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Perinatal exposure to 5-hydroxytryptophan induces reduction of 5HT concentration in the midbrain 5HT neurons of adult rats

Introduction: Serotonin (5-hydroxytryptamine, 5HT) is a biologically active amine with diverse roles in the mammalian organism. Drugs targeting 5HT system are widely used in treatments of affective and anxiety disorders. Although selective serotonin reuptake inhibitors represent a common choice of 5HT enhancers during pregnancy, the immediate 5HT precursor, 5-hydroxytryptophan (5HTP), is being increasingly offered as a natural and safer alternative to antidepressant medication. However, consequences of developmental exposure to increased 5HTP concentrations on brain serotonin homeostasis have not been studied in animal models or humans. In the present study, we examined a possible influence of perinatal treatment with 5HTP (25mg/kg) on the serotonin concentration in the midbrain 5HT neurons of adult rats. Methods: Wistar rats were treated with either 5HTP or saline, from gestational day 13 until birth by subcutaneous injections to pregnant females, and from postnatal day (PND) 1 until PND 21 by receiving subcutaneous injections themselves. 5HT levels in serum and cortex were measured by ELISA at the end of treatment. Brain samples from 3 females and 2 males in each group were collected from animals sacrificed on PND 70. The samples were stored in saccharose and 40 µm slices were cut on a microtome. 5-HT immunohistochemical staining was performed on the slices containing dorsal raphe nuclei using DAB (3, 3’-diaminobenzidine) chromogen, and visualized by light microscope with a 2x magnification. Images of slices were processed with Zeiss Zen 2 software, and plate levels were defined according to The rat brain atlas of Paxinos and Watson. The number and size of 5HT neurons, as well as mean signal intensity per cell (corresponding to 5HT concentration) were analyzed in plates 95, 96 and 97 of each animal using NIH ImageJ software. Results: Chronic 5HTP treatment significantly raised serum, but not cortical 5HT concentrations in pups. In adult animals, no differences were observed between the control and experimental animals regarding the number and size of the 5HT-containing cells in the examined area of the brain slices. However, mean signal intensity per cell was significantly lower in the experimental group (p<0.01), indicating a decrease in 5HT concentrations in animals perinatally treated with 5HTP. The result corresponded well to the earlier observations of the increased expression of midbrain monoamine oxydase (MAO) genes in these animals. Conclusion: We can conclude that transient perinatal hyperserotonemia, caused by the chronic 5HTP treatment, induced compensatory changes in the expression/activity of the elements controlling serotonin homeostasis in the midbrain 5HT neurons, resulting in a long-lasting/permanent reduction of 5HT concentration. Our results, obtained on an animal model, suggest a need for thorough examination of the potential effect of 5HTP treatment on the developing human brain and its possible neurological/behavioral consequences.

serotonin, 5-HTP, hyperserotonemia, raphe nuclei, immuno-staining

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