Cyto- and chemoarchitecture of the dorsal thalamus of the monotreme Tachyglossus aculeatus, the short beaked echidna

We have examined the cyto- and chemoarchitecture of the dorsal thalamus of the short beaked echidna (Tachyglossus aculeatus), using Nissl and myelin staining, immunoreactivity for parvalbumin, calbindin, calretinin and non-phosphorylated neurofilament protein (SMI-32 antibody), and histochemistry for acetylcholinesterase and NADPH diaphorase. Immunohistochemical methods revealed many nuclear boundaries, which were difficult to discern with Nissl staining. Parvalbumin immunoreactive somata were concentrated in the ventral posterior, reticular, posterior, lateral and medial geniculate nuclei, while parvalbumin immunoreactivity of the neuropil was present throughout all but the midline nuclei. Large numbers of calbindin immunoreactive somata were also found within the midline thalamic nuclei, and thalamic sensory relay nuclei. Immunoreactivity for calretinin was found in many small somata within the lateral geniculate “a” nucleus, with other labelled somata found in the lateral geniculate “b” nucleus, ventral posterior medial and ventral posterior lateral nuclei. Immunoreactivity with the SMI-32 antibody was largely confined to somata and neuropil within the thalamocortical relay nuclei (ventral posterior medial and lateral nuclei, lateral and medial geniculate nuclei and the posterior thalamic nucleus). In broad terms there were many similarities between the thalamus of this monotreme and that of eutheria (e.g. disposition of somatosensory thalamus, complementarity of parvalbumin and calbindin immunoreactive structures), but there were some unique features of the thalamus of the echidna. These include the relatively small size of the thalamic reticular nucleus and the preponderance of calbindin immunoreactive neurons over parvalbumin immunoreactive neurons in the ventral posterior nucleus.     

The distribution of calbindin, calretinin and parvalbumin immunoreactivity in the human thalamus

Calcium-binding proteins show a heterogeneous distribution in the mammalian central nervous system and are useful markers for identifying neuronal populations. The distribution of the three major calcium-binding proteins - calbindin-D28k (calbindin), calretinin and parvalbumin - has been investigated in eight neurologically normal human thalami using standard immunohistochemical techniques. Most thalamic nuclei show immunoreactive cell bodies for at least two of the three calcium-binding proteins; the only nucleus showing immunoreactivity for one calcium-binding protein is the centre médian nucleus (CM) which is parvalbumin-positive. Overall, the calcium-binding proteins show a complementary staining pattern in the human thalamus. In general terms, the highest density of parvalbumin staining is in the component nuclei of the ventral nuclear group (i.e. in the ventral anterior, ventral lateral and ventral posterior nuclear complexes) and in the medial and lateral geniculate nuclear groups. Moderate densities of parvalbumin staining are also present in regions of the mediodorsal nucleus (MD). By contrast, calbindin and calretinin immunoreactivity both show a similar distribution of dense staining in the thalamus which appears to complement the pattern of intense parvalbumin staining. That is, calbindin and calretinin staining is most dense in the rostral intralaminar nuclear group and in the patchy regions of the MD which show very low levels of parvalbumin staining. However, calbindin and calretinin also show low levels of staining in the ventral nuclear complex and in the medial and lateral geniculate bodies which overlaps with the intense parvalbumin staining in these regions. These results show that the calcium-binding proteins are heterogeneously distributed in a complementary fashion within the nuclei of the human thalamus. They provide further support for the concept recently proposed by Jones (Jones, E.G., 1998. Viewpoint: the core and matrix of thalamic organization. Neuroscience 85, 331-345) that the primate thalamus comprises of a matrix of calbindin immunoreactive cells and a superimposed core of parvalbumin immunoreactive cells which may have differential patterns of cortical projections.     

