Rotifer neuropharmacology--II. Synergistic effect of acetylcholine on local anesthetic activity in Brachionus calyciflorus (Rotifera, Aschelminthes).

A number of compounds showing general anesthetic action in the rotifer Brachionus calyciflorus were investigated in the presence of acetylcholine. Non-ionizing anesthetics, including tricaine, showed no interaction with acetylcholine. However, highly ionized compounds like the local anesthetics procaine and lidocaine, the muscarinic blocker and local anesthetic atropine, and the beta-adrenergic blocker propranolol showed a synergistic effect with acetylcholine. ACh increased the general anesthetic effect of these compounds in a statistically highly significant dose-dependent fashion. To account for the mechanism of this unusual and novel effect it is proposed that these compounds interact with the anesthetic binding site of the rotifer cholinoceptor ionophore in the open state. It is also proposed that non-ionizing compounds have a general membrane effect only. In addition to anesthesia, atropine and propranolol cause foot paralysis in B. calyciflorus. This other novel effect is also enhanced by acetylcholine as well as decamethonium, a neuromuscular blocker.