Effects of olfactory bulbectomy on NMDA receptor density in the rat brain:

Memory impairments in rodent depression models: A link with depression theories

More than 80% of depressed patients struggle with learning new tasks, remembering positive events, or concentrating on a single topic. These neurocognitive deficits accompanying depression may be linked to functional and structural changes in the prefrontal cortex and hippocampus. However, their mechanisms are not yet completely understood. We conducted a narrative review of articles regarding animal studies to assess the state of knowledge. First, we argue the contribution of changes in neurotransmitters and hormone levels in the pathomechanism of cognitive dysfunction in animal depression models. Then, we used numerous neuroinflammation studies to explore its possible implication in cognitive decline. Encouragingly, we also observed a positive correlation between increased oxidative stress and a depressive-like state with concomitant memory deficits. Finally, we discuss the undeniable role of neurotrophin deficits in developing cognitive decline in animal models of depression. This review reveals the complexity of depression-related memory impairments and highlights the potential clinical importance of gathered findings for developing more reliable animal models and designing novel antidepressants with procognitive properties.

Buspirone, a 5-HT1A agonist attenuates social isolation-induced behavior deficits in rats: a comparative study with fluoxetine

Social isolation is a potent stressor in both humans and animals that results in increased anger-like emotion, (anger in humans), aggression and suicidal ideation in humans [suicidal trait-related behavior in rats (STRB)]. The study's purpose was to compare the effects of buspirone (BUS) and fluoxetine (Flx) on social isolation-induced behavior deficits in rats. The male Wistar rats were randomized into six groups and caged individually for 14 days except for the non stress control (nSC) group. They were then divided into the following groups, stress control (SC), Flx (30), BUS (10), BUS (20) and BUS (40) and treated from day 14 to day 28. On the last day of treatment behavior parameters were recorded. Serum cortisol, blood pressure (BP) measurement, magnetic resonance imaging (MRI) of the rat's brain and brain-derived neurotrophic factor (BDNF) expression were performed. SC group showed a significant increase in anger-like emotion, aggression, irritability score, learned helplessness, increased cortisol level and reduced BDNF. These behavioral deficits were attenuated by BUS and Flx, Both were found to be equally beneficial in preventing anger-like emotions and aggression. Flx, which has been found to promote suicidal thoughts in people, did not reduce irritability in rats, showing that it did not affect it. BUS significantly improved all behavioral traits also reduced cortisol levels, significantly increased BDNF and normalized BP. Neuroimaging studies in SC brains showed a reduction in amygdala size compared to nSC, BUS treatment mitigated this reduction. Buspirone is effective in preventing social isolation induced behavioural-deficits.

Ghrelin restores memory impairment following olfactory bulbectomy in mice by activating hippocampal NMDA1 and MAPK1 gene expression

Ghrelin (Ghrl) is an orexigenic peptide with potential roles in the modulation of anxiety- and depressive-like symptoms induced by bilateral olfactory bulbectomy (OB) in rodents. In the present work, we assessed whether intrahippocampal Ghrl could reverse OB-induced depressive-like and amnesic effects by regulating molecular mechanisms related to neuroplasticity. Adult female albino Swiss mice were divided into sham and OB groups, and infused with saline (S) or Ghrl 0.03 nmol/μl, 0.3 nmol/μl, or 3 nmol/μl into the hippocampus before exposition to open-field test (OFT) and tail suspension test (TST) or immediately after training in the object recognition test (ORT). After test phase in ORT, animals were euthanized and their hippocampi were dissected to study the expression of genes related to memory. The OB-S animals presented hyperlocomotion in OFT, increased immobility in TST and memory impairment compared to sham-S (p < 0.05), but acute intrahippocampal infusion of Ghrl 0.3 nmol/μl produced an improvement on these parameters in OB animals (p < 0.05). In addition, this dose of Ghrl reversed OB-induced low expression of NMDA1 and MAPK1 iso1 and up-regulated the expression of CaMKIIa iso1 and iso2, and MAPK1 iso2 (p < 0.05). These results extend the existing literature regarding OB-induced behavioral and neurochemical changes, and provide mechanisms that could underlie the antidepressant effect of Ghrl in this model.

