Physiological studies of spinohypothalamic tract neurons in the lumbar enlargement of monkeys

History of Spinal Cord “Pain” Pathways Including the Pathways Not Taken

Traditional medical neuroanatomy/neurobiology textbooks teach that pain is generated by several ascending pathways that course in the anterolateral quadrant of the spinal cord, including the spinothalamic, spinoreticular and spinoparabrachial tracts. The textbooks also teach, building upon the mid-19th century report of Brown-Séquard, that unilateral cordotomy, namely section of the anterolateral quadrant, leads to contralateral loss of pain (and temperature). In many respects, however, this simple relationship has not held up. Most importantly, pain almost always returns after cordotomy, indicating that activation of these so-called “pain” pathways may be sufficient to generate pain, but they are not necessary. Indeed, Brown-Séquard, based on his own studies, eventually came to the same conclusion. But his new view of “pain” pathways was largely ignored, and certainly did not forestall Spiller and Martin's 1912 introduction of cordotomy to treat patients. This manuscript reviews the history of “pain” pathways that followed from the first description of the Brown-Séquard Syndrome and concludes with a discussion of multisynaptic spinal cord ascending circuits. The latter, in addition to the traditional oligosynaptic “pain” pathways, may be critical to the transmission of “pain” messages, not only in the intact spinal cord but also particularly after injury.

Widespread Pressure Delivered by a Weighted Blanket Reduces Chronic Pain: A Randomized Controlled Trial

Pleasant sensation is an underexplored avenue for modulation of chronic pain. Deeper pressure is perceived as pleasant and calming, and can improve sleep. Although pressure can reduce acute pain, its effect on chronic pain is poorly characterized. The current remote, double-blind, randomized controlled trial tested the hypothesis that wearing a heavy weighted blanket - providing widespread pressure to the body - relative to a light weighted blanket would reduce ratings of chronic pain, mediated by improvements in anxiety and sleep. Ninety-four adults with chronic pain were randomized to wear a 15-lb. (heavy) or 5-lb. (light) weighted blanket during a brief trial and overnight for one week. Measures of anxiety and chronic pain were collected pre- and post-intervention, and ratings of pain intensity, anxiety, and sleep were collected daily. After controlling for expectations and trait anxiety, the heavy weighted blanket produced significantly greater reductions in broad perceptions of chronic pain than the light weighted blanket (Cohen's f = .19, CI [-1.97, -.91]). This effect was stronger in individuals with high trait anxiety (P = .02). However, weighted blankets did not alter pain intensity ratings. Pain reductions were not mediated by anxiety or sleep. Given that the heavy weighted blanket was associated with greater modulation of affective versus sensory aspects of chronic pain, we propose that the observed reductions are due to interoceptive and social/affective effects of deeper pressure. Overall, we demonstrate that widespread pressure from a weighted blanket can reduce the severity of chronic pain, offering an accessible, home-based tool for chronic pain. The study purpose, targeted condition, study design, and primary and secondary outcomes were pre-registered in ClinicalTrials.gov (NCT04447885: "Weighted Blankets and Chronic Pain"). Perspective: This randomized-controlled trial showed that a 15-lb weighted blanket produced significantly greater reductions in broad perceptions of chronic pain relative to a 5-lb weighted blanket, particularly in highly anxious individuals. These findings are relevant to patients and providers seeking home-based, nondrug therapies for chronic pain relief.

