Changes in response properties of rostral ventromedial medulla neurons during prolonged inflammation: modulation by neurokinin-1 receptors

Descending Facilitation of Nociceptive Transmission From the Rostral Ventromedial Medulla Contributes to Hyperalgesia in Mice with Sickle Cell Disease

Sickle cell disease (SCD) is an inherited blood disorder that is associated with acute episodic and chronic pain. Mice with SCD have robust hyperalgesia mediated, in part, by sensitization of spinal dorsal horn neurons. However, underlying mechanisms are not fully understood. Since the rostral ventromedial medulla (RVM) is a major component of descending circuitry that modulates nociceptive transmission in the spinal cord, we examined if the RVM contributes to hyperalgesia in mice with SCD. Injection of lidocaine, but not vehicle, into the RVM eliminated mechanical and heat hyperalgesia in sickle (HbSS-BERK) mice without altering mechanical and heat sensitivity in naïve C57B mice. These data indicate that the RVM contributes to the maintenance of hyperalgesia in mice with SCD. In electrophysiological studies, we determined the changes in response properties of RVM neurons that might contribute to hyperalgesia in sickle mice. Recordings were made from single ON, OFF, and Neutral cells in the RVM of sickle and control (HbAA-BERK) mice. Spontaneous activity and responses of ON, OFF and Neutral cells evoked by heat (50 °C) and mechanical (26 g) stimuli applied to the hind paw were compared between sickle and control mice. Although there were no differences in the proportions of functionally-identified neurons or spontaneous activity between sickle and control mice, evoked responses of ON cells to heat and mechanical stimuli were increased approximately 3-fold in sickle mice as compared to control mice. Thus, the RVM contributes to hyperalgesia in sickle mice via a specific ON cell-dependent descending facilitation of nociceptive transmission.

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.

Neuronal Activities in the Rostral Ventromedial Medulla Associated with Experimental Occlusal Interference-Induced Orofacial Hyperalgesia

The imbalanced conditions of pronociceptive ON-cells and antinociceptive OFF-cells in the rostral ventromedial medulla (RVM) alter nociceptive transmission and play an important role in the development of chronic pain. This study aimed to explore the neuroplastic mechanisms of the RVM ON-cells and OFF-cells in a male rat model of experimental occlusal interference (EOI)-induced nociceptive behavior reflecting orofacial hyperalgesia and in modified models involving EOI removal at early and later stages. We recorded the mechanical head withdrawal thresholds, orofacial operant behaviors, and the activity of identified RVM ON-cells and OFF-cells in these rats. EOI-induced orofacial hyperalgesia could be relieved by EOI removal around postoperative day 3; this effect could be inhibited by intra-RVM microinjection of the κ-opioid receptor agonist U-69593. EOI removal around postoperative day 8 did not relieve the orofacial hyperalgesia, which could, however, be reversed by intra-RVM microinjection of the NK-1 (neurokinin-1) receptor antagonist L-733060. The activity of ON-cells and OFF-cells did not change during both the initial 3 and 6 d of EOI. When EOI was removed on postoperative day 3, OFF-cell responses decreased, contributing to the reversal of hyperalgesia. When EOI lasted for 8 d or was removed on postoperative day 8, spontaneous activity and stimulus-evoked responses of ON-cell increased, contributing to the maintained hyperalgesia. In contrast, when the EOI lasted for 14 d, OFF-cell responses decreased, possibly participating in the maintenance of hyperalgesia with persistent EOI. Our results reveal that adaptive changes in the RVM were associated with orofacial pain following EOI placement and removal.SIGNIFICANCE STATEMENT A considerable proportion of patients experience chronic orofacial pain throughout life despite the therapies given or removal of potential etiologic factors. However, current therapies lack effectiveness because of limited knowledge of the chronicity mechanisms. Using electrophysiological recording, combined with a behavioral test, we found that the prevailing descending facilitation in the rostral ventromedial medulla (RVM) participates in the maintenance of orofacial hyperalgesia following late removal of nociceptive stimuli, while the prevailing descending inhibition from the RVM may contribute to the reversal of orofacial hyperalgesia following early removal of nociceptive stimuli. Thus, variable clinical outcomes of orofacial pain may be associated with descending modulation, and an optimal window of time may exist in the management of chronic orofacial pain.

