Depression is a state marked by sadness, emptiness, a sense of worthlessness, and a loss of interest in most activities of the day. Other indicators include insomnia or hypersomnia, fatigue/loss of energy, restlessness or psychomotor retardation, inability to concentrate, thoughts of death and suicidal thoughts. .
As indicated by the study on the global burden of disease  depression remains, since 1990, one of the leading causes of disability in the world, reflecting a lack of progress in the fight against this condition. A study based on the World Health Organization’s Global Mental Health Surveys in 21 countries (four low/lower middle income, five upper middle income, one lower or upper middle income and 11 high income) , showed that one of the most common individual disorders among college students was major depressive disorder (MDD), with rates of 4.5-7.7% . Research on the prevalence of depressive symptoms among university students in Europe found rates, for female and male students, respectively, of 12.1-45.0%, with lower scores in Western countries, Germany 26, 7%/22.8%, Denmark 24.9%/12.1%, and higher scores in Eastern countries Poland 45.5%/27.3% and Bulgaria 42.9%/33, 8% . Similar results were found in a systematic review , with prevalence rates ranging from 10 to 85%, average prevalence of 30.6%, a rate which exceeds the 21.6% observed in the general population. Although epidemiological research shows great variability in the prevalence of depression between countries, the available data is consistent regarding higher rates of depression in women compared to men. [4, 6]. Depressive symptoms have a negative impact on quality of life (QOL)  and have been associated with school absenteeism and early school leaving [8, 9]poor concentration, poor general ability to work, and poor academic performance [10, 11].
Oxytocin (OT), a neuropeptide synthesized in the hypothalamus and released by the posterior pituitary in systematic circulation, has long been known as an important hormone involved in parturition and lactation. [10,11,12]. In addition to peripheral effects, in humans oxytocin is released from magnocellular excretory cells and binds to the amygdala, striatum, substantia nigra, hypothalamus . This central action seems to be involved in human behaviors.
Existing evidence suggests that serum oxytocin levels play a role in many aspects of social behavior, improving bonding and attachment [13,14,15] increased willingness to share emotions positive communication  and trust [18, 19] increase the ability to interpret mental states  and modulate attentional processes .
Some of the characteristics inherent in social behavior, such as openness to relationships or the regulation of stress, are also implicated in other behavioral and psychological processes such as depression, anxiety and post-traumatic stress disorder (PTSD). ). . Recent research has focused on the relevance of occupational therapy for developing, maintaining, and treating these disorders.
Research, including clinical trials, has been done in a variety of conditions, including schizophrenia, anxiety, and depression, and has shown an association of this hormone with individuals’ social skills, such as bonding, empathy, generosity and altruism. .
Mixed results have been found regarding the correlation between circulating plasma OT level and depressive symptomatology. Studies with non-human mammalian samples found that higher levels of OT, either by exogenous administration or endogenous release, increased positive hedonic states in rats, suggesting that OT helps to attenuate the severity of anhedonia seen in major depressive disorder (MDD). . In humans, low levels of OT have been associated with symptoms that characterize depression, such as loss of interest in maintaining interpersonal relationships. [25, 26]. In a population diagnosed with MDD, Scantamburlo et al.  found that plasma OT levels were reduced compared to individuals without a mood disorder diagnosis. Similarly, in a study by Gordon et al. in healthy university students, occupational therapy was negatively correlated with depressive symptoms, as measured by the Beck Depression Inventory (BDI), and psychological distress (i.e. depression and anxiety ) were found to be predictors of occupational therapy . Reinforcing these results, the study by Ozsoy et al.  who compared oxytocin levels between a sample of hospitalized patients with depressive disorder, bipolar affective disorder, depressive episode and a sample of healthy controls, found that serum oxytocin levels were decreased in patients by report to witnesses. These results are consistent with several other studies that have shown an inverse association between OT levels and depression. [30,31,32]. However, a recent meta-analysis that includes 64 studies with multiple psychiatric disorders, showed no significant difference in peripheral OT between healthy adult and MDD groups. . Additionally, Parker et al.  reported that plasma OT concentration was elevated in depressed subjects compared to healthy controls.
The effect of oxytocin has been proposed to be gender-specific, with OT being biologically more relevant for females. In fact, despite some mixed results, the inverse correlation between OT and stress and mood disorders was more consistently found in women. . For example, Yuen et al.  showed that depressed women had lower OT concentrations than depressed men, regardless of their psychotic status, and, unlike men, depressed women had lower OT concentrations than healthy control women .
More recently, Engel et al.  performed a systematic review and meta-analysis that specifically measured basal endogenous oxytocin concentrations in depressed patients and healthy controls. Evaluation of nine studies, included in the meta-analytic procedure, showed non-significant differences in basal endogenous oxytocin concentrations between depressed patients and healthy controls. However, significant heterogeneity of effects was detected, and the authors point out that study designs, hormonal assessments, and clinical and demographic factors could explain these results.
New studies have begun to focus on other patient characteristics, such as temperament and personality traits, which may contribute to the different results reported by various studies. Rothbart and Derryberry  defined temperament as constitutionally based individual differences in responsiveness and self-regulation in emotional, activation, and attention processes. This approach identified four central constructs of temperament: (1) Extraversion/Surgency, which consists of sociability and expressions of pleasure in anticipation of rewards or during high-intensity/novel activities and motor activity; (2) Negative affect, which includes discomfort, anger/frustration, sadness, fear, and low sociability; (3) Voluntary control, which refers to the ability to voluntarily suppress a predominant response to perform a subdominant response according to environmental requirements, detecting errors and planning; and (4) orienting sensitivity, including perceptual, associative, and general sensitivity to stimuli from the environment . It has been understood that temperament relates to individual differences that (a) have a strong genetic basis; (b) manifest early in life, and (c) are relatively stable throughout life [37, 38].
A large body of research has shown evidence of a link between temperament and depressive and anxiety disorders. For example, in a study with a sample of 4773 subjects, members of the base population, high levels of harm avoidance and pessimism were related to both depressed mood (effect size; d = 0.84 and d = 1.25, respectively) and depressive disorder (d = 0.68 and d = 0.68, respectively) . More recently, Katz et al. in a meta-analysis that aimed to quantify relationships between the temperament dimension of sensitivity, depression, and anxiety, found that sensitivity to punishment predicted depression (β = 0.37) and anxiety (β = 0.35), and that reward sensitivity predicted depression (β = −0.07).
Despite this evidence, to our knowledge, no research has investigated the effects of serum oxytocin levels and temperament dimensions on depressive symptoms in a healthy sample. The current research is part of an extensive study with university students from the Instituto Politécnico de Lisboa (IPL) to assess the prevalence of psychological distress, namely anxiety and depression and their correlates. The present study aimed to: i) assess the relationship between serum oxytocin, depression and temperament traits and ii) analyze the effect of serum oxytocin and temperament in depressive symptomatology, in a sample of 45 young healthy university students.