Convergence of cutaneous, muscular and visceral noxious inputs onto ventromedial thalamic neurons in the rat

Thalamus: The 'promoter' of endogenous modulation of pain and potential therapeutic target in pathological pain

More recently, the thalamic mediodorsal (MD) and ventromedial (VM) nuclei have been revealed to be functioned as 'nociceptive discriminator' in discriminating noxious and innocuous peripheral afferents, and exhibits distinct different descending controls of nociception. Of particularly importance, the function of thalamic nuclei in engaging descending modulation of nociception is 'silent' or inactive during the physiological state as well as in condition exposed to insufficient noxious stimulation. Once initiation by sufficient noxious or innocuous C-afferents associated with temporal and spatial summation, the thalamic MD and VM nuclei exhibit salient, different effects: facilitation and inhibition, on noxious mechanically and heat evoked nociception, respectively. Based on series of experimental evidence, we here summarize a novel hypothesis involving thalamic MD and VM nuclei functioned as 'promoter' in initiating descending facilitation and inhibition of pain with specific spatiotemporal characteristics. We further hypothesize that clinical remedy in targeting thalamic VM nucleus by enhancing its activities in recruiting inhibition alone or decreasing thalamic MD nucleus induced facilitation may provide promising way in effectively control of pathological pain.

Effect of acetyl-L-carnitine on hypersensitivity in acute recurrent caerulein-induced pancreatitis and microglial activation along the brain’s pain circuitry

BACKGROUND

Acute pancreatitis (AP) and recurring AP are serious health care problems causing excruciating pain and potentially lethal outcomes due to sepsis. The validated caerulein- (CAE) induced mouse model of acute/recurring AP produces secondary persistent hypersensitivity and anxiety-like behavioral changes for study.

AIM

To determine efficacy of acetyl-L-carnitine (ALC) to reduce pain-related behaviors and brain microglial activation along the pain circuitry in CAE-pancreatitis.

METHODS

Pancreatitis was induced with 6 hly intraperitoneal (i.p.) injections of CAE (50 µg/kg), 3 d a week for 6 wk in male C57BL/6J mice. Starting in week 4, mice received either vehicle or ALC until experiment’s end. Mechanical hyper-sensitivity was assessed with von Frey filaments. Heat hypersensitivity was determined with the hotplate test. Anxiety-like behavior was tested in week 6 using elevated plus maze and open field tests. Microglial activation in brain was quantified histologically by immunostaining for ionized calcium-binding adaptor molecule 1 (Iba1).

RESULTS

Mice with CAE-induced pancreatitis had significantly reduced mechanical withdrawal thresholds and heat response latencies, indicating ongoing pain. Treatment with ALC attenuated inflammation-induced hypersensitivity, but hypersensitivity due to abdominal wall injury caused by repeated intraperitoneal injections persisted. Animals with pancreatitis displayed spontaneous anxiety-like behavior in the elevated plus maze compared to controls. Treatment with ALC resulted in increased numbers of rearing activity events, but time spent in “safety” was not changed. After all the abdominal injections, pancreata were translucent if excised at experiment’s end and opaque if excised on the subsequent day, indicative of spontaneous healing. Post mortem histopathological analysis performed on pancreas sections stained with Sirius Red and Fast Green identified wide-spread fibrosis and acinar cell atrophy in sections from mice with CAE-induced pancreatitis that was not rescued by treatment with ALC. Microglial Iba1 immunostaining was significantly increased in hippocampus, thalamus (intralaminar nuclei), hypothalamus, and amygdala of mice with CAE-induced pancreatitis compared to naïve controls but unchanged in the primary somatosensory cortex compared to naïves.

CONCLUSION

CAE-induced pancreatitis caused increased pain-related behaviors, pancreatic fibrosis, and brain microglial changes. ALC alleviated CAE-induced mechanical and heat hypersensitivity but not abdominal wall injury-induced hypersensitivity caused by the repeated injections.

