Zinc-enriched amygdalo- and hippocampo-cortical connections to the inferotemporal cortices in macaque monkey

Cytology, architecture, development, and connections of the primate striatum: Hints for human pathology

Degeneration of neurons and circuits across the striatum shows stereotyped time-course and spatial topography patterns that are distinct for Huntington's disease, Parkinson's disease, or the Tauopathies. These patterns of neurodegeneration in humans have not yet been systematically related to developmental, connectional, cellular, and chemical factors studied in human and non-human primates, that may underlie potential differences in selective vulnerability across striatal sectors. Relating primate anatomy to human pathology could provide new venues for identifying molecular, cellular, and connectional factors linked to the degeneration of striatal neurons and circuits. This review describes and summarizes several developmental, cellular, structural, and connectional features of the primate striatum in relation to patterns of neurodegeneration in the striatum of humans and of non-human primate models. We review (1) the types of neurons in the primate striatum, (2) the cyto-, myelo-, and chemoarchitecture of the primate striatum, (3) the developmental origin of the striatum in light of modern patterning studies, (4) the organization of corticostriatal projections in relation to cortical types, and (5) the topography and time-course of neuron loss, glial reaction, and protein aggregation induced by neurodegenerative diseases in humans and in non-human primate models across striatal sectors and their corresponding cortical areas. We summarize current knowledge about key aspects of primate striatal anatomy and human pathology and indicate knowledge gaps that should be addressed in future studies. We aim to identify factors for selective vulnerability to neurodegeneration of striatal neurons and circuits and obtain hints that could help elucidate striatal pathology in humans.

Neuroanatomical and psychological considerations in temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy and is associated with a variety of structural and psychological alterations. Recently, there has been renewed interest in using brain tissue resected during epilepsy surgery, in particular 'non-epileptic' brain samples with normal histology that can be found alongside epileptic tissue in the same epileptic patients - with the aim being to study the normal human brain organization using a variety of methods. An important limitation is that different medical characteristics of the patients may modify the brain tissue. Thus, to better determine how 'normal' the resected tissue is, it is fundamental to know certain clinical, anatomical and psychological characteristics of the patients. Unfortunately, this information is frequently not fully available for the patient from which the resected tissue has been obtained - or is not fully appreciated by the neuroscientists analyzing the brain samples, who are not necessarily experts in epilepsy. In order to present the full picture of TLE in a way that would be accessible to multiple communities (e.g., basic researchers in neuroscience, neurologists, neurosurgeons and psychologists), we have reviewed 34 TLE patients, who were selected due to the availability of detailed clinical, anatomical, and psychological information for each of the patients. Our aim was to convey the full complexity of the disorder, its putative anatomical substrates, and the wide range of individual variability, with a view toward: (1) emphasizing the importance of considering critical patient information when using brain samples for basic research and (2) gaining a better understanding of normal and abnormal brain functioning. In agreement with a large number of previous reports, this study (1) reinforces the notion of substantial individual variability among epileptic patients, and (2) highlights the common but overlooked psychopathological alterations that occur even in patients who become "seizure-free" after surgery. The first point is based on pre- and post-surgical comparisons of patients with hippocampal sclerosis and patients with normal-looking hippocampus in neuropsychological evaluations. The second emerges from our extensive battery of personality and projective tests, in a two-way comparison of these two types of patients with regard to pre- and post-surgical performance.

