1 Introduction
According to the WHO, every year, about 700,000 people end their lives through suicide, making it the fourth leading cause of death in young people (datadot, 2024). According to estimates, for every suicide carried out, there are between 10 and 40 suicide attempts (Krug et al., 2002; Chang et al., 2011). Suicide turns out to be a real social problem, as it cannot leave public opinion indifferent. According to ISTAT (Italian National Institute of Statistics) data from 2016, 3780 committed suicides in Italy, 78.8% of whom were male. The death rate from suicide is 11.8 per 100,000 population for men and 3.0 per 100,000 population for women (Vichi et al., 2019). In Western countries, the suicide rate among men is three to four times higher than among women. This difference is likely to be the case because men use reliably more lethal means of taking their own lives. This difference is even more marked in men aged 65 years and older, who are likely to be ten times more than women (Sue et al., 2016). In the Eastern Mediterranean countries, on the other hand, female and male suicide rates are almost identical. In many countries, suicide rates appear to be higher in middle age or old age. The absolute number of suicides, however, is highest in the age group between 15 and 29 years old (Pitman et al., 2012; Yip et al., 2012). Suicide is also related to age, with young people under 25 and older adults interested the most in the risk of suicide, although it proportionally increases with age (OECD, 2012). In 2018, the ISTAT estimated that 3,820 people committed suicide, with the highest incidence in males between 35 and 64 years old. The main methods used to commit suicide are hanging, pesticide poisoning, and the use of firearms. In a survey (Ajdacic-Gross et al., 2008) carried out in 56 countries, hanging was by far the most common method used by 53% of male suicides and 39% of female suicides (O Connor et al., 2011; Suicide in the Province, 2022). Suicide presents numerous risk factors (Beghi et al., 2021; Risk and Protective Factors, 2023), such as individuals (previous suicide attempts, chronic psychiatric and physical pathologies, drug addiction), relationships (history of family suicide, isolation, bullying), community (violence, discrimination), and social-linked (spectacularization by the media, access to lethal means). One of the most important risk factors is the presence of psychiatric pathology, primarily major depression, followed by anxiety disorder, bipolar disorder, and drug addiction (Beghi et al., 2021). Also, genetics can influence the risk of committing a self-suppressive act (Brent and Melhem, 2008). Among psychiatric pathologies, it is clear that patients suffering from major depression present a significant risk of suicide or attempted suicide (Conejero et al., 2018; Fusar-Poli et al., 2021). Identifying risk factors can recognize a part of the population at high risk of committing extreme acts (Granier and Boulenger, 2002).
The neurobiology underlying suicide is exceptionally complex, not being the result of a single pathophysiological entity but of a multiplicity of behavioral, socio-environmental, and psychological causal factors that act together simultaneously. According to the literature, suicide follows the “diathesis-stress” model: a behavioral disorder derives from a genetic predisposition that makes one more vulnerable to stress, usually caused by life events (Mann, 2003; Van Heeringen and Mann, 2014; Zeigler-Hill and Shackelford, 2019). In particular, traumatic life events and stress acts as triggers for suicidal behavior (Dwivedi, 2012). Furthermore, the interaction of numerous biological systems is involved (Girardi and Pompili, 2015), including the neurotrophic system. Indeed, the pathogenesis of depression and suicidal behavior involves changes in neuronal plasticity (Garcia, 2002), reducing the brain’s ability to adapt.