Differential responses of neurons in the rat caudal ventrolateral medulla to visceral and somatic noxious stimuli and their alterations in colitis

Visceral and somatic types of pain have been reported to manifest crucial differences not only in the experience, but also in their peripheral and central processing. However, the precise neuronal mechanisms that responsible for the modality-specific transmission of pain signals, especially at the supraspinal level, remain unclear. Very little is known also about the potential involvement of such mechanisms in the development of viscero-somatic hyperalgesia. Therefore, in the present study performed on urethane-anesthetized adult male Wistar rats we examined responses of neurons in the caudal ventrolateral medulla (CVLM)-the first site for supraspinal processing of both internal and external pain signals-to visceral (colorectal distension, CRD) and somatic (squeezing of the tail) noxious stimulations and evaluated alterations in response properties of these cells after the induction of colitis. It has been found out that the CVLM of healthy control rats, along with harboring of cells excited by both stimulations (23.7%), contained neurons that were activated by either visceral (31.9%) or somatic noxious stimuli (44.4%). In inflamed animals, the percentages of the visceral and somatic nociceptive cells were decreased (to 18.3% and 34.3%, correspondingly) and the number of bimodal neurons was increased (up to 47.4%); these alterations were associated with substantially enhanced responses of both the modality-specific and convergent CVLM neurons not only to CRD, but also to squeezing of the tail. Under these conditions, visceral and somatic pain stimuli induced similar changes in arterial blood pressure and respiratory rate, whereas in the absence of intestinal inflammation noxious CRD and tail stimulation evoked predominantly divergent autonomic reactions. The data obtained can benefit to a deeper understanding of the neuronal mechanisms that underlie differential supraspinal processing of visceral and somatic noxious stimuli and can potentially contribute to the realization of specific cardiovascular and respiratory accompaniments inherent to a particular type of pain. Therewith, results of the study elucidate colitis-induced alterations in these mechanisms, which may be responsible for the combined development of visceral hypersensitivity and somatic hyperalgesia.

Descending monoaminergic pathways projecting to the spinal defecation center enhance colorectal motility in rats

The central regulating mechanisms of defecation, especially roles of the spinal defecation center, are still unclear. We have shown that monoamines including norepinephrine, dopamine, and serotonin injected into the spinal defecation center cause propulsive contractions of the colorectum. These monoamines are the main neurotransmitters of descending pain inhibitory pathways. Therefore, we hypothesized that noxious stimuli in the colorectum would activate the descending monoaminergic pathways projecting to the spinal defecation center and that subsequently released endogenous monoamine neurotransmitters would enhance colorectal motility. Colorectal motility was measured in rats anesthetized with α-chloralose and ketamine. As a noxious stimulus, capsaicin was administered into the colorectal lumen. To interrupt neuronal transmission in the spinal defecation center, antagonists of norepinephrine, dopamine, and/or serotonin receptors were injected intrathecally at the L6-S1 spinal level, where the spinal defecation center is located. Intraluminal administration of capsaicin, acting on the transient receptor potential vanilloid 1 channel, caused transient propulsive contractions. The effect of capsaicin was abolished by surgical severing of the pelvic nerves or thoracic spinal transection at the T4 level. Capsaicin-induced contractions were blocked by preinjection of D2-like dopamine receptor and 5-hydroxytryptamine subtype 2 and 3 receptor antagonists into the spinal defecation center. We demonstrated that intraluminally administered capsaicin causes propulsive colorectal motility through reflex pathways involving the spinal and supraspinal defecation centers. Our results provide evidence that descending monoaminergic neurons are activated by noxious stimulation to the colorectum, leading to facilitation of colorectal motility. NEW & NOTEWORTHY The present study demonstrates that noxious stimuli in the colorectum activates the descending monoaminergic pathways projecting to the spinal defecation center and that subsequently released endogenous monoamine neurotransmitters, serotonin and dopamine, enhance colorectal motility. Our findings provide a possible explanation of the concurrent appearance of abdominal pain and bowel disorder in irritable bowel syndrome patients. Thus the present study may provide new insights into understanding of mechanisms of colorectal dysfunction involving the central nervous system.

Optical inactivation of the anterior cingulate cortex modulate descending pain pathway in a rat model of trigeminal neuropathic pain created via chronic constriction injury of the infraorbital nerve

Purpose

The anterior cingulate cortex (ACC) plays a critical role in the initiation, development, and maintenance of neuropathic pain. Recently, the effects of optical stimulation on pain have been investigated, but the therapeutic effects of optical stimulation on trigeminal neuralgia (TN) have not been clearly shown. Here, we investigated the effects of optical inhibition of the ACC on TN lesions to determine whether the alleviation of pain affects behavior performance and thalamic neuron signaling.

