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

Rapid Antidepressant-Like Potential of Chaihu Shugan San Depends on Suppressing Glutamate Neurotransmission and Activating Synaptic Proteins in Hippocampus of Female Mice

OBJECTIVE

To explore the rapid antidepressant potential and the underlying mechanism of Chaihu Shugan San (CSS) in female mice.

METHODS

Liquid chromatography mass spectrometry (LC-MS)/MS was used to determine the content of main components in CSS to determine its stability. Female C57BL/6J mice were randomly divided into 4 groups, including control (saline), vehicle (saline), CSS (4 g/kg) and ketamine (30 mg/kg) groups. Mice were subjected to irregular stress stimulation for 4 weeks to establish the chronic mild stress (CMS) model, then received a single administration of drugs. Two hours later, the behavioral tests were performed, including open field test, tail suspension test (TST), forced swimming test (FST), novelty suppression feeding test (NSF), and sucrose preference test (SPT). Western blot analysis was used to detect the expression levels of N-methyl-D-aspartate receptor (NMDA) subtypes [N-methyl-D-aspartate receptor 1 (NR1), NR2A, NR2B], synaptic proteins [synapsin1 and post synaptic density protein 95 (PSD95)], and brain-derived neurotrophic factor (BDNF). Moreover, the rapid antidepressant effect of CSS was tested by pharmacological technologies and optogenetic interventions that activated glutamate receptors, NMDA.

RESULTS

Compared with the vehicle group, a single administration of CSS (4 g/kg) reversed all behavioral defects in TST, FST, SPT and NSF caused by CMS (P<0.05 or P<0.01). CSS also significantly decreased the expressions of NMDA subtypes (NR1, NR2A, NR2B) at 2 h in hippocampus of mice (all P<0.01). In addition, similar to ketamine, CSS increased levels of synaptic proteins and BDNF (P<0.05 or P<0.01). Furthermore, the rapid antidepressant effects of CSS were blocked by transient activation of NMDA receptors in the hippocampus (all P<0.01).

CONCLUSION

Rapid antidepressant effects of CSS by improving behavioral deficits in female CMS mice depended on rapid suppression of NMDA receptors and activation of synaptic proteins.

Hippocampal PACAP signaling activation triggers a rapid antidepressant response

BACKGROUND

The development of ketamine-like rapid antidepressants holds promise for enhancing the therapeutic efficacy of depression, but the underlying cellular and molecular mechanisms remain unclear. Implicated in depression regulation, the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is investigated here to examine its role in mediating the rapid antidepressant response.

METHODS

The onset of antidepressant response was assessed through depression-related behavioral paradigms. The signaling mechanism of PACAP in the hippocampal dentate gyrus (DG) was evaluated by utilizing site-directed gene knockdown, pharmacological interventions, or optogenetic manipulations. Overall, 446 mice were used for behavioral and molecular signaling testing. Mice were divided into control or experimental groups randomly in each experiment, and the experimental manipulations included: chronic paroxetine treatments (4, 9, 14 d) or a single treatment of ketamine; social defeat or lipopolysaccharides-injection induced depression models; different doses of PACAP (0.4, 2, 4 ng/site; microinjected into the hippocampal DG); pharmacological intra-DG interventions (CALM and PACAP6-38); intra-DG viral-mediated PACAP RNAi; and opotogenetics using channelrhodopsins 2 (ChR2) or endoplasmic natronomonas halorhodopsine 3.0 (eNpHR3.0). Behavioral paradigms included novelty suppressed feeding test, tail suspension test, forced swimming test, and sucrose preference test. Western blotting, ELISA, or quantitative real-time PCR (RT-PCR) analysis were used to detect the expressions of proteins/peptides or genes in the hippocampus.

RESULTS

Chronic administration of the slow-onset antidepressant paroxetine resulted in an increase in hippocampal PACAP expression, and intra-DG blockade of PACAP attenuated the onset of the antidepressant response. The levels of hippocampal PACAP expression were reduced in both two distinct depression animal models and intra-DG knockdown of PACAP induced depression-like behaviors. Conversely, a single infusion of PACAP into the DG region produced a rapid and sustained antidepressant response in both normal and chronically stressed mice. Optogenetic intra-DG excitation of PACAP-expressing neurons instantly elicited antidepressant responses, while optogenetic inhibition induced depression-like behaviors. The longer optogenetic excitation/inhibition elicited the more sustained antidepressant/depression-like responses. Intra-DG PACAP infusion immediately facilitated the signaling for rapid antidepressant response by inhibiting calcium/calmodulin-dependent protein kinase II (CaMKII)-eukaryotic elongation factor 2 (eEF2) and activating the mammalian target of rapamycin (mTOR). Pre-activation of CaMKII signaling within the DG blunted PACAP-induced rapid antidepressant response as well as eEF2-mTOR-brain-derived neurotrophic factor (BDNF) signaling. Finally, acute ketamine treatment upregulated hippocampal PACAP expression, whereas intra-DG blockade of PACAP signaling attenuated ketamine's rapid antidepressant response.

CONCLUSIONS

Activation of hippocampal PACAP signaling induces a rapid antidepressant response through the regulation of CaMKII inhibition-governed eEF2-mTOR-BDNF signaling.

Jiawei-Xiaoyao pill elicits a rapid antidepressant effect, dependent on activating CaMKII/mTOR/BDNF signaling pathway in the hippocampus

ETHNOPHARMACOLOGICAL RELEVANCE

Jiawei-Xiaoyao pill (JWX), a traditional Chinese medicine, was recorded in ancient Chinese medicine pharmacopoeia using for treatment of various diseases, including mood disorders. Current mainstream antidepressants have a disadvantage in delayed onset of action. The rapid antidepressant potential of JWX and the underlying mechanisms remain unclear.

AIM OF THE STUDY

We aimed to assess the rapid antidepressant potential of JWX, within the prescription dose range, and the distinct underlying neuroplasticity signaling mechanism.

MATERIALS AND METHODS

The rapid antidepressant response of JWX were determined using various behavioral paradigms, and in a corticosterone (CORT)-induced depression model in mice. The molecular neuroplasticity signaling and the expression of BDNF in the hippocampus was evaluated using immunoblotting and immunostaining. The contribution of specific signaling was investigated using pharmacological interventions.

