Tamara Zharkovsky

Developmental lead exposure impairs contextual fear conditioning and reduces adult hippocampal neurogenesis in the rat brain

The effects of developmental lead exposure on the emotional reactivity, contextual fear conditioning and neurogenesis in the dentate gyrus of 60–80 days‐old rats were studied. Wistar rat pups were exposed to 0.2% lead acetate via their dams’ drinking water from postnatal day (PND) 1 to PND 21 and directly via drinking water from weaning until PND 30. At PND 60 and 80 the level of anxiety and contextual fear conditioning were studied, respectively. At PND 80 all animals received injections of BrdU to determine the effects of Pb on the generation of new cells in the dentate gyrus of hippocampus and on their survival and differentiation patterns. The results of the present study demonstrate that developmental lead exposure induces persistent increase in the level of anxiety and inhibition of contextual fear conditioning. Developmental lead exposure reduced generation of new cells in the dentate gyrus and altered the pattern of differentiation of BrdU‐positive cells into mature neurons. A lower proportion of BrdU‐positive cells co‐expressed with the marker for mature neurons, calbindin. In contrast, the proportions of young not fully differentiated neurons and proportions of astroglial cells, generated from newly born cells, were increased in lead‐exposed animals.

Decreased Hippocampal Neurogenesis Following Olfactory Bulbectomy is Reversed by Repeated Citalopram Administration

1. Whereas much progress has been made in the treatment of depression, the exact pathogenetic mechanisms of the disorder are still poorly understood. It has been proposed that one possible mechanism could be a decrease in adult hippocampal neurogenesis.2. The olfactory bulbectomy (OB) in rats is widely accepted as an animal model of depression. In the present study, we investigated whether hippocampal neurogenesis is affected by an OB, and whether chronic citalopram, a serotonin selective reuptake inhibitor, counteracts OB-induced impairment of neurogenesis.3. Our study shows that OB decreases proliferation of the neuronal precursors in the dentate gyrus and retards their differentiation into mature granule neurons. In OB rats, repeated administration of citalopram restores reduced proliferative activity and enhances the differentiation of precursors into mature calbindin-positive neurons.4. The obtained data demonstrate that a citalopram-induced increase in neurogenesis in OB rats could be one possible mechanism by which antidepressants alleviate OB-induced depressive-like behavior.

Depression‐like behaviour in neural cell adhesion molecule (NCAM)‐deficient mice and its reversal by an NCAM‐derived peptide, FGL

The neural cell adhesion molecule (NCAM) plays a pivotal role in brain plasticity. Brain plasticity itself has a crucial role in the development of depression. The aim of this study was to analyze whether NCAM‐deficient (NCAM−/−) mice exhibit depression‐like behaviour and whether a peptide termed FGL, derived from the NCAM binding site for the fibroblast growth factor (FGF) receptor, is able to reverse the depression‐like signs in NCAM−/− mice. Our study showed that NCAM−/− mice demonstrated increased freezing time in the tail‐suspension test and reduced preference for sucrose consumption in the sucrose preference test, reduced adult neurogenesis in the dentate gyrus and reduced levels of the phosphorylated cAMP response element‐binding protein (pCREB) in the hippocampus. FGL administered acutely or repeatedly reduced depression‐like behaviour in NCAM−/− mice without having an effect on their wild‐type littermates. Repeated administration of FGL enhanced survival of the newly born neurons in NCAM−/− mice and increased the levels of pCREB in both NCAM+/+ and NCAM−/− mice. In conclusion, our data demonstrate that NCAM deficiency in mice results in a depression‐like phenotype which can be reversed by the acute or repeated administration of FGL. The results also suggest a role of the deficit in NCAM signalling through the FGF receptor in depression.

Up-regulation of lysosomal cathepsin L and autophagy during neuronal death induced by reduced serum and potassium

Serum and potassium deprivation‐induced neuronal death on the primary culture of rat cerebellar granule neurons is being widely used as an in vitro model of neurodegeneration and neuronal apoptosis. In our experiments, serum and potassium deprivation for 12 h induced neuronal death in ≈ 20% of cerebellar granule neurons as demonstrated by Trypan Blue assay. Neuronal death was accompanied by a transient increase in the intralysosomal cathepsin L activity, which preceded neuronal death. During this time, the lysosomal membrane integrity remained preserved and no leakage of cathepsin L into the cytosol was seen. Ultrastructural analysis revealed the appearance of multiple vacuoles and autophagosomes in the cytoplasmatic compartment of serum‐ and potassium‐deprived granule neurons. Addition of selective cathepsin L inhibitors or of the autophagy inhibitor 3‐methyladenine provided partial protection against serum and potassium deprivation‐induced death. Our data also show that combining cathepsin L inhibitors and caspase‐3 inhibitors leads to a synergistic neuroprotective effect against serum and potassium deprivation. The results of the current study suggest that activation of the autophagosomal–lysosomal compartment plays an important role in neuronal death induced by serum and potassium deprivation in cultured cerebellar granule cells.

