Current perspectives on the antidepressant-like effects of guanosine

Augmentation effect of ketamine by guanosine in the novelty-suppressed feeding test is dependent on mTOR signaling pathway

The ketamine's potential for the treatment of refractory depression and anxiety has been considered one the most important discoveries in the last years, however, repeated use of ketamine is limited due to its side/adverse effects. Therefore, the search for effective augmentation strategies that may reduce ketamine doses is welcome. Therefore, this study sought to augment the effect of ketamine by guanosine in the novelty-suppressed feeding (NSF) test, a behavioral paradigm able to detect depression/anxiety-related behavior. Acute administration of guanosine (0.05 mg/kg, p.o.), similar to ketamine (1 mg/kg, i.p.), produced a rapid behavioral response in mice submitted to NSF test. Moreover, the coadministration of sub-effective doses of guanosine (0.01 mg/kg, p.o.) and ketamine (0.1 mg/kg, i.p.) was effective in mice submitted to NSF test. Subsequently, the intracellular mechanism underpinning the augmentation effect of ketamine by guanosine was investigated. Our results suggest that augmentation response of ketamine by guanosine in the NSF test probably involves the activation of mTOR signaling, since the treatment with rapamycin (0.2 nmol/site, i.c.v., a selective mTOR inhibitor) completely abolished this effect. This augmentation strategy also increased mTOR phosphorylation (Ser²⁴⁴⁸) in the hippocampus, reinforcing the role of mTOR in this augmentation response. However, no changes in the p70S6K, PSD-95, GluA1, and synapsin immunocontents were found in the hippocampus of ketamine plus guanosine-treated mice. Overall, results provide evidence that guanosine is able to augment the effect of ketamine in the NSF test via mTOR activation, a finding that might have therapeutic implications for the management of depression/anxiety.

Novel Targets for Fast Antidepressant Responses: Possible Role of Endogenous Neuromodulators

The available medications for the treatment of major depressive disorder have limitations, particularly their limited efficacy, delayed therapeutic effects, and the side effects associated with treatment. These issues highlight the need for better therapeutic agents that provide more efficacious and faster effects for the management of this disorder. Ketamine, an N-methyl-D-aspartate receptor antagonist, is the prototype for novel glutamate-based antidepressants that has been shown to cause a rapid and sustained antidepressant effect even in severe refractory depressive patients. Considering the importance of these findings, several studies have been conducted to elucidate the molecular targets for ketamine's effect. In addition, efforts are under way to characterize ketamine-like drugs. This review focuses particularly on evidence that endogenous glutamatergic neuromodulators may be able to modulate mood and to elicit fast antidepressant responses. Among these molecules, agmatine and creatine stand out as those with more published evidence of similarities with ketamine, but guanosine and ascorbic acid have also provided promising results. The possibility that these neuromodulators and ketamine have common neurobiological mechanisms, mainly the ability to activate mechanistic target of rapamycin and brain-derived neurotrophic factor signaling, and synthesis of synaptic proteins in the prefrontal cortex and/or hippocampus is presented and discussed.

Neuromodulatory Effects of Guanine-Based Purines in Health and Disease

The function of guanine-based purines (GBPs) is mostly attributed to the intracellular modulation of heteromeric and monomeric G proteins. However, extracellular effects of guanine derivatives have also been recognized. Thus, in the central nervous system (CNS), a guanine-based purinergic system that exerts neuromodulator effects, has been postulated. The thesis that GBPs are neuromodulators emerged from in vivo and in vitro studies, in which neurotrophic and neuroprotective effects of these kinds of molecules (i.e., guanosine) were demonstrated. GBPs induce several important biological effects in rodent models and have been shown to reduce seizures and pain, stabilize mood disorder behavior and protect against gliomas and diseases related with aging, such as ischemia or Parkinson and Alzheimer diseases. In vitro studies to evaluate the protective and trophic effects of guanosine, and of the nitrogenous base guanine, have been fundamental for understanding the mechanisms of action of GBPs, as well as the signaling pathways involved in their biological roles. Conversely, although selective binding sites for guanosine have been identified in the rat brain, GBP receptors have not been still described. In addition, GBP neuromodulation may depend on the capacity of GBPs to interact with well-known membrane proteins in glutamatergic and adenosinergic systems. Overall, in this review article, we present up-to-date GBP biology, focusing mainly on the mechanisms of action that may lead to the neuromodulator role of GBPs observed in neurological disorders.