Evidence that levamisole, pyrantel, morantel, amidantel, deacylated amidantel and hycanthone may act on acetylcholine receptors of central neurones of Helix aspersa.

1. Intracellular recordings were made from identified neurones in the suboesophageal ganglionic mass of the snail, Helix aspersa. Neurones were classified as either "H" cells, inhibited by acetylcholine or "D" cells, excited by acetylcholine. 2. The actions of levamisole, morantel, pyrantel, amidantel, deacylated amidantel and hycanthone were investigated on these neurones and compared to that of acetylcholine. 3. Levamisole was 10.85 +/- 0.56 times less active than acetylcholine on "H" cells but more than 100 times less active on "D" cells. On "H" cells levamisole had a secondary gradual depolarizing effect which was irreversible and resulted in the loss of cell activity. 4. Morantel and pyrantel were 1.12 +/- 0.13 and 2.56 +/- 0.26 times respectively less active than acetylcholine on "D" cells and 5.16 +/- 0.6 and 3.53 +/- 0.63 times respectively less active than acetylcholine on "H" cells. 5. Amidantel was more than 100 times less active than acetylcholine on both "D" and "H" cells while its deacylated derivative was 26.0 +/- 1.0 and 76.0 +/- 3.25 times respectively less active than acetylcholine on "D" and "H" cells. 6. Hycanthone possessed weak inhibitory effects on "H" cells but also appeared to reduce the duration of acetylcholine inhibitory responses when applied immediately after the acetylcholine response had reached its maximum.