Inhibition of sigma-1 receptors substantially modulates GABA and glutamate transport in presynaptic nerve terminals

Abstract

Sigma-1 receptors (Sig-1Rs) have been implicated in many neurological and psychiatric disorders and are a novel target for the treatment of such disorders. Sig-1R expression/activity deficits are linked to neurodegeneration, whereas the mechanisms mediated by Sig-1R are still unclear. Here, presynaptic [³H]GABA and L-[¹⁴C]glutamate transport was analysed in rat brain nerve terminals (synaptosomes) in the presence of the Sig-1R antagonist NE-100. NE-100 at doses of 1 and 10 μM increased the initial rate of synaptosomal [³H]GABA uptake, whereas 50 and 100 μM NE-100 decreased this rate, exerting a biphasic mode of action.Antagonists of GABAA and GABAB receptors, flumazenil and saclofen, respectively, prevented an increase in [³H]GABA uptake caused by 10 μM NE-100. L-[¹⁴C]glutamate uptake was decreased by 10-100 μM NE-100. A decrease in the uptake of both neurotransmitters mediated by NE-100 (50-100 μM) may have resulted from simultaneous antagonist-induced membrane depolarization, which was measured using the potential-sensitive fluorescent dye rhodamine 6G. The extracellular level of [³H]GABA was decreased by 1-10 μM NE-100, but that of L-[¹⁴C]glutamate remained unchanged. The tonic release of [³H]GABA measured in the presence of NO-711 was not changed by the antagonist, suggesting that NE-100 did not disrupt membrane integrity. The KCl- and FCCP-induced transporter-mediated release of L-[¹⁴C]glutamate was decreased by the antagonist; this may underlie the neuroprotective action of the antagonist in hypoxia/ischaemia. NE-100 (10-100 μM) decreased the KCl-evoked exocytotic release of [³H]GABA and L-[¹⁴C]glutamate, whereas the induction of the release of both neurotransmitters by the Ca²⁺ ionophore ionomycin was not affected by the antagonist; therefore, the mitigation of KCl-evoked exocytosis was associated with the NE-100-induced dysfunction of potential-dependent Ca²⁺ channels. Therefore, the Sig-1R antagonist can specifically act in an acute manner at the presynaptic level through the modulation of GABA and glutamate uptake, transporter-mediated release and exocytosis.

Keywords: Antagonist NE-100; Brain nerve terminals; Exocytosis; Extracellular level; GABA; Glutamate; Sigma-1 receptors; Transporter-mediated uptake/release.