Patches in the striatum of squirrel monkeys are enriched with calretinin fibers but devoid of calretinin cell bodies

Disrupted striatal neuron inputs and outputs in Huntington's disease

Huntington's disease (HD) is a hereditary progressive neurodegenerative disorder caused by a CAG repeat expansion in the gene coding for the protein huntingtin, resulting in a pathogenic expansion of the polyglutamine tract in the N-terminus of this protein. The HD pathology resulting from the mutation is most prominent in the striatal part of the basal ganglia, and progressive differential dysfunction and loss of striatal projection neurons and interneurons account for the progression of motor deficits seen in this disease. The present review summarizes current understanding regarding the progression in striatal neuron dysfunction and loss, based on studies both in human HD victims and in genetic mouse models of HD. We review evidence on early loss of inputs to striatum from cortex and thalamus, which may be the basis of the mild premanifest bradykinesia in HD, as well as on the subsequent loss of indirect pathway striatal projection neurons and their outputs to the external pallidal segment, which appears to be the basis of the chorea seen in early symptomatic HD. Later loss of direct pathway striatal projection neurons and their output to the internal pallidal segment account for the severe akinesia seen late in HD. Loss of parvalbuminergic striatal interneurons may contribute to the late dystonia and rigidity.

The calretinin interneurons of the striatum: comparisons between rodents and primates under normal and pathological conditions

This paper reviews the major organizational features of calretinin interneurons in the dorsal striatum of rodents and primates, with some insights on the state of these neurons in Parkinson's disease and Huntington's chorea. The rat striatum harbors medium-sized calretinin-immunoreactive (CR⁺) interneurons, whereas the mouse striatum is pervaded by medium-sized CR⁺ interneurons together with numerous small and highly immunoreactive CR⁺ cells. The CR interneuronal network is even more elaborated in monkey and human striatum where, in addition to the small- and medium-sized CR⁺ interneurons, a set of large CR⁺ interneurons occurs. The majority of these giant CR⁺ interneurons, which are unique to the primate striatum, also display immunoreactivity for choline acetyltransferase (ChAT), a faithful marker of cholinergic neurons. The expression of CR and/or ChAT by the large striatal interneurons appears to be seriously compromised in Parkinson's disease and Huntington's chorea. The species differences noted above have to be considered to better understand the role of CR interneurons in striatal organization in both normal and pathological conditions.

Distribution and morphological characteristics of striatal interneurons expressing calretinin in mice: a comparison with human and nonhuman primates

Striatal interneurons display a morphological and chemical heterogeneity that has been particularly well characterized in rats, monkeys and humans. By comparison much less is known of striatal interneurons in mice, although these animals are now widely used as transgenic models of various neurodegenerative diseases. The present immunohistochemical study aimed at characterizing striatal interneurons expressing calretinin (CR) in mice compared to those in squirrel monkeys and humans. The mouse striatum contains both small (9-12 μm) and medium-sized (15-20 μm) CR+ cells. The small cells are intensely stained with a single, slightly varicose and moderately arborized process. They occur throughout the striatum (77±9 cells/mm(3)), but prevail in the area of the subventricular zone and subcallosal streak, with statistically significant anteroposterior and dorsoventral decreasing gradients. The medium-sized cells are less intensely immunoreactive and possess 2-3 long, slightly varicose and poorly branched dendrites. They are rather uniformly scattered throughout the striatum and three times more numerous (224±31 cells/mm(3)) than the smaller CR+ cells. Double immunostaining experiments with choline acetyltransferase (ChAT) as a cholinergic marker in normal and Drd1a-tdTomato/Drd2-EGFP double transgenic mice reveal that none of the small or medium-sized CR+ cells express ChAT or D1 and D2 dopamine receptors. In contrast, the striatum in human and nonhuman primates harbors small and medium-sized CR+/ChAT- cells, as well as large CR+/ChAT+ interneurons that are absent in mice. Such a difference between rodents and primates must be taken into consideration if one hopes to better understand the striatal function in normal and pathological conditions.

