Multisegmental A{delta}- and C-fiber input to neurons in lamina I and the lateral spinal nucleus

Reliability of task-based fMRI in the dorsal horn of the human spinal cord

The application of functional magnetic resonance imaging (fMRI) to the human spinal cord is still a relatively small field of research and faces many challenges. Here we aimed to probe the limitations of task-based spinal fMRI at 3T by investigating the reliability of spinal cord blood oxygen level dependent (BOLD) responses to repeated nociceptive stimulation across two consecutive days in 40 healthy volunteers. We assessed the test-retest reliability of subjective ratings, autonomic responses, and spinal cord BOLD responses to short heat pain stimuli (1s duration) using the intraclass correlation coefficient (ICC). At the group level, we observed robust autonomic responses as well as spatially specific spinal cord BOLD responses at the expected location, but no spatial overlap in BOLD response patterns across days. While autonomic indicators of pain processing showed good-to-excellent reliability, both β-estimates and z-scores of task-related BOLD responses showed poor reliability across days in the target region (gray matter of the ipsilateral dorsal horn). When taking into account the sensitivity of gradient-echo echo planar imaging (GE-EPI) to draining vein signals by including the venous plexus in the analysis, we observed BOLD responses with fair reliability across days. Taken together, these results demonstrate that heat pain stimuli as short as one second are able to evoke a robust and spatially specific BOLD response, which is however strongly variable within participants across time, resulting in low reliability in the dorsal horn gray matter. Further improvements in data acquisition and analysis techniques are thus necessary before event-related spinal cord fMRI as used here can be reliably employed in longitudinal designs or clinical settings.

The Historical Evolution of Topographical Mapping and Nomenclature of the Lateral Cervical and Lateral Spinal Nuclei

The intricate organization of nuclei within the dorsolateral funiculus of the spinal cord has long been an area of interest in the field of neuroanatomy. Numerous researchers have endeavored to determine the morphology, neurochemistry, connections, and physiology of the lateral cervical nucleus and lateral spinal nucleus throughout history. This manuscript charts the historical progression in the mapping, naming, and comprehension of the lateral cervical nucleus and lateral spinal nucleus across a variety of species, such as rats, mice, marmosets, rhesus monkeys, and humans. It synthesizes significant research spanning decades, which together shed light on the nuanced topography of these nuclei, starting from Theodor Ziehen's foundational work in 1903, through Molander's precise mappings, to the detailed contemporary mappings by modern scholars. Despite the wealth of research elucidating the mappings of these nuclei, there remains a need for further investigation into their roles and neurochemical characteristics.

Novel aspects of signal processing in lamina I

The most superficial layer of the spinal dorsal horn, lamina I, is a key element of the nociceptive processing system. It contains different types of projection neurons (PNs) and local-circuit neurons (LCNs) whose functional roles in the signal processing are poorly understood. This article reviews recent progress in elucidating novel anatomical features and physiological properties of lamina I PNs and LCNs revealed by whole-cell recordings in ex vivo spinal cord. This article is part of the Special Issue on "Ukrainian Neuroscience".

Contralateral Afferent Input to Lumbar Lamina I Neurons as a Neural Substrate for Mirror-Image Pain

Mirror-image pain arises from pathologic alterations in the nociceptive processing network that controls functional lateralization of the primary afferent input. Although a number of clinical syndromes related to dysfunction of the lumbar afferent system are associated with the mirror-image pain, its morphophysiological substrate and mechanism of induction remain poorly understood. Therefore, we used ex vivo spinal cord preparation of young rats of both sexes to study organization and processing of the contralateral afferent input to the neurons in the major spinal nociceptive projection area Lamina I. We show that decussating primary afferent branches reach contralateral Lamina I, where 27% of neurons, including projection neurons, receive monosynaptic and/or polysynaptic excitatory drive from the contralateral Aδ-fibers and C-fibers. All these neurons also received ipsilateral input, implying their involvement in the bilateral information processing. Our data further show that the contralateral Aδ-fiber and C-fiber input is under diverse forms of inhibitory control. Attenuation of the afferent-driven presynaptic inhibition and/or disinhibition of the dorsal horn network increased the contralateral excitatory drive to Lamina I neurons and its ability to evoke action potentials. Furthermore, the contralateral Aβδ-fibers presynaptically control ipsilateral C-fiber input to Lamina I neurons. Thus, these results show that some lumbar Lamina I neurons are wired to the contralateral afferent system whose input, under normal conditions, is subject to inhibitory control. A pathologic disinhibition of the decussating pathways can open a gate controlling contralateral information flow to the nociceptive projection neurons and, thus, contribute to induction of hypersensitivity and mirror-image pain.SIGNIFICANCE STATEMENT We show that contralateral Aδ-afferents and C-afferents supply lumbar Lamina I neurons. The contralateral input is under diverse forms of inhibitory control and itself controls the ipsilateral input. Disinhibition of decussating pathways increases nociceptive drive to Lamina I neurons and may cause induction of contralateral hypersensitivity and mirror-image pain.

Elucidating afferent-driven presynaptic inhibition of primary afferent input to spinal laminae I and X

Although spinal processing of sensory information greatly relies on afferent-driven (AD) presynaptic inhibition (PI), our knowledge about how it shapes peripheral input to different types of nociceptive neurons remains insufficient. Here we examined the AD-PI of primary afferent input to spinal neurons in the marginal layer, lamina I, and the layer surrounding the central canal, lamina X; two nociceptive-processing regions with similar patterns of direct supply by Aδ- and C-afferents. Unmyelinated C-fibers were selectively activated by electrical stimuli of negative polarity that induced an anodal block of myelinated Aβ/δ-fibers. Combining this approach with the patch-clamp recording in an ex vivo spinal cord preparation, we found that attenuation of the AD-PI by the anodal block of Aβ/δ-fibers resulted in the appearance of new mono- and polysynaptic C-fiber-mediated excitatory postsynaptic current (EPSC) components. Such homosegmental Aβ/δ-AD-PI affected neurons in the segment of the dorsal root entrance as well as in the adjacent rostral segment. In their turn, C-fibers from the L5 dorsal root induced heterosegmental AD-PI of the inputs from the L4 Aδ- and C-afferents to the neurons in the L4 segment. The heterosegmental C-AD-PI was reciprocal since the L4 C-afferents inhibited the L5 Aδ- and C-fiber inputs, as well as some direct L5 Aβ-fiber inputs. Moreover, the C-AD-PI was found to control the spike discharge in spinal neurons. Given that the homosegmental Aβ/δ-AD-PI and heterosegmental C-AD-PI affected a substantial percentage of lamina I and X neurons, we suggest that these basic mechanisms are important for shaping primary afferent input to the neurons in the spinal nociceptive-processing network.

