Depressing Antidepressant: Fluoxetine Affects Serotonin Neurons Causing Adverse Reproductive Responses in Daphnia magna

Abstract

Selective serotonin reuptake inhibitors (SSRIs) are widely used antidepressants. As endocrine disruptive contaminants in the environment, SSRIs affect reproduction in aquatic organisms. In the water flea Daphnia magna, SSRIs increase offspring production in a food ration-dependent manner. At limiting food conditions, females exposed to SSRIs produce more but smaller offspring, which is a maladaptive life-history strategy. We asked whether increased serotonin levels in newly identified serotonin-neurons in the Daphnia brain mediate these effects. We provide strong evidence that exogenous SSRI fluoxetine selectively increases serotonin-immunoreactivity in identified brain neurons under limiting food conditions thereby leading to maladaptive offspring production. Fluoxetine increases serotonin-immunoreactivity at low food conditions to similar maximal levels as observed under high food conditions and concomitantly enhances offspring production. Sublethal amounts of the neurotoxin 5,7-dihydroxytryptamine known to specifically ablate serotonin-neurons markedly decrease serotonin-immunoreactivity and offspring production, strongly supporting the effect to be serotonin-specific by reversing the reproductive phenotype attained under fluoxetine. Thus, SSRIs impair serotonin-regulation of reproductive investment in a planktonic key organism causing inappropriately increased reproduction with potentially severe ecological impact.

Tryptophan hydroxylase (TRH) loss of function mutations induce growth and behavioral defects in Daphnia magna

“…We previously found that individuals of D. magna exposed to SSRI fluoxetine and fluvoxamine mature earlier and produce more but smaller offspring 12 , 13 . Consistently, fluoxetine increases brain serotonin-immunoreactivity levels at low food conditions, bringing them to the maximal levels normally only observed under high food conditions, and enhancing offspring production concomitantly 14 . Sublethal amounts of the neurotoxin 5,7-dihydroxytryptamine, selectively killing serotonergic neurons, markedly decrease serotonin-immunofluorescence and offspring production in fluoxetine-treated animals, supporting that the observed effect on reproductive phenotype is modulated by serotonin levels 14 .…”

“…Consistently, fluoxetine increases brain serotonin-immunoreactivity levels at low food conditions, bringing them to the maximal levels normally only observed under high food conditions, and enhancing offspring production concomitantly 14 . Sublethal amounts of the neurotoxin 5,7-dihydroxytryptamine, selectively killing serotonergic neurons, markedly decrease serotonin-immunofluorescence and offspring production in fluoxetine-treated animals, supporting that the observed effect on reproductive phenotype is modulated by serotonin levels 14 . Producing small clutches of larger offspring is an adaptive response to food scarcity in natural conditions, since the larger the offspring the more resistant to starvation 15 .…”

“…In this paper, we used this approach to deepen on the study of the role of serotonin in D. magna reproductive and metabolic strategies. Furthermore using TRH knockouts lacking serotonin we also tested the premise that previously reported effects of the SSRI fluoxetine on reproductive and behavioral responses 12 , 14 , 16 in D. magna were related to serotonin changes. CRISPR-Cas9 mutated, TRH (−/−) clones showed no detectable levels of serotonin, tested both by serotonin inmunofluorescence in neurons of optica ganglia and brain, and by direct serotonin quantification in the whole animal.…”

Using a new high-throughput video-tracking platform to assess behavioural changes in Daphnia magna exposed to neuro-active drugs

“…The mechanisms of action of the SSRI fluoxetine on D. magna are better known than those of the remaining tested chemicals. Fluoxetine enhances brain serotonin activity in Daphnia (Campos et al, 2016), increases development and reproductive rates (Campos et al, 2012) and alters phototaxis behavior. Recent studies using knockout Daphnia individuals lacking serotonin showed that these animals had the opposite phenotype as those exposed to fluoxetine: animals matured latter, reproduced less and were more mobile than wild type animals (Rivetti et al, 2018).…”

Assessment of long term ecotoxicity of urban stormwaters using a multigenerational bioassay on Ceriodaphnia dubia: A preliminary study

“…[3,4,5] Endocrine disrupting activities can lead to adverse effects on exposed organisms, but also on their progeny, creating adverse effects on subsequent generations. [6,7] Nowadays, there is undoubtedly an important concern about the long term ecotoxicity assessment of single chemicals and of multi-contaminated matrixes and the scientific community is actively working to create harmonization (OECD guidelines) of multigenerational bioassays on cladocerans (D. magna)…”