Astrocytes in the rostral ventromedial medulla contribute to the maintenance of oro-facial hyperalgesia induced by late removal of dental occlusal interference

BACKGROUND

Astrocytes in the rostral ventromedial medulla (RVM) contribute to descending pain modulation, but their role in oro-facial pain induced by persistent experimental dental occlusal interference (PEOI) or following EOI removal (REOI) is unknown.

OBJECTIVE

To explore the involvement of RVM astrocytes in PEOI-induced oro-facial hyperalgesia or its maintenance following REOI.

METHODS

Male rats were randomly assigned into five groups: sham-EOI, postoperative day 6 and 14 of PEOI (PEOI 6 d and PEOI 14 d), postoperative day 6 following REOI on day 3 (REOI 3 d) and postoperative day 14 following REOI on day 8 (REOI 8 d). The nociceptive head withdrawal threshold (HWT) and activities of RVM ON- or OFF-cells were recorded before and after intra-RVM astrocyte gap junction blocker carbenoxolone (CBX) microinjection. RVM astrocytes were labelled immunohistochemically with glial fibrillary acidic protein (GFAP) and analysed semi-quantitatively.

RESULTS

Persistent experimental dental occlusal interference-induced oro-facial hyperalgesia, as reflected in decreased HWTs, was partially inhibited by REOI at day 3 but not at day 8 after EOI placement. Increased GFAP-staining area occurred only in REOI 8 d group in which CBX could inhibit the maintained hyperalgesia; CBX was ineffective in inhibiting hyperalgesia in PEOI 14 d group. OFF-cell activities showed no change, but the spontaneous activity and responses of ON-cells were significantly enhanced that could be suppressed by CBX in REOI 8 d group.

CONCLUSION

Rostral ventromedial medulla astrocytes may not participate in PEOI-induced oro-facial hyperalgesia or hyperalgesia inhibition by early REOI but are involved in the maintenance of oro-facial hyperalgesia by late REOI.

Regular physical activity reduces the percentage of spinally projecting neurons that express mu-opioid receptors from the rostral ventromedial medulla in mice

Introduction

Regular physical activity/exercise is an effective nonpharmacological treatment for individuals with chronic pain. Central inhibitory mechanisms, involving serotonin and opioids, are critical to analgesia produced by regular physical activity. The rostral ventromedial medulla (RVM) sends projections to the spinal cord to inhibit or facilitate nociceptive neurons and plays a key role in exercise-induced analgesia.

Objective

The goal of these studies was to examine if regular physical activity modifies RVM-spinal cord circuitry.

Methods

Male and female mice received Fluoro-Gold placed on the spinal cord to identify spinally projecting neurons from the RVM and the nucleus raphe obscurus/nucleus raphe pallidus, dermorphin-488 into caudal medulla to identify mu-opioid receptors, and were immunohistochemically stained for either phosphorylated-N-methyl-d-aspartate subunit NR1 (p-NR1) to identify excitatory neurons or tryptophan hydroxylase (TPH) to identify serotonin neurons. The percentage of dermorphin-488-positive cells that stained for p-NR1 (or TPH), and the percentage of dermorphin-488-positive cells that stained for p-NR1 (or TPH) and Fluoro-Gold was calculated. Physically active animals were provided running wheels in their cages for 8 weeks and compared to sedentary animals without running wheels. Animals with chronic muscle pain, induced by 2 intramuscular injections of pH 4.0, were compared to sham controls (pH 7.2).

Results

Physically active animals had less mu-opioid-expressing neurons projecting to the spinal cord when compared to sedentary animals in the RVM, but not the nucleus raphe obscurus/nucleus raphe pallidus. No changes were observed for TPH.

Conclusions

These data suggest that regular exercise alters central facilitation so that there is less descending facilitation to result in a net increase in inhibition.