RESULTS

A single dose of JWX induced rapid and persistent antidepressant effects in both the normal and chronic CORT-exposed mice. The phosphorylation of CaMKII, mTOR, ERK and the expressions of BDNF, synapsin1 and PSD95 increased at 30 min post JWX. JWX restored the expression of BDNF in the hippocampal dentate gyrus reduced by CORT-exposure. The rapid antidepressant effect and upregulation of BDNF expression by JWX was blunted by a mTOR antagonist, rapamycin, or a CaMKII antagonist, KN-93. CaMKII signaling blockade blunted mTOR signaling activated by JWX, but not vice versa.

CONCLUSION

JWX elicits a rapid antidepressant effect, via quickly stimulating CaMKII signaling, subsequently activating mTOR-BDNF signaling pathway, and thus enhancing hippocampal neuroplasticity.

Chronic Pb Exposure Induces Anxiety and Depression-like Behaviors in Mice via Excitatory Neuronal Hyperexcitability in Ventral Hippocampal Dentate Gyrus

Lead (Pb) is a widespread neurotoxic pollutant. Pb exposure is associated with mood disorders, with no well-established neural mechanisms elucidated. In the present study, we aimed to investigate whether excitatory neurons in the dentate gyrus subregion of the ventral hippocampus (vDG) played a key role in Pb-induced anxiety and depression-like behaviors. C57BL/6 mice were exposed to 100 ppm Pb starting on day 1 of pregnancy until experiments were performed using the offspring. Behavioral studies suggested that chronic Pb exposure triggered anxiety and depression-like behaviors. A combination of electrophysiological, optogenetic, and immunohistochemistry experiments was conducted. Results showed that Pb exposure resulted in excitatory neuronal hyperexcitability in vDG and that the behavioral deficits caused by Pb exposure could be rescued by inhibition of excitatory neuronal activity. Moreover, it was found that the action potential (AP) threshold of excitatory neurons was decreased by electrophysiological recordings. Our study demonstrates a significant role for excitatory neurons in vDG in Pb-induced anxiety and depression-like behaviors in mice, which is likely a result of decreased AP threshold. These outcomes can serve as an important basis for understanding mechanisms of anxiety and depression under environmental Pb exposure and help in the design of therapeutic strategies.

Changes in synaptic markers after administration of ketamine or psychedelics: a systematic scoping review

Background

Ketamine and psychedelics have abuse liability. They can also induce "transformative experiences" where individuals experience enhanced states of awareness. This enhanced awareness can lead to changes in preexisting behavioral patterns which could be beneficial in the treatment of substance use disorders (SUDs). Preclinical and clinical studies suggest that ketamine and psychedelics may alter markers associated with synaptic density, and that these changes may underlie effects such as sensitization, conditioned place preference, drug self-administration, and verbal memory performance. In this scoping review, we examined studies that measured synaptic markers in animals and humans after exposure to ketamine and/or psychedelics.

Methods

A systematic search was conducted following PRISMA guidelines, through PubMed, EBSCO, Scopus, and Web of Science, based on a published protocol (Open Science Framework, DOI: 10.17605/OSF.IO/43FQ9). Both in vivo and in vitro studies were included. Studies on the following synaptic markers were included: dendritic structural changes, PSD-95, synapsin-1, synaptophysin-1, synaptotagmin-1, and SV2A.

Results

Eighty-four studies were included in the final analyses. Seventy-one studies examined synaptic markers following ketamine treatment, nine examined psychedelics, and four examined both. Psychedelics included psilocybin/psilocin, lysergic acid diethylamide, N,N-dimethyltryptamine, 2,5-dimethoxy-4-iodoamphetamine, and ibogaine/noribogaine. Mixed findings regarding synaptic changes in the hippocampus and prefrontal cortex (PFC) have been reported when ketamine was administered in a single dose under basal conditions. Similar mixed findings were seen under basal conditions in studies that used repeated administration of ketamine. However, studies that examined animals during stressful conditions found that a single dose of ketamine counteracted stress-related reductions in synaptic markers in the hippocampus and PFC. Repeated administration of ketamine also counteracted stress effects in the hippocampus. Psychedelics generally increased synaptic markers, but results were more consistently positive for certain agents.

Conclusion

Ketamine and psychedelics can increase synaptic markers under certain conditions. Heterogeneous findings may relate to methodological differences, agents administered (or different formulations of the same agent), sex, and type of markers. Future studies could address seemingly mixed results by using meta-analytical approaches or study designs that more fully consider individual differences.

Antidepressant effects of repeated s-ketamine administration as NMDAR Antagonist: Involvement of CaMKIIα and mTOR signaling in the hippocampus of CUMS mice

With the approval of s-ketamine nasal spray as a novel antidepressant, its robust antidepressant effects have been intensively examined in clinical trials. However, the therapeutic efficacy and mechanisms of repeated intermittent drug administration remain unclear. In the present study, we applied a classic chronic unpredictable mild stress (CUMS) model to induce depressive-like behaviors of mice and evaluated the role of repeated s-ketamine administration (10 mg/kg, 7 consecutive days) in ameliorating depressive-like behaviors and modulating related molecular pathways. A battery of behavioral tests were performed to assess CUMS-induced depression. The protein expressions of GluN1, GluN2A, GluN2B, GluR1, CaMKIIα, phosphorylated CaMKIIα (p-CaMKIIα), BDNF, TrkB, phosphorylated TrkB (p-TrkB), mTOR, and phosphorylated mTOR (p-mTOR) as well as modification of synaptic ultrastructure was identified in hippocampal tissues. It turned out that s-ketamine manifested evident antidepressant effects with improved synaptic plasticity. Meanwhile, the results suggested that s-ketamine could differentially modulate glutamate receptors with upregulated GluN1 and GluR1 levels and downregulated GluN2B levels. CUMS-induced elevation of CaMKIIα phosphorylation and decline of BDNF, TrkB phosphorylation and mTOR could also be reversed through s-ketamine treatment. Together, our study provided evidence that selectively modulated glutamate receptors as well as CaMKIIα and mTOR signaling were involved in repeated s-ketamine administration.

