PASSIVE FLUOXETINE EXPOSURE DURING NURSING RESULTS IN DETECTABLE WHOLE-BRAIN NORFLUOXETINE CONCENTRATIONS, BUT NOT HYPERSEROTONERGIA, IN THE OFFSPRING OF FSL RATS
Authors: Steyn *Stephan Frederik , Viljoen Francois , Brand Linda 
Published: February 2025

Abstract

Background
Globally, 36% of women who have recently given birth, experience symptoms of depression and anxiety – statistics that have been worsened by the recent Covid-19 pandemic1. Effective antidepressant treatments are limited, with fluoxetine being a popular treatment option. Fluoxetine is expressed in the breast milk, yet it is unclear to what extent fluoxetine, or its active metabolite, norfluoxetine, reaches the brain of the developing child and what the effects of such exposure on the related neurobiological processes would be. Due to ethical considerations and practical restrictions, clinical investigations into the neurodevelopmental effects of passively administered antidepressants (via the breast milk) are problematic. Therefore, preclinical investigations into this topic are not only important but clinically relevant.

Aims & Objectives
We aimed to quantify the concentration of passively administered, i.e., via the breast milk during nursing, fluoxetine, and its active metabolite, norfluoxetine in the whole brains of exposed Flinders sensitive line (FSL) rats (an established rodent model of depression). We further aimed to establish if said exposure would associate with changes in whole-brain serotonergic function and redox status.

Methods
Adult FSL dams received fluoxetine (10 mg/kg/day), or placebo for fourteen days, beginning on postpartum day 04. Offspring (n = 16 per exposure group; 50:50 male: female) were passively exposed to fluoxetine until postnatal day 18 and euthanized on postnatal day 22. Fluoxetine, norfluoxetine, serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), and oxidized and reduced glutathione (GSH and GSSG) were then measured in the whole brains via liquid chromatography/mass spectrometry (LC-MS).

Results
Fluoxetine, was undetectable in the brain of FSL offspring, while norfluoxetine concentrations, averaged 41.28 ± 6.47 ng/g. Neither serotonin nor its metabolite (5-HIAA) was affected by passively administered fluoxetine in the juvenile brain. In terms of redox status, pups exposed to fluoxetine presented with a heightened oxidative stress, as evinced by a lower GSH/GSSG ratio.

Discussion & Conclusion
Although fluoxetine and norfluoxetine concentrations have been measured in breast milk and infant plasma, to the best of our knowledge, it has not been quantified in the juvenile brain until now. Our results are in line with clinical findings, suggesting the infant norfluoxetine/fluoxetine ratio to be elevated2,3, probably because of the prolonged half-life of norfluoxetine. Although only norfluoxetine was detected, this did not influence the central serotonin concentrations of offspring. However, it associated with increased oxidative stress, of which the pathophysiological significance remains to be established4,5. Taken together, our findings confirm that passively administered fluoxetine does reach the infant brain in the form of norfluoxetine and may manipulate processes of oxidative stress regulation. Further studies into the long-term bio-behavioural effects are however needed to effectively inform breast feeding mothers on the safety of antidepressant-use during the postpartum period.

References
1)Cameron et al. 2020. 276:765-74. DOI: 10.1016/j.jad.2020.07.081.
2)Epperson et al. 2003. Pediatrics, 112(5):e425. DOI: 10.1542/peds.112.5.e425.
3)Heikkinen et al. 2003. Clin Pharmacol Ther. 73(4):330-7. DOI: 10.1016/s0009-9236(02)17634-x.
4)Dalmizrak et al. 2019. Protein J. 38(5):515-24. DOI: 10.1007/s10930-019-09834-7.
5)De Andrade Silva et al. 2023. Neurochem Int. 162:105454. DOI: 10.1016/j.neuint.2022.105454.