Painful stimulation of a sensitized site in the forearm inhibits ipsilateral trigeminal nociceptive blink reflexes

Exposure to moderate levels of ultraviolet B radiation (UVB) is painless but nevertheless induces an inflammatory response that sensitizes primary afferent nociceptors. Subsequently, heating the UVB-treated site can sensitize spinal nociceptors. We used a repeated-measures design to determine whether heating the UVB-treated site also triggers ipsilateral inhibitory controls. Specifically, a 2-cm diameter site on the forearm of 20 participants was exposed to UVB at twice the minimum erythema dose. 48 h later mechanical and thermal sensitivity had increased at the UVB-treated site, indicating primary hyperalgesia. In addition, sensitivity to blunt pressure had increased in the ipsilateral forehead, implying activation of an ipsilateral supra-spinal pro-nociceptive mechanism. Despite this, the area under the curve of the ipsilateral nociceptive blink reflex decreased when the UVB-treated site was heated to induce moderate pain. Together, these findings suggest that the UVB treatment sensitized primary nociceptive afferents and generated an ipsilateral supra-spinal pro-nociceptive mechanism. In addition, sensitization to heat induced by the UVB treatment strengthened an ipsilateral anti-nociceptive process elicited by heat-pain. Infrequent but enduring discharge of sensitized primary nociceptive afferents, driven by inflammation after UVB exposure, might initiate a lateralized supra-spinal pro-nociceptive influence that heightens awareness of impending harm on the sensitized side of the body. In addition, a lateralized anti-nociceptive response triggered by an intense barrage of nociceptive signals may help to differentiate stronger from weaker sources of pain.

Novel Pharmacological Nonopioid Therapies in Chronic Pain

PURPOSE OF REVIEW

Opioid use and abuse has led to a worldwide opioid epidemic. And while opioids are clinically useful when appropriately indicated, they are associated with a wide range of dangerous side effects and whether they are effective at treating or eliminating chronic pain is controversial. There has long been a need for the development of nonopioid alternative treatments for patients that live in pain, and until recently, only a few effective treatments were available. Today, there are a wide range of nonopioid treatments available including NSAIDs, acetaminophen, corticosteroids, nerve blocks, SSRIs, neurostimulators, and anticonvulsants. However, these treatments are still not entirely effective at treating pain, which has sparked a new exploration of novel nonopioid pharmacotherapies.

RECENT FINDINGS

This manuscript will outline the most recent trends in novel nonopioid pharmacotherapy development including tramadol/dexketoprofen, TrkA inhibitors, tapentadol, opioid agonists, Nektar 181, TRV 130, ßarrestin2, bisphosphonates, antibodies, sodium channel blockers, NMDA antagonists, TRP receptors, transdermal vitamin D, AAK1 kinase inhibition, calcitonin gene-related peptide (CGRP), TRPV4 antagonists, cholecystokinin, delta opioid receptor, neurokinin, and gene therapy. The pharmacotherapies discussed in this manuscript outline promising opioid alternatives which can change the future of chronic pain treatment.

Hyperalgesia and sensitization of dorsal horn neurons following activation of NK-1 receptors in the rostral ventromedial medulla

Neurons in the rostral ventromedial medulla (RVM) project to the spinal cord and are involved in descending modulation of pain. Several studies have shown that activation of neurokinin-1 (NK-1) receptors in the RVM produces hyperalgesia, although the underlying mechanisms are not clear. In parallel studies, we compared behavioral measures of hyperalgesia to electrophysiological responses of nociceptive dorsal horn neurons produced by activation of NK-1 receptors in the RVM. Injection of the selective NK-1 receptor agonist Sar9,Met(O2)11-substance P (SSP) into the RVM produced dose-dependent mechanical and heat hyperalgesia that was blocked by coadministration of the selective NK-1 receptor antagonist L-733,060. In electrophysiological studies, responses evoked by mechanical and heat stimuli were obtained from identified high-threshold (HT) and wide dynamic range (WDR) neurons. Injection of SSP into the RVM enhanced responses of WDR neurons, including identified neurons that project to the parabrachial area, to mechanical and heat stimuli. Since intraplantar injection of capsaicin produces robust hyperalgesia and sensitization of nociceptive spinal neurons, we examined whether this sensitization was dependent on NK-1 receptors in the RVM. Pretreatment with L-733,060 into the RVM blocked the sensitization of dorsal horn neurons produced by capsaicin. c-Fos labeling was used to determine the spatial distribution of dorsal horn neurons that were sensitized by NK-1 receptor activation in the RVM. Consistent with our electrophysiological results, administration of SSP into the RVM increased pinch-evoked c-Fos expression in the dorsal horn. It is suggested that targeting this descending pathway may be effective in reducing persistent pain.NEW & NOTEWORTHY It is known that activation of neurokinin-1 (NK-1) receptors in the rostral ventromedial medulla (RVM), a main output area for descending modulation of pain, produces hyperalgesia. Here we show that activation of NK-1 receptors produces hyperalgesia by sensitizing nociceptive dorsal horn neurons. Targeting this pathway at its origin or in the spinal cord may be an effective approach for pain management.