Neurochemical characterization of the tree shrew dorsal striatum

The striatum is a major component of the basal ganglia and is associated with motor and cognitive functions. Striatal pathologies have been linked to several disorders, including Huntington’s, Tourette’s syndrome, obsessive–compulsive disorders, and schizophrenia. For the study of these striatal pathologies different animal models have been used, including rodents and non-human primates. Rodents lack on morphological complexity (for example, the lack of well defined caudate and putamen nuclei), which makes it difficult to translate data to the human paradigm. Primates, and especially higher primates, are the closest model to humans, but there are ever-increasing restrictions to the use of these animals for research. In our search for a non-primate animal model with a striatum that anatomically (and perhaps functionally) can resemble that of humans, we turned our attention to the tree shrew. Evolutionary genetic studies have provided strong data supporting that the tree shrews (Scadentia) are one of the closest groups to primates, although their brain anatomy has only been studied in detail for specific brain areas. Morphologically, the tree shrew striatum resembles the primate striatum with the presence of an internal capsule separating the caudate and putamen, but little is known about its neurochemical composition. Here we analyzed the expression of calcium-binding proteins, the presence and distribution of the striosome and matrix compartments (by the use of calbindin, tyrosine hydroxylase, and acetylcholinesterase immunohistochemistry), and the GABAergic system by immunohistochemistry against glutamic acid decarboxylase and Golgi impregnation. In summary, our results show that when compared to primates, the tree shrew dorsal striatum presents striking similarities in the distribution of most of the markers studied, while presenting some marked divergences when compared to the rodent striatum.

Immunohistochemical localization of AMPA-type glutamate receptor subunits in the striatum of rhesus monkey

Corticostriatal and thalamostriatal projections utilize glutamate as their neurotransmitter. Their influence on striatum is mediated, in part, by ionotropic AMPA-type glutamate receptors, which are heteromers composed of GluR1-4 subunits. While the cellular localization of AMPA-type subunits in the basal ganglia has been well characterized in rodents, the cellular localization of AMPA subunits in primate basal ganglia is not. We thus carried out immunohistochemical studies of GluR1-4 distribution in rhesus monkey basal ganglia in conjunction with characterization of each major neuron type. In striatum, about 65% of striatal neurons immunolabeled for GluR1, 75%-79% immunolabeled for GluR2 or GluR2/3, and only 2.5% possessed GluR4. All neurons the large size of cholinergic interneurons (mean diameter 26.1 microm) were moderately labeled for GluR1, while all neurons in the size range of parvalbuminergic interneurons (mean diameter 13.8 microm) were intensely rich in GluR1. Additionally, somewhat more than half of the neurons in the size range of projection neurons (mean diameter 11.6 microm) immunolabeled for GluR1, and about one third of these were very rich in GluR1. About half of the neurons the size of cholinergic interneurons were immunolabeled for GluR2, and the remainder of the neurons that were immunolabeled for GluR2 coincided with projection neurons in size and shape (GluR2 diameter=10.7 microm), indicating that the vast majority of striatal projection neurons possess immunodectible GluR2. Similar results were observed with GluR2/3 immunolabeling. Half of the neurons the size of cholinergic interneurons immunolabeled for GluR4 and seemingly all neurons in the size range of parvalbuminergic interneurons possessed GluR4. These results indicate that AMPA receptor subunit combinations for striatal projection neurons in rhesus monkey are similar to those for the corresponding neuron types in rodents, and thus their AMPA responses to glutamate are likely to be similar to those demonstrated in rodents.

Three-dimensional organization of dendrites and local axon collaterals of shell and core medium-sized spiny projection neurons of the rat nucleus accumbens

Medium-sized spiny projection neurons (MSN) in the head of the primate caudate nucleus are thought to have preferred dendritic orientations that tend to parallel the orientations of the striosomes. Moreover, recurrent axon collaterals of MSN in the rat dorsal striatum have been categorized into two types, i.e., restricted and widespread. The nucleus accumbens (Acb) has a highly complex compartmental organization, and the spatial organization of dendritic and axonal arbors of MSN has not yet been systematically studied. In this study, using single-cell juxtacellular labeling with neurobiotin as well as anterograde neuroanatomical tracing with biotinylated dextran amine, we investigated the three-dimensional (3D) organization of dendrites and axons of MSN of the rat Acb in relation to subregional (shell-core) and compartmental (patch-matrix) boundaries. Our results show that dendritic arbors of MSN in both the Acb shell and core subregions are preferentially oriented, i.e., they are flattened in at least one of the 3D-planes. The preferred orientations are influenced by shell-core and patch-matrix boundaries, suggesting parallel and independent processing of information. Dendritic orientations of MSN of the Acb core are more heterogeneous than those of the shell and the dorsal striatum, suggesting a more complex distribution of striatal inputs within the core. Although dendrites respect the shell-core and patch-matrix boundaries, recurrent axon collaterals may cross these boundaries. Finally, different degrees of overlap between dendritic and axonal arborizations of individual MSN were identified, suggesting various possibilities of lateral inhibitory interactions within and between, functionally distinct territories of the Acb.

