Cyto- and chemoarchitecture of the amygdala of a monotreme, Tachyglossus aculeatus (the short-beaked echidna)

Distribution and morphology of calbindin neurons in the Amygdaloid Complex of the marmoset monkey (callithrix jacchus)

The location and distribution of the calcium-binding protein calbindin-D28k (CB) has been considered to be of great value as a neuronal marker for identifying distinct brain regions and discrete neuronal populations. In the amygdaloid complex (AC), the balance of excitatory and inhibitory inputs is controlled by CB immunoreactive interneurons. Alterations of inhibitory mechanisms in the AC may play a role in the emotional symptomatology of neurological diseases like Alzheimer's and psychiatric disorders like posttraumatic stress disorder. The present investigation examined the distribution and morphology of CB-containing neurons, neuropils and fibers in marmoset monkey ACs by using immunohistochemical and morphometrical methods. We recognized four types of CB cells in the AC: type 1 (multipolar), type 2 (spherical or bipolar), type 3 (pyramidal) and type 4 (halo cells), a cell type specific to the marmoset located in the basal and central nuclei. We detected CB cells in all nuclei and areas of the AC, where most of the cells were present in the deep nuclei (lateral, basal, accessory basal and paralaminar). In the superficial nuclei (the nucleus of the lateral olfactory tract, medial nucleus, periamygdaloid cortex and cortical nuclei), the CB cells were abundant in layers 2 and 3. The intercalated nuclei contained small densely packed cells. The CB neuropils were particularly dense in layer 1 of the superficial nuclei, in the deep nuclei and in the amygdalohippocampal area. Large CB immunoreactive neurons in the white matter and fibers with varicosities were found in the myelin tracts that surrounded the AC. These findings are the first step in determining whether some of these cells are specifically disrupted in pathological states.

Quantitative analysis of forebrain pallial morphology in monotremes and comparison with that in therians

We have made quantitative volumetric analyses of cerebral cortical (pallial) structures in the brains of three species of monotreme (Ornithorhynchus anatinus, Tachyglossus aculeatus, Zaglossus bruijni) and compared the findings with similar measurements in a range of therian mammals (6 marsupials and 50 placentals). We have found that although the iso- and periallocortical grey matter volume of the monotremes is about what would be expected for their brain size, the proportion of iso- and periallocortical white matter in monotremes is substantially lower than that in the forebrains of therians. This suggests that the forebrains of the three monotremes have fewer association, commissural and/or projection connections than those of similarly sized forebrains of therian mammals. We also found that the iso- and periallocortex of the platypus is relatively smooth-surfaced compared to similarly sized brains of therian mammals, with a distinct caudal shift in the positioning of cortical white matter in the forebrain, consistent with expansion of the posterior thalamic radiation. Central laminated olfactory structures (anterior olfactory nucleus and piriform cortex) are large in the tachyglossid monotremes (Tachyglossus aculeatus and Zaglossus bruijni) and large in xenarthran placental mammals, suggesting convergence of the forebrain structure of monotreme formivores with that of similarly specialized therians like the xenarthrans Myrmecophaga tridactyla and Dasypus novemcinctus.

Regional chemoarchitecture of the brain of lungfishes based on calbindin D-28K and calretinin immunohistochemistry

Lungfishes are the closest living relatives of land vertebrates, and their neuroanatomical organization is particularly relevant for deducing the neural traits that have been conserved, modified, or lost with the transition from fishes to land vertebrates. The immunohistochemical localization of calbindin (CB) and calretinin (CR) provides a powerful method for discerning segregated neuronal populations, fiber tracts, and neuropils and is here applied to the brains of Neoceratodus and Protopterus, representing the two extant orders of lungfishes. The results showed abundant cells containing these proteins in pallial and subpallial telencephalic regions, with particular distinct distribution in the basal ganglia, amygdaloid complex, and septum. Similarly, the distribution of CB and CR containing cells supports the division of the hypothalamus of lungfishes into neuromeric regions, as in tetrapods. The dense concentrations of CB and CR positive cells and fibers highlight the extent of the thalamus. As in other vertebrates, the optic tectum is characterized by numerous CB positive cells and fibers and smaller numbers of CR cells. The so-called cerebellar nucleus contains abundant CB and CR cells with long ascending axons, which raises the possibility that it could be homologized to the secondary gustatory nucleus of other vertebrates. The corpus of the cerebellum is devoid of CB and CR and cells positive for both proteins are found in the cerebellar auricles and the octavolateralis nuclei. Comparison with other vertebrates reveals that lungfishes share most of their features of calcium binding protein distribution with amphibians, particularly with salamanders.

