Abstract:
The species of the cetacean and artiodactyl suborders, which make up the
cetartiodactyl order, have very different arousal thresholds and sleep-wake systems.
The aim of this study was to determine whether cetaceans or artiodactyls have
differently organized orexinergic arousal systems by examining the density of
orexinergic innervation to the cerebral cortex. This study provides a comparison of
orexinergic bouton density in the cerebral cortex of twelve cetartiodactyl species by
means of immunohistochemical staining and stereological analysis. It was observed
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 cetaceans compared to artiodactyls, even when
corrected for brain mass, neuron density, glial density and glial: neuron ratio. Glial
density was identified as the major determinant for the observed differences. It appears
a synergy exists between the orexinergic neurons and their projections, glial cells, and
the biochemical correlates of appetitive drive and arousal, but further studies need to
be performed to understand the full extent of the orexinergic system and its role in
sustained arousal.