Differential localization of the GluR1 and GluR2 subunits of the AMPA-type glutamate receptor among striatal neuron types in rats

Differences among the various striatal projection neuron and interneuron types in cortical input, function, and vulnerability to degenerative insults may be related to differences among them in AMPA-type glutamate receptor abundance and subunit configuration. We therefore used immunolabeling to assess the frequency and abundance of GluR1 and GluR2, the most common AMPA subunits in striatum, in the main striatal neuron types. All neurons projecting to the external pallidum (GPe), internal pallidum (GPi) or substantia nigra, as identified by retrograde labeling, possessed perikaryal GluR2, while GluR1 was more common in striato-GPe than striato-GPi perikarya. The frequency and intensity of immunostaining indicated the rank order of their perikaryal GluR1:GluR2 ratio to be striato-GPe>striatonigral>striato-GPi. Ultrastructural studies suggested a differential localization of GluR1 and GluR2 to striatal projection neuron dendritic spines as well, with GluR1 seemingly more common in striato-GPe spines and GluR2 more common in striato-GPi and/or striatonigral spines. Comparisons among projection neurons and interneurons revealed GluR1 to be most common and abundant in parvalbuminergic interneurons, and GluR2 most common and abundant in projection neurons, with the rank order for the GluR1:GluR2 ratio being parvalbuminergic interneurons>calretinergic interneurons>cholinergic interneurons>projection neurons>somatostatinergic interneurons. Striosomal projection neurons had a higher GluR1:GluR2 ratio than did matrix projection neurons. The abundance of both GluR1 and GluR2 in striatal parvalbuminergic interneurons and projection neurons is consistent with their prominent cortical input and susceptibility to excitotoxic insult, while differences in GluR1:GluR2 ratio among projection neurons are likely to yield differences in Ca(2+) permeability, desensitization, and single channel current, which may contribute to differences among them in plasticity, synaptic integration, and excitotoxic vulnerability. The apparent association of the GluR1 subunit with synaptic plasticity, in particular, suggests striato-GPe neuron spines as a particular site of corticostriatal synaptic plasticity, presumably associated with motor learning.

The striatum in the hedgehog tenrec: histochemical organization and cortical afferents

In order to get insight into the striopallidal organization in mammals with little differentiated brain the striatum of the lesser hedgehog tenrec (Afrotheria) was characterized histochemically and analysed with regard to its cortical afferents using axonal tracer substances. The majority of neocortical cells projecting to the striatum were found bilaterally in the layers 2 and 3 of the frontal hemisphere; caudalwards the relative number of cells increased somewhat in the upper layer 5. There was a topographical organization as far as the allocortical projections appeared confined to the ventral striatum, and the efferents from hippocampal, posterior paleocortical, somatosensory and audiovisual areas were distributed in largely different striatal territories. Projections from the anterior frontal cortex, on the other hand, terminated extensively upon the caudate-putamen and also involved the nucleus accumbens and the olfactory tubercle. In the latter region the molecular layer was especially involved. The entorhinal cortex also projected heavily to the olfactory tubercle but unlike other species it scarcely involved the nucleus accumbens. The cortical fibers were distributed in a relatively homogenous fashion within their striatal territory and there was little evidence for patches of high density terminations. Islands of low density labeling, however, were noted occasionally in the caudate-putamen. These islands were partly similar in size as the patches of neuropil staining obtained with anti-calretinin and anti-substance P. There were also hints for the presence of a shell-like region in the nucleus accumbens stained with anti-dopamine transporter and NADPh-diaphorase. The classical striosome-matrix markers such as calbindin, acetylcholinesterase and enkephalin, however, failed to reveal any compartmental organization.

