Alexandre Dayer

New GABAergic interneurons in the adult neocortex and striatum are generated from different precursors

Ongoing neurogenesis in the adult mammalian dentate gyrus and olfactory bulb is generally accepted, but its existence in other adult brain regions is highly controversial. We labeled newly born cells in adult rats with the S-phase marker bromodeoxyuridine (BrdU) and used neuronal markers to characterize new cells at different time points after cell division. In the neocortex and striatum, we found BrdU-labeled cells that expressed each of the eight neuronal markers. Their size as well as staining for γ-aminobutyric acid (GABA), glutamic acid decarboxylase 67, calretinin and/or calbindin, suggest that new neurons in both regions are GABAergic interneurons. BrdU and doublecortin-immunoreactive (BrdU+/DCX+) cells were seen within the striatum, suggesting migration of immature neurons from the subventricular zone. Surprisingly, no DCX+ cells were found within the neocortex. NG2 immunoreactivity in some new neocortical neurons suggested that they may instead be generated from the NG2+ precursors that reside within the cortex itself.

Regulation of adult neurogenesis by stress, sleep disruption, exercise and inflammation: Implications for depression and antidepressant action

Adult hippocampal neurogenesis, a once unorthodox concept, has changed into one of the most rapidly growing fields in neuroscience. The present report results from the ECNP targeted expert meeting in 2007 during which cellular plasticity changes were addressed in the adult brain, focusing on neurogenesis and apoptosis in hippocampus and frontal cortex. We discuss recent studies investigating factors that regulate neurogenesis with special emphasis on effects of stress, sleep disruption, exercise and inflammation, a group of seemingly unrelated factors that share at least two unifying properties, namely that they all regulate adult hippocampal neurogenesis and have all been implicated in the pathophysiology of mood disorders. We conclude that although neurogenesis has been implicated in cognitive function and is stimulated by antidepressant drugs, its functional impact and contribution to the etiology of depression remains unclear. A lasting reduction in neurogenesis following severe or chronic stress exposure, either in adult or early life, may represent impaired hippocampal plasticity and can contribute to the cognitive symptoms of depression, but is, by itself, unlikely to produce the full mood disorder. Normalization of reductions in neurogenesis appears at least partly, implicated in antidepressant action.

Volatile anesthetics rapidly increase dendritic spine density in the rat medial prefrontal cortex during synaptogenesis.

Background:Recent experimental observations suggest that, in addition to induce neuroapoptosis, anesthetics can also interfere with synaptogenesis during brain development. The aim of this study was to pursue this issue by evaluating the exposure time-dependent effects of volatile anesthetics on neuronal cytoarchitecture in 16-day-old rats, a developmental stage characterized by intense synaptogenesis in the cerebral cortex. Methods:Whistar rats underwent isoflurane (1.5%), sevoflurane (2.5%), or desflurane (7%) anesthesia for 30, 60, and 120 min at postnatal day 16, and the effect of these treatments on neuronal cytoarchitecture was evaluated 6 h after the initiation of anesthesia. Cell death was assessed using Fluoro-Jade B staining and terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick-end labeling assay. Ionotophoretic injections into layer 5 pyramidal neurons in the medial prefrontal cortex allowed visualization of dendritic arbor. Tracing of dendritic tree was carried out using the Neurolucida station (Microbrightfield, Williston, VT), whereas dendritic spines were analyzed using confocal microscopy. Results:Up to a 2-h-long exposure, none of the volatile drugs induced neuronal cell death or significant changes in gross dendritic arbor pattern of layer 5 pyramidal neurons in pups at postnatal day 16. In contrast, these drugs significantly increased dendritic spine density on dendritic shafts of these cells. Importantly, considerable differences were found between these three volatile agents in terms of exposure time-dependent effects on dendritic spine density. Conclusion:These new results suggest that volatile anesthetics, with different potencies and without inducing cell death, could rapidly interfere with physiologic patterns of synaptogenesis and thus might impair appropriate circuit assembly in the developing cerebral cortex.

