Sex-based changes in rat brain serotonin and behavior in a model of altitude-related vulnerability to treatment-resistant depression

Kynurenines Dynamics in the Periphery and Central Nervous System Steers Behavioral Deficits in Rats under Hypobaric Hypoxia

People travel to high-altitude regions as tourists, workers, and military personnel on duty. Despite the consistent 21% oxygen content in the atmosphere, ascending to higher altitudes results in a decrease in the partial pressure of oxygen, inducing a state known as hypobaric hypoxia (HH). HH is an environmental stress that is responsible for neuroinflammation and behavioral deficits (anxiety, depression, mood disturbance, etc.), but little is known about its metabolic pathways. The kynurenine pathway (KP) is a promising candidate to uncover the mysteries of HH stress, as it is an important regulator of the immune system and is associated with behavioral deficits. To investigate the role of KP under HH, the levels of KP metabolites in the serum, cerebrospinal fluid (CSF), and brain tissue (prefrontal cortex-PFC, neocortex, and hippocampus) of male Sprague-Dawley rats exposed to HH at 7620 m for 1, 3, and 7 days were estimated utilizing high-performance liquid chromatography (HPLC). The behavioral analogs for anxiety-like and depression-like behavior were assessed using the open field test and forced swim test, respectively. Upon HH exposure, crosstalk between the periphery and central nervous system and KP metabolite region-dependent differential expression in the brain were observed. KP metabolites showed a positive correlation with behavioral parameters. The results of our study are indicative that KP can be proposed as the etiology of behavioral deficits, and KP metabolite levels in serum or CSF can be used as plausible markers for anxiety-like and depression-like behaviors under HH stress with a scope of targeted therapeutic interventions.

Potential Therapeutic Effects of NAMPT-Mediated NAD Biosynthesis in Depression In Vivo

This study aimed to investigate the potential therapeutic effects of nicotinamide phosphoribosyltransferase (NAMPT)-mediated adenine dinucleotide (NAD) biosynthesis in depression models in vivo. Nampt^flox/flox mice were used to evaluate the role of NAMPT in depression. NAMPT and NAD levels in the prefrontal cortex (PFC) were measured, and depression-associated behavior, cognitive function, and social interaction were evaluated. The expression levels of BDNF, pCREB, CREB, monoamine neurotransmitters, and corticosterone (CORT) were also detected in the PFC. The contents of NAMPT and NAD decreased in the PFC in Nampt^flox/flox mice. Nampt^flox/flox mice showed depression-like behaviors, cognitive function deterioration, decreased social ability, and decreased dominance. Meanwhile, there were decreased expression levels of the pCREB/CREB ratio, but not BDNF, in the PFC. Levels of DA, 5-HT, and NE were decreased, and CORT was activated in the PFC of Nampt^flox/flox mice. Additionally, the role of NAMPT-NAD was examined in rats treated with nicotinamide riboside (NR) after being exposed to chronic unexpected mild stress (CUMS). NR reversed the decreased NAMPT expression in the PFC and HIP, and the NAD content in the PFC, but not HIP in rats with CUMS-induced depression. NR also improved depressive- and anxiolytic-like behaviors, locomotor activity, and cognitive function. BDNF expression and the pCREB/CREB ratio were significantly increased in both the PFC and HIP after NR treatment. The activation of CORT and decreased content of DA were reversed after NR treatment in the PFC. There was no difference in the 5-HT content among groups in both the PFC and HIP. Taken together, NAD synthesis induced by NAMPT could be associated with depression-like behaviors in mice, and the elevated NAD level by NR improved depression in rats.

The interplay of hypoxic and mental stress: Implications for anxiety and depressive disorders

Adequate oxygen supply is essential for the human brain to meet its high energy demands. Therefore, elaborate molecular and systemic mechanism are in place to enable adaptation to low oxygen availability. Anxiety and depressive disorders are characterized by alterations in brain oxygen metabolism and of its components, such as mitochondria or hypoxia inducible factor (HIF)-pathways. Conversely, sensitivity and tolerance to hypoxia may depend on parameters of mental stress and the severity of anxiety and depressive disorders. Here we discuss relevant mechanisms of adaptations to hypoxia, as well as their involvement in mental stress and the etiopathogenesis of anxiety and depressive disorders. We suggest that mechanisms of adaptations to hypoxia (including metabolic responses, inflammation, and the activation of chemosensitive brain regions) modulate and are modulated by stress-related pathways and associated psychiatric diseases. While severe chronic hypoxia or dysfunctional hypoxia adaptations can contribute to the pathogenesis of anxiety and depressive disorders, harnessing controlled responses to hypoxia to increase cellular and psychological resilience emerges as a novel treatment strategy for these diseases.