Materials and methods

TN lesions were established in animals by generating a chronic constriction injury of the infraorbital nerve, and the animals received injections of AAV-hSyn-eNpHR3.0-EYFP or a vehicle (phosphate-buffered saline [PBS]) in the ACC. The optical fiber was fixed into the ipsilateral ACC after the injection of adeno-associated virus plasmids or vehicle. Behavioral testing, consisting of responses to an air puff and cold allodynia, was performed, and thalamic neuronal activity was monitored following optical stimulation in vivo. Optical stimulation experiments were executed in three steps: during pre-light-off, stimulation-light-on, and post-light-off states. The role of the optical modulation of the ACC in response to pain was shown using a combination of optical stimulation and electrophysiological recordings in vivo.

Results

Mechanical thresholds and facial cold allodynia scores were significantly improved in the TN lesion group during optical stimulation compared to those in the control group. Thalamic neuronal activity, consisting of the firing rate (spikes/s) and burst rate (bursts/s), was also decreased during optical stimulation.

Conclusion

Reciprocal optical inhibition of the ACC can alleviate pain-associated behavior and decrease abnormal thalamic sensory neuron activity in the trigeminal neuropathic rat model. The descending pain pathway can modulate thalamic neurons from the ACC following optical stimulation.

Sensory processing of deep tissue nociception in the rat spinal cord and thalamic ventrobasal complex

Sensory processing of deep somatic tissue constitutes an important component of the nociceptive system, yet associated central processing pathways remain poorly understood. Here, we provide a novel electrophysiological characterization and immunohistochemical analysis of neural activation in the lateral spinal nucleus (LSN). These neurons show evoked activity to deep, but not cutaneous, stimulation. The evoked responses of neurons in the LSN can be sensitized to somatosensory stimulation following intramuscular hypertonic saline, an acute model of muscle pain, suggesting this is an important spinal relay site for the processing of deep tissue nociceptive inputs. Neurons of the thalamic ventrobasal complex (VBC) mediate both cutaneous and deep tissue sensory processing, but in contrast to the lateral spinal nucleus our electrophysiological studies do not suggest the existence of a subgroup of cells that selectively process deep tissue inputs. The sensitization of polymodal and thermospecific VBC neurons to mechanical somatosensory stimulation following acute muscle stimulation with hypertonic saline suggests differential roles of thalamic subpopulations in mediating cutaneous and deep tissue nociception in pathological states. Overall, our studies at both the spinal (lateral spinal nucleus) and supraspinal (thalamic ventrobasal complex) levels suggest a convergence of cutaneous and deep somatosensory inputs onto spinothalamic pathways, which are unmasked by activation of muscle nociceptive afferents to produce consequent phenotypic alterations in spinal and thalamic neural coding of somatosensory stimulation. A better understanding of the sensory pathways involved in deep tissue nociception, as well as the degree of labeled line and convergent pathways for cutaneous and deep somatosensory inputs, is fundamental to developing targeted analgesic therapies for deep pain syndromes.

Afferents to anterior cingulate areas 24a and 24b and midcingulate areas 24a' and 24b' in the mouse

Areas 24a and 24b of the anterior cingulate cortex (ACC) play a major role in cognition, emotion and pain. While their connectivity has been studied in primate and in rat, a complete mapping was still missing in the mouse. Here, we analyzed the afferents to the mouse ACC by injecting retrograde tracers in the ventral and dorsal areas of the ACC (areas 24a/b) and of the midcingulate cortex (MCC; areas 24a'/b'). Our results reveal inputs from five principal groups of structures: (1) cortical areas, mainly the orbital, medial prefrontal, retrosplenial, parietal associative, primary and secondary sensory areas and the hippocampus, (2) basal forebrain, mainly the basolateral amygdaloid nucleus, the claustrum and the horizontal limb of the diagonal band of Broca, (3) the thalamus, mainly the anteromedial, lateral mediodorsal, ventromedial, centrolateral, central medial and reuniens/rhomboid nuclei, (4) the hypothalamus, mainly the lateral and retromammillary areas, and (5) the brainstem, mainly the monoaminergic centers. The neurochemical nature of inputs from the diagonal band of Broca and brainstem centers was also investigated by double-labeling, showing that only a part of these afferents were cholinergic or monoaminergic. Comparisons between the areas indicate that areas 24a and 24b receive qualitatively similar inputs, but with different densities. These differences are more pronounced when comparing the inputs to ACC's areas 24a/24b to the inputs to MCC's areas 24a'/24b'. These results provide a complete analysis of the afferents to the mouse areas 24a/24b and 24a'/24b', which shows important similarity with the connectivity of homologous areas in rats, and brings the anatomical basis necessary to address the roles of cingulate areas in mice.

