A dorsolateral spinothalamic tract in macaque monkey

Role of noradrenergic and dopaminergic systems in the antinociceptive effect of N-(3-(phenylselanyl)prop-2-yn-1-yl)benzamide in mice

Pain has a negative impact on public health, reducing quality of life. Unfortunately, current treatments are not fully effective and have adverse effects. Therefore, there is a need to develop new analgesic compounds. Due to promising results regarding the antinociceptive effect of N-(3-(phenylselanyl)prop-2-in-1-yl)benzamide (SePB), this study aimed to evaluate the participation of the dopaminergic and noradrenergic systems in this effect in mice, as well as its toxicity. To this, the antagonists sulpiride (D2/D3 receptor antagonist, 5 mg/kg), SCH-23390 (D1 receptor antagonist, 0.05 mg/kg), prazosin (α1 adrenergic receptor antagonist, 0.15 mg/kg), yohimbine (α2-adrenergic receptors, 0.15 mg/kg) and propranolol (non-selective β-adrenergic antagonist, 10 mg/kg) were administered intraperitoneally to mice 15 min before SePB (10 mg/kg, intragastrically), except for propranolol (20 min). After 26 min of SePB administration, the open field test was performed for 4 min to assess locomotor activity, followed by the tail immersion test to measure the nociceptive response. For the toxicity test, animals received a high dose of 300 mg/kg of SePB. SePB showed an increase in the latency for nociceptive response in the tail immersion test, and this effect was prevented by SCH-23390, yohimbine and propranolol, indicating the involvement of D1, α2 and β-adrenergic receptors in the antinociceptive mechanism of the SePB effect. No changes were observed in the open field test, and the toxicity assessment suggested that SePB has low potential to induce toxicity. These findings contribute to understanding SePB's mechanism of action, with a focus on the development of new alternatives for pain treatment.

Hierarchical models of pain: Inference, information-seeking, and adaptive control

Computational models of pain consider how the brain processes nociceptive information and allow mapping neural circuits and networks to cognition and behaviour. To date, they have generally have assumed two largely independent processes: perceptual inference, typically modelled as an approximate Bayesian process, and action control, typically modelled as a reinforcement learning process. However, inference and control are intertwined in complex ways, challenging the clarity of this distinction. Here, we consider how they may comprise a parallel hierarchical architecture that combines inference, information-seeking, and adaptive value-based control. This sheds light on the complex neural architecture of the pain system, and takes us closer to understanding from where pain 'arises' in the brain.

Somatotopy and Organization of Spinothalamic Tracts in the Human Cervical Spinal Cord

BACKGROUND

Understanding spinothalamic tract anatomy may improve lesioning and outcomes in patients undergoing percutaneous cordotomy.

OBJECTIVE

To investigate somatotopy and anatomical organization of spinothalamic tracts in the human cervical spinal cord.

METHODS

Patients with intractable cancer pain undergoing cordotomy underwent preoperative and postoperative quantitative sensory testing for sharp pain and heat pain on day 1 and 7 after cordotomy. Intraoperative sensory stimulation was performed with computed tomography (CT) imaging to confirm the location of the radiofrequency electrode during cordotomy. Postoperative magnetic resonance (MR) imaging was performed to define the location of the lesion.

RESULTS

Twelve patients were studied, and intraoperative sensory stimulation combined with CT imaging revealed a somatotopy where fibers from the legs were posterolateral to fibers from the hand. Sharpness detection thresholds were significantly elevated in the area of maximum pain on postoperative day 1 (P = .01). Heat pain thresholds for all areas were not elevated significantly on postoperative day 1, or postoperative day 7. MR imaging confirmed that the cordotomy lesion was in the anterolateral quadrant, and in this location the lesion had a sustained effect on sharp pain but a transient impact on heat pain.

CONCLUSION

In the high cervical spinal cord, spinothalamic fibers mediating sharp pain for the arms are located ventromedial to fibers for the legs, and these fibers are spatially distinct from fibers that mediate heat pain.

