The distribution and morphological characteristics of catecholaminergic cells in the diencephalon and midbrain of the bottlenose dolphin (Tursiops truncatus)

Abstract

The present study describes the distribution and cellular morphology of catecholaminergic neurons in the diencephalon and midbrain of the bottlenose dolphin (Tursiops truncatus). Tyrosine hydroxylase immunohistochemistry was used to visualize these putatively dopaminergic neurons. The standard A1-A17, C1-C3, nomenclature is used for expediency; however, the neuroanatomical names of the various nuclei have also been given. Dolphins exhibit certain tyrosine hydroxylase immunoreactive (TH-ir) catecholaminergic neuronal groups in the midbrain (A8, A9, A10) and diencephalon (A11, A12, A14), however, no neuronal clusters clearly corresponding to the A13 and A15 groups could be identified. The subdivisions of these neuronal groups are in general agreement with those of other mammals, but there is a high degree of species specificity. First, three TH-ir neuronal groups not identified in other species were found: in the ventral lateral peri-aqueductal gray matter, posterior dorsal hypothalamus, and rostral mesencephalic raphe. Second, the normal components of the substantia nigra (A9 or pars compacta, A9 lateral or pars lateralis, A9 ventral or pars reticulata) were extremely cell sparse, but there was a substantial expansion of the A9 medial and A10 lateral subdivisions forming an impressive 'ventral wing' in the posterior substantia nigra. The findings of this and previous studies suggest a distinct evolutionary trend occurring in the neuromodulatory systems in mammals. The results are discussed in relation to motor control, thermoregulation, unihemispheric sleep, and dolphin cognition.

Fig. 1.

Drawings of the anatomy of the dolphin brain in mid-sagittal section (A) and the floor of the fourth ventricle (B). These figures are provided to allow orientation to the coronal sections of figure 2. As can be seen in A the hypothalamus is found ventral to the midbrain in the coronal plane due to maintenance of the cephalic flexure in the adult cetacean brain. These figures are redrawn from those of Breathnach [1960]. See list for abbreviations.

Fig. 2.

A series of coronal sections through the diencephalon and midbrain of the bottlenose dolphin showing the location of TH-immunopositive neurons found in the present study. A is the most rostral section, K the most caudal. Each subdivision of the catecholaminergic system has been drawn in a different color to allow ease of interpretation. The borders between cell groups, as defined by changes in color, were placed as close to the observed anatomical borders as possible. However, some of the borders between neuronal clusters were not as distinct as indicated here, thus the borders provided are meant more as a guide than as a stricture. Each colored dot equals one cell. The stippled regions indicate areas of high density TH-ir axonal staining and outline the medial forebrain bundle (A–C) and a bundle emanating from the locus coeruleus complex passing anteriorly to decussate across the posterior commissure (A–K). See list for abbreviations.

Fig. 2.

A series of coronal sections through the diencephalon and midbrain of the bottlenose dolphin showing the location of TH-immunopositive neurons found in the present study. A is the most rostral section, K the most caudal. Each subdivision of the catecholaminergic system has been drawn in a different color to allow ease of interpretation. The borders between cell groups, as defined by changes in color, were placed as close to the observed anatomical borders as possible. However, some of the borders between neuronal clusters were not as distinct as indicated here, thus the borders provided are meant more as a guide than as a stricture. Each colored dot equals one cell. The stippled regions indicate areas of high density TH-ir axonal staining and outline the medial forebrain bundle (A–C) and a bundle emanating from the locus coeruleus complex passing anteriorly to decussate across the posterior commissure (A–K). See list for abbreviations.

Fig. 3.

Photomicrographs of representative TH-ir neurons from the various subdivisions of the diencephalon and midbrain of the bottlenose dolphin. Both low power micrographs (A, C, E) which give an indication of density and high power micrographs (B, D, F) showing the cellular structure are provided for the A11 (A, B), A12 (C, D) and A14 (E, F) clusters that were observed. Note the large size of the A11 neuronal bodies (B) in comparison to those of the A12 (D) and A14 (F) neuronal bodies. Scale bar in E = 500 μm and applies to A, C and E; scale bar in F = 200 μm and applies to B, D and F.

Fig. 4.

Photomicrographs of representative TH-ir neurons from two subdivisions of the ventral tegmental region (A10 group) of the bottlenose dolphin and the ventral lateral periaqueductal gray matter cluster (VLpag). Both low-power micrographs (A, B, C) which give an indication of density and high-power micrographs (D, E, F) showing the cellular structure are provided for the A10 medial (A, E), A10 lateral (B, F) and VLpag (C, D) clusters that were observed. Note the similarity in size of the A10 medial and lateral neuronal bodies, but the larger size of the VLpag neuronal bodies. The density of neurons is lower in VLpag than the subdivisions of the A10 region. Scale bar in B = 500 μm and applies to A, B and C; scale bar in F = 200 μm and applies to D, E and F.

Fig. 5.

Photomicrographs of representative TH-ir neurons from the various subdivisions of the substantia nigra (A9 group) of the bottlenose dolphin. Both low-power micrographs (A, C, E, G) which give an indication of density and high-power micrographs (B, D, F, H) showing the cellular structure are provided for the A9 pars compacta (A, B), A9 ventral (C, D), A9 medial (E, F) and A9 lateral (G, H) clusters that were observed. Note the similarity in neuronal body size of all clusters, but the variations in density. In general it can be said that the normal subdivisions of the substantia nigra of the bottlenose dolphin are not well expressed. Scale bar in G = 500 μm and applies to A, C, E and G; scale bar in H = 200 μm and applies to B, D, F and H.

Fig. 6.

Photomicrographs of representative TH-ir neurons from the retrorubral region or A8 (A, B), and substantia nigra A9 ventral (pars reticulata) subdivision (C, D) of the bottlenose dolphin. Note the extremely low density and number of TH-ir neurons in the A8 region (A, B). The region of A9 ventral shown here exhibited striking striations (C, D) caused by the passage of the oculomotor nerve through this part of the substantia nigra (see also fig. 2F, G). Scale bar in C = 500 μm and applies to A, and C; scale bar in D = 200 μm and applies to B, and D.

Fig. 7.

Photomontage of the TH-ir neurons that make up the ventral wing of the midbrain catecholaminergic system of the bottlenose dolphin. Medial is to the left and dorsal is up. Note the relatively higher density of TH-ir neurons in this region as compared with the remainder of the substantia nigra (see figs. 5 and 6). Immediately below this ventral wing is the cerebral peduncle (cp) and the pons. Scale bar = 1 mm.

Fig. 8.

Photomicrographs of representative TH-ir neurons from two of the previously unidentified catecholaminergic groups of the bottlenose dolphin, the posterior dorsal hypothalamic cluster (A – low power and C – high power), and the rostral mesencephalic raphe cluster (B – low power and D – high power). Note the small size of these neurons in comparison to those of the substantia nigra and ventral tegmental area neurons (magnifications in figs. 4, 5 and 6 are the same as in this plate), giving them the appearance of dopaminergic interneurons, a supposition that is supported by the small size of the dendritic arborization. Scale bar in B = 500 μm and applies to A, and B; scale bar in D = 200 μm and applies to C, and D.