Calretinin immunoreactive structures in the human hippocampal formation

Cell numbers in the reflected blade of CA3 and their relation to other hippocampal principal cell populations across seven species

The hippocampus of many mammals contains a histoarchitectural region that is not present in laboratory mice and rats-the reflected blade of the CA3 pyramidal cell layer. Pyramidal cells of the reflected blade do not extend dendrites into the hippocampal molecular layer, and recent evidence indicates that they, like the proximal CA3 pyramids in laboratory rats and mice, partially integrate functionally with the dentate circuitry in pattern separation. Quantitative assessments of phylogenetic or disease-related changes in the hippocampal structure and function treat the reflected blade heterogeneously. Depending on the ease with which it can be differentiated, it is either assigned to the dentate hilus or to the remainder of CA3. Here, we investigate the impact that the differential assignment of reflected blade neurons may have on the outcomes of quantitative comparisons. We find it to be massive. If reflected blade neurons are treated as a separate entity or pooled with dentate hilar cells, the quantitative makeup of hippocampal cell populations can differentiate between species in a taxonomically sensible way. Assigning reflected blade neurons to CA3 greatly diminishes the differentiating power of all hippocampal principal cell populations, which may point towards a quantitative hippocampal archetype. A heterogeneous assignment results in a differentiation pattern with little taxonomic semblance. The outcomes point towards the reflected blade as either a major potential player in hippocampal functional and structural differentiation or a region that may have cloaked that hippocampi are more similarly organized across species than generally believed.

Differential distribution of inhibitory neuron types in subregions of claustrum and dorsal endopiriform nucleus of the short-tailed fruit bat

Few brain regions have such wide-ranging inputs and outputs as the claustrum does, and fewer have posed equivalent challenges in defining their structural boundaries. We studied the distributions of three calcium-binding proteins-calretinin, parvalbumin, and calbindin-in the claustrum and dorsal endopiriform nucleus of the fruit bat, Carollia perspicillata. The proportionately large sizes of claustrum and dorsal endopiriform nucleus in Carollia brain afford unique access to these structures' intrinsic anatomy. Latexin immunoreactivity permits a separation of claustrum into core and shell subregions and an equivalent separation of dorsal endopiriform nucleus. Using latexin labeling, we found that the claustral shell in Carollia brain can be further subdivided into at least four distinct subregions. Calretinin and parvalbumin immunoreactivity reinforced the boundaries of the claustral core and its shell subregions with diametrically opposite distribution patterns. Calretinin, parvalbumin, and calbindin all colocalized with GAD67, indicating that these proteins label inhibitory neurons in both claustrum and dorsal endopiriform nucleus. Calretinin, however, also colocalized with latexin in a subset of neurons. Confocal microscopy revealed appositions that suggest synaptic contacts between cells labeled for each of the three calcium-binding proteins and latexin-immunoreactive somata in claustrum and dorsal endopiriform nucleus. Our results indicate significant subregional differences in the intrinsic inhibitory connectivity within and between claustrum and dorsal endopiriform nucleus. We conclude that the claustrum is structurally more complex than previously appreciated and that claustral and dorsal endopiriform nucleus subregions are differentially modulated by multiple inhibitory systems. These findings can also account for the excitability differences between claustrum and dorsal endopiriform nucleus described previously.

An assessment of the existence of adult neurogenesis in humans and value of its rodent models for neuropsychiatric diseases

In sub-mammalian vertebrates like fishes, amphibians, and reptiles, new neurons are produced during the entire lifespan. This capacity diminishes considerably in birds and even more in mammals where it persists only in the olfactory system and hippocampal dentate gyrus. Adult neurogenesis declines even more drastically in nonhuman primates and recent evidence shows that this is basically extinct in humans. Why should such seemingly useful capacity diminish during primate evolution? It has been proposed that this occurs because of the need to retain acquired complex knowledge in stable populations of neurons and their synaptic connections during many decades of human life. In this review, we will assess critically the claim of significant adult neurogenesis in humans and show how current evidence strongly indicates that humans lack this trait. In addition, we will discuss the allegation of many rodent studies that adult neurogenesis is involved in psychiatric diseases and that it is a potential mechanism for human neuron replacement and regeneration. We argue that these reports, which usually neglect significant structural and functional species-specific differences, mislead the general population into believing that there might be a cure for a variety of neuropsychiatric diseases as well as stroke and brain trauma by genesis of new neurons and their incorporation into existing synaptic circuitry.

Revealing the Precise Role of Calretinin Neurons in Epilepsy: We Are on the Way

Epilepsy is a common neurological disorder characterized by hyperexcitability in the brain. Its pathogenesis is classically associated with an imbalance of excitatory and inhibitory neurons. Calretinin (CR) is one of the three major types of calcium-binding proteins present in inhibitory GABAergic neurons. The functions of CR and its role in neural excitability are still unknown. Recent data suggest that CR neurons have diverse neurotransmitters, morphologies, distributions, and functions in different brain regions across various species. Notably, CR neurons in the hippocampus, amygdala, neocortex, and thalamus are extremely susceptible to excitotoxicity in the epileptic brain, but the causal relationship is unknown. In this review, we focus on the heterogeneous functions of CR neurons in different brain regions and their relationship with neural excitability and epilepsy. Importantly, we provide perspectives on future investigations of the role of CR neurons in epilepsy.

Calretinin and parvalbumin in schizophrenia and affective disorders: a mini-review, a perspective on the evolutionary role of calretinin in schizophrenia, and a preliminary post-mortem study of calretinin in the septal nuclei

OBJECTIVE

The septal nuclei are important limbic regions that are involved in emotional behavior and connect to various brain regions such as the habenular complex. Both the septal nuclei and the habenular complex are involved in the pathology of schizophrenia and affective disorders.

METHODS

We characterized the number and density of calretinin-immunoreactive neurons in the lateral, medial, and dorsal subregions of the septal nuclei in three groups of subjects: healthy control subjects (N = 6), patients with schizophrenia (N = 10), and patients with affective disorders (N = 6).

RESULTS

Our mini-review of the combined role of calretinin and parvalbumin in schizophrenia and affective disorders summarizes 23 studies. We did not observe significant differences in the numbers of calretinin-immunoreactive neurons or neuronal densities in the lateral, medial, and dorsal septal nuclei of patients with schizophrenia or patients with affective disorders compared to healthy control subjects.