Neurotrophins are growth factors that play a crucial role in regulating structural and synaptic plasticity, maintaining neuronal functions, and modulating neurotransmission (Thoenen, 1995). Neurotrophins are essential for the survival, growth, and regeneration of different neuronal populations belonging to the central and peripheral nervous systems in the fetal and adult brain (Allen et al., 2013). The first neurotrophin discovered in the 1950s was Nerve Growth Factor (NGF) (Levi-Montalcini, 1987). Neurotrophin is transported from the production site to the nerve terminal via axonal retrograde flow during development. Neurons that present this retrograde flow survive; otherwise, they degenerate (“neurotrophic hypothesis”) (Hamburger et al., 1981; Ginty and Segal, 2002). There are five neurotrophic factors expressed in mammals: Brain-Derived Neurotrophic Factor (BDNF), Neurotrophins 3 and 4 (NT-3 and NT-4), Glial Derived Neurotrophic Factor (GDNF) and Ciliar Neurotrophic Factor (CNTF). Neurotrophins are initially synthesized as pro-neurotrophins, and then through proteolytic processes, they are converted into mature neurotrophins that interact with two classes of receptors (p75NRT and Trk) to act (Ibáñez and Simi, 2012; Neurotrophic, 2017). Neurotrophins also have numerous actions outside the nervous system, such as immune (Thorpe and Perez Polo, 1987; Otten et al., 1989; Donovan et al., 2000) and hematopoietic function (Thorpe et al., 1987). They also control some neuroendocrine functions (for example, the development of the female rat’s hypothalamus) (Ojeda et al., 1991) and are involved in the transmission of pain (Skaper, 2017). Furthermore, these molecules can increase cholinergic function and protect against neurodegeneration (Hock et al., 2000; Hoxha et al., 2018), hypothesizing a role in Alzheimer’s disease (Hefti, 1986). Similarly, the GDNF can be involved in Parkinson’s disease, as it acts on dopaminergic neurons (Yasuda and Mochizuki, 2010). Therefore, a pathological alteration of neurotrophic factors could determine defects in neural maintenance and regeneration, possible structural anomalies in the brain, and a reduction in neural plasticity, thus compromising the individual’s ability to adapt in critical situations.
BDNF (“Brain-Derived Neurotrophic Factor”) is the most widespread neurotrophin in the brain of mammals, both in adults and in the developing phase (Anisman et al., 2012). It is involved in neurogenesis, the development of neurons, neuronal homeostasis, the structural integrity and maintenance of neuronal plasticity in the adult brain (Altar et al., 1997; Bartrup et al., 1997; Edelmann et al., 2014; Panja and Bramham, 2014), synaptic connection, in the mechanisms that regulate learning and memory (Kang and Schuman, 1995), drug addiction, and in stress adaptation mechanisms. Numerous exogenous and endogenous stimuli (such as stress, physical activity, diet, and brain injury) regulate BDNF expression (Aid et al., 2007; Pruunsild et al., 2007). Several neuropathologies cause a reduction in BDNF protein levels and serum in patients’ brains (Autry and Monteggia, 2012; Borba et al., 2016). Unfortunately, it is unclear whether serum BDNF levels reflect brain levels, as studies contradict each other (Sartorius et al., 2009; Klein et al., 2011). However, serum BDNF levels should be higher than those in plasma and cerebrospinal fluid (Radka et al., 1996; Serra-Millàs, 2016). According to some clinical studies that examined the blood levels of BDNF in vivo, the BDNF can cross the blood-brain barrier in both directions via a high-capacity saturable transport system, which determines an early influx and rapid in the brain (Pan et al., 1998). BDNF concentrations can be measured in serum, plasma, or whole blood and appear highly correlated to those in cerebrospinal fluid. Therefore, BDNF levels in the blood could be related to the concentration of the same neurotrophin at the level of the cerebral cortex (Klein et al., 2011). The blood quantification of BDNF in vivo could be a marker of neuronal plasticity (Shimizu et al., 2003). Furthermore, BDNF also plays an essential role in neuroinflammation and aging, identifying a role in degenerative diseases such as Parkinson’s and Alzheimer’s (Lima et al., 2019). In humans, BDNF