellularly filled with neurobiotin and identified with double immunofluorescent labeling for both neurobiotin and serotonin. Under halothane anesthesia (0.75%), noxious thermal stimuli ⩾48°C activated almost all (88%) of the serotonergic neurons located within the LPGi (n=16). The increase in firing was clear (3.4±0.3spike/s: mean of responses above the population median) and sustained during the whole application of strong thermal noxious stimuli, with a high mean threshold (48.3±0.3°C) and large receptive fields. Recording of serotonergic neurons in the RMg (n=21) demonstrated that the proportion of strongly activated (>2spike/s) neurons (19% vs 59% for the LPGi) as well as the magnitude of the activation (2.1±0.4spike/s: mean of responses above the population median) to thermal noxious stimuli were significantly lower than in the LPGi (P<.05). Within the boundaries of both the LPGi and the RMg (B3 group), nonserotonergic neurons were also predominantly excited (75%) by noxious stimuli, and the resulting activation (7.9±1.2spike/s) was even greater than that of serotonergic neurons. Thermal noxious stimuli-induced activation of LPGi serotonergic cells probably plays a key role in serotonin-mediated modulations of cardiac baroreflex and transmission of nociceptive messages occurring under such intense noxious conditions.

Genetic differences in response properties of rostral ventromedial medulla neurons to the μ-opioid receptor agonist DAMGO in mouse inbred strains

BACKGROUND

Opioid sensitivity varies among individuals. Although opioids can act partly in the rostral ventromedial medulla (RVM), which has a major role in pain perception, individual differences in the functions of the RVM in response to opioids have not been elucidated. Pain-related behavior among inbred mouse strains may reflect individual differences in sensitivity to pain. We therefore investigated the changes in action potentials of RVM neurons in response to opioid in different mouse strains.

METHODS

Two inbred strains of mice (A/J and CBA/J) were used. Their behavior to noxious stimuli was measured after intracerebroventricular injection of the μ-opioid receptor agonist, DAMGO. Using an in vivo extracellular recording technique, action potentials from single RVM neurons and their functional type (ON-like, OFF-like, or NEUTRAL-like cell) were identified. Evoked responses of the RVM neurons to noxious stimuli were recorded before and after DAMGO administration.

RESULTS

The behavioral study showed that the dose-dependent antinociceptive effect in the A/J strain was significantly stronger than in the CBA/J strain. The electrophysiological study showed that the number of inhibitory OFF-like cells in A/J mice was significantly larger than in CBA/J mice (P<0.01), and that the evoked responses of neurons of A/J mice were inhibited significantly more than in CBA/J mice both for ON-like and OFF-like cells (P<0.01).

CONCLUSIONS

The strain differences in the physiological properties of RVM neurons corresponded to the behavioral strain differences. Genetic differences may contribute to the interindividual variation seen in opioid-induced analgesia.

Medullary circuits for nociceptive modulation

Neurons in the medullary raphe are critical to opioid analgesia through descending projections to the dorsal horn. Work in anesthetized rats led to the postulate that nociceptive suppression results from tonic activation of nociceptive-inhibiting neurons and tonic inhibition of nociceptive-facilitating neurons. However, morphine does not cause tonic changes in raphe neuronal firing in unanesthetized rodents. Recent work suggests that a drop in activity of nociceptive-inhibiting neurons synchronizes nociceptive circuits and a burst of activity in nociceptive-facilitating neurons facilitates withdrawal magnitude. After morphine, the phasic responses of raphe cells are suppressed along with nociceptive withdrawals. The results suggest a new model of brainstem modulation of nociception in which the medullary raphe facilitates nociceptive reactions when noxious input occurs and may modulate other functions between injurious events.