The role of the neuronal microenvironment in sensory function and pain pathophysiology

The high prevalence of pain and the at times low efficacy of current treatments represent a significant challenge to healthcare systems worldwide. Effective treatment strategies require consideration of the diverse pathophysiologies that underlie various pain conditions. Indeed, our understanding of the mechanisms contributing to aberrant sensory neuron function has advanced considerably. However, sensory neurons operate in a complex dynamic microenvironment that is controlled by multidirectional interactions of neurons with non-neuronal cells, including immune cells, neuronal accessory cells, fibroblasts, adipocytes, and keratinocytes. Each of these cells constitute and control the microenvironment in which neurons operate, inevitably influencing sensory function and the pathology of pain. This review highlights the importance of the neuronal microenvironment for sensory function and pain, focusing on cellular interactions in the skin, nerves, dorsal root ganglia, and spinal cord. We discuss the current understanding of the mechanisms by which neurons and non-neuronal cells communicate to promote or resolve pain, and how this knowledge could be used for the development of mechanism-based treatments.

Surround Inhibition Mediates Pain Relief by Low Amplitude Spinal Cord Stimulation: Modeling and Measurement

Low-frequency (<200 Hz), subperception spinal cord stimulation (SCS) is a novel modality demonstrating therapeutic efficacy for treating chronic neuropathic pain. When stimulation parameters were carefully titrated, patients experienced rapid onset (seconds-minutes) pain relief without paresthesia, but the mechanisms of action are unknown. Using an integrated computational model and in vivo measurements in urethane-anesthetized rats, we quantified how stimulation parameters (placement, pulse width, frequency, and amplitude) influenced dorsal column (DC) axon activation and neural responses in the dorsal horn (DH). Both modeled and recorded DC axons responded with irregular spiking patterns in response to low-amplitude SCS. Maximum inhibition of DH neurons occurred at ∼80% of the predicted sensory threshold in both modeled and recorded neurons, and responses were strongly dependent on spatially targeting of stimulation, i.e., the complement of DC axons activated, and on stimulation parameters. Intrathecal administration of bicuculline shifted neural responses to low-amplitude stimulation in both the model and experiment, suggesting that analgesia is dependent on segmental GABAergic mechanisms. Our results support the hypothesis that low-frequency subperception SCS generates rapid analgesia by activating a small number of DC axons which inhibit DH neuron activity via surround inhibition.

Precision spinal gene delivery-induced functional switch in nociceptive neurons reverses neuropathic pain

Second-order spinal cord excitatory neurons play a key role in spinal processing and transmission of pain signals to the brain. Exogenously-induced change in developmentally-imprinted excitatory neurotransmitter phenotype of these neurons to inhibitory has not yet been achieved. Here we use a subpial dorsal horn-targeted delivery of AAV (adeno-associated virus) vector(s) encoding GABA (gamma-Aminobutyric acid,) synthesizing-releasing inhibitory machinery in mice with neuropathic pain. Treated animals showed a progressive and complete reversal of neuropathic pain (tactile and brush-evoked pain behavior) which persisted for minimum 2.5 months post-treatment. The mechanism of this treatment effect results from the switch of excitatory to preferential inhibitory neurotransmitter phenotype in dorsal horn nociceptive neurons and a resulting increase in inhibitory activity in regional spinal circuitry after peripheral nociceptive stimulation. No detectable side effects (such as sedation, motor weakness or loss of normal sensation) were seen between 2-13 months post-treatment in naive adult mice, pigs and non-human primates. The use of this treatment approach may represent a potent and safe treatment modality in patients suffering from spinal cord- or peripheral nerve-injury induced neuropathic pain.

Presynaptic Interactions between Trigeminal and Cervical Nociceptive Afferents Supplying Upper Cervical Lamina I Neurons

Cervical and trigeminal afferents innervate neighboring cranial territories, and their convergence on upper cervical dorsal horn neurons provides a potential substrate for pain referral in primary headache syndromes. Lamina I neurons are central to this mechanism, as they relay convergent nociceptive input to supraspinal pain centers. Unfortunately, little is known about the interactions between trigeminal and cervical afferents supplying Lamina I neurons. Here, we used rats of both sexes to show that cervical and trigeminal afferents interact via presynaptic inhibition, where monosynaptic inputs to Lamina I neurons undergo unidirectional as well as reciprocal presynaptic control. This means that afferent-driven presynaptic inhibition shapes the way trigeminal and cervical Aδ-fiber and C-fiber input reaches Lamina I projection neurons (PNs) and local-circuit neurons (LCNs). We propose that this inhibition provides a feedforward control of excitatory drive to Lamina I neurons that regulates their convergent and cervical-specific or trigeminal-specific processing modes. As a consequence, disruption of the trigeminal and cervical afferent-driven presynaptic inhibition may contribute to development of primary headache syndromes.SIGNIFICANCE STATEMENT Cervical and trigeminal afferents innervate neighboring cranial territories, and their convergence on upper cervical dorsal horn neurons provides a potential substrate for pain referral in primary headache syndromes. Lamina I neurons are central to this mechanism as they relay convergent nociceptive input to supraspinal pain centers. Here, we show that cervical and trigeminal afferents interact via presynaptic inhibition, where inputs to Lamina I neurons undergo unidirectional as well as reciprocal control. The afferent-driven presynaptic inhibition shapes the trigeminocervical Aδ-fiber and C-fiber input to Lamina I neurons. This inhibition provides control of excitatory drive to Lamina I neurons that regulates their convergent and cervical-specific or trigeminal-specific processing modes. Disruption of this control may contribute to development of primary headache syndromes.

Needling Interventions for Sciatica: Choosing Methods Based on Neuropathic Pain Mechanisms-A Scoping Review

Sciatica is a condition often accompanied by neuropathic pain (NP). Acupuncture and dry needling are common treatments for pain, and the current literature supports acupuncture as an effective treatment for sciatica. However, it is unknown if the mechanisms of NP are considered in the delivery of needling interventions for sciatica. Our objective was to assess the efficacy and the effectiveness of needling therapies, to identify common needling practices and to investigate if NP mechanisms are considered in the treatment of sciatica. A scoping review of the literature on needling interventions for sciatica and a review of the literature on mechanisms related to NP and needling interventions were performed. Electronic literature searches were conducted on PubMed, MEDLINE, CINAHL and Cochrane Database of Systematic Reviews from inception to August, 2020 to identify relevant papers. Reference lists of included papers were also manually screened and a related-articles search through PubMed was performed on all included articles. Mapping of the results included description of included studies, summary of results, and identification of gaps in the existing literature. Ten articles were included. All studies used acupuncture for the treatment of sciatica, no studies on dry needling were identified. Current evidence supports the efficacy and effectiveness of acupuncture for sciatica, however, no studies considered underlying NP mechanisms in the acupuncture approach for sciatica and the rationale for using acupuncture was inconsistent among trials. This review reveals that neuropathic pain mechanisms are not routinely considered in needling approaches for patients with sciatica. Studies showed acupuncture to be an effective treatment for sciatic pain, however, further research is warranted to explore if needling interventions for sciatica and NP would be more effective if NP mechanisms are considered.