Extensive cortical functional connectivity of the human hippocampal memory system

The cortical connections of the human hippocampal memory system are fundamental to understanding its operation in health and disease, especially in the context of the great development of the human cortex. The functional connectivity of the human hippocampal system was analyzed in 172 participants imaged at 7T in the Human Connectome Project. The human hippocampus has high functional connectivity not only with the entorhinal cortex, but also with areas that are more distant in the ventral 'what' stream including the perirhinal cortex and temporal cortical visual areas. Parahippocampal gyrus TF in humans has connectivity with this ventral 'what' subsystem. Correspondingly for the dorsal stream, the hippocampus has high functional connectivity not only with the presubiculum, but also with areas more distant, the medial parahippocampal cortex TH which includes the parahippocampal place or scene area, the posterior cingulate including retrosplenial cortex, and the parietal cortex. Further, there is considerable cross connectivity between the ventral and dorsal streams with the hippocampus. The findings are supported by anatomical connections, which together provide an unprecedented and quantitative overview of the extensive cortical connectivity of the human hippocampal system that goes beyond hierarchically organised and segregated pathways connecting the hippocampus and neocortex, and leads to new concepts on the operation of the hippocampal memory system in humans.

Extensive Cortical Connectivity of the Human Hippocampal Memory System: Beyond the "What" and "Where" Dual Stream Model

The human hippocampus is involved in forming new memories: damage impairs memory. The dual stream model suggests that object "what" representations from ventral stream temporal cortex project to the hippocampus via the perirhinal and then lateral entorhinal cortex, and spatial "where" representations from the dorsal parietal stream via the parahippocampal gyrus and then medial entorhinal cortex. The hippocampus can then associate these inputs to form episodic memories of what happened where. Diffusion tractography was used to reveal the direct connections of hippocampal system areas in humans. This provides evidence that the human hippocampus has extensive direct cortical connections, with connections that bypass the entorhinal cortex to connect with the perirhinal and parahippocampal cortex, with the temporal pole, with the posterior and retrosplenial cingulate cortex, and even with early sensory cortical areas. The connections are less hierarchical and segregated than in the dual stream model. This provides a foundation for a conceptualization for how the hippocampal memory system connects with the cerebral cortex and operates in humans. One implication is that prehippocampal cortical areas such as the parahippocampal TF and TH subregions and perirhinal cortices may implement specialized computations that can benefit from inputs from the dorsal and ventral streams.

The ventral visual pathway: an expanded neural framework for the processing of object quality

Since the original characterization of the ventral visual pathway, our knowledge of its neuroanatomy, functional properties, and extrinsic targets has grown considerably. Here we synthesize this recent evidence and propose that the ventral pathway is best understood as a recurrent occipitotemporal network containing neural representations of object quality both utilized and constrained by at least six distinct cortical and subcortical systems. Each system serves its own specialized behavioral, cognitive, or affective function, collectively providing the raison d'être for the ventral visual pathway. This expanded framework contrasts with the depiction of the ventral visual pathway as a largely serial staged hierarchy culminating in singular object representations and more parsimoniously incorporates attentional, contextual, and feedback effects.

Hippocampal pyramidal cells: the reemergence of cortical lamination

The increasing resolution of tract-tracing studies has led to the definition of segments along the transverse axis of the hippocampal pyramidal cell layer, which may represent functionally defined elements. This review will summarize evidence for a morphological and functional differentiation of pyramidal cells along the radial (deep to superficial) axis of the cell layer. In many species, deep and superficial sublayers can be identified histologically throughout large parts of the septotemporal extent of the hippocampus. Neurons in these sublayers are generated during different periods of development. During development, deep and superficial cells express genes (Sox5, SatB2) that also specify the phenotypes of superficial and deep cells in the neocortex. Deep and superficial cells differ neurochemically (e.g. calbindin and zinc) and in their adult gene expression patterns. These markers also distinguish sublayers in the septal hippocampus, where they are not readily apparent histologically in rat or mouse. Deep and superficial pyramidal cells differ in septal, striatal, and neocortical efferent connections. Distributions of deep and superficial pyramidal cell dendrites and studies in reeler or sparsely GFP-expressing mice indicate that this also applies to afferent pathways. Histological, neurochemical, and connective differences between deep and superficial neurons may correlate with (patho-) physiological phenomena specific to pyramidal cells at different radial locations. We feel that an appreciation of radial subdivisions in the pyramidal cell layer reminiscent of lamination in other cortical areas may be critical in the interpretation of studies of hippocampal anatomy and function.