Distinction in the immunoreactivities of two calcium-binding proteins and neuronal birthdates in the first and higher-order somatosensory thalamic nuclei of mice: Evolutionary implications

Comparative embryonic studies are the most effective way to discern phylogenetic changes. To gain insight into the constitution and evolution of mammalian somatosensory thalamic nuclei, we first studied how calbindin (CB) and parvalbumin (PV) immunoreactivities appear during embryonic development in the first-order relaying somatosensory nuclei, i.e., the ventral posteromedial (VPM) and posterolateral (VPL) nuclei, and their neighboring higher-order modulatory regions, including the ventromedial or ventrolateral nucleus, posterior, and the reticular nucleus. The results indicated that cell bodies that were immunoreactive for CB were found earlier (embryonic day 12 [E12]) in the dorsal thalamus than were cells positive for PV (E14), and the adult somatosensory thalamus was characterized by complementary CB and PV distributions with PV dominance in the first-order relaying nuclei and CB dominance in the higher-order regions. We then labeled proliferating cells with [(3) H]-thymidine from E11 to 19 and found that the onset of neurogenesis began later (E12) in the first-order relaying nuclei than in the higher-order regions (E11). Using double-labeling with [(3) H]-thymidine autoradiography and CB or PV immunohistochemistry, we found that CB neurons were born earlier (E11-12) than PV neurons (E12-13) in the studied areas. Thus, similar to auditory nuclei, the first and the higher-order somatosensory nuclei exhibited significant distinctions in CB/PV immunohistochemistry and birthdates during embryonic development. These data, combined with the results of a cladistic analysis of the thalamic somatosensory nuclei, are discussed from an evolutionary perspective of sensory nuclei.

A systematic review of the evidence for central nervous system plasticity in animal models of inflammatory-mediated gastrointestinal pain

BACKGROUND

Abdominal pain frequently accompanies inflammatory disorders of the gastrointestinal tract (GIT), and animal models of GIT inflammation have been developed to explore the role of the central nervous system (CNS) in this process. Here, we summarize the evidence from animal studies for CNS plasticity following GIT inflammation.

METHODS

A systematic review was conducted to identify studies that: (1) used inflammation of GIT organs, (2) assessed pain or visceral hypersensitivity, and (3) presented evidence of CNS involvement. Two hundred and eight articles were identified, and 79 were eligible for analysis.

RESULTS

Rats were most widely used (76%). Most studies used adult animals (42%) with a bias toward males (74%). Colitis was the most frequently used model (78%) and 2,4,6-trinitrobenzenesulfonic acid the preferred inflammatory agent (33%). Behavioral (58%), anatomical/molecular (44%), and physiological (24%) approaches were used alone or in combination to assess CNS involvement during or after GIT inflammation. Measurement times varied widely (<1 h-> 2 wk after inflammation). Blinded outcomes were used in 42% studies, randomization in 10%, and evidence of visceral inflammation in 54%. Only 3 studies fulfilled our criteria for high methodological quality, and no study reported sample size calculations.

CONCLUSIONS

The included studies provide strong evidence for CNS plasticity following GIT inflammation, specifically in the spinal cord dorsal horn. This evidence includes altered visceromotor responses and indices of referred pain, elevated neural activation and peptide content, and increased neuronal excitability. This evidence supports continued use of this approach for preclinical studies; however, there is substantial scope to improve study design.

Perception and pain thresholds for cutaneous heat and cold, and rectal distension: associations and disassociations

BACKGROUND

Hypersensitivity to somatic or visceral pain has been reported in numerous clinical conditions such as fibromyalgia or the irritable bowel syndrome, and general hypersensitivity has been proposed to be the underlying mechanism. However, cross-modal relationships especially between somatic and visceral pain have rarely been investigated even in healthy volunteers. Furthermore, psychological influences on pain have rarely been characterized across modalities.

