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Abstract
As soon as zebrafish larvae start eating, they exhibit a marked aversion for bitter and acidic substances, as revealed by a consumption assay, in which fluorescent Tetrahymena serve as a feeding basis, to which various stimuli can be added. Bitter and acidic substances elicited an increase in mRNA accumulation of the immediate-early response gene egr-1, as revealed by in situ hybridization. Conversely, chemostimulants that did not induce aversion did not induce egr-1 response. Maximum labeling was observed in cells located in the oropharyngeal cavity and on the gill rakers. Gustatory areas of the brain were also labeled. Interestingly, when bitter tastants were repeatedly associated with food reward, zebrafish juveniles learned to ingest food in the presence of the bitter compound. After habituation, the acquisition of acceptance for bitterness was accompanied by a loss of egr-1 labeling. Altogether, our data indicate that egr-1 participates specifically in food aversion. The existence of reward-coupled changes in taste sensitivity in humans suggests that our results are relevant to situations in humans.
Highlights
Feeding is a complex behavior that relies on the integration of different sensations produced by gustatory, olfactory, visual, and oral somatosensory stimuli
A convenient model of taste aversion relies on conditioned taste aversion (CTA), which is a learned aversion to a taste stimulus that has been followed by illness
In order to set up an assay for the acquisition and regulation of taste aversion in fish, we have taken advantage of the fact that zebrafish larvae, at the age of only 5 d.p.f. readily prey on live Tetrahymena, by either chasing the prey or remaining stationary and sucking the prey into their mouths (Borla et al, 2002)
Summary
Feeding is a complex behavior that relies on the integration of different sensations produced by gustatory, olfactory, visual, and oral somatosensory stimuli. Sodium L-glutamate, which has been reported as hedonic for fish (Aihara et al, 2008) and can reverse the aversive effect of denatonium (our data) did www.frontiersin.org not induce egr-1 response, as suggested by the faint labeling of gills (Figure 4Ak).
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