Daphnia are remarkably versatile little beasts. Living in aquatic environments, they migrate to deeper water by day – to avoid predators and UV damage – and return to the surface when safe at night. Andrew Christie from the Mount Desert Island Biological Laboratory, USA, and his colleagues Matthew McCoole and Kevin Baer from the University of Louisiana at Monroe, USA, explain that this migration is probably driven by phototaxis – when animals are either attracted to or repelled by light. They add that the tiny crustaceans respond quickly to pollution, so their phototactic behaviour is routinely studied by ecotoxicologists, who use them as a model to understand how environmental change affects behaviour. Yet, shockingly, nothing was known about the neural basis of Daphnia's migrations. Knowing that the amine histamine plays a key role as a neurotransmitter in the visual systems of other arthropods, the team decided to make the first measurements to find out whether histamine contributes to Daphnia's phototactic response (p. 1773).
Collecting Daphnia magna and Daphnia pulex from lab-based colonies, the scientists were able to trace the tissues containing high levels of histamine using antibodies that recognise the amine. Scrutinising the crustaceans with a microscope, Christie and his colleagues could see that histamine was restricted to the animal's nervous system, and more specifically to the crustacean's eye, optic ganglion and brain.
Having confirmed that the neurotransmitter occurs in Daphnia's visual system, the team began searching through the recently published D. pulex genome to look for genes encoding a key enzyme involved in histamine synthesis and ion channels that could be activated by the neurotransmitter. Using Drosophila proteins as templates, the team identified three potential histidine decarboxylase proteins, ranging in length from 688 amino acids up to 747 amino acids, and two histamine-gated chloride channels in the D. pulex genome. So, the components for histaminergic signalling were in place, but the researchers needed direct evidence before they could be sure that Daphnia employ histamine signalling in phototaxis.
Knowing that the drug cimetidine blocks histamine signalling through its receptors, the team decided to see what effect the drug had on the crustacean's ability to respond to light. Together, they tested the reactions of juvenile Daphnia to a 10 min UV light exposure and found that most of the animals fled to the bottom of the enclosure. However, when the team added cimetidine to the water, the animals stopped descending to the bottom. The drug had robbed the animals of their ability to respond to light, although they quickly recovered phototaxis when the drug was absent.
‘Taken collectively, our results show that an extensive histaminergic system is present in Daphnia species, including the visual system, and that this amine is involved in the control of phototaxis in these animals,’ says Christie and his colleagues.