METHODS

Sixty-one healthy participants underwent testing of perception and pain thresholds for cutaneous thermode heat and cold, as well as for rectal balloon distension. Psychological testing for anxiety, depression, and pain experience (including catastrophizing and coping) as well as cardiac interoception was performed. Measurement quality and the correlations between the different modalities were examined.

KEY RESULTS

Significant correlations existed between the perception thresholds for cold/heat (τB  = -0.28, p = 0.002) and cold/distension (τB  = -0.21, p = 0.03) and for the pain thresholds for cold/heat (r = -0.61, p < 0.001) and heat/distension (r = 0.33, p = 0.01). No association was found between pain thresholds and anxiety, depression, psychological experience with and processing of pain, or cardiac interoception. Retest reliabilities for pain measurements were satisfying after short intertrial intervals (all intraclass correlation coefficients >0.8), but less so after longer intervals. The individuals contributing to the respective correlations differ between measurements.

CONCLUSIONS & INFERENCES

Moderate associations were found for pain thresholds across modalities. The strength of the associations and their stability over time warrants further investigation and might serve to increase the understanding of conditions affecting multiple pain modalities.

Reactive oxygen species are involved in group I mGluR-mediated facilitation of nociceptive processing in amygdala neurons

Recent biochemical and behavioral data implicate reactive oxygen species (ROS) in peripheral and spinal pain mechanisms. However, pain-related functions of ROS in the brain and mechanisms of pain-related ROS activation remain to be determined. Our previous studies showed that the amygdala plays a key role in emotional-affective pain responses and pain modulation. Hyperactivity of amygdala neurons in an animal pain model depends on group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5), but their signaling pathway remains to be determined. Here we tested the hypothesis that activation of group I mGluRs increases nociceptive processing in amygdala neurons through a mechanism that involves ROS. Extracellular single-unit recordings were made from neurons in the laterocapsular division of the central nucleus of the amygdala (CeLC) in anesthetized adult male rats. Administration of a group I mGluR agonist (DHPG) into the CeLC by microdialysis increased the responses to innocuous and noxious somatosensory (knee joint compression) and visceral (colorectal distention [CRD]) stimuli. A ROS scavenger (PBN) and a superoxide dismutase mimetic (TEMPOL) reversed the facilitatory effects of DHPG. An mGluR5 antagonist (MPEP) also inhibited the effects of DHPG on the responses to innocuous and noxious somatosensory and visceral stimuli, whereas an mGluR1 antagonist (LY367385) decreased only the responses to visceral stimulation. The results show for the first time that ROS mediate group I mGluR-induced facilitation of nociceptive processing in amygdala neurons. The antagonist data may suggest differential contributions of subtypes mGluR1 and mGluR5 to the processing of somatosensory and visceral nociceptive information in the amygdala.

Effects of 17beta-estradiol on responses of viscerosomatic convergent thalamic neurons in the ovariectomized female rat

Ovarian hormones have been shown to exert multiple effects on CNS function and viscerosomatic convergent activity. Ovariectomized (OVX) female rats were used in the present study to examine the long-term effects of proestrus levels of 17beta-estradiol (EB) delivered by a 60-day time-released subcutaneous pellet on the response properties of viscerosomatic convergent thalamic neurons. In addition, avoidance thresholds to mechanical stimulation for one of the convergent somatic territories, the trunk, was assessed using an electro-von Frey anesthesiometer before and at the end of the 6-wk post-OVX/implant period prior to the terminal electrophysiological experiments, which were done under urethane anesthesia. Rats implanted with an EB-containing pellet, relative to placebo controls, demonstrated 1) altered thalamic response frequencies and thresholds for cervix and vaginal but not colon stimulation; 2) some response variations for just the lateral group of thalamic subnuclei; and 3) altered thalamic response frequencies and thresholds for trunk stimulation. Thalamic response thresholds for trunk pressure in EB versus placebo rats were consistent with the avoidance thresholds obtained from the same groups. In addition, EB replacement affected visceral and somatic thresholds in opposite ways (i.e., reproductive-related structures were less sensitive to pressure, whereas somatic regions showed increased sensitivity). These results have obvious reproductive advantages (i.e., decreased reproductive organ sensitivity for copulation and increased trunk sensitivity for lordosis posturing), as well as possible clinical implications in women suffering from chronic pelvic pain syndromes and/or neuropathic pain.

