The effects of early-life adversity on fear memories in adolescent rats and their persistence into adulthood.

Adolescence is a developmental period characterized by extensive morphological and functional remodeling of the brain. The processes of brain maturation during this period may unmask malfunctions that originate earlier in life as a consequence of early-life stress (ELS). This is associated with the emergence of many psychopathologies during adolescence, particularly affective spectrum disorders. In the present study, we applied a maternal separation (MS) procedure (3h/day, on postnatal days 1-14) as a model of ELS to examine its effects on the acquisition, expression and extinction of fear memories in adolescent rats. Additionally, we studied the persistence of these memories into adulthood. We found that MS decreased the expression of both contextual (CFC) and auditory (AFC) fear conditioning in adolescent rats. Besides, MS had no impact on the acquisition of extinction learning. During the recall of extinction MS animals both, those previously subjected and not subjected to the extinction session, exhibited equally low levels of freezing. In adulthood, the MS animals (conditioned during adolescence) still displayed impairments in the expression of AFC (only in males) and CFC. Furthermore, the MS procedure had also an impact on the expression of CFC (but not AFC) after retraining in adulthood. Our findings imply that ELS may permanently affect fear learning and memory. The results also support the hypothesis that, depending on individual predispositions and further experiences, ELS may either lead to a resilience or a vulnerability to early- and late-onsets psychopathologies.

Early-life stress affects the structural and functional plasticity of the medial prefrontal cortex in adolescent rats.

Early life experiences are crucial factors that shape brain development and function due to their ability to induce structural and functional plasticity. Among these experiences, early-life stress (ELS) is known to interfere with brain development and maturation, increasing the risk of future psychopathologies, including depression, anxiety, and personality disorders. Moreover, ELS may contribute to the emergence of these psychopathologies during adolescence. In this present study, we investigated the effects of ELS, in the form of maternal separation (MS), on the structural and functional plasticity of the medial prefrontal cortex (mPFC) and anxiety-like behavior in adolescent male rats. We found that the MS procedure resulted in disturbances in mother-pup interactions that lasted until weaning and were most strongly demonstrated by increases in nursing behavior. Moreover, MS caused atrophy of the basal dendritic tree and reduced spine density on both the apical and basal dendrites in layer II/III pyramidal neurons of the mPFC. The structural changes were accompanied by an impairment of long-term potentiation processes and increased expression of key proteins, specifically glutamate receptor 1, glutamate receptor 2, postsynaptic density protein 95, αCa(2+) /calmodulin-dependent protein kinase II and αCa(2+)/calmodulin-dependent protein kinase II phosphorylated at residue Thr305, that are engaged in long-term potentiation induction and maintenance in the mPFC. We also found that the MS animals were more anxious in the light/dark exploration test. The results of this study indicate that ELS has a significant impact on the structural and functional plasticity of the mPFC in adolescents. ELS-induced adaptive plasticity may underlie the pathomechanisms of some early-onset psychopathologies observed in adolescents.

MK-801, a NMDA receptor antagonist, increases phosphorylation of histone H3 in the rat medial prefrontal cortex.

BACKGROUND: The present study investigated whether MK-801, when given in doses that cause psychomimetic effects in rats, could alter the phosphorylation of histone 3 (H3) at serine 10 (H3S10p) and the acetylation of H3 at lysine 14 (H3K14ac) in the medial prefrontal cortex (mPFC). These posttranslational modifications of H3 promote chromatin relaxation and increase the probability of gene expression. METHODS: Stereological counting, immunoblot analysis and confocal laser scanning microscopy. RESULTS: Treatment with MK-801 (0.4 mg/kg) evoked a time-dependent increase in the number of H3S10p positive nuclei in both the II/III and V/VI layers of the mPFC, reaching the peak of activation 30 min after injection. MK-801 treatment (0.4 mg/kg) failed to alter H3K14ac. These effects were confirmed by immunoblot analysis on tissue samples from the mPFC. Analysis of cortical cells expressing H3S10p positive nuclei revealed that constitutive and MK-801-induced expression of H3S10p was observed only in neurons and not in glia cells (H3S10p colocalized with NeuN but not with S-100ß). Moreover, it has been found that H3S10p is exclusively present in pyramidal (glutamate-positive) but not in cortical GABA-ergic interneurons (GABA-positive). The effects of MK-801 can be attenuated or blocked by the neuroleptic drug risperidone. In the cortical layer II/III, risperidone was effective at doses of 0.2 and 1 mg/kg, while it was only active at a dose of 1 mg/kg in the V/VI layer. Again, these stereological data were confirmed by immunoblot analysis. CONCLUSIONS: Our results indicate that MK-801 may increase the transcriptional activity of mPFC via the activation of the epigenetic program associated with H3S10p phosphorylation during the course of experimental psychosis.

