Neuroanatomy of the pain system and of the pathways that modulate pain

Multiple pathways for noxious information in the human spinal cord

To investigate the pathways of noxious information in the spinal cord in humans, we recorded cortical potentials following the stimulation of A-delta fibers using a YAG laser applied to two cutaneous points on the back at the C7 and Th10 level, 4cm to the right of the vertebral spinous process. A multiple source analysis showed that four sources were activated; the primary somatosensory cortex (SI), bilateral parasylvian region (Parasylvian), and cingulate cortex. The activity of the cingulate cortex had two components (N2/P2). The mean peak latencies of the activities obtained by C7 and Th10 stimulation were 166.9 and 186.0 ms (SI), 144.3 and 176.8 ms (contralateral Parasylvian), 152.7 and 185.5 ms (ipsilateral Parasylvian), 186.2 and 215.8 ms (N2), and 303.0 and 332.3 ms (P2). Estimated spinal conduction velocities (CVs) of the respective activities were 16.8, 9.3, 8.7, 10.1 and 10.7 m/s. CV of SI was significantly faster than the others (P<0.05). Therefore, our results suggested that noxious signals were conveyed through at least two distinct pathways of the spinal cord probably reaching distinct groups of thalamic nuclei. Further studies are required to clarify the functional significance of these two pathways.

Desvenlafaxine succinate: A new serotonin and norepinephrine reuptake inhibitor

The purpose of this study was to characterize a new chemical entity, desvenlafaxine succinate (DVS). DVS is a novel salt form of the isolated major active metabolite of venlafaxine. Competitive radioligand binding assays were performed using cells expressing either the human serotonin (5-HT) transporter (hSERT) or norepinephrine (NE) transporter (hNET) with K(i) values for DVS of 40.2 +/- 1.6 and 558.4 +/- 121.6 nM, respectively. DVS showed weak binding affinity (62% inhibition at 100 microM) at the human dopamine (DA) transporter. Inhibition of [3H]5-HT or [3H]NE uptake by DVS for the hSERT or hNET produced IC50 values of 47.3 +/- 19.4 and 531.3 +/- 113.0 nM, respectively. DVS (10 microM), examined at a large number of nontransporter targets, showed no significant activity. DVS (30 mg/kg orally) rapidly penetrated the male rat brain and hypothalamus. DVS (30 mg/kg orally) significantly increased extracellular NE levels compared with baseline in the male rat hypothalamus but had no effect on DA levels using microdialysis. To mimic chronic selective serotonin reuptake inhibitor treatment and to block the inhibitory 5-HT(1A) autoreceptors, a 5-HT(1A) antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclo hexanecarboxamide maleate salt (WAY-100635) (0.3 mg/kg s.c.), was administered with DVS (30 mg/kg orally). 5-HT increased 78% compared with baseline with no additional increase in NE or DA levels. In conclusion, DVS is a new 5-HT and NE reuptake inhibitor in vitro and in vivo that demonstrates good brain-to-plasma ratios, suggesting utility in a variety of central nervous system-related disorders.

Involvement of 5-HT(2) serotonergic receptors of the nucleus raphe magnus and nucleus reticularis gigantocellularis/paragigantocellularis complex neural networks in the antinociceptive phenomenon that follows the post-ictal immobility syndrome

The post-ictal immobility syndrome is followed by a significant increase in the nociceptive thresholds in animals and men. In this interesting post-ictal behavioral response, endogenous opioid peptides-mediated mechanisms, as well as cholinergic-mediated antinociceptive processes, have been suggested. However, considering that many serotonergic descending pathways have been implicated in antinociceptive reactions, the aim of the present work is to investigate the involvement of 5-HT(2)-serotonergic receptor subfamily in the post-ictal antinociception. The analgesia was measured by the tail-flick test in seven or eight Wistar rats per group. Convulsions were followed by statistically significant increase in the tail-flick latencies (TFL), at least for 120 min of the post-ictal period. Male Wistar rats were submitted to stereotaxic surgery for introduction of a guide-cannula in the rhombencephalon, aiming either the nucleus raphe magnus (NRM) or the gigantocellularis complex. In independent groups of animals, these nuclei were neurochemically lesioned with a unilateral microinjection of ibotenic acid (1.0 microg/0.2 microL). The neuronal damage of either the NRM or nucleus reticularis gigantocellularis/paragigantocellularis complex decreased the post-ictal analgesia. Also, in other independent groups, central administration of ritanserin (5.0 microg/0.2 microL) or physiological saline into each of the reticular formation nuclei studied caused a statistically significant decrease in the TFL of seizing animals, as compared to controls, in all post-ictal periods studied. These results indicate that serotonin input-connected neurons of the pontine and medullarly reticular nuclei may be involved in the post-ictal analgesia.

Isolating the modulatory effect of expectation on pain transmission: a functional magnetic resonance imaging study

We use a novel balanced experimental design to specifically investigate brain mechanisms underlying the modulating effect of expected pain intensity on afferent nociceptive processing and pain perception. We used two visual cues, each conditioned to one of two noxious thermal stimuli [ approximately 48 degrees C (high) or 47 degrees C (low)]. The visual cues were presented just before and during application of the noxious thermal stimulus. Subjects reported significantly higher pain when the noxious stimulus was preceded by the high-intensity visual cue. To control for expectancy effects, for one-half of the runs, the noxious thermal stimuli were accompanied by the cue conditioned to the other stimulus. Comparing functional magnetic resonance imaging blood oxygenation level-dependent activations produced by the high and low thermal stimulus intensities presented with the high-intensity visual cue showed significant activations in nociceptive regions of the thalamus, second somatosensory cortex, and insular cortex. To isolate the effect of expectancy, we compared activations produced by the two visual cues presented with the high-intensity noxious thermal stimulus; this showed significant differences in the ipsilateral caudal anterior cingulate cortex, the head of the caudate, cerebellum, and the contralateral nucleus cuneiformis (nCF). We propose that pain intensity expectancy modulates activations produced by noxious stimuli through a distinct modulatory network that converges with afferent nociceptive input in the nCF.

Magnetic resonance spectroscopy detects biochemical changes in the brain associated with chronic low back pain: a preliminary report

Magnetic resonance (MR) spectroscopy is a noninvasive technique that can be used to detect and measure the concentration of metabolites and neurotransmitters in the brain and other organs. We used in vivo (1)H MR spectroscopy in subjects with low back pain compared with control subjects to detect alterations in biochemistry in three brain regions associated with pain processing. A pattern recognition approach was used to determine whether it was possible to discriminate accurately subjects with low back pain from control subjects based on MR spectroscopy. MR spectra were obtained from the prefrontal cortex, anterior cingulate cortex, and thalamus of 32 subjects with low back pain and 33 control subjects without pain. Spectra were analyzed and compared between groups using a pattern recognition method (Statistical Classification Strategy). Using this approach, it was possible to discriminate between subjects with low back pain and control subjects with accuracies of 100%, 99%, and 97% using spectra obtained from the anterior cingulate cortex, thalamus, and prefrontal cortex, respectively. These results demonstrate that MR spectroscopy, in combination with an appropriate pattern recognition approach, is able to detect brain biochemical changes associated with chronic pain with a high degree of accuracy.