Neurodegeneration and production of the new cells in the dentate gyrus of juvenile rat hippocampus after a single administration of ethanol

Administration of ethanol during brain development induces widespread neuronal loss in various structures of the brain. Here, we show that a single administration of ethanol given during the early postnatal period can induce not only neuronal death, but also an increase in proliferation of the progenitor cells in the dentate gyrus of hippocampal formation in rats. Ethanol (1.5 or 3 g/kg, i.p.) administered to 10-day-old rats induced massive neuronal degeneration as evidenced by TUNEL assay in the dentate gyrus. The neuronal death induced by a high dose of ethanol (3 g/kg) was accompanied by an enhanced proliferation of the progenitor cells labeled by bromodeoxyuridine (BrdU, 50 mg/kg, i.p.) in dentate gyrus. One and 3 weeks following ethanol or saline administration, ethanol-treated rats still had significantly more BrdU-labeled cells than control animals. In ethanol-treated rats, a higher proportion of newly born cells acquired the phenotype of immature postmitotic neurons whereas the final differentiation into calbindin-expressing granule cells remained unchanged. The proportion of astroglial cells was also increased in ethanol-treated rats. Thus, ethanol given in high doses not only induces neurodegeneration but also initiates the process of neuro- and gliogenesis, which might be responsible for the neuronal and glial reorganization of the dentate gyrus.

Dysregulated CREB signaling pathway in the brain of neural cell adhesion molecule (NCAM)-deficient mice

The neural cell adhesion molecule (NCAM) mediates cell-cell interactions and plays an important role in processes associated with neural plasticity, including learning and memory formation. It has been shown that mice deficient in all isoforms of NCAM (NCAM-/- mice) demonstrate impairment in long-term plasticity at multiple hippocampal synapses, disrupted spatial learning, and impaired contextual and auditory-cued fear conditioning. The formation of long-term memory is associated with activation of transcription factor CREB (cAMP response element binding protein). The aims of this study were to investigate NCAM-mediated signaling transduction pathways and the levels of the phosphorylated (Ser133) active form of the CREB in the brain structures (the pre- and frontal cortex, basolateral amygdala, and hippocampus) involved in the memory formation in NCAM-deficient mice. Immunohistochemical analysis revealed reduced levels of pCREB in the prefrontal cortex (PFC), frontal cortex (FC), CA3 subregion of the hippocampus (CA3) and basolateral nucleus of amygdala (BLA) in NCAM-/- mice. NCAM-/- mice had also reduced levels of the phosphorylated CaMKII and CaMKIV in PFC/FC and the hippocampus, which are the downstream signaling molecules of NCAM. The levels of non-phosphorylated kinases did not differ from those seen in the wild-type mice. These results provide evidence that NCAM deficiency results in the dysregulation of CREB-mediated signaling pathways in the brain regions, which is related to the formation of memory.

Effects of repeated citalopram treatment on kainic acid-induced neurogenesis in adult mouse hippocampus

Previous studies have demonstrated that systemic administration of kainic acid (KA) triggers a cascade of neuroplastic changes in the hippocampus. Intensive neurodegeneration accompanied by immune response and enhanced neurogenesis following local or systemic KA administration in rats and mice has been reported. KA-induced enhancement in proliferative activity of neuronal and glial precursors results in the appearance of immature hyperactive neurons which could be regarded as evidence of dysregulated neural plasticity. In this study we attempted to investigate whether administration of selective serotonin reuptake inhibitor (SSRI) citalopram could inhibit KA-induced reactive gliosis and dysregulated neurogenesis in mice. The results of our study demonstrate that repeated administration of citalopram counteracted KA-induced reactive gliosis and reduced aberrant proliferative activity in the dentate gyrus of the mouse brain. We found that the population of BrdU-positive cells expressing markers for young neurons was decreased following repeated citalopram administration compared to KA-treated animals. These results suggest that repeated citalopram administration could prevent activation of aberrant neuroplasticity in the damaged hippocampus.

