Effect of an NMDA receptor antagonist on the wind-up of neurons in the trigeminal oralis subnucleus

Low-Dose Dextromethorphan for the Treatment of Fibromyalgia Pain: Results from a Longitudinal, Single-Blind, Placebo-Controlled Pilot Trial

Objective

Fibromyalgia (FM) is a debilitating chronic pain condition with few treatment options. Central sensitization and neuroinflammation have been forwarded as models of FM pathophysiology, both of which indicate dextromethorphan (DXM) as a potential treatment. DXM is an NMDA-receptor antagonist and microglial modulator with anti-neuroinflammatory properties at low doses. It is available for clinical use but has not been tested as a treatment for FM at low dosages. This study evaluated the effectiveness of DXM in treating FM-associated symptoms.

Methods

In a single-blind, placebo-controlled trial, 14 women meeting the 2010 American College of Rheumatology criteria for FM received a placebo for five weeks, followed by 20 mg DXM for ten weeks, while providing daily symptom reports on a 0–100 scale. Pain and physical activity were the primary and secondary outcomes, respectively. Daily symptom ratings during the last four weeks of placebo were contrasted with ratings during the last four weeks of the active treatment using generalized estimating equations (GEE).

Results

DXM was well tolerated, and treatment adherence was high. Baseline pain was reduced by at least 20% in six participants. Self-reported daily pain and physical activity in the entire cohort were not significantly different between the placebo and DXM conditions, and the primary hypotheses were not supported. Exploratory analyses using the entire placebo and DXM data showed that pain was significantly lower in the DXM condition than in the placebo condition (b=−9.933, p=0.013).

Discussion

A strong clinical effect of DXM was not observed at the 20mg/day dosage.

Signal transduction mechanisms underlying group I mGluR-mediated increase in frequency and amplitude of spontaneous EPSCs in the spinal trigeminal subnucleus oralis of the rat

Group I mGluRs (mGluR1 and 5) pre- and/or postsynaptically regulate synaptic transmission at glutamatergic synapses. By recording spontaneous EPSCs (sEPSCs) in the spinal trigeminal subnucleus oralis (Vo), we here investigated the regulation of glutamatergic transmission through the activation of group I mGluRs. Bath-applied DHPG (10 μM/5 min), activating the group I mGluRs, increased sEPSCs both in frequency and amplitude; particularly, the increased amplitude was long-lasting. The DHPG-induced increases of sEPSC frequency and amplitude were not NMDA receptor-dependent. The DHPG-induced increase in the frequency of sEPSCs, the presynaptic effect being further confirmed by the DHPG effect on paired-pulse ratio of trigeminal tract-evoked EPSCs, an index of presynaptic modulation, was significantly but partially reduced by blockades of voltage-dependent sodium channel, mGluR1 or mGluR5. Interestingly, PKC inhibition markedly enhanced the DHPG-induced increase of sEPSC frequency, which was mainly accomplished through mGluR1, indicating an inhibitory role of PKC. In contrast, the DHPG-induced increase of sEPSC amplitude was not affected by mGluR1 or mGluR5 antagonists although the long-lasting property of the increase was disappeared; however, the increase was completely inhibited by blocking both mGluR1 and mGluR5. Further study of signal transduction mechanisms revealed that PLC and CaMKII mediated the increases of sEPSC in both frequency and amplitude by DHPG, while IP₃ receptor, NO and ERK only that of amplitude during DHPG application. Altogether, these results indicate that the activation of group I mGluRs and their signal transduction pathways differentially regulate glutamate release and synaptic responses in Vo, thereby contributing to the processing of somatosensory signals from orofacial region.