Differential modulation of excitatory and inhibitory populations of superficial dorsal horn neurons in lumbar spinal cord by Aβ-fiber electrical stimulation

Activation of Aβ-fibers is fundamental to numerous analgesic therapies, yet its effects on dorsal horn neuronal activity remain unclear. We used multiphoton microscopy of the genetically encoded calcium indicator GCaMP6s to characterize the effects of Aβ-fiber electrical stimulation (Aβ-ES) on neural activity. Specifically, we quantified somatic responses evoked by C-fiber intensity stimulation before and after a 10-minute train of dorsal root Aβ-ES in superficial dorsal horn (SDH) neurons, in mouse lumbar spinal cord. Aβ-ES did not alter C-fiber-evoked activity when GCaMP6s was virally expressed in all neurons, in an intact lumbar spinal cord preparation. However, when we restricted the expression of GCaMP6s to excitatory or inhibitory populations, we observed that Aβ-ES modestly potentiated evoked activity of excitatory neurons and depressed that of inhibitory neurons. Aβ-ES had no significant effects in a slice preparation in either SDH population. A larger proportion of SDH neurons was activated by Aβ-ES when delivered at a root rostral or caudal to the segment where the imaging and C-fiber intensity stimulation occurred. Aβ-ES effects on excitatory and inhibitory populations depended on the root used. Our findings suggest that Aβ-ES differentially modulates lumbar spinal cord SDH populations in a cell type- and input-specific manner. Furthermore, they underscore the importance of the Aβ-ES delivery site, suggesting that Aβ stimulation at a segment adjacent to where the pain is may improve analgesic efficacy.

Cervical spinal cord injury-induced neuropathic pain in male mice is associated with a persistent pro-inflammatory macrophage/microglial response in the superficial dorsal horn

A significant portion of individuals living with traumatic spinal cord injury (SCI) experiences some degree of debilitating neuropathic pain (NP). This pain remains largely intractable in a majority of cases, due in part to an incomplete understanding of its underlying mechanisms. Central sensitization, an increase in excitability of pain transmission neurons located in superficial dorsal horn (sDH), plays a key role in development and maintenance of SCI-induced NP. Resident microglia and peripheral monocyte-derived macrophages (referred to collectively as MMΦ) are involved in promoting SCI-induced DH neuron hyperexcitability. Importantly, these MMΦ consist of populations of cells that can exert pro-inflammatory or anti-inflammatory signaling within injured spinal cord. It is critical to spatiotemporally characterize this heterogeneity to understand MMΦ contribution to NP after SCI. Given that a majority of SCI cases are cervical in nature, we used a model of unilateral C5/C6 contusion that results in persistent at-level thermal hyperalgesia and mechanical allodynia, two forms of NP-related behavior, in the forepaw. The aim of this study was to characterize the sDH MMΦ response within intact cervical spinal cord segments caudal to the lesion (i.e. the location of primary afferent nociceptive input from the forepaw plantar surface). Cervical SCI promoted a persistent MMΦ response in sDH that coincided with the chronic NP phenotype. Using markers of pro- and anti-inflammatory MMΦ, we found that the MMΦ population within sDH exhibited significant heterogeneity that evolved over time post-injury, including a robust and persistent increase in pro-inflammatory MMΦ that was especially pronounced at later times. C5/C6 contusion SCI also induced below-level thermal hyperalgesia and mechanical allodynia in the hindpaw; however, we did not observe a pronounced MMΦ response in sDH of L4/L5 spinal cord, suggesting that different inflammatory cell mechanisms occurring in sDH may be involved in at-level versus below-level NP following SCI. In conclusion, our findings reveal significant MMΦ heterogeneity both within and across pain transmission locations after SCI. These data also show a prominent and persistent pro-inflammatory MMΦ response, suggesting a possible role in DH neuron hyperexcitability and NP.

Processing of trigeminocervical nociceptive afferent input by neuronal circuity in the upper cervical lamina I

ABSTRACT

Afferents from the C2 spinal nerve (SN) and trigeminal nerve (TN) innervate neighboring cranial territories, and their convergence on the upper cervical dorsal horn neurons represents neural substrate of pain referral in primary headache disorders. Unfortunately, little is known about trigeminocervical input to the major spinal nociceptive projection area lamina I. Here, we used ex vivo brainstem-cervical cord preparation for the visually guided whole-cell recording from the upper cervical lamina I neurons. We show that 50% of them receive convergent monosynaptic input from both nerves, whereas 35% and 11% of neurons receive specific supply from the C2 SN and TN, respectively. Altogether, 10 distinct patterns of synaptic input from the C2 SN and TN to lamina I neurons could be identified. Although stimulation of both nerves evoked excitatory/inhibitory responses, more numerous pure inhibitory inputs arose from the TN. We show that cervical and trigeminal nociceptors converge on to lamina I projection and inhibitory neurons. Thus, trigeminocervical input in lamina I is processed in both nerve-specific and convergent circuitries. Afferent convergence on to inhibitory interneurons serves as a feedforward mechanism balancing excitatory drive to projection neurons. Disruption of this balance may cause pain in primary headache syndromes.

Projecting neurons in spinal dorsal horn send collateral projections to dorsal midline/intralaminar thalamic complex and parabrachial nucleus

Itch is an annoying sensation that always triggers scratching behavior, yet little is known about its transmission pathway in the central nervous system. Parabrachial nucleus (PBN), an essential transmission nucleus in the brainstem, has been proved to be the first relay station in itch sensation. Meanwhile, dorsal midline/intralaminar thalamic complex (dMITC) is proved to be activated with nociceptive stimuli. However, whether the PBN-projecting neurons in spinal dorsal horn (SDH) send collateral projections to dMITC, and whether these projections involve in itch remain unknown. In the present study, a double retrograde tracing method was applied when the tetramethylrhodamine-dextran (TMR) was injected into the dMITC and Fluoro-gold (FG) was injected into the PBN, respectively. Immunofluorescent staining for NeuN, substance P receptor (SPR), substance P (SP), or FOS induced by itch or pain stimulations with TMR and FG were conducted to provide morphological evidence. The results revealed that TMR/FG double-labeled neurons could be predominately observed in superficial laminae and lateral spinal nucleus (LSN) of SDH; Meanwhile, most of the collateral projection neurons expressed SPR and some of them expressed FOS in acute itch model induced by histamine. The present results implicated that some of the SPR-expressing neurons in SDH send collateral projections to the dMITC and PBN in itch transmission, which might be involved in itch related complex affective/emotional processing to the higher brain centers.

Spinal cord involvement in Lewy body-related α-synucleinopathies

Context: Lewy body (LB)-related α-synucleinopathy (LBAS) is the neuropathological hallmark of several neurodegenerative diseases such as Parkinson disease (PD), but it is also found in neurologically asymptomatic subjects. An abnormal accumulation of α-synuclein has been reported also in the spinal cord, but extent and significance of the spinal cord involvement are still poorly defined. Objective: We aimed to review the studies addressing the spinal cord involvement of LBAS in healthy subjects and in patients with PD or other neurodegenerative diseases. Methods: A MEDLINE search was performed using following terms: "spinal cord", " α-synucleinopathy", "α-synuclein", "Lewy body", "Parkinson's disease", "multiple system atrophy", "neurodegenerative disorder". Results: LBAS in the spinal cord is associated with that of the medullary reticular formation and locus ceruleus in the brainstem but not with that in the olfactory bulb and amygdala. The intermediolateral columns of the thoracic and sacral cord are the most frequently and severely affected region of the spinal cord. LBAS occurs in centrally projecting spinal cord neurons integrating pain, in particular from lower body periphery. It also involves the sacral parasympathetic nucleus innervating the smooth muscles of the bladder and distal colon and the Onuf's nucleus innervating the striated sphincters. The spinal cord lesions may thus play a crucial role in the genesis of frequent non-motor symptoms such as pain, urinary symptoms, bowel dysfunction, autonomic failure including orthostatic hypotension and sexual disturbances. Moreover, these may also contribute to the motor symptoms, since α-synuclein inclusions have been observed in the pyramidal tracts of patients with PD and multiple system atrophy. Conclusion: Recognition of this peculiar spinal cord pathology may help in the management of the related symptoms in subjects affected by α-synucleinopathies.