Cellular Circuits in the Brain and Their Modulation in Acute and Chronic Pain

Chronic, pathological pain remains a global health problem and a challenge to basic and clinical sciences. A major obstacle to preventing, treating, or reverting chronic pain has been that the nature of neural circuits underlying the diverse components of the complex, multidimensional experience of pain is not well understood. Moreover, chronic pain involves diverse maladaptive plasticity processes, which have not been decoded mechanistically in terms of involvement of specific circuits and cause-effect relationships. This review aims to discuss recent advances in our understanding of circuit connectivity in the mammalian brain at the level of regional contributions and specific cell types in acute and chronic pain. A major focus is placed on functional dissection of sub-neocortical brain circuits using optogenetics, chemogenetics, and imaging technological tools in rodent models with a view towards decoding sensory, affective, and motivational-cognitive dimensions of pain. The review summarizes recent breakthroughs and insights on structure-function properties in nociceptive circuits and higher order sub-neocortical modulatory circuits involved in aversion, learning, reward, and mood and their modulation by endogenous GABAergic inhibition, noradrenergic, cholinergic, dopaminergic, serotonergic, and peptidergic pathways. The knowledge of neural circuits and their dynamic regulation via functional and structural plasticity will be beneficial towards designing and improving targeted therapies.

Pruriceptive spinothalamic tract neurons: physiological properties and projection targets in the primate

Itch of peripheral origin requires information transfer from the spinal cord to the brain for perception. Here, primate spinothalamic tract (STT) neurons from lumbar spinal cord were functionally characterized by in vivo electrophysiology to determine the role of these cells in the transmission of pruriceptive information. One hundred eleven STT neurons were identified by antidromic stimulation and then recorded while histamine and cowhage (a nonhistaminergic pruritogen) were sequentially applied to the cutaneous receptive field of each cell. Twenty percent of STT neurons responded to histamine, 13% responded to cowhage, and 2% responded to both. All pruriceptive STT neurons were mechanically sensitive and additionally responded to heat, intradermal capsaicin, or both. STT neurons located in the superficial dorsal horn responded with greater discharge and longer duration to pruritogens than STT neurons located in the deep dorsal horn. Pruriceptive STT neurons discharged in a bursting pattern in response to the activating pruritogen and to capsaicin. Microantidromic mapping was used to determine the zone of termination for pruriceptive STT axons within the thalamus. Axons from histamine-responsive and cowhage-responsive STT neurons terminated in several thalamic nuclei including the ventral posterior lateral, ventral posterior inferior, and posterior nuclei. Axons from cowhage-responsive neurons were additionally found to terminate in the suprageniculate and medial geniculate nuclei. Histamine-responsive STT neurons were sensitized to gentle stroking of the receptive field after the response to histamine, suggesting a spinal mechanism for alloknesis. The results show that pruriceptive information is encoded by polymodal STT neurons in histaminergic or nonhistaminergic pathways and transmitted to the ventrobasal complex and posterior thalamus in primates.

Termination zones of functionally characterized spinothalamic tract neurons within the primate posterior thalamus

The primate posterior thalamus has been proposed to contribute to pain sensation, but its precise role is unclear. This is in part because spinothalamic tract (STT) neurons that project to the posterior thalamus have received little attention. In this study, antidromic mapping was used to identify individual STT neurons with axons that projected specifically to the posterior thalamus in Macaca fascicularis. Each axon was located by antidromic activation at low stimulus amplitudes (<30 microA) and was then surrounded distally by a grid of stimulating points in which 500-microA stimuli were unable to activate the axon antidromically, thereby indicating the termination zone. Several nuclei within the posterior thalamus were targets of STT neurons: the posterior nucleus, suprageniculate nucleus, magnocellular part of the medial geniculate nucleus, and limitans nucleus. STT neurons projecting to the ventral posterior inferior nucleus were also studied. Twenty-five posterior thalamus-projecting STT neurons recorded in lumbar spinal cord were characterized by their responses to mechanical, thermal, and chemical stimuli. Sixteen of 25 neurons were recorded in the marginal zone and the balance was located within the deep dorsal horn. Thirteen neurons were classified as wide dynamic range and 12 as high threshold. One-third of STT neurons projecting to posterior thalamus responded to noxious heat (50 degrees C). Two-thirds of those tested responded to cooling. Seventy-one percent responded to an intradermal injection of capsaicin. These data indicate that the primate STT transmits noxious and innocuous mechanical, thermal, and chemical information to multiple posterior thalamic nuclei.