NCAM-deficient mice show prominent abnormalities in serotonergic and BDNF systems in brain – Restoration by chronic amitriptyline

Mood disorders are associated with alterations in serotonergic system, deficient BDNF (brain-derived neurotrophic factor) signaling and abnormal synaptic plasticity. Increased degradation and reduced functions of NCAM (neural cell adhesion molecule) have recently been associated with depression and NCAM deficient mice show depression-related behavior and impaired learning. The aim of the present study was to investigate potential changes in serotonergic and BDNF systems in NCAM knock-out mice. Serotonergic nerve fiber density and SERT (serotonin transporter) protein levels were robustly reduced in the hippocampus, prefrontal cortex and basolateral amygdala of adult NCAM(-)(/-) mice. This SERT reduction was already evident during early postnatal development. [(3)H]MADAM binding experiments further demonstrated reduced availability of SERT in cell membranes of NCAM(-)(/-) mice. Moreover, the levels of serotonin and its major metabolite 5-HIAA were down regulated in the brains of NCAM(-)(/-) mice. NCAM(-)(/-) mice also showed a dramatic reduction in the BDNF protein levels in the hippocampus and prefrontal cortex. This BDNF deficiency was associated with reduced phosphorylation of its receptor TrkB. Importantly, chronic administration of antidepressant amitriptyline partially or completely restored these changes in serotonergic and BDNF systems, respectively. In conclusion, NCAM deficiency lead to prominent and persistent abnormalities in brain serotonergic and BDNF systems, which likely contributes to the behavioral and neurobiological phenotype of NCAM(-/-) mice.

PSA modification of NCAM supports the survival of injured retinal ganglion cells in adulthood.

Neural cell adhesion molecule (NCAM) is known as the cell surface glycoprotein, and it belongs to the immunoglobulin superfamily of adhesion molecules. Polysialic acid (PSA) is a carbohydrate attached to NCAM via either of two specific sialyltransferases: ST8SiaII and ST8SiaIV. Polysialylated neural cell adhesion molecule (PSA-NCAM) mediates cell interactions, plays a role in axon growth, migration, synaptic plasticity during development and cell regeneration. Some evidence has shown that PSA-NCAM supports the survival of neurons. It was demonstrated that PSA-NCAM is present in abundance in the retina during development and in adulthood. The aim of this study was to investigate whether PSA-NCAM promotes retinal ganglion cell (RGC) survival in transgenic mice with deficiencies in sialyltransferases or NCAM or after the administration of endoneuraminidase (Endo-N). RGC injury was induced by intravitreal administration of kainic acid (KA). These studies showed that injection of Endo-N after 14 days enhances the toxicity of KA to RGCs in wild-type (WT) mice by 18%. In contrast, in knockout mice (ST8SiaII-/-, ST8SiaIV-/-, NCAM-/-), survival of RGCs after KA injury did not change. Deficiencies of either ST8SiaII or ST8SiaIV did not influence the level of PSA-NCAM in the adult retina, however, in neonatal animals, decreased levels of PSA-NCAM were observed. In knockout ST8SiaII-/- adults, a reduced number of RGCs was detected, whereas in contrast, increased numbers of RGCs were noted in NCAM-/- mice. In conclusion, these data demonstrate that PSA-NCAM supports the survival of injured RGCs in adulthood. However, the role of PSA-NCAM in the adult retina requires further clarification.

Depression-like phenotype in NCAM-deficient mice

Depression is highly prevalent complex, heterogeneous disorder with possible serious physical, mental and socioeconomical consequences. There are several theories trying to explain the genesis of depression, one novel, links the depression with reduced brain plasticity. Neural cell adhesion (NCAM) plays important roles in the development of central nervous system and in the structural and functional plasticity in the CNS throughout life. NCAM is a member of the immunoglobulin superfamily of adhesion molecules is characterized by several immunoglobulin (Ig)-like domains. If the reduced neuronal plasticity is an important factor predisposing to the development of psychiatric disorders and depression in particular, than animals with reduced brain plasticity due to deficiency in NCAM should serve as a valuable model of depression. Mice (c56/BL strain) with genetic deficiency for NCAMs were subjected to two tests widely used for the assessment of depression-like phenotype: tails suspension test and sucrose preference test. NCAMdeficient mice demonstrated higher immobility time in the tail suspension test and lower preference for sucrose solution in sucrose consumption test as compared with their wild type littermates. The increased immobility time in tail suppression test was reversed by amitriptyline and citalopram treatment. Furthermore, a peptide with NCAMmimetic actions, FGL was also able to rescue “depression-like” phenotype in NCAM-deficient mice. Analysis of hippocampal neurogenesis in NCAMdeficient mice revealed a reduced survival and differentiation of the newly born cells into calbindinpositive granule neurons. At the same time point, larger proportion of young postmitotic neurons expressing Tuj1 was found in NCAM−/− mice. No changes in the proportion of BrdUpositive cells, which have been differentiated into glial phenotype, were found. In conclusion, NCAM-deficient mice demonstrate “depression-like” behaviour associated with reduced hippocampal neurogenesis and might serve as a genetic model of depression.