High-threshold primary afferent supply of spinal lamina X neurons

The spinal gray matter region around the central canal, lamina X, is critically involved in somatosensory processing and visceral nociception. Although several classes of primary afferent fibers terminate or decussate in this area, little is known about organization and functional significance of the afferent supply of lamina X neurons. Using the hemisected ex vivo spinal cord preparation, we show that virtually all lamina X neurons receive primary afferent inputs, which are predominantly mediated by the high-threshold Aδ- fibers and C-fibers. In two-thirds of the neurons tested, the inputs were monosynaptic, implying a direct targeting of the population of lamina X neurons by the primary nociceptors. Beside the excitatory inputs, 48% of the neurons also received polysynaptic inhibitory inputs. A complex pattern of interactions between the excitatory and inhibitory components determined the output properties of the neurons, one-third of which fired spikes in response to the nociceptive dorsal root stimulation. In this respect, the spinal gray matter region around the central canal is similar to the superficial dorsal horn, the major spinal nociceptive processing area. We conclude that lamina X neurons integrate direct and indirect inputs from several types of thin primary afferent fibers and play an important role in nociception.

The Pathways and Processes Underlying Spinal Transmission of Low Back Pain: Observations From Dorsal Root Ganglion Stimulation Treatment

BACKGROUND

Dorsal root ganglion stimulation (DRG-S) is a novel approach to treat chronic pain. Lead placement at L2 has been reported to be an effective treatment for axial low back pain (LBP) primarily of discogenic etiology. We have recently shown, in a diverse cohort including cases of multilevel instrumentation following extensive prior back surgeries, that DRG-S lead placement at T12 is another promising target. Local effects at the T12 DRG, alone, are insufficient to explain these results.

MATERIALS AND METHODS

We performed a literature review to explore the mechanisms of LBP relief with T12 DRG-S.

FINDINGS

Branches of individual spinal nerve roots innervate facet joints and posterior spinal structures, while the discs and anterior vertebrae are carried via L2, and converge in the dorsal horn (DH) of the spinal cord at T8-T9. The T12 nerve root contains cutaneous afferents from the low back and enters the DH of the spinal cord at T10. Low back Aδ and C-fibers then ascend via Lissauer's tract (LT) to T8-T9, converging with other low back afferents. DRG-S at T12, then, results in inhibition of the converged low back fibers via endorphin-mediated and GABAergic frequency-dependent mechanisms. Therefore, T12 lead placement may be the optimal location for DRG-S to treat LBP.

Primary afferent-driven presynaptic inhibition of C-fiber inputs to spinal lamina I neurons

Presynaptic inhibition of primary afferent terminals is a powerful mechanism for controlling sensory information flow into the spinal cord. Lamina I is the major spinal nociceptive projecting area and monosynaptic input from C-fibers to this region represents a direct pathway for transmitting pain signals to supraspinal centers. Here we used an isolated spinal cord preparation to show that this pathway is under control of the afferent-driven GABAergic presynaptic inhibition. Presynaptic inhibition of C-fiber input to lamina I projection and local-circuit neurons is mediated by recruitment of Aβ-, Aδ- and C-afferents. C-fiber-driven inhibition of C-fibers functions as a feedforward mechanism, by which the homotypic afferents control sensory information flow into the spinal cord and regulate degree of the primary nociceptive afferent activation needed to excite the second order neurons. The presynaptic inhibition of C-fiber input to lamina I neurons may be mediated by both synaptic and non-synaptic mechanisms, and its occurrence and extent are quite heterogeneous. This heterogeneity is likely to be reflective of involvement of lamina I neurons in diverse circuitries processing specific modalities of sensory information in the superficial dorsal horn. Thus, our results implicate both low- and high-threshold afferents in the modulation of C-fiber input into the spinal cord.

Distinct mechanisms of signal processing by lamina I spino-parabrachial neurons

Lamina I spino-parabrachial neurons (SPNs) receive peripheral nociceptive input, process it and transmit to the supraspinal centres. Although responses of SPNs to cutaneous receptive field stimulations have been intensively studied, the mechanisms of signal processing in these neurons are poorly understood. Therefore, we used an ex-vivo spinal cord preparation to examine synaptic and cellular mechanisms determining specific input-output characteristics of the neurons. The vast majority of the SPNs received a few direct nociceptive C-fiber inputs and generated one spike in response to saturating afferent stimulation, thus functioning as simple transducers of painful stimulus. However, 69% of afferent stimulation-induced action potentials in the entire SPN population originated from a small fraction (19%) of high-output neurons. These neurons received a larger number of direct Aδ- and C-fiber inputs, generated intrinsic bursts and efficiently integrated a local network activity via NMDA-receptor-dependent mechanisms. The high-output SPNs amplified and integrated the nociceptive input gradually encoding its intensity into the number of generated spikes. Thus, different mechanisms of signal processing allow lamina I SPNs to play distinct roles in nociception.

Differential suppression of the ipsi- and contralateral nociceptive reflexes in the neonatal rat spinal cord by agonists of µ-, δ- and κ-opioid receptors

Nociceptive discharges caused by the unilateral tissue damage are processed in the spinal cord by both ipsi- and contralateral neuronal circuits. The mechanisms of the neurotransmitter control of this bilateral excitation spread is poorly understood. Spinally administered opiates are known to suppress nociceptive transmission and nociceptive withdrawal reflexes. Here we investigated whether three major types of opioid receptors are involved in the bilateral control of the spinal nociceptive sensorimotor processing. Effects of the µ-, δ- and κ-opioid receptor agonists on the ipsi- and contralateral nociceptive reflexes were studied by recording slow ventral root potentials in an isolated spinal cord preparation of the new-born rat. Absolute levels of expression of the opioid genes were analyzed by the droplet digital PCR. Ipsi- and contralateral slow ventral root potentials were most strongly suppressed by the µ-opioid receptor agonist DAMGO, by 63% and 85%, followed by the κ-opioid receptor agonist U-50488H, by 44% and 73%, and δ-opioid receptor agonist leucine-enkephalin, by 27% and 49%, respectively. All these agonists suppressed stronger contra- than ipsilateral responses. Naloxone prevented effects of the agonists indicating that they act through opioid receptors, which, as we show, are expressed in the neonatal spinal cord at the levels similar to those in adults. Thus, opioid receptor agonists suppress the segmental nociceptive reflexes. Stronger contralateral effects suggest that the endogenous opioid system regulates sensorimotor processing in the spinal commissural pathways. These effects of opioids may be relevant for treatment of symmetric clinical pain symptoms caused by unilateral tissue injury.