CONCLUSIONS

Most post-mortem investigations of patients with schizophrenia have indicated significant abnormalities of parvalbumin-immunoreactive neurons in various brain regions including the hippocampus, the anterior cingulate cortex, and the prefrontal cortex in schizophrenia. This study also provides an explanation from an evolutionary perspective for why calretinin is affected in schizophrenia.

The vulnerability of calretinin-containing hippocampal interneurons to temporal lobe epilepsy

This review focuses on the vulnerability of a special interneuron type—the calretinin (CR)-containing interneurons—in temporal lobe epilepsy (TLE). CR is a calcium-binding protein expressed mainly by GABAergic interneurons in the hippocampus. Despite their morphological heterogeneity, CR-containing interneurons form a distinct subpopulation of inhibitory cells, innervating other interneurons in rodents and to some extent principal cells in the human. Their dendrites are strongly connected by zona adherentiae and presumably by gap junctions both in rats and humans. CR-containing interneurons are suggested to play a key role in the hippocampal inhibitory network, since they can effectively synchronize dendritic inhibitory interneurons. The sensitivity of CR-expressing interneurons to epilepsy was discussed in several reports, both in animal models and in humans. In the sclerotic hippocampus the density of CR-immunopositive cells is decreased significantly. In the non-sclerotic hippocampus, the CR-containing interneurons are preserved, but their dendritic tree is varicose, segmented, and zona-adherentia-type contacts can be less frequently observed among dendrites. Therefore, the dendritic inhibition of pyramidal cells may be less effective in TLE. This can be partially explained by the impairment of the CR-containing interneuron ensemble in the epileptic hippocampus, which may result in an asynchronous and thus less effective dendritic inhibition of the principal cells. This phenomenon, together with the sprouting of excitatory pathway axons and enhanced innervation of principal cells, may be involved in seizure generation. Preventing the loss of CR-positive cells and preserving the integrity of CR-positive dendrite gap junctions may have antiepileptic effects, maintaining proper inhibitory function and helping to protect principal cells in epilepsy.

The hippocampus of the eastern rock sengi: cytoarchitecture, markers of neuronal function, principal cell numbers, and adult neurogenesis

The brains of sengis (elephant shrews, order Macroscelidae) have long been known to contain a hippocampus that in terms of allometric progression indices is larger than that of most primates and equal in size to that of humans. In this report, we provide descriptions of hippocampal cytoarchitecture in the eastern rock sengi (Elephantulus myurus), of the distributions of hippocampal calretinin, calbindin, parvalbumin, and somatostatin, of principal neuron numbers, and of cell numbers related to proliferation and neuronal differentiation in adult hippocampal neurogenesis. Sengi hippocampal cytoarchitecture is an amalgamation of characters that are found in CA1 of, e.g., guinea pig and rabbits and in CA3 and dentate gyrus of primates. Correspondence analysis of total cell numbers and quantitative relations between principal cell populations relate this sengi to macaque monkeys and domestic pigs, and distinguish the sengi from distinct patterns of relations found in humans, dogs, and murine rodents. Calretinin and calbindin are present in some cell populations that also express these proteins in other species, e.g., interneurons at the stratum oriens/alveus border or temporal hilar mossy cells, but neurons expressing these markers are often scarce or absent in other layers. The distributions of parvalbumin and somatostatin resemble those in other species. Normalized numbers of PCNA+ proliferating cells and doublecortin-positive (DCX+) differentiating cells of neuronal lineage fall within the overall ranges of murid rodents, but differed from three murid species captured in the same habitat in that fewer DCX+ cells relative to PCNA+ were observed. The large and well-differentiated sengi hippocampus is not accompanied by correspondingly sized cortical and subcortical limbic areas that are the main hippocampal sources of afferents and targets of efferents. This points to intrinsic hippocampal information processing as the selective advantage of the large sengi hippocampus.

Variability of sclerosis along the longitudinal hippocampal axis in epilepsy: a post mortem study

Detailed neuropathological studies of the extent of hippocampal sclerosis (HS) in epilepsy along the longitudinal axis of the hippocampus are lacking. Neuroimaging studies of patients with temporal lobe epilepsy support that sclerosis is not always localised. The extent of HS is of relevance to surgical planning and poor outcomes may relate to residual HS in the posterior remnant. In 10 post mortems from patients with long histories of drug refractory epilepsy and 3 controls we systematically sampled the left and right hippocampus at seven coronal anatomical levels along the body to the tail. We quantified neuronal densities in CA1 and CA4 subfields at each level using Cresyl Violet (CV), calretinin (CR), calbindin (CB) and Neuropeptide Y (NPY) immunohistochemistry. In the dentate gyrus we graded the extent of granule cell dispersion, patterns of CB expression, and synaptic reorganisation with CR and NPY at each level. We identified four patterns of HS based on patterns of pyramidal and interneuronal loss and dentate gyrus reorganisation between sides and levels as follows: (1) symmetrical HS with anterior-posterior (AP) gradient, (2) symmetrical HS without AP gradient, (3) asymmetrical HS with AP gradient and (4) asymmetrical cases without AP gradient. We confirmed in this series that HS can extend into the tail. The patterns of sclerosis (classical versus atypical or none) were consistent between all levels in less than a third of cases. In conclusion, this series highlights the variability of HS along the longitudinal axis. Further studies are required to identify factors that lead to focal versus diffuse HS.

Loss and reorganization of calretinin-containing interneurons in the epileptic human hippocampus

Calretinin is expressed mainly in interneurons that specialize to innervate either principal cell dendrites or other interneurons in the human hippocampus. Calretinin-containing cells were shown to be vulnerable in animal models of ischaemia and epilepsy. In the human hippocampus, controversial data were published regarding their sensitivity in epilepsy. Therefore we aimed to reveal the fate of this cell type in human epileptic hippocampi. Surgically removed hippocampi of patients with drug-resistant temporal lobe epileptic (n = 44) were examined and compared to control (n = 8) samples with different post-mortem delays. The samples were immunostained for calretinin and the changes in the distribution, density and synaptic target selectivity of calretinin-positive cells were analysed. Control samples with post-mortem delays longer than 8 h resulted in a reduced number of immunolabelled cells compared to controls with short post-mortem delay. The number of calretinin-positive cells in the epileptic tissue was considerably decreased in correlation with the severity of principal cell loss. Preserved cells had segmented and shortened dendrites. Electron microscopic examination revealed that in controls, 23% of the calretinin-positive interneuronal terminals targeted calretinin-positive dendrites, whereas in the epileptic samples it was reduced to 3-5%. The number of contacts between calretinin-positive dendrites also dropped. The present quantitative data suggest that calretinin-containing cells in the human hippocampus are highly vulnerable, thus inhibition mediated by dendritic inhibitory cells and their synchronization by interneuron-specific interneurons may be impaired in epilepsy. We hypothesize that reorganization of the interneuron-selective cells may be implicated in the occurrence of seizures in non-sclerotic patients, where the majority of principal and non-principal cells are preserved.