Regional distribution of calretinin and calbindin-D28k expression in the brain of the urodele amphibian Pleurodeles waltl during embryonic and larval development

The sequence of appearance of calretinin and calbindin-D28k immunoreactive (CRir and CBir, respectively) cells and fibers has been studied in the brain of the urodele amphibian Pleurodeles waltl. Embryonic, larval and juvenile stages were studied. The early expression and the dynamics of the distribution of CBir and CRir structures have been used as markers for developmental aspects of distinct neuronal populations, highlighting the accurate extent of many regions in the developing brain, not observed on the basis of cytoarchitecture alone. CR and, to a lesser extent, CB are expressed early in the central nervous system and show a progressively increasing expression from the embryonic stages throughout the larval life and, in general, the labeled structures in the developing brain retain their ability to express these proteins in the adult brain. The onset of CRir cells primarily served to follow the development of the olfactory bulbs, subpallium, thalamus, alar hypothalamus, mesencephalic tegmentum, and distinct cell populations in the rhombencephalic reticular formation. CBir cells highlighted the development of, among others, the pallidum, hypothalamus, dorsal habenula, midbrain tegmentum, cerebellum, and central gray of the rostral rhombencephalon. However, it was the relative and mostly segregated distribution of both proteins in distinct cell populations which evidenced the developing regionalization of the brain. The results have shown the usefulness in neuroanatomy of the analysis during development of the onset of CBir and CRir structures, but the comparison with previous data has shown extensive variability across vertebrate classes. Therefore, one should be cautious when comparing possible homologue structures across species only on the basis of the expression of these proteins, due to the variation of the content of calcium-binding proteins observed in well-established homologous regions in the brain of different vertebrates.

Nitric oxide synthase-containing neurons in the amygdaloid nuclear complex of the rat

The nitric oxide-producing neurons in the rat amygdala (Am) were studied, using reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry. Almost all nuclei of the Am contained NADPHd-positive neurons and fibers, but the somatodendritic morphology and the intensity of staining of different subpopulations varied. The strongly stained neurons displayed labeling of the perikaryon and the dendritic tree with Golgi impregnation-like quality, whilst the dendrites of the lightly stained neurons were less successfully followed. Many strongly positive neurons were located in the external capsule and within the intraamygdaloid fiber bundles. A large number of small, strongly stained cells was present in the amygdalostriatal transition area. In the Am proper, a condensation of deeply stained cells occurred in the lateral amygdaloid nucleus. In the basolateral nucleus, the strongly NADPHd-positive neurons were few, and were located mainly along the lateral border of the nucleus. These cells clearly differed from the large, pyramidal, and efferent cells. The basomedial nucleus contained numerous positive cells but most of them were only lightly labeled. A moderate number of strongly stained neurons appeared in the medial division of the central nucleus, and a larger accumulation of strongly positive cells was present in the lateral and the capsular divisions. The medial amygdaloid nucleus contained numerous moderately stained neurons and displayed the strongest diffuse neuropil staining in Am. In the nucleus of the lateral olfactory tract, the first layer contained only NADPHd-stained axons, in the second layer, there were numerous moderately stained cells, and in the third layer, a few but deeply stained neurons. From the cortical nuclei, the most appreciable number of stained neurons was seen in the anterior cortical nucleus. The anterior amygdaloid area contained numerous NADPHd-positive neurons; in its dorsal part the majority of cells were only moderately stained, whereas in the ventral part the neurons were very strongly stained. The intercalated amygdaloid nucleus lacked NADPHd-positive neurons but an appreciable plexus of fine, tortuous axons was present. In the intra-amygdaloid part of the bed nucleus of the stria terminalis (st) some lightly stained cells were seen but along the entire course of st strongly stained neurons were observed. Some Am nuclei, and especially the central lateral nucleus and the intercalated nucleus, display considerable species differences when compared with the primate Am. The age-related changes of the nitrergic Am neurons, as well as their involvement in neurodegenerative diseases is discussed.

The anterior olfactory nucleus and piriform cortex of the echidna and platypus

The cyto- and chemoarchitecture of the anterior olfactory nucleus and piriform cortex of the short-beaked echidna and platypus were studied to determine: (1) if these areas contain chemically distinct subdivisions, and (2) if the chemoarchitecture of those cortical olfactory regions differs from therians. Nissl and myelin staining were applied in conjunction with enzyme reactivity for NADPH diaphorase and acetylcholinesterase, and immunoreactivity for calcium-binding proteins (parvalbumin, calbindin and calretinin) and tyrosine hydroxylase. Golgi impregnations were also available for the echidna. In the echidna, the anterior olfactory nucleus is negligible in extent and merges at very rostral levels with a four-layered piriform cortex. Several rostrocaudally running subregions of the echidna piriform lobe could be identified on the basis of Nissl staining and calcium-binding protein immunoreactivity. Laminar-specific differences in calcium-binding protein immunoreactivity and NADPH-d-reactive neuron distribution were also noted. Neuron types identified in echidna piriform cortex included pyramidal neurons predominating in layers II and III and non-pyramidal neurons (e.g., multipolar profusely spiny and neurogliaform cells) in deeper layers. Horizontal cells were identified in both superficial and deep layers. By contrast, the platypus had a distinct anterior olfactory nucleus and a three-layered piriform cortex with no evidence of chemically distinct subregions within the piriform cortex. Volume of the paleocortex of the echidna was comparable to prosimians of similar body weight and, in absolute volume, exceeded that for eutherian insectivores such as T. ecaudatus and E. europaeus. The piriform cortex of the echidna shows evidence of regional differentiation, which in turn suggests highly specialized olfactory function.