Programme

The conductance of extrasynaptic GABA_A channels in newborn rat hippocampal neurons is increased by alphaxalone and pregnanolone

GABA_A receptors are the major inhibitory neurotransmitter receptors in the brain. They form chloride ion channels that are opened by GABA and modulated by a variety of clinically important drugs such as anaesthetics, convulsants and barbiturates. GABA_A receptors are found both at the synapse and non-synaptically on the cell body. It has been suggested that non-synaptic GABA_A receptors may provide a tonic background inhibition of neurons. It is these receptors that are normally studied using isolated patch techniques. Neuroactive steroids are a group of potent modulators of GABA_A receptors. In this study single channel chloride currents activated by GABA were recorded in cell-attached and inside-out patches from cultured hippocampal neurons obtained from newborn rats after they were decapitated using protocols approved by the Animal Ethics Committee (JBM ) of the Australian National University and Heidelberg University. The effects of the anaesthetic neurosteroid alphaxalone and the endogenous steroid pregnanolone were tested on GABA_A channels. Alphaxalone at concentrations above 1 μM, and pregnanolone at 0.1 μM, increased the conductance of channels activated by GABA. Both steroids at higher concentrations could directly activate chloride channels and generally these channels exhibited a high conductance. The directly activated channels were modulated by diazepam and it was concluded that they were GABA_A channels. An important site of action of neurosteroids may be extrasynaptic GABA_A receptors.