Dopamine synergizes with caffeine to increase the heart rate of Daphnia .

Dopamine is a key neurotransmitter, and is widely used as a central nervous system (CNS) agent.  Dopamine plays an important role in humans, including a major role in reward and motivation behaviour. Several addictive drugs are well known to increase neuronal dopamine activity. We selected Daphnia , an important model organism, to investigate the effect(s) of selected CNS agents on heart rate. Dopamine's effects on Daphnia 's heart has not been previously reported. Caffeine is a well-known and widely consumed stimulant. Ethanol is well known for its effects on both neurological and physiological processes in mammals. We tested the effect of dopamine on the heart rate of Daphnia , and compared its effect with caffeine and ethanol alone and in combination. Both caffeine and dopamine were found to instantly increase the heart rate of Daphnia in a dose-dependent manner. Interestingly, caffeine synergized with dopamine to increase Daphnia 's heart rate. As ethanol decreased the heart rate of Daphnia and dopamine increased the heart rate of Daphnia, we wanted to test the effect of these molecules in combination . Indeed, Dopamine was able to restore the ethanol-induced decrease in the heart rate of Daphnia .  Effects of these CNS agents on Daphnia can possibly be correlated with similar effects in the case of mammals.