Role for Medullary Pain Facilitating Neurons in Secondary Thermal Hyperalgesia

Pain facilitation and activity-dependent plasticity in pain modulatory circuitry: role of BDNF-TrkB signaling and NMDA receptors

Pain modulatory circuitry in the brainstem exhibits considerable synaptic plasticity. The increased peripheral neuronal barrage after injury activates spinal projection neurons that then activate multiple chemical mediators including glutamatergic neurons at the brainstem level, leading to an increased synaptic strength and facilitatory output. It is not surprising that a well-established regulator of synaptic plasticity, brain-derived neurotrophic factor (BDNF), contributes to the mechanisms of descending pain facilitation. After tissue injury, BDNF and TrkB signaling in the brainstem circuitry is rapidly activated. Through the intracellular signaling cascade that involves phospholipase C, inositol trisphosphate, protein kinase C, and nonreceptor protein tyrosine kinases; N-methyl-D-aspartate (NMDA) receptors are phosphorylated, descending facilitatory drive is initiated, and behavioral hyperalgesia follows. The synaptic plasticity observed in the pain pathways shares much similarity with more extensively studied forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), which typically express NMDA receptor dependency and regulation by trophic factors. However, LTP and LTD are experimental phenomena whose relationship to functional states of learning and memory has been difficult to prove. Although mechanisms of synaptic plasticity in pain pathways have typically not been related to LTP and LTD, pain pathways have an advantage as a model system for synaptic modifications as there are many well-established models of persistent pain with clear measures of the behavioral phenotype. Further studies will elucidate cellular and molecular mechanisms of pain sensitization and further our understanding of principles of central nervous system plasticity and responsiveness to environmental challenge.

Sensitization of pain-modulating neurons in the rostral ventromedial medulla after peripheral nerve injury

Nerve injury can lead to mechanical hypersensitivity in both humans and animal models, such that innocuous touch produces pain. Recent functional studies have demonstrated a critical role for descending pain-facilitating influences from the rostral ventromedial medulla (RVM) in neuropathic pain, but the underlying mechanisms and properties of the relevant neurons within the RVM are essentially unknown. We therefore characterized mechanical responsiveness of physiologically characterized neurons in the RVM after spinal nerve ligation, a model of neuropathic pain that produces robust mechanical hyperalgesia and allodynia. RVM neurons were studied 7-14 d after spinal nerve ligation, and classified as "on-cells," "off-cells," or "neutral cells" using standard criteria of changes in firing associated with heat-evoked reflexes. On-cells are known to promote nociception, and off-cells to suppress nociception, whereas the role of neutral cells in pain modulation remains an open question. Neuronal and behavioral responses to innocuous and noxious mechanical stimulation were tested using calibrated von Frey filaments (4-100 g) applied to the hindpaws ipsilateral and contralateral to the injury, and in sham-operated and unoperated control animals. On- and off-cells recorded in nerve-injured animals exhibited novel responses to innocuous mechanical stimulation, and enhanced responses to noxious mechanical stimulation. Neuronal hypersensitivity in the RVM was correlated with behavioral hypersensitivity. Neutral cells remained unresponsive to cutaneous stimulation after nerve injury. These data demonstrate that both on- and off-cells in the RVM are sensitized to innocuous and noxious mechanical stimuli after nerve injury. This sensitization likely contributes to allodynia and hyperalgesia of neuropathic pain states.

Are opioid-sensitive neurons in the rostral ventromedial medulla inhibitory interneurons?