NMDA receptor-mediated modulation on glutamine synthetase and glial glutamate transporter GLT-1 is involved in the antidepressant-like and neuroprotective effects of guanosine

Guanosine has been reported to elicit antidepressant-like responses in rodents, but if these actions are associated with its ability to afford neuroprotection against glutamate-induced toxicity still needs to be fully understood. Therefore, this study investigated the antidepressant-like and neuroprotective effects elicited by guanosine in mice and evaluated the possible involvement of NMDA receptors, glutamine synthetase, and GLT-1 in these responses. We found that guanosine (0.05 mg/kg, but not 0.01 mg/kg, p. o.) was effective in producing an antidepressant-like effect and protecting hippocampal and prefrontocortical slices against glutamate-induced damage. Our results also unveiled that ketamine (1 mg/kg, but not 0.1 mg/kg, i. p, an NMDA receptor antagonist) effectively elicited antidepressant-like actions and protected hippocampal and prefrontocortical slices against glutamatergic toxicity. Furthermore, the combined administration of sub-effective doses of guanosine (0.01 mg/kg, p. o.) with ketamine (0.1 mg/kg, i. p.) promoted an antidepressant-like effect and augmented glutamine synthetase activity and GLT-1 immunocontent in the hippocampus, but not in the prefrontal cortex. Our results also showed that the combination of sub-effective doses of ketamine and guanosine, at the same protocol schedule that exhibited an antidepressant-like effect, effectively abolished glutamate-induced damage in hippocampal and prefrontocortical slices. Our in vitro results reinforce that guanosine, ketamine, or sub-effective concentrations of guanosine plus ketamine protect against glutamate exposure by modulating glutamine synthetase activity and GLT-1 levels. Finally, molecular docking analysis suggests that guanosine might interact with NMDA receptors at the ketamine or glycine/d-serine co-agonist binding sites. These findings provide support for the premise that guanosine has antidepressant-like effects and should be further investigated for depression management.

The antidepressant-like effect of guanosine involves the modulation of adenosine A1 and A2A receptors

Guanosine has been considered a promising candidate for antidepressant responses, but if this nucleoside could modulate adenosine A₁ (A₁R) and A_2A (A_2AR) receptors to exert antidepressant-like actions remains to be elucidated. This study investigated the role of A₁R and A_2AR in the antidepressant-like response of guanosine in the mouse tail suspension test and molecular interactions between guanosine and A₁R and A₂AR by docking analysis. The acute (60 min) administration of guanosine (0.05 mg/kg, p.o.) significantly decreased the immobility time in the tail suspension test, without affecting the locomotor performance in the open-field test, suggesting an antidepressant-like effect. This behavioral response was paralleled with increased A₁R and reduced A_2AR immunocontent in the hippocampus, but not in the prefrontal cortex, of mice. Guanosine-mediated antidepressant-like effect was not altered by the pretreatment with caffeine (3 mg/kg, i.p., a non-selective adenosine A₁R/A_2AR antagonist), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX - 2 mg/kg, i.p., a selective adenosine A₁R antagonist), or 4-(2-[7-amino-2-{2-furyl}{1,2,4}triazolo-{2,3-a}{1,3,5}triazin-5-yl-amino]ethyl)-phenol (ZM241385 - 1 mg/kg, i.p., a selective adenosine A_2AR antagonist). However, the antidepressant-like response of guanosine was completely abolished by adenosine (0.5 mg/kg, i.p., a non-selective adenosine A₁R/A_2AR agonist), N-6-cyclohexyladenosine (CHA - 0.05 mg/kg, i.p., a selective adenosine A₁ receptor agonist), and N-6-[2-(3,5-dimethoxyphenyl)-2-(methylphenyl)ethyl]adenosine (DPMA - 0.1 mg/kg, i.p., a selective adenosine A_2A receptor agonist). Finally, docking analysis also indicated that guanosine might interact with A₁R and A_2AR at the adenosine binding site. Overall, this study reinforces the antidepressant-like of guanosine and unveils a previously unexplored modulation of the modulation of A₁R and A_2AR in its antidepressant-like effect.

Guanosine as a promising target for fast-acting antidepressant responses

Although the rapid-onset and sustained antidepressant responses elicited by ketamine have gained considerable attention in recent years, it has some knock-on effects that limit its widespread clinical use. Therefore, ketamine is considered the prototype for the new generation of glutamate-based rapid-acting antidepressants. Within this context, it has been demonstrated that guanosine, an endogenous guanine-based purine, has overlapping mechanisms of action with ketamine and is effective in eliciting fast antidepressant-like responses and even potentiating ketamine's actions in preclinical studies. Here, we review the recent findings regarding the ability of guanosine to produce rapid-acting antidepressant-like effects and we provide an overview of the molecular mechanisms underlying its antidepressant-like actions. Moreover, the neurobiological mechanisms underpinning the ability of guanosine in boosting the antidepressant-like and pro-synaptogenic effects elicited by ketamine are also reported. Taken together, this review opens perspectives for the use of guanosine alone or in combination with ketamine for the management of treatment-resistant depression.

Chronic stress disrupts the homeostasis and progeny progression of oligodendroglial lineage cells, associating immune oligodendrocytes with prefrontal cortex hypomyelination

Major depressive disorder (MDD) is a chronic debilitating illness affecting yearly 300 million people worldwide. Oligodendrocyte-lineage cells have emerged as important neuromodulators in synaptic plasticity and crucial components of MDD pathophysiology. Using the repeated social defeat (RSDS) mouse model, we demonstrate that chronic psychosocial stress induces long-lasting losses and transient proliferation of oligodendrocyte-precursor cells (OPCs), aberrant differentiation into oligodendrocytes, and severe hypomyelination in the prefrontal cortex. Exposure to chronic stress results in OPC morphological impairments, excessive oxidative stress, and oligodendroglial apoptosis, implicating integrative-stress responses in depression. Analysis of single-nucleus transcriptomic data from MDD patients revealed oligodendroglial-lineage dysregulation and the presence of immune-oligodendrocytes (Im-OL), a novel population of cells with immune properties and myelination deficits. Im-OL were also identified in mice after RSDS, where oligodendrocyte-lineage cells expressed immune-related markers. Our findings demonstrate cellular and molecular changes in the oligodendroglial lineage in response to chronic stress and associate hypomyelination with Im-OL emergence during depression.

The role of serotonin neurotransmission in rapid antidepressant actions

RATIONALE

Ketamine has rapid antidepressant effects that represent a significant advance in treating depression, but its poor safety and tolerability limit its clinical utility. Accreting evidence suggests that serotonergic neurotransmission participates in the rapid antidepressant effects of ketamine and hallucinogens. Thus, understanding how serotonin contributes to these effects may allow identification of novel rapid antidepressant mechanisms with improved tolerability.