Brain NMDA Receptors in Schizophrenia and Depression

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP), dizocilpine (MK-801) and ketamine have long been considered a model of schizophrenia, both in animals and humans. However, ketamine has been recently approved for treatment-resistant depression, although with severe restrictions. Interestingly, the dosage in both conditions is similar, and positive symptoms of schizophrenia appear before antidepressant effects emerge. Here, we describe the temporal mechanisms implicated in schizophrenia-like and antidepressant-like effects of NMDA blockade in rats, and postulate that such effects may indicate that NMDA receptor antagonists induce similar mechanistic effects, and only the basal pre-drug state of the organism delimitates the overall outcome. Hence, blockade of NMDA receptors in depressive-like status can lead to amelioration or remission of symptoms, whereas healthy individuals develop psychotic symptoms and schizophrenia patients show an exacerbation of these symptoms after the administration of NMDA receptor antagonists.

Involvement of glutamatergic N-methyl-d-aspartate receptors in the expression of increased head-dipping behaviors in the hole-board tests of olfactory bulbectomized mice

Olfactory bulbectomized (OB) mice produce agitated anxiety-like behaviors in the hole-board test, which was expressed by an increase in head-dipping counts and a decrease in head-dipping latencies. However, the associated mechanisms remain unclear. In the present study, MK-801 (10, 100μg/kg), a selective N-methyl-d-aspartate (NMDA) receptor antagonist, significantly and dose-dependently suppressed the increased head-dipping behaviors in OB mice, without affecting sham mice. Similar results were obtained with another selective NMDA receptor antagonist D-AP5 treatment in OB mice. On the other hand, muscimol, a selective aminobutyric acid type A (GABAA) receptor agonist produced no effects on these hyperemotional behaviors in OB mice at a dose (100μg/kg) that produced anxiolytic-like effects in sham mice. Interestingly, glutamine contents and glutamine/glutamate ratios were significantly increased in the amygdala and frontal cortex of OB mice compared to sham mice. Based on these results, we concluded that the glutamatergic NMDA receptors are involved in the expression of increased head-dipping behaviors in the hole-board tests of OB mice. Accordingly, the changes in glutamatergic transmission in frontal cortex and amygdala may play important roles in the expression of these abnormal behaviors in OB mice.

Antidepressant-like activity of magnesium in the olfactory bulbectomy model is associated with the AMPA/BDNF pathway

RATIONALE

Numerous studies suggest agents that act on glutamatergic transmission as potential antidepressants. Preclinical and clinical evidence suggests that magnesium, an N-methyl-D-aspartate receptor blocker, may be useful in the treatment of depression.

OBJECTIVE

The aim of this study was to investigate the effects of magnesium on behavior; protein levels of GluN2A, GluN2B [N-methyl-D-aspartate receptor (NMDAR) subunits], GluA1 [α-amino-3-hydroxy-5 methyl-4-isoxazolepropionic acid (AMPA) subunit], phospho-Ser-831-GluA1 (P-S831), phospho-Ser-845-GluA1 (P-S845), and brain-derived neurotrophic factor (BDNF); and messenger RNA (mRNA) levels of GluN2A and GluN2B in different brain areas in the olfactory bulbectomy (OB) model of depression in rats.

METHODS

Magnesium was administered once daily for 14 days at three doses (10, 15, and 20 mg/kg, intraperitoneal) to sham and OB rats. Following treatment, open field and passive avoidance tests were performed in the sham and OB rats. After 24 h, the hippocampus, the prefrontal cortex (PFC), and the amygdala of rats treated with the most active dose (15 mg/kg) were harvested, and the protein and mRNA levels were determined.

RESULTS

Chronic administration of magnesium (15 and 20 mg/kg) reduced the number of trials required to learn passive avoidance and reduced the OB-induced hyperactivity. OB increased the P-S845 level in the hippocampus, which was reduced by magnesium treatment. Magnesium significantly increased the levels of BDNF, GluN2B, P-S831, and P-S845 protein (and mRNA) primarily in the PFC and the hippocampus in OB rats.

