Scientists mapped where a peptide called delta sleep‑inducing peptide (DSIP) is found in rat brains. It shows up in many brain regions linked to sleep, arousal, movement, and sensory processing, but the exact role of DSIP is still unknown.

Delta sleep-inducing peptide has been found in the peripheral circulation of animals entering slow-wave, or delta, sleep. An antiserum to this peptide was used to localize immunoreactive-like delta sleep-inducing peptide in the rat brain. The peptide was shown to have a rather widespread distribution. In the forebrain, the majority of these neurons were found to extend in a continuous rostral-caudal band in the ventral one-third of the brain from the primary olfactory cortex to the lateral hypothalamus. Neurons were also present in the basal ganglia, amygdala, septum, and thalamus. In the brainstem, the neurons were widespread and associated with the reticular formation, raphe nuclei, nuclei of the trigeminal complex, several auditory nuclei--nuclei of the lateral lemniscus, cochlear nuclei, and inferior colliculus--, cerebellum, locus ceruleus, periventricular gray, and vagal and hypoglossal nuclei. Immunoreactive fibers were, in general, difficult to demonstrate; they were seen mainly in the vicinity of the third ventricle and near blood vessels. The function of delta sleep-inducing peptide is unknown and its role in sleep is still under investigation. The distribution of delta sleep-inducing peptide in the present study suggests that the peptide is a component of several systems--arousal, locomotion, auditory, visual and sensory--both somatic and vestibular. The widespread distribution of the peptide, the lack of demonstrable immunoreactive fiber tracts, and the presence of these neurons in areas known to contain aminergic and peptidergic neurons, raises the possibility that neurons containing delta sleep-inducing peptide may exert their effect by projecting directly into blood vessels and/or interacting with neurons in their immediate vicinity.