Calretinin immunoreactivity in the thalamus of the squirrel monkey

The distribution of the calcium-binding protein, calretinin, in the thalamus of the squirrel monkey (Saimiri sciureus) was studied with immunohistochemical methods. Calretinin was found to be heterogeneously distributed in the primate thalamus and to occur only in specific neuronal populations of certain thalamic nuclei. Neuronal cells and fibers in midline nuclei and their dorsolateral extension, which includes the parataenial and central superior lateral nuclei, displayed the most intense calretinin immunoreactivity. The immunoreactivity for cells and fibers in the intralaminar nuclei was moderate rostrally but very weak caudally. The centre médian nucleus, together with the medial habenular nucleus, were virtually devoid of calretinin immunostaining. The mediodorsal nucleus displayed a markedly heterogeneous staining, with numerous clusters of labeled cells and fibers in its central parvicellular part. Cell and fiber immunoreactivity ranged from moderate to high in the nuclei of the anterior and lateral groups, but was very weak in the nuclei of the ventral and posterior groups. There was a small to moderate number of heterogeneously distributed calretinin-immunoreactive cells and fibers in the lateral and medial geniculate bodies, as well as in the reticular nucleus. The present study provides the first evidence for the existence of calretinin in primate thalamus, where this protein is distributed according to a highly heterogeneous pattern. This specific pattern of distribution suggests that calretinin may play a role that is complementary to those of the other calcium-binding proteins parvalbumin and calbindin D-28k in the thalamus of primates.     

Cyto- and chemoarchitecture of the sensory trigeminal nuclei of the echidna, platypus and rat

We have examined the cyto- and chemoarchitecture of the trigeminal nuclei of two monotremes using Nissl staining, enzyme reactivity for cytochrome oxidase, immunoreactivity for calcium binding proteins and non-phosphorylated neurofilament (SMI-32 antibody) and lectin histochemistry (Griffonia simplicifolia isolectin B4). The principal trigeminal nucleus and the oralis and interpolaris spinal trigeminal nuclei were substantially larger in the platypus than in either the echidna or rat, but the caudalis subnucleus was similar in size in both monotremes and the rat. The numerical density of Nissl stained neurons was higher in the principal, oralis and interpolaris nuclei of the platypus relative to the echidna, but similar to that in the rat. Neuropil immunoreactivity for parvalbumin was particularly intense in the principal trigeminal, oralis and interpolaris subnuclei of the platypus, but the numerical density of parvalbumin immunoreactive neurons was not particularly high in these nuclei of the platypus. Neuropil immunoreactivity for calbindin and calretinin was relatively weak in both monotremes, although calretinin immunoreactive somata made up a large proportion of neurons in the principal, oralis and interpolaris subnuclei of the echidna. Distribution of calretinin immunoreactivity and Griffonia simplicifolia B4 isolectin reactivity suggested that the caudalis subnucleus of the echidna does not have a clearly defined gelatinosus region. Our findings indicate that the trigeminal nuclei of the echidna do not appear to be highly specialized, but that the principal, oralis and interpolaris subnuclei of the platypus trigeminal complex are highly differentiated, presumably for processing of tactile and electrosensory information from the bill.     

Calretinin distribution in the thalamus of the rat: immunohistochemical and in situ hybridization histochemical analyses.

The distribution of calretinin-containing cells was examined by in situ hybridization histochemistry and compared with the immunohistochemical mapping of calretinin in the thalamus of the rat. Results revealed a close correspondence between the immunohistochemical localization of cell bodies and the messenger RNA label produced by the calretinin oligonucleotide probe. Calretinin cells were most prominent in the midline (paraventricular, reuniens, rhomboid) and intralaminar (central medial, paracentral) nuclei and in a group of cells along the rostral central gray which appeared continuous with the caudal extent of the midline nuclei. A subpopulation of calretinin cell bodies was also identified in the reticular nucleus. The mediorostral lateral posterior nucleus, subparafascicular, lateral geniculate and habenular nuclei also contained calretinin messenger RNA probe label. In contrast, no positive cells were found in the anterior, ventral or posterior thalamic nuclei. The distribution of calretinin cells did not correspond directly with that of other histochemical markers. Thus, the in situ hybridization histochemical and immunohistochemical results revealed calretinin as a unique identifying marker for distinct sets of thalamic neurons.     