-1154G/A and -2578C/A polymorphisms of the vascular endothelial growth factor gene in Tunisian Alzheimer patients in relation to beta-amyloid (1-42) and total tau protein

Recent evidences indicate that polymorphisms within the promoter region of the vascular endothelial growth factor (VEGF) gene may elevate the risk for Alzheimer's disease (AD). To further investigate, we have analyzed association between promoter polymorphisms of the VEGF gene in 93 AD patients and age and sex matched 113 controls from Tunisian population. The distribution of genotype and allele frequencies of the VEGF (-2578C/A) and (-1154G/A) polymorphisms did not differ significantly between AD and control groups (p>0.05). In the subgroup of ApoE varepsilon4 carriers, the -2578A was observed to be significantly higher in the AD patients than in the control individuals. After adjusting the data by gender, age and the ApoE varepsilon4 status using logistic regression, the -2578A allele was found to increase the risk for sporadic AD by 1.7-fold. The present study provides the evidence that the -2578A allele may be associated with the development of AD in the individuals with ApoE varepsilon4 allele. In addition, AD patients carrying the -2578A allele had lower Abeta42 (p=0.029) levels than those without this allele, particularly in subjects with ApoE varepsilon4 allele.

Parvalbumin-, calbindin-, and calretinin-immunoreactive hippocampal interneuron density in autism

BACKGROUND

There has been a long-standing interest in the possible role of the hippocampus in autism and both postmortem brain and neuroimaging studies have documented varying abnormalities in this limbic system structure.

AIMS

This study investigates the density of subsets of hippocampal interneurons, immunostained with the calcium binding proteins, calbindin (CB), calretinin (CR) and parvalbumin (PV) to determine whether specific subpopulations of interneurons are impacted in autism.

MATERIALS AND METHODS

Unbiased stereological techniques were used to quantify the neuronal density of these immunoreactive subpopulations of gamma-aminobutyric acid-ergic (GABAergic) interneurons analyzed in the CA and subicular fields in postmortem brain material obtained from five autistic and five age-, gender- and postmortem interval-matched control cases.

RESULTS

Results indicate a selective increase in the density of CB-immunoreactive interneurons in the dentate gyrus, an increase in CR-immunoreactive interneurons in area CA1, and an increase in PV-immunoreactive interneurons in areas CA1 and CA3 in the hippocampus of individuals with autism when compared with controls.

DISCUSSION/CONCLUSIONS

Although our sample size is small, these findings suggest that GABAergic interneurons may represent a vulnerable target in the brains of individuals with autism, potentially impacting upon their key role in learning and information processing. These preliminary findings further suggest the need for future more expanded studies in a larger number of postmortem brain samples from cases of autism and controls.

Postmortem changes in the neuroanatomical characteristics of the primate brain: hippocampal formation

Comparative studies of the structural organization of the brain are fundamental to our understanding of human brain function. However, whereas brains of experimental animals are fixed by perfusion of a fixative through the vasculature, human or ape brains are fixed by immersion after varying postmortem intervals. Although differential treatments might affect the fundamental characteristics of the tissue, this question has not been evaluated empirically in primate brains. Monkey brains were either perfused or acquired after varying postmortem intervals before immersion-fixation in 4% paraformaldehyde. We found that the fixation method affected the neuroanatomical characteristics of the monkey hippocampal formation. Soma size was smaller in Nissl-stained, immersion-fixed tissue, although overall brain volume was larger as compared to perfusion-fixed tissue. Nonphosphorylated high-molecular-weight neurofilament immunoreactivity was lower in CA3 pyramidal neurons, dentate mossy cells, and the entorhinal cortex, whereas it was higher in the mossy fiber pathway in immersion-fixed tissue. Serotonin-immunoreactive fibers were well stained in perfused tissue but were undetectable in immersion-fixed tissue. Although regional immunoreactivity patterns for calcium-binding proteins were not affected, intracellular staining degraded with increasing postmortem intervals. Somatostatin-immunoreactive clusters of large axonal varicosities, previously reported only in humans, were observed in immersion-fixed monkey tissue. In addition, calretinin-immunoreactive multipolar neurons, previously observed only in rodents, were found in the rostral dentate gyrus in both perfused and immersion-fixed brains. In conclusion, comparative studies of the brain must evaluate the effects of fixation on the staining pattern of each marker in every structure of interest before drawing conclusions about species differences.

Loss of GABAergic neurons in the subiculum and its functional implications in temporal lobe epilepsy

Clinical and experimental evidence suggest that the subiculum plays an important role in the maintenance of temporal lobe seizures. Using the pilocarpine-model of temporal lobe epilepsy (TLE), the present study examines the vulnerability of GABAergic subicular interneurons to recurrent seizures and determines its functional implications. In the subiculum of pilocarpine-treated animals, the density of glutamic acid decarboxylase (GAD) mRNA-positive cells was reduced in all layers. Our data indicate a substantial loss of parvalbumin-immunoreactive neurons in the pyramidal cell and molecular layer whereas calretinin-immunoreactive cells were predominantly reduced in the molecular layer. Though the subiculum of pilocarpine-treated rats showed an increased intensity of GAD65 immunoreactivity, the density of GAD65 containing synaptic terminals in the pyramidal cell layer was decreased indicating an increase in the GAD65 intensity of surviving synaptic terminals. We observed a decrease in evoked inhibitory post-synaptic currents that mediate dendritic inhibition as well as a decline in the frequency of miniature inhibitory post-synaptic currents (mIPSCs) that are restricted to the perisomatic region. The decrease in mIPSC frequency (-30%) matched with the reduced number of perisomatic GAD-positive terminals (-28%) suggesting a decrease of pre-synaptic GABAergic input onto pyramidal cells in epileptic animals. Though cell loss in the subiculum has not been considered as a pathogenic factor in human and experimental TLE, our data suggest that the vulnerability of subicular GABAergic interneurons causes an input-specific disturbance of the subicular inhibitory system.

