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.

Epidemiological studies show that altitude-of-residence is an independent risk factor for worsening rates of mood disorders, substance abuse, and suicide. Proton (1H) magnetic resonance spectroscopy (MRS) studies in rodent models of moderate-to-high altitude exposure have documented significant alterations in total creatine, glutamate, and myo-inositol, neurometabolites involved in bioenergetic homeostasis and neuronal/glial cell function. This preliminary study utilized 3 Tesla 1H MRS to study anterior cingulate cortex (ACC) and parietal-occipital cortex (POC) neurochemistry in healthy subjects residing in Utah (n = 19), Massachusetts (n = 10), and South Carolina (n = 10), to test the hypothesis that individuals residing at moderate altitude (Utah; 1,372 m) would show neurometabolite alterations vs. subjects living at sea level. Expressed as ratios to total N-acetyl aspartate (NAA), Utah participants showed lower ACC (p = 0.03) and POC (p < 0.01) total creatine, a trend towards lower ACC glutamate (p = 0.06), and lower POC myo-inositol (p = 0.02). Study limitations include small sample sizes and uncorrected multiple comparisons. To our knowledge, this is the first MRS investigation to identify potential neurochemical differences in individuals residing at moderate altitudes vs. sea level, warranting future 1H MRS studies in larger cohorts and across a range of altitudes-of-residence.

ObjectivePrescription opioid misuse and fatal overdoses have increased significantly over the last two decades. Living at altitude has been linked to greater reward benefits of other drugs of abuse, and living at altitude may also exacerbate the respiratory depression linked to opioid use. Therefore, we examined the relationships between living at altitude, and prescription opioid misuse and fatal overdoses.MethodState-level past year rates of prescription opioid misuse were retrieved from the Substance Abuse and Mental Health Services Administration. County-level overdose data were extracted from the Centers for Disease Control and Prevention. Multiple linear regression models were fit to determine the relationship between average state elevation and state rates of opioid misuse. Logistic regression models were fit to determine the relationship between county elevation and county-level fatal opioid overdose prevalence.ResultsAfter controlling for state opioid prescribing rates and other confounders, we identified a significant positive association between mean state altitude and state-level opioid misuse rates for women, but not men. We also found a significant positive association between county-level altitude and prevalence of fatal opioid overdose.ConclusionsLiving at altitude is thus demographically associated with increasing rates of misuse of prescription opioids, as well as of cocaine and methamphetamine. Animal studies suggest that the hypobaric hypoxia exposure involved with living at altitude may disrupt brain neurochemistry, to increase reward benefits of drugs of abuse. This increased misuse of both stimulants and opioids may increase likelihood of overdose at altitude, with overdoses by opioid use also potentially facilitated by altitude-related hypoxia.

Sheth, Chandni, Hendrik Ombach, Paul Olson, Perry F. Renshaw, and Shami Kanekar. Increased anxiety and anhedonia in female rats following exposure to altitude. High Alt Med Biol. 19:81-90, 2018.-Anxiety disorders are chronic, highly prevalent conditions, often comorbid with depression. Both anxiety and depression form major risk factors for suicide. Living at altitude is associated with higher rates of depression and suicide, leading us to address whether anxiety disorders may also be amplified at altitude. Using a novel translational animal model, we previously showed that depression-like behavior increases with altitude of housing in female, but not male rats. We now use this model to examine the effects of altitude on both anxiety-like behavior and anhedonia, a core symptom of depression. After housing for a week at sea level, 4500 or 10,000 ft, rats were evaluated for anxiety in the open-field test or the elevated plus maze, and anhedonia in the sucrose preference test. Another group was tested at baseline. Anxiety-like behavior increased in females housed at altitude. In females, lower sucrose preference was seen in those housed at 10,000 ft versus those at sea level. Males showed no change in anxiety or anhedonia across groups. These data suggest that living at moderate-high altitude may pose a risk factor for those vulnerable to anxiety disorders, with the potential to be particularly detrimental to females at altitude.