OBJECTIVE

The goal of this paper is to understand how serotonergic mechanisms participate in rapid antidepressant mechanisms.

METHODS

We review the relevance of serotonergic neurotransmission for rapid antidepressant effects and evaluate the role of 5-HT_1A, 5-HT_1B, 5-HT_2A, and 5-HT₄ receptors in synaptic plasticity, BDNF signaling, and GSK-3β activity. Subsequently, we develop hypotheses on the relationship of these receptor systems to rapid antidepressant effects.

RESULTS

We found that 5-HT_1A and 5-HT_1B receptors may participate in ketamine's rapid antidepressant mechanisms, while agonists at 5-HT_2A and 5-HT₄ receptors may independently behave as rapid antidepressants. 5-HT_1A, 5-HT_2A, and 5-HT₄ receptors increase synaptic plasticity in the cortex or hippocampus but do not consistently increase BDNF signaling. We found that 5-HT_1A and 5-HT_1B receptors may participate in rapid antidepressant mechanisms as a consequence of increased BDNF signaling, rather than a cause. 5-HT_2A and 5-HT₄ receptor agonists may increase BDNF signaling, but these relationships are tenuous and need more study. Finally, we found that ketamine and several serotonergic receptor systems may mechanistically converge on reduced GSK-3β activity.

CONCLUSIONS

We find it plausible that serotonergic neurotransmission participates in rapid antidepressant mechanisms by increasing synaptic plasticity, perhaps through GSK-3β inhibition.

Prophylactic efficacy of ketamine, but not the low-trapping NMDA receptor antagonist AZD6765, against stress-induced maladaptive behavior and 4E-BP1-related synaptic protein synthesis impairment

Ketamine enhances the resilience against stress-induced depressive-like behavior, but its prophylactic efficacy in anxiety-related behaviors remains to be elucidated. Moreover, there is a need for developing novel preventive strategies against depressive- and anxiety-like behavior. AZD6765, a low-trapping NMDA receptor antagonist, shares with ketamine common molecular targets and produces rapid-onset antidepressant effects, suggesting that it could be a prophylactic agent. Therefore, this study investigated the prophylactic effect of ketamine against the depressive- and anxiety-like behavior induced by chronic restraint stress (2 h/day, for 10 days) in mice. We also investigated if AZD6765 exerts a resilience-enhancing response against these maladaptive behaviors. The contribution of 4E-BP1-related synaptic proteins synthesis (PSD-95/GluA1) in the possible pro-resilience efficacy of ketamine and AZD6765 was investigated. A single administration of ketamine (5 mg/kg, i.p.), but not AZD6765 (1 or 5 mg/kg, i.p.), given 1 week before the stress protocol, was effective in preventing stress-induced depressive-like behavior in the tail suspension test and splash test. Ketamine administered at 1 and 5 mg/kg (i.p.), but not AZD6765 (1 or 5 mg/kg, i.p.), prevented stress-induced anxiety-related self-grooming alterations. Stress-induced reduction on 4E-BP1 phosphorylation and PSD-95 and GluA1 immunocontent in the prefrontal cortex was prevented by ketamine (5 mg/kg, i.p.), but not AZD6765 (1 or 5 mg/kg, i.p.). The results indicate that ketamine, but not AZD6765, exerts a pro-resilience response against stress-induced maladaptive behavior, reinforcing that it could be a prophylactic agent to manage individuals at-risk to develop MDD and anxiety.

Guanosine boosts the fast, but not sustained, antidepressant-like and pro-synaptogenic effects of ketamine by stimulating mTORC1-driven signaling pathway

The mTORC1-dependent dendritic spines formation represents a key mechanism for fast and long-lasting antidepressant responses, but it remains to be determined whether this mechanism may account for the ability of guanosine in potentiating ketamine's actions. Here, we investigated the ability of ketamine plus guanosine to elicit fast and sustained antidepressant-like and pro-synaptogenic effects in mice and the role of mTORC1 signaling in these responses. The combined administration of subthreshold doses of ketamine (0.1 mg/kg, i.p.) and guanosine (0.01 mg/kg, p.o.) caused a fast (1 h - 24 h), but not long-lasting (7 days) reduction in the immobility time in the tail suspension test. This behavioral effect was paralleled by a rapid (started in 1 h) and transient (back to baseline in 24 h) increase on BDNF, p-Akt (Ser⁴⁷³), p-GSK-3β (Ser⁹), p-mTORC1 (Ser²⁴⁴⁸), p-p70S6K (Thr³⁸⁹) immunocontent in the hippocampus, but not in the prefrontal cortex. Conversely, ketamine plus guanosine increased PSD-95 and GluA1 immunocontent in the prefrontal cortex, but not the hippocampus after 1 h, whereas increased levels of these proteins in both brain structures were observed after 24 h, but these effects did not persist after 7 days. The combined administration of ketamine plus guanosine raised the dendritic spines density in the ventral hippocampal DG and prefrontal cortex after 24 h Rapamycin (0.2 nmol/site, i.c.v.) abrogated the antidepressant-like effect and pro-synaptogenic responses triggered by ketamine plus guanosine. These results indicate that guanosine may boost the antidepressant-like effect of ketamine for up to 24 h by a mTORC1-dependent mechanism.

The Mechanisms Behind Rapid Antidepressant Effects of Ketamine: A Systematic Review With a Focus on Molecular Neuroplasticity

The mechanism of action underlying ketamine's rapid antidepressant effects in patients with depression, both suffering from major depressive disorder (MDD) and bipolar disorder (BD), including treatment resistant depression (TRD), remains unclear. Of the many speculated routes that ketamine may act through, restoring deficits in neuroplasticity may be the most parsimonious mechanism in both human patients and preclinical models of depression. Here, we conducted a literature search using PubMed for any reports of ketamine inducing neuroplasticity relevant to depression, to identify cellular and molecular events, relevant to neuroplasticity, immediately observed with rapid mood improvements in humans or antidepressant-like effects in animals. After screening reports using our inclusion/exclusion criteria, 139 publications with data from cell cultures, animal models, and patients with BD or MDD were included (registered on PROSPERO, ID: CRD42019123346). We found accumulating evidence to support that ketamine induces an increase in molecules involved in modulating neuroplasticity, and that these changes are paired with rapid antidepressant effects. Molecules or complexes of high interest include glutamate, AMPA receptors (AMPAR), mTOR, BDNF/TrkB, VGF, eEF2K, p70S6K, GSK-3, IGF2, Erk, and microRNAs. In summary, these studies suggest a robust relationship between improvements in mood, and ketamine-induced increases in molecular neuroplasticity, particularly regarding intracellular signaling molecules.