The itch-producing agents histamine and cowhage activate separate populations of primate spinothalamic tract neurons

Itch is an everyday sensation, but when associated with disease or infection it can be chronic and debilitating. Several forms of itch can be blocked using antihistamines, but others cannot and these constitute an important clinical problem. Little information is available on the mechanisms underlying itch that is produced by nonhistaminergic mechanisms. We examined the responses of spinothalamic tract neurons to histaminergic and, for the first time, nonhistaminergic forms of itch stimuli. Fifty-seven primate spinothalamic tract (STT) neurons were identified using antidromic activation techniques and examined for their responses to histamine and cowhage, the nonhistaminergic itch-producing spicules covering the pod of the legume Mucuna pruriens. Each examined neuron had a receptive field on the hairy skin of the hindlimb and responded to noxious mechanical stimulation. STT neurons were tested with both pruritogens applied in a random order and we found 12 that responded to histamine and seven to cowhage. Each pruritogen-responsive STT neuron was activated by the chemical algogen capsaicin and two-thirds responded to noxious heat stimuli, demonstrating that these neurons convey chemical, thermal, and mechanical nociceptive information as well. Histamine or cowhage responsive STT neurons were found in both the marginal zone and the deep dorsal horn and were classified as high threshold and wide dynamic range. Unexpectedly, histamine and cowhage never activated the same cell. Our results demonstrate that the spinothalamic tract contains mutually exclusive populations of neurons responsive to histamine or the nonhistaminergic itch-producing agent cowhage.

The organisation of the stress response, and its relevance to chiropractors: a commentary

The stress response is a natural reaction by the body, against potentially harmful stimuli to enhance the chance for survival. Persistent activation of the stress response can cause changes to homeostatic mechanisms. The study of stress neurophysiology, in the evaluation of the manifestation of disease in the body, suggests that these chronic changes have detrimental effects on sub cortical structures. Furthermore, there is much scientific support for the notion that chronic activation of supraspinal systems will lead to maladaptation of homeostatic mechanisms, causing the impairment of processes within the body, and ultimately leading to visceral disorders. The chiropractic profession for many years has alluded to chronic change of neurophysiological pathways as a potential explanation of visceral disorders, but the profession has typically described these in terms of somatovisceral or viscerosomatic reflex activity. Change in supraspinal neurophysiological efferent activity is increasingly being used to explain "stress" related disease. The chiropractic profession should consider investigating such stress responses by conducting spinal manipulative therapy trials that evaluate supraspinal effects of manipulation. Such research may help elucidate key mechanisms associated with the change of visceral disorders noted by some chiropractors following manipulative therapy.

Mechanisms underlying development of spatially distributed chronic pain (fibromyalgia)

Chronic fibromyalgia (FM) pain is prevalent (estimated as high as 13%), predominantly affects women, and is associated with a variety of focal pain conditions. Ongoing FM pain is referred to deep tissues and is described as widespread but usually is maximally located within a restricted region such as the shoulders. Palpation of deep tissues reveals an enhanced nociceptive sensitivity that is not restricted to regions of clinical pain. Similarly, psychophysical testing reveals allodynia and hyperalgesia for cutaneous stimulation at locations beyond regions of clinical pain referral. The combination of widely distributed clinical pain and generalized hypersensitivity is highly disabling, but no satisfactory treatment is regularly prescribed. A thorough understanding of mechanisms will likely be required to develop and document adequate therapies. The generalized hypersensitivity associated with FM has focused considerable interest on central (CNS) mechanisms for the disorder. These include central sensitization, central disinhibition and a dysfunctional hypothalamic-pituitary-adrenal (HPA) axis. However, the central effects associated with FM can be produced by a peripheral source of pain. Chronic nociceptive input induces central sensitization, magnifying pain, and it activates the HPA and the sympathetic nervous system. Chronic sympathetic activation indirectly sensitizes peripheral nociceptors and sets up a vicious cycle. Thus, it appears that central mechanisms of FM pain are dependent on abnormal peripheral input(s) for development and maintenance of this condition. A substantial literature defines peripheral-CNS-peripheral interactions that are integral to FM pain. These reciprocal actions and related phenomena of relevance to FM pain are reviewed here, leading to suggestions for testing of therapeutic approaches.