Loss of spinal mu-opioid receptor is associated with mechanical allodynia in a rat model of peripheral neuropathy

The present study investigated whether the loss of spinal mu-opioid receptors following peripheral nerve injury is related to mechanical allodynia. We compared the quantity of spinal mu-opioid receptor and the effect of its antagonists, such as naloxone and CTOP, on pain behaviors in two groups of rats that showed extremely different severity of mechanical allodynia 2 weeks following partial injury of tail-innervating nerves. One group (allodynic group) exhibited robust signs of mechanical allodynia after the nerve injury, whereas the other group (non-allodynic group) showed little allodynia despite having suffered the same nerve injury. In addition, we investigated the quantity of spinal mu-opioid receptor and the effect of its antagonists on pain behaviors after the rats had recovered from mechanical allodynia 16 weeks following nerve injury. Immunohistochemical and Western blot analyses at 2 weeks after nerve injury indicated that spinal mu-opioid receptor content was more reduced in the allodynic group compared to the non-allodynic group. Intraperitoneal naloxone (2 mg/kg, i.p.) and intrathecal CTOP (10 microg/rat, i.t.) administration dramatically induced mechanical allodynia in the non-allodynic group. However, as in naïve animals, neither the loss of spinal mu-opioid receptors nor antagonist-induced mechanical allodynia was observed in the rats that had recovered from mechanical allodynia. These results suggest that the loss of spinal mu-opioid receptors following peripheral nerve injury is related to mechanical allodynia.

Increased cholecystokinin release in the rat anterior cingulate cortex during carrageenan-induced arthritis

Several human and animal studies indicate that the anterior cingulate cortex (ACC) plays an important role in the affective component of pain. The neuropeptide cholecystokinin (CCK) is especially abundant in the ACC. CCK has been suggested to be involved in the mediation of anxiety and in the modulation of opioid effects in the spinal cord and medulla oblongata. However, its possible role in pain transmission or modulation in the brain is far less clear. In this study, a model of subchronic inflammatory pain in rats, carrageenan-induced monoarthritis, was used to study the effect of pain on the release of CCK-like immunoreactivity (CCK-LI) in the ACC. Pain-related behaviour quantified by weight bearing and stance scoring, as well as inflammation measured by ankle oedema, was increased for at least 24 h after carrageenan injection with a maximum at 5 h. Using microdialysis in freely moving rats, extracellular concentrations of CCK-LI was measured in the ACC during a time period when the animals showed significant pain behaviour. In animals with carrageenan-induced arthritis, both basal and potassium-evoked release of CCK-LI were significantly increased compared to controls. HPLC analysis of dialysates from the ACC during potassium stimulation showed that the main part of the immunoreactive material was sulphated CCK-8. Because CCK has been implicated in anxiety, we suggest that an altered CCK-ergic activity in the ACC may be of importance for the affective component of pain, but an involvement in the modulation of nociception is also possible.

Antidepressants and cognitive-behavioral therapy for symptom syndromes

Somatic symptoms are common in primary care and clinicians often prescribe antidepressants as adjunctive therapy. There are many possible reasons why this may work, including treating comorbid depression or anxiety, inhibition of ascending pain pathways, inhibition of prefrontal cortical areas that are responsible for "attention" to noxious stimuli, and the direct effects of the medications on the syndrome. There are good theoretical reasons why antidepressants with balanced norepinephrine and serotonin effects may be more effective than those that act predominantly on one pathway, though head-to-head comparisons are lacking. For the 11 painful syndromes review in this article, cognitive-behavioral therapy is most consistently demonstrated to be effective, with various antidepressants having more or less randomized controlled data supporting or refuting effectiveness. This article reviews the randomized controlled trial data for the use of antidepressant and cognitive-behavior therapy for 11 somatic syndromes: irritable bowel syndrome, chronic back pain, headache, fibromyalgia, chronic fatigue syndrome, tinnitus, menopausal symptoms, chronic facial pain, noncardiac chest pain, interstitial cystitis, and chronic pelvic pain. For some syndromes, the data for or against treatment effectiveness is relatively robust, for many, however, the data, one way or the other is scanty.

Adenylyl cyclase type 5 (AC5) is an essential mediator of morphine action

Opioid drugs produce their pharmacological effects by activating inhibitory guanine nucleotide-binding regulatory protein-linked mu, delta, and kappa opioid receptors. One major effector for these receptors is adenylyl cyclase, which is inhibited upon receptor activation. However, little is known about which of the ten known forms of adenylyl cyclase are involved in mediating opioid actions. Here we show that all of the major behavioral effects of morphine, including locomotor activation, analgesia, tolerance, reward, and physical dependence and withdrawal symptoms, are attenuated in mice lacking adenylyl cyclase type 5 (AC5), a form of adenylyl cyclase that is highly enriched in striatum. Furthermore, the behavioral effects of selective mu or delta opioid receptor agonists are lost in AC5-/- mice, whereas the behavioral effects of selective kappa opioid receptor agonists are unaffected. These behavioral data are consistent with the observation that the ability of a mu or delta opioid receptor agonist to suppress adenylyl cyclase activity was absent in striatum of AC5-/- mice. Together, these results establish AC5 as an important component of mu and delta opioid receptor signal transduction mechanisms in vivo and provide further support for the importance of the cAMP pathway as a critical mediator of opioid action.

Origins of tactile vulnerability in high-risk infants

PURPOSE

This study examined the degree to which birthweight, gestational age, fetal drug exposure, hazardous postnatal medical events, and a mother's predisposition toward touch predicted the likelihood of an infant's tactile vulnerability during nursing care.

SUBJECTS

The convenience sample included 99 hospitalized infants in their second week of life and their mothers.

DESIGN

The design was correlational.

METHODS

A standardized observational assessment was used to code the infant's physiological and behavioral responses to touch during 4 different episodes of nursing care. Chart review was performed to collect information regarding fetal drug exposure, birthweight, gestational age, and postnatal medical events. Mothers completed a questionnaire regarding their own comfort with using touch and being touched.

MEASURES

The Tactile Vulnerability Assessment measured infant reactivity to and tolerance for touch during nursing care. The Postnatal Complications Scale identified hazardous medical conditions and treatments experienced by the infant during the first 2 weeks of life, and the Physical Contact Assessment evaluated a mother's predisposition toward touch.

RESULTS

The number of hazardous postnatal medical events experienced by an infant was the primary predictor of tactile vulnerability, accounting for 28% of the variance in the infant's likelihood of developing hyperarousal and distress from touch during care. Illicit fetal drug exposure and maternal predisposition toward touch were also significant predictors.

CONCLUSIONS

The significant relationship of postnatal medical events to tactile vulnerability suggests that it may be associated with the presence of infant pain or injury to the infant's central nervous system. Symptoms of drug withdrawal and the impact of teratogens on arousal regulation may also be implicated in greater distress from touch by infants. In addition, findings suggest the potential for inherited transmission of a vulnerability to touch. Assessment of infant vulnerability, including discussion with parents about their views of the infant's needs regarding touch, may enhance problem identification and early intervention to reduce infant distress and further medical problems.

