Transient, Activity Dependent Inhibition of Transmitter Release from Low Threshold Afferents Mediated by GABA_A Receptors in Spinal Cord Lamina III/IV

Abstract

BackgroundPresynaptic GABAA receptors (GABAARs) located on central terminals of low threshold afferent fibers are thought to be involved in the processing of touch and possibly in the generation of tactile allodynia in chronic pain. These GABAARs mediate primary afferent depolarization (PAD) and modulate transmitter release. The objective of this study was to expand our understanding of the presynaptic inhibitory action of GABA released onto primary afferent central terminals following afferent stimulation.ResultsWe recorded evoked postsynaptic excitatory responses (eEPSCs and eEPSPs) from lamina III/IV neurons in spinal cord slices from juvenile rats (P17–P23, either sex), while stimulating dorsal roots. We investigated time and activity dependent changes in glutamate release from low threshold A fibers and the impact of these changes on excitatory drive. Blockade of GABAARs by gabazine potentiated the second eEPSC during a train of four afferent stimuli in a large subset of synapses. This resulted in a corresponding increase of action potential firing after the second stimulus. The potentiating effect of gabazine was due to inhibition of endogenously activated presynaptic GABAARs, because it was not prevented by the blockade of postsynaptic GABAARs through intracellular perfusion of CsF. Exogenous activation of presynaptic GABAARs by muscimol depressed evoked glutamate release at all synapses and increased paired pulse ratio (PPR).ConclusionsThese observations suggest that afferent driven release of GABA onto low threshold afferent terminals is most effective following the first action potential in a train and serves to suppress the initial strong excitatory drive onto dorsal horn circuitry.

Counter-stimuli Inhibit GRPR Neurons via GABAergic Signaling in the Spinal Cord

“…That GRPR neurons receive both direct C/Aδ and indirect high threshold inputs from primary afferents is in line with earlier studies in primates and humans showing that primary afferent pruriceptors are high threshold C/A fibers (Handwerker et al, 1987;Johanek et al, 2008;Ringkamp et al, 2011;Schmelz et al, 1997). It is surprising that we failed to find significant A inputs onto GRPR neurons using transverse spinal cord slices, a preparation that has been shown to be suitable for studying mono-and polysynaptic responses mediated by Aδ fibers (Betelli et al, 2015;Torsney and MacDermott, 2006). However, we cannot exclude the possibility that synaptic circuits activating superficial GRPR neurons (sometimes selected because of the stronger fluorescent signal) were not entirely preserved, thereby contributing to an underestimation of the 13 amount of Aδ inputs received by GRPR neurons in the present study.…”

Pain Inhibits GRPR Neurons via GABAergic Signaling in the Spinal Cord

“…GRPR neurons receive both direct and indirect high threshold C/Aδ excitatory inputs from primary afferents, in line with earlier studies in primates and humans showing that primary afferent pruriceptors are high threshold C/A fibers 40–43 . However, it is surprising that we failed to find significant Aβ inputs onto GRPR neurons using transverse spinal cord slices, a preparation that has been shown to be suitable for studying mono- and polysynaptic responses mediated by Aβ fibers 44,45 . However, it is possible that synaptic circuits activating superficial GRPR neurons (sometimes selected because of the stronger fluorescent signal) were not preserved in our preparation, thereby contributing to an underestimation of the amount of Aβ inputs received by GRPR neurons in the present study.…”

“…Slice preparation and electrophysiological recordings were performed as previously described 45 . Briefly, GRPR- eGFP mice (P16-P25) were anesthetized with isoflurane and decapitated, the spinal cord and vertebrae were rapidly removed and placed in ice-cold dissecting Krebs’ solution (composition in mM:125 NaCl, 2.5 KCl, 1.25 NaH 2 PO 4 , 26 NaHCO 3 , 25 glucose, 6 MgCl 2 , 1.5 CaCl 2 , and 1 kynurenic acid, pH 7.4, 320 mOsm), bubbled with 95% O 2 , 5% CO 2 .…”

“…Spinal cord slices were prepared from GRPR- eGFP mice (P16-P25), as previously described 45 . Patch-clamp recording in whole-cell configuration was performed on visually identified fluorescent GRPR- eGFP neurons at room temperature 63 .…”

Chloride – The Underrated Ion in Nociceptors

“…In chicken embryo DRGs, uptake of [14C]2deoxyglucose as a marker of excitatory activity is facilitated in response to glycine stimulation, and taurine -a GlyR (but also GABA A ) agonist -causes depolarizing responses in frog primary afferents (Saji and Obata, 1981;Padjen et al, 1989). In contrast, GlyRs are not involved in presynaptic modulation of transmitter release in rat spinal cord sections and neither taurine nor glycine show any detectable agonist activity in mammalian DRG neurons (Robertson, 1989;Valeyev et al, 1996Valeyev et al, , 1999Betelli et al, 2015).…”