The peripheral and central mechanisms underlying itch

Itch is one of the most distressing sensations that substantially impair quality of life. It is a cardinal symptom of many skin diseases and is also caused by a variety of systemic disorders. Unfortunately, currently available itch medications are ineffective in many chronic itch conditions, and they often cause undesirable side effects. To develop novel therapeutic strategies, it is essential to identify primary afferent neurons that selectively respond to itch mediators as well as the central nervous system components that process the sensation of itch and initiate behavioral responses. This review summarizes recent progress in the study of itch, focusing on itch-selective receptors, signaling molecules, neuronal pathways from the primary sensory neurons to the brain, and potential decoding mechanisms based on which itch is distinguished from pain. [BMB Reports 2016; 49(9): 474-487]

A novel animal model of graded neuropathic pain: utility to investigate mechanisms of population heterogeneity

The mechanisms underlying neuropathic pain are not well understood, resulting in unsatisfactory treatment outcomes for many patients. Animal models underpin much of the current understanding of pain mechanisms due to their perceived ability to mimic pain hypersensitivities; however, are limited by their binomial approach (pain vs. control), which does not reflect the clinical heterogeneity in nociceptive hypersensitivity. We modified the chronic constriction injury model by varying the number of sciatic nerve chromic gut sutures. Each Sprague Dawley rat received 4 pieces of chromic gut to control for the inflammatory challenge posed by the gut. Treatment groups were neuronal sutures (N), subcutaneous sutures (S) N0S0, N0S4, N1S3, N2S2 and N4S0. At postoperative (PO) day 29, there was a 'dose-response' relationship between the number of perineural sutures and von Frey threshold (N0S4<N1S3<N2S2<N4S0, P<0.05). This graded model was applied to investigate lumbar dorsal spinal cord glial activation marker expression. Microglial CD11b expression was positively correlated with graded allodynia in the ipsilateral dorsal horn (P<0.05, r(2)>0.9) and associated in the dorsolateral funiculus (DLF; P=0.10, r(2)>0.8) at PO day 14. Astrocyte GFAP expression was positively associated with graded allodynia in the ipsilateral dorsal horn (P=0.18, r(2)>0.6) and ipsilateral DLF (P<0.05, r(2)>0.9). DLF glial activation may represent a contributor to contralateral pain. Our novel graded model has a dynamic range, allowing sensitive detection of interactions and subtle influences on neuropathic pain processing.

Psychophysics of CNS Pain-Related Activity: Binary and Analog Channels and Memory Encoding

The forebrain neuronal system signaling pain has been poorly characterized. The pain pathway afferent to the thalamus may be a labeled line consisting of neurons in the pain-signaling pathway to the brain (spinothalamic tract, STT) that respond only to painful stimuli. It has recently been proposed that the STT contains a series of analog-labeled lines, each signaling a different aspect of the internal state of the body (interoception), for example, visceral/cold/itch sensations. In this view, pain is the unpleasant emotion produced by disequilibrium of the internal state. The authors now show that stimulation of an STT receiving zone (thalamic principal somatic sensory nucleus, ventral caudal) in awake humans produces two different exteroceptive responses. The first is a binary response signaling the presence of painful stimuli. The second is an analog response in which nonpainful and painful sensations are graded with intensity of the stimulus. Such stimulation can evoke both the sensory and emotional components of previously experienced pain. These results illustrate the diverse functions of human pain signaling pathways.

Physiological changes in primate somatosensory thalamus induced by deafferentation are dependent on the spinal funiculi that are sectioned and time following injury

The importance of spike bursts in thalamo-cortical processing of sensory information has received an increasing amount of interest over the past several years. Previously it has been reported that short high-frequency spike trains (3-8 action potentials occurring at 67-167 Hz), or spike bursts, are increased in both human and non-human primate thalamus following deafferentation. Here we examine the effects of lesion of the ventral spinal quadrant alone versus combined lesion of the ventral and dorsal spinal quadrants on the evoked and spontaneous spike trains in thalamic neurons. A total of 1175 neurons were sampled from 13 animals, three intact, six with ventral quadrant lesions (three with prolonged survival and three with short-term survival after spinal lesion) and four with combined ventral and dorsal quadrant lesions. Detailed analysis was conducted on 256 of these neurons, which revealed that thalamic neurons of animals with ventral quadrant lesions had elevated burst and non-burst spike rates while neurons from animals with combined ventral-dorsal lesions showed two types of change. Neurons in the forelimb areas showed increased bursts without a change in non-burst activity, while neurons in lateral VPL without receptive fields showed very low non-burst activity, but high burst spike rates. The magnitude of the effects produced by ventral-lateral spinal lesions was more pronounced in the short-term survival animals than in the long-term survival animals. These results show that the effects of deafferentation on the physiological properties of thalamic neurons are dependent on the afferent tract or tracts that are lesioned and the time after lesion.

