The Complexly Parcellated, Yet Quantitatively Reduced, Orexinergic/Hypocretinergic System of Humans

Area of Science:

• Neuroscience
• Comparative Anatomy
• Primatology

Background:

• Understanding human neuroanatomy requires phylogenetic context, especially for therapeutic interventions based on animal models.
• Orexinergic (hypocretinergic) neurons play vital roles in regulating various physiological functions.

Purpose of the Study:

• To determine the parcellation and quantify orexinergic neurons in the hypothalami of humans and various primate species.
• To compare the organization and number of orexinergic neurons across different primate lineages.

Main Methods:

• Examined hypothalami from humans and multiple primate species (strepsirrhines, platyrrhines, cercopithecoids, hominoids).
• Mapped and quantified orexinergic neuron clusters within the hypothalamus.
• Correlated neuron numbers with brain mass across species.

Main Results:

• Non-human primates exhibited three distinct orexinergic neuron clusters, similar to most mammals.
• Hominoids displayed an additional cluster (optic tract cluster extension).
• Humans uniquely possessed a parvocellular cluster in the dorsomedial hypothalamus, resulting in the most complex parcellation.
• Orexinergic neuron number in non-human primates strongly correlated with brain mass.
• Humans have significantly fewer orexinergic neurons (approx. 74,300) than predicted for their brain mass.

Conclusions:

• The human orexinergic system exhibits the most complex parcellation among studied primates, suggesting potential functional specializations.
• Quantitatively, the human orexinergic system is paradoxical, with a lower neuronal count than expected for its brain size.