Neurochemical organization of the human basal ganglia: anatomofunctional territories defined by the distributions of calcium-binding proteins and SMI-32

The distribution of the calcium-binding proteins calbindin-D28K (CB), parvalbumin (PV) and calretinin (CR), and of the nonphosphorylated neurofilament protein (with SMI-32) was investigated in the human basal ganglia to identify anatomofunctional territories. In the striatum, gradients of neuropil immunostaining define four major territories: The first (T1) includes all but the rostroventral half of the putamen and is characterized by enhanced matriceal PV and SMI-32 immunoreactivity (-ir). The second territory (T2) encompasses most part of the caudate nucleus (Cd) and rostral putamen (PuT), which show enhanced matriceal CB-ir. The third and fourth territories (T3 and T4) comprise rostroventral parts of Cd and PuT characterized by complementary patch/matrix distributions of CB- and CR-ir, and the accumbens nucleus (Acb), respectively. The latter is separated into lateral (prominently enhanced in CB-ir) and medial (prominently enhanced in CR-ir) subdivisions. In the pallidum, parallel gradients also delimit four territories, T1 in the caudal half of external (GPe) and internal (GPi) divisions, characterized by enhanced PV- and SMI-32-ir; T2 in their rostral half, characterized by enhanced CB-ir; and T3 and T4 in their rostroventral pole and in the subpallidal area, respectively, both expressing CB- and CR-ir but with different intensities. The subthalamic nucleus (STh) shows contrasting patterns of dense PV-ir (sparing only the most medial part) and low CB-ir. Expression of CR-ir is relatively low, except in the medial, low PV-ir, part of the nucleus, whereas SMI-32-ir is moderate across the whole nucleus. The substantia nigra is characterized by complementary patterns of high neuropil CB- and SMI-32-ir in pars reticulata (SNr) and high CR-ir in pars compacta (SNc) and in the ventral tegmental area (VTA). The compartmentalization of calcium-binding proteins and SMI-32 in the human basal ganglia, in particular in the striatum and pallidum, delimits anatomofunctional territories that are of significance for functional imaging studies and target selection in stereotactic neurosurgery.

Chemical anatomy of striatal interneurons in normal individuals and in patients with Huntington's disease

This paper reviews the major anatomical and chemical features of the various types of interneurons in the human striatum, as detected by immunostaining procedures applied to postmortem tissue from normal individuals and patients with Huntington's disease (HD). The human striatum harbors a highly pleomorphic population of aspiny interneurons that stain for either a calcium-binding protein (calretinin, parvalbumin or calbindin D-28k), choline acetyltransferase (ChAT) or NADPH-diaphorase, or various combinations thereof. Neurons that express calretinin (CR), including multitudinous medium and a smaller number of large neurons, are by far the most abundant interneurons in the human striatum. The medium CR+ neurons do not colocalize with any of the known chemical markers of striatal neurons, except perhaps GABA, and are selectively spared in HD. Most large CR+ interneurons display ChAT immunoreactivity and also express substance P receptors. The medium and large CR+ neurons are enriched with glutamate receptor subunit GluR2 and GluR4, respectively. This difference in AMPA GluR subunit expression may account for the relative resistance of medium CR+ neurons to glutamate-mediated excitotoxicity that may be involved in HD. The various striatal chemical markers display a highly heterogeneous distribution pattern in human. In addition to the classic striosomes/matrix compartmentalization, the striosomal compartment itself is composed of a core and a peripheral region, each subdivided by distinct subsets of striatal interneurons. A proper knowledge of all these features that appear unique to humans should greatly help our understanding of the organization of the human striatum in both health and disease states.