Impact of early-life stress on the medial prefrontal cortex functions - a search for the pathomechanisms of anxiety and mood disorders.

Although anxiety and mood disorders (MDs) are the most common mental diseases, the etiologies and mechanisms of these psychopathologies are still a matter of debate. The medial prefrontal cortex (mPFC) is a brain structure that is strongly implicated in the pathophysiology of these disorders. A growing number of epidemiological and clinical studies show that early-life stress (ELS) during the critical period of brain development may increase the risk for anxiety and MDs. Neuroimaging analyses in humans and numerous reports from animal models clearly demonstrate that ELS affects behaviors that are dependent on the mPFC, as well as neuronal activity and synaptic plasticity within the mPFC. The mechanisms engaged in ELS-induced changes in mPFC function involve alterations in the developmental trajectory of the mPFC and may be responsible for the emergence of both early-onset (during childhood and adolescence) and adulthood-onset anxiety and MDs. ELS-evoked changes in mPFC synaptic plasticity may constitute an example of metaplasticity. ELS may program brain functions by affecting glucocorticoid levels. On the molecular level, ELS-induced programming is registered by epigenetic mechanisms, such as changes in DNA methylation pattern, histone acetylation and microRNA expression. Vulnerability and resilience to ELS-related anxiety and MDs depend on the interaction between individual genetic predispositions, early-life experiences and later-life environment. In conclusion, ELS may constitute a significant etiological factor for anxiety and MDs, whereas animal models of ELS are helpful tools for understanding the pathomechanisms of these disorders.

Impact of maternal separation on neural cell adhesion molecules expression in dopaminergic brain regions of juvenile, adolescent and adult rats.

Stressful experiences in the early stages of life can influence brain development and maturation, and they can also increase the risk for some psychiatric disorders; however, the specific mechanisms of this effect are still poorly understood. Neural cell adhesion molecules (NCAM 120, 140, 180 kDa) are known to play an important role in normal brain development and synaptic plasticity. Therefore, we decided to investigate whether maternal separation (MS) in rats, a paradigm which models an early life stress, has any impact on the expression of NCAM proteins in the juvenile, adolescent and adult brains of both male and female rats. Specifically, we focused our efforts on the brain regions associated with dopaminergic neurotransmission. In juvenile rats, MS decreased the levels of NCAM-140 in the substantia nigra (SN) of females and NCAM-180 in the ventral tegmental area of males. During adolescence, a reduction in NCAM-180 levels in the SN and medial prefrontal cortex (mPFC) of MS females was revealed. Finally, in adulthood, a decrease in NCAM-180 expression was observed in the mPFC of MS males. The results that we obtained indicate that early life stress can affect maturation and NCAM-driven plasticity in dopaminergic brain areas at different stages of ontogenesis and with a sex-specific manner.

ERK signalling pathway is not involved in PSA-NCAM-dependent alterations of hippocampal plasticity evoked by CB1 receptor activation.