A low-dose combination of ketamine and guanosine counteracts corticosterone-induced depressive-like behavior and hippocampal synaptic impairments via mTORC1 signaling

Ketamine exhibits rapid and sustained antidepressant responses, but its repeated use may cause adverse effects. Augmentation strategies have been postulated to be useful for the management/reduction of ketamine's dose and its adverse effects. Based on the studies that have suggested that ketamine and guanosine may share overlapping mechanisms of action, the present study investigated the antidepressant-like effect of subthreshold doses of ketamine and guanosine in mice subjected to repeated administration of corticosterone (CORT) and the role of mTORC1 signaling for this effect. The ability of the treatment with ketamine (0.1 mg/kg, i.p.) plus guanosine (0.01 mg/kg, p.o.) to counteract the depressive-like behavior induced by CORT (20 mg/kg, p.o., for 21 days) in mice, was paralleled with the prevention of the CORT-induced reduction on BDNF levels, Akt (Ser⁴⁷³) and GSK-3β (Ser⁹) phosphorylation, and PSD-95, GluA1, and synapsin immunocontent in the hippocampus. No changes on mTORC1 and p70S6K immunocontent were found in the hippocampus and prefrontal cortex of any experimental group. No alterations on BDNF, Akt/GSK-3β, mTORC1/p70S6K, and synaptic proteins were observed in the prefrontal cortex of mice. The antidepressant-like and pro-synaptogenic effects elicited by ketamine plus guanosine were abolished by the pretreatment with rapamycin (0.2 nmol/site, i.c.v., a selective mTORC1 inhibitor). Our results showed that the combined administration of ketamine and guanosine at low doses counteracted CORT-induced depressive-like behavior and synaptogenic disturbances by activating mTORC1 signaling. This study supports the notion that the combined administration of guanosine and ketamine may be a useful therapeutic strategy for the management of MDD.

TRPV4 inhibitor HC067047 produces antidepressant-like effect in LPS-induced depression mouse model

Inflammation is a crucial component that contributes to the pathogenesis of major depressive disorder. It has been revealed that the nonselective cation channel transient receptor potential vanilloid 4 (TRPV4) profoundly affects a variety of physiological processes, including inflammation. However, its roles and mechanisms in LPS-induced depression are still unclear. Here, for the first time, we found that there was a significant increase in TRPV4 in the hippocampus in a depression mouse model induced by LPS. TRPV4 inhibitor HC067047 or knockdown the hippocampal TRPV4 with TRPV4 shRNA could effectively rescue the aberrant behaviors. Furthermore, TRPV4 inhibitor HC067047 reduced the activation of astrocyte and microglia, decreased expression of CaMKII-NLRP3 inflammasome and increased the expression of neurogenesis marker DCX in the hippocampus. In addition, enhanced neuroinflammation in the serum was also reversed by TRPV4 inhibitor HC067047. Thus, we consider that TRPV4 has an important role in contributing to the depression-like behavior following LPS-induced systemic inflammation.

Merazin Hydrate Produces Rapid Antidepressant Effects Depending on Activating mTOR Signaling by Upregulating Downstream Synaptic Proteins in the Hippocampus

Major depressive disorder has become an increasingly serious disease in the world. However, convenient antidepressants have low efficacy and slow onset defects, which is dangerous for suicidal tendency patients. Nowadays, rapid antidepressant research has become the focus. Merazin hydrate (MH), a component of the natural herb Fructus Aurantii, has been shown to produce rapid antidepressant-like effects in animal models. However, the mechanism of its rapid antidepressant-like effects was still elusive like that of ketamine. The study aimed to reveal the relationship between the rapid antidepressant-like effects of MH and mTOR signaling, which is closely related to rapid antidepressants. The results showed that a single administration of MH was capable of reversing the behavioral defects at 2 h in two classic depressive models including learned helplessness (LH) and chronic mild stress (CMS). Moreover, the phosphorylated expression of mTOR, reduced by LH or CMS, was upregulated after a single administration of MH, and the expressions of BDNF and synaptic proteins in the hippocampus were also reversed 2 h later, similar to ketamine. Moreover, LH increased the expressions of eNOS, IL-10, and TNF-α in serum, which were all reversed by a single dose of MH at 2 h, similar to ketamine. Furthermore, we used rapamycin, an antagonist of mTOR, to confirm whether the rapid antidepressant-like effects of MH depend on mTOR or not. We found that inhibiting the activation of mTOR blocked the rapid antidepressant-like effects of MH, which also inhibited the upregulation of expressions of BDNF and PSD95. To sum up, the rapid antidepressant effect of MH depended on the activation of mTOR to regulate downstream BNDF and synaptic protein expressions.

Low doses of ketamine and guanosine abrogate corticosterone-induced anxiety-related behavior, but not disturbances in the hippocampal NLRP3 inflammasome pathway

RATIONALE

Guanosine has been shown to potentiate ketamine's antidepressant-like actions, although its ability to augment the anxiolytic effect of ketamine remains to be determined.

OBJECTIVE

This study investigated the anxiolytic-like effects of a single administration with low doses of ketamine and/or guanosine in mice subjected to chronic administration of corticosterone and the role of NLRP3-driven signaling.

METHODS

Corticosterone (20 mg/kg, p.o.) was administered for 21 days, followed by a single administration of ketamine (0.1 mg/kg, i.p.), guanosine (0.01 mg/kg, p.o.), or ketamine (0.1 mg/kg, i.p.) plus guanosine (0.01 mg/kg, p.o.). Anxiety-like behavior and NLRP3-related targets were analyzed 24 h following treatments.

RESULTS

Corticosterone reduced the time spent in the open arms and the central zone in the elevated plus-maze test and open-field test, respectively. Corticosterone raised the number of unsupported rearings and the number and time of grooming, and decreased the latency to start grooming in the open-field test. Disturbances in regional distribution (increased rostral grooming) and grooming transitions (increased aborted and total incorrect transitions) were detected in corticosterone-treated mice. These behavioral alterations were accompanied by increased immunocontent of Iba-1, ASC, NLRP3, caspase-1, TXNIP, and IL-1β in the hippocampus, but not in the prefrontal cortex. The treatments with ketamine, guanosine, and ketamine plus guanosine were effective to counteract corticosterone-induced anxiety-like phenotype, but not disturbances in the hippocampal NLRP3 pathway.