Precerebellar and vestibular nuclei of the short-beaked echidna (<Emphasis Type="Italic">Tachyglossus aculeatus</Emphasis>)

The monotremes are a unique group of living mammals, which diverged from the line leading to placental mammals at least 125 million years ago. We have examined the organization of pontine, inferior olivary, lateral reticular and vestibular nuclei in the brainstem of the short-beaked echidna (Tachyglossus aculeatus) to determine if the cyto- and chemoarchitecture of these nuclei are similar to that in placental mammals and marsupials. We have used Nissl staining in conjunction with enzyme-histochemistry for acetylcholinesterase, cytochrome oxidase and NADPH diaphorase as well as immunohistochemistry for non-phosphorylated neurofilament protein (SMI-32 antibody) and calcium binding proteins (parvalbumin, calbindin, calretinin). Homologies could be established between the arch shaped inferior olivary complex of the echidna and the principal, dorsal and medial accessory subdivisions of the therian inferior olivary complex. The pontine nuclei of the echidna included basilar and reticulotegmental components with similar cyto- and chemarchitectural features to therians and there were magnocellular and subtrigeminal components of the lateral reticular nucleus, also as seen in therians. Subdivisions and chemoarchitecture of the vestibular complex of the echidna were both similar to that region in rodents. In all three precerebellar nuclear groups studied and in the vestibular nucleus organization, the cyto- and chemoarchitecture of the echidna was very similar to that seen in therian mammals and no "primitive" or "reptilian" features were evident.     

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

We have examined the cyto- and chemoarchitecture of the temporal and extended amygdala in the brain of a monotreme (the short-beaked echidna Tachyglossus aculeatus) using Nissl and myelin staining, enzyme histochemistry for acetylcholine esterase and NADPH diaphorase, immunohistochemistry for calcium binding proteins (parvalbumin, calbindin and calretinin) and tyrosine hydroxylase. While the broad subdivisions of the eutherian temporal amygdala were present in the echidna brain, there were some noticeable differences. No immunoreactivity for parvalbumin or calretinin for somata was found in the temporal amygdala of the echidna. The nucleus of the lateral olfactory tract could not be definitively identified and the medial nucleus of amygdala appeared to be very small in the echidna. Calbindin immunoreactive neurons were most frequently found in the ventrolateral part of the lateral nucleus, intraamygdaloid parts of the bed nucleus of the stria terminalis and the lateral part of the central nucleus. Neurons strongly reactive for NADPH diaphorase with filling of the dendritic tree were found mainly scattered through the cortical, central and lateral subnuclei, while neurons showing only somata reactivity for NADPH diaphorase were concentrated in the basomedial and basolateral subnuclei. Most of the components of the extended amygdala of eutherians could also be identified in the echidna. Volumetric analysis indicated that the temporal amygdala in both the platypus and echidna is small compared to the same structure in both insectivores and primates, with the central and medial components of the temporal amygdala being particularly small.     

Calcium-binding proteins as markers of layer-I projecting vs. deep layer-projecting thalamocortical neurons: a double-labeling analysis in the rat