CONCLUSION

For the first time, the present results demonstrate the antidepressant-like activity of magnesium in the OB animal model of depression and indicate the potential involvement of the AMPA/BDNF pathway in this activity.

Roles of glutamate signaling in preclinical and/or mechanistic models of depression

Accumulating evidence suggests that the glutamatergic system plays important roles in the pathophysiology and treatment of major depressive disorder (MDD). Abnormalities in the glutamatergic system are definitely observed in this disorder, and certain glutamatergic agents exhibit antidepressant effects in patients with MDD. In this review, we summarize the preclinical findings suggesting the involvement of glutamate signaling in the pathophysiology and treatment of MDD. Preclinical animal models for depression are often characterized by changes in molecules related to glutamatergic signaling. Some antidepressants exert their effects by affecting glutamatergic system components in animals. Animals with genetically modified glutamatergic function exhibit depression-like behaviors or anti-depressive behavior. In addition, several types of glutamatergic agents have shown antidepressant-like effects in preclinical models for depression. Many types of glutamate receptors (NMDA, AMPA, and metabotropic glutamate receptors) or transporters appear to be involved in the etiology of depression or in the mechanisms of action of antidepressants. These functional proteins related to glutamate signal transduction are potential targets for a new generation of antidepressants with fast-onset effects, such as the NMDA antagonist ketamine.

Sigma receptors: potential targets for a new class of antidepressant drug

Despite the widespread and devastating impact of depression on society, our current understanding of its pathogenesis is limited. Likewise, existing treatments are inadequate, providing relief to only a subset of people suffering from depression. The search for more effective antidepressant drugs includes the investigation of new molecular targets. Among them, current data suggests that sigma receptors are involved in multiple processes effecting antidepressant-like actions in vivo and in vitro. This review summarizes accumulated evidence supporting a role for sigma receptors in antidepressant effects and provides a conceptual framework for delineating their potential roles over the course of antidepressant treatment.

Riluzole rapidly attenuates hyperemotional responses in olfactory bulbectomized rats, an animal model of depression

Growing evidence indicates that the glutamatergic neurotransmitter system is central to the neurobiology and treatment of depression. Riluzole, a drug currently used to slow the progression of amyotrophic lateral sclerosis (ALS), directly affects the glutamatergic system. In this study, we investigated the effects of riluzole in olfactory bulbectomy (OBX) rats, an animal model of depression. The olfactory bulbs in rats were removed by suction. The emotionality of rats was measured by scoring their responses to given stimuli, i.e., attack, startle, struggle, and fight responses. The OBX rats chronically treated with vehicle for 7 days at 14 days following surgery showed significant increases in emotionality responses. Single (1st day administration) and subchronic (7th day administration) riluzole treatment (1-10 mg/kg, po) significantly and dose-dependently reduced hyperemotional responses in OBX rats. Both single and subchronic riluzole treatment (10 mg/kg, po) had no significant effects on the emotional responses in sham operated rats. In addition, we demonstrated that single riluzole treatment (10 mg/kg, po) significantly decreased extracellular glutamate levels in medial prefrontal cortex of OBX rats by in vivo microdialysis. We provide the first experimental evidence that riluzole rapidly attenuated hyperemotional responses in OBX rats, an animal model of depression.

Is subcortical-cortical midline activity in depression mediated by glutamate and GABA? A cross-species translational approach

Major depressive disorder has recently been characterized by abnormal resting state hyperactivity in anterior midline regions. The neurochemical mechanisms underlying resting state hyperactivity remain unclear. Since animal studies provide an opportunity to investigate subcortical regions and neurochemical mechanisms in more detail, we used a cross-species translational approach comparing a meta-analysis of human data to animal data on the functional anatomy and neurochemical modulation of resting state activity in depression. Animal and human data converged in showing resting state hyperactivity in various ventral midline regions. These were also characterized by abnormal concentrations of glutamate and gamma-aminobutyric acid (GABA) as well as by NMDA receptor up-regulation and AMPA and GABA receptor down-regulation. This cross-species translational investigation suggests that resting state hyperactivity in depression occurs in subcortical and cortical midline regions and is mediated by glutamate and GABA metabolism. This provides insight into the biochemical underpinnings of resting state activity in both depressed and healthy subjects.