Reversible attenuation of neuropathic-like manifestations in rats by lesions or local blocks of the intralaminar or the medial thalamic nuclei

BACKGROUND AND AIM

Thalamic somatosensory nuclei have been classified into medial and lateral systems based on their role in nociception. An imbalance between these two systems may result in abnormal somatic sensations and spontaneous pain. This study aims to investigate the effects of transient or permanent block of the medial and intralaminar nuclear groups on the neuropathic-like behavior in a rat model for mononeuropathy.

METHODS

Neuropathy was induced on one hind paw in different groups of rats following the spared nerve injury model. When the resulting hyperalgesia and allodynia (tactile and cold) reached a maximum plateau, the rats received either chemical or electrolytic lesion or lidocaine (2%) microperfusion, placed in the various thalamic nuclear groups.

RESULTS

All procedures produced transient but significant decrease of neuropathic manifestations. The magnitude and duration of decrease depended on the type and the site of the block. These effects can be ranked in increasing order as follows, electrolytic<chemical<lidocaine micro-perfusion according to the procedure, and as rostro-medial<ventro-median<parafascicular nuclei, according to the site of the block. Thermal hyperalgesia was the most affected while cold allodynia showed the least attenuation. Neuropathic manifestations returned to their pre-lesion levels after 2-3 weeks, along with frequently observed delayed hyper-responsiveness to the hotplate test.

CONCLUSION

The observed results demonstrate the involvement of the medial and intralaminar thalamic nuclei in the processing of neuropathic-like manifestations, and the reversibility of the effects suggests the flexibility of the neural network involved in supraspinal processing of nociceptive information.

Generalized hyperalgesia in patients with chronic tension-type headache

Increased pain sensitivity in the central nervous system may play an important role in the pathophysiology of chronic tension-type headache (CTTH). Previous studies using pain thresholds as a measure of central pain sensitivity have yielded inconsistent results and only a few studies have examined perception of muscle pain without involvement of adjacent tissues. It has been suggested that suprathreshold testing might be more sensitive than threshold measurements in evaluation of central hyperexcitability in CTTH. The aim of the study was to compare pain ratings to suprathreshold single and repetitive (2 Hz) electrical stimulation of muscle and skin in cephalic (temporal and trapezius) and extracephalic (anterior tibial) regions between patients with CTTH and healthy subjects. In addition, we aimed to examine gender differences in pain ratings to suprathreshold stimulation and degree of temporal summation of pain between patients and controls. Pain ratings to both single and repetitive suprathreshold stimulation were higher in patients than in controls in both skin and muscle in all examined cephalic and extracephalic regions (P < 0.04). Pain ratings to both single and repetitive suprathreshold electrical stimulation were significantly higher in female compared with male patients (P < 0.001) and in female compared with male controls (P < or = 0.001). The degree of temporal summation of muscular and cutaneous pain tended to be higher in patients than in controls but the differences were not statistically different. This study provides evidence for generalized increased pain sensitivity in CTTH and strongly suggests that pain processing in the central nervous system is abnormal in this disorder. Furthermore, it indicates that suprathreshold stimulation is more sensitive than recording of pain thresholds for evaluation of generalized pain perception.

The lateral ventromedial thalamic nucleus spreads nociceptive signals from the whole body surface to layer I of the frontal cortex

Neurons within the lateral ventromedial thalamic nucleus (VMl) convey selectively nociceptive information from all parts of the body. The present experiments were performed in rats and were designed to determine the organization of cortical projections from VMl neurons. In a first series of experiments, these cells were characterized electrophysiologically and individually labelled in a Golgi-like manner following juxtacellular electrophoresis of biotin-dextran. In a second experimental series, topical applications of the tracers fluorogold and tetramethylrhodamine-labelled dextran were placed into both the rostral-most and caudal areas of layer I of the dorsolateral frontal cortex, respectively. All VMl nociceptive neurons were fusiform and their full dendritic arborizations were bipolar, extending in the lateromedial axis. VMl cells are thus particularly well located to receive widespread nociceptive inputs via a brainstem link, viz. the medullary subnucleus reticularis dorsalis. VMl neurons driven by 'whole body' nociceptive receptive fields project to the rostral part of the layer I of the dorsolateral frontal cortex. These projections are widespread because double-labelling data showed a great number of VMl neurons labelled from both rostral and caudal dorsolateral cortices. The VMl comprises a homogeneous, organized subset of thalamic neurons that allow any signals of pain to modify cortical activity in a widespread manner, by interacting with the entire layer I of the dorsolateral neocortex.

