(-)-Bicuculline methiodide

(-)-Bicuculline methiodide is an antagonist of γ-aminobutyric acid-A (GABA_A) receptor, with IC50 value of 500 μM for GABA binding [1]. 

Three classes of GABA receptors have been characterized based on their pharmacology, namely GABA_A, GABA_B and GABA_C receptors, among which GABA_A and GABA_C receptors are ligand-gated chloride (Cl^-) channels. GABA receptors play important roles in central nervous system by responding to the inhibitory neurotransmitter GABA [2]. 

On synaptic membranes of chick cerebellum at birth, (-)-bicuculline methiodide acted as a less potent convulsant than (+)-Bicuculline methiodide which was more effective in displacing GABA binding, with an IC50 value of 10 μM [1]. In C. elegans, (-)-bicuculline methiodide at the concentration of 10 μM did not inhibit UNC-49B homomeric or UNC-49B/C heteromeric receptors which are UNC-49 subunits most closely related to the bicuculline- and benzodiazepine-insensitive RDL GABA receptors of insects. (-)-Bicuculline methiodide was used to confirm that the UNC-49 receptors were pharmacologically distinct from the mammalian GABA_A receptors [3]. 

References:

[1]. Fiszer De Plazas S. Ontogenesis of GABA receptor sites in chick embryo cerebellum. Brain Research, 1982, 255(2): 263-275.

[2]. Goutman J D, Calvo D J. Studies on the mechanisms of action of picrotoxin, quercetin and pregnanolone at the GABA rho 1 receptor. British Journal of Pharmacology, 2004, 141(4): 717-727.

[3]. Bamber B A, Twyman R E, Jorgensen E M. Pharmacological characterization of the homomeric and heteromeric UNC-49 GABA receptors in C. elegans. British Journal of Pharmacology, 2003, 138(5): 883-893.