286 Heterogeneity of GABAA-receptors: impact on terminology Mohler, H. Zurich, Switzerland Manuscript not received. Proposal of a new GABAA-benzodiazepine receptor nomenclature based on the subunit composition of GABA A receptors Sieghart, W. Department of Biochemical Psychiatry, Universit?itsklinik fiir Psychiatrie, Vienna, Austria Key words: GABA h receptors, Nomenclature, Subunit composition GABA A receptors are iigand gated chloride ion channels and the site of action of a variety of clinically important drugs. Thus, it has been demonstrated that benzodiazepines, some barbiturates, cage convulsant compounds and some steroids modulate the function of these receptors by their interaction with GABA A receptor associated specific allosteric binding sites. Molecular biological studies have so far identified 6a-, 4p-, 2T- and 18-subunit of the GABAA receptor, and expression studies have indicated that GABA A receptors with correct pharmacology can only be obtained reproducibly when a-, p- and T-subunits are present simultaneously in a cell. Other studies have demonstrated that the pharmacological and electrophysiological properties vary significantly with the subunit composition of the GABAA receptors. Given the considerable overlap in the regional distribution of the various subunits and their mRNAs, the existence of a large variety of different subunit combinations and thus different GABA A receptors is possible. Even if not all possible combinations of GABAA receptor subunits are expressed in vivo, it can be expected that the number of identified GABA A receptors with distinct properties will dramatically increase over the next few years in parallel with the development of selective new compounds. Such a large number of expected GABAA receptors in a rapidly developing field cannot be handled by a simple numbering system naming receptors according to the order of their discovery. This always causes confusion in the literature since it never can be avoided that different receptors are discovered at approximately the same time and thus get numbered identically. In addition, this system of naming receptors according to the order of their discovery depends on the awareness and acceptance by the scientific community of even preliminary data on a possibly new receptor subtype. If these data finally turn out to be wrong, the gap in the numbering system has to be filled with another newly discovered receptor subtype. This again would cause some confusion in the literature. A second reason for avoiding a simple numbering system is given by the existence of several different allosteric binding sites on the GABA h receptor complex. At present, the only compounds which are able to distinguish between different GABA a receptors are benzodiazepine receptor ligands. Binding of these compounds seems to be predominantly influenced by the type of the ct- and 7-subunit present in the GABAA receptor. It can be assumed, however, that depending on the subunit composition there are differences not only in the benzodiazepine but also in the GABA-, barbiturate-, steroid- and picrotoxin/TBPT-binding site of GABA A receptors. If the binding properties of these sites are more strongly influenced by