Neuroplasticity of Supraspinal Structures Associated with Pathological Pain

Peripheral nerve and spinal cord injuries, along with other painful syndromes such as fibromyalgia, diabetic neuropathy, chemotherapeutic neuropathy, trigeminal neuralgia, complex regional pain syndrome, and/or irritable bowel syndrome, cause several neuroplasticity changes in the nervous system along its entire axis affecting the different neuronal nuclei. This paper reviews these changes, focusing on the supraspinal structures that are involved in the modulation and processing of pain, including the periaqueductal gray matter, red nucleus, locus coeruleus, rostral ventromedial medulla, thalamus, hypothalamus, basal ganglia, cerebellum, habenula, primary, and secondary somatosensory cortex, motor cortex, mammillary bodies, hippocampus, septum, amygdala, cingulated, and prefrontal cortex. Hyperexcitability caused by the modification of postsynaptic receptor expression, central sensitization, and potentiation of presynaptic delivery of neurotransmitters, as well as the reduction of inhibitory inputs, changes in dendritic spine, neural circuit remodeling, alteration of gray matter, and upregulation of proinflammatory mediators (e.g., cytokines) by reactivation of astrocytes and microglial cells are the main functional, structural, and molecular neuroplasticity changes observed in the above supraspinal structures, associated with pathological pain. Studying these changes in greater depth may lead to the implementation and improvement of new therapeutic strategies against pathological pain. Anat Rec, 300:1481-1501, 2017. © 2017 Wiley Periodicals, Inc.

Activity correlations between on-like and off-like cells of the rostral ventromedial medulla and simultaneously recorded wide-dynamic-range neurons of the spinal dorsal horn in rats

Considerable evidence supports the notion that on- and off-cells of the rostral ventromedial medulla (RVM) facilitate and depress, respectively, spinal nociceptive transmission. This notion stems from a covariation of on- or off-cell activities and spinal nocifensive reflexes. Such covariation could theoretically be due to their independently responding to a common source, or to an RVM-derived modulation of ventral horn neurons. Here, we tested whether on- and off-cells indeed modulate spinal nociceptive neurons. In deeply anesthetized rats, unitary recordings were simultaneously made from an RVM on-like or off-like cell and a spinal nociceptive neuron that shared a receptive field (RF) at a hind paw. Action potential firing in RVM/spinal neuron pairs was highly correlated, positively for on-like cells and negatively for off-like cells, both during ongoing activity and during application of calibrated noxious pressure to the RF. Microinjection of morphine into RVM induced a correlated decrease in on-like cell/spinal neuron ongoing activity and response to noxious stimulation. RVM morphine induced changes in off-like cell activity that were not correlated with spinal neuronal activity. These results suggest that on-cells exert a positive modulation upon spinal nociceptive neurons, upstream to ventral horn circuits and plausibly at the origin of nociceptive information that eventually reaches the cerebral cortex. On-cells may in this manner contribute to inflammation- and neuropathy-induced increases in withdrawal reflexes. Most significantly, on-cell modulation of nociceptive neurons may be a key factor in clinical pain conditions such as hyperalgesia and allodynia.

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.

Increased neuronal expression of neurokinin-1 receptor and stimulus-evoked internalization of the receptor in the rostral ventromedial medulla of the rat after peripheral inflammatory injury

This study examined possible mechanisms by which Substance P (Sub P) assumes a pronociceptive role in the rostral ventromedial medulla (RVM) under conditions of peripheral inflammatory injury, in this case produced by intraplantar (ipl) injection of complete Freund's adjuvant (CFA). In saline- and CFA-treated rats, neurokinin-1 receptor (NK1R) immunoreactivity was localized to neurons in the RVM. Four days after ipl injection of CFA, the number of NK1R-immunoreactive neurons in the RVM was increased by 30%, and there was a concomitant increase in NK1R-immunoreactive processes in CFA-treated rats. Although NK1R immunoreactivity was increased, tachykinin-1 receptor (Tacr1) mRNA was not increased in the RVM of CFA-treated rats. To assess changes in Sub P release, the number of RVM neurons that exhibited NK1R internalization was examined in saline- and CFA-treated rats following noxious heat stimulation of the hind paws. Only CFA-treated rats that experienced noxious heat stimulation exhibited a significant increase in the number of neurons showing NK1R internalization. These data suggest that tonic Sub P release is not increased as a simple consequence of peripheral inflammation, but that phasic or evoked release of Sub P in the RVM is increased in response to noxious peripheral stimulation in a persistent inflammatory state. These data support the proposal that an upregulation of the NK1R in the RVM, as well as enhanced release of Sub P following noxious stimulation, underlie the pronociceptive role of Sub P under conditions of persistent inflammatory injury.

