Spinal nerve ligation decreases γ‐aminobutyric acid_B receptors on specific populations of immunohistochemically identified neurons in L5 dorsal root ganglion of the rat

Abstract

This study examined the distribution of γ-aminobutyric acid (GABA)B receptors on immunohistochemically identified neurons, and levels of GABAB(1) and GABAB(2) mRNA, in the L4 and L5 dorsal root ganglia (DRG) of the rat in the absence of injury and 2 weeks after L5 spinal nerve ligation. In uninjured DRG, GABAB(1) immunoreactivity colocalized exclusively with the neuronal marker (NeuN) and did not colocalize with the satellite cell marker S-100. The GABAB(1) subunit colocalized to >97% of DRG neurons immunoreactive (IR) for neurofilament 200 (N52) or calcitonin gene-related peptide (CGRP), or labeled by isolectin B4 (IB4). Immunoreactivity for GABAB(2) was not detectable. L5 spinal nerve ligation did not alter the number of GABAB(1)-IR neurons or its colocalization pattern in the L4 DRG. However, ligation reduced the number of GABAB(1)-IR neurons in the L5 DRG by ≈38% compared with sham-operated and naïve rats. Specifically, ligation decreased the number of CGRP-IR neurons in the L5 DRG by 75%, but did not decrease the percent colocalization of GABAB(1) in those that remained. In the few IB4-positive neurons that remained in the L5 DRG, colocalization of GABAB(1)-IR decreased to 75%. Ligation also decreased levels of GABAB(1) and GABAB(2) mRNA in the L5, but not the L4 DRG compared with sham-operated or naïve rats. These findings indicate that the GABAB receptor is positioned to presynaptically modulate afferent transmission by myelinated, unmyelinated, and peptidergic afferents in the dorsal horn. Loss of GABAB receptors on primary afferent neurons may contribute to the development of mechanical allodynia after L5 spinal nerve ligation.

G protein βγ subunits inhibit TRPM3 ion channels in sensory neurons

“…We next examined whether GPCR inhibition of TRPM3 was specific to opioid receptors or could be extended to other G i/o -coupled receptors expressed on sensory neurons. The metabotropic G i/o -coupled receptors for GABA, GABA B1 and GABA B2 , are expressed by a high percentage (60–90%) of peripheral sensory neurons ( Charles et al, 2001 ; Cuny et al, 2012 ; Engle et al, 2012 ) and recently, activation of GABA B1 was shown to modulate activity of TRPV1 channels ( Hanack et al, 2015 ). Treatment of DRG neurons with the selective GABA B agonist baclofen (100 µM), significantly reduced the amplitude of the second PS response to R = 12 ± 2% compared to R = 58 ± 6% in control experiments (p<0.001, Figure 3a,b ) and abolished (R < 5%) the PS-evoked [Ca 2+ ] i responses in 67% (n = 130/194) of neurons.…”

Effects of baclofen on mechanical noxious and innocuous transmission in the spinal dorsal horn of the adult rat: in vivo patch‐clamp analysis

“…For example, reductions in levels of GABA (Castro‐Lopes et al ., ; Ibuki et al ., ; Eaton et al ., ) and its receptors (Bhisitkul et al ., ; Castro‐Lopes et al ., ) have been reported after axotomy and chronic constriction injury of the sciatic nerve. In addition, a loss of GABAergic neurons and a decrease in amount of GABA‐synthesising enzyme followed by a reduction in GABA levels were observed after chronic constriction injury or spared nerve injury in the SG (Moore et al ., ; Scholz et al ., ) and the DRG (Engle et al ., ), although a contradictory result was recently reported (Polgar et al ., ).…”

Effect of Electroacupuncture on Thermal Pain Threshold and Expression of Calcitonin-Gene Related Peptide, Substance P and γ-Aminobutyric Acid in the Cervical Dorsal Root Ganglion of Rats with Incisional Neck Pain

“…The GABAB1 subunit has been shown to be co-localised to >97% of DRG neurons immunoreactive (IR) for CGRP. Loss of GABAB receptors on primary afferent neurons may lead to mechanical allodynia, 19 while application of GABA agonists to the DRG attenuates pain behaviour in rats with neuropathic pain. 20 …”