Calretinin expression in hilar mossy cells of the hippocampal dentate gyrus of nonhuman primates and humans

Mossy cells, the major excitatory neurons of the hilus of the dentate gyrus constitutively express calretinin in several rodent species, including mouse and hamster, but not in rats. Several studies suggest that mossy cells of the monkey dentate gyrus are calretinin-positive, but others have reported mossy cells in monkeys to be devoid of detectable calretinin-like immunoreactivity. In the present study, the hilar region was investigated throughout the entire longitudinal extent of the hippocampal dentate gyrus in both Old World and New World monkeys, as well as in humans. In the examined four monkey species, mossy cells were found to be calretinin-positive at the uncal pole and at variable length within the main body of the dentate gyrus but not in the tail part. The associational pathway, formed by axons of mossy cells in the inner dentate molecular layer was calretinin-positive in more caudal sections, suggesting that mossy cell axon terminals may contain calretinin, whereas mossy cell somata may contain calretinin in a concentration too low to be detected by immunocytochemistry. In contrast, human mossy cells appear to be devoid of calretinin immunoreactivity in both their somata and their axon terminals. Taken together, mossy cells of nonhuman primates and humans exhibit different expression pattern for calretinin whereas they show similarities in neurochemical content, such as the cocaine and amphetamine-related transcript peptide.

The dentate gyrus in Alzheimer's disease

As part of the hippocampus, the dentate gyrus is considered to play a crucial role in associative memory. The reviewed data suggest that the dentate gyrus withstands the formation of plaques, tangles and neuronal death until late stages of Alzheimer's disease (AD). However, changes related to a disconnecting process, and more subtle intrinsic alterations, may contribute to disturbances in memory and learning observed in early stages of AD.

Axonal sprouting of GABAergic interneurons in temporal lobe epilepsy

Temporal lobe epilepsy is one of the most common forms of epilepsy. Numerous contributing factors and compensatory mechanisms have been associated with temporal lobe epilepsy. One feature found in both humans and animal models is sprouting of hippocampal principal cell axons, which suggests that axonal sprouting may be a general phenomenon associated with temporal lobe epilepsy. This article highlights the evidence showing that hippocampal GABAergic interneurons also undergo axonal sprouting in temporal lobe epilepsy. The caveats and unanswered questions associated with the current data and the potential physiological consequences of reorganizations in GABAergic circuits are discussed.

Impaired and repaired inhibitory circuits in the epileptic human hippocampus

This review focuses on the epileptic human temporal lobe, primarily on recent findings related to changes in hippocampal GABAergic interneuron circuits that have a central role in epileptogenesis. Relying on correlations to animal studies, we provide a functional interpretation of the different changes in perisomatic inhibition (controlling output synchrony) and dendritic inhibition (controlling input plasticity), and the potential consequences of the loss of interneuron-selective interneurons. The highly heterogeneous, but specific, alterations of GABAergic interneuron circuits have important implications for the pharmacotherapy of epilepsy.

A novel population of calretinin-positive neurons comprises reelin-positive Cajal-Retzius cells in the hippocampal formation of the adult domestic pig

Calretinin-containing neurons in the hippocampal formation, including the subiculum, presubiculum, parasubiculum, and entorhinal cortex, were visualized with immunocytochemistry. Calretinin immunoreactivity was present exclusively in non-principal cells. The largest immunoreactive cell population was found in the outer half of the molecular layer of the dentate gyrus and in the stratum lacunosum-moleculare of Ammon's horn. A proportion of these cells were also immunoreactive for reelin, a Cajal-Retzius cell marker. Similar calretinin-positive cells were found in the molecular layer of the subicular complex and entorhinal cortex. In the parasubiculum, a few immunoreactive bipolar and multipolar cells could be observed in the superficial and deep pyramidal cell layers. In the entorhinal cortex, bipolar and multipolar calretinin-positive cells were frequent in layer II, and large numbers of multipolar cells in layer V were immunoreactive. Electron microscopic analysis showed that somata of calretinin-positive cells contained either round nuclei with smooth nuclear envelopes or nuclei with multiple deep infoldings. Immunoreactive dendrites were smooth varicose, and the apposing axon terminals formed both symmetric and asymmetric synapses. Zonula adherentia were observed between calretinin-positive dendrites. Calretinin-positive axon terminals formed two types of synapses. Axon terminals with asymmetric synapses were found close to the hippocampal fissure, whereas axon terminals forming symmetric synapses innervated spiny dendrites in both the molecular layer of the dentate gyrus and in stratum lacunosum-moleculare of Ammon's horn. Calretinin-positive axon terminals formed both symmetric and asymmetric synapses with calretinin-positive dendrites. In conclusion, calretinin-positive neurons form two major subpopulations in the adult domestic pig hippocampus: (1) a gamma-aminobutyric acid (GABA)ergic subpopulation of local circuit neurons that innervates distal dendrites of principal cells in both the dentate gyrus and in Ammon's horn; and (2) Cajal-Retzius type cells close to the hippocampal fissure, as well as in the molecular layer of the subicular complex and entorhinal cortex.

Co-expression of calretinin and γ-aminobutyric acid in neurons of the entorhinal cortex of the common marmoset monkey

The gamma-aminobutyric acid (GABA)-containing interneuron population in the entorhinal cortex has been shown to consist of several subpopulations. In addition to GABA, these neurons contain another neurochemical substance, such as a neuropeptide or a calcium binding protein. In the present study, we examined the co-localization of calretinin and GABA in the entorhinal cortex of the common marmoset Callithrix jacchus, a New World monkey. Although the function of calretinin remains unclear, there are indications that it might have a protective role against cell death in a number of neuropathological diseases. Furthermore, it might have a regulatory role in the neurotransmission of GABAergic neurons. In contrast to the rat brain, sparse data exist regarding the degree of co-expression of these two markers in the monkey brain. Using immunofluorescence and confocal laser scanning microscopy, we found that an average of 56% of the calretinin-positive neurons in the monkey entorhinal cortex contained GABA, whereas about 27% of the GABA-positive neurons co-expressed calretinin. Interestingly, these numbers were higher in the superficial layers of the entorhinal cortex in comparison with the deep layers. However, no differences were found in co-localization percentages between the different entorhinal subfields. In general, the degree of co-localization was higher in comparison to findings in the rat entorhinal cortex. The higher amount of co-localization observed in the present study might reflect species differences between the primate and the non-primate brain.

