Strychnine activates neuronal a7 nicotinic receptors after mutations in the leucine ring and transmitter binding site domains

Recent work has shown that strychnine, the potent and selective antagonist of glycine receptors, is also an antagonist of nicotinic acetylcholine (AcCho) receptors including neuronal homomeric alpha 7 receptors, and that mutating Leu-247 of the alpha 7 nicotinic AcCho receptor-channel domain (L247T alpha 7; mut1) converts some nicotinic antagonists into agonists. Therefore, a study was made of the effects of strychnine on Xenopus oocytes expressing the chick wild-type alpha 7 or L247T alpha 7 receptors, In these oocytes, strychnine itself did not elicit appreciable membrane currents but reduced the currents elicited by AcCho in a reversible and dose-dependent manner. In sharp contrast, in oocytes expressing L247T alpha 7 receptors with additional mutations at Cys-189 and Cys-190, in the extracellular N-terminal domain (L247T/C189-190S alpha 7; mut2), micromolar concentrations of strychnine elicited inward currents that were reversibly inhibited by the nicotinic receptor blocker a-bungarotoxin, Single-channel recordings showed that strychnine gated mut2-channels with two conductance levels, 56 pS and 42 pS, and with kinetic properties similar to AcCho-activated channels. We conclude that strychnine is a modulator, as well as an activator, of some homomeric nicotinic alpha 7 receptors, After injecting oocytes with mixtures of cDNAs encoding mut1 and mut2 subunits, the expressed hybrid receptors were activated by strychnine, similar to the mut2, and had a high affinity to AcCho like the mut1, A pentameric symmetrical model yields the striking conclusion that two identical alpha 7 subunits may be sufficient to determine the functional properties of alpha 7 receptors.