AMPA- and P2X7-receptor-mediated facilitation of [ 3H] d-aspartate release from nerve terminals isolated from the rat caudal brainstem

Abstract

Glutamate (GLU) plays a key role in the transmission and modulation of sensory input to the trigeminal caudal nuclei (TCN). In the present study, we investigated the regulation of previously taken-up [ 3H] d-aspartate ([ 3H] d-ASP) release from nerve terminals isolated from rat caudal brainstem, in particular from the zone containing the TCN. TCN neurons can be considered integrative relay neurons linking peripheral and central pain mechanisms. Understanding the mechanisms that control the release of GLU in this area could lead to more effective treatment of migraines and other types of pain associated with the trigeminal nerve. In isolated rat caudal brainstem synaptosomes, exposure to AMPA dose-dependently potentiated [K +] e-stimulated release of [ 3H] d-ASP (maximum increase: 218 ± 13.08%; EC 50: 1.60 ± 0.08 μM). This effect was inhibited by selective AMPA-receptor antagonists (competitive [NBQX] and non-competitive [GYKI52466]) but not by the kainate receptor subunit antagonists NS102 and ACET. AMPA-evoked responses were significantly enhanced by preventing AMPA receptor desensitization with cyclothiazide (10 μM). Basal release of [ 3H] d-ASP was stimulated by millimolar concentrations of ATP (maximum increase: 197.80 ± 11.85%; EC 50: 545 ± 3.15 μM) and by the selective P2X7-receptor agonist benzoylbenzoyl-ATP. ATP also potentiated the release of [ 3H] d-ASP induced by depolarization. Its effect on basal [ 3H] d-ASP release was inhibited by the selective P2X7-receptor antagonist A-438079 and by the non-selective antagonist PPADS, but it was only partially suppressed by the ionotropic purinergic receptor antagonist TNP-ATP. Our findings demonstrate that glutamatergic nerve terminals in rat caudal brainstem express AMPA receptors that can facilitate [ 3H] d-ASP during terminal depolarization and P2X7 receptors that can also enhance this release under basal conditions.