Is (R)-ketamine a potential therapeutic agent for treatment-resistant depression with less detrimental side effects? A review of molecular mechanisms underlying ketamine and its enantiomers.

Approximately one-third of individuals with major depressive disorder are resistant to conventional antidepressants (i.e., monoamine-based therapies), and, even among respondents, a proper therapeutic effect may require weeks of treatment. Ketamine, a racemic mixture of the two enantiomers, (R)-ketamine and (S)-ketamine, is an N-methyl-d-aspartate receptor (NMDAR) antagonist and has been shown to have rapid-acting antidepressant properties in patients with treatment-resistant depression (TRD). Although (R)-ketamine has a lower affinity for NMDAR, it presents greater potency and longer-lasting antidepressant properties, with no major side effects, than racemic ketamine or (S)-ketamine in preclinical findings. Thereby, ketamine and its enantiomers have not only an antagonistic effect on NMDAR but also a strong synaptogenic-modulatory effect, which is impaired in TRD pathophysiology. In this review, we summarize the current evidence regarding the modulation of neurotransmission, neuroplasticity, and neural network activity as putative mechanisms of these rapid-acting antidepressants, highlighting differences on intracellular signaling pathways of synaptic proteins such as mammalian target of rapamycin (mTOR), extracellular signal-regulated kinase (ERK) and brain-derived neurotrophic factor (BDNF). In addition, we discuss probable mechanisms involved in the side effects of ketamine and its enantiomers.

Agmatine as a potential therapeutic intervention in bipolar depression: the preclinical landscape.

INTRODUCTION: Present antidepressant treatments are only helpful in a quarter of patients with bipolar depression, and new strategies are warranted. Increasing evidence suggests that accelerated polyamine metabolism is associated with the pathophysiology of depression. Polyamines regulate stress responses, inflammation, and neuronal signaling in the central and enteric nervous system. Agmatine is a promising target of altered polyamine metabolism considering its unique ability to regulate intracellular polyamine content and neuroprotective effects. Areas covered: This review discusses the polyamine system and its relationship to the central and enteric nervous system, focusing on results from preclinical studies supporting the relationship between agmatine and the pathophysiology of depression. We also discussed the main mechanisms underlying the antidepressant and neuroprotective effects of agmatine. Expert opinion: Our review points out the possible relationship between polyamines and the pathophysiology of depression. It discusses the efficacy of agmatine in several models of depressive-like behaviour, and suggests that it may prove to be an efficacious adjunctive treatment in bipolar depression. Furthermore, it discusses a proposed pathway linking systemic inflammation, observed in a subset of bipolar disorder patients, to abnormal polyamine metabolism and associated changes in the epithelial gut barrier and blood-brain barrier.