Treatment-resistant depression, a chronic condition that affects 30% of depressed patients on antidepressants, is highly linked to suicidal behavior. Chronic hypoxia exposure via living at altitude (hypobaric hypoxia) or with chronic hypoxic diseases is demographically linked to increased risk for depression and suicide. We previously demonstrated that housing rats at altitude for a week incrementally increases depression-like behavior in the forced swim test (FST) in females, but not males. In animal models, high altitude exposure reduces brain serotonin, and selective serotonin reuptake inhibitors (SSRIs) can lose efficacy when brain serotonin levels are low. To address whether residence at moderate altitude is detrimental to SSRI function, we examined SSRI efficacy in the FST after a week of housing rats at altitudes of 4500 ft. or 10,000 ft. as compared to at sea level. In females, the tricyclic antidepressant desipramine (positive control) functioned well in all groups, increasing latency to immobility and decreasing immobility, by increasing climbing. However, the SSRIs fluoxetine, paroxetine and escitalopram were ineffective in females in all groups: only paroxetine improved swimming in the FST as expected of a SSRI, while all three unexpectedly reduced climbing. Fluoxetine was also ineffective in male rats. Sertraline was the only SSRI with antidepressant efficacy at altitude in both females and males, increasing swimming, climbing and latency to immobility, and reducing immobility. Hypobaric hypoxia thus appears to be detrimental to efficacy of the SSRIs fluoxetine, paroxetine and escitalopram, but not of sertraline. Unlike the other SSRIs, sertraline can improve both serotonergic and dopaminergic transmission, and may be less impacted by a hypoxia-induced serotonin deficit. A targeted approach may thus be necessary for successful antidepressant treatment in patients with depression who live at altitude or with chronic hypoxic diseases, and that sertraline may be the SSRI of choice for prescription for this population.

Rates of depression and suicide are higher in people living at altitude, and in those with chronic hypoxic disorders like asthma, chronic obstructive pulmonary disorder (COPD), and smoking. Living at altitude exposes people to hypobaric hypoxia, which can lower rat brain serotonin levels, and impair brain bioenergetics in both humans and rats. We therefore examined the effect of hypobaric hypoxia on depression-like behavior in rats. After a week of housing at simulated altitudes of 20,000 ft, 10,000 ft, or sea level, or at local conditions of 4500 ft (Salt Lake City, UT), Sprague Dawley rats were tested for depression-like behavior in the forced swim test (FST). Time spent swimming, climbing, or immobile, and latency to immobility were measured. Female rats housed at altitude display more depression-like behavior in the FST, with significantly more immobility, less swimming, and lower latency to immobility than those at sea level. In contrast, males in all four altitude groups were similar in their FST behavior. Locomotor behavior in the open field test did not change with altitude, thus validating immobility in the FST as depression-like behavior. Hypobaric hypoxia exposure therefore induces depression-like behavior in female rats, but not in males.

Residing at high altitude may lead to reduced blood oxygen saturation in the brain and altered metabolism in frontal cortical brain areas, probably due to chronic hypobaric hypoxia. These changes may underlie the increased rates of depression and suicidal behavior that have been associated with life at higher altitudes. To test the hypothesis that hypobaric hypoxia is responsible for development of mood disorders due to alterations in neurochemistry, we assessed depression-like behavior in parallel to levels of brain metabolites in rats housed at simulated altitude. 32 female Sprague Dawley rats were housed either in a hypobaric hypoxia chamber at 10,000 ft of simulated altitude for 1 week or at local conditions (4500 ft of elevation in Salt Lake City, Utah). Depression-like behavior was assessed using the forced swim test (FST) and levels of neurometabolites were estimated by in vivo proton magnetic resonance spectroscopy in the frontal cortex, the striatum and the hippocampus at baseline and after a week of exposure to hypobaric hypoxia. After hypoxia exposure the animals demonstrated increased immobility behavior and shortened latency to immobility in the FST. Elevated ratios of myo-inositol, glutamate, and the sum of myo-inositol and glycine to total creatine were observed in the frontal cortex of hypoxia treated rats. A decrease in the ratio of alanine to total creatine was also noted. This study shows that hypoxia induced alterations in frontal lobe brain metabolites, aggravated depression-like behavior and might be a factor in increased rates of psychiatric disorders observed in populations living at high altitudes.