Activation of rostral ventromedial medulla neurons by noxious stimulation of cutaneous and deep craniofacial tissues

The rostral ventromedial medulla (RVM) projects to the medullary and spinal dorsal horns and is a major source of descending modulation of nociceptive transmission. Traditionally, neurons in the RVM are classified functionally as on, off, and neutral cells on the basis of responses to noxious cutaneous stimulation of the tail or hind paw. On cells facilitate nociceptive transmission, off cells are inhibitory, whereas neutral cells are unresponsive to noxious stimuli and their role in pain modulation is unclear. Classification of RVM neurons with respect to stimulation of craniofacial tissues is not well defined. In isoflurane-anesthetized male rats, RVM neurons first were classified as on (25.5%), off (25.5%), or neutral (49%) cells by noxious pinch applied to the hind paw. Pinching the skin overlying the temporomandibular joint (TMJ) altered the proportions of on (39.2%), off (42.2%), and neutral (19.6%) cells. To assess the response of RVM cells to specialized craniofacial inputs, adenosine triphosphate (ATP; 0.01-1 mM) was injected into the TMJ and capsaicin (0.1%) was applied to the ocular surface. TMJ and ocular surface stimulation also resulted in a reduced proportion of neutral cells compared with hind paw pinch. Dose-effect analyses revealed that on and off cells encoded the intra-TMJ concentration of ATP. These results suggest that somatotopy plays a significant role in the functional classification of RVM cells and support the notion that neutral cells likely are subgroups of on and off cells. It is suggested that a portion of RVM neurons serve different functions in modulating craniofacial and spinal pain conditions.

Loss of neurons in rostral ventromedial medulla that express neurokinin-1 receptors decreases the development of hyperalgesia

It is well known that neurons in the rostral ventromedial medulla (RVM) are involved in descending modulation of nociceptive transmission in the spinal cord. It has been shown that activation of neurokinin-1 receptors (NK-1Rs) in the RVM, which are presumably located on pain facilitating ON cells, produces hyperalgesia whereas blockade of NK-1Rs attenuates hyperalgesia. To obtain a better understanding of the functions of NK-1R expressing neurons in the RVM, we selectively ablated these neurons by injecting the stable analog of substance P (SP), Sar(9),Met(O2)(11)-Substance P, conjugated to the ribosomal toxin saporin (SSP-SAP) into the RVM. Rats received injections of SSP-SAP (1 μM) or an equal volume of 1 μM of saporin conjugated to artificial peptide (Blank-SAP). Stereological analysis of NK-1R- and NeuN-labeled neurons in the RVM was determined 21-24 days after treatment. Withdrawal responses to mechanical and heat stimuli applied to the plantar hindpaw were determined 5-28 days after treatment. Withdrawal responses were also determined before and after intraplantar injection of capsaicin (acute hyperalgesia) or complete Freund's adjuvant (CFA) (prolonged hyperalgesia). The proportion of NK-1R-labeled neurons in the RVM was 8.8 ± 1.3% in naïve rats and 8.1 ± 0.8% in rats treated with Blank-SAP. However, injection of SSP-SAP into the RVM resulted in a 90% decrease in NK-1R-labeled neurons. SSP-SAP did not alter withdrawal responses to mechanical or heat stimuli under normal conditions, and did not alter analgesia produced by morphine administered into the RVM. In contrast, the duration of nocifensive behaviors produced by capsaicin and mechanical and heat hyperalgesia produced by capsaicin and CFA were decreased in rats pretreated with SSP-SAP as compared to those that received Blank-SAP. These data support our earlier studies using NK-1R antagonists in the RVM and demonstrate that RVM neurons that possess the NK-1R do not play a significant role in modulating acute pain or morphine analgesia, but rather are involved in pain facilitation and the development and maintenance of hyperalgesia.

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.