Association of spinothalamic lamina I neurons and their ascending axons with calbindin-immunoreactivity in monkey and human

The calbindin-immunoreactivity of spinothalamic (STT) lamina I neurons and their ascending axons was examined in two experiments. In the first experiment, lamina I STT neurons in macaque monkeys were double-labeled for calbindin and for retrogradely transported WGA*HRP following large (n=2) or small (n=1) injections that included the posterior thalamus. Most, but not all (78%) of the contralateral retrogradely labeled lamina I STT cells were positive for calbindin. Calbindin-immunoreactivity was not selectively associated with any particular anatomical type of lamina I STT cell; 82% of the fusiform cells, 78% of the pyramidal cells and 67% of the multipolar cells were double-labeled. In the second experiment, oblique transverse sections from upper cervical spinal segments of three macaque monkeys, one squirrel monkey and five humans were stained for calbindin-immunoreactivity. In each case, a distinct bundle of fibers was densely stained in the middle of the lateral funiculus. This matches the location of anterogradely labeled ascending lamina I axons observed in prior work in cats and monkeys, and it matches the location of the classically described 'lateral spinothalamic tract' in humans. This bundle had variable shape across cases, an observation that might have clinical significance. These findings support the view that lamina I STT neurons are involved in spinal cordotomies that reduce pain, temperature and itch sensations.

Functional plasticity in primate somatosensory thalamus following chronic lesion of the ventral lateral spinal cord

The long-term consequences of thoracic spinothalamic tract lesion on the physiological properties of neurons in the ventral posterior lateral nucleus of the thalamus in monkeys were assessed. Neurons responding to both compressive and phasic brush stimuli (multireceptive neurons), but not brush-specific (low-threshold) neurons, in the partially deafferented thalamus showed increased spontaneous activity, increased responses evoked by cutaneous stimuli and larger mean receptive field size than the same types of cells in the thalamus with intact innervation. The spike train properties of both the spontaneous and evoked discharges of cells were also altered so that there was an increased incidence of spike-bursts in cells of deafferented thalamus. These changes were widespread in the thalamus, and included cells in both the fully innervated forelimb representation and the partially denervated hindlimb representation ipsilateral to the lesion. The spontaneous and evoked spike trains in the ipsilateral thalamus also show increased frequency of both spike-burst and non-burst events compared to the intact thalamus. These results indicate that chronic spinothalamic tract lesion produces widespread changes in the physiological properties of a discrete cell population of the thalamus.The findings in this study indicate that the thalamic processing of somatosensory information conveyed by the lemniscal system is altered by transection of the spinothalamic tract. This change in sensory processing in the thalamus would result in altered cortical processing of innocuous somatosensory inputs following deafferentation and so possibly contribute to the generation of the central pain syndrome.

Evidence for segregated pain and temperature conduction within the spinothalamic tract

The lateral spinothalamic tract, located in the anterolateral quadrant of the white matter of the spinal cord, is one of the most important structures in transmitting pain within the central nervous system. It has been known for almost a century that destruction of fibers in this tract results in analgesia contralateral to the lesion. The effectiveness and clinical importance of interruption of the lateral spinothalamic tract has been proven in many studies. Today cordotomies are still a useful neurosurgical treatment modality, especially when pain can no longer be sufficiently controlled by analgesic drugs. Although analgesia on the contralateral side is the desired effect, one must also expect to cause disturbance in temperature sensation when performing a cordotomy. The authors' observations showed that after a cordotomy the dermatome level of analgesia can be variable within certain limits, which is in accordance with the literature. Surprisingly, however, the loss of temperature sensation may differ significantly from the loss of pain sensation. It was also found to be possible to perform a successful cordotomy without altering the sensation of temperature at all. This indicates that pain and temperature sensations may be conducted via separate pathways. Possible mechanisms underlying this phenomenon are discussed.

