Serotonin depletion reduces both acquisition and expression of context-conditioned fear

The serotonin precursor 5-HTP and the serotonin releaser fenfluramine but not the serotonin reuptake inhibitor escitalopram impair acquisition and expression of context-conditioned fear

Whereas preclinical and clinical studies suggest serotonin to play an important role in the regulation of anxiety, it remains unclear if its dominating impact is anxiety-dampening or anxiety-provoking. In male rats exposed to a well-established animal model of anxiety, i.e., context-conditioned freezing, we previously found serotonin depletion prior to acquisition and/or expression of conditioned fear to result in a reduction in freezing. We now explored the effect of increasing synaptic levels of serotonin in the same paradigm. Two compounds eliciting a robust increase in extracellular serotonin levels - the serotonin precursor 5-hydroxytryptophan (5-HTP) and the serotonin releaser d-fenfluramine - were administered i) prior to acquisition, ii) immediately after acquisition, or iii) prior to expression of conditioned fear; for comparison, a serotonin reuptake inhibitor (SSRI) escitalopram was also tested. Both 5-HTP (≥ 25 mg/kg) and fenfluramine (≥ 0.5 mg/kg), but not escitalopram (3, 10 and 30 mg/kg), decreased the expression of conditioned fear in a dose-dependent manner. At higher dosage, both 5-HTP (≥ 250 mg/kg) and fenfluramine (5 mg/kg), but not escitalopram (30 mg/kg), impaired acquisition of fear conditioning as well as freezing displayed during presentation of electric foot shocks. In contrast, neither 5-HTP nor fenfluramine impacted expression of conditioned fear when administered immediately after the exposure to shocks. The results underline the importance of an intact and physiologically regulated serotonergic transmission for fear conditioning in rats.

Age-Related Alterations in the Level and Metabolism of Serotonin in the Brain of Males and Females of Annual Turquoise Killifish (Nothobranchius furzeri)

The annual turquoise killifish (Nothobranchius furzeri) is a laboratory model organism for neuroscience of aging. In the present study, we investigated for the first time the levels of serotonin and its main metabolite, 5-hydroxyindoleacetic acid, as well as the activities of the key enzymes of its synthesis, tryptophan hydroxylases, and degradation, monoamine oxidase, in the brains of 2-, 4- and 7-month-old male and female N. furzeri. The marked effect of age on the body mass and the level of serotonin, as well as the activities of tryptophan hydroxylases and monoamine oxidase in the brain of killifish were revealed. The level of serotonin decreased in the brain of 7-month-old males and females compared with 2-month-old ones. A significant decrease in the tryptophan hydroxylase activity and an increase in the monoamine oxidase activity in the brain of 7-month-old females compared to 2-month-old females was shown. These findings agree with the age-related alterations in expression of the genes encoding tryptophan hydroxylases and monoamine oxidase. N. furzeri is a suitable model with which to study the fundamental problems of age-related changes of the serotonin system in the brain.

Serotonin disruption at gestation alters expression of genes associated with serotonin synthesis and reuptake at weaning

RATIONALE

Serotonin (5-HT) is a monoamine neuromodulator that plays a key role in the organization of the central nervous system. 5-HT alterations may be associated to the emergence of social deficits and psychiatric disorders, including anxiety, depression, and substance abuse disorders. Notably, disruption of the 5-HT system during sensitive periods of development seems to exert long-term consequences, including altered anxiety responses and problematic use of alcohol.

OBJECTIVE

We analyzed, in mice, the effects of transient 5-HT depletion at gestation (a developmental stage when medial prefrontal cortex (mPFC) 5-HT levels depend exclusively on placental 5-HT availability) on 5-HT central synthesis and reuptake at weaning. We also explored if 5-HT disruption at the embryonic stage influences behavioral outcomes that may serve as a proxy for autistic- or anxiety-like phenotypes.

METHODS

C57/BL6 male and female mice, born from dams treated with a 5-HT synthesis inhibitor (PCPA; 4-Chloro-DL-phenylalanine methyl ester hydrochloride) at gestational days (G)13.5-16.5, were subjected to a behavioral battery that assesses social preference and novelty, compulsive behavior, stereotypies, and ethanol's anti-anxiety effects, at postnatal days (P) 21-28. Afterwards, expression of the genes that encode for 5-HT synthesis (Tph2) and SERT (5-HT transporter) were analyzed in mPFC via real-time RT-PCR. Dopamine 2 receptor (D2R) expression was also analyzed via RT-PCR to further explore possible effects of PCPA on dopaminergic transmission.

RESULTS

Transient 5-HT disruption at G13.5-16.5 reduced Tph2 expression of both male and female mice in mPFC at P23. Notably, female mice also exhibited higher SERT expression and reduced D2R expression in mPFC. Mice derived from 5-HT depleted dams displayed heightened compulsive behavior at P21, when compared to control mice. Alcohol anti-anxiety effects at early adolescence (P28) were exhibited by mice derived from 5-HT depleted dams, but not by control counterparts. No social deficits or stereotyped behaviors were observed.

CONCLUSION

Transient 5-HT inhibition at gestation resulted in altered expression of genes involved in 5-HT synthesis and reuptake in mPFC at weaning, a period in which the 5-HT system is still developing. These alterations may exert lingering effects, which translate to significant compulsivity and heightened sensitivity to the anxiolytic effects of alcohol at early adolescence.