The thalamus contains two main populations of projection neurons that selectively innervate different elements of the cortical microcircuit: the well-known "specific" or "core" (C-type) cells that innervate cortical layer IV, and, the "matrix" (M-type) cells that innervate layer I. Observations in different mammal species suggest that this may be a conserved, basic organizational principle of thalamocortical networks. Fragmentary observations in primate sensory nuclei suggest that M-type and C-type cells might be distinguished by their selective expression of calcium binding-proteins. In adult rats, we tested this proposal in a systematic manner throughout the thalamus. Applying Fast-Blue (FB) to a large swath of the pial surface in the lateral aspect of the cerebral hemisphere we labeled a large part of the M-type cell populations in the thalamus and subsequently examined FB co-localization with calbindin or parvalbumin immunoreactivity in thalamic neuron somata. FB-labeled cells were present in large numbers in the ventromedial, interanteromedial, posterior, lateral posterior and medial geniculate nuclei. Distribution of the FB-labeled neuron somata was roughly coextensive with that of the calbindin immunolabeled somata, while parvalbumin immunoreactive somata were virtually absent from dorsal thalamus. Co-localization of FB and calbindin immunolabeling ranged from >95% in the ventromedial and interanteromedial nuclei, to 30% in the dorsal lateral geniculate. Moreover, in the ventromedial and interanteromedial nuclei nearly all of the calbindin-immunoreactive neurons were also labeled with FB. In most other nuclei, however, a major population of M-type cells cannot be identified with calbindin immunolabeling. Consistent with studies in primates and carnivores, present data show that in rats M-type cells are numerous and widely distributed across the rat thalamus; however, calbindin is expressed only by a fraction, albeit a large one, of these cells.     

Thalamic nuclei in the opossum Monodelphis domestica

We investigated nuclear divisions of the thalamus in the gray short-tailed opossum (Monodelphis domestica) to gain detailed information for further developmental and comparative studies. Nissl and myelin staining, histochemistry for acetylcholinesterase and immunohistochemistry for calretinin and parvalbumin were performed on parallel series of sections. Many features of the Monodelphis opossum thalamus resemble those in Didelphis and small eutherians showing no particular sensory specializations, particularly in small murid rodents. However, several features of thalamic organization in Monodelphis were distinct from those in rodents. In the opossum the anterior and midline nuclear groups are more clearly separated from adjacent structures than in eutherians. The dorsal lateral geniculate nucleus (LGNd) starts more rostrally and occupies a large part of the lateral wall of the thalamus. As in other marsupials, two cytoarchitectonically different parts, alpha and beta are discernible in the LGNd of the opossum. Each of them may be subdivided into two additional bands in acetylcholinesterase staining, while in murid rodents the LGNd consists of a homogeneous mass of cells. Therefore, differentiation of the LGNd of the Monodelphis opossum is more advanced than in murid rodents. The medial geniculate body consists of three nuclei (medial, dorsal and ventral) that are cytoarchitectonically distinct and stain differentially for parvalbumin. The relatively large size of the MG and LGNd points to specialization of the visual and auditory systems in the Monodelphis opossum. In contrast to rodents, the lateral dorsal and lateral posterior nuclei in the opossum are poorly differentiated cytoarchitectonically.     

Delineation of thalamic nuclei immunoreactive for calcium-binding proteins in and around the posterior pole of the ventral posterior complex

An atlas of serial sections stained alternately for one of the three calcium-binding proteins, calbindin, calretinin or parvalbumin, or for markers that demarcate borders of thalamic nuclei in and around the posterior pole of the ventral posterior thalamic nucleus of a macaque monkey is presented. The concentrated focal zone of calbindin-immunoreactive fiber ramifications considered by others to form a specific pain and temperature relay nucleus is shown to be located entirely within the confines of the ventral posterior medial (VPM) nucleus. It contains a large population of calbindin-immunoreactive cells and overlaps a region of dense cell and fiber immunoreactivity for parvalbumin. In other parts of the ventral posterior complex, calbindin and parvalbumin immunoreactivity is complementary rather than co-extensive. It is unlikely that the zone of intense calbindin immunoreactivity in VPM forms the only thalamic relay for noxious thermal and mechanical inputs to the cerebral cortex.     