Wind-up in lamina I spinoparabrachial neurons: a role for reverberatory circuits

Wind-up is a frequency-dependent increase in the response of spinal cord neurons, which is believed to underlie temporal summation of nociceptive input. However, whether spinoparabrachial neurons, which likely contribute to the affective component of pain, undergo wind-up was unknown. Here, we addressed this question and investigated the underlying neural circuit. We show that one-fifth of lamina I spinoparabrachial neurons undergo wind-up, and provide evidence that wind-up in these cells is mediated in part by a network of spinal excitatory interneurons that show reverberating activity. These findings provide insight into a polysynaptic circuit of sensory augmentation that may contribute to the wind-up of pain's unpleasantness.

Low- and high-threshold primary afferent inputs to spinal lamina III antenna-type neurons

The dorsal horn of the spinal cord (laminae I-VI) processes diverse modalities of nociceptive and nonnociceptive sensory information. Antenna-type neurons with cell bodies located in lamina III and large dendritic trees extending from the superficial lamina I to deep lamina IV are best shaped for the integration of a wide variety of inputs arising from primary afferent fibers and intrinsic spinal circuitries. Although the somatodendritic morphology, the hallmark of antenna neurons, has been well studied, little is still known about the axon structure and basic physiological properties of these cells. Here, we did whole-cell recordings in a rat (P9-P12) spinal cord preparation with attached dorsal roots to examine the axon course, intrinsic firing properties, and primary afferent inputs of antenna cells. Nine antenna cells were identified from a large sample of biocytin-filled lamina III neurons (n = 46). Axon of antenna cells showed intensive branching in laminae III-IV and, in half of the cases, issued dorsally directed collaterals reaching lamina I. Antenna cells exhibited tonic and rhythmic firing patterns; single spikes were followed by hyperpolarization or depolarization. The neurons received monosynaptic inputs from the low-threshold Aβ afferents, Aδ afferents, as well as from the high-threshold Aδ, and C afferents. When selectively activated, C-fiber-driven monosynaptic and polysynaptic excitatory postsynaptic potentials were sufficiently strong to evoke firing in the neurons. Thus, lamina III antenna neurons integrate low-threshold and nociceptive high-threshold primary afferent inputs and can function as wide dynamic range neurons able to directly connect deep dorsal horn with the major nociceptive projection area lamina I.

A single GABA neuron receives contacts from myelinated primary afferents of two adjacent peripheral nerves. A possible role in neuropathic pain

GAD67-EGFP mice were used in a series of experiments to provide anatomical evidence for the role of the reduction in myelinated primary afferent input to GABA spinal neurons in the production of neuropathic pain following peripheral L5 nerve injury. First, we confirmed that L5 injury in these mice produced mechanical and thermal hyperalgesia in the ipsilateral foot. Second, we injected a mixture of cholera toxin subunit-B (CTb) and isolectin B4 (IB4) in the sciatic nerve to selectively label its myelinated and unmyelinated primary afferents. Results showed that primary afferents of sciatic nerve extend from L2-L6 spinal segments. Third, we determined the central terminations of myelinated primary afferents of L4 and L5 spinal nerves following CTb injection in either nerve. The myelinated primary afferents of both nerves terminated in the corresponding and two to three rostral spinal segments with some fibers descending to terminate in the segment caudal to the level at which they entered indicating an intermingling of their terminals at the dorsal horn of the spinal cord. Fourthly, we injected CTb in L5 nerve and CTb HRP-conjugate in L4 nerve. Confocal microscopy and subsequent image analyses showed that individual EGFP-labeled neurons in L4 segment receive myelinated primary afferent contacts from both L4 and L5 nerves. Eliminating inputs from L5 nerve following its injury would result in less involvement of spinal GABA neurons which would very likely initiate neuronal sensitization in L4 segment. This could lead to the development of hyperalgesia in response to the stimulation of the adjacent uninjured L4 nerve.

Regulation of Nociceptive Glutamatergic Signaling by Presynaptic Kv3.4 Channels in the Rat Spinal Dorsal Horn

Presynaptic voltage-gated K⁺ (Kv) channels in dorsal root ganglion (DRG) neurons are thought to regulate nociceptive synaptic transmission in the spinal dorsal horn. However, the Kv channel subtypes responsible for this critical role have not been identified. The Kv3.4 channel is particularly important because it is robustly expressed in DRG nociceptors, where it regulates action potential (AP) duration. Furthermore, Kv3.4 dysfunction is implicated in the pathophysiology of neuropathic pain in multiple pain models. We hypothesized that, through their ability to modulate AP repolarization, Kv3.4 channels in DRG nociceptors help to regulate nociceptive synaptic transmission. To test this hypothesis, we investigated Kv3.4 immunoreactivity (IR) in the rat cervical superficial dorsal horn (sDH) in both sexes and implemented an intact spinal cord preparation to investigate glutamatergic synaptic currents from second order neurons in the sDH under conditions that selectively inhibit the Kv3.4 current. We found presynaptic Kv3.4 IR in peptidergic and nonpeptidergic nociceptive fibers of the sDH. The Kv3.4 channel is hypersensitive to 4-aminopyridine and tetraethylammonium (TEA). Accordingly, 50 μm 4-aminopyridine and 500 μm TEA significantly prolong the AP, slow the maximum rate of repolarization in small-diameter DRG neurons, and potentiate monosynaptic excitatory postsynaptic currents (EPSCs) in dorsal horn laminae I and II through a presynaptic mechanism. In contrast, highly specific inhibitors of BK, Kv7, and Kv1 channels are less effective modulators of the AP and have little to no effect on EPSCs. The results strongly suggest that presynaptic Kv3.4 channels are major regulators of nociceptive synaptic transmission in the spinal cord.SIGNIFICANCE STATEMENT Intractable neuropathic pain can result from disease or traumatic injury and many studies have been conducted to determine the underlying pathophysiological changes. Voltage-gated ion channels, including the K⁺ channel Kv3.4, are dysregulated in multiple pain models. Kv3.4 channels are ubiquitously expressed in the dorsal root ganglion (DRG), where they are major regulators of DRG excitability. However, little is known about the ionic mechanisms that regulate nociceptive synaptic transmission at the level of the first synapse in the spinal cord, which is critical to pain transmission in both intact and pathological states. Here, we show that Kv3.4 channels have a significant impact on glutamatergic synaptic transmission in the dorsal horn, further illuminating its potential as a molecular pain therapeutic target.

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.

Selective Radiofrequency Stimulation of the Dorsal Root Ganglion (DRG) as a Method for Predicting Targets for Neuromodulation in Patients With Post Amputation Pain: A Case Series

OBJECTIVE

While spinal cord stimulation (SCS) has established itself as an accepted and validated treatment for neuropathic pain, there are a number of conditions where it has experienced less, long-term success: post amputee pain (PAP) being one of them. Dorsal root ganglion (DRG) stimulation has shown great promise, particularly in conditions where traditional SCS has fallen short. One major difference between DRG stimulation and traditional SCS is the ability to provide focal stimulation over targeted areas. While this may be a contributing factor to its superiority, it can also be a limitation insofar stimulating the wrong DRG(s) can lead to failure. This is particularly relevant in conditions like PAP where neuroplastic maladaptation occurs causing the pain to deviate from expected patterns, thus creating uncertainty and variability in predicting targets for stimulation. We propose selective radiofrequency (RF) stimulation of the DRG as a method for preoperatively predicting targets for neuromodulation in patients with PAP.

