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

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.

Keywords: beaked whales; hypothalamus; paraventricular nucleus; suprachiasmatic nucleus; supraoptic nucleus; toothed whales.

Figure 1

Gross anatomy of the hypothalamus and the adjoining structures. Cuvier’s beaked whale, transverse plane (a) and Atlantic spotted dolphin, midsagittal plane (b). Red rectangle in (b) indicates the hypothalamic region. a, Interthalamic adhesion; ac, anterior commissure; cc, corpus callosum; f, fornix; H, Hypothalamus; Mes, mesencephalon; Met, metencephalon; oc, optic chiasm; pc, posterior commissure; T, Thalamus; III, third ventricle; blue arrows, hypothalamic sulci.

Figure 2

Scheme of the basal surface of the BW brain (a). Sampling from the same brain shown in Figure 1a; Blainville’s beaked whale (b,c). Subdivisions of the hypothalamic regions (ventral view); the 4 regions of the hypothalamus are: preoptic (Pre), suprachiasmatic (Supra), tuberal (Tu), and mammillary (Mam). The blue arrows indicate the rudimentary mammillary bodies (b). Anterior view of figure (b) showing the supraoptic area with PVN and SON (c). Plots of the preoptic (d), supraoptic (e), and tuberal (f) left regions of the BWs brain (d–f). In (f) the purple area corresponds to the SCN. 2, optic nerve; 6, abducens nerve; 7, facial nerve; 8, vestibulocochlear nerve; OT, olfactory tubercle; ot, optic tract; P, pons; TL, temporal lobe.

Figure 3

Microphotographs of Nissl and immunolabeled brain section of the PVN (a–c, blue arrows), SONr (d,e,g–i, orange arrows), SONp (d–f, green arrows), and SCN (h,i, purple arrows). Blainville’s BW: (a,b,d–f,h,i). Cuvier’s BW: (c,g). Blainville’s BW is more represented due to the higher quality of its tissues and subsequently, the immunolabeling. (b,e), and (h) are Nissl stained. Higher magnification of PVN and SON. Magnocellular (green arrows) and parvocellular (red arrows) neurons of the PVN; the polygonal morphology of both populations and the larger size of the magnocellular population are observed (l). Magnocellular neurons of polygonal morphology of the SONr (m) and SONp (n) regions. The big cells of the magnocellular group, here observed in the PVN, but also in the SON, are morphologically comparable, with Nissl material dispersed peripherally and, sometimes, colloidal cytoplasmic inclusions associated with neurosecretion products (orange circles, o). Figures (l–o) were Nissl stained. Immunostaining of a PVN magnocellular neuron (p). Magnocellular “horizontal” neurons of SONr (q) and magnocellular “vertical” neurons of SONp (r). Histological figures between p and r were immunostained against ADH. Magnocellular neurons of polygonal morphology of the PVN and ependymal cells (purple arrows, s); Thionine staining. Mild neuronal immunoreactivity against CRF in the PVN; magnocellular (green arrows) and parvocellular (red arrows) neurons (t). Both SON and PVN are densely vascularized, and magnocellular neurons are often associated with blood vessels (orange arrow); ADH immunostaining (u).