The present study investigated the potential role of the extracellular signal-regulated kinase (ERK) pathway in the alternation of polysialylated neural cell adhesion molecule (PSA-NCAM) expression and proliferation rates in the dentate gyrus (DG) evoked by activation of the CB1 receptor. When given at a dose of 0.1 mg/kg, the CB1 receptor agonist, 3-(1,1-dimethylheptyl)-11-hydroxy-Delta(8)-tetrahydrocannabinol (HU-210), increased the levels of the phosphorylated forms of ERK (pERK1 and pERK2) in the hippocampus when measured 30 min after injection. This HU-210-induced effect was inhibited by alpha-{amino[(4-aminophenyl)thio]methylene}-2-(trifluoromethyl) benzeneacetonitrile (SL327, 30 mg/kg) - an inhibitor of mitogen-activated protein kinase kinase (MEK1/2), the upstream kinase of ERK - given 1 h before HU-210 administration. Additionally, SL327 alone significantly attenuated the basal level of both pERK1 and pERK2. HU-210 (0.1 mg/kg) decreased the number of PSA-NCAM-immunoreactive (IR) cells but did not affect the rate of proliferation, which was analyzed as the number of Ki-67-IR cells measured in the DG 2 days after HU-210 administration. The data indicated that SL327 (30 mg/kg) alone decreased the number of PSA-NCAM-IR cells 2 days after treatment. Joint administration of SL327 and HU-210 decreased the number of PSA-NCAM cells more robustly than did the administration of either alone. In addition, SL327 did not decrease the number of Ki-67-IR cells, while pretreatment with SL327 1 h before HU-210 administration did. These results suggest that stimulation of the ERK cascade caused by CB1 receptor activation is not involved in hippocampal plasticity governed by PSA-NCAM expression.

Repeated risperidone treatment increases the expression of NCAM and PSA-NCAM protein in the rat medial prefrontal cortex.

The present study investigates whether the anti-schizophrenic drug risperidone may evoke changes in the expression of NCAM/PSA-NCAM proteins, an indispensable element in the remodeling of synaptic arrangements, in the medial prefrontal cortex (mPFC). Rats were treated with risperidone (0.2 mg/kg, i.p.) either once or repeatedly (once a day, for 21 days). The expression of NCAM and PSA-NCAM proteins was analyzed via western blot and immunohistochemistry at intervals of 3 h and 3, 6, and 9 days after the single or the last risperidone dose. Repeated (but not acute) administration of risperidone was found to increase the expression of NCAM-180, NCAM-140 and PSA-NCAM proteins at 3 or 6 days after treatment. PSA-NCAM immunoreactivity was found in cell bodies, perisomatic-like sites, and in the neuropil of the mPFC. Neither single nor repeated risperidone administration changed the number of PSA-NCAM neurons in the mPFC. In contrast, the repeated risperidone treatment increased the number of PSA-NCAM perisomatic-like sites and the length density of PSA-NCAM positive neuropil at 3 days after the last injection. The data obtained indicate that risperidone, given repeatedly, may promote the remodeling of the structure of presumably GABA-ergic interneurons and that it may evoke the rearrangement of the synaptic contact in the mPFC.

Olfactory bulbectomy and amitriptyline treatment influences mGlu receptors expression in the mouse brain hippocampus.

Olfactory bulbectomy (OB) is an established animal model of depression that has been investigated mostly in rats. As in human major depression, OB induces behavioral alterations that can be ameliorated by chronic antidepressant treatment. Furthermore, it was shown that OB induces changes of various protein receptor levels in brain areas that are important in antidepressant therapy. In the present study, we investigated the effects of OB and amitriptyline (AMI) treatment on the expression of metabotropic glutamate receptors (mGluR) in the mouse hippocampus using the western blot method. AMI was given for 14 days, in a dose of 10 mg/kg, intraperitoneally. The levels of most subtypes of mGlu receptors, e.g., mGlu1a (mGluR group I), mGlu2/3 (mGluR group II), mGlu4, and mGlu7 (mGluR group III) receptors, were measured. Additionally, immunohistochemical stainings were made in slices of the mouse hippocampus. It was found that OB induced an increase in mGluR1a-immunoreactivity (IR), which was abolished by AMI treatment in the hippocampus. The removal of the olfactory bulbs caused a decrease in the level of mGlu2/3 receptors in the hippocampus, which was reversed after AMI administration. MGluR4-IR was decreased in the hippocampus in all the groups studied. A decrease of mGluR7-IR was observed in the OB group, and the effect was abolished by the administration of AMI. However, decreases in the level of mGlu2/3 and mGlu7 receptors were observed after AMI administration. The results obtained indicate an influence of OB on mGlu receptors levels in the hippocampus, and the OB-induced effect can be reversed by chronic AMI treatment in the case of mGlu1a, mGlu2/3 and mGlu7 receptors.