Ethanol alters the expression of ion channel genes in Daphnia pulex

Physiological endpoints in daphnid acute toxicity tests

Daphnids are freshwater crustaceans used in toxicity tests. Although lethality and immobilisation are the most commonly used endpoints in those tests, more sensitive parameters are required for determination of sublethal acute effects of toxicants. The use of various physiological endpoints in daphnids is considered as a low-cost and simple alternative that meets the 3R's rule (Replacement, Reduction, Refinement) criteria. However, currently there is no review-based evaluation of their applicability in toxicity testing. This paper presents the results on the most commonly determined physiological parameters of Daphnia in ecotoxicological studies and human drug testing, such as feeding activity, thoracic limb movement, heart rate, cardiac area, respiratory activity, compound eye, mandible movements and post-abdominal claw contractions. Furthermore, their applicability as promising endpoints in the assessment of water quality or drug testing is discussed.

ZD7288 and mibefradil inhibit the myogenic heartbeat in Daphnia magna indicating its dependency on HCN and T-type calcium ion channels

Daphnia magna heartbeat is myogenic-originating within the animal's heart. However, the mechanism for this myogenic automaticity is unknown. The mechanism underlying the automaticity of vertebrate myogenic hearts involves cells (pacemaker cells), which have a distinct set of ion channels that include hyperpolarization activated cyclic nucleotide-gated (HCN) and T-type calcium ion channels. We hypothesized that these ion channels also underlie the automatic myogenic heartbeat of Daphnia magna. The drugs, ZD7288 and mibefradil dihydrochloride, block HCN and T-type calcium ion channels respectively. Application of these drugs, in separate experiments, show that they inhibit the heartbeat of Daphnia magna in a dose-dependent manner. Calculation of the percent difference between the heart rate of pretreatment (before drug application) and heart rate following drug application (post-treatment) allowed us to graph a dose-response curve for both ZD7288 and mibefradil, revealing that ZD7288 produces a greater effect on decreasing heart rate. This indicates the HCN ion channels play a foremost role in generating Daphnia magna heartbeat. Our results show conclusively that HCN and T-type calcium ion channels underlie the automatic myogenic heartbeat in Daphnia magna-and suggest a conserved mechanism for generating myogenic heartbeat within the animal kingdom. Thus, Daphnia magna represents a credible model system for further exploration of cardiac physiology.