An essential role for DeltaFosB in the nucleus accumbens in morphine action

The transcription factor DeltaFosB is induced in the nucleus accumbens (NAc) and dorsal striatum by the repeated administration of drugs of abuse. Here, we investigated the role of DeltaFosB in the NAc in behavioral responses to opiates. We achieved overexpression of DeltaFosB by using a bitransgenic mouse line that inducibly expresses the protein in the NAc and dorsal striatum and by using viral-mediated gene transfer to specifically express the protein in the NAc. DeltaFosB overexpression in the NAc increased the sensitivity of the mice to the rewarding effects of morphine and led to exacerbated physical dependence, but also reduced their sensitivity to the analgesic effects of morphine and led to faster development of analgesic tolerance. The opioid peptide dynorphin seemed to be one target through which DeltaFosB produced this behavioral phenotype. Together, these experiments demonstrated that DeltaFosB in the NAc, partly through the repression of dynorphin expression, mediates several major features of opiate addiction.

Retrograde analyses of spinothalamic projections in the macaque monkey: Input to ventral posterior nuclei

The distribution of retrogradely labeled spinothalamic tract (STT) neurons was analyzed in monkeys following variously sized injections of cholera toxin subunit B (CTb) in order to determine whether different STT termination sites receive input from different sets of STT cells. This report focuses on STT input to the ventral posterior lateral nucleus (VPL) and the subjacent ventral posterior inferior nucleus (VPI), where prior anterograde tracing studies identified scattered STT terminal bursts and a dense terminal field, respectively. In cases with small or medium-sized injections in VPL, labeled STT cells were located almost entirely in lamina V (in spinal segments consistent with the mediolateral VPL topography); few cells were labeled in lamina I (<8%) and essentially none in lamina VII. Large and very large injections in VPL produced marked increases in labeling in lamina I, associated first with spread into VPI and next into the posterior part of the ventral medial nucleus (VMpo), and abundant labeling in lamina VII, associated with spread into the ventral lateral (VL) nucleus. Small injections restricted to VPI labeled many STT cells in laminae I and V with an anteroposterior topography. These observations indicate that VPL receives STT input almost entirely from lamina V neurons, whereas VPI receives STT input from both laminae I and V cells, with two different topographic organizations. Together with the preceding observation that STT input to VMpo originates almost entirely from lamina I, these findings provide strong evidence that the primate STT consists of anatomically and functionally differentiable components.

Antinociceptive effect of spinally administered cannabinergic and 2-adrenoceptor drugs on the formalin test in rat: possible interactions

The current experiments were designed to study the antinociceptive effects of intrathecal (i.t.) administration of cannabinoid CB1 receptor and 2-adrenoceptor drugs in the nociceptive processing and also their receptor interactions. Different doses of a cannabinoid receptor agonist, CP 55,940, and an 2-adrenoceptor agonist, clonidine induced a dose-dependent antinociception in both phases of the formalin test.CP 55,940-induced antinociception was reduced by pretreatment of a selective cannabinoid CB1 receptor antagonist, SR 141716A, but not by pretreatment with an 2-adrenoceptor antagonist, yohimbine in both phases of the test. However, yohimbine and SR 141716A attenuated the antinociception induced by clonidine in the early phase but not in the late phase of the test. While SR 141716A by itself did not influence pain behaviour, the reversal effect of clonidine by SR 141716A indicate that clonidine stimulate the release of endocannabinoid(s). In conclusion, our findings may suggest that: (1) spinal cannabinoid and 2-adrenoceptor systems are able to induce antinociception in both phases of formalin test, and (2) the cannabinoid system may be involved in the antinociception induced by adrenoceptors in the early phase.

Intrathecal sensory neuron-specific receptor agonists bovine adrenal medulla 8-22 and (Tyr6)-γ2-msh-6-12 inhibit formalin-evoked nociception and neuronal Fos-like immunoreactivity in the spinal cord of the rat

The finding that sensory neuron-specific G-protein-coupled receptor mRNA is solely expressed in small primary sensory neurons suggests involvement of the receptor in nociceptive modulation. The present study was designed to assess effects of intrathecal administration of bovine adrenal medulla 8-22 and (Tyr6)-gamma2-MSH-6-12, selective sensory neuron-specific receptor agonists, on nocifensive behaviors and expression of spinal c-Fos-like immunoreactivity evoked by intraplantar injection of 2.5% formalin in rats. The agonists were administered 10 min before (pretreatment) and/or after (post-treatment) injection of formalin. Pretreatment with bovine adrenal medulla 8-22 dose-dependently (3, 10 and 30 nmol) decreased time lifting and licking the paw mainly in the second phase. Intrathecal bovine adrenal medulla 8-22 (30 nmol) remarkably suppressed nocifensive behaviors in the first and second phases and the expression of formalin-evoked c-Fos-like immunoreactivity in laminae I-II and V-VI of the spinal dorsal horn at L4-5. Moreover, naloxone (20 microg, intrathecal) failed to antagonize the inhibitory effects of bovine adrenal medulla 8-22. Post-treatment with bovine adrenal medulla 8-22 also exerted inhibition on the second phase behaviors in a dose-dependent manner with a similar efficacy observed in pretreatment groups. Furthermore, post-treatment with (Tyr6)-gamma2-MSH-6-12 (0.5, 1.5 and 5 nmol) also suppressed formalin-evoked nocifensive behaviors in the second phase and c-Fos-like immunoreactivity in the spinal dorsal horn similar with bovine adrenal medulla 8-22. Our results suggest that sensory neuron-specific receptor may play an important role in modulation of spinal nociceptive transmission. This is the first to demonstrate that activation of sensory neuron-specific receptor produces analgesia in the persistent pain model.

Assessment of gastrointestinal sensation--a review

Understanding the mechanisms of symptoms in patients with gastrointestinal disorders remains a great challenge. One of the major problems facing clinicians in this area is the limited information gained from subjective outcome measures commonly used to assess these conditions. To address this, various stimulation and recording techniques, commonly used by neurologists, have been adapted to study gastrointestinal sensory processing. This review article provides an overview of this expanding research area and discusses the advantages and disadvantages of each approach.

Calcium Channels As Therapeutic Targets in Neuropathic Pain

Injury to the nerve can produce changes in dorsal horn function and pain. This facilitated processing may be mediated in part by voltage-sensitive calcium channels. Activation of these channels increases intracellular calcium, thereby mediating transmitter release and activating cascades serving to alter membrane excitability and initiate protein transcription. Molecular techniques reveal the complexity and multiplicity of these channels. At the spinal level, blocking of several of these calcium channels, notably those of the N type, can prominently alter pain behavior. These effects are consistent with the high levels of expression on primary afferents and dorsal horn neurons of these channels. More recently, agents binding to auxiliary subunits such as the alpha2delta of these calcium channels diminish excitability of the membrane without completely blocking channel function. Drugs that bind to this site, highly expressed in the superficial dorsal horn, will diminish neuropathic pain states. Continuing developments in our understanding of these channel functions promises to advance the control of aberrant spinal functions initiated by nerve injury.