Dorsal thalamic connections of the ventral lateral geniculate nucleus of rats

In order to understand better the organisation of the ventral lateral geniculate nucleus of the ventral thalamus, this paper has examined the patterns of connections that this nucleus has with various nuclei of the dorsal thalamus in rats. Injections of biotinylated dextran or cholera toxin subunit B were made into the parafascicular, central lateral, posterior thalamic, medial dorsal, lateral dorsal, lateral posterior, dorsal lateral geniculate, anterior, ventral lateral, ventrobasal and medial geniculate nuclei of Sprague-Dawley rats and their brains were processed using standard tracer detection methods. Three general patterns of ventral lateral geniculate connectivity were seen. First, the parafascicular, central lateral, medial dorsal, posterior thalamic and lateral dorsal nuclei had heavy connections with the parvocellular (internal) lamina of the ventral lateral geniculate nucleus. This geniculate lamina has been shown previously to receive heavy inputs from many functionally diverse brainstem nuclei. Second, the visually related dorsal lateral geniculate and lateral posterior nuclei had heavy connections with the magnocellular (external) lamina of the ventral lateral geniculate nucleus. This geniculate lamina has been shown by previous studies to receive heavy inputs from the visual cortex and the retina. Finally, the anterior, ventral lateral, ventrobasal and medial geniculate nuclei had very sparse, if any, connections with the ventral lateral geniculate nucleus. Overall, our results strengthen the notion that one can package the ventral lateral geniculate nucleus into distinct visual (magnocellular) and non-visual (parvocellular) components.     

Parvalbumin and calbindin are differentially distributed within primary and secondary subregions of the mouse auditory forebrain

The calcium binding proteins parvalbumin and calbindin are thought to differentially regulate physiological functions and often show complementary distributions in the CNS. Our goal was to determine parvalbumin and calbindin distributions in the different subdivisions of mouse auditory thalamus and auditory cortex. Following fixation, FVB mouse brains (postnatal days 38-80) were sectioned along coronal and horizontal planes, then processed for parvalbumin and calbindin immunohistochemistry (antibodies: parvalbumin pa-235, calbindin-d-28k cl-300). Strong complementary differences in calcium binding protein distributions were found in mouse auditory thalamus. The ventral division of the medial geniculate, which is the principal relay to primary auditory cortex, exhibited dense parvalbumin but weak calbindin immunoreactivity. In contrast, most of the 'secondary' auditory thalamic regions surrounding the ventral division showed strong calbindin and lighter parvalbumin levels. Thus, the mouse auditory thalamus is composed of a parvalbumin positive 'core' surrounded by a calbindin positive 'shell'. Parvalbumin immunoreactivity was also more prominent in the primary auditory cortex than in the secondary belt auditory cortex. Calbindin immunoreactivity in auditory cortex was less clearly divided along primary/secondary lines, especially in supragranular layers. However, within infragranular layers, there was heavier staining in belt areas than in primary auditory cortex. In auditory thalamus, parvalbumin labeling was largely confined to the neuropil, whereas calbindin labeling involved somata and neuropil. In auditory cortex, somata and neuropil were positive for both proteins.In summary, the calcium binding proteins parvalbumin and calbindin were found to be differentially distributed within the primary and non-primary regions of mouse auditory forebrain. These differences in protein distribution may contribute to the distinct types of physiological responses that occur in the primary vs. non-primary areas.     

The thalamus of the Amazon spiny rat Proechimys guyannensis,an animal model of resistance to epilepsy,and pilocarpine-induced long-term changes of protein expression

The thalamus of the spiny rat Proechimys guyannensis (casiragua), a common rodent of the Amazon basin, was investigated with immunohistochemistry, using as markers GABA and glutamic acid decarboxylase, and calcium binding proteins. As in all mammals, GABAergic neurons containing also parvalbumin filled the reticular nucleus, and GABAergic cells were seen in the dorsal lateral geniculate nucleus. At variance with the laboratory rat, GABAergic and parvalbumin-containing neurons were also seen in the laterodorsal and anterodorsal nuclei, in which the two markers were co-distributed. Calbindin-immunopositive cells were widely distributed in dorsal thalamic domains, except for the intralaminar nuclei, and prevailed in the laterodorsal nucleus. The distribution of calretinin-immunopositive neurons was more restricted, and they were especially concentrated in the laterodorsal and midline nuclei.At variance with the laboratory rat, in which systemic pilocarpine administration induces status epilepticus and results in chronic limbic epilepsy, pilocarpine elicited in casiragua an acute seizure that was not followed by spontaneous seizures up to 1 month, when changes were evaluated in the thalamus using also image analysis. Parvalbumin immunostaining in reticular nucleus neurons and in the dorsal thalamus neuropil, and the number of parvalbumin-positive and GABAergic cells in the laterodorsal and anterodorsal nuclei, exhibited an increase with respect to controls. Calbindin immunostaining was also enhanced, whereas calretinin immunostaining was mostly reduced, but was preserved in midline neurons. The findings show, after an acute seizure induced in an animal model of anti-convulsant mechanisms, regional long-term neurochemical alterations that could reflect functional changes in inhibitory and excitatory thalamic neurons.     