CONCLUSIONS

Our study provides novel evidence that low doses of ketamine and/or guanosine reverse corticosterone-induced anxiety-like behavior and shows that the NLRP3 inflammasome pathway is likely unrelated to this response.

Antidepressant-like effect of guanosine involves activation of AMPA receptor and BDNF/TrkB signaling

Guanosine is a purine nucleoside that has been shown to exhibit antidepressant effects, but the mechanisms underlying its effect are not well established. We investigated if the antidepressant-like effect induced by guanosine in the tail suspension test (TST) in mice involves the modulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, voltage-dependent calcium channel (VDCC), and brain-derived neurotrophic factor (BDNF)/tropomyosin receptor kinase B (TrkB) pathway. We also evaluated if the antidepressant-like effect of guanosine is accompanied by an acute increase in hippocampal and prefrontocortical BDNF levels. Additionally, we investigated if the ability of guanosine to elicit a fast behavioral response in the novelty suppressed feeding (NSF) test is associated with morphological changes related to hippocampal synaptogenesis. The antidepressant-like effect of guanosine (0.05 mg/kg, p.o.) in the TST was prevented by DNQX (AMPA receptor antagonist), verapamil (VDCC blocker), K-252a (TrkBantagonist), or BDNF antibody. Increased P70S6K phosphorylation and higher synapsin I immunocontent in the hippocampus, but not in the prefrontal cortex, were observed 1 h after guanosine administration. Guanosine exerted an antidepressant-like effect 1, 6, and 24 h after its administration, an effect accompanied by increased hippocampal BDNF level. In the prefrontal cortex, BDNF level was increased only 1 h after guanosine treatment. Finally, guanosine was effective in the NSF test (after 1 h) but caused no alterations in dendritic spine density and remodeling in the ventral dentate gyrus (DG). Altogether, the results indicate that guanosine modulates targets known to be implicated in fast antidepressant behavioral responses (AMPA receptor, VDCC, and TrkB/BDNF pathway).

M2-AChR Mediates Rapid Antidepressant Effects of Scopolamine Through Activating the mTORC1-BDNF Signaling Pathway in the Medial Prefrontal Cortex

Background: Scopolamine, a non-selective muscarinic acetylcholine receptor (M1~5-AChR) antagonist, has rapid and robust antidepressant effects in humans and other species. However, which of the five M-AChRs mediates these therapeutic effects has not been fully identified. Several studies implicate M2-AChR as a potential antidepressant target of scopolamine. This study aimed to explore the role of M2-AChR in scopolamine's antidepressant-like effects and determine the underlying mechanisms. Methods: We used the classic novelty suppressed feeding test (NSFT), open field test (OFT) and forced swim test (FST) to observe antidepressant-related behaviors of normal rats, medial prefrontal cortex (mPFC) neuron silenced rats and M2-AChR knockdown rats treated with scopolamine. In a further experiment, the M2 cholinergic receptor antagonist methoctramine (MCT) was injected intracerebroventricularly into normal rats. Levels of mTORC1 and brain-derived neurotrophic factor (BDNF) in the mPFC of animals were analyzed by Western blotting. Results: Consistent with previous studies, mPFC was required for the antidepressant-like effects of scopolamine, and intracerebroventricular injection of MCT into rats could produce similar antidepressant-like effects. Use of AAV-shRNA to knock down M2-AChR in the mPFC resulted in the antidepressant-like effects of scopolamine being blunted. Furthermore, Western blotting demonstrated increased expression of mTORC1 signaling and BDNF in MCT-treated rats. Conclusion: Our results indicate that M2-AChR in the mPFC mediates the antidepressant-like effects of scopolamine by increasing the expression of BDNF and activating the mTORC1 signaling pathway.

A single administration of ascorbic acid rapidly reverses depressive-like behavior and hippocampal synaptic dysfunction induced by corticosterone in mice

Ketamine is the prototype for glutamate-based fast-acting antidepressants. The establishment of ketamine-like drugs is still a challenge and ascorbic acid has emerged as a candidate. This study investigated the ascorbic acid's ability to induce a fast antidepressant-like response and to improve hippocampal synaptic markers in mice subjected to chronic corticosterone (CORT) administration. CORT was administered for 21 days, followed by a single administration of ascorbic acid (1 mg ∕Kg, p.o.), ketamine (1 mg ∕Kg, i.p.) or fluoxetine (10 mg ∕Kg, p.o.) in mice. Depressive-like behavior, hippocampal synaptic proteins immunocontent, dendrite spines density in the dentate gyrus (DG) were analyzed 24 h following treatments. The administration of ascorbic acid or ketamine, but not fluoxetine, counteracted CORT-induced depressive-like behavior in the tail suspension test (TST). CORT administration reduced PSD-95, GluA1, and synapsin (synaptic markers) immunocontent, and these alterations were reversed by ascorbic acid or ketamine, but only ketamine reversed the CORT-induced reduction on GluA1 immunocontent. In the ventral and dorsal DG, CORT decreased filopodia-, thin- and stubby-shaped spines, while ascorbic acid and ketamine abolished this alteration only in filopodia spines. Ascorbic acid and ketamine increased mushroom-shaped spines density in ventral and dorsal DG. Therefore, the results show that a single administration of ascorbic acid, in a way similar to ketamine, rapidly elicits an antidepressant-like response and reverses hippocampal synaptic deficits caused by CORT, an effect associated with increased levels of synaptic proteins and dendritic remodeling.