Effects of milnacipran and fluvoxamine on hyperemotional behaviors and the loss of tryptophan hydroxylase-positive cells in olfactory bulbectomized rats

RATIONALE

It has been reported that many of the behavioral and serotonergic neuronal changes observed in olfactory bulbectomy (OBX) were improved by subchronic administration of a variety of antidepressants.

OBJECTIVE

We examined the effects of subchronic treatment with milnacipran, a dual serotonin and noradrenaline reuptake inhibitors (SNRIs) and fluvoxamine, selective serotonin reuptake inhibitors (SSRI) in the OBX-induced hyperemotional behaviors and tryptophan hydroxylase (TPH), rate-limiting enzyme of 5-HT.

MATERIALS AND METHODS

The olfactory bulbs were removed by suction. Drugs were administered p.o. once daily for 8 days beginning 14 days post-surgery. The hyperemotionality behaviors of OBX rats were measured by rating scale and in the elevated plus-maze test.

RESULTS

OBX rats, after milnacipran or fluvoxamine treatment, showed significant decrease in the score of hyperemotional responses on 7th day as compared with vehicle-treated OBX rats. In addition, milnacipran and fluvoxamine in OBX rats respectively produced a significant increase in the percentage of time spent in and number of entries into open arms in the elevated plus maze test. Furthermore, when 5-HTnergic neuronal function was examined using antibodies against tryptophan hydroxylase (TPH) following the behavioral tests, fluvoxamine significantly reversed the loss of TPH-positive cells produced by OBX in the dorsal raphe.

CONCLUSION

We demonstrated that chronic treatment with milnacipran or fluvoxamine was effective to improve both the hyperemotional behavior and the loss of TPH-positive cells seen in OBX rats.

Differential roles of amygdaloid nuclei in the anxiolytic- and antidepressant-like effects of the V1b receptor antagonist, SSR149415, in rats

RATIONALE

SSR149415 ((2S, 4R)-1-[5-chloro-1-[(2,4-dimethoxyphenyl)sulfonyl]-3-(2-methoxyphenyl)-2-oxo-2,3-dihydro-1H-indol-3-yl]-4-hydroxy-N,N-dimethyl-2-pyrrolidinecarboxamide), the first selective nonpeptide vasopressin V1b receptor antagonist has been shown to induce antidepressant-and anxiolytic-like effects following systemic administration, whereas intraseptal infusion of the drug engender antidepressant-but not anxiolytic-like effects.

OBJECTIVES

Based on recent evidence that V1b receptors are located within the amygdaloid complex, a structure which is well known for its modulatory role of emotional processes, the possible involvement of the different amygdaloid nuclei in the anxiolytic- and/or antidepressant-like effects of SSR149415 was examined.

METHODS

Male Sprague-Dawley or Wistar rats were infused with SSR149415 into the central (CeA), the basolateral (BlA), or the medial (MeA) nucleus of the amygdala and tested 10 min after microinjection in the elevated plus-maze or the forced-swimming test, two models typically used for assessing the anxiolytic and antidepressant effects of drugs, respectively.

RESULTS

Microinjection of SSR149415 into the BlA (1-10 ng), but not into the CeA or the MeA, increased the percentage of time spent in the open arms of the elevated plus-maze, indicating anxiolytic-like effects. Furthermore, in the forced-swimming test, microinjection of the drug into the CeA (1, 10, and 100 ng), BlA (1-10 ng), or MeA (100 ng) decreased immobility, an effect which is indicative of an antidepressant-like action. Together, these findings indicate that while the antidepressant-like effects of SSR149415 are mediated by different amygdaloid nuclei, its anxiolytic-like effects appear to involve only the basolateral nucleus of the amygdala. Moreover, these results add further evidence to the role of extrahypothalamic vasopressinergic systems in the control of emotional responses.