Nociceptive craniofacial muscle primary afferent neurons synapse in both the rostral and caudal brain stem

Limited information is available on muscle afferent neurons with fine fibers despite their presumed participation in musculoskeletal disorders, including temporomandibular disorders. To study these neurons, intracellular recordings were made from the central axons of slowly conducting muscle afferent neurons in anesthetized rats. After intraaxonal impalement, axons were characterized by masseter nerve stimulation, receptive field testing, muscle stretching and intramuscular injection of hypertonic saline. Intracellular recordings were made from 310 axons (conduction velocity: 6.5-60(M)/s, mean = 27.3(M)/s; following frequency: 27-250 Hz, mean = 110Hz). No neurons responded to cutaneous palpation or muscle stretching. Some axons (n = 34) were intracellularly stained with biotinamide. These neurons were classified as group II/III noxious mechanoreceptors because their mechanical threshold exceeded 15 mN, and conduction velocities ranged from 12 to 40.2(M)/s (mean = 25.3(M)/s). Two morphological types were recognized by using an object-based, three-dimensional colocalization methodology to locate synapses. One type (IIIHTM(Vp-Vc)) possessed axon collaterals that emerged along the entire main axon and synapsed in the trigeminal principal sensory nucleus and spinal trigeminal subnuclei oralis (Vo), interpolaris (Vi), and caudalis (Vc). A second type (IIIHTM(Vo-Vc)) possessed axon collaterals that synapsed only in caudal Vo, Vi, and Vc. Our previous studies show that muscle spindle afferent neurons are activated by innocuous stimuli and synapse in the rostral and caudal brain stem; here we demonstrate that nociceptive muscle mechanoreceptor afferent axons also synapse in rostral and caudal brain stem regions. Traditional dogma asserts that the most rostral trigeminal sensory complex exclusively processes innocuous somatosensory information, whereas caudal portions receive nociceptive sensory input; the data reported here do not support this paradigm.

Bidirectional modulation of windup by NMDA receptors in the rat spinal trigeminal nucleus

Activation of afferent nociceptive pathways is subject to activity-dependent plasticity, which may manifest as windup, a progressive increase in the response of dorsal horn nociceptive neurons to repeated stimuli. At the cellular level, N-methyl-d-aspartate (NMDA) receptor activation by glutamate released from nociceptive C-afferent terminals is currently thought to generate windup. Most of the wide dynamic range nociceptive neurons that display windup, however, do not receive direct C-fibre input. It is thus unknown where the NMDA mechanisms for windup operate. Here, using the Sprague-Dawley rat trigeminal system as a model, we anatomically identify a subpopulation of interneurons that relay nociceptive information from the superficial dorsal horn where C-fibres terminate, to downstream wide dynamic range nociceptive neurons. Using in vivo electrophysiological recordings, we show that at the end of this pathway, windup was reduced (24 +/- 6%, n = 7) by the NMDA receptor antagonist AP-5 (2.0 fmol) and enhanced (62 +/- 19%, n = 12) by NMDA (1 nmol). In contrast, microinjections of AP-5 (1.0 fmol) within the superficial laminae increased windup (83 +/- 44%, n = 9), whereas NMDA dose dependently decreased windup (n = 19). These results indicate that NMDA receptor function at the segmental level depends on their precise location in nociceptive neural networks. While some NMDA receptors actually amplify pain information, the new evidence for NMDA dependent inhibition of windup we show here indicates that, simultaneously, others act in the opposite direction. Working together, the two mechanisms may provide a fine tuning of gain in pain.

Co-localization of mu opioid receptor and N-methyl-D-aspartate receptor in the trigeminal dorsal horn

Antagonists acting at the N-methyl-D-aspartate (NMDA) receptor can block the development of tolerance to the analgesic effects of [mu ] opioid receptor (MOR) ligands, such as morphine, and can also enhance the analgesic efficacy of opioids. These findings have led to the hypothesis that interactions between NMDA receptor and MOR ligands may be due to the co-localization of these receptors on neurons in the dorsal horn. We used dual immunogold and immunoperoxidase immunocytochemistry for MOR1 and NMDAR1 to determine the degree of co-localization of these receptors in neurons of the trigeminal dorsal horn. By use of electron microscopy, we found that both receptors were primarily located in dendrites and to a lesser extent in perikarya, axons, axon terminals, and glia. With regard to the degree of co-localization in dendrites, 63% of MOR1-labeled dendrites also contained NMDAR1, whereas 61% of NMDAR1-labeled dendrites also contained MOR1. Most of the dual-labeled profiles (94%) were classified as dendrites, with the remainder being axons, axon terminals, or perikarya. These results suggest that direct interactions between MOR and NMDA receptor ligands are likely mediated through shared dendritic targets in the dorsal horn. Less frequently, we found evidence for modulation of afferents to MOR-containing neurons through presynaptic NMDA receptors.