Developmental Stage-dependent persistent impact of propofol anesthesia on dendritic spines in the rat medial prefrontal cortex

Background:Recent observations demonstrate that anesthetics rapidly impair synaptogenesis during neuronal circuitry development. Whether these effects are lasting and depend on the developmental stage at which these drugs are administered remains, however, to be explored. Methods:Wistar rats received propofol anesthesia at defined developmental stages during early postnatal life. The acute and long-term effects of these treatments on neuronal cytoarchitecture were evaluated by Neurolucida and confocal microscopy analysis after iontophoretic injections of Lucifer Yellow into layer 5 pyramidal neurons in the medial prefrontal cortex. Quantitative electron microscopy was applied to investigate synapse density. Results:Layer 5 pyramidal neurons of the medial prefrontal cortex displayed intense dendritic growth and spinogenesis during the first postnatal month. Exposure of rat pups to propofol at postnatal days 5 and 10 significantly decreased dendritic spine density, whereas this drug induced a significant increase in spine density when administered at postnatal days 15, 20, or 30. Quantitative electron microscopy revealed that the propofol-induced increase in spine density was accompanied by a significant increase in the number of synapses. Importantly, the propofol-induced modifications in dendritic spine densities persisted up to postnatal day 90. Conclusion:These new results demonstrate that propofol anesthesia can rapidly induce significant changes in dendritic spine density and that these effects are developmental stage-dependent, persist into adulthood, and are accompanied by alterations in synapse number. These data suggest that anesthesia in the early postnatal period might permanently impair circuit assembly in the developing brain.

Anesthetics rapidly promote synaptogenesis during a critical period of brain development

Experience-driven activity plays an essential role in the development of brain circuitry during critical periods of early postnatal life, a process that depends upon a dynamic balance between excitatory and inhibitory signals. Since general anesthetics are powerful pharmacological modulators of neuronal activity, an important question is whether and how these drugs can affect the development of synaptic networks. To address this issue, we examined here the impact of anesthetics on synapse growth and dynamics. We show that exposure of young rodents to anesthetics that either enhance GABAergic inhibition or block NMDA receptors rapidly induce a significant increase in dendritic spine density in the somatosensory cortex and hippocampus. This effect is developmentally regulated; it is transient but lasts for several days and is also reproduced by selective antagonists of excitatory receptors. Analyses of spine dynamics in hippocampal slice cultures reveals that this effect is mediated through an increased rate of protrusions formation, a better stabilization of newly formed spines, and leads to the formation of functional synapses. Altogether, these findings point to anesthesia as an important modulator of spine dynamics in the developing brain and suggest the existence of a homeostatic process regulating spine formation as a function of neural activity. Importantly, they also raise concern about the potential impact of these drugs on human practice, when applied during critical periods of development in infants.

Excess of serotonin affects embryonic interneuron migration through activation of the serotonin receptor 6

The discovery that a common polymorphism (5-HTTLPR, short variant) in the human serotonin transporter gene (SLC6A4) can influence personality traits and increase the risk for depression in adulthood has led to the hypothesis that a relative increase in the extracellular levels of serotonin (5-HT) during development could be critical for the establishment of brain circuits. Consistent with this idea, a large body of data demonstrate that 5-HT is a strong neurodevelopmental signal that can modulate a wide variety of cellular processes. In humans, serotonergic fibers appear in the developing cortex as early as the 10th gestational week, a period of intense neuronal migration. In this study we hypothesized that an excess of 5-HT could affect embryonic cortical interneuron migration. Using time-lapse videometry to monitor the migration of interneurons in embryonic mouse cortical slices, we discovered that the application of 5-HT decreased interneuron migration in a reversible and dose-dependent manner. We next found that 5-HT6 receptors were expressed in cortical interneurons and that 5-HT6 receptor activation decreased interneuron migration, whereas 5-HT6 receptor blockade prevented the migratory effects induced by 5-HT. Finally, we observed that interneurons were abnormally distributed in the cerebral cortex of serotonin transporter gene (Slc6a4) knockout mice that have high levels of extracellular 5-HT. These results shed new light on the neurodevelopmental alterations caused by an excess of 5-HT during the embryonic period and contribute to a better understanding of the cellular processes that could be modulated by genetically controlled differences in human 5-HT homeostasis.