mu-Opioid agonists frequently activate output neurons in the brain via disinhibition, that is, by inhibiting "secondary cells," which results in disinhibition of "primary cells," considered to be output neurons. Secondary cells are generally presumed to be inhibitory interneurons that serve only to regulate the activity of the output neurons. However, studies of the opioid-sensitive neurons in the rostral ventromedial medulla, a region with a well-documented role in nociceptive modulation, indicate that the opioid-inhibited neurons in this region (termed "on-cells" when recorded in vivo) have a distinct functional role that parallels and opposes the output of the subset of RVM neurons that are activated following opioid administration, the "off-cells." The aim of the present study was to analyze the relative timing of on- and off-cell reflex-related firing in the rostral ventromedial medulla to help determine whether on-cells are likely to function as inhibitory interneurons in this region. On- and off-cells display complementary firing patterns during noxious-evoked withdrawal: off-cells stop firing and on-cells show a burst of activity. If on-cells are inhibitory interneurons mediating the off-cell pause, the on-cells would be expected to begin their reflex-related discharge before the off-cells cease firing. To examine this we recorded activity of on- and off-cell pairs during heat-evoked paw or tail withdrawal in lightly anesthetized rats. For each cell pair, we measured the onsets of the off-cell pause and the on-cell burst. Contrary to what would be expected if on-cells were inhibitory interneurons, off-cells typically ceased firing before on-cells began reflex-related firing, with a mean 481 (+/-69) ms lag between the final off-cell spike and the first on-cell spike. This suggests that on-cells do not mediate the off-cell pause, and points instead to presynaptic mechanisms in opioid-mediated disinhibition of medullary output neurons. These data also support an independent role for on-cells in pain modulation.

Pronociceptive changes in response properties of rostroventromedial medullary neurons in a rat model of peripheral neuropathy

The spared nerve injury (SNI) model of peripheral neuropathy produces a robust and long-lasting hypersensitivity. Previous behavioural studies suggest that brainstem-spinal pathways originating in or relaying through the rostroventromedial medulla (RVM) contribute to neuropathic hypersensitivity. We determined whether SNI induces changes in response properties of RVM neurons that might influence descending modulation of nociception. RVM neurons included in the study were classified into presumably pronociceptive ON-cells and antinociceptive OFF-cells (giving excitatory or inhibitory responses to noxious stimulation, respectively). Spontaneous activity and the response to cold, pinch and colorectal distension were assessed under light anaesthesia in the rat, 1 week and 8 weeks following nerve injury or sham operation. Spontaneous activity was increased 1 week but not 8 weeks after nerve injury in ON-cells but decreased in OFF-cells at both time points. In the SNI group, cold-evoked responses were enhanced particularly in ON-cells, independent of the postoperative time point. Responses of ON-cells to pinch and visceral stimulation were enhanced 8 weeks but not 1 week following nerve injury, whereas OFF-cell responses to pinch or colorectal distension were not changed. The results indicate that SNI induces pronociceptive changes in spontaneous activities of ON-cells and OFF-cells and peripherally evoked responses of ON-cells that vary with the postoperative time point. Increased ON-cell activity and decreased OFF-cell activity in the RVM are likely to enhance spinal nociception in a tonic fashion, whereas increased responses of ON-cells to peripheral stimulation are likely to enhance ascending nociceptive signals by a positive feedback following peripheral noxious stimulation.

Rostral ventromedial medulla control of spinal sensory processing in normal and pathophysiological states

Complex networks of pathways project from various structures in the brain to modulate spinal processing of sensory input in a top-down fashion. The rostral ventromedial medulla (RVM) in the brainstem is one major final common output of this endogenous modulatory system and is involved in the relay of sensory information between the spinal cord and brain. The net output of descending neurons that exert inhibitory and facilitatory effects will determine whether neuronal activity in the spinal cord is increased or decreased. By pharmacologically blocking RVM activity with the local anesthetic lignocaine, and then measuring evoked responses of dorsal horn neurons to a range of applied peripheral stimuli, our aim was to determine the prevailing descending influence operating in normal anesthetized animals and animals with experimental neuropathic pain. The injection of 0.8 microl 2% lignocaine into the RVM caused a reduction in deep dorsal horn neuronal responses to electrical and natural stimuli in 64% of normal animals and in 81% of spinal-nerve-ligated (SNL) animals. In normal animals, responses to noxious input were predominantly reduced, while in SNL animals, reductions in spinal cord activity induced by intra-RVM lignocaine further included responses to non-noxious stimuli. This suggests that in terms of activity at least, if not number, descending facilitations are the predominant RVM influence that impacts the spinal cord in normal animals. Moreover, the increase in the proportion of neurons showing a post-lignocaine reduction in dorsal horn activity in SNL rats suggests that the strength of these facilitatory influences increases after neuropathy. This predominant inhibitory spinal effect following the injection of lignocaine into the RVM may be due to blockade of facilitatory On cells.