PERSPECTIVE

Pharmacologic studies showing the role of spinal voltage-sensitive calcium channels in neuropathic pain models provide evidence suggesting their applicability in human pain states.

Control over brain activation and pain learned by using real-time functional MRI

If an individual can learn to directly control activation of localized regions within the brain, this approach might provide control over the neurophysiological mechanisms that mediate behavior and cognition and could potentially provide a different route for treating disease. Control over the endogenous pain modulatory system is a particularly important target because it could enable a unique mechanism for clinical control over pain. Here, we found that by using real-time functional MRI (rtfMRI) to guide training, subjects were able to learn to control activation in the rostral anterior cingulate cortex (rACC), a region putatively involved in pain perception and regulation. When subjects deliberately induced increases or decreases in rACC fMRI activation, there was a corresponding change in the perception of pain caused by an applied noxious thermal stimulus. Control experiments demonstrated that this effect was not observed after similar training conducted without rtfMRI information, or using rtfMRI information derived from a different brain region, or sham rtfMRI information derived previously from a different subject. Chronic pain patients were also trained to control activation in rACC and reported decreases in the ongoing level of chronic pain after training. These findings show that individuals can gain voluntary control over activation in a specific brain region given appropriate training, that voluntary control over activation in rACC leads to control over pain perception, and that these effects were powerful enough to impact severe, chronic clinical pain.

Control over brain activation and pain learned by using real-time functional MRI

If an individual can learn to directly control activation of localized regions within the brain, this approach might provide control over the neurophysiological mechanisms that mediate behavior and cognition and could potentially provide a different route for treating disease. Control over the endogenous pain modulatory system is a particularly important target because it could enable a unique mechanism for clinical control over pain. Here, we found that by using real-time functional MRI (rtfMRI) to guide training, subjects were able to learn to control activation in the rostral anterior cingulate cortex (rACC), a region putatively involved in pain perception and regulation. When subjects deliberately induced increases or decreases in rACC fMRI activation, there was a corresponding change in the perception of pain caused by an applied noxious thermal stimulus. Control experiments demonstrated that this effect was not observed after similar training conducted without rtfMRI information, or using rtfMRI information derived from a different brain region, or sham rtfMRI information derived previously from a different subject. Chronic pain patients were also trained to control activation in rACC and reported decreases in the ongoing level of chronic pain after training. These findings show that individuals can gain voluntary control over activation in a specific brain region given appropriate training, that voluntary control over activation in rACC leads to control over pain perception, and that these effects were powerful enough to impact severe, chronic clinical pain.

Facilitation of enkephalins-induced delta-opioid behavioral responses by chronic amisulpride treatment

The endogenous opioid system is known to have a great influence on the dopaminergic system. Conversely, blockade of the dopaminergic system in D2 receptor knock-out mice triggers an increase in enkephalin supporting the important physiological relationship between both systems. Therefore, the aim of this study was to investigate whether or not chronic treatment with the specific D2 antagonist amisulpride (20mg/kg, i.p., twice daily for 5 days) could lead to a facilitation of behavioral effects of enkephalins, protected from their enzymatic degradation by the dual inhibitor N-[(R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-oxopropyl]-l-phenylalanine benzyl ester (RB101) (5mg/kg, i.v.) in mice. RB101 induced an increase in locomotor activity, antidepressant-like effects in the forced swim test, and antinociceptive effects in the hot-plate test. Chronic treatment with amisulpride potentiated the action of RB101 and this effect seemed to be restricted to behavioral responses induced by opioids acting on delta-opioid receptors (locomotor activity and forced swim test). This was confirmed by the use of the selective delta-opioid receptor agonist, (+)-4-[alpha-R*)-alpha-((2S*,5R*)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80; 2.5mg/kg, i.p.), and antagonist, naltrindole (5mg/kg, i.p.). Considering the involvement of delta-opioid receptors in mood regulation, the interaction between amisulpride and RB101 could lead to a new therapeutic approach in the treatment of some mood disorders.

Neonatal immune challenge alters nociception in the adult rat

Intense pain or intense peripheral inflammation experienced during development can have pronounced effects upon adult pain sensation. However, little is known about the more commonly encountered mild systemic inflammation, such as that experienced with mild illness. Neonatal exposure to lipopolysaccharide (LPS), an established model of immune system activation, has been shown to affect febrile and cyclooxygenase-2 (COX-2) responses to a similar exposure in adulthood. Adult LPS also elicits a range of sickness behaviours, including enhanced responses to painful stimuli. We, therefore, hypothesized that adult sensation and pain responses could be affected by a neonatal LPS challenge. Male and female Sprague-Dawley rats were administered LPS at postnatal day 14 and were tested in adulthood for nociceptive responses to thermal and mechanical stimuli using, respectively, a plantar test apparatus and von Frey filaments, before and after adult LPS. Expression of dorsal root ganglion and lumbar spinal cord COX-2 was also examined. Animals treated as neonates with saline showed the expected hypersensitivity to painful stimuli after adult LPS as well as enhanced spinal cord COX-2. Neonatally LPS-treated rats, however, showed a significantly different profile. They displayed enhanced baseline nociception and elevated basal spinal cord COX-2 compared with neonatally saline-treated rats. Also, rather than the expected hyperalgesia after adult LPS, no changes in nociceptive responses and a reduction in spinal cord COX-2 expression were observed. These findings have important implications for the understanding of pain and its management and highlight the importance of the neonatal period in the development of pain pathways.

The physiology and processing of pain: a review

Despite the many advances in our understanding of the mechanisms underlying pain processing, pain continues to be a major healthcare problem in the United States. Each day, millions of Americans are affected by both acute and chronic pain conditions, costing in excess of $100 billion for treatment-related costs and lost work productivity. Thus, it is imperative that better treatment strategies be developed. One step toward improving pain management is through increased knowledge of pain physiology. Within the nervous system, there are several pathways that transmit information about pain from the periphery to the brain. There is also a network of pathways that carry modulatory signals from the brain and brainstem that alter the incoming flow of pain information. This article provides a review to the physiology and processing of pain.

Laser evoked potentials and prepulse inhibition of the blink reflex in patients with Wallenberg's syndrome

Spinothalamic tract lesions in patients with Wallenberg's syndrome can be demonstrated by abnormalities in the laser evoked potentials (LEPs) to stimulation of the affected side. However, before reaching the structures generating LEPs, laser stimuli can induce effects at a subcortical level. We examined LEPs and laser-induced prepulse inhibition of the blink reflex in seven patients with Wallenberg's syndrome within a month after the infarct. All patients had abnormally elevated thresholds for temperature and pain sensation, and for pinprick pain induced by laser stimuli, in the affected vs the non-affected side. LEPs to stimulation of the affected side were abnormal because of absent, reduced or delayed responses. However, the same laser stimuli that were unable to induce LEPs generated normal inhibition of the blink reflex response when applied 250ms before a trigeminal nerve electrical stimulus. The percentage inhibition induced in the R2 response of the blink reflex by laser stimulation of the affected side was not different from that induced by stimulation of the non-affected side, or in control subjects. These results are compatible with either a different pathway for prepulse inhibition and evoked potentials or a reduced energy requirement of the sensory input generating prepulse inhibition in comparison to that generating evoked potentials.