is considered a promising biomarker for various psychiatric pathologies (Chen et al., 2017). In some animal models, mice heterozygous for the BDNF gene showed increased aggression and anxiety, significant weight gain, and memory impairment, suggesting that its depletion could be associated with the development of some psychiatric symptoms (Lindholm and Castracn, 2014). This hypothesis also seems supported by the fact that there is an increase in BDNF levels after treatment of psychiatric pathologies (Nuernberg et al., 2016; Castrén and Monteggia, 2021). Patients with major depression show an appreciable reduction in serum levels of BDNF (Shimizu et al., 2003), probably due to a reduction of the protein in the brain (Karege et al., 2005a). There is also a single nucleotide polymorphism of the BDNF gene (Val66Met) that is associated with the severity of depressive symptoms and memory deficits (Youssef et al., 2018; Zhao et al., 2018), as well as a higher risk of suicide attempts (Zai et al., 2012). Typically, treatment with antidepressant drugs increases BDNF levels in serum and plasma (Molendijk et al., 2011). Some post-mortem studies have shown that treatment-resistant patients had significantly low levels of BDNF, especially in areas such as the hippocampus, which otherwise is generally rich (Colla et al., 2007; Brunoni et al., 2014). Other studies have demonstrated reductions in levels of BDNF mRNA and its protein from post-mortem brain samples of depressed patients, in particular in the hippocampus and amygdala (Guilloux et al., 2012; Ray et al., 2014). The reduction of serum BDNF levels has also been ascertained in other psychiatric pathologies, such as schizophrenia (Ray et al., 2014), autism (Katoh Semba et al., 2007), and bipolar disorder (Fernandes et al., 2015). Furthermore, in animal models, BDNF levels in the prefrontal cortex and hippocampus were increased after pharmacological treatment with mood stabilizers (Yasuda et al., 2009; Jornada et al., 2010). In the literature, there are numerous studies carried out on the brains of suicidal victims, which observe a reduction in the levels of BDNF protein and its receptor in the hippocampus (Karege et al., 2005a; University, 2021) and the prefrontal cortex (Zheng et al., 2016; Misztak et al., 2020), as well as in its mRNA (Hamburger et al., 1981). Thus, BDNF levels could represent an interesting biological marker of suicidal behavior.
Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophin expressed primarily during neuronal development and differentiation. In the adult, its expression decreases and remains limited to some regions, such as the cortex, the hippocampus, the substantia nigra, and the striatum nucleus (Duarte Azevedo et al., 2020). GDNF promotes the differentiation of dopaminergic (Christophersen et al., 2007) and serotonergic (Ducray et al., 2006) neurons. Furthermore, one of its most important roles is the protection of neurons (Hochstrasser et al., 2011; Uzdensky et al., 2013; Jaumotte and Zigmond, 2014) from oxidative stress and inflammation (Pascual et al., 2008). GDNF levels appear to be reduced in patients who have Alzheimer’s disease, leading to the hypothesis of its use as a biomarker for the pathology [ (Sharif et al., 2021; Kimhy et al., 2015)]. Although there are not many studies in the literature relating to the involvement of GDNF in the suicidal phenomenon, there is evidence linking this neurotrophin to the onset of mood disorders. In fact, according to some studies, there are lower serum GDNF levels in patients suffering from depression, with a subsequent increase after pharmacological treatment (Lin and Tseng, 2015; Wang et al., 2023). In a post-mortem study, an increase in GDNF was observed in the parietal cortex (site of emotion regulation (Anderson et al., 2004)) of patients with depressive disorder (Michel et al., 2008), while in other studies, a reduction in the expression of its mRNA was measured (Otsuki et al., 2008). In the literature, GDNF levels increase following ischemic or inflammatory damage (Wei et al., 2000; Michel et al., 2007) as neuronal resilience and plasticity compromise, hypothesizing that the increase in GDNF is an adaptive and compensatory response to neuronal damage (Chao and Lee, 1999; Tokumine et al., 2003).
