In the last year we have continued to investigate the genetic and molecular mechanisms underlying neuropsychiatric diseases. Our primary focus has been on the serotonergic system and we have continued to use the serotonin transporter (5-HTT) knockout mouse model developed in this laboratory as a model of disrupted serotonergic function. Previously we described a behavioral phenotype of this mouse as one displaying an increase in anxiety. Further behavioral characterization has shown that this mouse also exhibits robust lack of behavioral despair. Behavioral despair in animal models is believed to parallel depressive states reported in humans and drugs that alleviate depression in humans decrease behavioral despair in animal models. Thus these mice represent an animal model that reflects an absence of depression and are a useful model in which to study the molecular mechanism that may be acting to withstand or recover from depressive states. In particular, this animal may be useful in elucidating the effects of chronic selective serotonin transporter inhibitors (SSRIs) that underlie remission of depression. Serotonin function has been linked to impulsivity and aggression. There is an inverse relationship between serotonin function and aggression. The 5-HTT knockout mice show a decrease in aggression. We have discovered that although 5-HTT knockout mice show a gene-dose depletion of serotonin in whole tissues preparations, serotonin levels in the extracellular space are elevated reflecting the lack of transporters to remove serotonin from the extracellular space. Thus the lack of aggression and behavioral despair in the 5-HTT knockout mice may be a result of increased availability of serotonin in the extracellular space. As previously reported there is a decrease in 5-HT1A expression and function in 5-HTT knockout mice. We suggest that this downregulation of the 5-HT1A receptors is due to the excess 5-HT in the synapse. Furthermore we have discovered that in addition to 5-HT1A receptor alterations these mice also show altered expression of 5-HT2A/C receptors in a regionally specific manner. 5-HT2A/C receptors are increased in the amygdala, a brain region associated with aggression. The role of serotonin and its pre- and post- synaptic receptors along with associated intracellular signally pathways in the expression of anxiety, depression, and aggression are being investigated. We have also made progress in the study of the interaction of serotonin in neuroendocrine and other neurotransmitter functions. As stress can be a major factor in the development of depression and anxiety we studied the stress responses in 5-HTT knockout mice. 5-HTT knockout mice display a neuroendocrine profile that may parallel that produced by chronic stress. Although basal levels of plasma epinephrine are unchanged in 5-HTT knockout mice, there is an enhanced secretion of epinephrine in to the plasma with coincidental depletion in adrenal tissues as well as depletion in of pituitary ACTH. We are investigating the hypothesis that serotonin plays a critical role in the ability to adapt to stress. As stress often results in a heightened state of anxiety, which can be moderated by drugs that act on both the serotonin and GABA systems. We have begun investigating the interaction of serotonin and the GABA system. 5-HTT knockout mice display altered GABAergic function at both the behavioral and molecular levels. We are testing the hypothesis that this altered GABA function may reflect a lack of ability of these mice to adapt to stress as well as the hypothesis that excess extracelluar serotonin during development may alter the GABA system. We have also fine-tuned our surgical and HPLC procedures to allow us to correlate alterations of neurotransmitters and hormones in the brain and other tissues with that in plasma. As there are few peripheral measures of central serotonergic function, these techniques will allow us to describe possible correlations of neuroendocrine alterations in the central nervous system with that of the periphery. This will be critical in correlating basic science data collected in animals with that from human subjects. Serotonin may have a trophic effect on neuronal cells and SSRIs have been shown to increase the viability of certain neuronal cells. Our colleagues have shown that 5-HTT knockout mice are more resistant to oxidative stress. The 5-HTT knockout mice develop in the absence of serotonin transporter from embryonic day one and thus, represent a potent model in which study the developmental effects of chronic serotonin transporter blockade as well as extracellular serotonin excess. With this in mind, we are investigating the role of the serotonin transporter and certain neurotrophic factors such as BDNF in neuronal function, survival, and development. We have successfully developed a mouse neuronal stem cell line and are currently developing a neuronal stem cell line derived from 5-HTT knockout mice in order to further investigate the role of the serotonin transporter in the developing nervous system. Although anxiety is often, but not always, co-morbid with depression, we are interested in this model as it relates to an anxious but not depressed animal. Un-teasing this relationship may be advantageous in appropriate treatment for depression as well as anxiety. We are continuing to study the molecular mechanisms and relationship between serotonin in depression and anxiety as well as investigating the developmental effects of the lack of 5-HTT.
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