Differences between somatosensory-evoked potentials recorded from the ventral posterolateral thalamic nucleus, primary somatosensory cortex and vertex in the rat

Somatosensory-evoked potential (SEP) components recorded over the primary somatosensory cortex (SI) and vertex in the rat within the 10-30 ms latency range were characterised with respect to the anatomy and function of the primary somatosensory pathway. To this aim, these components were compared to SEP components in the similar latency range recorded from the ventral posterolateral thalamic (VPL) nucleus, a nucleus known to be part of the subcortical structure of the primary somatosensory pathway and were described with respect to their stimulus-response characteristics and their response to the mu-opioid agonist fentanyl. The VPL positive (P)11-negative (N)18-P22 and SI P13-N18-P22 differed with respect to peak occurrence (P11 versus P13, respectively) and waveform morphology, but did not differ with respect to stimulus-response characteristics and their response to fentanyl. When compared to the vertex P15-N19-P26, the VPL P11-N18-P22 and SI P13-N18-P22 complex follow a relatively fast acquisition in stimulus intensity-response and were affected significantly less to increasing stimulus frequencies and to fentanyl. These results demonstrated that when compared to the VPL-SEP and SI-SEP, the Vx-SEP was modulated differently by the experimental conditions. It is suggested that this may be related to involvement of neural structures within different functional somatosensory pathways.

Nonpeptidergic Circuits Feel Your Pain

Pain consists of both a sensory component and an affective component. In this issue of Neuron, Braz and colleagues genetically targeted the transneuronal tract tracer wheat germ agglutinin (WGA) to nonpeptidergic nociceptive neurons. They found that these neurons give rise to a specialized multisynaptic circuit that links pain signals in the periphery to limbic/affective regions of the brain.

A brainstem substrate for analgesia elicited by intraoral sucrose

Previous studies demonstrated that nursing or intraoral infusion of certain components of mother's milk (e.g. sugars and fats) produces calming and opiate receptor-dependent analgesia in newborn rats and humans. However, the neural circuitry underlying such analgesia is unknown. The aim of the present study was to specify the central pathways by which taste stimuli engage neural antinociceptive mechanisms. For this purpose, midcollicular transactions were used to investigate the role of the forebrain in analgesia elicited by intraoral infusion of 0.2 M sucrose in neonatal rats. Sucrose-induced analgesia persisted, and was enhanced, following midcollicular transection, indicating that it did not require neural circuits confined to the forebrain. Fos immunohistochemistry was used to identify brainstem neurons activated by a brief (90 s) intraoral infusion of a small volume (90 microl, 0.2M) of sucrose or a salt solution (0.1 M ammonium chloride) in 10-day-old rat pups. Compared with control groups (intact, cannula, distilled water), both sucrose and ammonium chloride induced Fos expression in the rostral nucleus tractus solitarius, the first relay in the ascending gustatory pathway. Sucrose also elicited Fos expression in several brainstem areas associated with centrally mediated analgesia, including the periaqueductal gray and the nucleus raphe magnus. Taken together, these findings demonstrate that analgesia elicited by intraoral sucrose does not require involvement of the forebrain. Intraoral sucrose activates neurons in the periaqueductal gray and nucleus raphe magnus, two key brainstem sites critically involved in descending pain modulation.

Analysis of the dorsal spinal cord synaptic architecture by combined proteome analysis and in situ hybridization

The proteomic analysis of tissue samples is an analytical challenge, because identified gene products not only have to be assigned to subcellular structures, but also to cell subpopulations. We here report a strategy of combined subcellular proteomic profiling and in situ hybridization to assign proteins to subcellular sites in subsets of cells within the dorsal region of rat spinal cord. With a focus on synaptic membranes, which represent a complex membrane protein structure composed of multiple integral membrane proteins and networks of accessory structural proteins, we also compared different two-dimensional gel electrophoresis systems for the separation of the proteins. Using MALDI mass spectrometric protein identification based on peptide mass fingerprints, we identified in total 122 different gene products within the different synaptic membrane subfractions. The tissue structure of the dorsal region of the spinal cord is complex, and different layers of neurons can be distinguished neuroanatomically. Proteomic data combined with an in situ hybridization analysis for the detection of mRNA was used to assign selected gene products, namely the optical atrophy protein OPA-1, the presynaptic cytomatrix protein KIAA0378/CAST1, and the uncharacterized coiled-coil-helix-coiled-coil-helix domain containing protein 3 (hypothetical protein FLJ20420), to cell subsets of the dorsal area of the spinal cord. Most striking, KIAA0378/CAST1 mRNA was found only sparsely within the dorsal horn of the spinal cord, but highly abundant within the dorsal root ganglion. This finding, combined with the identification of KIAA0378/CAST1 within the synaptic membrane fraction of the spinal cord at the protein level, are consistent with the reported presynaptic localization of CAST, predominantly within the tissue we investigated primarily attributable to primary afferent sensory neurons. Our approach may be of use in broader studies to characterize the proteomes of neural tissue.

Enhancement of NMDA receptor phosphorylation of the spinal dorsal horn and nucleus gracilis neurons in neuropathic rats

NR1 is an essential component of functional NMDA receptors and can be activated by phosphorylation. It is suggested that phosphorylation of NR1 (pNR1) contributes to central sensitization after intradermal capsaicin injection. The present study investigates whether increases of spinal pNR1 are correlated to central sensitization and thus pain behaviors in neuropathic pain. Neuropathic rats were produced by L5 spinal nerve ligation, mechanical thresholds of the paw were measured, and then the L4/5 spinal cords and the nucleus gracilis (NG) were removed and immunostained for pNR1. The results showed that the number of pNR1-immunoreactive neurons was significantly increased in the ipsilateral cord, at 3, 7, and 28 days after nerve ligation and these increases coincide with mechanical allodynia. The increase of pNR1-immunoreactive neurons in the NG was observed only at 28 days after the nerve ligation. Western blot analyses confirmed the significant increase of pNR1 protein in spinal dorsal horn after nerve ligation. A protein kinase A inhibitor, H89, moderately reversed mechanical allodynia in 7 day neuropathic rats. Many pNR1-immunoreactive neurons were identified as projection neurons by retrograde tracer. The data suggest that PKA mediated NMDA receptor phosphorylation plays an important role in spinal nerve ligation induced neuropathic pain.

