During the development of the brain a large number of regulatory signals are responsible for the migration of neurons and the subsequent formation of neuronal connections. One of these signals that has a modulatory function during neurodevelopment is serotonin. Throughout life, but mainly during development, environmental factors influence brain structure and function. It has been shown that when environmental factors affect serotonin levels during development, several changes in the brain and in particular in the cortex can be observed. The ontogeny of the cortex starts early in development and around birth, the six-layered structure of the cortex can be distinguished. It has been reported that in the postnatal brain reelin controls dendritic maturation of cortical pyramidal neurons upon serotonergic activation of the serotonin 5-HT 3 receptor on Cajal-Retzius cells. However, how environmental factors affect this regulatory pathway involved in cortical development and what the consequences are is to date unknown. In this dissertation, I have investigated the consequences of alterations in serotonergic signaling during neurodevelopment. I specifically focused on the effect of alterations in a serotonin 5-HT 3 receptor-mediated regulatory pathway involved in postnatal dendritic maturation of cortical pyramidal neurons.
In the first two chapters I focused on the question: How do environmental influences interfere with the ongoing development of the cortex? In chapter 2, we used the maternal care model to investigate the effect of early-life experience on cortical development. In this study, we found that differential maternal during the first week of life had a lifelong influence on dendritic maturation and spine density of cortical layer 2/3 pyramidal neurons. By recording sEPSC's we showed that as a consequence of differential maternal care, an increase or decrease in dendritic complexity resulted in a corresponding increase or decrease in amplitude of the recorded events. The observation that differential maternal care also influenced cortical reelin levels, indicated that differential maternal care affected cortical development via a reelin-dependent regulatory pathway.
In chapter 3, we investigated the effect of an increase in serotonin levels due to prenatal exposure of the SSRI fluoxetine on cortical development. In this study, we found that alterations in serotonin levels during neurodevelopment had a lifelong effect on cortical development and that prenatal fluoxetine exposure resulted in a hypocomplex dendritic tree of cortical layer 2/3 pyramidal neurons. More importantly, we showed that the effect of prenatal fluoxetine exposure on dendritic complexity and anxiety-related behavior was absent in 5-HT 3A receptor knockout mice. In addition, we showed that in organotypic brain slices of mice prenatally exposed to fluoxetine, the decrease in dendritic complexity could be reversed by treating these slices with a 5-HT 3 receptor antagonist. Together, these results indicate that the 5-HT 3 receptor plays a critical role in modulating the effects of alterations in serotonin levels on cortical development and anxiety-related behavior.
After showing that environmental influences induce lifelong changes in cortical development via a reelin-dependent pathway and that the 5-HT 3 receptor plays an essential role in modulating these effects, I focused on the question: Are these changes in cortical development also accompanied by alterations in the columnar organization of the cortex? In chapter 4, we investigated the organization of dendritic bundles in the somatosensory cortex of 5-HT 3A receptor knockout mice. By analyzing the spatial organization of MAP -2 immunostained apical dendrites in tangential sections from layer 3 of the somatosensory cortex, we compared dendritic bundle properties of 5-HT 3A receptor knockout mice with wildtype mice. In addition, we analyzed in both groups the distribution of reelin-rich Cajal-Retzius cells in tangential sections from the somatosensory cortex, to determine whether the distribution of these Cajal-Retzius cells was related to the position of dendritic bundles. In this study, we showed that in the somatosensory cortex of both groups apical dendrites are organized in dendritic bundles, but that the average surface of the dendritic bundles in 5-H 3A receptor knockout mice was larger than in wildtype mice. Furthermore, we showed that the distribution of reelin-rich Cajal-Retzius cells was random while the distribution of dendritic bundles was regular. Together with the previously observed differences in dendritic complexity of cortical layer 2/3 pyramidal neurons and cortical reelin levels, these findings suggest another important role for the 5-HT 3 receptor in cortical development.
Based on previous findings which showed that depleting the serotonergic innervation to the cortex in the brains of neonatal mice resulted in changes in cortical organization, but also in changes in social behavior, in chapter 5, I addressed the question: Do 5-HT 3A receptor knockout mice show impaired social behavior? In this study, we showed that although both male and female 5-HT 3A receptor knockout mice showed deficits in social communication, other deficits in social behavior were sex-specific. Together, these results suggest that, similar to other models in which serotonergic signaling is changed during neurodevelopment, 5-HT 3A receptor knockout mice show impaired social behavior.
In SUMMARY, by showing that alterations in serotonergic signaling during neurodevelopment result in lifelong changes in dendritic maturation and dendritic bundle organization in the cortex together with behavioral deficits via a 5-HT 3 receptor-dependent pathway, our results indicate that the serotonin 5-HT 3 receptor plays an essential role in modulating cortical development.