This project is aimed at investigating the genetic mechanisms that act across lifespan to regulate 5-HT system function and determine how these mechanisms impact postnatal 5-HT modulated behaviors. The basis of these studies is the discovery of a Pet-1 dependent transcriptional program that regulates 5-HT-modulated behaviors and newly developed 5-HT neuron-specific and temporally controlled conditional targeting approaches. Our approaches have enabled an investigation of 5-HT neuron-specific transcriptional mechanisms in purified 5-HT neurons and functional studies of serotonergic genes at any stage of life with reliable spatial and temporal control of gene ablation. Many questions remain unanswered about the mechanisms through which Pet-1 controls 5-HT neuron development and ultimately 5-HT modulated behaviors. For example, although our previous studies have shown that Pet-1 is needed for the coordinate induction of genes (Tph2, AADC, Sert, Vmat2, Htr1A, Htr1B, MaoB) directly responsible for 5-HT synthesis, reuptake, autoinhibition and metabolism it is not known whether Pet-1's function is restricted to these genes. It is also not known whether Pet-1 regulates different sets of genes in different populations of 5-HT neurons and whether it is solely an activator of serotonergic gene expression or whether it, perhaps, also simultaneously represses expression of some genes during the development of 5-HT neurons. Although we have shown that Pet-1 is needed in adult 5-HT neurons for maintenance of Tph2 and Sert the critical periods for Pet-1-directed transcription in essential 5-HT neuron functions such as 5-HT neuron firing and 5-HT modulated behaviors are poorly understood. We will investigate whether transcriptional regulation by Pet-1 is required for control of 5-HT signaling at different stages of life including the neonatal to adolescent period. The neonatal period (P5-P20) is a period in which 5-HT signaling has been shown to be critical for normal adult emotional behaviors while the adolescent period coincides with the onset of 5-HT related disorders such as schizophrenia and stress-related emotional disturbances. Finally, the importance of 5-HT synthesis in neonatal and adult 5-HT neurons for 5-HT modulated behaviors has never been adequately addressed because of the lack of suitable methods. We will use our new temporal conditional targeting approaches to test specific hypotheses about the early postnatal and adulthood requirements for 5-HT synthesis in the behaving mouse. Together, our new aims will investigate the downstream network of Pet-1 regulated genes, the requirements for intrinsic transcriptional mechanisms, and 5-HT synthesis at malleable developmental transition periods that are known to be critical periods for the determination of mental health relevant behaviors.