Received for publication September 28, 2004.
Revised February 8, 2005.
Accepted for publication February 16, 2005.

Developmental regulation of -carboline-induced inhibition of glycine-evoked responses depends on glycine receptor subunit expression

Abstract

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

Key words: Glycine, Ion channel regulation

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