Hydrocarbon molar water solubility predicts NMDA vs. GABAA receptor modulation

Abstract

BackgroundMany anesthetics modulate 3-transmembrane (such as NMDA) and 4-transmembrane (such as GABAA) receptors. Clinical and experimental anesthetics exhibiting receptor family specificity often have low water solubility. We hypothesized that the molar water solubility of a hydrocarbon could be used to predict receptor modulation in vitro.MethodsGABAA (α1β2γ2s) or NMDA (NR1/NR2A) receptors were expressed in oocytes and studied using standard two-electrode voltage clamp techniques. Hydrocarbons from 14 different organic functional groups were studied at saturated concentrations, and compounds within each group differed only by the carbon number at the ω-position or within a saturated ring. An effect on GABAA or NMDA receptors was defined as a 10% or greater reversible current change from baseline that was statistically different from zero.ResultsHydrocarbon moieties potentiated GABAA and inhibited NMDA receptor currents with at least some members from each functional group modulating both receptor types. A water solubility cut-off for NMDA receptors occurred at 1.1 mM with a 95% CI = 0.45 to 2.8 mM. NMDA receptor cut-off effects were not well correlated with hydrocarbon chain length or molecular volume. No cut-off was observed for GABAA receptors within the solubility range of hydrocarbons studied.ConclusionsHydrocarbon modulation of NMDA receptor function exhibits a molar water solubility cut-off. Differences between unrelated receptor cut-off values suggest that the number, affinity, or efficacy of protein-hydrocarbon interactions at these sites likely differ.

GABA_A Receptor Modulation by Phenyl Ring Compounds Is Associated with a Water Solubility Cut-Off Value

“…Therefore, the bulk aqueous drug concentration necessary to compete with water in a low-affinity GABA A -binding site would be different than in an analogous NMDA-binding site. Consequently, the GABA A receptor molar water solubility cut-off value is predicted to be different than the cut-off value previously reported for NMDA receptors [5] .…”

“…NMDA receptors also exhibit a cut-off effect that is associated with the hydrocarbon molar water solubility [5] . Similar amphipathic, water-filled binding pockets have been postulated to explain why poorly soluble drugs fail to inhibit NMDA receptors.…”

“…Capable of modulating of GABA A receptor function at even low aqueous phase concentrations ( table 1 ), propofol exhibits high-affinity binding in addition to other possible low-affinity interactions. In vitro, injectable anesthetics such as propofol and its structurally similar congeners can modulate other unrelated receptors, including NMDA receptors, given sufficient drug molar water solubility and high aqueous drug concentrations [5] . However, high affinity drug-receptor binding at targets relevant to anesthetic action increases drug potency in vivo; consequently, the probability of low-affinity receptor interactions with low drug concentrations is reduced.…”

“…Whole cell currents were studied with a -80 mV holding potential using standard 2-electrode voltage clamping techniques. Methods for measuring whole-cell GABA A receptor currents have been described [5,[11][12][13] . Briefly, pairs of identical concentrations of the phenyl compound were prepared in glass syringes containing frog ringer's (FR) solution (115 mmol/l NaCl, 2.5 mmol/l KCl, 1.8 mmol/l CaCl 2 , and 10 mmol/l HEPES in 18.2 MΩ H 2 O, filtered and adjusted to pH 7.4) or FR + agonist (FR-GABA; FR plus 20-40 μmol/l, GABA, equal to an EC [10][11][12][13][14][15][16][17][18][19][20].…”

“…Low-affinity binding sites are postulated to be amphipathic and water filled pockets within the receptor [3,4] . At least in the case of N-methyl-D-aspartate (NMDA) receptors, this low-affinity protein binding is subject to a cut-off effect associated with molar water solubility, but not molecular structure or size [5] .…”