Reelin-expressing neurons in the postnatal and adult human hippocampal formation

Reelin plays a major role in the development of laminated brain structures. In the developing neocortex and hippocampus, Reelin is secreted by Cajal-Retzius cells in the marginal zone. In the present report, we characterize Reelin-immunoreactive neurons in the perinatal and adult human hippocampal formation. Two main populations of cells are described: Cajal-Retzius cells and interneurons. Cajal-Retzius cells are defined as neurons that coexpress Reelin and p73, a nuclear protein of the p53 family. Colocalization experiments of p73 with calcium-binding proteins indicate that most Cajal-Retzius cells express calretinin, but not calbindin. Cajal-Retzius cell density decreases dramatically during the postnatal period, although a few Reelin/p73-positive neurons are still found in the adult. At birth, Reelin-positive, p73-negative neurons are present in all layers of the hippocampal formation. Their morphology and localization indicate that they belong to a heterogeneous population of interneurons. They are numerous in the strata lacunosum-moleculare and radiatum of CA1-CA3, in the hilus, and in the molecular layer of the dentate gyrus, but less common in stratum oriens and alveus, and rare in the principal cell layers. Subpopulations of Reelin-positive interneurons express calretinin or calbindin. The packing density of Reelin-positive cells decreases postnatally, which may be related to the disappearance of Cajal-Retzius cells and to the growth of the hippocampal formation. The presence of Reelin-immunoreactive cells in the adult hippocampal formation indicates that Reelin is not restricted to development but that it may have additional functions in adult life.

A selective decrease in the relative density of parvalbumin-immunoreactive neurons in the hippocampus in schizophrenia

Neuropathological studies have demonstrated deficits of GABAergic interneurons in the hippocampus in schizophrenia. and selective deficits in some GABAergic sub-populations defined by calcium-binding proteins (CBPs) have been reported in the cortex in schizophrenia. In the present study, the relative densities of cells immunoreactive for the CBPs parvalbumnin (PV) and calretinin (CR) were determined in hippocampal tissue sections taken from patients with schizophrenia, bipolar disorder and major depression and from matched control subjects (15 per group). No significant difference in the density of CR-immunoreactive neurons was found between subject groups. Relative to normal controls, schizophrenic patients showed a significant and profound deficit in the relative density of PV-immunoreactive neurons in all hippocampal sub-fields. These reductions were more apparent in male than female schizophrenic patients, and were unrelated to antipsychotic drug treatment, age or duration of illness. The density of PV-immunoreactive neurons did not differ significantly from controls in the depression group, although a trend toward decreased relative density of PV-immunoreactive neurons was apparent in bipolar disorder that reached significance in one sub-field. The findings provide further evidence to support a profound and selective abnormality of a sub-population of GABAergic neurons in the hippocampus in schizophrenia.

Hippocampal fields in the hedgehog tenrec. Their architecture and major intrinsic connections

The Madagascan lesser hedgehog tenrec was investigated to get insight into the areal evolution of the hippocampal formation in mammals with poorly differentiated brains. The hippocampal subdivisions were analyzed using cyto- and chemoarchitectural criteria; long associational and commissural connections were demonstrated with tracer techniques. The hedgehog tenrec shows a well differentiated dentate gyrus, CA3 and CA1. Their major intrinsic connections lie within the band of variations known from other species. The dentate hilar region shows calretinin-positive mossy cells with extensive projections to the molecular layer. The calbindin- and enkephalin-positive granule mossy fibers form a distinct endbulb and do not invade the CA1 as reported in the erinaceous hedgehog. Isolated granule cells with basal dendrites were also noted. A CA2 region is hard to identify architecturally; its presence is suggested due to its contralateral connections. Subicular and perisubicular regions are clearly present along the dorsal aspects of the hemisphere, but we failed to identify them unequivocally along the caudal and ventral tip of the hippocampus. A temporal portion of the subiculum, if present, differs in its chemoarchitecture from its dorsal counterpart. The perisubicular region, located medially adjacent to the dorsal subiculum may be equivalent to the rat's presubiculum; evidence for the presence of a parasubiculum was rather weak.

The spiny rat Proechimys guyannensis as model of resistance to epilepsy: chemical characterization of hippocampal cell populations and pilocarpine-induced changes

At variance with pilocarpine-induced epilepsy in the laboratory rat, pilocarpine administration to the tropical rodent Proechimys guyannensis (casiragua) elicited an acute seizure that did not develop in long-lasting status epilepticus and was not followed by spontaneous seizures up to 30 days, when the hippocampus was investigated in treated and control animals. Nissl staining revealed in Proechimys a highly developed hippocampus, with thick hippocampal commissures and continuity of the rostral dentate gyri at the midline. Immunohistochemistry was used to study calbindin, parvalbumin, calretinin, GABA, glutamic acid decarboxylase, and nitric oxide synthase expression. The latter was also investigated with NADPH-diaphorase histochemistry. Cell counts and densitometric evaluation with image analysis were performed. Differences, such as low calbindin immunoreactivity confined to some pyramidal cells, were found in the normal Proechimys hippocampus compared to the laboratory rat. In pilocarpine-treated casiraguas, stereological cell counts in Nissl-stained sections did not reveal significant neuronal loss in hippocampal subfields, where the examined markers exhibited instead striking changes. Calbindin was induced in pyramidal and granule cells and interneuron subsets. The number of parvalbumin- or nitric oxide synthase-containing interneurons and their staining intensity were significantly increased. Glutamic acid decarboxylase(67)-immunoreactive interneurons increased markedly in the hilus and decreased in the CA1 pyramidal layer. The number and staining intensity of calretinin-immunoreactive pyramidal cells and interneurons were significantly reduced. These findings provide the first description of the Proechimys hippocampus and reveal marked long-term variations in protein expression after an epileptic insult, which could reflect adaptive changes in functional hippocampal circuits implicated in resistance to limbic epilepsy.

