Department of Physics, Chemistry and Biology (IFM)

I will discuss the results first regarding the monoamine manipulations, before moving on to the effect of parasite status.

Serotonin manipulation by fluoxetine

The results of serotonin manipulations are consistent with previous studies, where organisms exposed to fluoxetine became less bold (Dzieweczynski et al. 2016, Björklund 2017, Eisenreich et al. 2017, Saaristo et al. 2017). Despite this, zebra fish (Danio rerio) exposed to fluoxetine became bolder during a novel environment assay (Singer et al. 2016). The concentrations of fluoxetine that have been previously used in behavioural and personality experiments are rather wide, with concentrations ranging from 10ng L^-1 to 10mg L^-1, a difference of one million (Brooks et al. 2003, Painter et al. 2009, Sebire et al. 2015, Dzieweczynski et al. 2016, Singer et al. 2016, Winberg & Thörnqvist 2016, Martin et al. 2017). The dose used in the current experiment (200 ng L^-1) is an intermediate and it is ecologically relevant, which might have not been the appropriate to elicit clearer behavioural changes in the three-spined sticklebacks used. According to the previous, the effects of serotonin manipulation seem to depend, for example, on the species, the drug and the dose chosen (Olsén et al. 2014).

Dopamine manipulation by ropinirole

Literature where ropinirole has been used for similar purposes as in this experiment is scarce. The dose response curve experiment showed limited effects of the pharmaceutical on the behaviour of the exposed three-spined sticklebacks: An intermediate dose (2,500 ng L^-1) was the dose showing the greatest effect on behaviour relative to control. Mediterranean field cricket injected with 10µl of an 8.6 g L^-1 solution of ropinirole showed no effects on activity, exploration, boldness and aggressiveness (Abbey-Lee et al. submitted). These results are consistent with the results of the present study, where fish exposed to 2,500 ng L^-1 of ropinirole did not show any behavioural difference compared to unexposed fish. Despite this, and as for the fluoxetine treatment, the dose of ropinirole used in the present study can be considered quite low, so it cannot be discarded that higher doses of the same drug might be able to generate behavioural changes in three-spined sticklebacks. 

Simultaneous manipulations of serotonin and dopamine

Simultaneous exposure to fluoxetine and ropinirole did not have any effect on the behaviours measured. In this case, the result that fish exposed to fluoxetine became shier, while fish exposed to both fluoxetine and ropinirole did not, suggests an interaction between the two drugs. In fact, SSRI have been demonstrated to be able to interact with dopaminergic cells (Zhou et al. 2005). Even though I cannot make a strong conclusion about the combined effect and further research is needed on this, a counter effect of ropinirole on fluoxetine effects on behaviour is a plausible explanation for my results.

Effects of G. anomala on fish behaviour

In the current study, fish without parasites exposed to fluoxetine became shier the longer they were exposed to the drug, compared to fish with parasites. Several studies suggest that parasites may generate shifts in the behaviours of their hosts (Moore 2002). The reasons for these behavioural changes are still not clear. Some of the explanations given so far are impairment of swimming functions, high energetic drain and direct monoamine manipulations by the parasite. With my results, I cannot draw a clear link between parasite status and behavioural alterations. However, that only fish without parasites were affected by fluoxetine may indicate that infection with Glugea anomala might generate changes in the monoamine profile that could have counteract the effects of fluoxetine. However, the possible interactions between drugs and parasites are overlooked and warrant further investigation.  

According to the results of my work I can conclude that the serotonergic system seems to be involved in explaining variability of boldness-related behaviours in three-spined sticklebacks. Further, individuals can behave differently depending on their parasite status, which in turn can affect how serotonin manipulations affect them. Dopaminergic pathways seem not to affect the behaviours measured in three-spined sticklebacks. However, dopamine may interact and inhibit serotonergic pathways, counteracting its effects on behaviour. Finally, since the dose of fluoxetine used in this study is similar to the doses found in waste water, the populations of wild three-spined sticklebacks might suffer from similar effects of fluoxetine exposure.