Descending projections from the rostral ventromedial medulla (RVM) to trigeminal and spinal dorsal horns are morphologically and neurochemically distinct

Neurons in the rostral ventromedial medulla (RVM) are thought to modulate nociceptive transmission via projections to spinal and trigeminal dorsal horns. The cellular substrate for this descending modulation has been studied with regard to projections to spinal dorsal horn, but studies of the projections to trigeminal dorsal horn have been less complete. In this study, we combined anterograde tracing from RVM with immunocytochemical detection of the GABAergic synthetic enzyme, GAD67, to determine if the RVM sends inhibitory projections to trigeminal dorsal horn. We also examined the neuronal targets of this projection using immunocytochemical detection of NeuN. Finally, we used electron microscopy to verify cellular targets. We compared projections to both trigeminal and spinal dorsal horns. We found that RVM projections to both trigeminal and spinal dorsal horn were directed to postsynaptic profiles in the dorsal horn, including somata and dendrites, and not to primary afferent terminals. We found that RVM projections to spinal dorsal horn were more likely to contact neuronal somata and were more likely to contain GAD67 than projections from RVM to trigeminal dorsal horn. These findings suggest that RVM neurons send predominantly GABAergic projections to spinal dorsal horn and provide direct input to postsynaptic neurons such as interneurons or ascending projection neurons. The RVM projection to trigeminal dorsal horn is more heavily targeted to dendrites and is only modestly GABAergic in nature. These anatomical features may underlie differences between trigeminal and spinal dorsal horns with regard to the degree of inhibition or facilitation evoked by RVM stimulation.

Neuronal loss in the rostral ventromedial medulla in a rat model of neuropathic pain

Cell death has been reported in the CNS in models of neuropathic pain (Sugimoto et al., 1990; Whiteside and Munglani, 2001; Scholz et al., 2005; Fuccio et al., 2009). In our present study, we examined the effects of spinal nerve ligation (SNL) on the number of neurons in the rostral ventromedial medulla (RVM), a brainstem region involved in modulation of nociception. In rats receiving SNL, we found that the number of RVM neurons decreased by 23% in the side ipsilateral to the surgery. The loss of RVM neurons was also associated with a bilateral increase in the number of glia as well as bilateral activation of both astrocytes and microglia. Administration of tauroursodeoxycholic acid (TUDCA), which reportedly inhibits apoptosis, significantly reduced the loss of neurons, the increase in glia, and the mechanical hypersensitivity induced by SNL. Among RVM neurons, we found that serotonergic (5-hydroxytryptamine, 5-HT) neurons decreased by 35% ipsilateral to SNL. Consistent with these findings, the density of 5-HT-immunoreactive varicosities in the superficial dorsal horn of the spinal cord was 15-30% lower, ipsilateral to SNL. To test the function of the remaining 5-HT neurons, we administered the 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). Interestingly, after 5,7-DHT, mechanical withdrawal thresholds increased significantly. We conclude that nerve injury induces death of antinociceptive RVM neurons that can be reduced or abolished by TUDCA. We propose that the loss of RVM neurons shifts the balance of descending control from pain inhibition to pain facilitation.

Opioid receptor modulation of GABAergic and serotonergic spinally projecting neurons of the rostral ventromedial medulla in mice

The rostral ventromedial medulla (RVM) is an important site of opioid actions and forms part of an analgesic pathway that projects to the spinal cord. The neuronal mechanisms by which opioids act within this brain region remain unclear, particularly in relation to the neurotransmitters GABA and serotonin. In the present study, we examined serotonergic and GABAergic immunoreactivity, identified using immunohistochemistry for tryptophan hydroxylase (TPH) and glutamate decarboxylase (GAD), in combination with in vitro whole cell patch clamping to investigate the role of opioids on the mouse RVM with identified projections to the spinal cord. Tyr-d-Ala-Gly-N-Me-Phe-Gly-ol enkephalin (DAMGO) produced μ-opioid receptor-mediated outward currents in virtually all TPH-immunoreactive projecting neurons and GAD-immunoreactive nonprojecting neurons (87% and 86%). The other groups of RVM neurons displayed mixed responsiveness to DAMGO (40-68%). Deltorphin II and U-69593 produced δ- and κ-opioid receptor-mediated outward currents in smaller subpopulations of RVM neurons, with many of the δ-opioid responders forming a subpopulation of μ-opioid-sensitive GABAergic nonprojecting neurons. These findings are consistent with prior electrophysiological and anatomic studies in the rat RVM and indicate that both serotonergic and GABAergic RVM neurons mediate the actions of μ-opioids. Specifically, μ-opioids have a direct postsynaptic inhibitory influence over both GABAergic and serotonergic neurons, including those that project to the dorsal spinal cord.