Effects of prenatal exposure to fluoxetine on circadian rhythmicity in the locomotor activity and neuropeptide Y and 5-HT expression in male and female adult Wistar rats

Serotonin (5-HT) reuptake inhibitors, such as fluoxetine, are the most prescribed antidepressant for maternal depression. In this sense, it exposes mothers and the brains of infants to increased modulatory and trophic effects of serotonergic neurotransmission. 5-HT promotes essential brain changes throughout its development, which include neuron migration, differentiation, and organization of neural circuitries related to emotional, cognitive, and circadian behavior. Early exposure to the SSRIs induces long-term effects on behavioral and neural serotonergic signalization. We have aimed to evaluate the circadian rhythm of locomotor activity and the neurochemical content, neuropeptide Y (NPY) and 5-HT in three brain areas: intergeniculate leaflet (IGL), suprachiasmatic nuclei (SCN) and raphe nuclei (RN), at two zeitgebers (ZT6 and ZT18), in male and female rat's offspring early exposed (developmental period GD13-GD21) to fluoxetine (20mg/kg). First, we have conducted daily records of the locomotor activity rhythm using activity sensors coupled to individual cages over four weeks. We have lastly evaluated the immunoreactivity of NPY in both SCN and IGL, and as well the 5-HT expression in the dorsal and medial RN. In summary, our results showed that (1) prenatal fluoxetine affects phase entrainment of the rest/activity rhythm at ZT6 and ZT18, more in male than female specimens, and (2) modulates the NPY and 5-HT expression. Here, we show male rats are more susceptible to phase entrainment and the NPY and 5-HT misexpression compared to female ones. The sex differences induced by early exposure to fluoxetine in both the circadian rhythm of locomotor activity and the neurochemical expression into SCN, IGL, and midbrain raphe are an important highlight in the present work. Thus, our results may help to improve the knowledge on neurobiological mechanisms of circadian rhythms and are relevant to understanding the "broken brains" and behavioral abnormalities of offspring early exposed to antidepressants.

Postnatal depletion of serotonin affects the morphology of neurons and the function of the hippocampus in male rats

Serotonin (5-HT) is an essential neurotransmitter for the refined organization of the cerebral cortex. Studies have suggested that altered serotonin signaling contributes to cognitive impairment and psychiatric disorders. However, the exact role of this neurotransmitter on the development of hippocampal neurons is not recognized. Here we aimed to examine the effects of the para- chlorophenylalanine (PCPA; 100mg/kg/daily, s.c during the postnatal days 10-20), a reversible inhibitor of 5-HT synthesis, on the serotonin level of the hippocampal and prefrontal cortex. We also focused on the morphology of the neurons in the hippocampus and spatial learning and memory. Our results indicated that the administration of PCPA led to a decrease in serotonin levels in the hippocampus and prefrontal cortex. Postnatal serotonin depletion also induced subtle alterations in the neuronal populations of the hippocampus and impaired spatial memory in the adulthood period of life. We found that critical developmental periods of serotonin depletion caused degeneration and swelling of neurons as well as significant neuronal loss in the hippocampal CA1, CA3, and dentate gyrus (DG) areas. Thus, serotonin, a strikingly important neurotransmitter, can affect neuronal morphology, development, and hippocampal-dependent memory.

Repeated episodes of transient reduction of oxygen exposure simulating aircraft cabin conditions enhance resilience to stress in mice

Pilots and crew of domestic flights are exposed to transient periods of mild reductions of partial pressure of inspired oxygen each day, and this might have functional consequence on their performance in the long range. Here, we exposed mice to mild reductions of oxygen exposure (ROE) four times per day for 21 days by lowering oxygen partial pressure to levels corresponding to an altitude of about 2300 m, which is the quote of pressurization of the air cabin. Four groups of mice were studied: unstressed or stressed mice exposed to ROE or normoxic conditions. Mice were exposed to chronic unpredictable stress (CUS) for 28 days, and ROE was delivered in the last 21 days of CUS. In normoxic mice, CUS caused anhedonia in the sucrose preference test, anxiety-like behaviour in the open field test, learning impairment in the Morris water maze, reduced hippocampal neurogenesis, increased serum corticosterone levels and increased expression of depression-related genes (Pclo, Mthfr and Grm5) in the hippocampus. All these changes were reversed by ROE, which had little or no effect in unstressed mice. These findings suggest that ROE simulating air cabin conditions of domestic flights may enhance resilience to stress improving mood, anxiety and learning ability.

Sex-Based Impact of Creatine Supplementation on Depressive Symptoms, Brain Serotonin and SSRI Efficacy in an Animal Model of Treatment-Resistant Depression

Background: Rates of major depressive disorder (MDD) increase with living at altitude. In our model, rats housed at moderate altitude (in hypobaric hypoxia) exhibit increased depression-like behavior, altered brain serotonin and a lack of antidepressant response to most selective serotonin reuptake inhibitors (SSRIs). A forebrain deficit in the bioenergetic marker creatine is noted in people living at altitude or with MDD. Methods: Rats housed at 4500 ft were given dietary creatine monohydrate (CRMH, 4% w/w, 5 weeks) vs. un-supplemented diet, and impact on depression-like behavior, brain bioenergetics, serotonin and SSRI efficacy assessed. Results: CRMH significantly improved brain creatine in a sex-based manner. At altitude, CRMH increased serotonin levels in the female prefrontal cortex and striatum but reduced male striatal and hippocampal serotonin. Dietary CRMH was antidepressant in the forced swim test and anti-anhedonic in the sucrose preference test in only females at altitude, with motor behavior unchanged. CRMH improved fluoxetine efficacy (20 mg/kg) in only males at altitude: CRMH + SSRI significantly improved male striatal creatine and serotonin vs. CRMH alone. Conclusions: Dietary CRMH exhibits sex-based efficacy in resolving altitude-related deficits in brain biomarkers, depression-like behavior and SSRI efficacy, and may be effective clinically for SSRI-resistant depression at altitude. This is the first study to link CRMH treatment to improving brain serotonin.