Central sensitization of nociceptive neurons in trigeminal subnucleus oralis depends on integrity of subnucleus caudalis

Our recent studies have shown that application to the tooth pulp of the inflammatory irritant mustard oil (MO) produces a prolonged (>40 min) "central sensitization" reflected in neuroplastic changes in the mechanoreceptive field (RF) and response properties of nociceptive brain stem neurons in subnuclei oralis (Vo) and caudalis (Vc) of the trigeminal spinal tract nucleus. In view of the previously demonstrated ascending modulatory influence of Vc on Vo, our aim was to determine whether the Vo neuroplastic changes induced by MO application to the tooth pulp depend on an ascending influence from Vc. In chloralose/urethan-anesthetized rats, MO application to the pulp produced significant increases in Vo nociceptive neuronal orofacial RF size and responses to mechanical noxious stimuli that lasted as long as 40-60 min. These changes were not affected by vehicle (saline) microinjected into Vc at 20 min after MO application, but 0.3 microl of a 5 mM CoCl(2) solution microinjected into the ipsilateral Vc produced a reversible blockade of the MO-induced Vo neuroplastic changes. A similar volume and concentration of CoCl(2) solution injected into subnucleus interpolaris of the trigeminal spinal tract nucleus did not affect the MO-induced neuroplastic changes in Vo. These findings indicate that inflammatory pulp-induced central sensitization in Vo is dependent on the functional integrity of Vc.

Low doses of N-methyl-D-aspartate antagonists in superficial laminae of medulla oblongata facilitate wind-up of convergent neurones

In this study, a trigeminal model was used in which high threshold C-fibre-evoked activities of convergent neurones located in the subnucleus oralis of the trigeminal complex are modulated through the superficial part, the substantia gelatinosa, of the subnucleus caudalis. The two subnuclei are located 3 mm apart, therefore, it was possible to inject dizocilpine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, into either the superficial or the deep parts of subnucleus caudalis without interfering with ongoing recording of convergent neurones in subnucleus oralis. A differential NMDA-dependent modulation of wind-up was observed according to the dose and the injection target. (1) The injections of small non-diffusible doses (0.12 microg) of dizocilpine into the superficial part of subnucleus caudalis facilitated wind-up. The effect peaked at 25 min with a mean increase above control of 173+/-31%. Injection (0.5 microg) of either the less active enantiomer dizocilpine or saline into superficial subnucleus caudalis had no significant effect on subnucleus oralis convergent neurones. This suggests that NMDA-dependent interneurones, probably located in substantia gelatinosa of subnucleus caudalis, exert, in normal conditions, an inhibitory control on wind-up of convergent subnucleus oralis neurones. (2) The injection of larger doses (0.5 microg) into the superficial part of subnucleus caudalis induced a predominant inhibitory effect on wind-up. The mean peak effect at 15 min was 46+/-7% compared to control (100%). Small and large doses of dizocilpine injected into the deep part of subnucleus caudalis had a predominant inhibitory effect. The inhibition of wind-up of subnucleus oralis neurones after injection of NMDA receptor antagonists in superficial or deep subnucleus caudalis indicates that wind-up may be due, at least in part, to NMDA activation at synapses that do not involve the recorded convergent neurones.

Superficial and deep convergent nociceptive neurons are differentially affected by N-methyl-D-aspartate applied on the brainstem surface of the rat medullary dorsal horn

The activation of N-methyl-D-aspartate receptors is implicated in the spinal and trigeminal processing of nociceptive information conveyed by convergent (wide dynamic range) neurons and particularly in C-fiber-evoked responses elicited by repetitive and high-intensity electrical stimulation of the neuronal receptive field. In this study, the effects of intrathecal NMDA application on the electrically evoked nociceptive responses of trigeminal subnucleus caudalis convergent neurons have been investigated. The total C-fiber-evoked activity triggered by 30 successive stimuli was divided into two components: the C-fiber input response and the 'wind-up' response. Application of 0.1 microg (in 50 microl) of NMDA evoked a bi-directional effect on the total C-fiber-evoked activity of 19 neurons tested. A significant increase in the total C-fiber-evoked activity was observed 15-25 min after the NMDA application for nine neurons located in superficial laminae II and III. In contrast, a significant decrease in the total C-fiber-evoked activity was observed 5-25 min after the NMDA application for 10 neurons located more deeply, in lamina V. The NMDA-induced modifications seen in the total C-fiber-evoked activity were likely a reflection of a significant increase or decrease in neuronal activity evoked by the C-fiber input rather than wind-up of the responses since the latter was not significantly modified by the NMDA application. These results provide evidence for a possible inhibitory role for NMDA-dependent interneurons of the superficial laminae of the medullary dorsal horn on the nociceptive activity of deep convergent neurons.

