5HT and circadian gene and protein networks of the brain have extensive, reciprocal interactions that demonstrably impact human mental health. The master circadian pacemaker of the brain, the suprachiasmatic nucleus (SCN), receives direct serotonergic innervation and, in turn, makes polysynaptic output to the mid-brain serotonergic nuclei. At the gene network level, 5HT signaling genes such as SERT and SHTRs are expressed in the SCN, whereas circadian clock genes are expressed in serotonergic neurons of the raphe nuclei. In Project 6: Interactions of 5HT and Circadian Signaling Networks, Doug McMahon combines his lab's expertise with that of multiple Conte Center investigators to examine the effects of genetic variation in, and environmental manipulation of, the serotonergic system on the cellular and molecular properties of serotonergic neurons, SCN circuitry and a readily quantified behavior, circadian ocomotor rhythms. McMahon's overall hypothesis is that the serotonergic and circadian networks of the brain are interlocked through genetic, physiological and developmental mechanisms, and that perturbation in one system affects the function of the other, resulting in behavioral alterations associated with human affective disorders. He will test this hypothesis using mouse models with genetic alterations in specific signaling molecules of the serotonergic and circadian systems as well as epigenetic manipulations. Specifically, he will use electrophysiological, molecular, behavioral and real-time gene expression imaging endpoints to define the impact on: (/) 5HT neuron function of genetic manipulation of circadian and 5HT gene networks;(//) circadian function of genetic manipulation/variation in 5HT gene networks;(Hi) 5HT and circadian function of environmental manipulation of circadian and 5HT network development. The long-term goal of the proposed project is to gain an understanding of the specific mechanistic links between the serotonergic and circadian networks of the brain. Such an understanding will provide an expanded basis fo understanding the etiology, pathophysiology and therapeutic intervention into human mood disorders, in which dysregulation of circadian and serotonergic function are co-mingled. The experiments proposed in Project 6 will determine the functional consequences for 5HT neurons of altering the 5HT molecular signaling network, characterizing the impact of circadian gene network on 5HT function and defining the changes in the neural substrate for a defined 5HT-modulated behavior. Through these efforts, Project 6 will identify novel gene interactions to be further explored in the collaborating projects of the Conte Center, generating novel hypotheses regarding the molecular, cellular and behavioral outcomes of alterations in 5HT recepto and transporter genes and further elucidating the mechanisms involved in defining the 5HT neuron network and the influence of the 5HT network on its neural targets.