GABA_B receptors-mediated tonic inhibition of glutamate release from Aβ fibers in rat laminae III/IV of the spinal cord dorsal horn

Abstract

Presynaptic GABAB receptors (GABABRs) are highly expressed in dorsal root ganglion neurons and spinal cord dorsal horn. GABABRs located in superficial dorsal horn play an important antinociceptive role, by acting at both pre- and postsynaptic sites. GABABRs expressed in deep dorsal horn could be involved in the processing of touch sensation and possibly in the generation of tactile allodynia in chronic pain. The objective of this study was to characterize the morphological and functional properties of GABABRs expressed on Aβ fibers projecting to lamina III/IV and to understand their role in modulating excitatory synaptic transmission. We performed high-resolution electron microscopic analysis, showing that GABAB2 subunit is expressed on 71.9% of terminals in rat lamina III-IV. These terminals were engaged in axodendritic synapses and, for the 46%, also expressed glutamate immunoreactivity. Monosynaptic excitatory postsynaptic currents, evoked by Aβ fiber stimulation and recorded from lamina III/IV neurons in spinal cord slices, were strongly depressed by application of baclofen (0.1–2.5 µM), acting as a presynaptic modulator. Application of the GABABR antagonist CGP 55845 caused, in a subpopulation of neurons, the potentiation of the first of two excitatory postsynaptic currents recorded with the paired-pulse protocol, showing that GABABRs are endogenously activated. A decrease in the paired-pulse ratio accompanied the effect of CGP 55845, implying the involvement of presynaptic GABABRs. CGP 55845 facilitated only the first excitatory postsynaptic current also during a train of four consecutive stimuli applied to Aβ fibers. These results suggest that GABABRs tonically inhibit glutamate release from Aβ fibers at a subset of synapses in deep dorsal horn. This modulation specifically affects only the early phase of synaptic excitation in lamina III-IV neurons.

Impaired chromaffin cell excitability and exocytosis in autistic Timothy syndrome TS2‐neo mouse rescued by L‐type calcium channel blockers

“…Ultrathin sections were cut with an ultramicrotome (EM UC6, Leica Mikrosysteme Vertrieb GmbH, Wetzlar, Germany), collected on uncoated nickel grids (200 mesh) and counterstained with lead citrate (10 min) (Salio et al . ). Grids were observed with a JEM‐1010 transmission electron microscope (JEOL, Tokyo, Japan) equipped with a side‐mounted CCD camera (Mega View III, Olympus Soft Imaging System, GmbH, Muenster, Germany).…”

“…After washing in PB, the glands were post-fixed in osmium ferrocyanide (1 volume of 2% aqueous osmium tetroxide: 1 volume of 3% potassium ferrocyanide) for 1 h at 4°C, dehydrated for 15 min in increasing concentrations of acetone (30%, 60%, 90%, 100%), and then embedded in Spurr resin in 0.5 ml Eppendorf tubes. Ultrathin sections were cut with an ultramicrotome (EM UC6, Leica Mikrosysteme Vertrieb GmbH, Wetzlar, Germany), collected on uncoated nickel grids (200 mesh) and counterstained with lead citrate (10 min) (Salio et al 2017). Grids were observed with a JEM-1010 transmission electron microscope (JEOL, Tokyo, Japan) equipped with a side-mounted CCD camera (Mega View III, Olympus Soft Imaging System, GmbH, Muenster, Germany).…”

“…5D). Conventional electron microscopy was used to view the ultrastructure of CCs in intact adrenal medulla glands of WT and TS2-neo mice (Salio et al 2017). Transition electron microscopy images exhibited remarkable structural changes at both the nuclear and cytoplasmic levels (Fig.…”

Modulation of mechanosensory vibrissal responses in the trigeminocervical complex by stimulation of the greater occipital nerve in a rat model of trigeminal neuropathic pain

“…Moreover, GABA would be acting on presynaptic GABA B receptors, giving a negative feedback signal on the inhibitory afferent (reviewed in [107]). A similar GABA-mediated negative feedback has been shown to occur on glutamate release by Ab terminals in laminae III-IV [108].…”

“…With the GABA A receptors blocked by Bic, GABA could still activate GABA B receptors by the extrasynaptic diffusion of the transmitter, not just on a cell postsynaptic to the interneurons, but on other neurons as well [116]. These cells thus become less excitable by the activation of postsynaptic GABA B receptors [117], as will the vibrissal afferents themselves, since presynaptic GABA B receptors are expressed in terminals of Aβ bers in the spinal laminae III-IV [108]. The circuit diagram sketched in Fig.…”