Abstract

In this work, we show that β-carbolines, which are known negative allosteric modulators of GABA_A receptors, inhibit glycine-induced currents of embryonic mouse spinal cord and hippocampal neurons. In both cell types, β-carboline-induced inhibition of glycine receptor (GlyR)-mediated responses decreases with time in culture. Single-channel recordings show that the major conductance levels of GlyR unitary currents shifts from high levels (≥50 pS) in 2 to 3 days in vitro (DIV) neurons to low levels (<50 pS) in 11 to 14 DIV neurons, assessing the replacement of functional homomeric GlyR by heteromeric GlyR. In cultured spinal cord neurons, the disappearance of β-carboline inhibition of glycine responses and high conductance levels is almost complete in mature neurons, whereas a weaker decrease in β-carboline-evoked glycine response inhibition and high conductance level proportion is observed in hippocampal neurons. To confirm the hypothesis that the decreased sensitivity of GlyR to β-carbolines depends on β subunit expression, Chinese hamster ovary cells were permanently transfected either with GlyR α2 subunit alone or in combination with GlyR β subunit. Single-channel recordings revealed that the major conductance levels shifted from high levels (≥50 pS) in GlyR-α2-transfected cells to low levels (<50 pS) in GlyR-α2+β-containing cells. Consistently, both picrotoxin- and β-carboline-induced inhibition of glycine-gated currents were significantly decreased in GlyR-α2+β-transfected cells compared with GlyR-α2-containing cells. In summary, we demonstrate that the incorporation of β subunits in GlyRs confers resistance not only to picrotoxin but also to β-carboline-induced inhibition. Furthermore, we also provide evidence that hippocampal neurons undergo in vitro a partial maturation process of their GlyR-mediated responses.