Molecular Pharmacology, Vol 12, 999-1006, Copyright © 1976 by the American Society for Pharmacology and Experimental Therapeutics

Effects of Eserine and Neostigmine on the Interaction of -Bungarotoxin with Aplysia Acetylcholine Receptors

¹ Neurobiology Department, Armed Forces Radiobiology Research Institute, and Laboratory of Biochemical Genetics, National Heart and Lung Institute, Bethesda, Maryland 20014, and Bermuda Biological Station, Bermuda

Binding of [¹²⁵I]-bungarotoxin to acetylcholine receptors of ganglionic homogenate of the marine mollusc Aplysia is blocked by the anticholinesterases eserine (I₅₀ = 4 µM) and neostigmine (I₅₀ = 0.2 mM). The classical acetylcholine antagonist d-tubocurarine blocks with an I₅₀ of 2 µM. Eserine (I₅₀ = 3.2 µM) and neostigmine (I₅₀ > 1 mM) also block toxin binding to a solubilized receptor preparation. In contrast to their relative potency in blocking toxin binding, neostigmine is a more potent inhibitor of Aplysia acetylcholinesterase (I₅₀ = 14 nM) than is eserine (I₅₀ = 250 nM). -Bungarotoxin does not affect esterase activity or interfere with the ability of eserine to block the esterase. The response to acetylcholine recorded through intracellular microelectrodes is blocked by -bungarotoxin. Neither eserine nor neostigmine blocks the acetylcholine response; rather, they prolong and increase it, as expected from their effects on the esterase. Eserine (0.1 mM) blocks the -bungarotoxin inhibition of the physiological acetylcholine response. These results indicate that eserine and neostigmine block the binding of -bungarotoxin by interacting with a site which is different from both the esterase and the cholinergic sites of the acetylcholine receptor.