This study identifies another large class of compounds (surfactants) that modulate the function of GABAA
, glycine, and NMDA receptors in a manner that is qualitatively similar to isoflurane and ethanol (potentiation of GABAA
and glycine receptors, inhibition of NMDA receptors) (–): in 19 of 21 cases, surfactants modulated these receptors in an anesthetic-like manner. We therefore cannot reject the hypothesis that interfacial activity is a sufficient condition for anesthetic-like modulation of these receptors. Results from other studies support the present results. For example, an anionic alkylbenzene sulfonate surfactant with a 12 carbon tail potentiates α2
glycine, and α1
receptors, and inhibits DL--amino-3-hydroxy-5-methyl-4-isoxalonepropionic acid (AMPA) GluR3 receptor function (22
) and NMR spectroscopy (10
) show that small uncharged inhaled anesthetics accumulate at interfaces. To the extent that surfactants and volatile anesthetics may act by a common mechanism to modulate channel function, surfactants may prove to be valuable probes of anesthetic action. At what interface might the actions described here of the surfactants be produced? As for volatile anesthetics, our results are consistent with either an action at a protein-water or a membrane-water interface. However, some observations seem to make the protein-water site of action less likely for surfactants. Given the range of surfactant charges, and the diversity of effects on the mutant channels we observed, if the surfactants act by binding to channel proteins, then different surfactants probably bind to different sites on the protein and yet produce similar modulatory effects. This would seem to be unlikely. Alternately, the hydrophilic region of the surfactants may interfere with charge interactions between the protein and head group regions of the membrane, or affect electrical potentials at the bilayer interface which may indirectly affect channel function. A theory has been proposed by which volatile anesthetics alter mechanical properties (stresses) of membranes as a result of their interfacial activity (23
). This theory may also provide an explanation of our findings, particularly in light of the well-known effects of surfactants on surface tension (24
Why do some compounds accumulate at interfaces? As noted previously, the favorable interactions of the hydrophobic tail and hydrophilic head groups of a surfactant with the hydrophobic and aqueous phases present at an interface promote the concentration of surfactant at the interface. These interactions should be smaller for inhaled anesthetics which in general have smaller hydrophobic tails.
The hexane/water interface was used as a model interface in initial molecular dynamic calculations. A consideration of the energetics of transfer of solute from water to hexane reveals another reason why these solutes accumulate at this interface. This transfer may be considered to consist of four steps (for a detailed discussion of the thermodynamics of solvation, see reference (25
)). In the first step, a space is made for the solute in the phase to which it will be transferred. The second step requires breaking contacts between the solute and water. These two steps cost energy. The third step is insertion of the solute into its new chemical environment. In the fourth step, the cavity formed by removal of the solute from water is closed. These latter two steps are energetically favorable, because contacts are formed between molecules. It has been shown via simulations at hexane/water interfaces that for inhaled anesthetics, the balance between the cost of opening a cavity (which is least favorable in water) and the free energy of solvation (which is most favorable in water) is particularly important (9
). These energies change in opposite directions in transferring the anesthetic from water across the interface. In particular, the minimal energetic cost of cavity formation at the interface, which results from the tendency of the immiscible phases to separate, favors the interfacial location of these small solutes at hexane/water interfaces.
Two polyhydroxyalkanes, each containing four −OH groups separated by six carbons from each other were recently synthesized and their anesthetic actions studied in tadpoles (26
). These compounds were designed to be larger than the cavities anesthetics have been predicted to bind to in proteins. The alcohol groups did however render these compounds exceptionally interfacially active because each −OH tethers the molecule to the membrane interface, making the molecule more potent on a molar basis than alcohols with fewer −OH groups if interfacial activity determines anesthetic potency. That these compounds had the predicted anesthetic effects in tadpoles is consistent with a membrane mediated mechanism of anesthesia (26
The length of the tail of the two anionic surfactants we studied affected the response of the glycine receptor. The 12 carbon anionic surfactant SDS inhibited and the 8 carbon anionic surfactant SOS potentiated glycine receptor function. Such a chain-length dependent crossover from potentiation to inhibition has been reported for a related cys-loop receptor, the neuronal nicotinic actylcholine receptor, in which short chain alcohols potentiate α4
receptor function, while longer chain alcohols inhibit currents through these receptors (27
). Possibly a similar effect occurs with application of anionic surfactants to glycine receptors.
Homologous mutations, from serine to isoleucine at position 270 of the GABAA receptor α1 subunit, and position 267 of the α1 subunit of the glycine receptor, attenuated the modulatory effect of both anionic surfactants (SDS and SOS) on α1β2γ2s GABAA and α1 glycine receptors. This may indicate a shared mechanism between these compounds and isoflurane and ethanol, whose modulatory effects are also reduced by these mutations. However, for other surfactants with differing carbon chain lengths and head group charges, no clear pattern was observed on mutant receptors in comparison to wild type receptors, as conjectured.
Our results may have implications for the development of new anesthetics. Both surfactants and inhaled anesthetics are interfacially active, and our results suggest that they modulate receptor function similarly. This raises the possibility that interfacial activity may be used to predict function. To achieve sufficient interfacial concentration in the central nervous system to modulate receptor function, a compound must also cross the blood brain barrier. While uncharged inhaled anesthetics readily penetrate the blood brain barrier, large charged surfactants will not. Development or identification of interfacially active compounds that can cross the blood brain barrier may produce leads for new anesthetics. Possibly, some small metabolites which modulate channel function like anesthetics, and have anesthetic-like effects in animals fulfill these conditions (19
). We would predict that they are interfacially active and in particular that they will affect physical properties of lipids at interfaces.