The core-shell dichotomy of nucleus accumbens in the rhesus monkey as revealed by double-immunofluorescence and morphology of cholinergic interneurons

Double-immunolabelling experiments for the combinations, calretinin (CR)-calbindin, CR-tyrosine hydroxylase (TH) and calbindin-TH, were performed in rhesus monkeys to compare the chemical organization of the nucleus accumbens (ACC) in primates and rodents. Additionally, the soma sizes and numbers of primary dendrites of cholinergic neurons in the subregions of ACC were compared with those of caudate-putamen. Our findings subserve the shell-core concept also in the primate ACC, as like in the rat, CR immunoreactivity (-ir) due to intense neuropil labelling is very strong in the shell of rhesus monkey, but poor in the core. The staining intensity of this marker decreases in dorsoventral direction. An almost complementary pattern was noted in sections of the monkey ACC immunostained for both calbindin and TH. The cholinergic interneurons of the nucleus caudatus-putamen are clearly distinguished from those of the ACC and insula Calleja magna by their much bigger soma sizes and higher numbers of primary dendrites. Cholinergic neurons of the shell were found to be slightly, but significantly, larger than those of the core that also subserves subdivision of the primate ACC into shell and core. A low proportion of tyrosine-hydroxylase-immunostained cells, already previously described below the rostral ACC, co-expressed CR but not calbindin. A CR-immunoreactive neuronal population, intermingled with these cells, extends as a stripe medially to the ACC along the septal part of corpus callosum into the lateral septal area. The presumed origin of CR-immunoreactive fibres in the shell of ACC is discussed.

Evidence for a deficit in cholinergic interneurons in the striatum in schizophrenia

Neurochemical and functional abnormalities of the striatum have been reported in schizophrenic brains, but the cellular substrates of these changes are not known. We hypothesized that schizophrenia may involve an abnormality in one of the key modulators of striatal output, the cholinergic interneuron. We measured the densities of cholinergic neurons in the striatum in schizophrenic and control brains in a blind analysis, using as a marker of this cell population immunoreactivity for choline acetyltransferase, the synthetic enzyme of acetylcholine. As an independent marker, we used immunoreactivity for calretinin, a protein which is co-localized with choline acetyltransferase in virtually all of the cholinergic interneurons of the striatum. A significant decrease in choline acetyltransferase-positive and calretinin-positive cell densities was found in the schizophrenic cases compared with controls in the striatum as a whole [for the choline acetyltransferase-positive cells: controls: 3.21 +/- 0.48 cells/mm2 (mean +/- S.D.), schizophrenics: 2.43 +/- 0.68 cells(mm2; P < 0.02]. The decrease was patchy in nature and most prominent in the ventral striatum (for the choline acetyltransferase-positive cells: controls: 3.47 +/- 0.59 cells/mm2, schizophrenics: 2.52 +/- 0.64 cells/ mm2; P < 0.005) which included the ventral caudate nucleus and nucleus accumbens region. Three of the schizophrenic cases with the lowest densities of cholinergic neurons had not been treated with neuroleptics for periods from more than a month to more than 20 years. A decrease in the number or function of the cholinergic interneurons of the striatum may disrupt activity in the ventral striatal-pallidal-thalamic-prefrontal cortex pathway and thereby contribute to abnormalities in function of the prefrontal cortex in schizophrenia.

Calretinin-immunoreactive neurons in primate pedunculopontine and laterodorsal tegmental nuclei

Single- and double-antigen localization procedures were used to study the distribution, morphological characteristics and chemical phenotype of neurons containing the calcium-binding protein calretinin in the pedunculopontine and laterodorsal tegmental nuclei of the cynomolgus monkey (Macaca fascicularis). Calretinin was detected in neurons that belonged to a highly heteromorphic and widely distributed subpopulation of the pedunculopontine and laterodorsal tegmental nuclei in the cynomolgus monkey. Double-immunostaining experiments revealed that about 12% of these calretinin-containing neurons displayed immunoreactivity for another calcium-binding protein, Calbindin-D28k. The calretinin/Calbindin-D28k double-labeled neurons had small to medium-sized perikarya, from which emerged a bipolar or multipolar dendritic arborization. Calretinin was also present in approximately 8% of the cholinergic neurons of the pedunculopontine/laterodorsal nuclear complex, as visualized on single sections immunostained for both calretinin and choline acetyltransferase. These calretinin/choline acetyltransferase double-labeled neurons displayed markedly different sizes and shapes, and occurred preferentially in the pars compacta and dissipata of the pedunculopontine tegmental nucleus. Numerous calretinin-immunoreactive fibers were also present within and around the superior cerebellar peduncle. Some of these varicose fibers closely surrounded large non-immunoreactive neurons, as well as large neurons staining positively for choline acetyltransferase. This study provides the first evidence for the existence of calretinin-immunoreactive neurons within the primate pedunculopontine and laterodorsal tegmental nuclei. Our data suggest that calretinin may play a role in the function of the pedunculopontine/laterodorsal nuclear complex by acting either alone or in conjunction with acetylcholine or Calbindin-D28k.

