Sight of conspecific images induces changes in neurochemistry in zebrafish

Abstract

Zebrafish are gaining popularity in behavioural brain research as this species combines practical simplicity with system complexity. The dopaminergic system has been thoroughly investigated using mammals. Dopamine plays important roles in motor function and reward. Zebrafish have dopamine receptors homologous to mammalian counterparts, and dopamine receptor antagonists as well as alcohol have been shown to exert significant effects on this species as measured using HPLC or behavioural methods. The sight of conspecifics was previously shown to be rewarding in zebrafish but whether this stimulus affects the dopaminergic system has not been studied. Here, we present animated images of zebrafish to the experimental zebrafish subject for varying lengths of time and quantify the amount of dopamine, DOPAC, serotonin and 5HIAA extracted from the subject's brain immediately after the stimulus presentation using HPLC with electrochemical detection. We find conspecific images to induce a robust behavioural response (attraction) in experimental zebrafish. Importantly, dopamine and DOPAC levels significantly increased in response to the presentation of conspecific images but not to scrambled images. Last, serotonin and 5HIAA levels did not significantly change in response to the conspecific images. We conclude that our findings, together with pervious studies, now conclusively demonstrate that the behavioural response induced by the appearance of conspecifics is mediated, at least partly, by the dopaminergic system in zebrafish.

Figure 1

The distance from the stimulus presentation screen significantly differs between stimulus groups. Mean ± S.E.M. are shown. n = 10 for each group. The grey symbols correspond to the 30 sec intervals during which no stimulus is shown. The black symbols correspond to the 30 sec intervals during which either the scrambled images (a rectangle containing the same pixels as in the zebrafish image but in a random, scrambled manner) or images of zebrafish were shown. The stimulus condition (i.e. which image and for how long was shown) is indicated above the graphs. The horizontal line shows random chance, i.e. 25 cm from the stimulus side. The vertical dashed line shows the interval during which the side of the stimulus presentation was switched. Note that fish in the no stimulus group had no significant side preference. Also note that fish in the scrambled stimulus group also did not swim significantly below chance level in response to the image presentation. Last, note the effect of zebrafish images (last three graphs).

Figure 2

The change in the distance from the stimulus screen in response to the presentation of stimuli is significantly below random chance when the stimulus is the images of zebrafish but not when it is the scrambled image. Mean ± S.E.M. are shown. n = 10 for each group. Random chance is indicated by 0 on the Y axis. Note that a negative value represents reduction of distance in response to image presentation and a positive value an increase.

Figure 3

Dopamine levels significantly increase in response to the presentation of zebrafish images but not in response to the scrambled images. Mean ± S.E.M. are shown. n = 10 for each group. Data are expressed as weight (ng) of neurochemical per weight (mg) of total brain protein. The colour coding of the stimulus conditions is indicated by the legend. Note that the strongest effect is seen after 10 min of zebrafish image presentation.

Figure 4

DOPAC levels significantly increase in response to the presentation of zebrafish images but not in response to the scrambled images. Mean ± S.E.M. are shown. n = 10 for each group. Data are expressed as weight (ng) of neurochemical per weight (mg) of total brain protein. The colour coding of the stimulus conditions is indicated by the legend. Note that the strongest effect is seen after 10 min of zebrafish image presentation.

Figure 5

Serotonin levels significantly decrease in response to the presentation of the scrambled images but no change is seen in response to the zebrafish images. Mean ± S.E.M. are shown. n = 10 for each group. Data are expressed as weight (ng) of neurochemical per weight (mg) of total brain protein. The colour coding of the stimulus conditions is indicated by the legend.

Figure 6

5HIAA levels significantly decrease in response to the presentation of the scrambled images but no change is seen in response to the zebrafish images. Mean ± S.E.M. are shown. n = 10 for each group. Data are expressed as weight (ng) of neurochemical per weight (mg) of total brain protein. The colour coding of the stimulus conditions is indicated by the legend.