Picrotoxin-like channel blockers of GABAA receptors.

P icrotoxin (PTX) is the prototypic antagonist of GABAA receptors (GABARs), the primary mediators of inhibitory neurotransmission (rapid and tonic) in the nervous system. Picrotoxinin (Fig. 1A), the active ingredient in this plant convulsant, structurally does not resemble GABA, a simple, small amino acid, but it is a polycylic compound with no nitrogen atom. The compound somehow prevents ion flow through the chloride channel activated by GABA in the GABAR, a member of the cys-loop, ligand-gated ion channel superfamily. Unlike the competitive GABAR antagonist bicuculline, PTX is clearly a noncompetitive antagonist (NCA), acting not at the GABA recognition site but perhaps within the ion channel. Thus PTX appears to be an excellent example of allosteric modulation, which is extremely important in protein function in general and especially for GABAR (1). Recent advances in structural modeling of GABAR (Fig. 1 B and C) are consistent with action of PTX and analogous convulsants as NCAs. In a recent issue of PNAS, Chen et al. (2) describe how numerous drugs in this category, with a variety of pharmacological effects, can interact with the same domain of the GABAR protein within the ion channel. PTX has an interesting history as a drug and research tool. Many naturally occurring and synthetic convulsive agents, also blockers of GABA-mediated inhibition, have some rather famous uses, such as stimulants, convulsants, chemical warfare agents, and insecticides, to name a few (3, 4). PTX is isolated from plants of the moonseed family, Menispermaceae, and its close relatives tutin and coriamyrtin, from the New Zealand tutu plant Coriaria arborea, known as a ‘‘loco weed’’ that caused occasional poisonings in cows and even in people. PTX was shown to be a stimulant and could cause tonic-clonic convulsions at fairly low doses, but was listed in the Merck Index as late as the 1970s as an ‘‘antidote for barbiturate overdose’’ (5). It was shown to antagonize inhibitory pathways in the nervous system activated by GABA, and GABAenhancing drugs like barbiturates and benzodiazepines reverse its action. In vitro, PTX inhibited GABA-activated inhibitory currents in neurons and crayfish muscle (4, 6). Radioactive [3H]dihydropicrotoxinin bound to specific sites in crayfish muscle and mammalian brain that could be identified as GABARs (7). Casida and Palmer (8) have synthesized and studied a series of synthetic potent neurotoxic chemicals, the ‘‘cage convulsants.’’ These compounds were developed as insecticides, aimed at the common target, acetylcholineresterase, like the well known malathion. These cage convulsants were not inhibitors of this enzyme, but they were equally toxic, like nerve gas, but were acting on another target in the nervous system. Instead of an acetylcholine (ACh) target, the cage convulsants are noncompetitive GABAR antagonists acting at the PTX site: they inhibit GABAR currents and synapses in mammalian neurons and inhibit [3H]dihydropicrotoxinin binding to GABAR sites in brain membranes (7, 9). A potent example, t-butyl bicyclophosphorothionate, is a major research tool used to assay GABARs by radio-ligand binding (10). This drug target appears to be the site of action of the experimental convulsant pentylenetetrazol (1, 4) and numerous polychlorinated hydrocarbon insecticides, including dieldrin, lindane, and fipronil, compounds that have been applied in huge amounts to the environment with major agricultural economic impact (2). Some of the other potent toxicants insecticides were also radiolabeled and used to characterize receptor action, allowing structure–function analysis of the chemical family on GABAR in both insects and mammals, and to define the receptor site (2, 4, 8). The point of ref. 2 is that this series of compounds has the same molecular target (‘‘receptor’’) despite considerable differences in chemical structure. Thujone, a constituent of oil of wormwood, found in the beverage absinthe, was also shown by Casida and colleagues (11) to target the GABAR channels. Absinthe is the green-colored alcoholic beverage popular in fin de siecle Paris, now illegal because of its toxicity. Absinthe was not only the wildly popular favorite of Parisians (in addition to wine, of course, but there were more absinthe parlors than bread stores!), but considered then and now to impart a special creative stimulus to the cultural elite of the day. To paraphrase my commentary (12), why would a drug with toxic and convulsant actions possibly be considered pleasant or at least desirable? Thujone, like PTX, is excitatory on the brain (analeptic) [not a depressant, like marijuana]. Such an agent may produce mood elevation and antidepressant, anxiety-generating, and alerting effects, as opposed to the anxiolytic, sedative, and amnestic effects of GABA-enhancing drugs like benzodiazepines and ethanol (1). Do not forget, however, that in absinthe one is balancing the stimula-