Serotonin release in rat hippocampus examined by in vivo voltammetry: serotonergic function and tryptophan availability

Neurometabolic effects of sweetened solution intake during adolescence related to depressive-like phenotype in rats

OBJECTIVE

Exposure to artificial sweeteners, such as aspartame, during childhood and adolescence has been increasing in recent years. However, the safe use of aspartame has been questioned owing to its potentially harmful effects on the developing brain. The aim of this study was to test whether the chronic consumption of aspartame during adolescence leads to a depressive-like phenotype and to investigate the possible mechanisms underlying these behavioral changes.

METHODS

Adolescent male and female rats were given unlimited access to either water, solutions of aspartame, or sucrose in their home cages from postnatal day 21 to 55.

RESULTS

Forced swim test revealed that both chronic aspartame and sucrose intake induced depressive-like behaviord, which was more pronounced in males. Additionally, repeated aspartame intake was associated with increased cerebrospinal fluid (CSF) aspartate levels, decreased hippocampal neurogenesis, and reduced activation of the hippocampal leptin signaling pathways in males. In females, we observed a main effect of aspartame: reducing PI3K/AKT one of the brain-derived neurotrophic factor pathways; aspartame also increased CSF aspartate levels and decreased the immunocontent of the GluN2A subunit of the N-methyl-d-aspartic acid receptor.

CONCLUSION

The findings revealed that repeated aspartame intake during adolescence is associated with a depressive-like phenotype and changes in brain plasticity. Interestingly, males appear to be more vulnerable to the adverse neurometabolic effects of aspartame than females, demonstrating a sexually dimorphic response. The present results highlighted the importance of understanding the effects caused by the constant use of this artificial sweetener in sensitive periods of development and contribute to regulation of its safe use.

Improving therapeutics in anorexia nervosa with tryptophan

A growing body of evidence suggests that our diet is an important contributing factor in the development, management and prevention of a number of psychiatric illnesses. Tryptophan, an essential amino acid, is the sole precursor of neurotransmitter 5-hydroxytryptamine (5-HT; serotonin). Administration of tryptophan can boost serotonin neurotransmission to produce therapeutically important effects in serotonin deficiency disorders. Anorexia nervosa (AN) an eating disorder associated with high levels of psychiatric comorbidity including psychosis, hyperactivity, depression and anxiety has highest lethality of all psychiatric illnesses. Evidence suggests that excessive dieting and food restriction can decrease brain tryptophan and serotonin in AN patients to precipitate depression, psychosis and hyperactivity. There are currently no FDA approved pharmacological treatments available for AN patients; antidepressants and antipsychotics, largely used to treat associated psychiatric comorbidities are also not very effective. The aim of this non-systematic review article is to evaluate and document a potential importance of tryptophan supplementation in improving therapeutics in AN patients.

Differential effects of predator stress and the antidepressant tianeptine on physiological plasticity in the hippocampus and basolateral amygdala

Stress can profoundly affect memory and alter the functioning of the hippocampus and amygdala. Studies have also shown that the antidepressant tianeptine can block the effects of stress on hippocampal and amygdala morphology and synaptic plasticity. We examined the effects of acute predator stress and tianeptine on long-term potentiation (LTP; induced by 100 pulses in 1 s) and primed burst potentiation (PB; a low threshold form of LTP induced by only five physiologically patterned pulses) in CA1 and in the basolateral nucleus (BLA) of the amygdala in anesthetized rats. Predator stress blocked the induction of PB potentiation in CA1 and enhanced LTP in BLA. Tianeptine blocked the stress-induced suppression of PB potentiation in CA1 without affecting the stress-induced enhancement of LTP in BLA. In addition, tianeptine administered under non-stress conditions enhanced PB potentiation in the hippocampus and LTP in the amygdala. These findings support the hypothesis that acute stress impairs hippocampal functioning and enhances amygdaloid functioning. The work also provides insight into the actions of tianeptine with the finding that it enhanced electrophysiological measures of plasticity in the hippocampus and amygdala under stress, as well as non-stress, conditions.

Altered forebrain neurotransmitter responses to immobilization stress following 3,4-methylenedioxymethamphetamine

(+/-)3,4-Methylenedioxymethamphetamine (MDMA, "ecstasy") is an increasingly popular drug of abuse that acts as a neurotoxin to forebrain serotonin neurons. The neurochemical effects of the serotonin depletion following high doses of MDMA were investigated in response to acute immobilization stress. Male rats were treated with a neurotoxic dosing regimen of MDMA (10 mg/kg, i.p. every 2 h for four injections) or equivalent doses of saline. Seven days after treatment, in vivo microdialysis was used to assess extracellular dopamine and serotonin in the dorsal hippocampus and prefrontal cortex during 1 h of immobilization stress. In saline treated control rats, serotonin in the hippocampus and serotonin and dopamine in the prefrontal cortex were increased during immobilization stress. Rats pretreated with MDMA, however, showed blunted neurotransmitter responses in the hippocampus and the prefrontal cortex. In the drug pretreated rats, basal serotonin levels in the hippocampus, but not the prefrontal cortex, were lower compared to saline pretreated controls. Stress-induced increases in plasma corticosterone and body temperature were not affected by the pretreatment condition. From these studies we suggest that depletion of serotonin stores in terminal regions with the neurotoxin MDMA compromises the ability of the serotonergic neurons to activate central systems that respond to stressful stimuli. This altered responsiveness may have implications for long-term functional consequences of MDMA abuse as well as the interactions between the serotonergic system and stress.

Psychological stress repeatedly blocks hippocampal primed burst potentiation in behaving rats

Primed burst (PB) potentiation is a long-term increase in CA1 population spike amplitude produced by brief physiologically patterned electrical stimulation of the hippocampal commissure. Exposure of rats to a novel environment resulted in a blockade of short-term (Post-tetanic potentiation, PTP) and long-term (PB potentiation) plasticity in all cases (n = 6). When the animals had extensive exposure to the environment (14 consecutive days), PTP and PB potentiation occurred. With placement of the rats in a second novel environment, once again, neither PTP nor PB potentiation was observed. Placement of rats in each of the two novel environments produced a significant increase in serum corticosterone levels, while placement in the familiar environment produced no increase in response. These findings support the hypothesis that hippocampal plasticity is repeatedly susceptible to modulation by the stress of forced exposure to a novel environment.