Visceral nociceptive input to the area of the medullary lateral reticular nucleus ascends in the lateral spinal cord

In halothane-anesthetized rats, neurons stereotaxically located in the region of the medullary lateral reticular nucleus (LRN) and responsive to urinary bladder distension (UBD) were characterized using extracellular electrodes. Most neurons excited by UBD were also excited by noxious stimuli applied to bilateral receptive fields comprising at least half of the body surface. These bilateral nociceptive specific (bNS) neurons exhibited graded responses to graded intensities of UBD. Neuronal responses to noxious UBD were highly positively correlated with responses to noxious colorectal distension, suggesting a convergence of visceral sensory information in the area of LRN. Bilateral lateral mid-cervical spinal cord lesions virtually abolished activity of bNS neurons evoked by noxious UBD, while dorsal midline lesions had no significant effect. These data support a role for neurons in the region of the LRN in visceral nociception and implicate traditional lateral spinal cord pain pathways in the transmission of visceral information to caudal ventrolateral medullary structures.

Comparison of responses of ventral posterolateral and posterior complex thalamic neurons in naive rats and rats with hindpaw inflammation: mu-opioid receptor mediated inhibitions

The aim of the present study was to compare the effects of morphine on thalamic neuronal responses in naive rats and rats with carrageenan-induced hindpaw inflammation. Multiple single unit ventral posterolateral (VPL) and posterior complex (Po) activity was recorded and mechanically- (7 g, 14 g, 21 g, 60 g and 80 g) evoked responses of VPL and Po neurones were measured in naive rats and rats with carrageenan (100 microl, 2%)-induced hindpaw inflammation. Effects of systemic (0.5 mg kg(-1)) and intra-thalamic (66 microM, 250 nL) morphine on neuronal responses were determined. Mechanically-evoked (60 g) nociceptive responses of VPL neurones were significantly larger in inflamed rats (29 +/- 4 spikes s(-1)) compared to naive rats (19 +/- 2 spikes s(-1), P < 0.05). Systemic morphine inhibited 7 g-evoked responses of VPL neurones in inflamed (24 +/- 8% control, P < 0.01), but not in naive rats (123 +/- 3% control). Frank noxious-evoked responses of VPL neurones in inflamed rats were less sensitive to the effects of systemic and intra-thalamic morphine, compared to naive rats (P < 0.05 for both). These data provide evidence for altered evoked responses of neurones at the level of VPL, but not at Po, during hindpaw inflammation and suggest that thalamic sites of action contribute to the effects of systemic morphine.

Quantitative analysis of ubiquitin-immunoreactivity in the midbrain periaqueductal gray matter with regard to the causes of death in forensic autopsy

The aim of the present study was to examine Ub-immunoreactivity in the midbrain periaqueductal gray matter (PGM), which is involved in pain processing and modulation, in forensic autopsy cases (n=273) in relation to the causes of death: acute deaths from blunt injuries (n=75), sharp weapon injuries (n=36), fatal asphyxiation (n=22), drownings (n=16: freshwater, n=9; saltwater, n=7), fire fatalities (n=64), poisoning (n=12), hyperthermia (n=5), hypothermia (n=5), delayed deaths from blunt head injury (n=8), acute cardiac deaths (n=24), and acute cerebrovascular strokes (n=6). The Ub-immunoreactivity was clearly observed in the nuclei of the PGM neurons, showing no postmortem interference or age-dependency. A higher value was observed in blunt injuries, fire fatalities and also in saltwater drowning, hyperthermia and delayed head injury deaths. These findings suggest a complicated mechanism for the ubiquitination of PGM neurons, to which multiple factors including the intensity and duration of pains possibly under alert consciousness, traumatic and metabolic neurodegeneration may contribute.

In vivo measurement of noradrenaline in the locus coeruleus of rats during the formalin test: A microdialysis study

The locus coeruleus is involved in the regulation of the sense of pain. To demonstrate the changes in noradrenaline level in the locus coeruleus during the formalin test, a microdialysis probe was implanted into the left locus coeruleus of rats. Formalin was subcutaneously injected into the plantar surface of the right hind paw and pain ratings were recorded. The concentrations of noradrenaline and its metabolite 3-methoxy-4-hydroxyphenylethylenglycol (MHPG) were measured. The results showed an almost four-fold elevation in noradrenaline release in the early phase of the formalin test; levels return to baseline in the late phase. Levels of MHPG changed in a similar fashion.

Electrical stimulation of the anterior cingulate cortex reduces responses of rat dorsal horn neurons to mechanical stimuli

The anterior cingulate cortex (ACC) is involved in the affective and motivational aspect of pain perception. Behavioral studies show a decreased avoidance behavior to noxious stimuli without change in mechanical threshold after stimulation of the ACC. However, as part of the neural circuitry of behavioral reflexes, there is no evidence showing that ACC stimulation alters dorsal horn neuronal responses. We hypothesize that ACC stimulation has two phases: a short-term phase in which stimulation elicits antinociception and a long-term phase that follows stimulation to change the affective response to noxious input. To begin testing this hypothesis, the purpose of this study was to examine the response of spinal cord dorsal horn neurons during stimulation of the ACC. Fifty-eight wide dynamic range spinal cord dorsal horn neurons from adult Sprague-Dawley rats were recorded in response to graded mechanical stimuli (brush, pressure, and pinch) at their respective receptive fields, while simultaneous stepwise electrical stimulations (300 Hz, 0.1 ms, at 10, 20, and 30 V) were applied in the ACC. The responses to brush at control, 10, 20, and 30 V, and recovery were 14.2 +/- 1.4, 12.3 +/- 1.2, 10.9 +/- 1.2, 10.3 +/- 1.1, and 14.1 +/- 1.4 spikes/s, respectively. The responses to pressure at control, 10, 20, and 30 V, and recovery were 39.8 +/- 4.7, 25.6 +/- 3.0, 25.0 +/- 3.0, 21.6 +/- 2.4, and 34.2 +/- 3.7 spikes/s, respectively. The responses to pinch at control, 10, 20, and 30 V, and recovery were 40.7 +/- 3.8, 30.6 +/- 3.1, 27.8 +/- 2.8, 27.2 +/- 3.2, and 37.4 +/- 3.9 spikes/s, respectively. We conclude that electrical stimulation of the ACC induces significant inhibition of the responses of spinal cord dorsal horn neurons to noxious mechanical stimuli. The stimulation-induced inhibition begins to recover as soon as the stimulation is terminated. These results suggest differential short-term and long-term modulatory effects of the ACC stimulation on nociceptive circuits.

Recent advances in the neurophysiology of chronic pain

The chronic pain syndrome patient has become the 'leper' of emergency medicine. There are no emergency medicine guidelines and minimal research into managing this challenging group of patients.

OBJECTIVE

To summarize the recent advances in laboratory research into the development of chronic pain that have relevance to emergency management. When the level of supporting evidence is low, it is imperative that emergency physicians understand the physiology that underpins those expert opinions upon which they base their treatment strategies.

METHODS

Literature was searched via Medline, Cochrane, Cinahl, and PsycINFO from 1996 to 2004, under 'chronic pain and emergency management'. Medline from 1996 was searched for 'chronic pain and prevention', 'chronic pain and emergency' and 'chronic pain'. Bibliographies were manually searched for older keynote articles.

RESULTS

Advances in understanding the biochemical changes of chronic pain are paralleled by lesser known advances in delineation of the corticol processing.

CONCLUSIONS

Drug manipulation causes complex action and reaction in chronic pain. Emergency physicians must also optimize cognitive and behavioural aspects of treatment to successfully manage this systemic disease.