Cell proliferation is influenced by bulbectomy and normalized by imipramine treatment in a region-specific manner

Growing evidence indicates that alterations of neuroplasticity may contribute to the pathophysiology of depression. In contrast, various antidepressants increase adult hippocampal neurogenesis and block the effects of stress. These findings result in the 'neurogenesis hypothesis of depression'. The present study seeks to determine out whether cell proliferation is altered in the hippocampus, subventricular zone (SVZ), and basolateral amygdala of adult rats exposed to bilateral olfactory bulbectomy, another established model of depression and, if so, how imipramine effects bulbectomy-induced changes of cell genesis. Bulbectomy results in a significant reduction of cell proliferation in the hippocampus and SVZ, an effect that is normalized by subchronic doses of imipramine. Moreover, an increase in cell genesis in the basolateral amygdala, which is not affected by imipramine, is demonstrated. TUNEL staining indicates an enhanced apoptosis after bulbectomy in the SVZ that cannot be reduced by imipramine. Cell death rates in the hippocampus and amygdala are not affected by bulbectomy. The opposing effects of bulbectomy and imipramine treatment in the hippocampus and amygdala demonstrate that these structures of the limbic system, both integrated in emotional processing, react quite differently with regard to neuroplasticity. Further to this, we discuss a possible link between the pathogenesis of depression and changed neuronal plasticity in the SVZ.

Decreased calcium/calmodulin-dependent protein kinase II and protein kinase C activities mediate impairment of hippocampal long-term potentiation in the olfactory bulbectomized mice

Olfactory bulbectomized (OBX) mice showed significant impairment of learning and memory-related behaviors 14 days after olfactory bulbectomy, as measured by passive avoidance and Y-maze tasks. We here observed a large impairment of hippocampal long-term potentiation (LTP) in the OBX mice. Concomitant with decreased acetylcholinesterase expression, protein kinase C (PKC)alpha autophosphorylation and NR1(Ser-896) phosphorylation significantly decreased in the hippocampal CA1 region of OBX mice. Both PKCalpha and NR1(Ser-896) phosphorylation significantly increased following LTP in the control mice, whereas increases were not observed in OBX mice. Like PKC activities, calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation significantly decreased in the hippocampal CA1 region of OBX mice as compared with that of control mice. In addition, increased CaMKII autophosphorylation following LTP was not observed in OBX mice. Finally, the impairment of CaMKII autophosphorylation was closely associated with reduced pGluR1(Ser-831) phosphorylation, without change in synapsin I (site 3) phosphorylation in the hippocampal CA1 region of OBX mice. Taken together, in OBX mice NMDA receptor hypofunction, possibly through decreased PKCalpha activity, underlies decreased CaMKII activity in the post-synaptic regions, thereby impairing LTP induction in the hippocampal CA1 region. Both decreased PKC and CaMKII activities with concomitant LTP impairment account for the learning disability observed in OBX mice.

Animal models of depression: olfactory lesions affect amygdala, subventricular zone, and aggression

Psychiatric or depressed patients show alterations in both olfactory projection areas and mucosa. In rodents, ablation of olfactory bulbs causes a depression-like syndrome, useful to test antidepressant agents. We studied in mice the behavioral symptoms and neuroanatomical correlates after mucosal damage or ablation of the olfactory bulb. Our results are based on a battery of tests exploiting anxious, aggressive, and depressive behavior, on morphological and immunohistochemical analysis. We found similar results in both sensory-damaged and bulbectomized animals, with a behavioral dissociation concerning different forms of aggression. These findings do not support a simple downregulation of social interactions in damaged mice. The most prominent modifications in the brains of sensory damaged and bulbectomized mice are detected in the subventricular zone (SVZ), the source area of neural stem cells, and in the content of cAMP-dependent protein kinase within the amygdala, suggesting a central role of this structure in the functional modulation of behavior.