Percutaneous cervical cordotomy: a review of 181 operations on 146 patients with a study on the location of "pain fibers" in the C-2 spinal cord segment of 29 cases

The authors present a review of 146 patients who underwent 181 percutaneous cervical cordotomies for intractable pain. In addition, an anatomical-clinical correlation was carried out for 29 of these patients. It was found that the fibers subserving pain sensation in the C-2 segment lie in the anterolateral funiculus between the level of the denticulate ligament and a line drawn perpendicularly from the medial angle of the ventral gray-matter horn to the surface of the cord. The best analgesic results have been obtained by creating lesions that extend 5.0 mm deep to the surface of the cord and destroy about 20% of the hemicord. There is a somatotopic organization with sacral fibers running ventromedially and cervical fibers running dorsolaterally. The authors believe that the ascending fibers subserving the distinct sensations of pain induced by tissue damage and pinprick, although mixed (overlapping) in the anterolateral funiculus of the spinal cord, are physiologically distinct from one another. Whereas some cordotomies, both in the current series and as reported in the literature, may affect these functions differentially, optimum pain relief seems to be obtained only when pinprick sensation is also abolished in the affected segments. Evoked pain sensation is not abolished by cordotomy, but its threshold is greatly raised. When pathological pain is completely abolished, so is pinprick sensation. However, in a number of cases where pathological pain was only partially alleviated, pinprick sensation remained intact. The significance of these and other cases reported in the literature is discussed. The importance of clinically distinguishing between pain caused by tissue damage and pinprick sensation is emphasized, as well as that between return of pre-existing or new tissue-damage pain and painful dysesthesia.

Substance P innervation of the rat and cat thalamus. II. Cells of origin in the spinal cord

Evidence in the preceding paper suggests that fibers and terminals immunopositive for substance P (SP) in somatosensory thalamic nuclei are part of the spinothalamic tract (STT). In this paper, more direct evidence on this point is provided by immunocytochemistry for SP on the cervical spinal cord, alone or combined with the retrograde transport of colloidal gold-labeled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase (WGAapoHRP-Au). In cats and rats pretreated with colchicine and/or anterolateral chordotomy (to increase SP content in cell bodies), many small to large cell bodies are SP-immunopositive especially in laminae I and V, but also in more ventral laminae of the upper cervical cord. SP neurons are also present in the dorsolateral funiculus (in the lateral spinal nucleus, LSN, in rats) but not in the lateral cervical nucleus or in the internal basilar nucleus. In both species there is a considerable degree of overlap in the distribution of SP-positive neurons and that of STT neurons. SP immunocytochemistry in rats after WGAapoHRP-Au injection in the somatosensory thalamus reveals SP-positive STT neurons in LSN, in lamina I and in lamina V, and, to a lesser extent, in more ventral laminae. These results demonstrate that SP is a marker and/or neuromediator for some STT neurons. Together with the evidence discussed in the preceding paper, the results also suggest that SP-positive neurons may be involved in the transmission of nociceptive input.

Spinal distribution of ascending lamina I axons anterogradely labeled with Phaseolus vulgaris leucoagglutinin (PHA-L) in the cat

The location of the ascending axons of spinal lamina I cells was studied in cats that received injections of Phaseolus vulgaris leucoagglutinin (PHA-L) in the superficial dorsal horn of the cervical or lumbosacral enlargement. Lamina I axons that could be ascribed to the spinothalamic tract (STT) were of particular interest. The cases were divided into three sets: in seven optimal cases the injections were restricted to lamina I; in ten nominal cases the injections involved laminae I-II or laminae I-III and occasionally lamina IV; and in eight mixed cases laminae I-V were injected. Since ipsilateral propriospinal and bilateral supraspinal axons originate from laminae I and V, but only ipsilateral propriospinal axons from laminae II-IV, this categorization facilitated a comparative analysis. Ascending axons labeled immunohistochemically with avidin/Texas Red were observed in oblique transverse sections from the C1, C3/4, T6, T12, and L3/4 levels. Incidental axonal labeling occurred in the ipsilateral dorsal columns because of passing primary afferent fiber uptake and, in nominal and mixed cases with involvement of laminae III-IV, in the superficial dorsolateral funiculus at the location of the spinocervical tract. Ipsilateral ascending lamina I axons in optimal cases were located in Lissauer's tract and in the white matter adjacent to the dorsal horn. Since these appeared to terminate in lamina I, and few remained at C1, they were ascribed to propriospinal projections. Contralateral ascending lamina I axons in optimal and nominal cases were distributed throughout the dorsal and ventral portions of the lateral funiculus (LF), but, despite considerable variability between animals in their location and dispersion, they were consistently concentrated in the middle of the LF (i.e., at the level of the central canal). This concentration was observed in a slightly more ventral location at C1, and a similar but weaker concentration of lamina I axons was located slightly more dorsally in C1 on the ipsilateral side. These supraspinal lamina I projections were ascribed to the spinomesencephalic tract (SMT) and to the STT. In mixed cases, additional ascending axons ascribed to lamina V cells were labeled in the ventrolateral and ventral funiculi. Many labeled axons were found in this region following a large injection of biocytin into lumbosacral laminae V-VIII in a supplementary case. These results thus together support previous descriptions of a dorsoventral distribution of STT axons according to laminar origin, but they contradict recent reports that lamina I axons ascend in the dorsolateral funiculus.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of spinal neurons by afferent fibers in the ventral roots of rats

