Comparison of projections of the dorsal and median raphe nuclei, with some functional considerations

It is well established that the brainstem contains discrete groups of serotonin-containing neurons with extensive axonal processes that distribute throughout the neuroaxis. Serotonergic neurons have been implicated in a range of functions including sleep/wakefulness, feeding, affect/emotion, thermoregulation, and cognitive behaviors. We describe the projections and some functional properties of the two major serotonergic cell groups of the brain, the dorsal raphe (DR) and the median raphe (MR) nuclei. DR fibers distribute widely throughout the forebrain to dopamine-containing nuclei of the ventral midbrain, the lateral hypothalamus, the midline thalamus, amygdala, the dorsal and ventral striatum and adjoining regions of the basal forebrain, and most of the cortex. By contrast with DR, MR is a midline/paramidline system of projections. Specifically, MR fibers mainly distribute to forebrain structures lying on or close to the midline including the medial mammillary and supramammillary nuclei, posterior and perifornical nuclei of the hypothalamus, midline and intralaminar nuclei of the thalamus, lateral habenula, medial zona incerta, diagonal band nuclei, septum and hippocampus. Overall, MR projections to the cortex are light. With few exceptions, DR and MR project to separate, non-overlapping regions of the forebrain — or, in effect, DR and MR share the serotonergic innervation of the forebrain. We discuss roles for DR in sleep/wake control, feeding/appetite and mood/affect, and for MR in the control of electroencephalographic (EEG) activity of the hippocampus — or states of hippocampal EEG desynchronization.