MK-801 suppresses muricidal behavior but not locomotion in olfactory bulbectomized rats: involvement of NMDA receptors

In rats, olfactory bulbectomy (OBX) causes changes in glutamatergic function in the amygdala (AMG) and induces mouse-killing behavior (MKB). The medial AMG (mAMG) plays an important role in the initiation and maintenance of OBX-induced MKB. In the present study, systemic injection or intra-mAMG perfusion of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801) was used to determine the effects of MK-801, a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, on the expression of OBX-induced MKB in male Wistar rats that had undergone OBX 1 month previously. The effects of MK-801 on locomotion in OBX rats were also examined using the open-field test. Intraperitoneal injection of MK-801 at doses of 0.10 and 0.15 mg/kg resulted in reversible suppression of MKB, the effect being maximal within 1 h after drug treatment, then gradually disappearing over 6 h. Locomotor distance in OBX rats was not affected using 0.10 mg/kg of MK-801, but increased after treatment with 0.15 mg/kg of MK-801; both doses, however, caused the rats to spend longer in the central area of the open field. MKB was also reversibly suppressed by local perfusion of 1 mM MK-801 at a rate of 1 microl/min into the mAMG through microdialysis probes. These results suggest that NMDA receptors, at least, in the mAMG, are involved in the expression of OBX-induced MKB.

Herpes virus-mediated preproenkephalin gene transfer in the ventral striatum mimics behavioral changes produced by olfactory bulbectomy in rats

The syndrome of behavioral, physiological, and neurochemical changes caused by ablation of the olfactory bulbs (OBX) in rats serves as a reliable and well-validated model of depression. Previous experiments have demonstrated that OBX leads to increased expression of the preproenkephalin (ENK) gene in the olfactory tubercle (OT) portion of the ventral striatum in rats. The aim of the present experiments was to investigate the role of OBX-induced ENK overexpression in the OT in the behavioral abnormalities exhibited by bulbectomized rats. A recombinant herpes virus carrying human preproENK cDNA was used to manipulate ENK gene expression in the OT of bulbectomized and sham-operated rats. Motivational deficits were assessed by the sucrose preference test, and 'agitation-like' behaviors were measured with the novel open field and footshock-induced freezing tests. ENK gene transfer in sham-operated rats mimicked some of the effects of OBX; it decreased freezing behavior in response to mild footshock and produced behavioral activation in the open field. In another experiment, virally mediated ENK gene transfer into the OT of intact rats decreased footshock-induced freezing, and this effect was reversed by naltrexone administration. PreproENK gene transfer into the OT did not produce analgesic effects in the tail-flick test. No effects on freezing behavior were observed following preproENK gene transfer into the frontal cortex. An additional experiment revealed that naltrexone administration attenuated the OBX-induced abnormality in freezing behavior. The results indicate that overexpression of the preproENK gene in the ventral striatum may mediate the 'agitation-like' behavior exhibited by bulbectomized rats.

Olfactory bulbectomy protects hippocampal pyramidal neurons against excitotoxic death

The olfactory system is functionally linked to the hippocampus, and odors can modify the activity of hippocampal neurons. Because hippocampal neurons are selectively vulnerable to death in several prominent neurodegenerative conditions, we tested the hypothesis that activity in olfactory pathways can modify the sensitivity of hippocampal neurons to excitotoxic damage. We report that rats subjected to olfactory bulbectomy exhibit a decrease in the vulnerability of hippocampal pyramidal neurons to excitotoxic injury. Four-month-old male Sprague-Dawley rats were subjected to bilateral olfactory bulbectomy or a sham operation. Three months later the rats were given a unilateral infusion of kainic acid in the dorsal hippocampus and were euthanized 24 h later. There was a threefold increase in the number of CA3 neurons that survived kainic acid administration in the bulbectomized rats compared to sham-operated rats. These findings provide the first evidence that olfactory input affects the vulnerability of neurons to excitotoxic death.