Glial responses to neonatal hypoxic-ischemic injury in the rat cerebral cortex

Neurogenesis is nearly completed after birth, whereas gliogenic activities remain intense during the postnatal period in the developing rat cortex. These include involution of radial glia, proliferation of astrocytes and oligodendrocytes and myelin formation. Little is known about the effects of hypoxic–ischemic (HI) injury on these critical postnatal processes. Here we explored the glial reactions to mild HI injury of the neonatal rat cerebral cortex at P3. We show that the HI lesion results in disruption of the normal radial glia architecture, which was paralleled by an increase in GFAP immunopositive reactive astrocytes. The morphology of these latter cells and the fact that they were immunolabelled for both nestin and GFAP suggest an accelerated transformation of radial glia into astrocytes. In addition, BrdU/GFAP immunostaining revealed a significant increase of double‐labelled cells indicating an acute proliferation of astrocytes after HI. This enhanced proliferative activity of astrocytes persisted for several weeks. We found an elevated number and increased mitotic activity of both NG2‐positive oligodendrocyte progenitors and RIP‐positive oligodendrocytes after injury. These findings imply that glial responses are central to cortical tissue remodelling following neonatal ischemia and represent a potential target for therapeutic approaches.

Adverse effects of methylene blue on the central nervous system

Background:An increasing number of clinical observations suggest adverse neurologic outcome after methylene blue (MB) infusion in the setting of parathyroid surgery. Hence, the aim of the current study was to investigate the potentially neurotoxic effects of MB using a combination of in vivo and in vitro experimental approaches. Methods:Isoflurane-anesthetized adult rats were used to evaluate the impact of a single bolus intravascular administration of MB on systemic hemodynamic responses and on the minimum alveolar concentration (MAC) of isoflurane using the tail clamp test. In vivo, MB-induced cell death was evaluated 24 h after MB administration using Fluoro-Jade B staining and activated caspase-3 immunohistochemistry. In vitro, neurotoxic effects of MB were examined in hippocampal slice cultures by measuring excitatory field potentials as well as propidium iodide incorporation after MB exposure. The impact of MB on dendritic arbor was evaluated in differentiated single cell neuronal cultures. Results:Bolus injections of MB significantly reduced isoflurane MAC and initiated widespread neuronal apoptosis. Electrophysiologic recordings in hippocampal slices revealed a rapid suppression of evoked excitatory field potentials by MB, and this was associated with a dose-dependent effect of this drug on cell death. Dose–response experiments in single cell neuronal cultures revealed that a 2-h-long exposure to MB at non–cell-death-inducing concentrations could still induce significant retraction of dendritic arbor. Conclusions:These results suggest that MB exerts neurotoxic effects on the central nervous system and raise questions regarding the safety of using this drug at high doses during parathyroid gland surgery.

GABA Regulates Dendritic Growth by Stabilizing Lamellipodia in Newly Generated Interneurons of the Olfactory Bulb

The initial formation and growth of dendrites is a critical step leading to the integration of newly generated neurons into postnatal functional networks. However, the cellular mechanisms and extracellular signals regulating this process remain mostly unknown. By directly observing newborn neurons derived from the subventricular zone in culture as well as in olfactory bulb slices, we show that ambient GABA acting through GABAA receptors is essential for the temporal stability of lamellipodial protrusions in dendritic growth cones but did not interfere with filopodia dynamics. Furthermore, we provide direct evidence that ambient GABA is required for the proper initiation and elongation of dendrites by promoting the rapid stabilization of new dendritic segments after their extension. The effects of GABA on the initial formation of dendrites depend on depolarization and Ca2+ influx and are associated with a higher stability of microtubules. Together, our results indicate that ambient GABA is a key regulator of dendritic initiation in postnatally generated olfactory interneurons and offer a mechanism by which this neurotransmitter drives early dendritic growth.

New interneurons in the adult neocortex: small, sparse, but significant?

During the last decade, the intense study of adult hippocampal neurogenesis has led to several new lines of inquiry in the field of psychiatry. Although it is generally believed that adult mammalian neurogenesis is restricted to the hippocampus and olfactory bulb, a growing number of studies have described new neurons in the adult neocortex in both rodents and nonhuman primates. Interestingly, all of the new neurons observed in these studies have features of interneurons rather than pyramidal cells, the largest neuronal population of the neocortex. In this review, we discuss features of these interneurons that may explain why cortical neurogenesis has been so difficult to detect. In addition, these features suggest ways that production of even a small numbers of new neurons in the adult cortex could make a significant impact on neocortical function.