Chemical phenotype of calretinin interneurons in the human striatum

We recently reported the existence of a new class of aspiny interneurons characterized by their immunoreactivity for the calcium-binding protein calretinin (CR) in human striatum. This group is composed of numerous medium-sized (10-20 microm) neurons with poorly branched dendrites and a smaller number of large-sized (24-42 microm) neurons with highly ramified dendrites. We further demonstrated the selective sparing of the medium-sized, but not all the large-sized, CR+ striatal neurons in Huntington's disease. In the present study, we applied a double-antigen localization method to postmortem striatal tissue obtained from normal individuals to further characterize the chemical phenotype of these two subsets of CR+ neurons. Our results reveal that in the medium-sized neurons, CR is not colocalized with any of the following current markers of striatal neurons: calbindin, parvalbumin, beta-nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), or choline acetyltransferase (ChAT). Furthermore, quantitative estimates show that the medium-sized CR+ neurons are by far the most abundant type of interneurons in the human striatum. In contrast, CR is colocalized with ChAT in about 80% of the large-sized CR+ neurons. Thus, the medium-sized CR+ neurons appear to form a distinct class of striatal interneurons, whereas most of the large-sized CR+ neurons belong to the population of giant cholinergic neurons. This study has provided the first exhaustive characterization of the chemical phenotype of the CR + neurons in the human striatum.

Calretinin-immunoreactive neurons in the human thalamus

The distribution of the calcium-binding protein calretinin in the thalamus of normal human individuals was studied with immunohistochemistry. Calretinin immunoreactivity was weak in the geniculate bodies and in nuclei of the ventral and posterior groups, moderate in the reticular nucleus and in nuclei of the anterior, medial, and lateral groups, and strong in nuclei of the midline group and anterior intralaminar nuclei. The mediodorsal nucleus was unique among thalamic nuclei because it contained a wide variety of intensely immunostained perikarya embedded in a moderately-labelled neuropil. The reticular nucleus displayed several small and uniformly distributed neuronal clusters composed of immunostained perikarya lying in a moderately-labelled neuropil. Intense and uniform immunostaining was observed in all midline nuclei and in the anterior intralaminar nuclei, including the paracentral and central lateral nuclei. These nuclei, which harboured numerous intensely-stained perikarya lying in a dense immunoreactive neuropil, were the most strongly-immunoreactive structures of the entire human thalamus. At the level of the posterior intralaminar nuclei, the central median nucleus was virtually free of immunostaining whereas the parafascicular nucleus was moderately labelled. The nucleus submedius located just beneath the central median/parafascicular complex displayed a very intense calretinin immunostaining. This study has provided evidence for the presence of the protein calretinin in the human thalamus. The pattern of distribution of calretinin, as delineated in the present study, suggests that this calcium-binding protein may participate in various subcortical and cortical thalamic systems involved in the modulation of emotional and motivational states.

Calretinin gene expression in the human thalamus

The localization and levels of expression of the calcium-binding protein calretinin (CR) in the human thalamus was studied with an in situ hybridization method applied to formalin-fixed postmortem material from normal individuals. The riboprobe used was generated from a specific fragment of human CR cDNA. As visualized on X-ray film autoradiographs, high levels of CR gene transcript occurred in several thalamic nuclei, including the reticular nucleus, mediodorsal nucleus, rostral intralaminar nuclei (paracentral, central medial and central lateral) and several midline nuclei (paraventricular, reuniens and medioventral nuclei). In the reticular nucleus, neurons expressing CR mRNA were few in number but formed dense and widely distributed clusters. In contrast, virtually all neurons in the rostral intralaminar and midline nuclei expressed very high levels of CR mRNA and formed a prominent rim around the mediodorsal nucleus, which contained scattered clusters of labeled neurons. The caudal intralaminar nuclei, principally the centromedian nucleus, displayed very few neurons expressing CR mRNA. Only the medial part of the parafascicular nucleus expressed moderate levels of CR mRNA. The nuclei of the ventral group (ventral anterior, lateral and posterior nuclei) were virtually devoid of CR gene transcript. This highly heterogeneous pattern of mRNA expression suggests that CR may be heavily involved in the function of the so-called non-specific nuclei, but not in that of the specific relay nuclei of the human thalamus. The data also demonstrate that the presence of CR gene transcript can easily be detected on formalin-fixed sections of the human brain.