METHODS

We present four patients with PAP of the lower extremities. RF stimulation was used to selectively stimulate individual DRG's, creating areas of paresthesias to see which most closely correlated/overlapped with the painful area(s). RF stimulation to the DRG's that resulted in the desirable paresthesia coverage in the residual or the missing limb(s) was recorded as "positive." Trial DRG leads were placed based on the positive RF stimulation findings.

RESULTS

In each patient, stimulating one or more DRG(s) produced paresthesias patterns that were contradictory to know dermatomal patterns. Upon completion of a one-week trial all four patients reported 60-90% pain relief, with coverage over the painful areas, and opted for permanent implant.

CONCLUSIONS

Mapping the DRG via RF stimulation appears to provide improved accuracy for determining lead placement in the setting of PAP where pain patterns are known to deviate from conventional dermatomal mapping.

Serotonergic mechanisms of trigeminal meningeal nociception: Implications for migraine pain

Serotonergic mechanisms play a central role in migraine pathology. However, the region-specific effects of serotonin (5-HT) mediated via multiple types of receptors in the nociceptive system are poorly understood. Using extracellular and patch-clamp recordings, we studied the action of 5-HT on the excitability of peripheral and central terminals of trigeminal afferents. 5-HT evoked long-lasting TTX-sensitive firing in the peripheral terminals of meningeal afferents, the origin site of migraine pain. Cluster analysis revealed that in majority of nociceptive fibers 5-HT induced either transient or persistent spiking activity with prevailing delta and theta rhythms. The 5-HT3-receptor antagonist MDL-72222 or 5-HT1B/D-receptor antagonist GR127935 largely reduced, but their combination completely prevented the excitatory pro-nociceptive action of 5-HT. The 5-HT3 agonist mCPBG activated spikes in MDL-72222-dependent manner but the 5HT-1 receptor agonist sumatriptan did not affect the nociceptive firing. 5-HT also triggered peripheral CGRP release in meninges, which was blocked by MDL-72222.5-HT evoked fast membrane currents and Ca²⁺ transients in a fraction of trigeminal neurons. Immunohistochemistry showed expression of 5-HT3A receptors in fibers innervating meninges. Endogenous release of 5-HT from degranulated mast cells increased nociceptive firing. Low pH but not histamine strongly activated firing. 5-HT reduced monosynaptic inputs from trigeminal Aδ- and C-afferents to the upper cervical lamina I neurons and this effect was blocked by MDL-72222. Consistent with central inhibitory effect, 5-HT reduced CGRP release in the brainstem slices. In conclusion, 5-HT evokes powerful pro-nociceptive peripheral and anti-nociceptive central effects in trigeminal system transmitting migraine pain.

Diverse firing properties and Aβ-, Aδ-, and C-afferent inputs of small local circuit neurons in spinal lamina I

Spinal lamina I is a key element of the pain processing system, which integrates primary afferent input and relays it to supraspinal areas. More than 90% of neurons in this layer are local circuit neurons, whose role in the signal processing is poorly understood. We performed whole-cell recordings in a spinal cord preparation with attached dorsal roots to examine morphological features and physiological properties of small local circuit neurons (n = 47) in lamina I. Cells successfully filled with biocytin (n = 17) had fusiform (n = 10), flattened (n = 4), and multipolar (n = 3) somatodendritic morphology; their axons branched extensively and terminated in laminae I-III. Intrinsic firing properties were diverse; in addition to standard tonic (n = 16), adapting (n = 7), and delayed (n = 6) patterns, small local circuit neurons also generated rhythmic discharges (n = 6) and plateau potentials (n = 10), the latter were suppressed by the L-type Ca(2+)-channel blocker nifedipine. The neurons received monosynaptic inputs from Aδ and C afferents and could generate bursts of spikes on the root stimulation. In addition, we identified lamina I neurons (n = 7) with direct inputs from the low-threshold Aβ afferents, which could be picked up by ventral dendrites protruding to lamina III. Stimulation of afferents also evoked a disynaptic inhibition of neurons. Thus, small local circuit neurons exhibit diverse firing properties, can generate rhythmic discharges and plateau potentials, and their dendrites extending into several laminae allow broad integration of Aβ-, Aδ-, and C-afferent inputs. These properties are required for processing diverse modalities of nociceptive inputs in lamina I and may underlie spinal sensitization to pain.

Monosynaptic convergence of somatic and visceral C-fiber afferents on projection and local circuit neurons in lamina I: a substrate for referred pain

Spinal lamina I is the first site in the central nervous system where somatic and visceral pathways monosynaptically converge onto projection and local circuit neurons.

Referred pain is a phenomenon of feeling pain at a site other than the site of the painful stimulus origin. It arises from a pathological mixing of nociceptive processing pathways for visceral and somatic inputs. Despite numerous studies based on unit recordings from spinal and supraspinal neurons, the exact mechanism and site of this mixing within the central nervous system are not known. Here, we selectively recorded from lamina I neurons, using a visually guided patch-clamp technique, in thoracic spinal cord preparation with preserved intercostal (somatic) and splanchnic (visceral) nerves. We show that somatic and visceral C fibers converge monosynaptically onto a group of lamina I neurons, which includes both projection and local circuit neurons. Other groups of lamina I neurons received inputs from either somatic or visceral afferents. We have also identified a population of lamina I local circuit neurons showing overall inhibitory responses upon stimulation of both nerves. Thus, the present data allow us to draw two major conclusions. First, lamina I of the spinal cord is the first site in the central nervous system where somatic and visceral pathways directly converge onto individual projection and local circuit neurons. Second, the mechanism of somatovisceral convergence is complex and based on functional integration of monosynaptic and polysynaptic excitatory as well as inhibitory inputs in specific groups of neurons. This complex pattern of convergence provides a substrate for alterations in the balance between visceral and somatic inputs causing referred pain.

Predictive Factors for Successful Outcome of Pulsed Radiofrequency Treatment in Patients with Intractable Lumbosacral Radicular Pain

BACKGROUND

In a previous prospective study on pulsed radiofrequency (PRF) treatment adjacent to the lumbar dorsal root ganglion (DRG) for patients with chronic lumbosacral radicular pain, we reported success in 55.4% of the patients at 6 months. Identification of predictors for success after PRF may improve outcome. We assessed the predictors of PRF in patients with chronic intractable lumbosacral radicular pain.

METHODS

Patients with monosegmental chronic lumbosacral radicular pain of L5 or S1 first received a selective nerve root block at the corresponding level. Independent of the result of this block a PRF treatment at the same level was performed. At 6 weeks, 3 months, and 6 months after the procedure the outcome was evaluated.

RESULTS

A positive diagnostic nerve root block and age ≥ 55 were predictive factors for successful outcome at 6 months, while disability was a negative predictor.The use of failed back surgery syndrome, gender, duration of pain, Numerical Rating Scale, level and side of treatment, DN4, and RAND-36 as predictors for success was not supported.