Effect of neonatal capsaicin treatment on neural activity in the medullary dorsal horn of neonatal rats evoked by electrical stimulation to the trigeminal afferents: an optical, electrophysiological, and quantitative study

To elucidate which glutamate receptors, NMDA or non-NMDA, have the main role in synaptic transmission via unmyelinated afferents in the trigeminal subnucleus caudalis (the medullary dorsal horn), and to examine the early functional effects of neonatal capsaicin treatment to the subnucleus caudalis, optical recording, field potential recording, and quantitative study using electron micrographs were employed. A medulla oblongata isolated from a rat 5--7 days old was sectioned horizontally 400-microm thick or parasagittally and stained with a voltage-sensitive dye, RH482 or RH795. Single-pulse stimulation with high intensity to the trigeminal afferents evoked optical responses mainly in the subnucleus caudalis. The optical signals were composed of two phases, a fast component followed by a long-lasting component. The spatiotemporal properties of the optical signals were well correlated to those of the field potentials recorded simultaneously. The fast component was eliminated by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 10 microM), while the long-lasting component was not. The latter increased in amplitude under a condition of low Mg(2+) but was significantly reduced by DL-2-amino-5-phosphonovaleric acid (AP5; 30 microM). Neonatal capsaicin treatment also reduced the long-lasting component markedly. In addition, the decreases in the ratio of unmyelinated axons to myelinated axons and in the ratio of unmyelinated axons to Schwann cell subunits of trigeminal nerve roots both showed significant differences (P<0.05, Student's t-test) between the control group and the neonatal capsaicin treatment group. This line of evidence indirectly suggests that synaptic transmission via unmyelinated afferents in the subnucleus caudalis is mediated substantially by NMDA glutamate receptors and documented that neonatal capsaicin treatment induced a functional alteration of the neural transmission in the subnucleus caudalis as well as a morphological alteration of primary afferents within several days after the treatment.

Neuroplasticity Induced by Tooth Pulp Stimulation in Trigeminal Subnucleus Oralis Involves NMDA Receptor Mechanisms

We have recently demonstrated that application of the mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant increases in jaw muscle electromyographic (EMG) activity and neuroplastic changes in trigeminal (V) subnucleus caudalis. Since subnucleus oralis (Vo) as well as caudalis receives projections from molar pulp afferents and is also an integral brain stem relay of afferent input from orofacial structures, we tested whether MO application to the exposed pulp induces neuroplastic changes in oralis neurons and whether microinjection of MK-801, a noncompetitive NMDA antagonist, into the Vo influences the pulp/MO-induced neuroplastic changes in chloralose/urethan-anesthetized rats. Single neuronal activity was recorded in Vo, and neurons classified as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), nociceptive-specific (NS), deep (D), or skin/mucosa and deep (S + D). The spontaneous activity, mechanoreceptive field (RF) size, mechanical threshold, and response to suprathreshold mechanical stimuli applied to the neuronal RF were assessed prior to and throughout a 40- to 60-min period after MO application to the maxillary molar pulp. In animals pretreated with saline microinjection (0.3 μl) into the Vo, MO application to the pulp produced a significant increase in spontaneous activity, expansion of the pinch or deep RF, decrease in the mechanical threshold, and increase in response to suprathreshold mechanical stimuli of the nociceptive (WDR, NS, and S + D) neurons except for those nociceptive neurons having their RF only in the intraoral region. The pulpal application of MO did not produce any significant neuroplastic changes in LTM neurons. Furthermore, in animals pretreated with MK-801 microinjection (3 μg/0.3 μl) into the Vo, MO application to the pulp did not produce any significant changes in the RF and response properties of nociceptive neurons. In other animals pretreated with saline (0.3 μl) or MK-801 (3 μg/0.3 μl) microinjected into the Vo, mineral oil application to the pulp did not produce any significant changes in RF and response properties of nociceptive neurons. These findings indicate that the application of MO to the tooth pulp can induce significant neuroplastic changes in oralis nociceptive neurons and that central NMDA receptor mechanisms may be involved in these neuroplastic changes.