The forced swim test (FST) is a behavioral test in rodents which was developed in 1978 by Porsolt and colleagues as a model for predicting the clinical efficacy of antidepressant drugs. A modified version of the FST added the classification of active behaviors into swimming and climbing, in order to facilitate the differentiation between serotonergic and noradrenergic classes of antidepressant drugs. The FST is now widely used in basic research and the pharmaceutical screening of potential antidepressant treatments. It is also one of the most commonly used tests to assess depressive-like behavior in animal models. Despite the simplicity and sensitivity of the FST procedure, important differences even in baseline immobility rates have been reported between different groups, which complicate the comparison of results across studies. In spite of several methodological papers and reviews published on the FST, the need still exists for clarification of factors which can influence the procedure. While most recent reviews have focused on antidepressant effects observed with the FST, this one considers the methodological aspects of the procedure, aiming to summarize issues beyond antidepressant action in the FST. The previously published literature is analyzed for factors which are known to influence animal behavior in the FST. These include biological factors, such as strain, age, body weight, gender and individual differences between animals; influence of preconditioning before the FST: handling, social isolation or enriched environment, food manipulations, various kinds of stress, endocrine manipulations and surgery; schedule and routes of treatment, dosage and type of the drugs as well as experimental design and laboratory environmental effects. Consideration of these factors in planning experiments may result in more consistent FST results.

In mouse olfactory epithelium (OE), pituitary adenylate cyclase-activating peptide (PACAP) protects against axotomy-induced apoptosis. We used mouse OE to determine whether PACAP protects neurons during exposure to the inflammatory cytokine TNFα. Live slices of neonatal mouse OE were treated with 40 ng/ml TNFα ± 40 nM PACAP for 6 h and dying cells were live-labeled with 0.5% propidium iodide. TNFα significantly increased the percentage of dying cells while co-incubation with PACAP prevented cell death. PACAP also prevented TNFα-mediated cell death in the olfactory placodal (OP) cell lines, OP6 and OP27. Although OP cell lines express all three PACAP receptors (PAC1, VPAC1,VPAC2), PACAP's protection of these cells from TNFα was mimicked by the specific PAC1 receptor agonist maxadilan and abolished by the PAC1 antagonist PACAP6-38. Treatment of OP cell lines with blockers or activators of the PLC and AC/MAPKK pathways revealed that PACAP-mediated protection from TNFα involved both pathways. PACAP may therefore function through PAC1 receptors to protect neurons from cell death during inflammatory cytokine release in vivo as would occur upon viral infection or allergic rhinitis-associated injury.

One premise regarding the mechanism of injury‐evoked neuroregeneration is that injured cells induce the release of neurotrophic factors to trigger neurogenesis. Extracellular purine nucleotides exert multiple neurotrophic actions in the central nervous system mediated via activation of purinergic receptors. However, whether purinergics have a neurotrophic role in the olfactory neuroepithelium has not been investigated. Thus, we monitored the ATP‐induced release of neuropeptide Y (NPY), a neuropeptide that increases neuroproliferation in the olfactory epithelium. To visualize NPY release, slices of olfactory epithelium from neonatal mice were cultured on nitrocellulose paper. Immunoassays of the nitrocellulose demonstrated NPY immunoreactivity in regions corresponding to the olfactory epithelium of the nasal cavity. One hour of exposure to exogenous ATP (100, 500 μM) significantly increased the number of olfactory epithelium slices that released NPY from 25% ± 6% to 60% ± 7% or 71% ± 10% ( P = 0.001). The purinergic receptor antagonists pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulfonic acid (PPADS; 25 μM) and suramin (100 μM) significantly reduced the number of olfactory epithelium slices exhibiting ATP‐evoked NPY release to 18% ± 11% ( P = 0.004), indicating that NPY release is mediated by activation of purinergic receptors. Released NPY was quantified by enzyme and radioimmunoassays. Exogenous ATP or UTP significantly increased the amount of NPY released. Overall, this study demonstrates that purinergic receptor activation mediates the release of neurotrophic factor NPY in the olfactory epithelium and provides pharmacological targets to promote regeneration of damaged olfactory epithelium. © 2008 Wiley‐Liss, Inc.