Selective gene expression in regions of primate neocortex: implications for cortical specialization

The neocortex, which is characteristic of mammals, has evolved to play important roles in cognitive and perceptual functions. The localization of different functions in different regions of the neocortex was well established within the last century. Studies on the formation of the neocortex have advanced at the molecular level, thus clarifying the mechanisms that control neural or glial cell differentiation and sensory projections. However, mechanisms that underlie cortical area specialization remain unsolved. To address this problem, our approach has been to isolate and characterize the genes that are selectively expressed in particular subsets of neocortical areas in primates; these areas are most distinctive among mammals. By differential display and restriction landmark cDNA scanning (RLCS) methods, we have identified two major classes of genes that are specifically expressed in the adult macaque monkey neocortical areas: one is expressed in the primary sensory areas, particularly, in the primary visual cortex (V1) and the other is expressed in the association areas. The genes that show these specific expression patterns are limited to only several gene families among our large-scale screening. In this review, I first describe the isolation and characterization of these genes, along with another class of genes specifically expressed in motor areas. Then, I discuss their functional significance in the primate neocortex. Finally, I discuss the implication of these gene expression patterns in neocortical specialization in primates and possible future research directions.

Different structural correlates for verbal memory impairment in temporal lobe epilepsy with and without mesial temporal lobe sclerosis

OBJECTIVES

Memory impairment is one of the most prominent cognitive deficits in temporal lobe epilepsy (TLE). The overall goal of this study was to explore the contribution of cortical and hippocampal (subfield) damage to impairment of auditory immediate recall (AIMrecall), auditory delayed recall (ADMrecall), and auditory delayed recognition (ADMrecog) of the Wechsler Memory Scale III (WMS-III) in TLE with (TLE-MTS) and without hippocampal sclerosis (TLE-no). It was hypothesized that volume loss in different subfields determines memory impairment in TLE-MTS and temporal neocortical thinning in TLE-no.

METHODS

T1 whole brain and T2-weighted hippocampal magnetic resonance imaging and WMS-III were acquired in 22 controls, 18 TLE-MTS, and 25 TLE-no. Hippocampal subfields were determined on the T2 image. Free surfer was used to obtain cortical thickness averages of temporal, frontal, and parietal cortical regions of interest (ROI). MANOVA and stepwise regression analysis were used to identify hippocampal subfields and cortical ROI significantly contributing to AIMrecall, ADMrecall, and ADMrecog.

RESULTS

In TLE-MTS, AIMrecall was associated with cornu ammonis 3 (CA3) and dentate (CA3&DG) and pars opercularis, ADMrecall with CA1 and pars triangularis, and ADMrecog with CA1. In TLE-no, AIMrecall was associated with CA3&DG and fusiform gyrus (FUSI), and ADMrecall and ADMrecog were associated with FUSI.

CONCLUSION

The study provided the evidence for different structural correlates of the verbal memory impairment in TLE-MTS and TLE-no. In TLE-MTS, the memory impairment was mainly associated by subfield-specific hippocampal and inferior frontal cortical damage. In TLE-no, the impairment was associated by mesial-temporal cortical and to a lesser degree hippocampal damage.

Pathway-specific utilization of synaptic zinc in the macaque ventral visual cortical areas

