María Ángeles Gómez-Climent

Chronic Fluoxetine Treatment Increases the Expression of PSA-NCAM in the Medial Prefrontal Cortex

Recent hypotheses suggest that changes in neuronal structure and connectivity may underlie the etiology of depression. The medial prefrontal cortex (mPFC) is affected by depression and shows neuronal remodeling during adulthood. This plasticity may be mediated by the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), which is intensely expressed in the adult mPFC. As the expression of PSA-NCAM is increased by serotonin in other cerebral regions, antidepressants acting on serotonin reuptake may influence PSA-NCAM expression and thus counteract the effects of depression by modulating neuronal structural plasticity. Using immunohistochemistry, we have studied the relationship between serotoninergic fibers and PSA-NCAM expressing neurons in the adult rat mPFC and the expression of serotonin receptors in these cells. The effects of fluoxetine treatment for 14 days on mPFC PSA-NCAM expression have also been analyzed. Although serotoninergic fibers usually do not contact PSA-NCAM immunoreactive neurons, most of these cells express 5-HT3 receptors. In general, chronic fluoxetine treatment induces significant increases in the number of PSA-NCAM immunoreactive neurons and in neuropil immunostaining and coadministration of the 5-HT3 antagonist ondansetron blocks the effects of fluoxetine on PSA-NCAM expression. These results indicate that fluoxetine, acting through 5-HT3 receptors, can modulate PSA-NCAM expression in the mPFC. This modulation may mediate the structural plasticity of this cortical region and opens new perspectives on the study of the molecular bases of depression.

A Population of Prenatally Generated Cells in the Rat Paleocortex Maintains an Immature Neuronal Phenotype into Adulthood

New neurons in the adult brain transiently express molecules related to neuronal development, such as the polysialylated form of neural cell adhesion molecule, or doublecortin (DCX). These molecules are also expressed by a cell population in the rat paleocortex layer II, whose origin, phenotype, and function are not clearly understood. We have classified most of these cells as a new cell type termed tangled cell. Some cells with the morphology of semilunar-pyramidal transitional neurons were also found among this population, as well as some scarce cells resembling semilunar, pyramidal. and fusiform neurons. We have found that none of these cells in layer II express markers of glial cells, mature, inhibitory, or principal neurons. They appear to be in a prolonged immature state, confirmed by the coexpression of DCX, TOAD/Ulip/CRMP-4, A3 subunit of the cyclic nucleotide-gated channel, or phosphorylated cyclic adenosine monophosphate response element-binding protein. Moreover, most of them lack synaptic contacts, are covered by astroglial lamellae, and fail to express cellular activity markers, such as c-Fos or Arc, and N-methyl-d-aspartate or glucocorticoid receptors. We have found that none of these cells appear to be generated during adulthood or early youth and that most of them have been generated during embryonic development, mainly in E15.5.

The Polysialylated Form of the Neural Cell Adhesion Molecule (PSA-NCAM) Is Expressed in a Subpopulation of Mature Cortical Interneurons Characterized by Reduced Structural Features and Connectivity

Principal neurons in the adult cerebral cortex undergo synaptic, dendritic, and spine remodeling in response to different stimuli, and several reports have demonstrated that the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) participates in these plastic processes. However, there is only limited information on the expression of this molecule on interneurons and on its role in the structural plasticity of these cells. We have found that PSA-NCAM is expressed in mature interneurons widely distributed in all the extension of the cerebral cortex and have excluded the expression of this molecule in most principal cells. Although PSA-NCAM expression is generally considered a marker of immature neurons, birth-dating analyses reveal that these interneurons do not have an adult or perinatal origin and that they are generated during embryonic development. PSA-NCAM expressing interneurons show reduced density of perisomatic and peridendritic puncta expressing different synaptic markers and receive less perisomatic synapses, when compared with interneurons lacking this molecule. Moreover, they have reduced dendritic arborization and spine density. These data indicate that PSA-NCAM expression is important for the connectivity of interneurons in the adult cerebral cortex and that its regulation may play an important role in the structural plasticity of inhibitory networks.

Chronic antidepressant treatment induces contrasting patterns of synaptophysin and PSA-NCAM expression in different regions of the adult rat telencephalon.

Structural modifications occur in the brain of severely depressed patients and they can be reversed by antidepressant treatment. Some of these changes do not occur in the same direction in different regions, such as the medial prefrontal cortex, the hippocampus or the amygdala. Differential structural plasticity also occurs in animal models of depression and it is also prevented by antidepressants. In order to know whether chronic fluoxetine treatment induces differential neuronal structural plasticity in rats, we have analyzed the expression of synaptophysin, a protein considered a marker of synaptic density, and the expression of the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), a molecule involved in neurite and synaptic remodeling. Chronic fluoxetine treatment increases synaptophysin and PSA-NCAM expression in the medial prefrontal cortex and decreases them in the amygdala. The expression of these molecules is also affected in the entorhinal, the visual and the somatosensory cortices.

PSA-NCAM expression in the human prefrontal cortex.