Effect of nor-trimebutine on neuronal activation induced by a noxious stimulus or an acute colonic inflammation in the rat

Nor-trimebutine is the main metabolite of trimebutine that is used in the treatment of patients with irritable bowel syndrome. Nor-trimebutine has a blocking activity on sodium channels and a potent local anesthetic effect. These properties were used to investigate the effect of nor-trimebutine on spinal neuronal activation induced by models of noxious somato-visceral stimulus and acute colonic inflammation. Nor-trimebutine was administered in rats either subcutaneously 30 min before intraperitoneal administration of acetic acid or intracolonically 30 min before intracolonic infusion of trinitrobenzenesulfonic acid. Abdominal contractions were counted for 1 h as a marker of abdominal pain. c-fos expression was used as a marker of neuronal activation and revealed by immunohistochemistry 1h after intraperitoneal acetic acid injection and 2 h after colonic inflammation. Nor-trimebutine decreased Fos expression in the thoraco-lumbar (peritoneal irritation) and lumbo-sacral (colonic inflammation) spinal cord in laminae I, IIo V, VII and X. This effect was also observed in the sacral parasympathetic nucleus after colonic inflammation. Nor-trimebutine induced a significant decrease of abdominal contractions following intraperitoneal acetic acid injection. These data may explain the effectiveness of trimebutine in the therapy of abdominal pain in the irritable bowel syndrome.

Afferent pain pathways: a neuroanatomical review

Painful experience is a complex entity made up of sensory, affective, motivational and cognitive dimensions. The neural mechanisms involved in pain perception acts in a serial and a parallel way, discriminating and locating the original stimulus and also integrating the affective feeling, involved in a special situation, with previous memories. This review examines the concepts of nociception, acute and chronic pain, and also describes the afferent pathways involved in reception, segmental processing and encephalic projection of pain stimulus. The interaction model of the cerebral cortex areas and their functional characteristics are also discussed.

Comparison of responses of primate spinothalamic tract neurons to pruritic and algogenic stimuli

We investigated the role of mechanosensitive spinothalamic tract (STT) neurons in mediating 1) the itch evoked by intradermal injection of histamine, 2) the enhanced sense of itch evoked by innocuous stroking (alloknesis), and 3) the enhanced pain evoked by punctate stimulation (hyperalgesia) of the skin surrounding the injection site. Responses to intradermal injections of histamine and capsaicin were compared in STT neurons recorded in either the superficial or the deep dorsal horn of the anesthetized monkey. Each neuron was identified by antidromic activation from the ventral posterior lateral nucleus of thalamus and classified by its initial responses to mechanical stimuli as wide dynamic range (WDR) or high-threshold (HT). Approximately half of the WDRs and one of the HTs responded weakly to histamine, some with a duration > 5 min, the maximal time allotted. WDRs but not HTs exhibited a significant increase in response to punctate stimulation after histamine consistent with their possible role in mediating histamine-induced hyperalgesia. Neither type of neuron exhibited significant changes in response to stroking, consistent with their unlikely role in mediating alloknesis. Furthermore, nearly all STT neurons exhibited vigorous and persistent responses to capsaicin, after which they became sensitized to stroking and to punctate stimulation. We conclude that the STT neurons in our sample are more likely to contribute to pain, allodynia, and hyperalgesia than to itch and alloknesis.