Currently, many studies on the possible link between neurotrophins and suicide are carried out in vivo or plasma (Kim et al., 2007; Salas Magana et al., 2017; Sonal and Raghavan, 2018). Further interesting data could come from post-mortem studies carried out directly on the brains of suicidal subjects. An essential element is that many of these studies focus on the link between neurotrophins and neuro-psychiatric pathology. However, not all subjects who commit suicide suffer from a diagnosed psychiatric pathology. A recent literature review (De Simone et al., 2022) shows a paucity of experimental work on BDNF and GDNF. It shows that the altered regulation of BDNF, such as the Val66Met polymorphism, can favor the onset of psychiatric disorders linked to stress (Felmingham et al., 2013; Zhao et al., 2018; Miao et al., 2020), leading to an increase in suicidal risk according to some works (Zai et al., 2012; Paska et al., 2013), or no correlation with suicide, according to another study (Ratta-apha et al., 2013). Some of the studies examining the level of BDNF in post-mortem brain samples showed reduced values compared to controls, regardless of psychiatric diagnosis, in the prefrontal cortex and hippocampus (Dwivedi et al., 2003; Karege et al., 2005b; Ducray et al., 2006; Hochstrasser et al., 2011), as well as in the amygdala (Ray et al., 2014) and the anterior cingulate cortex (Tripp et al., 2012) of subjects suffering from major depressive disorder. Schneider et al. (2015) also evaluated subjects suffering from depression, detecting increased methylation of the BDNF promoter in the frontal cortex, in agreement with other studies (Keller et al., 2010; Kang et al., 2013). Another interesting finding, supported by further evidence (Pan et al., 1998; Klein et al., 2011), is that the brain level of BDNF was higher in suicidal subjects suffering from major depression on pharmacological therapy compared to non-suicidal controls (Gadad et al., 2021). As regards GDNF, in the study by Michel et al. (Michel et al., 2007), it was not possible to demonstrate a statistically significant increase in the levels of this neurotrophin in different brain areas of depressed patients taking antidepressant drug therapy.
The results of this systematic review partially support the hypothesis that a lower level of neurotrophins is connected to an increased risk of suicide. The identification of a suicide biomarker remains a challenge for the scientific and forensic community. The objective of the present study is to analyze the correlation between suicide and the degree of expression of BDNF and GDNF on autopsy samples in specific brain areas. These neurotrophic factors could have an important role both in the prevention of suicidal events in the population at high risk for anti-conservative behavior, allowing early action to limit this risk preventively, and in the search for a new potential pharmacological target. In the literature, there are no valuable markers for the identification of suicidal risk, so BDNF and GDNF could be promising for identifying suicidal risk in people with well-defined risk factors.
4 Discussion
There are few studies in the literature relating to the measure of BDNF and GDNF on post-mortem samples of suicidal subjects, especially those without psychiatric pathologies. Our work aims to contribute to science to identify molecules useful as markers of suicide risk. These neurotrophins could also be potential pharmacological targets for researching and developing new targeted therapies.
The sample examined in our study was composed of twenty subjects who died by suicide and ten controls who died of non-suicidal causes, matched for sex and age, who underwent autopsy examination. 90% of subjects were male (18 of 20), 10% were female (2 of 20), 13 of them (65%) were under 50 years old, and 7 (35%) were over 50 years old. Furthermore, 6 out of 20 individuals (30%) did not have any diagnosis of psychiatric disorder. However, they had been involved in recent stressful events, for example, bereavement, job loss, and imprisonment. The remaining 14 cases (70%), however, presented a psychiatric pathology; of these, six were affected by mood disorders (five from major depressive disorder, one from bipolar disorder), eight from other types of diagnosed psychiatric pathology (pathological gambling, substance use disorder, problematic use of alcohol, autism spectrum disorder, self-harm, adjustment disorder). Five of the 20 cases analyzed (25%) were subjects subjected to a prison regime, another risk factor.
