Calciumbinding And Structural Proteins

There are many protein molecules that bind Ca2+ and whose distribution is instructive in brain delineations. The most frequently used are the vitamin D-dependent calcium-binding protein calbindin D-28K, parvalbu-min, S-100, and calretinin. The functional significance of these molecules remains unclear, and the main interest in them is derived from the large variability

Figure 1 Photomicrographs of a series of coronal sections stained for acetylcholinesterase (AChE) that reveals the other wise vague boundaries of the dorsomedial nucleus at three rostrocaudal levels of the human (A-C) and monkey (D-F) hypothalamus. Stereotaxic coordinates are indicated on the left side of the photomicrographs for the human. Note the area between DM and Pa labeled in B as Sub Pa in the human but absent in the monkey. Also note an AChE-negative compact DM prominent in the monkey (D) but not in the human (A). Scale bar=1 mm.

Figure 1 Photomicrographs of a series of coronal sections stained for acetylcholinesterase (AChE) that reveals the other wise vague boundaries of the dorsomedial nucleus at three rostrocaudal levels of the human (A-C) and monkey (D-F) hypothalamus. Stereotaxic coordinates are indicated on the left side of the photomicrographs for the human. Note the area between DM and Pa labeled in B as Sub Pa in the human but absent in the monkey. Also note an AChE-negative compact DM prominent in the monkey (D) but not in the human (A). Scale bar=1 mm.

Figure 2 Adjacent coronal sections of the human hypothalamus processed alternately with immuno/enzyme histochemistry for corticotropin-releasing factor (CRF), acetylcholinesterase (AChE), and tyrosin hydroxylase (TH) that reveal the subnuclei organization of the human Pa. In each photomicrograph the third ventricle is to the right. Subnuclear boundaries are indicated by a dashed line in the CRF immunoreacted section. Scale bar=0.5 mm for all photomicrographs.

Figure 2 Adjacent coronal sections of the human hypothalamus processed alternately with immuno/enzyme histochemistry for corticotropin-releasing factor (CRF), acetylcholinesterase (AChE), and tyrosin hydroxylase (TH) that reveal the subnuclei organization of the human Pa. In each photomicrograph the third ventricle is to the right. Subnuclear boundaries are indicated by a dashed line in the CRF immunoreacted section. Scale bar=0.5 mm for all photomicrographs.

with which these molecules are distributed in different brain structures. Thus, above all, calcium-binding proteins are chemical markers differentiating brain structures with high resolution and consistency across species.

In the brain, calcium-binding proteins were successfully used to reveal the organization of the human thalamus, striatum, and hypothalamus. In the thalamus and hypothalamus, the content and morphological distribution of calbindin D-28K, parvalbumin, and calretinin varied greatly and consistently between different areas and cell groups. For example, distributions of calbindin and calretinin complement each other in differentiating the shell and core regions of the accumbens nucleus, whereas parvalbumin is a distinct marker of the ventral pallidum. The distribution of calcium-binding proteins has been utilized in two ways; one related to the presence of the specific protein and another related to the structural boundaries this protein reveals. The first approach infers homology on the basis of similar chemical content of the putative homolog across species. For example, neurons of both the rat and the human medial preoptic nucleus (MPO), which is thought to be important for the regulation of sexual behavior, contain calbindin D-28K. The second approach infers homology on the basis of unambiguous borders as revealed by the distribution of these substances.

Among structural proteins, neuroanatomists often use the distribution of labeling by a monoclonal antibody directed against nonphosphorylated epitopes on neurofilament B protein [Sternberger Monoclonal Inc. 32 (SMI32)]. Immunohistochemical staining for SMI32 has been successfully used as an anatomical marker in the cortex, thalamus, and hypothalamus of the monkey and human. Hof suggested that the large size of SMI32-positive neurons found in the monkey and human cortex reflects the large distances to their neuronal targets. Recently, the pattern of SMI32 staining in the medial preoptic nucleus of the hypothalamus, an important regulatory center for sexual behavior, helped to establish a homology between the lateral MPO in the rat, monkey, and human hypothalamus (Fig. 3).

Occasionally, homologous nuclei in different species do not correspond fully in their chemical profile. For example, no one would dispute the homology of the basal nucleus of Meynert in the rat and human established on the basis of cyto- and chemoarchitec-ture. However, with respect to some substances, including calbindin D-28K, these cells have opposite profiles in the rat and human. However, the use of multiple markers, including calcium-binding proteins, affords greater confidence about the borders of this nucleus, revealing proportions and position that correspond to the homologous structure in the rat.

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