Central neuronal plasticity, low back pain and spinal manipulative therapy

OBJECTIVE

Recent experimental evidence demonstrating neuronal/synaptic plasticity and, in particular, long-term potentiation (LTP) and long-term depression (LTD) in spinal neurons is reviewed. The implications of these studies for possible mechanistic explanations of low back pain and its remediation by spinal manipulative therapy (SMT) are explored. Brief descriptions of LTP and LTD and elaboration of the key roles of calcium, glutamate, and glutamate receptors in LTP/LTD are provided as separate appendices.

DATA SOURCES

The referenced articles regarding LTP/LTD in spinal cord neurons and neuronal plasticity, in general, were identified from accumulated review of the neuroscience literature. Publications cited from chiropractic sources relevant to central neuronal plasticity and LTP/LTD were identified using the Index to Chiropractic Literature and informal review.

STUDY SELECTION

Experimental studies examining LTP/LTD mechanisms in spinal neurons and more general references useful as an introduction to central neuronal plasticity and LTP/LTD are included.Data Extraction Experimental evidence presented in this review has been previously published and illustrates neuronal plasticity from an animal model for low back pain.

DATA SYNTHESIS

Both in vitro and in vivo evidence identifying LTP and LTD in dorsal horn nociceptive neurons is reviewed. Of special interest are studies showing LTP in response to intense noxious stimulation and reports that Adelta-mechanosensitive afferent activation can reverse an existing LTP condition in dorsal horn neurons.

CONCLUSIONS

The potential involvement of LTP in low back pain is discussed and a role for LTD in spinal manipulative therapy is proposed. The need for future studies is identified in the areas of spatial and temporal changes in symptomatology post-SMT of the low back; combining, sequencing, and comparing several therapeutic approaches; and demonstrating LTD in spinal cord neurons post-SMT-like stimulation.

Unilateral spinal nerve ligation leads to an asymmetrical distribution of mast cells in the thalamus of female but not male mice

Mast cells are restricted to the leptomeninges and thalamus of healthy mice. These populations are increased by stress and highly sensitive to reproductive hormones. To examine the influence of nociception, a form of stress, on thalamic mast cells, we ligated the left fifth lumbar spinal nerve of male and female mice to induce hyperalgesia. Two, 7 and 14 days later, mice were killed and thalami examined histologically using toluidine blue stain. The total number of thalamic mast cells was not influenced by ligation of the spinal nerve compared to sham-operation in either female or male mice. However, in females, the percent of thalamic mast cells located on the side of the thalamus contralateral to the ligation was greater on days 2 and 7, coincident with mechanical hyperalgesia. At these times, areas in which mast cells were most dense contralateral to nerve-injury included the posterior (Po) and lateral geniculate (LG) nuclei compared to their symmetrical distribution in sham-operated mice. These data suggest that local nociceptive signals to each side of the thalamus rather than stress hormones influence the location of mast cells during the development of allodynia and hyperalgesia. In addition, both hyperalgesia and mast cell distribution induced by nerve-ligation differ in females compared to males, reflecting a novel neuroimmune response to pain within the CNS.

Propofol suppresses synaptic responsiveness of somatosensory relay neurons to excitatory input by potentiating GABA(A) receptor chloride channels

Propofol is a widely used intravenous general anesthetic. Propofol-induced unconsciousness in humans is associated with inhibition of thalamic activity evoked by somatosensory stimuli. However, the cellular mechanisms underlying the effects of propofol in thalamic circuits are largely unknown. We investigated the influence of propofol on synaptic responsiveness of thalamocortical relay neurons in the ventrobasal complex (VB) to excitatory input in mouse brain slices, using both current- and voltage-clamp recording techniques. Excitatory responses including EPSP temporal summation and action potential firing were evoked in VB neurons by electrical stimulation of corticothalamic fibers or pharmacological activation of glutamate receptors. Propofol (0.6 - 3 microM) suppressed temporal summation and spike firing in a concentration-dependent manner. The thalamocortical suppression was accompanied by a marked decrease in both EPSP amplitude and input resistance, indicating that a shunting mechanism was involved. The propofol-mediated thalamocortical suppression could be blocked by a GABAA receptor antagonist or chloride channel blocker, suggesting that postsynaptic GABAA receptors in VB neurons were involved in the shunting inhibition. GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) were evoked in VB neurons by electrical stimulation of the reticular thalamic nucleus. Propofol markedly increased amplitude, decay time, and charge transfer of GABAA IPSCs. The results demonstrated that shunting inhibition of thalamic somatosensory relay neurons by propofol at clinically relevant concentrations is primarily mediated through the potentiation of the GABAA receptor chloride channel-mediated conductance, and such inhibition may contribute to the impaired thalamic responses to sensory stimuli seen during propofol-induced anesthesia.

Thalamic thermo-algesic transmission: ventral posterior (VP) complex versus VMpo in the light of a thalamic infarct with central pain

The respective roles of the ventral posterior complex (VP) and of the more recently described VMpo (posterior part of the ventral medial nucleus) as thalamic relays for pain and temperature pathways have recently been the subject of controversy. Data we obtained in one patient after a limited left thalamic infarct bring some new insights into this debate. This patient presented sudden right-sided hypesthesia for both lemniscal (touch, vibration, joint position) and spinothalamic (pain and temperature) modalities. He subsequently developed right-sided central pain with allodynia. Projection of 3D magnetic resonance images onto a human thalamic atlas revealed a lesion involving the anterior two thirds of the ventral posterior lateral nucleus (VPL) and, to a lesser extent, the ventral posterior medial (VPM) and inferior (VPI) nuclei. Conversely, the lesion did not extend posterior and ventral enough to concern the putative location of the spinothalamic-afferented nucleus VMpo. Neurophysiological studies showed a marked reduction (67%) of cortical responses depending on dorsal column-lemniscal transmission, while spinothalamic-specific, CO2-laser induced cortical responses were only moderately attenuated (33%). Our results show that the VP is definitely involved in thermo-algesic transmission in man, and that its selective lesion can lead to central pain. However, results also suggest that much of the spino-thalamo-cortical volley elicited by painful heat stimuli does not transit through VP, supporting the hypothesis that a non-VP locus lying more posteriorly in the human thalamus is important for thermo-algesic transmission.