A critical review of the role of the proposed VMpo nucleus in pain

The evidence presented by Craig and his colleagues for an important projection from lamina I spinothalamic tract neurons to a renamed thalamic nucleus (the posterior part of the ventral medial nucleus or VMpo), as well as to the ventrocaudal medial dorsal and the ventral posterior inferior thalamic nuclei, is critically reviewed. Of particular concern is the denial of an important nociceptive lamina I projection to the ventrobasal complex. Contrary evidence is reviewed that strongly favors a role of spinothalamic projections from both lamina I and deep layers of the dorsal horn to the ventrobasal complex and other thalamic nuclei and from there to the SI and SII somatosensory cortices in the sensory-discriminative processing of pain and temperature information.

Trigeminohypothalamic and reticulohypothalamic tract neurons in the upper cervical spinal cord and caudal medulla of the rat

Sensory information that arises in orofacial organs facilitates exploratory, ingestive, and defensive behaviors that are essential to overall fitness and survival. Because the hypothalamus plays an important role in the execution of these behaviors, sensory signals conveyed by the trigeminal nerve must be available to this brain structure. Recent anatomical studies have shown that a large number of neurons in the upper cervical spinal cord and caudal medulla project directly to the hypothalamus. The goal of the present study was to identify the types of information that these neurons carry to the hypothalamus and to map the route of their ascending axonal projections. Single-unit recording and antidromic microstimulation techniques were used to identify 81 hypothalamic-projecting neurons in the caudal medulla and upper cervical (C(1)) spinal cord that exhibited trigeminal receptive fields. Of the 72 neurons whose locations were identified, 54 were in laminae I-V of the dorsal horn at the level of C(1) (n = 22) or nucleus caudalis (Vc, n = 32) and were considered trigeminohypothalamic tract (THT) neurons because these regions are within the main projection territory of trigeminal primary afferent fibers. The remaining 18 neurons were in the adjacent lateral reticular formation (LRF) and were considered reticulohypothalamic tract (RHT) neurons. The receptive fields of THT neurons were restricted to the innervation territory of the trigeminal nerve and included the tongue and lips, cornea, intracranial dura, and vibrissae. Based on their responses to mechanical stimulation of cutaneous or intraoral receptive fields, the majority of THT neurons were classified as nociceptive (38% high-threshold, HT, 42% wide-dynamic-range, WDR), but in comparison to the spinohypothalamic tract (SHT), a relatively high percentage of low-threshold (LT) neurons were also found (20%). Responses to thermal stimuli were found more commonly in WDR than in HT neurons: 75% of HT and 93% of WDR neurons responded to heat, while 16% of HT and 54% of WDR neurons responded to cold. These neurons responded primarily to noxious intensities of thermal stimulation. In contrast, all LT neurons responded to innocuous and noxious intensities of both heat and cold stimuli, a phenomenon that has not been described for other populations of mechanoreceptive LT neurons at spinal or trigeminal levels. In contrast to THT neurons, RHT neurons exhibited large and complex receptive fields, which extended over both orofacial ("trigeminal") and extracephalic ("non-trigeminal") skin areas. Their responses to stimulation of trigeminal receptive fields were greater than their responses to stimulation of non-trigeminal receptive fields, and their responses to innocuous stimuli were induced only when applied to trigeminal receptive fields. As described for SHT axons, the axons of THT and RHT neurons ascended through the contralateral brain stem to the supraoptic decussation (SOD) in the lateral hypothalamus; 57% of them then crossed the midline to reach the ipsilateral hypothalamus. Collateral projections were found in the superior colliculus, substantia nigra, red nucleus, anterior pretectal nucleus, and in the lateral, perifornical, dorsomedial, suprachiasmatic, and supraoptic hypothalamic nuclei. Additional projections (which have not been described previously for SHT neurons) were found rostral to the hypothalamus in the caudate-putamen, globus pallidus, and substantia innominata. The findings that nonnociceptive signals reach the hypothalamus primarily through the direct THT route, whereas nociceptive signals reach the hypothalamus through both the direct THT and the indirect RHT routes suggest that highly prioritized painful signals are transferred in parallel channels to ensure that this critical information reaches the hypothalamus, a brain area that regulates homeostasis and other humoral responses required for the survival of the organism.