Circadian modulation of learning ability in a disease vector insect, Rhodnius prolixus.

Despite the drastic consequences it may have on the transmission of parasites, the ability of disease vectors to learn and retain information has just begun to be characterised. The kissing bug Rhodnius prolixus, a vector of Chagas disease, is an excellent model, particularly because conditioning the proboscis extension response (PER) constitutes a valuable paradigm to study their cognitive abilities under carefully controlled conditions. Another characteristic of these bugs is the temporal organisation of their different activities in a bimodal endogenous daily rhythm. This offers the opportunity to address the implication of the circadian system in learning and memory. Using aversive conditioning of the PER, we tested whether the ability of kissing bugs to learn and remember information varies during the day. We found that bugs perform well during the night, but not during the day: their ability to acquire information - but not their ability to retrieve it - is modulated by time. When the bugs were kept under constant conditions in order to analyse the origin of this rhythm, the rhythm continued to free run, showing its endogenous and truly circadian nature. These results are the first to evince the effect of the circadian system on the learning abilities of disease vectors and one of the few in insects in general.