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.

Segmental inhibition of cutaneous heat sensation and of laser-evoked potentials by experimental muscle pain

The aim of the study was to evaluate the effect of tonic muscle pain evoked by injection of 5% hypertonic saline in the right brachioradialis muscle on the somatosensory sensation of laser-evoked heat pain and laser-evoked potentials. The heat pain pathways were studied in 9 healthy human subjects by recording the scalp potentials evoked by CO(2) laser stimuli delivered on four sites: the skin above the right brachioradialis muscle (ipsilateral local pain), the wrist area where muscle pain was referred in all subjects (ipsilateral referred pain), and two areas on the left arm symmetrical to both local and referred pain (contralateral local pain and contralateral referred pain). Laser-evoked potentials were obtained from 31 scalp electrodes before saline injection, during saline infusion (bolus injection with 0.3 ml saline infused over 20 s, followed by a steady infusion rate of 30 ml/h for the next 25 min), and 20 min after muscle pain had disappeared. While the early N1/P1 component (around 130 ms and 145 ms of latency after stimulation of the skin over the brachioradialis muscle and the wrist, respectively) was not affected by muscle pain, the amplitudes of the later vertex laser-evoked potentials (N2 latency of around 175 ms and 210 ms after stimulation of the skin over the brachioradialis muscle and the wrist, respectively; P2 latency of around 305 ms and 335 ms after stimulation of the skin over the brachioradialis muscle and the wrist, respectively) evoked from ipsilateral local pain, ipsilateral referred pain, and contralateral local pain sites were significantly decreased during muscle pain compared with the baseline recording, while they recovered after pain had disappeared. At the same stimulation sites, the rating of the laser-evoked pain sensation was reduced significantly during muscle pain as compared with the baseline and it recovered after pain had disappeared. On the contrary, muscle pain did not show any effect on both laser-evoked pain and laser-evoked potential amplitude when the contralateral referred pain site was stimulated. The muscle pain inhibitory effect on both heat pain sensation and laser-evoked potential amplitude is probably mediated by an ipsilateral and contralateral segmental mechanism which acts also on the referred pain area, while more general inhibitory mechanisms, such as a distraction effect or a diffuse noxious inhibitory control, are excluded by the absence of any effect of muscle pain on laser-evoked pain and laser-evoked potentials obtained from a remote site, such as the contralateral referred pain area. Since muscle pain induced by hypertonic saline injection is very similar to clinical pain, our results can be useful in understanding the pathophysiology of the somatosensory modifications which can be observed in patients with musculoskeletal pain syndromes.

Thalamic modulation of visceral nociceptive processing in adult rats with neonatal colon irritation

Visceral pain originates from visceral organs in response to a noxious stimulus which, if prolonged, may lead to chronic changes in the neural network mediating visceral nociception. For instance, colon inflammation enhances the responses of neurons in the thalamus to colorectal distension (CRD), whereas lesion in the dorsal column (DC) reverses this neuronal sensitization, suggesting that the thalamus and the DC play major roles in chronic visceral pain. In this study, we used adult rats sensitized with neonatal painful colon irritation to reveal the contribution of the thalamus and the DC to neuronal hyperexcitability in a model of chronic visceral pain. We recorded the responses of lumbosacral neurons to CRD in control rats and in rats with colon irritation following stimulation or inactivation of the thalamus, and after DC lesion. Our results show that, first, neuronal responses to CRD decreased following thalamic stimulation in control rats, whereas, in rats with colon irritation, responses either decreased or increased; second, DC lesion attenuated or enhanced these effects in the positively or in the negatively modulated group of neurons, respectively; third, lidocaine injection in the thalamus reduced the responses to CRD in some of the neurons recorded in rats with colon irritation, but had no effect on those in control rats. Therefore, it is reasonable to speculate that plasticity in rats with colon irritation that may underlie chronic pain is sustained by feedback loops ascending in the DC and engaging the thalamus.