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

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.