Synaptic zinc is an activity-related neuromodulator, enriched in hippocampal mossy fibers and a subset of glutamatergic cortical projections, exclusive of thalamocortical or corticothalamic. Some degree of pathway specificity in the utilization of synaptic zinc has been reported in rodents. Here, we use focal injections of the retrograde tracer sodium selenite to identify zinc-positive (Zn+) projection neurons in the monkey ventral visual pathway. After injections in V1, V4, and TEO areas, neurons were detected preferentially in several feedback pathways but, unusually, were restricted to deeper layers without involvement of layers 2 or 3. Temporal injections resulted in more extensive labeling of both feedback and intratemporal association pathways. The Zn+ neurons had a broader laminar distribution, similar to results from standard retrograde tracers. After anterograde tracer injection in area posterior TE, electron microscopic analysis substantiated that a proportion of feedback synapses was co-labeled with zinc. Nearby injections, Zn+ intrinsic neurons concentrated in layer 2, but in temporal areas were also abundant in layer 6. These results indicate considerable pathway and laminar specificity as to which cortical neurons use synaptic zinc. Given the hypothesized roles of synaptic zinc, this is likely to result in distinct synaptic properties, possibly including differential synaptic plasticity within or across projections.

Paraneoplastic antigen-like 5 gene (PNMA5) is preferentially expressed in the association areas in a primate specific manner

To understand the relationship between the structure and function of primate neocortical areas at a molecular level, we have been screening for genes differentially expressed across macaque neocortical areas by restriction landmark cDNA scanning (RLCS). Here, we report enriched expression of the paraneoplastic antigen-like 5 gene (PNMA5) in association areas but not in primary sensory areas, with the lowest expression level in primary visual cortex. In situ hybridization in the primary sensory areas revealed PNMA5 mRNA expression restricted to layer II. Along the ventral visual pathway, the expression gradually increased in the excitatory neurons from the primary to higher visual areas. This differential expression pattern was very similar to that of retinol-binding protein (RBP) mRNA, another association-area-enriched gene that we reported previously. Additional expression analysis for comparison of other genes in the PNMA gene family, PNMA1, PNMA2, PNMA3, and MOAP1 (PNMA4), showed that they were widely expressed across areas and layers but without the differentiated pattern of PNMA5. In mouse brains, PNMA1 was only faintly expressed and PNMA5 was not detected. Sequence analysis showed divergence of PNMA5 sequences among mammals. These findings suggest that PNMA5 acquired a certain specialized role in the association areas of the neocortex during primate evolution.

Differential expression patterns of occ1-related genes in adult monkey visual cortex

We have previously revealed that occ1 is preferentially expressed in the primary visual area (V1) of the monkey neocortex. In our attempt to identify more area-selective genes in the macaque neocortex, we found that testican-1, an occ1-related gene, and its family members also exhibit characteristic expression patterns along the visual pathway. The expression levels of testican-1 and testican-2 mRNAs as well as that of occ1 mRNA start of high in V1, progressively decrease along the ventral visual pathway, and end of low in the temporal areas. Complementary to them, the neuronal expression of SPARC mRNA is abundant in the association areas and scarce in V1. Whereas occ1, testican-1, and testican-2 mRNAs are preferentially distributed in thalamorecipient layers including “blobs,” SPARC mRNA expression avoids these layers. Neither SC1 nor testican-3 mRNA expression is selective to particular areas, but SC1 mRNA is abundantly observed in blobs. The expressions of occ1, testican-1, testican-2, and SC1 mRNA were downregulated after monocular tetrodotoxin injection. These results resonate with previous works on chemical and functional gradients along the primate occipitotemporal visual pathway and raise the possibility that these gradients and functional architecture may be related to the visual activity–dependent expression of these extracellular matrix glycoproteins.

Complementary circuits connecting the orbital and medial prefrontal networks with the temporal, insular, and opercular cortex in the macaque monkey

