Orexinergic bouton density is lower in the cerebral cortex of cetaceans compared to artiodactyls

The species of the cetacean and artiodactyl suborders, which constitute the order Cetartiodactyla, exhibit very different sleep phenomenology, with artiodactyls showing typical bihemispheric slow wave and REM sleep, while cetaceans show unihemispheric slow wave sleep and appear to lack REM sleep. The aim of this study was to determine whether cetaceans and artiodactyls have differently organized orexinergic arousal systems by examining the density of orexinergic innervation to the cerebral cortex, as this projection will be involved in various aspects of cortical arousal. This study provides a comparison of orexinergic bouton density in the cerebral cortex of twelve Cetartiodactyla species (ten artiodactyls and two cetaceans) by means of immunohistochemical staining and stereological analysis. It was found that the morphology of the axonal projections of the orexinergic system to the cerebral cortex was similar across all species, as the presence, size and proportion of large and small orexinergic boutons were similar. Despite this, orexinergic bouton density was lower in the cerebral cortex of the cetaceans studied compared to the artiodactyls studied, even when corrected for brain mass, neuron density, glial density and glial:neuron ratio. Results from correlational and principal component analyses indicate that glial density is a major determinant of the observed differences between artiodactyl and cetacean cortical orexinergic bouton density.