Immunoreactivity for calcium-binding proteins defines subregions of the vestibular nuclear complex of the cat

The vestibular nuclear complex (VNC) is classically divided into four nuclei on the basis of cytoarchitectonics. However, anatomical data on the distribution of afferents to the VNC and the distribution of cells of origin of different efferent pathways suggest a more complex internal organization. Immunoreactivity for calcium-binding proteins has proven useful in many areas of the brain for revealing structure not visible with cell, fiber or Golgi stains. We have looked at the VNC of the cat using immunoreactivity for the calcium-binding proteins calbindin, calretinin and parvalbumin. Immunoreactivity for calretinin revealed a small, intensely stained region of cell bodies and processes just beneath the fourth ventricle in the medial vestibular nucleus. A presumably homologous region has been described in rodents. The calretinin-immunoreactive cells in this region were also immunoreactive for choline acetyltransferase. Evidence from other studies suggests that the calretinin region contributes to pathways involved in eye movement modulation but not generation. There were focal dense regions of fibers immunoreactive to calbindin in the medial and inferior nuclei, with an especially dense region of label at the border of the medial nucleus and the nucleus prepositus hypoglossi. There is anatomical evidence that suggests that the likely source of these calbindin-immunoreactive fibers is the flocculus of the cerebellum. The distribution of calbindin-immunoreactive fibers in the lateral and superior nuclei was much more uniform. Immunoreactivity to parvalbumin was widespread in fibers distributed throughout the VNC. The results suggest that neurochemical techniques may help to reveal the internal complexity in VNC organization.     

Gamma-aminobutyrate-like immunoreactivity in the thalamus of the cat

Serial sections of the cat's thalamus were incubated with a purified antiserum raised against gamma-aminobutyric acid conjugated to bovine serum albumin by distilled glutaraldehyde. This serum has been extensively characterized and appears to react selectively with fixed gamma-aminobutyric acid in brain tissue treated with glutaraldehyde. Adjoining sections were stained with thionin and served as invaluable guides for a correct evaluation of the immunolabelling pattern. In the neuropil the intensity of the immunostaining varies considerably between thalamic nuclei and even between nuclear subdivisions. The neuropil staining appears particularly dense in the nuclei parataenialis, periventricularis, centralis medialis, reuniens, rhomboideus, habenularis lateralis, centrum medianum, parafascicularis, subparafascicularis, submedius, dorsal and ventral parts of the lateral geniculate body, the dorsal part of the medial geniculate body, the posterior complex, suprageniculate nucleus, pulvinar and parts of the lateral posterior nucleus. The pulvinar/lateralis posterior complex shows a particularly well-differentiated staining pattern which closely matches Updyke's [Updyke (1983) J. comp. Neurol. 219, 143-181] parcellation of this region. In several thalamic nuclei or subareas--and notably in those relay nuclei which are known to project upon non-primary sensory cortical areas--the immunostained neuropil is characterized by many puncta encircling an unstained profile. With few exceptions all thalamic nuclei displayed immunoreactive nerve cell bodies. Several examples were found of a mismatch between the number of such cells and the staining intensity of the neuropil. Thus the nuclei periventricularis, parafascicularis, subparafascicularis, parataenialis, limitans and centrum medianum although being very rich in neuropil staining have practically no immunostained perikarya. Rough estimates were made of the size and the proportion of gamma-aminobutyric acid labelled neurons in all major--and some minor--thalamic nuclei and their subdivisions. In some thalamic nuclei, notably the nuclei reticularis, anterior dorsalis, lateralis dorsalis, centralis lateralis, ventralis posterior and the dorsal lateral geniculate body, the population of immunoreactive neurons is distinctly heterogeneous with regard to soma size. The findings are discussed with regard to previous immunocytochemical studies of the distribution of gamma-aminobutyric acid and its synthesizing enzyme in the thalamus. Particular emphasis is put on the great species differences which appear to exist in this respect.     