To exclude the possible influence of psychopharmacological active ingredients on the expression of BDNF and GDNF, the authors excluded patients on therapy with antidepressant and antipsychotic drugs and those who tested positive for narcotic and psychoactive substances in the toxicological analyses. Therefore, subjects who committed suicide due to overdose were also excluded. Only three of the 20 subjects studied were taking mild therapy based on non-neuroleptic or antidepressant drugs (mainly benzodiazepines) whose active ingredients, based on the results of toxicological analyses, were still within the therapeutic dosage range.
11 out of 20 subjects (55%) died from hanging [the most used method, according to literature data (Baldari et al., 2021)], 4 out of 20 (20%) from precipitation, 2 out of 20 (10%) from stab wounds, 2 in 20 (10%) by drowning and 1 in 20 (5%) by gunshot wounds.
The results of our work, which used an immunohistochemical method widely used in the field of forensic pathology (Baldari et al., 2021), highlighted a statistically significant reduction in BDNF levels in the various brain regions examined in all twenty suicidal subjects compared to the ten controls. In particular, the areas most affected are the prefrontal cortex and cingulate gyrus (p < 0.0001), to a slightly lesser extent, hippocampus and basal nuclei (p < 0.001). These data agree with the literature (Middlemas et al., 1991; Karege et al., 2002; Dwivedi et al., 2003; Karege et al., 2005b; Tripp et al., 2012), which reports significant decreases in BDNF levels in the prefrontal cortex, hippocampus, and cingulate gyrus. Furthermore, it is interesting to consider that the global examination of the different brain areas highlighted a statistically significant reduction in BDNF in suicidal subjects compared to controls (p < 0.0001).
The encephalic expression of GDNF was increased in all 20 study subjects. In particular, the authors observed a statistically significant (p < 0.0001) increase in GDNF in suicidal subjects compared to controls at the prefrontal cortex, hippocampus, and cingulate gyrus. In the basal nuclei of suicidal subjects, however, there was a slight increase in GDNF levels compared to controls, which was not statistically significant. Interestingly, the global examination of the different brain areas highlighted an increase in GDNF in suicidal subjects, compared to controls, which was statistically significant (p < 0.0001).
Our research is unique, as we analyzed the expression of GDNF on the brain tissue of suicidal subjects without therapy. The few studies in the literature on GDNF have evaluated its concentration in the peripheral blood of patients with depressive or, more generally, mood disorders.
The results obtained agree with the data of another work (Rosa et al., 2006), which documented an increase in GDNF in the peripheral blood of patients who have bipolar disorder in the depressive phase. Thus, increased BDNF synthesis could be a characteristic of acute episodes of mood disorders, although the literature is unclear (Takebayashi et al., 2006; Otsuki et al., 2008).
An important point of the study is that we have only selected autoptic cases of subjects who have not undergone drug therapy. In literature, most of the studies concern subjects on antidepressant drug therapy, finding a reduction in the expression of GDNF in plasma or blood (Maheu et al., 2015). On the contrary, in our study, the GDNF molecule in the brain is increased. It suggests that central GDNF signaling may represent a novel target for antidepressant treatment (Maheu et al., 2015).
In our study, we evaluate neurotrophic expression at the brain tissue, while other studies evaluate it at the plasma peripheral level (Sun et al., 2019). It would be interesting to conduct a multidisciplinary clinic-based assessment to observe whether the reduction in GDNF correlates with an improvement in symptoms.
The authors analyzed three further subgroups of the 20 cases: six subjects suffering from psychiatric pathology, eight subjects suffering from mood disorders, and six normal subjects. This analysis revealed a statistically significant reduction (p < 0.0001) in the expression of BDNF in all psychiatric subjects compared to the control group in all encephalic areas. In contrast, no statistically significant variations appeared in comparison between depressed individuals and those with other mental disorders. The same result was obtained with the GNDF, with a statistically significant increase (p < 0.0001) in all brain areas of psychiatric subjects compared to the control group. In contrast, there were no statistically significant variations in the comparison between depressed individuals and those with other psychic disorders.