The serotonergic system is characterized by high centrality and is implicated in affective disorders and their treatment. During longitudinal antidepressant (fluoxetine) administration in mice, we optogenetically stimulate the ascending serotonergic system, and infer brain activity via fMRI. We resolve serotonergic excitability and functional connectivity at 5 time points and show that a network representation is needed to adequately model the data. For acute and chronic sessions we document significantly increased serotonergic excitability, and distinctly multivariate effects on serotonergic transmission. Finally, we find a full brain function recovery to baseline after fluoxetine withdrawal.