Brain-derived neurotrophic factor (BDNF) is critical for neuroplasticity, synaptic transmission, and neuronal survival. Studies have implicated it in the pathophysiology of depression, as its expression is significantly reduced in brain areas such as the prefrontal cortex and hippocampus in patients with depression. Our narrative review focuses on the relationship between BDNF, ketamine, and esketamine, specifically by summarizing human studies investigating BDNF variations in patients treated with these two drugs. BDNF plays a pivotal role in neuroplasticity and neurotrophic mechanisms that can be enhanced by traditional antidepressants, which have been shown to increase BDNF levels both peripherally and in targeted brain regions. Ketamine and its S-enantiomer, esketamine, exert both rapid and sustained antidepressant effects through activation of glutamate-related pathways, with neurotrophic effects involving BDNF, as demonstrated in experimental studies. However, clinical findings have shown mixed results, with most indicating an increase in plasma BDNF in patients treated with intravenous ketamine, although some studies contradict these findings. In addition to this, there are few studies of BDNF and esketamine. Currently, the limited number of studies suggests the need for further research, including larger sample sizes and investigations of BDNF and intranasal esketamine, which has been approved by several regulatory agencies for the treatment of treatment-resistant depression.

Over the past two decades, immune–inflammatory dysregulation has emerged as a central paradigm in the biology of mood disorders. Patients with major depression (MDD) and bipolar disorder (BD) frequently display low-grade systemic inflammation. Elevated C-reactive protein (CRP), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) identify clinically relevant subgroups of patients characterized by greater severity, cognitive impairment, and poor treatment response. Changes in the gut microbiota and disruptions of the blood–brain barrier (BBB) act as important gateways through which systemic immune activity can influence the brain. At the intracellular level, pattern-recognition receptors activate convergent hubs including NF-κB, JAK/STAT, and MAPK cascades, while the NLRP3 inflammasome integrates mitochondrial dysfunction and oxidative stress with IL-1β release and pyroptosis. These pathways converge on glial dysregulation, impaired BDNF/TrkB signaling, and kynurenine pathway (KP) alterations, fostering excitotoxicity and synaptic deficits. Translational studies demonstrate that elevated CRP and IL-6 predict poor antidepressant outcomes. Anti-inflammatory agents such as infliximab and celecoxib show efficacy in specific subgroups of patients. Emerging multi-omics approaches identify immuno-metabolic biotypes, supporting the rationale for biomarker-guided stratification. These findings define an ‘inflammatory biotype’ of mood disorders and highlight the need for biomarkers and precision-based trials to guide treatment.

Schizophrenia (SCZ) and obsessive–compulsive disorder (OCD) typically have distinct diagnostic criteria and treatment approaches. SCZ is characterized by delusions, hallucinations, disorganized speech, and cognitive impairments, while OCD involves persistent, intrusive thoughts (obsessions) and repetitive behaviors (compulsions). The co-occurrence of these disorders increases clinical complexity and poses significant challenges for diagnosis and treatment. Epidemiological studies indicate a significant overlap, with prevalence rates of comorbid OCD in SCZ patients ranging from 12% to 25%, which is higher than in the general population. Etiological hypotheses suggest shared genetic, neurobiological, and environmental factors, with genetic studies identifying common loci and pathways, such as glutamatergic and dopaminergic systems. Neuroimaging studies reveal both overlapping and distinct neural abnormalities, indicating shared and unique neurobiological substrates. Environmental factors, like early life stressors and urbanicity, also contribute to the comorbidity. The overlapping clinical features of both disorders complicate diagnosis. Treatment approaches include combining SSRIs with antipsychotics and cognitive behavioral therapy (CBT). The complexity of SCZ and OCD comorbidity underscores the need for a dimensional, spectrum-based perspective on psychiatric disorders, alongside traditional categorical approaches, to improve diagnosis and treatment outcomes.

