Loss of GABAergic Interneurons in Laminae I–III of the Spinal Cord Dorsal Horn Contributes to Reduced GABAergic Tone and Neuropathic Pain after Spinal Cord Injury

Abstract

In this study we explore if loss of GABAergic inhibitory interneurons in the superficial dorsal horn of the spinal cord contributes to reduced GABAergic tone and neuropathic pain following spinal cord injury (SCI). A moderate contusion injury to T11 resulted in the development of mechanical hyperalgesia and thermal hyperalgesia below the level of the lesion in gad1:GFP mice that were alleviated by IP administration of the GABA transporter antagonist tiagabine. Six weeks following SCI a decreased number of GFP(+) neurons were observed in the dorsal horn of SCI animals relative to sham mice. Tissue from a mouse 2 weeks post-SCI was subsequently observed to express activated caspase-3, indicative of apoptosis, co-localized to some GFP(+) GABAergic neurons. Glutamate decarboxylase (GAD)65 and GAD67 immunohistochemical staining was reduced in the dorsal horn of SCI animals. This observation was confirmed in Western blots showing reduced immunoreactivity for GAD67, as well as GABA transporter (GAT)1. Reversal of post-SCI neuropathic pain by tiagabine suggests that reduced GABAergic tone may contribute to hyperalgesia symptoms. This is supported by the subsequent observation that SCI reduced the number of GFP(+) inhibitory neurons, and the finding that some GABAergic GFP(+) neurons undergo cell death at a time point consistent with the development of neuropathic pain following SCI. Concordantly, reductions in both GAD65 and GAD67 and GAT1 immunoreactivity also support the observation of a loss of GABAergic inhibition and the associated spinal interneurons.

Lamina specific loss of inhibition may lead to distinct neuropathic manifestations: a computational modeling approach

“…As a result, the response of the P-Neuroid to "noxious" stimulation had an increase of ~17 and ~78% when the inhibitory influence exerted by the IC-(lamina II) and Gly-Neuroids (lamina III), respectively, was significantly reduced (50%), but it had a decrease of ~14% when the inhibitory tone provided by the I-Neuroid (lamina II) was reduced in the same proportion (Figure 4c). The latter apparently contradicts previous studies arguing that loss of inhibitory control at lamina II may lead to abnormal pain sensations (Meisner et al, 2010;Takazawa and MacDermott, 2010b). However, it can be seen from Figure 1 that inhibition of the I-Neuroid (which represents the islet cell reported by Lu and Perl (2003)) "disinhibits" the IC-Neuroid (the inhibitory central cell reported by Zheng et al (2010)), which in turn inhibits the afferent input from the Aδ-Neuroid to the vertical cell-Neuroid (V-Neuroid), thereby resulting in a diminished response of the P-Neuroid.…”

SNO-ing at the Nociceptive Synapse?

“…Thus, persistent nitrosylation of Drp1 may play a causal role in NO-mediated neurotoxicity (Barsoum et al, 2006). Although mitochondrial fission in the context of chronic pain is unexplored, there is emerging evidence that neuronal loss after axonal injury contributes to the development and maintenance of nerve injury-evoked neuropathic pain (Joseph and Levine, 2004;Scholz et al, 2005;Ilnytska et al, 2006;Meisner et al, 2010).…”

Abnormal cutaneous flexor reflex activity during controlled isometric plantarflexion in human spinal cord injury spasticity syndrome

“…41 Loss of segmental and descending Pl-TA CR modulation after SCI Several candidate pathophysiological mechanisms could mediate the loss of physiological inhibition of the Pl-TA CR observed during controlled isometric plantarflexion in subjects with the SCI spasticity syndrome. 43 At the spinal level, dysfunction of spinal inhibitory mechanisms mediated by gamma-aminobutyric acid is related to cutaneous hyperreflexia after SCI, 44,45 whereas other structural spinal changes are associated with reflex dysfunction as a consequence of the loss of supraspinal descending control mechanisms. 21 Damage to descending supraspinal pathways, 20,46 including the corticospinal 47,48 or extrapyramidal control systems, 19,46,49 also may contribute specifically to the change in CR modulation observed during controlled plantarflexion.…”