Plateau potential-dependent windup of the response to primary afferent stimuli in rat dorsal horn neurons

In the spinal cord, repetitive stimulation of nociceptive afferent fibres induces a progressive build-up of dorsal horn neuron (DHN) responses. This 'action potential windup' is used as a cellular model of central sensitization to pain. It partly relies on synaptic plasticity, being reduced after blocking NMDA and neurokinin receptors. Using intracellular recordings in a slice preparation of the rat spinal cord, we have analysed the implication of an additional non-synaptic component of windup. Primary afferent fibres were electrically stimulated in the dorsal root. Of 47 responding deep DHNs, 17 (36%) produced action potential windup and afterdischarge during consecutive periods of repeated stimuli (0.4-1 Hz) activating high- (n = 13 neurons) and low-threshold (n = 6 neurons) afferent fibres. When the NMDA receptors were blocked, the rate of windup did not change. In all neurons, there was an absolute correlation between expression of windup and the production of calcium-dependent plateau potentials. Sensitization of the DHN response, similar to the synaptically induced windup, was obtained by repetitive intracellular injection of depolarizing current pulses. This intracellularly induced windup had the same pharmacology as the plateau potential. Synaptically induced windup was also abolished by nifedipine, an L-type calcium-channel blocker. Expression of plateau properties in DHNs is therefore a critical component of windup, operating downstream of synaptic processes. Being associated with calcium influx, generation of plateau potentials could be a link between short-term plasticity and the long-term modification of DHN excitability associated with central sensitization.

The effect of fentanyl, DNQX and MK-801 on dorsal horn neurones responsive to colorectal distension in the anaesthetized rat

Certain dorsal horn neurones respond in a graded manner to noxious colorectal distension (CRD). Morphine inhibits these responses in the spinalized rat, but the role of excitatory amino acids in baseline visceral nociceptive transmission is less clear. This study examines the effect of the mu-opiate receptor agonist fentanyl, and the non-NMDA and NMDA antagonists DNQX and MK-801, respectively, on such responses to CRD in the sodium pentobarbitone-anaesthetized rat. Male rats were prepared for extracellular recording from the lumbosacral spinal cord. 90 neurones responsive to CRD, located throughout the dorsal horn, were classified according to their response duration and latency to 60 mmHg distension, as SL-A (short latency-abrupt; 59%), SL-S (short latency-sustained; 23%), L-L (long-latency; 10%) and Inhib (inhibited; 8%). Convergent cutaneous receptive fields were mapped for 79/90 neurones and classified as LT (low threshold), WDR (wide dynamic range) or HT (high threshold). CRD (20-100 mm Hg) elicited graded responses in most neurones. In 6/6 SL-S neurones, fentanyl (1-8 microg kg-1) dose-dependently inhibited the response to 60 mm Hg CRD, in a naloxone-sensitive manner, with an ID50 value (+/-95% confidence limits) of 2.48 (1.7-3. 7) microg kg-1. In 6/6 SL-A neurones, fentanyl had no significant effect on the response to CRD. DNQX (0.03-3 mg kg-1) produced a dose-dependent inhibition of the response to CRD in 5/5 SL-A neurones, with an ID50 value of 0.32 (0.01-41.1) mg kg-1. MK-801 (0. 03-0.3 mg kg-1) had no significant effect on responses to CRD in 6/6 SL-A neurones. The differential inhibitory effects of fentanyl on two neuronal subtypes may indicate functional differences. In SL-A neurones AMPA/kainate, but not NMDA receptors are involved in mediating baseline nociceptive neurotransmission.

