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

Exploring the Relationship Between Brain Neurochemistry, Cervical Impairments and Pain Sensitivity in People with Migraine, Whiplash-Headache, Low Back Pain and Healthy Controls: A Secondary Analysis of a Cross-Sectional Case-Control Study

Background/Objectives: Gamma-Aminobutyric Acid (GABA) and glutamate are the main inhibitory and excitatory neurochemicals of the central nervous system. Recently, increased GABA+ (GABA+ macromolecules) and Glx (glutamate and glutamine) levels have been reported in migraine. Conversely, decreased GABA+ and Glx levels have been reported in conditions such as chronic musculoskeletal pain and other chronic widespread pain conditions. This has led to the hypothesis that unique neurochemical profiles may underpin different headache and pain conditions. What is currently unknown is how neurochemical levels correlate with different clinical presentations of local and widespread pain sensitivity. The aims of this study were therefore to (i) explore the relationship between brain neurochemicals and clinical presentations of different headache and pain conditions and (ii) use a novel approach to explore how participants cluster based on their neurochemical profiles and explore the clinical characteristics of the participants in these neurochemical clusters. Methods: In this exploratory secondary analysis of a cross-sectional study, participants with migraine (n = 20), whiplash-headache (n = 20), and low back pain (n = 20), and healthy controls (n = 21) completed pain, disability and psychological distress questionnaires, received Magnetic Resonance Spectroscopy (MEGAPRESS), and underwent cervical musculoskeletal and quantitative sensory testing. Participants were classified based on cervical musculoskeletal impairment, increased cervical pain sensitivity, and central sensitization. Correlations between neurochemical levels and clinical classifications were explored. Cluster analysis was used to determine how participants grouped based on their neurochemical profiles. Pain, disability and psychological distress scores and clinical classifications were then compared between the resultant clusters. Post hoc testing explored increased cervical pain sensitivity within the clusters. Results: GABA+ levels moderately correlated with increased cervical pain sensitivity (r2 = 0.31, p = 0.006), with no other significant correlations. Cluster analysis revealed three neurochemical profiles, Cluster 1 (Low GABA+ levels) had moderate disability, Cluster 2 (highest Glx levels) had the lowest pain and disability, and Cluster 3 (highest GABA+ levels) had the highest pain and disability. Post hoc testing demonstrated that the cluster with the highest GABA+ levels (Cluster 3) had the most cervical pain sensitivity. Conclusions: This study suggests that considering the pain condition or presence of central sensitization alone is not sufficient to explain GABA+ and Glx levels. Our findings suggest that increased cervical pain sensitivity might be more reflective of GABA+ levels than pain condition or central sensitization and would benefit from further investigation to further elucidate the relationship between brain neurochemicals and clinical characteristics of pain sensitivity.

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".

Neuralgia in the occipital region associated with ipsilateral trigeminal herpes zoster: Three case reports

BACKGROUND

Nerve fibers related to pain and temperature sensation in the trigeminal nerve territory converge with the upper cervical spinal nerves from the level of the lower medulla oblongata to the upper cervical cord. This structure is called the trigemino-cervical complex and may cause referred pain in the territory of the trigeminal or upper cervical spinal nerves.

CASE SERIES

Here, we report three cases of paroxysmal neuralgia in the occipital region with mild conjunctivitis or a few reddish spots in the ipsilateral trigeminal nerve territory. The patients exhibited gradual progression of these reddish spots evolving into vesicles over the course of several days, despite the absence of a rash in the occipital region. The patients were diagnosed with trigeminal herpes zoster and subsequently received antiherpetic therapy. Remarkably, the neuralgia in the occipital region showed gradual amelioration or complete resolution before the treatment, with no sequelae reported in the occipital region.

DISCUSSION

The trigemino-cervical complex has the potential to cause neuralgia in the occipital region, as referred pain, caused by trigeminal herpes zoster. These cases suggest that, even if conjunctivitis or reddish spots appear to be trivial in the trigeminal nerve territory, trigeminal herpes zoster should be considered when neuralgia occurs in the ipsilateral occipital region.

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.