Photoperiod Programs Dorsal Raphe Serotonergic Neurons and Affective Behaviors

The serotonergic raphe nuclei of the midbrain are principal centers from which serotonin neurons project to innervate cortical and sub-cortical structures. The dorsal raphe nuclei receive light input from the circadian visual system [1] and indirect input from the biological clock nuclei [2, 3]. Dysregulation of serotonin neurotransmission is implicated in neurobehavioral disorders, such as depression and anxiety [4], and alterations in the serotonergic phenotype of raphe neurons have dramatic effects on affective behaviors in rodents [5]. Here, we demonstrate that day length (photoperiod) during development induces enduring changes in mouse dorsal raphe serotonin neurons—programming their firing rate, responsiveness to noradrenergic stimulation, intrinsic electrical properties, serotonin and norepinephrine content in the midbrain, and depression/anxiety-related behavior in a melatonin receptor 1 (MT1)-dependent manner. Our results establish mechanisms by which seasonal photoperiods may dramatically and persistently alter the function of serotonin neurons. •Summer-like long photoperiods increase serotonin neuron excitability and firing•Long photoperiods increase serotonin and norepinephrine levels in the midbrain•Long photoperiods during development induce lasting increases in firing rate•Knockout of the MT1 receptor negates the observed photoperiodic changes Day length, or photoperiod, influences depression and anxiety behaviors, but the neurobiological mechanisms are unknown. Here, Green et al. show that photoperiod can have enduring effects on the firing rate of mouse brain serotonin neurons, on monoamine content in the midbrain, and on affective behaviors that depend on melatonin signaling.