Distribution of calretinin, calbindin-D28k and parvalbumin in the hypothalamus of the squirrel monkey

The immunohistochemical approach was used to study the distribution of three calcium-binding proteins of the 'EF hand' family, namely calretinin, calbindin-D28k and parvalbumin, in the preoptico-hypothalamic complex of the squirrel monkey (Saimiri sciureus). These three calcium-binding proteins were found to be heterogeneously distributed in the primate hypothalamus. Neurons expressing high levels of calretinin immunoreactivity were particularly abundant in the infundibular (arcuate) nucleus, the suprachiasmatic nucleus, the lateral area and the dorsomedial nucleus of the hypothalamus. Neurons displaying immunoreactivity for calbindin-D28k were especially numerous in the medial preoptic area and diagonal band nucleus, as well as in the magnocellular subdivision of the paraventricular nucleus, the suprachiasmatic nucleus, the supraoptic nucleus, the infundibular nucleus, the ventromedial nucleus and the mammillary bodies of the hypothalamus. Fibers displaying intense immunoreactivity for either calretinin or calbindin-D28k were very abundant in the median eminence of the hypothalamus. In contrast to calretinin- and calbindin-D28k, parvalbumin was largely absent from the primate preoptico-hypothalamic complex. Parvalbumin-immunoreactive neurons occurred in significant number only in the most lateral portion of the medial mammillary nucleus in the squirrel monkey. The results of the present study suggest that calretinin and calbindin-D28k may act, either in concert or in a complementary manner, so as to participate in some specific aspects of the multifarious role of the hypothalamus in primates. In contrast to the other two calcium-binding proteins, parvalbumin is unlikely to be involved in a significant manner in hypothalamic functions in primates.

Striatal interneurons in Huntington's disease: selective increase in the density of calretinin-immunoreactive medium-sized neurons

The marked atrophy of the striatum seen in Huntington's disease (HD) is largely due to a massive neuronal loss that affects the striatal projection neurons more severely than the local circuit neurons. We recently reported the existence of a new class of interneurons characterized by their immunoreactivity for the calcium-binding protein calretinin in the human striatum. In the present immunohistochemical study, we compared the distribution and relative density of the calretinin-expressing interneurons in the striata of four normal individuals and four patients with HD (grade 1 to 3). The population of calretinin-containing interneurons comprised (a) a small subset of large (17- to 44-microns), multipolar neurons with five to seven long, aspiny, and highly branched dendrites and (b) a large number of medium-sized (8- to 18-microns), round-to-oval neurons with two to three long, varicose, and poorly branched dendrites. Both types of chemospecific neurons occurred throughout the striatum in all specimens examined, but the density of the medium-sized neurons was much higher in patients with HD than in controls. A quantitative analysis showed a significant (p < 0.01) twofold increase in the density of the striatal medium-sized neurons and a similar decrease in the density of the large neurons in patients with HD compared with controls. This differential effect on the densities of the two types of interneurons suggests that calretinin may protect the medium-sized but not the large neurons against neurodegeneration in HD.

Calcium-binding proteins in primate basal ganglia

This paper describes the distribution of the calcium-binding proteins calbindin-D28k. Parvalbumin and calretinin in primate basal ganglia. The data derive from immunocytochemical studies undertaken in squirrel monkeys (Saimiri sciureus) and in normal human individuals. In the striatum, calbindin labels medium-sized spiny projection neurons whereas parvalbumin and calretinin mark two separate classes of aspiny interneurons. The striatal matrix compartment is markedly enriched with calbindin while striatal patches (striosomes) display a calretinin-rich neuropil. In the pallidum, virtually all neurons contain parvalbumin but none express calbindin. Calretinin occurs only in a small subpopulation of both large and small pallidal neurons. In the subthalamic nucleus, there exists a multitude of parvalbumun-positive cells and fibers but the number of calretinin and calbindin-positive neuronal elements is small. In the substantia nigra/ventral tegmental area complex, calbindin and calretinin occur principally in dopaminergic neurons of the dorsal tier of the pars compacta and in those of the ventral tegmental area. Parvalbumin is strictly confined to the GABAergic neurons of the pars reticulata and lateralis. Calbindin-rich fibers abound in the pars reticulata and lateralis, while calretinin-positive axons are confined to the pars compacta. These results indicate that calbindin and parvalbumin are distributed according to a strikingly complementary pattern in primate basal ganglia. Calretinin is less ubiquitous but occurs in all basal ganglia components where it labels distinct subsets of neurons. Such highly specific patterns of distribution indicate that calbindin, parvalbumin and calretinin may work in synergy within primate basal ganglia.