Intracellular recording and labeling of mossy cells and proximal CA3 pyramidal cells in macaque monkeys

Little is known about the morphological characteristics and intracellular electrophysiological properties of neurons in the primate hippocampus and dentate gyrus. We have therefore begun a program of studies using intracellular recording and biocytin labeling in hippocampal slices from macaque monkeys. In the current study, we investigated mossy cells and proximal CA3 pyramidal cells. As in rats, macaque mossy cells display fundamentally different traits than proximal CA3 pyramidal cells. Interestingly, macaque mossy cells and CA3 pyramidal neurons display some morphological differences from those in rats. Macaque monkey mossy cells extend more dendrites into the molecular layer of the dentate gyrus, have more elaborate thorny excrescences on their proximal dendrites, and project more axon collaterals into the CA3 region. In macaques, three types of proximal CA3 pyramidal cells are found: classical pyramidal cells, neurons with their dendrites confined to the CA3 pyramidal cell layer, and a previously undescribed cell type, the "dentate" CA3 pyramidal cell, whose apical dendrites extend into and ramify within the hilus, granule cell layer, and molecular layer of the dentate gyrus. The basic electrophysiological properties of mossy cells and proximal CA3 cells are similar to those reported for the rodent. Mossy cells have a higher frequency of large amplitude spontaneous depolarizing postsynaptic potentials, and proximal CA3 pyramidal cells are more likely to discharge bursts of action potentials. Although mossy cells and CA3 pyramidal cells in macaque monkeys display many morphological and electrophysiological features described in rodents, these findings highlight significant species differences, with more heterogeneity and the potential for richer interconnections in the primate hippocampus.

Human and monkey fetal brain development of the supramammillary-hippocampal projections: a system involved in the regulation of theta activity

The supramammillary (SUM)-hippocampal pathway plays a central role in the regulation of theta rhythm frequency. We followed its prenatal development in eight Cynomolgus monkeys (Macaca fascicularis) from embryonic day E88 to postnatal day 12 (term 165 days) and in eight human fetuses from 17.5 to 40 gestational weeks, relying on neurochemical criteria established in the adult (Nitsch and Leranth [1993] Neuroscience 55:797-812). We found that 1) SUM afferents reached the dentate juxtagranular and CA2 pyramidal cell layers at midgestation in human fetuses, earlier than in monkeys (two-thirds of gestation [E109]). They co-expressed calretinin, substance P, and acetylcholinesterase but not gamma-aminobutyric acid (GABA) or glutamic acid decarboxylase (GAD); 2) the presumed parent neurons in the monkey SUM expressed calretinin or both calretinin and substance P; 3) most of them were surrounded by GAD-containing terminals that might correspond to the septo-SUM feedback pathway (Leranth et al. [1999] Neuroscience 88:701); and 4) in addition, a large band of calretinin-labeled terminals that did not co-express substance P, GAD, or acetylcholinesterase was present in the deepest one-third of the dentate molecular layer in both the Cynomolgus monkey and human fetuses. It persisted in the adult monkey but not in adult human hippocampus; it remains questionable whether it originates in the SUM. In conclusion, the early ingrowth of the excitatory SUM-hippocampal system in human and non-human primates may contribute to the prenatal activity-dependent development of the hippocampal formation. The possibility and the functional importance of an in utero generation of hippocampal theta-like activity should also be considered.

Aberrant trajectory of entorhino-dentate axons in the mutant Shaking Rat Kawasaki: a Dil-labelling study

The Shaking Rat Kawasaki (SRK) is a neurological mutant that exhibits abnormalities of cell migration and lamination, with many similarities to the mouse reeler mutant. We recently used lamina-specific antibody staining to show that despite severe aberrations in the laminar organization of the SRK dentate gyrus, the entorhinal terminal field in the outer dentate molecular layer appeared relatively normal (Woodhams & Terashima, 1999, J. Comp. Neurol. 409 p57). However, neurofilament immunostaining suggested that entorhino-dentate afferents take an abnormal trajectory in reaching their appropriate targets, the granule cells dendrites. In the present study, anterograde tracing with the carbocyanine dye 1, 1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) has been used to delineate directly the path that entorhinal axons take to the dentate gyrus, confirming that in SRK entorhinal axons do indeed reach their appropriate terminal fields in the molecular layer, with laminar segregation between projections from the lateral and medial entorhinal cortices. However, these fibres fail to cross the hippocampal fissure between the subiculum and the dentate gyrus, coursing instead parallel to it until they curve round the deepest point of the fissure in field CA3. Similar findings were seen in the murine reeler mutant. Insertion of DiI crystals into the entorhinal cortex of neonatal rats also retrogradely labelled the developmentally transient Cajal-Retzius cells at the hippocampal fissure; these survive for longer in SRK than in normal littermates. The presence of a marked astrogliosis at the SRK hippocampal fissure may play a part in determining the abnormal trajectory taken by entorhino-dentate afferents in this mutant.

Changes in the distribution and connectivity of interneurons in the epileptic human dentate gyrus

The distribution, size, dendritic morphology and synaptic connections of calbindin-, calretinin- and substance P receptor-positive interneurons and pathways have been examined in control and epileptic human dentate gyrus. In the epileptic dentate gyrus, calbindin-containing interneurons are preserved, but their dendrites become elongated and spiny, and several cell bodies appear hypertrophic. The relative laminar distribution of calretinin-containing cells did not change, but their number was considerably reduced. The calretinin-positive axonal bundle at the top of the granule cell layer originating from the supramammillary nucleus expanded, forming a dense network in the entire width of the stratum moleculare. Substance P receptor-immunopositive cells were partially lost in epileptic samples, and in addition, the laminar distribution and dendritic morphology of the surviving cells differed considerably from the controls. In the control human dentate gyrus, the majority of substance P receptor-positive cells can be seen in the hilus, while most are present in the stratum moleculare in the epileptic tissue. Their synaptic input is also changed. The extent of individual pathological abnormalities correlates with each other in most cases. Our data suggest, that although a large proportion of inhibitory interneurons are preserved in the epileptic human dentate gyrus, their distribution, morphology and synaptic connections differ from controls. These functional alterations of inhibitory circuits in the dentate gyrus are likely to be compensatory changes with a role to balance the enhanced excitatory input in the region.