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.

Central modulation of pain

It has long been appreciated that the experience of pain is highly variable between individuals. Pain results from activation of sensory receptors specialized to detect actual or impending tissue damage (i.e., nociceptors). However, a direct correlation between activation of nociceptors and the sensory experience of pain is not always apparent. Even in cases in which the severity of injury appears similar, individual pain experiences may vary dramatically. Emotional state, degree of anxiety, attention and distraction, past experiences, memories, and many other factors can either enhance or diminish the pain experience. Here, we review evidence for "top-down" modulatory circuits that profoundly change the sensory experience of pain.

Differential localization of vesicular glutamate transporters and peptides in corneal afferents to trigeminal nucleus caudalis

Trigeminal afferents convey nociceptive information from the corneal surface of the eye to the trigeminal subnucleus caudalis (Vc). Trigeminal afferents, like other nociceptors, are thought to use glutamate and neuropeptides as neurotransmitters. The current studies examined whether corneal afferents contain both neuropeptides and vesicular glutamate transporters. Corneal afferents to the Vc were identified by using cholera toxin B (CTb). Corneal afferents project in two clusters to the rostral and caudal borders of the Vc, regions that contain functionally distinct nociceptive neurons. Thus, corneal afferents projecting to these two regions were examined separately. Dual immunocytochemical studies combined CTb with either calcitonin gene-related peptide (CGRP), substance P (SP), vesicular glutamate transporter 1 (VGluT1), or VGluT2. Corneal afferents were more likely to contain CGRP than SP, and corneal afferents projecting to the rostral region were more likely to contain CGRP than afferents projecting caudally. Overall, corneal afferents were equally likely to contain VGluT1 or VGluT2. Together, 61% of corneal afferents contained either VGluT1 or VGluT2, suggesting that some afferents lack a VGluT. Caudal corneal afferents were more likely to contain VGluT2 than VGluT1, whereas rostral corneal afferents were more likely to contain VGluT1 than VGluT2. Triple-labeling studies combining CTb, CGRP, and VGluT2 showed that very few corneal afferents contain both CGRP and VGluT2, caudally (1%) and rostrally (2%). These results suggest that most corneal afferents contain a peptide or a VGluT, but rarely both. Our results are consistent with a growing literature suggesting that glutamatergic and peptidergic sensory afferents may be distinct populations.

The role of kappa-opioid receptor activation in mediating antinociception and addiction

The kappa-opioid receptor (KOR), a member of the opioid receptor family, is widely expressed in the central nervous system and peripheral tissues. Substantial evidence has shown that activation of KOR by agonists and endogenous opioid peptides in vivo may produce a strong analgesic effect that is free from the abuse potential and the adverse side effects of mu-opioid receptor (MOR) agonists, such as morphine. In addition, activation of the KOR has also been shown to exert an inverse effect on morphine-induced adverse actions, such as tolerance, reward, and impairment of learning and memory. Therefore, the KOR has received much attention in the effort to develop alternative analgesics to MOR agonists and agents for the treatment of drug addiction. However, KOR agonists also produce several severe undesirable side effects such as dysphoria, water diuresis, salivation, emesis, and sedation in nonhuman primates, which may limit the clinical utility of KOR agonists for pain and drug abuse treatment. This article will review the role of KOR activation in mediating antinociception and addiction. The possible therapeutic application of kappa-agonists in the treatment of pain and drug addiction is also discussed.

Molecular depletion of descending serotonin unmasks its novel facilitatory role in the development of persistent pain