Nootkatone improves anxiety- and depression-like behavior by targeting hyperammonemia-induced oxidative stress in D-galactosamine model of liver injury

Acute or chronic liver injury is closely related to hyperammonemia, which will result in oxidative stress and damage to nerve cells, and these factors are vital to the development of anxiety and depression. In this study, the effect of Nootkatone (NKT) on the anxiety- and depression-like behavioral changes in mice induced by liver injury was investigated. Liver injury was induced by D-galactosamine (D-GalN; 350 mg/kg) three times a week for 4 weeks. NKT (5 mg/kg or 10 mg/kg) was given as co-treatment daily for 4 weeks. NKT (5 mg/kg) co-treatment remarkably ameliorates D-GalN-induced anxiety- and depression-like behaviors as evident from the results of sucrose preference test, forced swimming test, tail suspension test, and novelty suppressed feeding test. Results showed that NKT could induce an elevation in serum alanine transaminase and aspartate transaminase level, alleviate the oxidative stress induced by hyperammonemia through activating Keap1/Nrf2/HO-1 antioxidant pathways, decrease the expression of inducible nitric oxide synthase and NOX2 in hippocampus and prefrontal cortex, enhance the vitality of superoxide dismutase, catalase, and glutathione levels in serum, liver, and brain, and significantly reduce the generation of malondialdehyde. At the same time, NKT also reduces the level of ammonia in serum and brain and upgrades the activity of glutamine synthetase in the hippocampus and prefrontal cortex. Taken together, the present results suggested that NKT has a significant antidepressant effect through modulation of oxidative stress induced by D-GalN administration.

Iridoids from Gardeniae fructus ameliorates depression by enhancing synaptic plasticity via AMPA receptor-mTOR signaling

ETHNOPHARMACOLOGICAL RELEVANCE

Gardeniae fructus is a traditional Chinese herb which exerts antidepressant effect. However, its effective constituent and potential mechanism are still unknown.

AIM OF THE STUDY

To examine whether iridoids, a type of monoterpenoids from Gardeniae fructus (IGF), exerts antidepressant effect by enhancing synaptic plasticity via AMPA receptor-mTOR signaling.

MATERIALS AND METHODS

The antidepressant effect of IGF (15 mg/kg; 30 mg/kg; 45 mg/kg) was investigated in spatial restraint stress (SRS)-induced mice. The expression levels of AMPA receptor-mTOR signaling and synaptic proteins were measured by Western blot, dendritic spine density was observed in Golgi staining. AMPA receptor (AMPAR) inhibitor NBQX and mTOR inhibitor Rapamycin were employed to determine the roles of AMPAR and mTOR signaling in IGF-induced antidepressant effects.

RESULTS

After IGF administration, the expression of the AMPA glutamate receptor Glutamate Receptor 1 (GluA1) was inhibited in SRS mice. We also observed a trend toward the up-regulation of the mammalian target of Rapamycin (mTOR) protein kinase, p70 ribosomal protein S6K (P70S6K) and eukaryotic translation initiation factor 4E-binding protein 1 (4EBP1). The protein levels of Synapsin-1 and PSD-95 were decreased after SRS challenge, along with declined dendritic spine density, which were all reversed with IGF treatment. Furthermore, the treatment efficacy of IGF were blocked with AMPA receptor inhibitor NBQX or mTOR inhibitor Rapamycin.

CONCLUSION

IGF exerted antidepressive-like effects by stimulating AMPAR-mTOR signaling regulated synaptic plasticity enhancement. This work provided an important basis for developing IGF and Gardeniae fructus as potential anti-depressants.

Antidepressant-Like Effects of Chronic Guanosine in the Olfactory Bulbectomy Mouse Model

Major depressive disorder (MDD) leads to pervasive changes in the health of afflicted patients. Despite advances in the understanding of MDD and its treatment, profound innovation is needed to develop fast-onset antidepressants with higher effectiveness. When acutely administered, the endogenous nucleoside guanosine (GUO) shows fast-onset antidepressant-like effects in several mouse models, including the olfactory bulbectomy (OBX) rodent model. OBX is advocated to possess translational value and be suitable to assess the time course of depressive-like behavior in rodents. This study aimed at investigating the long-term behavioral and neurochemical effects of GUO in a mouse model of depression induced by bilateral bulbectomy (OBX). Mice were submitted to OBX and, after 14 days of recovery, received daily (ip) administration of 7.5 mg/kg GUO or 40 mg/kg imipramine (IMI) for 45 days. GUO and IMI reversed the OBX-induced hyperlocomotion and recognition memory impairment, hippocampal BDNF increase, and redox imbalance (ROS, NO, and GSH levels). GUO also mitigated the OBX-induced hippocampal neuroinflammation (IL-1, IL-6, TNF-α, INF-γ, and IL-10). Brain microPET imaging ([¹⁸F]FDG) shows that GUO also prevented the OBX-induced increase in hippocampal FDG metabolism. These results provide additional evidence for GUO antidepressant-like effects, associated with beneficial neurochemical outcomes relevant to counteract depression.

Venlafaxine protects against chronic stress-related behaviors in mice by activating the mTORC1 signaling cascade

BACKGROUND

Recent studies have suggested the role of mammalian target of rapamycin complex 1 (mTORC1) in the pathophysiology of depression. Although venlafaxine was thought to be a serotonin and norepinephrine reuptake inhibitor (SNRI), its pharmacological mechanism remain elusive. In this study, the effects of venlafaxine on the mTORC1 system were studied in both chronic unpredictable mild stress (CUMS) and chronic social defeat stress (CSDS) models.

METHOD

First, we examined whether repeated venlafaxine treatment reversed the effects of CUMS and CSDS on the mTORC1 signaling cascade in both the hippocampus and medial prefrontal cortex (mPFC). Second, several selective pharmacological inhibitors of the mTORC1 system, including rapamycin, LY294002 and U0126, were used together to determine whether the protective effects of venlafaxine against the CUMS and CSDS models were prevented by mTORC1 system blockade. Finally, genetic knockdown of mTORC1 by mTORC1-shRNA was further adopted to test whether mTORC1 was necessary for the anti-stress effects of venlafaxine in mice.

RESULT

Our results showed that the decreasing effects of CUMS and CSDS on the mTORC1 signaling cascade in the hippocampus and mPFC were restored by venlafaxine, and the use of rapamycin, LY294002, U0126 and mTORC1-shRNA fully abolished the anti-stress actions of venlafaxine in mice.

CONCLUSION

The mTORC1 system is involved in the pharmacological mechanism of venlafaxine.

