The locus coeruleus of toothed whales: a microscopical description of the brain largest catecholaminergic nucleus

Abstract

The brain is still an undiscovered world; we are just starting to grasp the mysteries of neuroscience. Moreover, there is a great lack of information concerning neuroanatomy of Cetaceans. One of the main difficulties is to obtain fresh brain samples of such unique animals with so big brains. Locus coeruleus (LC) is a densely packed cluster of norepinephrine producer neurons, located in the upper part of the rostral rhombencephalon, near the floor of the fourth ventricle. It´s the largest catecholaminergic nucleus of the brain ad it supplies norepinephrine to the entire central nervous system. LC is involved in attention, behavioural activation and arousal and it is of great interest nowadays for its involvement and neuronal loss in Alzheimer's and Parkinson's diseases. Until today the only examined cetacean species for this nucleus was a Tursiops truncatus (Manger et al., 2003). Seven animals of six different species of the suborder Odontoceti (Mesoplodon densirostris, Globicephala macrorhynchus, Grampus griseus, Stenella coeruleoalba (n=2), Stenella frontalis and Delphinus delphis) were used for this study. Serial, rostro-caudal, 50 μm-thick coronal sections of the rostral rhombencephalon (from the most caudal level of the caudal colliculus to the rostral myelencephalon) were made using a sliding freezing microtome. Serial sections were later stained for Nissl substance with thionin. Immunoperoxidase staining was carried out on free-floating sections that were immunocytochemically stained for Tyrosine Hydroxylase (TH) and Corticotropin Releasing Factor. LC extended from the caudal level of the motor nucleus of the trochlear nerve to the rostral level of the motor nucleus on the trigeminal nerve. TH-immunopositive neurons were mainly three to four sided large polygons with three to four primary dendrites. This is the first description of LC in these species and we are interested in a future study of its involvement in acute stress response in live stranded Cetaceans.