Treating depression in adolescents is a significant challenge, and major depressive disorder (MDD) with suicidal ideation and treatment-resistant depression (TRD) are common and potentially devastating to optimal psychological and physical development in this age group. Suicide is among the leading causes of youth mortality, and TRD occurs in up to 40% of adolescents with MDD. TRD involves severe, persistent symptoms that are hard to treat, significantly reducing functioning and quality of life. We conducted a literature search focusing on key terms related to ketamine and esketamine for MDD with suicidal ideation and TRD in adolescents, aiming to review the potential utility of these molecules in adolescents for these conditions. Ketamine has shown efficacy in reducing depressive symptoms in adolescents with TRD. Esketamine has shown efficacy in reducing depressive symptoms and treating suicidal ideation in adolescents. Both ketamine and esketamine have demonstrated favorable safety and tolerability profiles. Using these drugs for serious conditions like adolescent MDD with suicidal thoughts and TRD can effectively treat symptoms, reduce self-harm and suicide risks, and provide a window for longer-term therapeutic interventions. The prompt and effective treatment of TRD could improve adolescents’ quality of life. However, more research is needed to optimize treatment protocols and evaluate long-term effects.

Bipolar disorder (BD) is characterized by the occurrence of manic/hypomanic and depressive episodes, with the latter having a significant impact on morbidity, mortality and overall quality of life. Current guidelines for bipolar depression provide limited treatment options, with only a few approved therapies. Despite these limitations, approximately 50–60% of individuals diagnosed with BD are prescribed antidepressants. However, the use of these medications remains controversial due to risks of manic induction, rapid cycling, and symptom destabilization. This review explores the neurotransmitter mechanisms underpinning the phases of BD, focusing on monoamines and assessing the efficacy and safety of different antidepressant medications in the treatment of bipolar depression. Norepinephrine and dopamine have been identified as neurotransmitters associated with both depressive and manic poles, with a proposed deficit in depression and an increase in mania. The evidence indicates that serotonin is deficient during depressive phases, yet its imbalance also manifests in mania. Selective serotonin reuptake inhibitors (SSRIs), which primarily increase serotonin levels, are generally safer than tricyclic antidepressants (TCAs) and show promising—though not definitive—results, especially when combined with mood stabilizers. Other newer-generation antidepressants may also have potential for the treatment of bipolar depression. The heterogeneity of mood disorders poses a significant challenge in the diagnosis of BD, which is often ambiguous and complex. The natural mood fluctuations associated with BD, in conjunction with the frequent comorbidities such as anxiety, render the treatment of this condition particularly challenging, particularly in the context of antidepressant therapy. While clinical trials are conducted with the utmost rigor, they frequently fail to account for the intricacies of the real-world context due to the strict inclusion criteria. The identification of predictors of effective antidepressant use, such as symptom severity and comorbid conditions, has the potential to enhance treatment outcomes. Future research should aim to identify individualized predictors and deepen understanding of mood disorder spectra to optimize antidepressant use in bipolar depression.

Background/Objectives: OCD is a chronic psychiatric disorder, often requiring long-term pharmacological treatment. Although selective serotonin reuptake inhibitors (SSRIs) are considered first-line agents, 40 to 60% of patients show only partial or no response when treated at standard dosages. In such cases, supratherapeutic doses of SSRIs have been proposed as an alternative strategy. However, the evidence supporting this approach remains limited and fragmented. This review aims to evaluate the rationale, clinical efficacy, tolerability, and practical considerations associated with high-dose SSRI use in OCD. Methods: A structured narrative review was conducted using targeted literature searches in PubMed and Scopus. Studies were included if they reported on the use of high-dose SSRIs (citalopram, escitalopram, fluoxetine, fluvoxamine, paroxetine, or sertraline) in patients with OCD and provided efficacy and/or tolerability data. Clinical trials, observational studies, and case reports were all reviewed. Results: Evidence shows that higher doses of SSRIs are significantly more effective than low or medium doses in reducing OCD symptoms—especially in individuals who have only partially responded to standard treatment. Smaller clinical studies and case reports have also demonstrated that supratherapeutic dosing, beyond typical regulatory limits, can be both effective and well tolerated in treatment-resistant OCD. Conclusions: High-dose SSRI treatment may be a valuable option for selected OCD patients who do not respond to standard therapy. However, careful patient selection, regular monitoring, and further controlled studies are necessary to better define its long-term safety and effectiveness. In this context, increasingly advanced technologies—such as therapeutic drug monitoring and pharmacogenetic testing for relevant polymorphisms—may support more individualized and safer treatment strategies.