The aim of the present study was to investigate whether a GABAB receptor agonist could modulate ATP-activated neuronal excitability of nociceptive TRG neurons using perforated whole-cell patch-clamp and immunohistochemical techniques. Immunohistochemical analysis revealed that 86% of P2X3 receptor-immunoreactive, small-diameter TRG neurons co-expressed GABAB receptor. Under voltage-clamp conditions (Vh=-60mV), application of ATP activated the inward current in acutely isolated rat TRG neurons in a dose-dependent manner (10-50 μM) and this current could be blocked by pyridoxal-phosphate-6-azophenyl-27,47-disulfonic acid (PPADS) (10 μM), a selective P2 purinoreceptor antagonist. The peak amplitude of ATP-activated currents was significantly inhibited after application of GABAB receptor agonist, baclofen (10-50 μM), in a concentration-dependent and reversible manner. The baclofen-induced inhibition of ATP-activated current was abolished by co-application of 3-amino-2 (4-chlorophenyl)-2hydroxypropysufonic acid) saclofen, a GABAB receptor antagonist (50 μM). Under current-clamp conditions, application of 20 μM ATP significantly depolarized the membrane potential resulting in increased mean action potential frequencies, and these ATP-induced effects were significantly inhibited by baclofen and these effects were antagonized by co-application of saclofen. Together, the results suggested that GABAB receptor activation could inhibit the ATP-induced excitability of small-diameter TRG neurons activated through the P2X3 receptor. Thus, the interaction between P2X3 and GABAB receptors of small-diameter TRG neuronal cell bodies is a potential therapeutic target for the treatment of trigeminal nociception.
Keywords: ATP; CGRP; DRG; EDTA; EGTA; GABA; GABA(B) receptor; HEPES; N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid; P(2)X3 receptor; PIPES; PPADS; Paracrine; SGCs; SP; TRG; TRGs; TTX-R; Trigeminal ganglion; Whole-cell patch clamping; adenosine triphosphate; calcitonin gene-related peptide; dorsal root ganglion; ethylene glycol-bis-β-aminoethyl ether N,N,N′,N′-tetraaceticacid; ethylenediaminetetraaceticacid; piperazine-N,N′-bis [2-ethanesulfonic acid]; pyridoxal-phosphate-6-azophenyl-27,47-disulfonic acid; satellite glial cells; substance P; tetrodotoxin-resistant; trigeminal ganglia; trigeminal ganglion; γ-aminobutyric acid.
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