Brain Research, 330 (1985) 135-140 135 Elsevier BRE 10613 Securinine Alkaloids: A New Class of GABA Receptor Antagonist J. A. BEUTLER 1, E. W. KARBON 1, A. N. BRUBAKER 2, R. MALIK 3, D. R. CURTIS 3 and S. J. ENNA 1 1Departments of Pharmacology and of Neurobiology and Anatomy, Universityof Texas Medical School at Houston, P.O. Box 20708, Houston, TX, 2Department of Pharmacal Sciences, Auburn UniversitySchool of Pharmacy, Auburn, AL (U.S.A.), 3Department of Pharmacology, John Curtin School of Medical Research, Canberra (Australia) (Accepted June 26th, 1984) Key words: securinine -- GABA receptors -- bicuculline -- GABA receptor antagonist Experiments were undertaken to determine the site of action of securinine and related convulsant indolizidines. All of these com- pounds induced tonic seizures in mice, with CD50 values ranging from 11 to 87 mg/kg. The CDs0 for bicuculline was found to be 8 mg/kg. Equilibrium binding assays revealed that securinine and dihydrosecurinine inhibit [3H]GABA binding to rat brain mem- branes with an IC50 of approximately 50 aM, which is some 7 times less potent than bicuculline. Allosecurinine and virosecurinine have IC50 values greater than 1 mM. Both dihydrosecurinine and securinine inhibited GABA-stimulated benzodiazepine binding in rat brain membranes, though they were somewhat weaker than bicuculline in this respect. Other binding assays revealed that securi- nine and its analogs were inactive as inhibitors of bicuculline-insensitive GABA binding, benzodiazepine, cholinergic muscarinic, and fl-adrenergic receptor binding. In addition, while thiocyanate ion increased the apparent binding potency of bicucuiline 10-fold, it had little effect on that of securinine. Extracellular electrophysiologicalstudies on neurons in the cat spinal cord indi6ated that securinine and dihydrosecurinine blocked the inhibitory action of GABA while having no effect on that of glycine. Allo- and virosecurinine were much less active as GABA receptor antagonists in this test. These results suggest that, like bicuculline, securinine and dihydrosecuri- nine are selective antagonists of GABA recognition sites on mammalian central neurons. INTRODUCTION While suspected for many years, the role of y-ami- nobutyric acid (GABA) as a transmitter in the mam- malian central nervous system could not be charac- terized until selective antagonists of central GABA recognition sites were identified6,21. Two major chemical classes of GABA antagonist have been re- ported: sesquiterpene lactones, such as picrotoxinin, and phthalide isoquinoline alkaloids, such as bicucul- line. Although the mechanism of action of picrotoxi- nin and bicuculline may differS. 19 (but see Barker et al.l), the structure-activity requirements for interac- tion with GABA recognition sites have been difficult to define, given the limited number of receptor an- tagonists. The shrub Securinega suffructicosa (Euphorbia- ceae) contains a group of convulsant alkaloids having a fused indolizidine ring structure 18. Of the 16 alka- loids known, securinine and two of its isomers, viro- securinine and allosecurinine, have been studied pharmacologically12,17,18. While securinine-induced seizures have been described as 'strychnine-like '10, there is no direct evidence that these substances are glycine receptor antagonists6, and therefore their mechanism of action remains unknown. The present study was undertaken to examine securinine and its congeners as potential glycine or GABA receptor an- tagonists in the mammalian central nervous system. The convulsant activity of these substances was com- pared with their biochemical and neuropharmacolog- ical properties. The results suggest that securinine al- kaloids are selective antagonists at central receptors associated with the inhibitory effect of GABA on neurons, and thus may be useful for assessing the pharmacological characteristics of GABA receptors. Correspondence: S. J. Enna, Department of Pharmacology, University of Texas Medical School at Houston, P.O. Box 20708, Hous- ton, TX, U.S.A. 0006-8993/85/$03.30 (~) 1985 Elsevier Science Publishers B.V. (Biomedical Division)