CONCLUSIONS

Successful outcome after PRF adjacent to the DRG, in patients with intractable chronic lumbosacral radicular pain, is more likely in patients ≥ 55 years, with limited disability and after a positive diagnostic nerve root block. A combination of all these factors creates a fair predictive value (AUC: 0.73).

Neurons in the lateral part of the lumbar spinal cord show distinct novel axon trajectories and are excited by short propriospinal ascending inputs

The role of spinal dorsal horn propriospinal connections in nociceptive processing is not yet established. Recently described, rostrocaudally oriented axon collaterals of lamina I projection and local-circuit neurons (PNs and LCNs) running in the dorsolateral funiculus (DLF) may serve as the anatomical substrate for intersegmental processing. Putative targets of these axons include lateral dendrites of superficial dorsal horn neurons, including PNs, and also neurons in the lateral spinal nucleus (LSN) that are thought to be important integrator units receiving, among others, visceral sensory information. Here we used an intact spinal cord preparation to study intersegmental connections within the lateral part of the superficial dorsal horn. We detected brief monosynaptic and prolonged polysynaptic excitation of lamina I and LSN neurons when stimulating individual dorsal horn neurons located caudally, even in neighboring spinal cord segments. These connections, however, were infrequent. We also revealed that some projection neurons outside the dorsal grey matter and in the LSN have distinct, previously undescribed course of their projection axon. Our findings indicate that axon collaterals of lamina I PNs and LCNs in the DLF rarely form functional connections with other lamina I and LSN neurons and that the majority of their targets are on other elements of the dorsal horn. The unique axon trajectories of neurons in the dorsolateral aspect of the spinal cord, including the LSN do not fit our present understanding of midline axon guidance and suggest that their function and development differ from the neurons inside lamina I. These findings emphasize the importance of understanding the connectivity matrix of the superficial dorsal horn in order to decipher spinal sensory information processing.

Axon diversity of lamina I local-circuit neurons in the lumbar spinal cord

Spinal lamina I is a key area for relaying and integrating information from nociceptive primary afferents with various other sources of inputs. Although lamina I projection neurons have been intensively studied, much less attention has been given to local-circuit neurons (LCNs), which form the majority of the lamina I neuronal population. In this work the infrared light-emitting diode oblique illumination technique was used to visualize and label LCNs, allowing reconstruction and analysis of their dendritic and extensive axonal trees. We show that the majority of lamina I neurons with locally branching axons fall into the multipolar (with ventrally protruding dendrites) and flattened (dendrites limited to lamina I) somatodendritic categories. Analysis of their axons revealed that the initial myelinated part gives rise to several unmyelinated small-diameter branches that have a high number of densely packed, large varicosities and an extensive rostrocaudal (two or three segments), mediolateral, and dorsoventral (reaching laminae III-IV) distribution. The extent of the axon and the occasional presence of long, solitary branches suggest that LCNs may also form short and long propriospinal connections. We also found that the distribution of axon varicosities and terminal field locations show substantial heterogeneity and that a substantial portion of LCNs is inhibitory. Our observations indicate that LCNs of lamina I form intersegmental as well as interlaminar connections and may govern large numbers of neurons, providing anatomical substrate for rostrocaudal "processing units" in the dorsal horn.

Peripherally driven low-threshold inhibitory inputs to lamina I local-circuit and projection neurones: a new circuit for gating pain responses

Spinal lamina I is a key element of the pain processing system which relays primary afferent input to supraspinal areas. However, little is known about how the signal is modulated by its intrinsic network including local-circuit neurones (LCNs) and much less numerous anterolateral tract projection neurones (PNs). Here, we used whole-cell patch clamp recordings in an isolated spinal cord preparation to examine properties of identified LCNs (n = 85) and PNs (n = 73) in their functionally preserved local networks. Forty LCNs showed spontaneous rhythmic firing (2-7 Hz) at zero current injection, which persisted in the presence of blockers of fast synaptic transmission. In the remaining cases, most LCNs and PNs fired tonically in response to depolarizing current injections. We identified LCNs and PNs receiving low-threshold primary afferent-driven inhibitory inputs, which in many cases were disynaptic and temporally preceded classical high-threshold excitatory inputs. This direct inhibitory link between low-threshold afferents and PNs can function as a postsynaptic gate controlling the nociceptive information flow in the spinal cord. The LCNs were found to be integrated into the superficial dorsal horn network by their receipt of monosynaptic and disynaptic inputs from other lamina I and II neurones. One-third of LCNs and two-thirds of PNs tested responded to substance P application. Thus, substance P released by a noxious afferent stimulation may excite PNs in two ways: directly, and via the activation of presynaptic LCN circuitries. In conclusion, we have described important properties of identified lamina I neurones and their roles in a new circuit for gating pain responses.

Loss of the Reelin-signaling pathway differentially disrupts heat, mechanical and chemical nociceptive processing

The Reelin-signaling pathway regulates neuronal positioning during embryonic development. Reelin, the extracellular matrix protein missing in reeler mutants, is secreted by neurons in laminae I, II and V, binds to Vldl and Apoer2 receptors on nearby neurons, and tyrosine phosphorylates the adaptor protein Disabled-1 (Dab1), which activates downstream signaling. We previously reported that reeler and dab1 mutants had significantly reduced mechanical and increased heat nociception. Here we extend our analysis to chemical, visceral, and cold pain and importantly, used Fos expression to relate positioning errors in mutant mouse dorsal horn to changes in neuronal activity. We found that noxious mechanical stimulation-induced Fos expression is reduced in reeler and dab1 laminae I-II, compared to wild-type mice. Additionally, mutants had fewer Fos-immunoreactive neurons in the lateral-reticulated area of the deep dorsal horn than wild-type mice, a finding that correlates with a 50% reduction and subsequent mispositioning of the large Dab1-positive cells in the mutant lateral-reticulated area. Furthermore, several of these Dab1 cells expressed Fos in wild-type mice but rarely in reeler mutants. By contrast, paralleling the behavioral observations, noxious heat stimulation evoked significantly greater Fos expression in laminae I-II of reeler and dab1 mutants. We then used the formalin test to show that chemical nociception is reduced in reeler and dab1 mutants and that there is a corresponding decrease in formalin-induced Fos expression. Finally, neither visceral pain nor cold-pain sensitivity differed between wild-type and mutant mice. As differences in the nociceptor distribution within reeler and dab1 mutant dorsal horn were not detected, these differential effects observed on distinct pain modalities suggest that dorsal horn circuits are organized along modality-specific lines.