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 mèdian nucleus, together with the medial habenular nucleus, were virtually devoid of calretinine 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 calretlnin 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.

Calretinin is largely localized to a unique population of striatal interneurons in rats

Previous studies have reported the presence of the calcium binding protein calretinin in neurons in the striatal part of the basal ganglia in rats and primates. In the present study, immunofluorescence double-labeling techniques and immunofluorescence combined with retrograde labeling were used in rats to determine whether calretinin is found in any of the known types of striatal neurons. The results showed that a small fraction of the calretinin-containing neurons (< 10%) contain parvalbumin, but none of the calretinin-containing striatal neurons contained markers for the other two major types of striatal interneurons (i.e., choline acetyltransferase-containing cholinergic neurons and somatostatin-containing neurons). Additionally, calretinin was not found in projection neurons, using either calbindin or DARPP32 as immunofluorescent markers of striatal projections neurons in general, or using retrograde labeling to specifically identify either striatonigral or striatopallidal neurons. Thus, calretinin appears to be largely found in a unique population of striatal interneurons in rats. This population appears to be about one third the abundance of any of the previously identified populations of striatal interneurons.

Calretinin as a marker of specific neuronal subsets in primate substantia nigra and subthalamic nucleus

Neurons immunoreactive for the calcium-binding protein calretinin (CR) were visualized in the substantia nigra (SN) pars compacta (SNc), reticulata (SNr) and lateralis (SNl), the ventral tegmental area (VTA) and the subthalamic nucleus (STN) of squirrel monkeys. The density of CR perikarya was high in VTA, moderate in SNc, low in SNr/SNl, and very low in STN. The SNc/VTA complex also displayed a CR-rich neuropil. Some CR perikarya and fibers occurred in specific sectors of STN. These data suggest that CR may play a role in specific neuronal subpopulations in all components of primate basal ganglia.

Compartmental organization of calretinin in the rat striatum

The present study examined the compartmental distribution of a calcium-binding protein, calretinin, in the rat striatum. Calretinin-immunoreactive cells were homogeneously scattered throughout the striatum, but calretinin-immunoreactive fibers were clustered as patches in the medial, central and ventral caudoputamen and in the lateral nucleus accumbens. These patches corresponded to striosomes, identified by immunostaining for calbindin-D28 K in adjacent sections. In the medial nucleus accumbens, calretinin-immunoreactive fibers showed diffuse distribution with occasional islands of calretinin-poor zones. These islands contained tightly packed, Nissl-stained cells, which have been previously shown to correspond to mu-opiate receptor-rich patches. Calretinin-positive fibers fill striosomes/patches in the caudoputamen and in the lateral nucleus accumbens and avoid them in the medial nucleus accumbens.

Calretinin-immunoreactive neurons in the human striatum

The human striatum contains two types of neurons displaying immunoreactivity for the calcium-binding protein calretinin (CR): (1) large (22, 44 microns>), multipolar neurons with 5-7 long, aspiny and tightly branched dendrites, and (2) medium-sized (9-18 microns), round-to-oval neurons with 2-3 long, varicose and poorly branched dendrites. These CR neurons represent only a small proportion of the total neuronal population and they are heterogeneously distributed in the striatum. The large CR neurons are more numerous in the putamen than in the caudate nucleus, whereas the inverse is true for the medium-sized CR neurons. The ratio of large- to medium-size CR neurons is 1:4 in the putamen compared to 1:6 in the caudate nucleus. The existence of these two distinct subsets of chemospecific striatal neurons suggest that CR may play an important role in the intrinsic organization of the human striatum.