Organization of galanin-like immunoreactive neuronal systems in weakly electric fish (Apteronotus leptorhynchus).

Immunohistochemical procedures were used to investigate the distribution of galanin-like immunoreactive neuronal somata, fiber pathways and apparent termination fields in the gymnotiform brain. Immunoreactive somata were observed only in the hypothalamus and were confined to preoptic, lateral and caudal hypothalamic regions. Within these areas, positive cells tended to be most concentrated in juxtaventricular nuclei. Dense immunoreactive fiber systems originating from hypothalamic regions were seen to project in separate or coalescing fiber bundles to the basal telencephalan, thalamus/tuberal diencephalon, midbrain and brainstem. The density of positive axons and boutons was quite variable, but regions which displayed the most massive network of axons included structures within the hypothalamus itself (anterior periventricular preoptic nucleus, caudal and lateral hypothalamus), ventral telencephalon (superior and ventral subdivisions), thalamic/tuberal areas (central posterior nucleus and tuberal neuropil within the ventral territory of the prepacemaker nucleus) and brainstem nuclei (dorsal reticular nucleus and the medial paralemniscal nucleus). Within these areas axons appeared more randomly distributed and varicose than along fiber tracs, and in counterstained sections were occasionally seen in apposition to unstained neuronal cell bodies and dendrites. In addition, a system of fibers was seen in the neurointermediate lobe of the pituitary. It is concluded that galanin-like immunoreactive neurons in the gymnotiform brain have a more restricted distribution than those in mammals, and that the major fiber systems emanating from the hypothalamus resemble the diverse projections of the tuberomammillary nucleus of higher vertebrates. The anatomy of galanin-like immunoreactive systems in the apteronotid brain suggests a role in neuroendocrine regulation and an involvement with anatomical areas controlling aggressive and courtship behaviour.     

Connections to the lateral posterior-pulvinar thalamic complex from the reticular and ventral lateral geniculate thalamic nuclei: a topographical study in the cat

The projections from the reticular thalamic nucleus and the ventral lateral geniculate nucleus to the lateral posterior-pulvinar thalamic complex were studied in the adult cat using the retrograde transport of horseradish peroxidase. Small, stereotaxically guided injections of the enzyme were placed in the various nuclei of this complex, including the pulvinar, lateralis intermedius oralis, lateralis intermedius caudalis, lateralis posterior lateralis, lateralis posterior medialis and lateralis medialis nuclei. The distribution of labeled neurons indicates that these nuclei receive topographically organized projections from the reticular and ventral lateral geniculate nuclei. The pulvinar nucleus receives only very scarce projections from the reticular thalamic nucleus originating in its posterodorsal and posteroventral sectors. The reticular projection to the nucleus lateralis intermedius oralis is even sparser. The nuclei lateralis intermedius caudalis, lateralis posterior lateralis and lateralis posterior medialis receive substantial projections from the suprageniculate sector of the reticular thalamic nucleus. The nucleus lateralis medialis receives an abundant projection from the three sectors (suprageniculate, pregeniculate and infrageniculate) of the reticular thalamic nucleus. Except for the lateralis intermedius caudalis, all nuclei of the lateral posterior-pulvinar complex receive consistent projections from the ventral lateral geniculate nucleus, the nucleus lateralis medialis receiving the densest one. Our findings suggest that visual, auditory, somatosensory, motor and limbic impulses from thalamic nuclei and from primary sensory and association cortical areas modulate the activity of the nucleus lateralis medialis via the reticular thalamic nucleus. The remaining nuclei of the lateral posterior-pulvinar complex are mainly modulated by sectors of the reticular thalamic nucleus that receive afferent connections from visual structures. The intrathalamic projections arising from the ventral lateral geniculate nucleus may be the way through which visuomotor inputs reach the different components of the lateral posterior-pulvinar thalamic complex.     

