Nuclear parcellation and numbers of orexinergic neurons in six species of birds.

The orexinergic/hypocretinergic system appears to be a key promoter of arousal and food intake. Studies of this system in birds have revealed an organizational invariance of the two hypothalamic clusters forming this system, with a strong allometric relationship between orexinergic neuronal numbers and brain mass. Using orexin-A immunohistochemistry, we describe the distribution, morphology and nuclear parcellation of orexinergic neurons within the hypothalami of a further five bird species (helmeted guinea fowl, domestic duck, domestic goose, inca tern, and Caribbean flamingo) where this system had not been previously investigated and iterates the study of this system in the domestic chicken. These birds broaden the phylogenetic studies of this system in birds, with brains ranging in size from 2.8 to 12 g. As in previous studies, the orexinergic neurons were organized in two clusters, a densely packed paraventricular hypothalamic nucleus cluster located within the medial hypothalamus and a more loosely packed lateral hypothalamic cluster in the lateral hypothalamus. In the Caribbean flamingo, orexin-A immunopositive neurons were observed in the medial eminence. Stereological analysis revealed a strong correlation between brain mass and the total number of orexinergic neurons. Thus, unlike mammals, where variances in orexinergic cluster organization have been often noted, in the birds studied to date an organizational consistency is observed, to the exception of one variance noted in the Caribbean flamingo, despite the differences in brain and body mass, phylogenetic relationships, and life histories of the species studied.