Molecular and neuronal substrates for general anaesthetics

General anaesthetics are much more selective than is usually appreciated and may act by binding to only a small number of targets in the central nervous system. At surgical concentrations their principal effects are on ligand-gated (rather than voltage-gated) ion channels, with potentiation of postsynaptic inhibitory channel activity best fitting the pharmacological profile observed in general anaesthesia. Although the role of second messengers remains uncertain, it is now clear that anaesthetics act directly on proteins rather than on lipids.

GABA(A) (gamma-aminobutyric acid(A)) receptors are molecular substrates for the regulation of vigilance, anxiety, muscle tension, epileptogenic activity and memory functions, which is evident from the spectrum of actions elicited by clinically effective drugs acting at their modulatory benzodiazepine-binding site. Here we show, by introducing a histidine-to-arginine point mutation at position 101 of the murine alpha1-subunit gene, that alpha1-type GABA(A) receptors, which are mainly expressed in cortical areas and thalamus, are rendered insensitive to allosteric modulation by benzodiazepine-site ligands, whilst regulation by the physiological neurotransmitter gamma-aminobutyric acid is preserved. alpha1(H101R) mice failed to show the sedative, amnesic and partly the anticonvulsant action of diazepam. In contrast, the anxiolytic-like, myorelaxant, motor-impairing and ethanol-potentiating effects were fully retained, and are attributed to the nonmutated GABA(A) receptors found in the limbic system (alpha2, alpha5), in monoaminergic neurons (alpha3) and in motoneurons (alpha2, alpha5). Thus, benzodiazepine-induced behavioural responses are mediated by specific GABA(A) receptor subtypes in distinct neuronal circuits, which is of interest for drug design.