The present investigation was aimed to search for spinal neurons which are activated by afferent fibers in the ventral roots, and once found, to study their receptive field and rostral projection. After dorsal roots L1-S4 on the left side were cut, ventral roots were stimulated to record extracellular responses of the neurons using glass microelectrodes filled with Fast green FCF. Thirty-nine neurons were activated by stimulating ventral roots at 2.5-26.7 x threshold (T) of the lowest threshold fibers of the ventral root. Six neurons were classified as wide dynamic range neurons and 16 high threshold neurons according to their response patterns to the periphery. Seventeen neurons were unresponsive to cutaneous stimulation, suggesting they had innervated visceral organs. Most of the neurons (71%) were located in layer V and VI. In 7 neurons (18%), rostral projection was confirmed by collision block method and ability to respond to high frequency stimuli with constant latency.

Effects of electrical stimulation of vagal afferents on spinothalamic tract cells in the rat

The effects of electrical stimulation of cervical vagal afferents (VAS) on the background activity and on the responses of 25 spinothalamic tract (STT) neurons to noxious stimuli were studied in anesthetized rats. Background (spontaneous) activity of 9 (36%) STT neurons was inhibited by all intensities of VAS. 6 (24%) units were facilitated at lesser and inhibited at greater intensities of VAS, 5 (20%) units were only facilitated by all intensities of VAS, and 5 (20%) units were not affected by VAS. Responses of 8 (36%) STT neurons to noxious stimuli were only inhibited by VAS, 9 (41%) were facilitated at lesser and inhibited at greater intensities of VAS, and 5 units (23%) were only facilitated by VAS. There were no significant differences in VAS-produced modulatory effects between STT neurons and 16 unidentified lumbar spinal dorsal horn neurons studied under the same conditions. These results reveal that descending facilitatory and inhibitory pathways engaged by activation of vagal afferents modulate rostrally projecting nociceptive transmission neurons in the spinal cord, constituting an important regulatory network for nociception.

Primate spinothalamic pathways: I. A quantitative study of the cells of origin of the spinothalamic pathway

In six monkeys spinothalamic (STT) cells were retrogradely labeled by injecting 2% wheat germ agglutinin-conjugated horseradish peroxidase into the somatosensory thalamus. Following a 5-day survival period, the animals were perfused and the tissue was removed and processed with the tetramethyl benzidine technique. In all animals there were HRP-labeled STT cells in all segments of the spinal cord. In one old world monkey, the injection included most of the thalamus and resulted in 18.235 estimated total number of STT cells. Of this total, 35% were located in the upper cervical segments (C1-C3), 18% were located in C4-C8, 19% were in the thoracic spinal cord with most found in T1-T3; 6% were in L1-L3, 13% were in L4-L7, and 7% were in the coccygeal segments. Of the total labeled STT cells, 17% were found in the spinal cord ipsilateral to the thalamic injections; 53% of these cells were located in C1-C3 primarily in lamina VIII. The percentage of label found in the contralateral lower cervical region laminae I-III (43-50%), IV-VI (33-48%), and VII-X (8-17%) was similar among three animals with similar thalamic injections. The distributions of the shapes of the labeled STT cells were similar for each lamina between the lower cervical and lower lumbar regions. The mean diameter of the labeled STT cells varied with spinal cord segment and lamina. The lamina I STT cells were the smallest. In the cervical spinal cord, lamina VIII STT cells had the largest diameters, while in the lumbar region laminae IV-VI had the largest STT cells.