NMDA-NO signaling in the dorsal and ventral hippocampus time-dependently modulates the behavioral responses to forced swimming stress

Nitric Oxide Synthase inhibition counteracts the stress-induced DNA methyltransferase 3b expression in the hippocampus of rats

It has been postulated that the activation of NMDA receptors (NMDAr) and nitric oxide (NO) production in the hippocampus is involved in the behavioral consequences of stress. Stress triggers NMDAr-induced calcium influx in limbic areas, such as the hippocampus, which in turn activates neuronal NO synthase (nNOS). Inhibition of nNOS or NMDAr activity can prevent stress-induced effects in animal models, but the molecular mechanisms behind this effect are still unclear. In this study, cultured hippocampal neurons treated with NMDA or dexamethasone showed an increased of DNA methyltransferase 3b (DNMT3b) mRNA expression, which was blocked by pre-treatment with nNOS inhibitor nω -propyl-l-arginine (NPA). In rats submitted to the Learned Helplessness paradigm (LH), we observed that inescapable stress increased DNMT3b mRNA expression at 1h and 24h in the hippocampus. The NOS inhibitors 7-NI and aminoguanidine (AMG) decreased the number of escape failures in LH and counteracted the changes in hippocampal DNMT3b mRNA induced in this behavioral paradigm. Altogether, our data suggest that NO produced in response to NMDAr activation following stress upregulates DNMT3b in the hippocampus.

Prelimbic neuronal nitric oxide synthase inhibition exerts antidepressant-like effects independently of BDNF signalling cascades

OBJECTIVES

NMDA antagonists and nitric oxide synthase (NOS) inhibitors induce antidepressant-like effects and may represent treatment options for depression. The behavioural effects of NMDA antagonists seem to depend on Tyrosine kinase B receptor (TrkB) activation by BDNF and on mechanistic target of rapamycin (mTOR), in the medial prefrontal cortex (mPFC). However, it is unknown whether similar mechanisms are involved in the behavioural effects of NOS inhibitors. Therefore, this work aimed at determining the role of TrkB and mTOR signalling in the prelimbic area of the ventral mPFC (vmPFC-PL) in the antidepressant-like effect of NOS inhibitors.

METHODS

Pharmacological treatment with LY235959 or ketamine (NMDA antagonists), NPA or 7-NI (NOS inhibitors), BDNF, K252a (Trk antagonist) and rapamycin (mTOR inhibitor) injected systemically or into vmPFC-PL followed by behavioural assessment.

RESULTS

We found that bilateral injection of BDNF into the vmPFC-PL induced an antidepressant-like effect, which was blocked by pretreatment with K252a and rapamycin. Microinjection of LY 235959 into the vmPFC-PL induced antidepressant-like effect that was suppressed by local rapamycin but not by K252a pretreatment. Microinjection of NPA induced an antidepressant-like effect insensitive to both K252a and rapamycin. Similarly, the antidepressant-like effects of a systemic injection of ketamine or 7-NI were not affected by blockade of mTOR or Trk receptors in the vmPFC-PL.

CONCLUSION

Our data support the hypothesis that NMDA blockade induces an antidepressant-like effect that requires mTOR but not Trk signalling into the vmPFC-PL. The antidepressant-like effect induced by local NOS inhibition is independent on both Trk and mTOR signalling in the vmPFC-PL.

Perinatal exposure to the SSRI paroxetine alters the methylome landscape of the developing dentate gyrus

Evidence in humans and rodents suggests that perinatal exposure to selective serotonin reuptake inhibitor (SSRI) antidepressants can have serious long-term consequences in offspring exposed in utero or infancy via breast milk. In spite of this, there is limited knowledge of how perinatal SSRI exposure impacts brain development and adult behaviour. Children exposed to SSRIs in utero exhibit increased internalizing behaviour and abnormal social behaviour between the ages of 3 and 6, and increased risk of depression in adolescence; however, the neurobiological changes underlying this behaviour are poorly understood. In rodents, perinatal SSRI exposure perturbs hippocampal gene expression and alters adult emotional behaviour (including increased depression-like behaviour). The present study demonstrates that perinatal exposure to the SSRI paroxetine leads to DNA hypomethylation and reduces DNA methyltransferase 3a (Dnmt3a) mRNA expression in the hippocampus during the second and third weeks of life. Next-generation sequencing identified numerous differentially methylated genomic regions, including altered methylation and transcription of several dendritogenesis-related genes. We then tested the hypothesis that transiently decreasing Dnmt3a expression in the early postnatal hippocampus would mimic the behavioural effects of perinatal SSRI exposure. We found that siRNA-mediated knockdown of Dnmt3a in the dentate gyrus during the second to third week of life produced greater depression-like behaviour in adult female (but not male) offspring, akin to the behavioural consequences of perinatal SSRI exposure. Overall, these data suggest that perinatal SSRI exposure may increase depression-like behaviours, at least in part, through reduced Dnmt3a expression in the developing hippocampus.

Nitric oxide signalling and antidepressant action revisited

Studies about the pathogenesis of mood disorders have consistently shown that multiple factors, including genetic and environmental, play a crucial role on their development and neurobiology. Multiple pathological theories have been proposed, of which several ultimately affects or is a consequence of dysfunction in brain neuroplasticity and homeostatic mechanisms. However, current clinical available pharmacological intervention, which is predominantly monoamine-based, suffers from a partial and lacking response even after weeks of continuous treatment. These issues raise the need for better understanding of aetiologies and brain abnormalities in depression, as well as developing novel treatment strategies. Nitric oxide (NO) is a gaseous unconventional neurotransmitter, which regulates and governs several important physiological functions in the central nervous system, including processes, which can be associated with the development of mood disorders. This review will present general aspects of the NO system in depression, highlighting potential targets that may be utilized and further explored as novel therapeutic targets in the future pharmacotherapy of depression. In particular, the review will link the importance of neuroplasticity mechanisms governed by NO to a possible molecular basis for the antidepressant effects.

