Organization of the Sleep-Related Neural Systems in the Brain of the Harbour Porpoise (Phocoena phocoena)

The present study provides the first systematic immu-nohistochemical neuroanatomical investigation of thesystems involved in the control and regulation of sleepin an odontocete cetacean, the harbor porpoise ( Pho-coena phocoena). The odontocete cetaceans show anunusual form of mammalian sleep, with unihemisphericslow waves, suppressed REM sleep, and continuousbodily movement. All the neural elements involved insleep regulation and control found in bihemisphericsleeping mammals were present in the harbor porpoise,with no specific nuclei being absent, and no novelnuclei being present. This qualitative similarity ofnuclear organization relates to the cholinergic, norad-renergic, serotonergic, and orexinergic systems and isextended to the g-aminobutyric acid (GABA)ergic ele-ments involved with these nuclei. Quantitative analysisof the cholinergic and noradrenergic nuclei of the pon-tine region revealed that in comparison with othermammals, the numbers of pontine cholinergic(126,776) and noradrenergic (122,878) neurons aremarkedly higher than in other large-brained bihemi-spheric sleeping mammals. The diminutive telencephaliccommissures (anterior commissure, corpus callosum,and hippocampal commissure) along with an enlargedposterior commissure and supernumerary pontine cho-linergic and noradrenergic neurons indicate that thecontrol of unihemispheric slow-wave sleep is likely tobe a function of interpontine competition, facilitatedthrough the posterior commissure, in response to uni-lateral telencephalic input related to the drive for sleep.In addition, an expanded peripheral division of the dor-sal raphe nuclear complex appears likely to play a rolein the suppression of REM sleep in odontocete ceta-ceans. Thus, the current study provides several clues tothe understanding of the neural control of the unusualsleep phenomenology present in odontocete cetaceans.