The Nature of the Local Anesthetic Binding Site

Mutation of specific amino acids of the Na⁺ channel has allowed definition of the regions that interact with local anesthetics. The major functional protein of the Na⁺ channel (the α-subunit) is composed of four homologous "domains" (DI to DIV), each of which contains six helical regions (S1 to S6) that span the core of the membrane ( Fig. 14-9A ). Each domain also has a loop, termed the "P region," that links the extracellular ends of its S5 and S6 segments; P regions extend inward between the transmembrane regions such that when the α-subunit folds together, each P loop contributes a quarter of the cylindrical, ion "selectivity pore," the narrowest passage of an open channel ( Fig. 14-9B ). Voltage sensitivity is derived from the positively charged S4 segments, which slide or swing "outward" in response to membrane depolarization. By linkages still unknown, this movement of S4 results in a rearrangement of the S6 segments to form the inner, cytoplasmic entry to the channel. Closed to open channel gating results from S6's movements, whereas inactivation gating appears to result from binding of the cytoplasmic loop between D-3 and D-4 to the cytoplasmic opening.

Local anesthetics bind in the "inner vestibule" of the Na⁺ channel ( Fig. 14-9C ). Mutations of amino acids in the S6 segments of D-1, D-3, and D-4 all modify local anesthetic action, thus suggesting that these regions can form a pharmacophere small enough to simultaneously contact the drug or that the local anesthetic molecule rapidly moves among these three segments. Mutation of loci on the Na⁺ channel closer to the external opening also modify local anesthetic blockade, consistent with the ability of extracellular Na⁺ ions to antagonize blockade by these drugs and the ability of extracellular H⁺ to protonate them and thus prolong and potentiate the drug-bound states.

The blocking rate constant for local anesthetics is higher for the more hydrophobic molecules, which suggests that drug molecules can reach the binding site (and depart from it) through a "hydrophobic" pathway. This pathway could be from the membrane phase laterally, into the channel, or through hydrophobic amino acid residues that limit access through the closed pore pathway. The slow blockade of closed and inactivated channels appears to use such a hydrophobic access, thus accounting for tonic inhibition.

In brief, hydrophobicity delivers the drug to the receptor, and charge keeps it there.