Heterosynaptic long-term potentiation at GABAergic synapses of spinal lamina I neurons

Neurons in spinal dorsal horn lamina I play a pivotal role for nociception that critically depends on a proper balance between excitatory and inhibitory inputs. Any modification in synaptic strength may challenge this delicate balance. Long-term potentiation (LTP) at glutamatergic synapses between nociceptive C-fibers and lamina I neurons is an intensively studied cellular model of pain amplification. In contrast, nothing is presently known about long-term changes of synaptic strength at inhibitory synapses in the spinal dorsal horn. Using a spinal cord-dorsal root slice preparation from rats, we show that conditioning stimulation of primary afferent fibers with a stimulating protocol that induces LTP at C-fiber synapses also triggered LTP at GABAergic synapses (LTP(GABA)). This LTP(GABA) was heterosynaptic in nature and was mediated by activation of group I metabotropic glutamate receptors. Opening of ionotropic glutamate receptor channels of the AMPA/KA or NMDA subtype was not required for LTP(GABA). Paired-pulse ratio, coefficient of variation, and miniature IPSCs analysis revealed that LTP(GABA) was expressed presynaptically. Nitric oxide as a retrograde messenger signal mediated this increase of GABA release at spinal inhibitory synapses. This novel form of synaptic plasticity in spinal nociceptive circuits may be an essential mechanism to maintain the relative balance between excitation and inhibition and to improve the signal-to-noise ratio in nociceptive pathways.

Non-peptidergic small diameter primary afferents expressing VGluT2 project to lamina I of mouse spinal dorsal horn

BACKGROUND

Unmyelinated primary afferent nociceptors are commonly classified into two main functional types: those expressing neuropeptides, and non-peptidergic fibers that bind the lectin IB4. However, many small diameter primary afferent neurons neither contain any known neuropeptides nor bind IB4. Most express high levels of vesicular glutamate transporter 2 (VGluT2) and are assumed to be glutamatergic nociceptors but their terminations within the spinal cord are unknown. We used in vitro anterograde axonal tracing with Neurobiotin to identify the central projections of these putative glutamatergic nociceptors. We also quantitatively characterised the spatial arrangement of these terminals with respect to those that expressed the neuropeptide, calcitonin gene-related peptide (CGRP).

RESULTS

Neurobiotin-labeled VGluT2-immunoreactive (IR) terminals were restricted to lamina I, with a medial-to-lateral distribution similar to CGRP-IR terminals. Most VGluT2-IR terminals in lateral lamina I were not labeled by Neurobiotin implying that they arose mainly from central neurons. 38 ± 4% of Neurobiotin-labeled VGluT2-IR terminals contained CGRP-IR. Conversely, only 17 ± 4% of Neurobiotin-labeled CGRP-IR terminals expressed detectable VGluT2-IR. Neurobiotin-labeled VGluT2-IR or CGRP-IR terminals often aggregated into small clusters or microdomains partially surrounding intrinsic lamina I neurons.

CONCLUSIONS

The central terminals of primary afferents which express high levels of VGluT2-IR but not CGRP-IR terminate mainly in lamina I. The spatial arrangement of VGluT2-IR and CGRP-IR terminals suggest that lamina I neurons receive convergent inputs from presumptive nociceptors that are primarily glutamatergic or peptidergic. This reveals a previously unrecognized level of organization in lamina I consistent with the presence of multiple nociceptive processing pathways.

Changes in synaptic transmission of substantia gelatinosa neurons in a rat model of lumbar radicular pain revealed by in vivo patch-clamp recording

Little is known about the pathophysiological mechanisms of radicular pain. We investigated changes in synaptic transmission of substantia gelatinosa (SG) neurons after an injury to the L5 nerve root using in vivo patch-clamp recording. A total of 141 SG neurons were recorded at L4 and L5 segmental levels of the spinal cord in root constriction rats and sham-operated control rats. At L4 and L5 segmental levels, SG neurons without a receptive field were observed only in root constriction rats, and the frequencies of spontaneous action potential firings in SG neurons were higher in the root constriction group than in the control group. At the L5 segmental level, the frequencies and amplitudes of spontaneous excitatory postsynaptic currents (EPSCs) as well as the proportion of multireceptive neurons among SG neurons was higher in the root constriction group than in the control group. At the L4 segmental level, the frequencies and amplitudes of spontaneous EPSCs were increased in the root constriction group, but the proportions of cell types did not change. The mean amplitudes of EPSCs evoked by mechanical stimuli at L4 and L5 segmental levels were larger in the root constriction group than in the control group. The results indicated that injuring the nerve root led to characteristic excitatory synaptic transmission in SG neurons at each segmental level and changed sensory processing in SG neurons at the segment to which the injured nerve projected. These changes could lead to spontaneous pain, mechanical allodynia, and hyperalgesia contributing to the pathogenesis of radicular pain.

Monosynaptic excitatory inputs to spinal lamina I anterolateral-tract-projecting neurons from neighbouring lamina I neurons

Spinal lamina I receives nociceptive primary afferent input to project through diverse ascending pathways, including the anterolateral tract (ALT). Large projection neurons (PNs) form only a few per cent of the cell population in this layer, and little is known about their local input from other lamina I neurons. We combined single-cell imaging in the isolated spinal cord, paired recordings, 3-D reconstructions of biocytin-labelled neurons and computer simulations to study the monosynaptic input to large ALT-PNs from neighbouring (somata separated by less than 80 μm) large lamina I neurons. All 11 connections identified were excitatory. We have found that an axon of a presynaptic neuron forms multiple synapses on an ALT-PN, and both Ca(2+)-permeable and Ca(2+)-impermeable AMPA receptors are involved in transmission. The monosynaptic EPSC latencies (1-12 ms) are determined by both post- and presynaptic factors. The postsynaptic delay, resulting from the electrotonic EPSC propagation in the dendrites of an ALT-PN, could be 4 ms at most. The presynaptic delay, caused by the spike propagation in a narrow highly branched axon of a local-circuit neuron, can be about 10 ms for neighbouring ALT-PNs and longer for more distant neurons. In many cases, the EPSPs evoked by release from a lamina I neuron were sufficient to elicit a spike in an ALT-PN. Our data show that ALT-PNs can receive input from both lamina I local-circuit neurons and other ALT-PNs. We suggest that lamina I is a functionally interconnected layer. The intralaminar network described here can amplify the overall output from the principal spinal nociceptive projection area.

Long and short multifunicular projections of sacral neurons are activated by sensory input to produce locomotor activity in the absence of supraspinal control

Afferent input from load and joint receptors has been shown to reactivate the central pattern generators for locomotion (CPGs) in spinal cord injury patients and thereby improve their motor function and mobility. Elucidation of the pathways interposed between the afferents and CPGs is critical for the determination of the capacity of sensory input to activate the CPGs when the continuity of the white matter tracts is impaired following spinal cord injury. Using electrophysiological recordings, confocal imaging studies of spinal neurons and surgical manipulations of the white matter, we show that the capacity of sacrocaudal afferent (SCA) input to produce locomotor activity in isolated rat spinal cords depends not only on long ascending pathways, but also on recruitment of sacral proprioneurons interposed between the second order neurons and the hindlimb CPGs. We argue that large heterogeneous populations of second-order and proprioneurons whose crossed and uncrossed axons project rostrally through the ventral, ventrolateral/lateral and dorsolateral white matter funiculi contribute to the generation of the rhythm by the stimulated sacrocaudal input. The complex organization and multiple projection patterns of these populations enable the sacrocaudal afferent input to activate the CPGs even if the white matter pathways are severely damaged. Further studies are required to clarify the mechanisms involved in SCA-induced locomotor activity and assess its potential use for the rescue of lost motor functions after spinal cord injury.