In vivo effects of serotonin and fluoxetine on cardio-ventilatory functions in the shore crab Carcinus maenas (L. 1758).
Serotonin (5-HT) takes a key position in regulating vital functions, such as cardio-ventilatory activity, locomotion and behaviour. Selective serotonin reuptake inhibitors (SSRIs) modulate the serotonergic system and thus affect these functions. Rhythmic behaviours, such as cardio-ventilatory activity, are controlled by central pattern generators, which in turn are regulated by 5-HT. In crustaceans, 5-HT also regulates the synthesis and secretion of crustacean hyperglycaemic hormone, a pleiotropic hormone involved in the mobilisation and release of glucose into the haemolymph, thus stimulating the animal's activity. As a matter of consequence, SSRIs may affect cardio-ventilatory activity. In order to examine how the SSRIs affect fundamental physiological parameters based on rhythmic behaviours in decapods, cardio-respiratory activity in the shore crab Carcinus maenas was assessed after pericardial injection of a single dose of either 0.5 μM, 0.75 μM or 1 μM fluoxetine, respectively. Simultaneous recordings of heart and scaphognathite movements in both brachial chambers were conducted by measuring impedance changes in the respective body compartments. Injection of 5-HT had an immediate effect on cardio-ventilatory activities and strongly upregulated both cardiac and ventilatory activities. Fluoxetine showed similar effects, entailing moderate tachycardia and increased ventilation rates. Compared to 5-HT, these effects were delayed in time and much less pronounced. Metabolism of fluoxetine into the active compound nor-fluoxetine might account for the delayed action, whereas compensatory regulation of cardio-ventilatory frequencies and amplitudes are likely to explain the attenuation of the responses compared to the strong and immediate increase by 5-HT. Overall, the results suggest increased 5-HT levels in invertebrates following fluoxetine exposure, which are able to disturb physiological functions regulated by 5-HT, such as cardiac and respiratory activity.