Recent studies indicate that persistent pain after tissue or nerve injury is accompanied by an enhanced net descending facilitatory drive that contributes to an amplification and spread of pain. Although 5-HT-containing neurons in the rostral ventromedial medulla (RVM) provide the major descending serotonergic projection to the spinal cord, it is not clear whether the neurotransmitter 5-HT itself released from RVM-spinal neurons contributes to descending pain modulation. In the present study, we determined the role of the descending 5-HT in rat nocifensive behaviors after persistent pain by selectively depleting functional phenotypes of 5-HT in RVM neurons with regional shRNA interference (RNAi) of tryptophan hydroxylase-2 (Tph-2), the rate-limiting enzyme in the synthesis of neuronal 5-HT. Compared to negative control shRNA, Tph-2 shRNA induced significantly prolonged downregulation of Tph-2 in the RVM and 5-HT in spinal dorsal horn. The 5-HT-depleted rats showed normal pain sensitivity in responses to acute noxious stimulation. However, the same RNAi treatment attenuated formalin-induced spontaneous nocifensive responses and tissue or nerve injury-induced allodynia and hyperalgesia. Furthermore, in control shRNA-treated animals, intra-RVM microinjection of brain-derived neurotrophic factor produced a reversible hyperalgesia, which was completely prevented by Tph-2 RNAi pretreatment. Descending inhibition induced by intra-RVM electrical stimulation, but not microinjection of the mu- or kappa-opioid receptor agonists in control shRNA-treated animals was eliminated in 5-HT-depleted rats. These results indicate that the descending 5-HT from the RVM is an important contributor to pain facilitation during the development of persistent pain, and may not mediate opioid-induced descending inhibition in acute pain.

Dynorphin, stress, and depression

Stress is most often associated with aversive states. It rapidly induces the release of hormones and neuropeptides including dynorphin, which activates kappa opioid receptors (KORs) in the central and peripheral nervous systems. In animal models, many aversive effects of stress are mimicked or exacerbated by stimulation of KORs in limbic brain regions. Although KOR signaling during acute stress may increase physical ability (by producing analgesia) and motivation to escape a threat (by producing aversion), prolonged KOR signaling in response to chronic or uncontrollable stress can lead to persistent expression of behavioral signs that are characteristic of human depressive disorders (i.e., "prodepressive-like" signs). Accumulating evidence suggests that KORs contribute to the progressive amplification (sensitization) of stress-induced behaviors that occurs with repeated exposure to stress. Many of the aversive effects of stress are blocked by KOR antagonists, suggesting that these agents may have potential as therapeutics for stress-related conditions such as depression and anxiety disorders. This review summarizes current data on how KOR systems contribute to the acute (rapid), delayed, and cumulative molecular and behavioral effects of stress. We focus on behavioral paradigms that provide insight on interactions between stress and KOR function within each of these temporal categories. Using a simplified model, we consider the time course and mechanism of KOR-mediated effects in stress and suggest future directions that may be useful in determining whether KOR antagonists exert their therapeutic effects by preventing the development of stress-induced behaviors, the expression of stress-induced behaviors, or both.

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.

Periaqueductal gray neurons project to spinally projecting GABAergic neurons in the rostral ventromedial medulla

The analgesic effects of morphine are mediated, in part, by periaqueductal gray (PAG) neurons that project to the rostral ventromedial medulla (RVM). Although much of the neural circuitry within the RVM has been described, the relationship between RVM neurons and PAG input and spinal output is not known. The objective of this study was to determine whether GABAergic output neurons from the PAG target RVM reticulospinal neurons. Immunocytochemistry and confocal microscopy revealed that PAG neurons project extensively to RVM neurons projecting to the spinal cord, and two-thirds of these reticulospinal neurons appear to be GABAergic (contain GAD67 immunoreactivity). The majority (71%) of PAG fibers that contact RVM reticulospinal GAD67-immunoreactive neurons also contained GAD67 immunoreactivity. Thus, there is an inhibitory projection from PAG to inhibitory RVM reticulospinal neurons. However, there were also PAG projections to the RVM that did not contain GAD67 immunoreactivity. Additional experiments were conducted to determine whether the heterogeneity in this projection can be explained by the electrophysiological character of the RVM target neurons. PAG projections to electrophysiologically defined and juxtacellularly filled ON, OFF, and Neutral cells in the RVM were examined. Similar to the pattern reported above, both GAD67- and non-GAD67-immunoreactive PAG neurons project to RVM ON, OFF, and Neutral cells in the RVM. These inputs include a GAD67-immunoreactive projection to a GAD67-immunoreactive ON cell and non-GAD67 projections to GAD67-immunoreactive OFF cells. This pattern is consistent with PAG neurons producing antinociception by direct excitation of RVM OFF cells and inhibition of ON cells.

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.