Genetic susceptibility and development of chronic non-malignant back pain

Previous data indicate that persistent pain states often involve sensitization within the central nervous system (CNS). Many recently described human genetic variants may affect these central processes. Genetic variability influences both synthesis and function of proteins affecting the plasticity of the CNS. Hence, individual genetic variability may be important to understand the development of many persistent pain conditions including chronic nonmalignant back pain. In this review we argue that genotyping of each patient may be a valuable complement to diagnosis of back disorders. This may be important for future prescription of medicine to individuals predisposed for persistent pain. Increased understanding of genetic variability may also improve multidisciplinary and cognitive-behavioral approaches to management of persistent pain. Translation of this information from the laboratory into clinical application will be important for future prevention as well as treatment of long-lasting non-malignant pain conditions.

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

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

Chronic morphine exposure increases the proportion of on-cells in the rostral ventromedial medulla in rats

Chronic opiate exposure produces tolerance and hypersensitivity to mechanical and thermal stimulation that involves increased pain facilitation from the rostral ventromedial medulla (RVM). The aim of the present study was to determine the effect of sustained systemic morphine exposure on RVM neurons. Three cell types in the RVM have been described: on-cells, off-cells and neutral cells. The activity of on-cells increases in response to noxious stimulation, whereas the activity of off-cells decreases following noxious stimulation. Neutral cells remain relatively unaffected. In lightly anesthetized rats, systematic exploration throughout the RVM using single-unit extracellular recordings was used to examine both the relative proportion and the neuronal properties of the different cell classes in chronic morphine and placebo treated animals. Seven days after implanting either morphine (150 mg, s.c.) or placebo pellets a total of four electrode penetrations through the RVM were made in each animal at identical coordinates along midline. Neuronal responses related to radiant heat-evoked paw withdrawals were recorded. When compared to placebo treated rats, chronic morphine increased the number of on-cells and decreased the number of neutral cells, while the number of off-cells remained unchanged. Chronic morphine exposure had no effect on the spontaneous or heat-evoked discharges in on-, off-, or neutral cells. These results indicate that chronic morphine may sensitize a subpopulation of RVM neurons to noxious stimulation, which would be expected to increase descending facilitation and promote tolerance and chronic morphine-induced paradoxical pain.

NK-1 Receptors Modulate the Excitability of on Cells in the Rostral Ventromedial Medulla

The role of neurokinin-1 (NK-1) receptors in the rostral ventromedial medulla (RVM) was studied using extracellular single-unit recording combined with microiontophoresis. In rats, on- and off-type neurons were identified using noxious heat or mechanical stimuli applied to the tail. Responses evoked by iontophoretic application of N-methyl-d-aspartate (NMDA) were determined before and after intraplantar injection of capsaicin or iontophoretic application of substance P. In off cells, capsaicin produced an extended pause in ongoing activity but did not alter the subsequent spontaneous discharge rate or NMDA-evoked responses. In contrast, spontaneous discharge rates of on cells increased after capsaicin, and their responses to NMDA increased >100% above control values. The increased responses to NMDA after capsaicin were attenuated by iontophoretic application of the selective NK-1 receptor antagonist L-733,060. Similarly to capsaicin, iontophoretic application of the selective NK-1 receptor agonist, [Sar9,Met(O2)11]-substance P (SM-SP), increased the spontaneous discharge rate and NMDA-evoked responses of on cells by >100% of control values. These effects were antagonized by L-733,060. Immunohistochemical studies showed that a subset of neurons in the RVM labeled NK-1 receptors and that nearly all of these neurons were immunoreactive for the NMDAR1 subunit of the NMDA receptor. These results demonstrate that activation of NK-1 receptors in the RVM enhances responses of on cells evoked by NMDA. It is suggested that activation of NK-1 receptors in the RVM and the ensuing sensitization of on cells may contribute to the development of central sensitization and hyperalgesia after tissue injury and inflammation.

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

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