[The GABA(A) receptor family: possibilities for the development of better anesthetics]

[Intern(euron)al affairs : The role of specific neocortical interneuron classes in the interaction between acetylcholine and GABAergic anesthetics]

Acetylcholine is a neuromodulator which is released throughout the central nervous system and plays an essential role in consciousness and cognitive processes including attention and learning. Due to its 'activating' effect on the neuronal and behavioral level its interaction with anesthetics has long been of interest to anesthesiologists. It is widely held that a reduction of the release of acetylcholine by general anesthetics constitutes part of the anesthetic effect. This notion is backed by numerous human and animal studies, but is also in seeming contradiction to findings that acetylcholine activates specific classes of inhibitory neurons: if acetylcholine excites elements within the neuronal network responsible for the release of the inhibitory neurotransmitter γ-aminobutyric acid (GABA), its withdrawal should diminish, not enhance, the effect of anesthetics.Focusing on cortical circuits, we present an overview of recent advances in cellular neurophysiology, particularly the interactions between inhibitory neuron classes, which provide insights on the interaction between acetylcholine and GABA.

Identification and characterization of anesthetic targets by mouse molecular genetics approaches

PURPOSE

It is now generally accepted that proteins are the primary targets of general anesthetics. However, the demonstration that the activity of a protein is altered by general anesthetics at clinically relevant concentrations in vitro does not provide direct evidence that this target mediates pharmacological actions of general anesthetics. Here we report on advances that have been made in identifying the contribution of individual ligand-gated ion channels to defined anesthetic endpoints using molecular mouse genetics.

PRINCIPAL FINDINGS

Gamma-aminobutyric acid (GABA)(A) receptor subtypes defined by the presence of the α1, α4, α5, β2, and β3 subunits and two-pore domain potassium channels (TASK-1, TASK-3, and TREK) have been discovered to mediate, at least in part, the hypnotic, immobilizing or amnestic actions of intravenous and volatile general anesthetics. Moreover, using tissues from genetically modified mice, specific functions of GABA(A) receptor subtypes in cortical and spinal neuronal networks were identified.

CONCLUSION

Genetically modified mice have been very useful for research on mechanisms of anesthesia and have contributed to the functional identification of general anesthetic targets and of the role of these targets in neuronal networks.

Metabolism of a 14C/3H-labeled GABAA receptor partial agonist in rat, dog and human liver microsomes: Evaluation of a dual-radiolabel strategy

The metabolism of 2-{[2-(3-fluoropyrid-2-yl)-1H-imidazol-1-yl]methyl}-1-propyl-5-cyano-1H-benzimidazole (1), a potent subtype-selective GABA(A) receptor partial agonist, was investigated in rat, dog and human liver microsomes. Due to its significant metabolic cleavage at C(8) observed in preliminary biotransformation studies with non-radiolabeled 1, both [(14)C]1 and [(3)H]1 were synthesized with respective radioisotopes placed on either side of C(8) to determine if all microsomal metabolites formed after C(8)N-dealkylation of 1 (or its core-intact metabolites) could be detected and quantified adequately. Both radiolabeled forms of 1, used separately in mono-radiolabel studies in cross-species microsomes and concomitantly in dual-radiolabel studies in rat microsomes, permitted the detection and quantification of all metabolites of 1, and a combination of radioactive and mass spectral data allowed structural elucidation of its Phase I metabolites. As expected, the sum of (14)C-only metabolites equaled that of (3)H-only metabolites in all incubations. In-line radiometric analysis worked extremely well (and was very reproducible) for quantifying either (14)C- or (3)H-compounds within separate incubations when using mono-radiolabeled 1. However, although the in-line radiodetector provided a comprehensive qualitative metabolic profile using dual-radiolabled 1, its inability to exclude completely (14)C- from (3)H-generated counts caused a degree of ambiguity pertaining to metabolite quantification. Thus, off-line liquid scintillation counting of collected dual-radiolabeled incubation LC-fractions was employed to quantify both (14)C- and (3)H-metabolites simultaneously, while in-line radiodetection was only used for qualitative analyses accompanying MS and MS/MS experiments. These studies demonstrated the analytical feasibility of using a dual-radiolabel approach for subsequent in vivo ADME studies with 1.