Neuronal nitric oxide synthase and affective disorders

Affective disorders including major depressive disorder (MDD), bipolar disorder (BPD), and general anxiety affect more than 10% of population in the world. Notably, neuronal nitric oxide synthase (nNOS), a downstream signal molecule of N-methyl-D-aspartate receptors (NMDARs) activation, is abundant in many regions of the brain such as the prefrontal cortex (PFC), hippocampus, amygdala, dorsal raphe nucleus (DRN), locus coeruleus (LC), and hypothalamus, which are closely associated with the pathophysiology of affective disorders. Decreased levels of the neurotransmitters including 5-hydroxytryptamine or serotonin (5-HT), noradrenalin (NA), and dopamine (DA) as well as hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis are common pathological changes of MDD, BPD, and anxiety. Increasing data suggests that nNOS in the hippocampus play a crucial role in the etiology of MDD whereas nNOS-related dysregulation of the nitrergic system in the LC is closely associated with the pathogenesis of BPD. Moreover, hippocampal nNOS is implicated in the role of serotonin receptor 1 A (5-HTR1 A) in modulating anxiety behaviors. Augment of nNOS and its carboxy-terminal PDZ ligand (CAPON) complex mediate stress-induced anxiety and disrupting the nNOS-CAPON interaction by small molecular drug generates anxiolytic effect. To date, however, the function of nNOS in affective disorders is not well reviewed. Here, we summarize works about nNOS and its signal mechanisms implicated in the pathophysiology of affective disorders. On the basis of this review, it is suggested that future research should more fully focus on the role of nNOS in the pathomechanism and treatment of affective disorders.

Anticonvulsant effect of minocycline on pentylenetetrazole-induced seizure in mice: involvement of nitric oxide and N-methyl-d-aspartate receptor

Anticonvulsant effects of minocycline have been explored recently. This study was designed to examine the anticonvulsant effect of acute administration of minocycline on pentylenetetrazole-induced seizures in mouse considering the possible role of the nitric oxide/N-methyl-d-aspartate (NMDA) pathway. We induced seizure using intravenous administration of pentylenetetrazole. Our results showed that acute administration of minocycline increased the seizure threshold. Furthermore, co-administration of subeffective doses of the nonselective nitric oxide synthase (NOS) inhibitor NG-l-arginine methyl ester (10 mg/kg) and the neuronal NOS inhibitor 7-nitroindazole (40 mg/kg) enhanced the anticonvulsant effect of subeffective doses of minocycline (40 mg/kg). We found that inducible NOS inhibitor aminoguanidine (100 mg/kg) had no effect on the antiseizure effect of minocycline. Moreover, l-arginine (60 mg/kg), as a NOS substrate, reduced the anticonvulsant effect of minocycline. We also demonstrated that pretreatment with the NMDA receptor antagonists ketamine (0.5 mg/kg) and MK-801 (0.05 mg/kg) increased the anticonvulsant effect of subeffective doses of minocycline. Results showed that minocycline significantly decreased the hippocampal nitrite level. Furthermore, co-administration of a neuronal NOS inhibitor like NMDA receptor antagonists augmented the effect of minocycline on the hippocampal nitrite level. In conclusion, we revealed that anticonvulsant effect of minocycline might be, at least in part, due to a decline in constitutive hippocampal nitric oxide activity as well as inhibition of NMDA receptors.

Hippocampal mammalian target of rapamycin is implicated in stress-coping behavior induced by cannabidiol in the forced swim test

BACKGROUND

Cannabidiol is a non-psychotomimetic compound with antidepressant-like effects. However, the mechanisms and brain regions involved in cannabidiol effects are not yet completely understood. Brain-derived neurotrophic factor/tropomyosin-receptor kinase B/mammalian target of rapamycin (BDNF-TrkB-mTOR) signaling, especially in limbic structures, seems to play a central role in mediating the effects of antidepressant drugs.

AIM

Since it is not yet known if BDNF-TrkB-mTOR signaling in the hippocampus is critical to the antidepressant-like effects of cannabidiol, we investigated the effects produced by cannabidiol (10/30/60 nmol/0.2 µL) micro-injection into the hippocampus of mice submitted to the forced swim test and to the open field test.

METHODS

Independent groups received intra-hippocampal injections of rapamycin (mTOR inhibitor, 0.2 nmol/0.2 µL) or K252 (Trk antagonist, 0.01 nmol/0.2 µL), before the systemic (10 mg/kg) or hippocampal (10 nmol/0.2µL) injection of cannabidiol, and were submitted to the same tests. BDNF levels were analyzed in the hippocampus of animals treated with cannabidiol (10 mg/kg).

RESULTS

Systemic cannabidiol administration induced antidepressant-like effects and increased BDNF levels in the dorsal hippocampus. Rapamycin, but not K252a, injection into the dorsal hippocampus prevented the antidepressant-like effect induced by systemic cannabidiol treatment (10 mg/kg). Differently, hippocampal administration of cannabidiol (10 nmol/0.2 µL) reduced immobility time, an effect that was blocked by both rapamycin and K252a local microinjection.

CONCLUSION

Altogether, our data suggest that the hippocampal BDNF-TrkB-mTOR pathway is vital for cannabidiol-induced antidepressant-like effect when the drug is locally administered. However, other brain regions may also be involved in cannabidiol-induced antidepressant effect upon systemic administration.