Synaptic conversion of chloride-dependent synapses in spinal nociceptive circuits: roles in neuropathic pain

Moderate to severe acute pain disturbs motor cortex intracortical inhibition and facilitation in orthopedic trauma patients: A TMS study

OBJECTIVE

Primary motor (M1) cortical excitability alterations are involved in the development and maintenance of chronic pain. Less is known about M1-cortical excitability implications in the acute phase of an orthopedic trauma. This study aims to assess acute M1-cortical excitability in patients with an isolated upper limb fracture (IULF) in relation to pain intensity.

METHODS

Eighty-four (56 IULF patients <14 days post-trauma and 28 healthy controls). IULF patients were divided into two subgroups according to pain intensity (mild versus moderate to severe pain). A single transcranial magnetic stimulation (TMS) session was performed over M1 to compare groups on resting motor threshold (rMT), short-intracortical inhibition (SICI), intracortical facilitation (ICF), and long-interval cortical inhibition (LICI).

RESULTS

Reduced SICI and ICF were found in IULF patients with moderate to severe pain, whereas mild pain was not associated with M1 alterations. Age, sex, and time since the accident had no influence on TMS measures.

DISCUSSION

These findings show altered M1 in the context of acute moderate to severe pain, suggesting early signs of altered GABAergic inhibitory and glutamatergic facilitatory activities.

Contributions of Nociresponsive Area 3a to Normal and Abnormal Somatosensory Perception

Traditionally, cytoarchitectonic area 3a of primary somatosensory cortex (SI) has been regarded as a proprioceptive relay to motor cortex. However, neuronal spike-train recordings and optical intrinsic signal imaging, obtained from nonhuman sensorimotor cortex, show that neuronal activity in some of the cortical columns in area 3a can be readily triggered by a C-nociceptor afferent drive. These findings indicate that area 3a is a critical link in cerebral cortical encoding of secondary/slow pain. Also, area 3a contributes to abnormal pain processing in the presence of activity-dependent reversal of gamma-aminobutyric acid A receptor-mediated inhibition. Accordingly, abnormal processing within area 3a may contribute mechanistically to generation of clinical pain conditions. PERSPECTIVE: Optical imaging and neurophysiological mapping of area 3a of SI has revealed substantial driving from unmyelinated cutaneous nociceptors, complementing input to areas 3b and 1 of SI from myelinated nociceptors and non-nociceptors. These and related findings force a reconsideration of mechanisms for SI processing of pain.

Neuroinflammation, neuroautoimmunity, and the co-morbidities of complex regional pain syndrome

Complex Regional Pain Syndrome (CRPS) is associated with non-dermatomal patterns of pain, unusual movement disorders, and somatovisceral dysfunctions. These symptoms are viewed by some neurologists and psychiatrists as being psychogenic in origin. Recent evidence, however, suggests that an autoimmune attack on self-antigens found in the peripheral and central nervous system may underlie a number of CRPS symptoms. From both animal and human studies, evidence is accumulating that neuroinflammation can spread, either anterograde or retrograde, via axonal projections in the CNS, thereby establishing neuroinflammatory tracks and secondary neuroinflammatory foci within the neuraxis. These findings suggest that neuroinflammatory lesions, as well as their associated functional consequences, should be evaluated during the differential diagnosis of non-dermatomal pain presentations, atypical movement disorders, as well as other "medically unexplained symptoms", which are often attributed to psychogenic illness.

Measuring GABAergic inhibitory activity with TMS-EEG and its potential clinical application for chronic pain

Chronic pain is debilitating disorder in which the underlying pathophysiology is still unknown. Impaired cortical inhibition is one mechanism that is associated with chronic pain. Cortical inhibition refers to a neurophysiological process in which gamma-aminobutyric acid (GABA) inhibitory interneurons selectively attenuate the activity of pyramidal neurons in the cortex. Previous studies have capitalized on the ability of transcranial magnetic stimulation (TMS) to index cortical inhibition by stimulating the motor cortex and measuring the resulting peripheral motor evoked potentials with electromyography. Chronic pain has been shown to induce changes in cortical inhibition within the motor cortex using TMS. Electroencephalography (EEG) studies also demonstrate that gamma (30-50 Hz) oscillations in the prefrontal and somatosensory cortex are associated with the experience of pain. As gamma oscillations are mediated by GABA, the combination of TMS with EEG allows for the examination of the relationship between cortical inhibition, gamma and chronic pain. In this paper, we summarize the evidence of impaired GABAergic and gamma oscillations in chronic pain patients. We then demonstrate TMS-EEG as a reliable method in which to record cortical inhibition directly from the prefrontal cortex to examine the modulatory effect of GABAB receptor inhibition on cortical oscillations. Finally, the modulation of GABA and gamma oscillations with repetitive TMS will be suggested as the possible mechanism through which rTMS exerts its therapeutic effects in the treatment of pain. The aim of this paper, therefore, is to present the TMS-EEG as a potential method through which to better classify, diagnose and treat chronic pain.