The prefrontal cortex (PFC) of adult rodents is capable of undergoing neuronal remodeling and neuroimaging studies in humans have revealed that the structure of this region also appears affected in different psychiatric disorders. However, the cellular mechanisms underlying this plasticity are still unclear. The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) may mediate these structural changes through its anti-adhesive properties. PSA-NCAM participates in neurite outgrowth and synaptogenesis and changes in its expression occur parallel to neuronal remodeling in certain regions of the adult brain. PSA-NCAM is expressed in the hippocampus and temporal cortex of adult humans, but it has not been studied in the PFC. Employing immunohistochemistry on sections from the rostromedial superior frontal gyrus we have found that PSA-NCAM is expressed in the human PFC neuropil following a laminated pattern and in a subpopulation of mature neurons, which lack doublecortin expression. Most of these cells have been identified as interneurons expressing calbindin. The expression of PSA-NCAM in the human PFC is similar to that of rodents. Since this molecule has been linked to the neuronal remodeling found in experimental models of depression, it may also participate in the structural plasticity described in the PFC of depressed patients.

Chronic non-invasive glucocorticoid administration decreases polysialylated neural cell adhesion molecule expression in the adult rat dentate gyrus

The expression of the polysialylated neural cell adhesion molecule (PSA-NCAM) is increased in the hippocampus after chronic restraint stress (CRS) and may play a permissive role in structural changes that include dendrite reorganization in dentate gyrus (DG) and CA3 pyramidal neurons and suppression of neurogenesis in DG. We report that chronic oral corticosterone (CORT) administration decreases the number of PSA-NCAM immunoreactive granule neurons in the adult rat dentate gyrus, and the available evidence suggests that this is an indirect effect of CORT, possibly involving excitatory amino acids, that may not be directly related to neurogenesis. Because CORT treatment reduces but does not eliminate PSA-NCAM expression, the present results do not exclude a permissive role for PSA-NCAM in CORT or CRS-induced structural plasticity in hippocampus.

Effects of chronic fluoxetine treatment on the rat somatosensory cortex: activation and induction of neuronal structural plasticity.

Recent hypotheses support the idea that disruption of normal neuronal plasticity mechanisms underlies depression and other psychiatric disorders, and that antidepressant treatment may counteract these changes. In a previous report we found that chronic fluoxetine treatment increases the expression of the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), a molecule involved in neuronal structural plasticity, in the somatosensory cortex. In the present study we intended to find whether, in fact, cell activation and neuronal structural remodeling occur in parallel to changes in the expression of this molecule. Using immunohistochemistry, we found that chronic fluoxetine treatment caused an increase in the expression of the early expression gene c-fos. Golgi staining revealed that this treatment also increased spine density in the principal apical dendrite of pyramidal neurons. These results indicate that, apart from the medial prefrontal cortex or the hippocampus, other cortical regions can respond to chronic antidepressant treatment undergoing neuronal structural plasticity.

Differential evolution of PSA-NCAM expression during aging of the rat telencephalon

Changes in the ability of neuronal networks to undergo structural remodeling may be involved in the age-associated cognitive decline. The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) declines dramatically during postnatal development, but persists in several regions of the young-adult rat telencephalon, where it participates, through its anti-adhesive properties, in neuronal structural plasticity. However, PSA-NCAM expression during aging has only been studied in the dentate gyrus and the piriform cortex layer II, where it is strongly downregulated in adult (middle-aged) individuals. Using immunohistochemistry, we have observed that in most of the telencephalic areas studied the number of PSA-NCAM expressing cells and the intensity of PSA-NCAM expression in the neuropil remains stable during aging. Old rats only show decreases in the number of PSA-NCAM expressing cells in the lateral amygdala and retrosplenial cortex, and in neuropil expression of stratum lucidum. Given the role of PSA-NCAM in neuronal plasticity, the present results indicate that, even during aging, many regions of the CNS may display neurite, spine or synaptic remodeling.

Olfactory bulbectomy, but not odor conditioned aversion, induces the differentiation of immature neurons in the adult rat piriform cortex

The piriform cortex layer II of young-adult rats presents a population of prenatally generated cells, which express immature neuronal markers, such as the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) or doublecortin (DCX), and display structural characteristics of immature neurons. The number of PSA-NCAM/DCX expressing cells in this region decreases markedly as age progresses, suggesting that these cells differentiate or die. Since the piriform cortex receives a major input from the olfactory bulb and participates in olfactory information processing, it is possible that the immature neurons in layer II are affected by manipulations of the olfactory bulb or olfactory learning. It is not known whether these cells can be induced to differentiate and, if so, what would be their fate. In order to address these questions, we have performed unilateral olfactory bulbectomy (OBX) and an olfactory learning paradigm (taste-potentiated odor aversion, TPOA), in young-adult rats and have studied the expression of different mature and immature neuronal markers, as well as the presence of cell death. We have found that 14 h after OBX there was a dramatic decrease in the number of both PSA-NCAM and DCX expressing cells in piriform cortex layer II, whereas that of cells expressing NeuN, a mature neuronal marker, increased. By contrast, the number of cells expressing glutamate decarboxylase, isoform 67 (GAD67), a marker for interneurons, decreased slightly. Additionally, we have not found evidence of numbers of dying cells high enough to justify the disappearance of immature neurons. Analysis of animals subjected to TPOA revealed that this paradigm does not affect PSA-NCAM expressing cells. Our results strongly suggest that OBX can induce the maturation of immature neurons in the piriform cortex layer II and that these cells do not become interneurons. By contrast, these cells do not seem to play a crucial role in olfactory memory.