Localization of the A kinase anchoring protein AKAP79 in the human hippocampus

The phosphorylation state of the proteins, regulated by phosphatases and kinases, plays an important role in signal transduction and long-term changes in neuronal excitability. In neurons, cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and calcineurin (CN) are attached to a scaffold protein, A kinase anchoring protein (AKAP), thought to anchor these three enzymes to specific sites of action. However, the localization of AKAP, and the predicted sites of linked phosphatase and kinase activities, are still unknown at the fine structural level. In the present study, we investigated the distribution of AKAP79 in the hippocampus from postmortem human brains and lobectomy samples from patients with intractable epilepsy, using preembedding immunoperoxidase and immunogold histochemical methods. AKAP79 was found in the CA1, presubicular and subicular regions, mostly in pyramidal cell dendrites, whereas pyramidal cells in the CA3, CA2 regions and dentate granule cells were negative both in postmortem and in surgical samples. In some epileptic cases, the dentate molecular layer and hilar interneurons also became immunoreactive. At the subcellular level, AKAP79 immunoreactivity was present in postsynaptic profiles near, but not attached to, the postsynaptic density of asymmetrical (presumed excitatory) synapses. We conclude that the spatial selectivity for the action of certain kinases and phosphatases regulating various ligand- and voltage-gated channels may be ensured by the selective presence of their anchoring protein, AKAP79, at the majority of glutamatergic synapses in the CA1, but not in the CA2/CA3 regions, suggesting profound differences in signal transduction and long-term synaptic plasticity between these regions of the human hippocampus.

Molecular neuropathology of human mesial temporal lobe epilepsy

With the recent progress in surgical treatment modalities, human brain tissue from patients with intractable focal epilepsies will increasingly become available for studies on the molecular pathology, electrophysiological changes and pathogenesis of human focal epilepsies. An inherent problem for studies on human temporal lobe epilepsy (TLE) is the lack of suitable controls. Strategies to alleviate this obstacle include the use of human post mortem samples, hippocampus from experimental animals and, in particular, the comparative analysis of surgical specimens from patients with Ammon's horn sclerosis (AHS) and with focal temporal lesions but anatomically preserved hippocampal structures. In this review we focus on selected aspects of the molecular neuropathology of TLE: (1) the potential impact of persisting calretinin-immunoreactive neurons with Cajal-Retzius cell morphology, (2) astrocytic tenascin-C induction and redistribution as potential regulator of aberrant axonal sprouting and (3) alterations of Ca2+ -mediated hippocampal signalling pathways. The diverse and complex changes described so far in human TLE specimens require a systematic interdisciplinary approach to distinguish primary, epileptogenic alterations and secondary, compensatory mechanisms in the pathogenesis of human temporal lobe epilepsies.

Synaptic loss reflected by secretoneurin-like immunoreactivity in the human hippocampus in Alzheimer's disease

Secretoneurin is a recently described peptide derived by endoproteolytic processing from secretogranin II, previously named chromogranin C. In this study, we have investigated the distribution of secretoneurin-like immunoreactivity in the human hippocampus in controls and in Alzheimer's disease patients, and compared the staining pattern to that of calretinin. Secretoneurin-like immunoreactivity is present throughout the hippocampal formation. At the border of the dentate molecular layer and the granule cell layer, a band of dense secretoneurin immunostaining appeared. In this part, as in the area of the CA2 sector, the high density of secretoneurin-immunoreactivity coincided with calretinin-like immunoreactivity. The mossy fibre system displayed a moderate density of secretoneurin-immunoreactivity. In the entorhinal cortex, a particularly high density of secretoneurin-immunoreactivity was observed. The density of secretoneurin-like immunoreactivity was significantly reduced in the innermost part of the molecular layer and in the outer molecular layer of the dentate gyrus in Alzheimer's disease. For calretinin-like immunoreactivity, a less pronounced decrease was found in the innermost part of the molecular layer. About 40-60% of neuritic plaques were secretoneurin-immunopositive. This study shows that secretoneurin is distinctly distributed in the human hippocampus and that significant changes of secretoneurin-like immunoreactivity occur in Alzheimer's disease, reflecting synaptic loss.

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 immunoreactivity in the mouse dentate gyrus: II. Mossy cells, with special reference to their dorsoventral difference in calretinin immunoreactivity

In our previous study we revealed the presence of clustered large calretinin-immunoreactive multipolar cells in the ventral hilus of the mouse dentate gyrus and indicated that they might be mossy cells, the principal neurons in the dentate hilus. In the present study we confirmed this identification with several methods and analysed further in detail. In Golgi-impregnated samples mossy cells were easily identified by their locations and characteristic thorny excrescences on their proximal dendrites. Golgi-impregnated mossy cells were observed not only in the ventral hilus but also in the dorsal hilus, where no calretinin-immunoreactive large multipolar cells were encountered. Interestingly, mossy cells exhibited dorsoventral differences in the size and complexity of thorny excrescences; mossy cells at the dorsal and middle levels had larger and more complex thorny excrescences, which covered dendritic shafts for a longer distance, while ventral mossy cells had smaller, simpler and shorter thorny excrescences. Confocal laser scanning light microscopic observations at a high magnification showed that the vast majority of calretinin-immunoreactive large neurons in the ventral hilus displayed the thorny excrescences characteristic to mossy cells. Mossy cells identified with the intracellular injection of Lucifer Yellow were calretinin-immunoreactive. Electron microscopic observations clearly revealed that calretinin-immunoreactive elements showed structural features of mossy cells such as thorny excrescences receiving typical synapses from mossy fibre terminals. At the supragranular zone, a well-known target zone of mossy cell axons, a dense calretinin-immunoreactive band was seen, where numerous calretinin-immunoreactive punctae and fibres were packed. Electron microscopic observations revealed that these calretinin-immunoreactive axon terminals in the supragranular zone made asymmetrical synapses on presumed granule cell dendritic spines. Tracer injection studies and lesion experiments indicated that the supragranular calretinin-immunoreactive axon terminals mainly originated from the large calretinin-immunoreactive multipolar cells in the ipsilateral ventral hilus. Fluorescent double immunostaining for calretinin and glutamate receptor 2/3 (GluR2/3) revealed that all large calretinin-immunoreactive hilar cells in the ventral level were GluR2/3-immunoreactive and almost all intensely GluR2/3-immunoreactive hilar cells in the ventral level were calretinin-immunoreactive. In addition intensely GluR2/3-immunoreactive but calretinin-negative large cells were encountered in the dentate hilus at the dorsal level. On the basis of these observations, we concluded that large calretinin-immunoreactive cells in the ventral hilus of the mouse dentate gyrus were really mossy cells and that mossy cells at the dorsal level were calretinin negative. The present study revealed that mouse mossy cells show the dorsoventral difference in the calretinin immunoreactivity and thus they are chemically heterogeneous.

