Although monoaminergic-targeted drugs have prompted great advances in the development of treatments for depression, the need for new options persists, since these drugs still have a delayed clinical effect and most patients do not respond properly to them. Recently, the observation of the antidepressant effects of ketamine brought on a new wave of studies regarding the comprehension of the neurobiology of depression and the development of new and more effective antidepressant drugs.

Thus, in this paper, we present a historical review of the development of monoaminergic antidepressant drugs and the role of ketamine as the introductory agent of a new era in the research of the neurobiology of depression.

Firstly, we review how the pharmacological treatment for major depression started, and we point out the main drugs discovered, the researchers involved, and how the studies developed have contributed to the understanding of the neurobiology of depression. Secondly, the major problems regarding the clinical efficacy and acceptance of these drugs are discussed, and the introduction of the glutamatergic system as a target for antidepressant drugs is presented. Finally, we review how ketamine revealed itself as an exciting option towards obtaining pharmacological agents to treat depression, through the understanding of biological markers.

Ketamine contributed to confirm that different targets of the glutamatergic system and neurotrophic pathways are strictly related to the neurobiology of depression. There are several antidepressant drugs based on ketamine’s mechanism of action already in the pipeline, and glutamatergic-targeted antidepressants may be on the market in the near future.

Lack of good animal models for affective disorders, including major depression and bipolar disorder, is noted as a major bottleneck in attempts to study these disorders and develop better treatments. We suggest that an important approach that can help in the development and use of better models is attention to variability between model animals.

Differences between mice strains were studied for some decades now, and sex differences get more attention than in the past. It is suggested that one factor that is mostly neglected, individual variability within groups, should get much more attention. The importance of individual differences in behavioral biology and ecology was repeatedly mentioned but its application to models of affective illness or to the study of drug response was not heavily studied. The standard approach is to overcome variability by standardization and by increasing the number of animals per group.

Possibly, the individuality of specific animals and their unique responses to a variety of stimuli and drugs, can be helpful in deciphering the underlying biology of affective behaviors as well as offer better prediction of drug responses in patients.

This study aimed to explore effects of adjunctive treatment with N-acetyl cysteine (NAC) on markers of inflammation and neurogenesis in bipolar depression.

This is a secondary analysis of a placebo-controlled randomised trial. Serum samples were collected at baseline, week 8, and week 32 of the open-label and maintenance phases of the clinical trial to determine changes in interleukin (IL)-6, IL-8, IL-10, tumour necrosis factor-α (TNF-α), C-reactive protein (CRP) and brain-derived neurotrophic factor (BDNF) following adjunctive NAC treatment, and to explore mediation and moderator effects of the listed markers.

Levels of brain-derived neurotrophic factor (BDNF), tumour necrosis factor-α (TNF-α), C-reactive protein (CRP), interleukins (IL) -6, 8, or 10 were not significantly changed during the course of the trial or specifically in the open-label and maintenance phases. There were no mediation or moderation effects of the biological factors on the clinical parameters.

The results suggest that these particular biological parameters may not be directly involved in the therapeutic mechanism of action of adjunctive NAC in bipolar depression.

Erythropoietin (EPO) has been suggested to improve metabolism and also cognition, but human studies are scarce. This randomised controlled trial aimed to investigate whether EPO treatment influences body composition and fat and glycated haemoglobin (HbA1c) and fasting glucose, and whether these changes would be associated with previous observed cognitive benefits of EPO.

In total, 84 non-obese patients with treatment-resistant unipolar depression or bipolar disorder in remission were randomised to 8 weekly EPO (40,000 IU) or saline (NaCl 0.9%) infusions in a double-blind, parallel-group design. Patients underwent dual X-ray absorptiometry scans at baseline and week 14 (6 weeks after treatment completion). Cognitive measures were assessed and fasting levels of cholesterol, lipoprotein fractions, triacylglycerides, glucose and HbA1c were obtained at baseline, week 9 and follow-up week 14.

In total, 79 patients had complete pre- and post-treatment data (EPO: N=40, saline: N=39). EPO had no cumulative effect on body composition and markers of fat metabolism. The EPO-treated group exhibited significantly lower HbA1c levels after 8 weeks treatment [F(1, 80)=8.51, p=0.005], however, 6 weeks after treatment termination a significantly higher fasting glucose levels [F(1, 79)=5.85, p=0.02] and HbA1c levels [F(1, 79)=5.85, p=0.02] were seen. The latter increase in HbA1c was further significantly correlated with a better cognitive outcome on verbal memory (r=0.25, p=0.03).

Repeated EPO infusions had no cumulative effect on body composition in this cohort of patients with affective disorders, however, EPO modulated HbA1c and fasting glucose and this was associated with patients’ improvement of verbal memory.

Our recent single-photon emission computed tomography (SPECT) study of patients with late-onset Alzheimer’s disease (AD) revealed that regional cerebral blood flow (rCBF) was reduced in the frontal, temporal, and limbic lobes, and to a lesser degree in the parietal and occipital lobes. Moreover, these patients’ scores on the Alzheimer’s Disease Assessment Scale-cognitive subscale (ADAS-cog) were significantly correlated with rCBF in some gyri of the frontal, parietal, and limbic lobes. Our present study aimed to understand how vascular factors and metabolic disease influenced the relationship between rCBF and ADAS-cog scores.

We divided late-onset AD patients into two groups according to their Hachinski Ischemic Score (HIS), low vascular risk patients had values of ≤4 (n=25) and high vascular risk patients had scores ≥5 (n=15). We examined rCBF using brain perfusion SPECT data.

The degrees and patterns of reduced rCBF were largely similar between late-onset AD patients in both groups, regardless of HIS values. Cognitive function was significantly associated with rCBF among late-onset AD patients with low vascular risk (HIS≤4), but not among those with high vascular risk (HIS≥5). Furthermore, metabolic diseases, such as hypertension and diabetes mellitus, disrupted the relationships between hypoperfusion and cognitive impairments in late-onset AD patients.

Factors other than hypoperfusion, such as hypertension and diabetes mellitus, could be involved in the cognitive dysfunction of late-onset AD patients with high vascular risk.

To explore the correlations between observer ratings and instrumental parameters across domains of psychomotor functioning in depression.

In total, 73 patients with major depressive disorder underwent extensive psychomotor and clinical testing. Psychomotor functioning was assessed with (i) an observer-rated scale (the CORE measure) and also objectively with (ii) 24-h actigraphy, and (iii) a fine motor drawing task.

Observer ratings of retardation correlated with instrumental assessments of fine and gross motor functioning. In contrast, observer ratings of agitation did not correlate with observer ratings of retardation or with the instrumental measures. These associations were partly influenced by age and, to a lesser extent, by depression severity.

Psychomotor disturbance is a complex concept with different manifestations in depressed patients. Although observer ratings of retardation correspond well with instrumental measures of the motor domains, objective measurement of agitation and other aspects of psychomotor disturbance require further research.