The locus coeruleus complex of the bottlenose dolphin (Tursiops truncatus) as revealed by tyrosine hydroxylase immunohistochemistry

Abstract

Using tyrosine hydroxylase immunohistochemistry we examined the structure of the pontine, or rostral rhombencephalic, catecholaminergic cells groups, which may be collectively termed the locus coeruleus complex (LC), in the bottlenose dolphin. The present study is the first to describe the LC in a cetacean species and, at 1.3 kg, represents the largest non-human brain to date in which the LC has been investigated. We identified four catecholaminergic cell groups in the dorsal pontine tegementum and peri-aqueductal gray matter: A6 dorsal (locus coeruleus), A6 ventral (locus coeruleus alpha), A7 (subcoeruleus), and A5 (fifth arcuate nucleus). No patterns of cellular distribution, nuclear subdivision, or cellular morphology indicate specialization of the LC, which might have been anticipated because of the large absolute brain size and unihemispheric sleep phenomenology of cetaceans.

Figure 1.

Diagrams of the cellular distribution of the tyrosine hydroxylase immunoreactive cells which make up the locus coeruleus complex of the bottlenose dolphin. Sections are approximately 1 mm apart and are shown from rostral (a) to caudal (i). Each dot represents a single immunoreactive cell. cg, central, or peri-aqueductal gray matter; D, dorsal; dec c.p., decussation of the cerebral peduncle or brachium conjunctivum; PTF, pontine tegemental field.

Figure 2.

High power photomicrographs of cells in three of the subdivisions described in the present study. (a) Tyrosine hydroxylase immunoreactive cells in the A6 dorsal subdivision. Note the high cellular density and extensive local plexus formed by the dendrites of these cells. (b) Cells of the A6 ventral subdivision. Note the lower cellular density and the similarly reduced density of the local dendritic plexus. (c) Cells of the A7 subdivision. Here the cellular density is lowest, however, this varies across this subdivision, with the periphery exhibiting the lowest cellular density. The density of the dendritic plexus is also significantly lower. Scale bar = 50 µm, applies to all plates.