Ultrastructural localization of neurotransmitter immunoreactivity in mossy cell axons and their synaptic targets in the rat dentate gyrus

Electrophysiologically identified and intracellularly biocytin-labeled mossy cells in the dentate hilus of the rat were studied using electron microscopy and postembedding immunogold techniques. Ultrathin sections containing a labeled mossy cell or its axon collaterals were reacted with antisera against the excitatory neurotransmitter glutamate and against the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). From single- and double-immunolabeled preparations, we found that 1) mossy cell axon terminals made asymmetric contacts onto postsynaptic targets in the hilus and stratum moleculare of the dentate gyrus and showed immunoreactivity primarily for glutamate, but never for GABA; 2) in the hilus, glutamate-positive mossy cell axon terminals targeted GABA-positive dendritic shafts of hilar interneurons and GABA-negative dendritic spines; and 3) in the inner molecular layer, the mossy cell axon formed asymmetric synapses with dendritic spines associated with GABA-negative (presumably granule cell) dendrites. The results of this study support the view that excitatory (glutamatergic) mossy cell terminals contact GABAergic interneurons and non-GABAergic neurons in the hilar region and GABA-negative granule cells in the stratum moleculare. This pattern of connectivity is consistent with the hypothesis that mossy cells provide excitatory feedback to granule cells in a dentate gyrus associational network and also activate local hilar inhibitory elements.

Correlated axonal sprouting and dendritic spine formation during kainate-induced neuronal morphogenesis in the dentate gyrus of adult mice

Several examples of structural plasticity in the adult brain have been provided in the hippocampus, among which the most striking concerns axonal remodeling of the dentate gyrus granule cells. We have recently demonstrated that a single injection of kainic acid into the dorsal hippocampus of adult mice triggers a conspicuous morphogenetic response of granule cells. Cellular labeling with biocytin 1, 2, and 4 weeks after injection of kainate revealed a progressive increase in dendritic thickness and length (up to 2.5-times), combined with an increase in the number of dendritic spines. This correlation resulted in the conservation of total spine density. No modifications of the dendritic arborization pattern were noted. In addition to dendritic changes, the number of axonal profiles observed within the hypertrophied granular layer and the inner part of the molecular layer appeared dramatically increased. Timm staining and anterograde labeling of two of the main extra-hippocampal afferent systems (i.e., septal, entorhinal) evidenced sprouting of mossy fibers and of septal afferents. Entorhinal fibers were not obviously modified. As revealed by calretinin-immunohistochemistry, commissural afferents also responded by an extensive sprouting. In addition, increases of dendritic spine number and dendritic length were noticeably greater in portions of dendrites that receive mossy fiber collaterals and septal and hypothalamic afferents, than in the external portion which receives entorhinal afferents. Although qualitative, this correlation suggests a relationship between kainate-induced structural plasticity of mature granule cells and the specific capacities of afferent systems to elaborate axon collaterals.

Neurons co-localizing calretinin immunoreactivity and reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) activity in the hippocampus and dentate gyrus of the rat

Co-localization of calretinin immunoreactivity and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) activity was studied in the rat hippocampus and dentate gyrus. Neurons co-expressing both markers (CR/NADPH-d) were observed throughout the hippocampus and dentate gyrus. However, they were more abundant in the stratum pyramidale and radiatum of CA3, stratum pyramidale of CA1, and in the juxtagranular zone of the hilus. The NADPH-d activity appeared in 37% of the calretinin immunoreactive neurons in CA3, 42% in CA1, and 36% in the dentate gyrus, whereas calretinin immunoreactivity occurred in 41% of the NADPH-d positive neurons in the hippocampus, and 16% in the dentate gyrus. The morphology and location of the double marked cells could not be used as a characteristic of the co-localizing neurons. The heavily stained NADPH-d neurons occurring mainly in CA1 do not show calretinin immunoreactivity. NADPH-d fiber swellings could be observed in close apposition to calretinin immunoreactive neurons and dendrites, suggesting synaptic contacts. It has been reported that calretinin immunoreactivity and NADPH-d activity co-localize infrequently in other areas such as the neocortex, striatum, hypothalamus and tegmental nucleus. The relatively high proportion of double marked cells found in the hippocampus and dentate gyrus could be indicative of the importance of the CR/NADPH-d interneurons in the circuitries of these areas.

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.

Distribution of neurofilament protein and calcium-binding proteins parvalbumin, calbindin, and calretinin in the canine hippocampus

Neurofilament protein and calcium-binding proteins parvalbumin, calbindin, and calretinin are present in morphologically distinct neuronal subpopulations in the mammalian cerebral cortex. Immunohistochemical studies of the hippocampal formation and neocortex have demonstrated that while neurofilament protein and calbindin are localized in subsets of pyramidal neurons, the three calcium-binding proteins are useful markers to differentiate non-overlapping populations of interneurons. To date, most studies have been performed in rodents and primates. In the present analysis, we analyzed the distribution of these proteins in the canine hippocampus. Neurofilament protein was present in large multipolar neurons in the hilus and in pyramidal neurons in the CA3 field, whereas pyramidal neurons in the CA1 field and subiculum were less intensely immunoreactive. Parvalbumin immunoreactivity was observed in large multipolar neurons in the hilus and throughout the CA3-CA1 fields, in a few pyramidal-shaped neurons in the CA1 field and subiculum, and had a distinct neuropil staining pattern in the granule cell layer and stratum pyramidale of the Ammon's horn. Calbindin immunoreactivity displayed a strong labeling of the granule cells and mossy fibers and was also observed in a population of moderately immunoreactive neurons in the CA1 field and subiculum. Calretinin immunoreactivity was relatively weaker overall. The inner molecular layer in the dentate gyrus had a distinct band of labeling, the stratum lacunosum/moleculare contained a punctate neuropil staining, and there were a few small multipolar neurons in the hilus, CA3-CA1 fields, and subiculum. Comparison of the staining patterns observed in the dog hippocampus with those in human, macaque monkeys and rats revealed that although there are some subregional differences among these taxa, the dog may constitute a valuable large animal model for the study of certain neurological conditions that affect humans, in spite of the phylogenetic distance between carnivores and primates.