Modulatory effects of the serotonergic and histaminergic systems on reaction to light in the crustacean Gammarus pulex

• Serotonin modulates reaction to light in the crustacean Gammarus pulex.
• In vivo pharmacology provides evidence for the activation of 5-HT receptor in phototaxis.
• Additive effect of an antihistamine suggests that both 5-HT and histamine mediate phototaxis.
• Our interpretation is based on pharmacological profiles of vertebrate and invertebrate receptors.
• Classical drugs could be used to compare 5-HT induced and parasite-induced behavioural syndromes.

Serotonin modulates reaction to light in many animals. In the freshwater amphipod Gammarus pulex, exogenous administration of serotonin induces a transient reversal of photic behaviour from strong photophobia to photophily. We have elucidated further the neuromodulation of photic behaviour in G. pulex by using in vivo pharmacology and behavioural testing. Using several mammalian 5-HT receptor antagonists and agonists, we provide evidence for a role of serotonin receptors in the 5-HT-dependent regulation of G. pulex photic behaviour, possibly involving 5-HTR2 subtype. Serotonin-induced photophily was blocked by the 5-HT receptor antagonists, mianserin, cyproheptadine, and ritanserin, but not by ketanserin and methiothepin. In addition, two agonists at 5-HT receptor, mCPP and α-methylserotonin, partially mimicked 5-HT-induced photophily. The natural photophobia of G. pulex was also decreased by exogenous administration of the histamine receptor antagonist cimetidine, suggesting that photic behaviour may be controlled by more complex interactions between serotonergic and histaminergic systems.