Sensitization after Brief Pre-Exposure in a Moth Figure 3. Trajectometry analyses of the walking pathways of PHE pre-exposed males confronted with different concentrations of PHE. Kinetic parameters describing different characteristics of the movement of the males on the locomotion compensator were calculated: A) ‘‘latency’’ (time until males activated), B) ‘‘walked distance’’ (displacement in any direction), C) ‘‘walked time’’ (active time) and D) ‘‘mean speed’’ (‘‘walked distance’’ divided by ‘‘walked time’’). The numbers of analyzed trajectories are n = 7 and n = 19 for C(hex) and PHE 0.1FE; n = 18 and n = 25 for C(hex) and PHE 0.25FE; n = 18 and n = 26 for C(hex) and PHE 0.5FE, respectively. The boundaries of the box indicate the 25th and 75th percentiles, whiskers indicate the 90th and 10th percentiles and black dots show outliers. Full and dashed lines within the box mark the median and mean respectively. Median tests (p,0.05) were carried out to reveal differences between different tested concentrations and pre-exposures. Only ‘‘walked distance’’ and ‘‘mean speed’’ increased in PHE pre-exposed male moths, but were not dependent on pheromone concentration. ‘‘Latency’’ and ‘‘walked time’’ changed with pheromone concentration but not after pre-exposure. doi:10.1371/journal.pone.0034141.g003 Effects of brief gustatory exposure on proboscis extension responses to sucrose and its neurobiological basis Proboscis extension responses to sucrose, especially at low doses, occurred more frequently in pre-exposed S. littoralis than in control males. The effect of pre-exposure to both sucrose and quinine on sucrose responses was highly similar, in spite of quinine being a feeding deterrent (i.e. an aversive signal) in phytophagous insects [26,37–39], whereas sucrose is an appetitive stimulus. To our knowledge, this is the first time that a brief exposure to tastants is shown to increase behavioural responses to the same or even a different chemical stimulus after one whole day. In other insects, brief exposure to sucrose elicited short-term sensitization as shown by PER responses in honey bees [40] and fruitflies [7]. In infant rats, pre-exposure to tastants (quinine, sucrose) has been shown to enhance responsiveness to these substances, discussed as a form of taste familiarity [41]. Interestingly, pre-exposure was repeated many times in this latter study, while a single pre-exposure was sufficient in ours. In humans, a repeated or even a single exposure to fructose caused a sensitization to glucose, which lasted several days [42]. Our electrophysiological characterization of sucrose-sensitive antennal sensilla revealed no statistical differences in dose-response curves between sensilla of males pre-exposed to sucrose, quinine and water (control). These results are in agreement with data obtained by electroantennogram recordings in S. littoralis with sex pheromone stimulation, which did not reveal differences after brief pre-exposure [19]. Given these observations, we can exclude the peripheral level as a major origin of modulation of behavioural gustatory sensitivity after pre-exposure to sucrose and quinine. Cross-modal effects of pre-exposure and their neurobiological basis In our behavioural experiments we revealed reciprocal cross- modal effects of pre-exposure between olfactory and gustatory stimuli. Both oriented sex pheromone responses and PER responses PLoS ONE | www.plosone.org 5 March 2012 | Volume 7 | Issue 3 | e34141