γ-Hydroxybutyrate receptor function determined by stimulation of rubidium and calcium movements from NCB-20 neurons

gamma-Hydroxybutyrate is derived from GABA in brain and plays specific functional roles in the CNS. It is thought to exert a tonic inhibitory control on dopamine and GABA release in certain brain areas, through specific gamma-hydroxybutyrate receptors. Apart from modifying certain calcium currents, the specific transduction mechanism induced by stimulation of gamma-hydroxybutyrate receptors remains largely unknown. We investigated the possible contribution of K+ channels to the hyperpolarization phenomena generally induced by gamma-hydroxybutyrate in brain, by monitoring Rb-86(+) movements in a neuronal cell line (NCB-20 cells), which expresses gamma-hydroxybutyrate receptors. Physiological concentrations of gamma-hydroxybutyrate (5-25 muM) induce a slow efflux of Rb-86(+), which peaks at 5-15 min and returns to baseline levels 20 min later after constant stimulation. This effect can be reproduced by the gamma-hydroxybutyrate receptor agonist NCS-356 and blocked by the gamma-hydroxybutyrate receptor antagonist 6,7,8,9-tetrahydro-5-[H]-benzocycloheptene-5-ol-4-ylidene. The GABA(B) receptor antagonist CGP 55,845 has no effect on gamma-hydroxybutyrate-induced Rb-86(+) efflux. The pharmacology of this gamma-hydroxybutyrate-dependent efflux of Rb-86(+) is in favor of the involvement of tetraethylammonium and charybdotoxin insensitive, apamin sensitive Ca2+ activated K+ channels, identifying them as small conductance calcium activated channels. We demonstrated a gamma-hydroxybutyrate dose-dependent entry of calcium ions into NCB-20 neuroblastoma cells at resting potential. Electrophysiological data showed that this Ca2+ entry corresponded mainly to a left-hand shift of the current/voltage relation of the T-type calcium channel. This process must at least partially trigger small conductance calcium activated channel activation leading to gamma-hydroxybutyrate-induced hyperpolarization. (C) 2003 IBRO. Published by Elsevier Science Ltd. All rights reserved.