Ketamine, but not guanosine, as a prophylactic agent against corticosterone-induced depressive-like behavior: Possible role of long-lasting pro-synaptogenic signaling pathway

Ketamine has been reported to exert a prophylactic effect against stress-induced depressive-like behavior by modulating the guanosine-based purinergic system. However, the molecular pathways underlying its prophylactic effect and whether guanosine also elicits a similar effect remain to be determined. Here, we investigated the prophylactic effect of ketamine and guanosine against corticosterone (CORT – 20 mg/kg, p.o.)-induced depressive-like behavior in mice. Furthermore, we characterized if the prophylactic response may be associated with mTORC1-driven signaling in the hippocampus and prefrontal cortex. A single administration of ketamine (5 mg/kg, i.p.), but not guanosine (1 or 5 mg/kg, p.o.), given 1 week before the pharmacological stress prevented CORT-induced depressive-like behavior in the tail suspension test (TST) and splash test (SPT). Fluoxetine treatment for 3 weeks did not prevent CORT-induced behavioral effects. A single administration of subthreshold doses of ketamine (1 mg/kg, i.p.) plus guanosine (5 mg/kg, p.o.) partially prevented the CORT-induced depressive-like behavior in the SPT. Additionally, CORT reduced Akt (Ser⁴⁷³) and GSK-3β (Ser⁹) phosphorylation and PSD-95, GluA1, and synapsin immunocontent in the hippocampus, but not in the prefrontal cortex. No alterations on mTORC1/p70S6K immunocontent were found in both regions in any experimental group. CORT-induced reductions on PSD-95, GluA1, and synapsin immunocontent were prevented only by ketamine treatment. Collectively, these findings suggest that ketamine, but not guanosine, exerts a prophylactic effect against depressive-like behavior, an effect associated with the stimulation of long-lasting pro-synaptogenic signaling in the hippocampus.

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CORT induces depressive-like behavior and hippocampal synaptogenic markers deficits.

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Ketamine prevents CORT-induced behavioral and hippocampal synaptogenic alterations.

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Guanosine or fluoxetine are unable to prevent the alterations induced by CORT.

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Ketamine plus guanosine partially prevent CORT-induced reduced self-care behavior.

Guanosine potentiates the antidepressant-like effect of subthreshold doses of ketamine: Possible role of pro-synaptogenic signaling pathway

Background Augmentation therapies may be effective strategies to potentiate the ketamine's actions with lower potential for knock-on effects. Thus, this study investigated the ability of combined administration of guanosine plus ketamine to elicit an antidepressant-like effect associated with mTOR pathway modulation. The ability of this combined administration to exert an antidepressant-like effect in a model of depression was also evaluated. Methods Mice were administered with subthreshold doses of ketamine (0.1 mg/kg, i.p.) and guanosine (0.01 mg/kg, p.o.) and submitted to the tail suspension test, and immunoblotting analyses (p-mTOR, p-p70S6K, PSD-95, GluA1, and synapsin) in the hippocampus and prefrontal cortex. The antidepressant-like effect of ketamine plus guanosine in mice subjected to administration of corticosterone (20 mg/kg, p.o., 21 days) was also evaluated. Results Ketamine plus guanosine treatment elicited an antidepressant-like effect, which was associated with increased mTOR (Ser²⁴⁴⁸) and p70S6K (Thr³⁸⁹) phosphorylation in the hippocampus, but not in the prefrontal cortex. Furthermore, increased PSD-95 and GluA1 immunocontent were observed in the prefrontal cortex, but not in the hippocampus of ketamine plus guanosine-treated mice. Reinforcing the notion that guanosine may potentiate the ketamine's behavioral response, a single administration of subthreshold doses of ketamine plus guanosine counteracted the corticosterone-induced depressive-like behavior. Conclusions Our results indicate that guanosine potentiates the antidepressant-like effect of subthreshold doses of ketamine, an effect likely associated with the stimulation of synaptogenic pathway in the hippocampus and prefrontal cortex, although with a different profile. The augmentation effect of ketamine by guanosine could have therapeutic relevance for patients with treatment-resistant depression.

Guanosine fast onset antidepressant-like effects in the olfactory bulbectomy mice model

The treatment of major depressive disorder (MDD) is still a challenge. In the search for novel antidepressants, glutamatergic neuromodulators have been investigated as possible fast-acting antidepressants. Innovative studies suggest that the purine cycle and/or the purinergic signaling can be dysregulated in MDD, and the endogenous nucleoside guanosine has gained attention due to its extracellular effects. This study aimed to verify if guanosine produces fast-onset effects in the well-validated, reliable and sensitive olfactory bulbectomy (OBX) model of depression. The involvement of the mTOR pathway, a key target for the fast-onset effect of ketamine, was also investigated. Results show that a single i.p. injection of guanosine, or ketamine, completely reversed the OBX-induced anhedonic-like behavior 24 or 48 h post treatment, as well as the short-term recognition memory impairment 48 h post treatment. The antidepressant-like effects of guanosine and ketamine were completely abolished by rapamycin. This study shows, for the first time, that guanosine, in a way similar to ketamine, is able to elicit a fast antidepressant response in the OBX model in mice. The results support the notion that guanosine represents a new road for therapeutic improvement in MDD.

Ascorbic acid presents rapid behavioral and hippocampal synaptic plasticity effects

Growing evidence has suggested that ascorbic acid may exhibit rapid anxiolytic and antidepressant-like effects. In this study the effects of a single administration of ascorbic acid (1 mg/kg, p.o.), ketamine (1 mg/kg, i.p., a fast-acting antidepressant) and fluoxetine (10 mg/kg, p.o., conventional antidepressant) were investigated on: a) behavioral performance in the novelty suppressed feeding (NSF) test; b) hippocampal synaptic protein immunocontent; c) dendritic spine density and morphology in the dorsal and ventral dentate gyrus (DG) of the hippocampus and d) hippocampal dendritic arborization. Ascorbic acid or ketamine, but not fluoxetine, decreased the latency to feed in the NSF test in mice. This effect was accompanied by increased p70S6K (Thr³⁸⁹) phosphorylation 1 h after ascorbic acid or ketamine treatment, although only ascorbic acid increased synapsin I immunocontent. Ketamine administration increased the dendritic spine density in the dorsal DG, but none of the treatments affected the maturation of dendritic spines in this region. In addition, both ascorbic acid and ketamine increased the dendritic spine density in the ventral DG, particularly the mature spines. Sholl analysis demonstrated no effect of any treatment on hippocampal dendritic arborization. Altogether, the results provide evidence that the behavioral and synaptic responses observed following ascorbic acid administration might occur via the upregulation of synaptic proteins, dendritic spine density, and maturation in the ventral DG, similar to ketamine. These findings contribute to understand the cellular targets implicated in its antidepressant/anxiolytic behavioral responses and support the notion that ascorbic acid may share with ketamine the ability to increase synaptic function.

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.