The Hypothalamus of the Beaked Whales: The Paraventricular, Supraoptic, and Suprachiasmatic Nuclei

SIMPLE SUMMARY: Beaked whales are singular and unconventional marine mammals, living in deep offshore waters. There is a scarce, almost absent, number of neuroanatomical studies on these toothed whales. The hypothalamus is a small brain region and it serves as the primary connection between the nervous and endocrine systems. This region is responsible for maintaining the body in a steady state of equilibrium known as homeostasis. The hypothalamic paraventricular and supraoptic nuclei of the Cuvier’s beaked whale and Blainville’s beaked whale are characterized here. A hypothalamic suprachiasmatic nucleus, the central biological clock, is also described for the first time in the two animals. The paraventricular nucleus occupied the preoptic region and the anterior or suprachiasmatic regions. The supraoptic nucleus was located in the preoptic, supraoptic, and tuberal regions. The suprachiasmatic nucleus was located in the ventromedial extremity of the tuberal hypothalamic region, occupying the median eminence of the hypothalamus. This study adds new insights and sets the stage for future investigations into the brains of beaked whales. ABSTRACT: The hypothalamus is the body’s control coordinating center. It is responsible for maintaining the body’s homeostasis by directly influencing the autonomic nervous system or managing hormones. Beaked whales are the longest divers among cetaceans and their brains are rarely available for study. Complete hypothalamic samples from a female Cuvier’s beaked whale and a male Blainville’s beaked whale were processed to investigate the paraventricular (PVN) and supraoptic (SON) nuclei, using immunohistochemical staining against vasopressin. The PVN occupied the preoptic region, where it reached its maximum size, and then regressed in the anterior or suprachiasmatic region. The SON was located from the preoptic to the tuberal hypothalamic region, encompassing the optical structures. It was composed of a retrochiasmatic region (SONr), which bordered and infiltrated the optic tracts, and a principal region (SONp), positioned more medially and dorsally. A third vasopressin-positive nucleus was also detected, i.e., the suprachiasmatic nucleus (SCN), which marked the end of the SON. This is the first description of the aforementioned nuclei in beaked whales—and in any marine mammals—as well as their rostro-caudal extent and immunoreactivity. Moreover, the SCN has been recognized for the first time in any marine mammal species.