Activation of alpha-2 adrenergic receptors stimulates GABA release by astrocytes

Abstract

Norepinephrine is one of the key neurotransmitters in the hippocampus, but its role in the functioning of the neuroglial networks remains unclear. Here we show that norepinephrine suppresses NH₄ Cl-induced oscillations of the intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in hippocampal neurons. We found that the inhibitory effect of norepinephrine against ammonium-induced [Ca²⁺ ]_i oscillations is mediated by activation of alpha-2 adrenergic receptors. Furthermore, UK 14,304, an agonist of alpha-2 adrenergic receptors, evokes a biphasic [Ca²⁺ ]_i elevation in a minor population of astrocytes. This elevation consists of an initial fast, peak-shaped [Ca²⁺ ]_i rise, mediated by G_iβγ subunit and subsequent PLC-induced mobilization of Ca²⁺ from internal stores, and a plateau phase, mediated by a Ca²⁺ influx from the extracellular medium through store-operated and TRPC3 channels. We show the correlation between the Ca²⁺ response in astrocytes and suppression of [Ca²⁺ ]_i oscillations in neurons. The inhibitory effect of UK 14,304 is abolished in the presence of gallein, an inhibitor of G_βγ -signaling. In turn, application of the agonist in the presence of the PLC inhibitor decreases the frequency and amplitude of [Ca²⁺ ]_i oscillations in neurons but does not suppress them. The same effect is observed in the presence of bicuculline, a GABA(A) receptor antagonist. We demonstrate that UK 14,304 application increases the frequency and amplitude of slow outward chloride currents in neurons, indicating the release of GABA by astrocytes. Thus, our findings indicate that the activation of astrocytic alpha-2 adrenergic receptors stimulates GABA release from astrocytes via G_iβγ subunit-associated signaling pathway, contributing to the suppression of neuronal activity.

Keywords: GABA; alpha-2 adrenergic receptors; astrocytes; hyperammonemia; norepinephrine; βγ-signaling.