Species - Dependent Cardiac And Motor Responses To Cholinergic Stimulation In Daphnia Pulex And Daphnia Magna

The role of acetylcholine (ACh) in regulating the activity of the heart and `feeding current\u27 driven by the beating thoracic appendages of Daphnia pulex and Daphnia magna was evaluated using acetylcholinesterase inhibitors (AChE-I) and muscarinic receptor agonists. Single animals, tethered to a stainless steel pin, were tested in a watertight aquatic chamber that allowed free movement of appendages and swimming antennae. Heart contraction rate and the rate of thoracic appendage beating were quantified optically by measuring fluctuating changes in light-intensity caused by movement. Physostigmine, neostigmine, oxotremorine, pilocarpine as well as nicotine were used to study ACh and AChE. Atropine was used to attempt to block responses caused by the drugs used. The findings strongly support the role of ACh in the regulation of appendage beat rate, and suggest that AChE inhibition results in sufficiently high ACh levels to affect the rate of rhythmic appendage beating