Preclinical neuropharmacology of naratriptan

The basic CNS neuropharmacology of naratriptan is reviewed here. Naratriptan is a second-generation triptan antimigraine drug, developed at a time when CNS activity was thought not to be relevant to its therapeutic effect in migraine. It was, however, developed to be a more lipid-soluble, more readily absorbed and less readily metabolized variant on preexisting triptans and these variations conferred on it a higher CNS profile. Naratriptan is a 5-HT(1B/1D) receptor agonist with a highly selective action on migraine pain and nausea, without significant effect on other pain or even other trigeminal pain. Probable sites of therapeutic action of naratriptan include any or all of: the cranial vasculature; the peripheral terminations of trigeminovascular sensory nerves; the first-order synapses of the trigeminovascular sensory system; the descending pain control system; and the nuclei of the thalamus. Naratriptan may prevent painful dilatation of intracranial vessels or reverse such painful dilatation. Naratriptan can prevent the release of sensory peptides and inhibit painful neurogenic vasodilatation of intracranial blood vessels. At the first order synapse of the trigeminal sensory system, naratriptan can selectively suppress neurotransmission from sensory fibers from dural and vascular tissue, while sparing transmission from other trigeminal fibers, probably through inhibition of neuropeptide transmitter release. In the periaqueductal gray matter and in the nucleus raphe magnus, naratriptan selectively activates inhibitory neurons which project to the trigeminal nucleus and spinal cord and which exert inhibitory influences on trigeminovascular sensory input. Naratriptan has also a therapeutic effect on the nausea of migraine, possibly exerting its action at the level of the nucleus tractus solitarius via the same mechanisms by which it inhibits trigeminovascular nociceptive input. The incidence of naratriptan-induced adverse effects in the CNS is low and it is not an analgesic for pain other than that of vascular headache. In patients receiving selective serotonin uptake inhibitors (SSRIs) naratriptan may cause serotonin syndrome-like behavioral side effects. The mechanism of action involved in the production of behavioral and other CNS side effects of naratriptan is unknown.

Differences between primary somatosensory cortex- and vertex-derived somatosensory-evoked potentials in the rat

The somatosensory-evoked potential (SEP) elicited by high-intensity stimulation potentially provides a reliable indicator of analgesic efficacy since it reflects the level of activation of the nociceptive system. In the present study, components in the 10-30-ms latency range of SEPs recorded over the primary somatosensory cortex (SI-SEPs) and vertex (Vx-SEP) in the rat were characterized and compared. SEPs were elicited by electrical tail-base stimulation, and SI-SEPs and Vx-SEPs were recorded simultaneously. Responses to increasing stimulus intensity and stimulus frequency while awake and responses to bolus injection of fentanyl, thiopental, and ketamine were investigated. The SI-SEP positive component (P) occurring at 12 ms after stimulation (P12) showed a significantly lower intensity threshold and was significantly less affected by increasing stimulus frequency and by administration of the different drugs when compared to the Vx-SEP P15. The fact that a single stimulus modality results in different signal characteristics dependent on the recording site supports the view that different neural mechanisms involved in primary processing of somatosensory information are responsible for the generation of the SI-SEP P12 and Vx-SEP P15, respectively. This differentiation between SI-SEPs and Vx-SEPs potentially has distinct consequences using the SEP to evaluate nociception and analgesia in the rat model.

Alterations in spinal cord gene expression after hindpaw formalin injection

Heme oxygenase type 2 (HO-2) is an enzyme that uses heme as a substrate to produce iron, biliverdin, and carbon monoxide (CO). This enzyme participates in regulation of nociceptive signal transmission in spinal cord tissue. We set out to identify genes undergoing alterations in expression in a model of inflammatory pain and to determine whether HO-2 participates in that regulation. After the hindpaw injection of formalin in mice, we measured changes in expression of immediate early genes including c-fos, c-jun, jun B, nerve growth factor induced genes (NGFI-A and NGFI-B) and activity-related cytoskeletal protein (ARC) using real-time PCR. The mRNA corresponding to these genes increased in abundance in the first hour after formalin injection and then slowly declined. Changes in the abundance of prodynorphin, extracellular signal related kinases (ERK1 and ERK2) and N-methyl-D-aspartate (NMDA) receptor R1 subunit mRNA generally peaked between 8 and 12 hr after formalin injection. In HO-2 null mutant mice, the enhancement of expression was less for all genes studied. We went on to quantify gene expression in superficial dorsal horn tissue using laser capture microdissection followed by RNA amplification and real-time PCR. The results confirmed that the changes in gene expression were occurring in regions of the spinal cord involved in nociceptive processing. We conclude that the hindpaw injection of formalin leads to enhanced early and late expression of many genes in spinal cord dorsal horn tissue, and that this enhancement of expression relies to a degree on the presence of HO-2.

Efficacy of duloxetine, a potent and balanced serotonin-norepinephrine reuptake inhibitor in persistent pain models in rats

5-Hydroxytryptamine (serotonin) (5-HT) and norepinephrine (NE) are implicated in modulating descending inhibitory pain pathways in the central nervous system. Duloxetine is a selective and potent dual 5-HT and NE reuptake inhibitor (SNRI). The ability of duloxetine to antagonize 5-HT depletion in para-chloramphetamine-treated rats was comparable with that of paroxetine, a selective serotonin reuptake inhibitor (SSRI), whereas its ability to antagonize NE depletion in alpha-methyl-m-tyrosine-treated rats was similar to norepinephrine reuptake inhibitors (NRIs), thionisoxetine or desipramine. In this paradigm, duloxetine was also more potent than other SNRIs, including venlafaxine or milnacipran and amitriptyline. Low doses of the SSRI paroxetine or the NRI thionisoxetine alone did not have an effect on late phase paw-licking pain behavior in the formalin model of persistent pain; however, when combined, significantly attenuated this pain behavior. Duloxetine (3-15 mg/kg intraperitoneal) significantly attenuated late phase paw-licking behavior in a dose-dependent manner in the formalin model and was more potent than venlafaxine, milnacipran, and amitriptyline. These effects of duloxetine were evident at doses that did not cause neurologic deficits in the rotorod test. Duloxetine (5-30 mg/kg oral) was also more potent and efficacious than venlafaxine and milnacipran in reversing mechanical allodynia behavior in the L5/L6 spinal nerve ligation model of neuropathic pain. Duloxetine (3-30 mg/kg oral) was minimally efficacious in the tail-flick model of acute nociceptive pain. These data suggest that inhibition of both 5-HT and NE uptake may account for attenuation of persistent pain mechanisms. Thus, duloxetine may have utility in treatment of human persistent and neuropathic pain states.

Role of nociceptin in the modulation of nociception in the arcuate nucleus of rats

Neuropeptide nociceptin/orphanin FQ is the endogenous ligand for the opioid-receptor-like receptor 1 (ORL1), mediating essential functions in the central and peripheral nervous systems. The present study was performed to investigate the role of nociceptin and ORL1 receptor in nociception and morphine-induced antinociception in the arcuate nucleus of hypothalamus in rats. Hindpaw withdrawal latencies (HWL) were measured by hot-plate and Randall Selitto tests. The HWL to both thermal and mechanical stimulation decreased significantly after intra-arcuate nucleus injection of nociceptin in a dose-dependent manner. The effect of nociceptin was blocked significantly by subsequent intra-arcuate nucleus administration of [Nphe(1)]nociceptin(1-13)-NH(2), an ORL1 receptor antagonist. Furthermore, an intra-arcuate nucleus injection of nociceptin dramatically attenuated the antinociceptive effect induced by morphine either injected in the same site or applied intraperitoneally. These results suggest that nociceptin in the arcuate nucleus induces a hyperalgesic effect by acting on ORL1 receptors. The present study also demonstrates an interaction between nociceptin and opioids in the arcuate nucleus of the hypothalamus.