Acute and chronic craniofacial pain: brainstem mechanisms of nociceptive transmission and neuroplasticity, and their clinical correlates

This paper reviews the recent advances in knowledge of brainstem mechanisms related to craniofacial pain. It also draws attention to their clinical implications, and concludes with a brief overview and suggestions for future research directions. It first describes the general organizational features of the trigeminal brainstem sensory nuclear complex (VBSNC), including its input and output properties and intrinsic characteristics that are commensurate with its strategic role as the major brainstem relay of many types of somatosensory information derived from the face and mouth. The VBSNC plays a crucial role in craniofacial nociceptive transmission, as evidenced by clinical, behavioral, morphological, and electrophysiological data that have been especially derived from studies of the relay of cutaneous nociceptive afferent inputs through the subnucleus caudalis of the VBSNC. The recent literature, however, indicates that some fundamental differences exist in the processing of cutaneous vs. other craniofacial nociceptive inputs to the VBSNC, and that rostral components of the VBSNC may also play important roles in some of these processes. Modulatory mechanisms are also highlighted, including the neurochemical substrate by which nociceptive transmission in the VBSNC can be modulated. In addition, the long-term consequences of peripheral injury and inflammation and, in particular, the neuroplastic changes that can be induced in the VBSNC are emphasized in view of the likely role that central sensitization, as well as peripheral sensitization, can play in acute and chronic pain. The recent findings also provide new insights into craniofacial pain behavior and are particularly relevant to many approaches currently in use for the management of pain and to the development of new diagnostic and therapeutic procedures aimed at manipulating peripheral inputs and central processes underlying nociceptive transmission and its control within the VBSNC.

Evidence for subnucleus interpolaris in craniofacial muscle pain mechanisms demonstrated by intramuscular injections with hypertonic saline

The subnucleus interpolaris (Vi) has been identified as a major recipient for trigeminal ganglionic input from jaw muscles, and contains neurons with nociceptive properties similar to those in the subnucleus caudalis (Vc). Therefore, Vi may be another important site for processing craniofacial muscle nociception. The aims of present study were to define functional properties of Vi neurons that receive input from masseter muscle afferents by characterizing their responses to electrical, mechanical, and to chemical stimulation of the muscle. Ninety cells were identified as masseter muscle units in 11 adult cats. Most of these units (79%) received additional inputs from orofacial skin. Following the intramuscular injection of 5% hypertonic saline, 49% of the cells showed a significant modulation of either the resting discharge and/or responses to innocuous mechanical stimulation on their cutaneous receptive fields (RFs). The most common response to saline injection was an induction or facilitation of resting discharge which declined as an exponential decay function, returning to pre-injection level within 3-4 min. Forty-five percent of the muscle units that were tested with mechanical stimulation (13/29) showed a prolonged inhibition of mechanically-evoked responses. In most cases (8/13), the inhibitory response was accompanied by initial facilitation. The observations that Vi contained a population of neurons that receive small diameter muscle afferent inputs, responded to noxious mechanical stimulation on the muscle and to a chemical irritant that is known to produce pain in humans provide compelling evidence for the involvement of Vi in craniofacial muscle pain mechanisms.

Effects of lesions in the trigeminal oralis and caudalis subnuclei on different orofacial nociceptive responses in the rat

Behavioural responses to two different orofacial noxious stimulations were analysed following lesion of spinal trigeminal subnucleus oralis (Sp5O) in the rat. Lesions were obtained by intranuclear microinjections of quinolinic acid (0.4 microliter of 60 nmol/microliter solution). The control groups received microinjection of saline. Noxious stimulation was a subcutaneous injection of formalin into the upper lip or electrical stimulation of the tooth pulp. The measured behavioural responses were duration of rubbing induced by the formalin injection and thresholds of the jaw-opening reflex (JOR), head rotation (HR) and face rubbing (FR) evoked by the pulp stimulation. In addition, formalin injection was also performed in two groups of rats that had received intranuclear injection of quinolinic acid or saline into rostral subnucleus caudalis (Sp5C). Rubbing duration was not significantly modified by the lesion of Sp5O, whereas a significant decrease occurred after the lesion of rostral Sp5C. After the lesion of Sp5O, an increase in the threshold of JOR was observed whereas the thresholds of HR and FR were not significantly modified. These results suggest that Sp5O is not necessary for the processing and relay of nociceptive inputs triggered by intense stimulations of oral and perioral areas. However further experiments are needed to reconcile these results with the relevant data obtained from cell recording experiments which indicate the existence, in Sp5O, of neuronal activities related to the sensory discriminative aspect of intense nociception.