Prenatal development of calbindin immunoreactivity in the dorsal thalamus of the rat.

The distribution of calbindin immunoreactivity was studied in the developing rat dorsal thalamus at embryonic days 14, 16, 18 and 20. At early stages (days 14-16), calbindin is expressed throughout the dorsal thalamic cell mass. Most intense labeling occurs in cells adjacent to the ventricular surface, in a spatial gradient reflecting the well-known outside-in generation pattern. Between days 16 and 20, calbindin-positive periventricular cells are redistributed in the dorsal thalamus according to two different patterns. They first become oriented tangentially within the periventricular layer, and diminish in number at the central locus where midline thalamic fusion occurs at 18 days. Periventricular calbindin immunoreactivity becomes restricted to a ring of late-born cells surrounding the gray commissure. Recognizable portions of this ring-shaped primordium will mature forming n.paratenialis, n.reuniens, n.paraventricularis, and n.subparafascicularis magnocellularis. Simultaneously, a massive contingent of radially-oriented, fusiform, calbindin-positive young neurons extends from the periventricular ring-shaped aggregate to the lateral brain surface at the caudoventral pole of the dorsal thalamus at embryonic days 17/18. These cells surround the primordium of the medial geniculate body, participating in the constitution of its marginal zone, and invade the lateral posterior nucleus, accumulating within its caudomedial part. Other portions of this stream form the parvocellular subparafascicular nucleus and the peripeduncular nucleus. The observed patterns of calbindin expression suggest that dorsal thalamic postmitotic neurons transiently express the marker during initial phases of axogenesis, whereas a specific, late-born population expresses calbindin continuously into adulthood. This late subpopulation displays migratory behavior, and finally subdivides into several nuclei of the mature midline, superficial and posterior thalamus.     

Evidence for a projection from the perireticular thalamic nucleus to the dorsal thalamus in the adult rat and ferret

Summary During early development, the perireticular thalamic nucleus is very large (i.e. has many cells) and has a strong projection to the dorsal thalamus and to the cerebral neocortex. By adulthood, the nucleus has much reduced in size and only a few cells remain. It is not clear whether these perireticular cells that remain into adulthood maintain their connections with the dorsal thalamus and with the neocortex. This study examines this issue by injecting neuronal tracers into various nuclei of the dorsal thalamus (dorsal lateral geniculate nucleus, medial geniculate complex, ventroposteromedial nucleus, lateral posterior nucleus, posterior thalamic nucleus) and into different areas of the neocortex (somatosensory, visual, auditory). After injections of tracer into the individual nuclei of the rat and ferret dorsal thalamus, retrogradely-labelled perireticular cells are seen. In general, after each injection, the retrogradely-labelled perireticular cells lie immediately adjacent to a group of retrogradely-labelled reticular cells. For instance, after injections into the medial geniculate complex, perireticular cells adjacent to the auditory reticular sector are retrogradely-labelled, whilst after an injection into the dorsal lateral geniculate nucleus, retrogradely-labelled perireticular cells adjacent to the visual reticular sector are seen. By contrast, injections of tracer into various areas of the rat and ferret neocortex result in no retrogradely-labelled cells in the perireticular nucleus. Thus, unlike during perinatal development when perireticular cells project to both neocortex and dorsal thalamus, perireticular cells in the adult seem to project to the dorsal thalamus only: the perireticular projection to the neocortex appears to be entirely transient.