The origin and termination of axonal connections between the orbital and medial prefrontal cortex (OMPFC) and the temporal, insular, and opercular cortex have been analyzed with anterograde and retrograde axonal tracers, injected in the OMPFC or temporal cortex. The results show that there are two distinct, complementary, and reciprocal neural systems, related to the previously defined "orbital" and "medial" prefrontal networks. The orbital prefrontal network, which includes areas in the central and lateral part of the orbital cortex, is connected with vision-related areas in the inferior temporal cortex (especially area TEav) and the fundus and ventral bank of the superior temporal sulcus (STSf/v), and with somatic sensory-related areas in the frontal operculum (OPf) and dysgranular insular area (Id). No connections were found between the orbital network and auditory areas. The orbital network is also connected with taste and olfactory cortical areas and the perirhinal cortex and appears to be involved in assessment of sensory objects, especially food. The medial prefrontal network includes areas on the medial surface of the frontal lobe, medial orbital areas, and two caudolateral orbital areas. It is connected with the rostral superior temporal gyrus (STGr) and the dorsal bank of the superior temporal sulcus (STSd). This region is rostral to the auditory parabelt areas, and there are only relatively light connections between the auditory areas and the medial network. This system, which is also connected with the entorhinal, parahippocampal, and cingulate/retrosplenial cortex, may be involved in emotion and other self-referential processes.

Differential modes of termination of amygdalothalamic and amygdalocortical projections in the monkey

The amygdala complex participates in multiple systems having to do with affective processes. It has been implicated in human disorders of social and emotional behavior, such as autism. Of the interconnected functional networks, considerable research in rodents and primates has focused on connections between the amygdala and orbitofrontal cortex (OFC). The amygdala projects to OFC by both a direct amygdalocortical (AC) pathway and an indirect pathway through mediodorsal thalamus. In the rat, retrograde tracer experiments indicate that the AC and amygdalothalamic (AT) pathways originate from separate populations, and may therefore convey distinctive information, although the characteristics of these pathways remain unclear. To investigate this issue in monkeys we made anterograde tracer injections in the basolateral amygdala complex (BLC; n = 3). Three distinctive features were found preferentially associated with the AT or AC pathways. First, AT terminations are large (average diameter = 3.5 microm; range = 1.2-7.0 microm) and cluster around proximal dendrites, in contrast with small-bouton AC terminations. Second, AT terminations form small arbors (diameter approximately 0.1 mm), while AC are widely divergent (often >1.0 mm long). The AT terminations features are reminiscent of large bouton, "driver" corticothalamic terminations. Finally, AC but not AT terminations are positive for zinc (Zn), a neuromodulator associated with synaptic plasticity. From these results we suggest that AC and AT terminations originate from distinct populations in monkey as well as in rodent. Further work is necessary to determine the degree and manner of their segregation and how these subsystems interact within a broader connectivity network.

Cytoarchitectonic and chemoarchitectonic subdivisions of the perirhinal and parahippocampal cortices in macaque monkeys

Although the perirhinal and parahippocampal cortices have been shown to be critically involved in memory processing, the boundaries and extent of these areas have been controversial. To produce a more objective and reproducible description, the architectonic boundaries and structure of the perirhinal (areas 35 and 36) and parahippocampal (areas TF and TH) cortices were analyzed in three macaque species, with four different staining methods [Nissl and immunohistochemistry for parvalbumin, nonphosphorylated neurofilaments (with SMI-32), and the m2 muscarinic acetylcholine receptor]. We further correlated the architectonic boundary of the parahippocampal cortex with connections to and from different subregions of anterior area TE and with previously published connections with the prefrontal cortex and temporal pole (Kondo et al. [2005] J. Comp. Neurol. 493:479-509). Together, these data provided a clear delineation of the perirhinal and parahippocampal areas, although it differs from previous descriptions. In particular, we did not extend the perirhinal cortex into the temporal pole, and the lateral boundaries of areas 36 and TF with area TE were placed more medially than in other studies. The lateral boundary of area TF in Macaca fuscata was located more laterally than in Macaca fascicularis or Macaca mulatta, although there was no difference in architectonic structure. We recognized a caudal, granular part of the parahippocampal cortex that we termed "area TFO." This area closely resembles the laterally adjacent area TE and the caudally adjacent area V4 but is clearly different from the more rostral area TF. These areas are likely to have distinct functions.

Neocortical areas, layers, connections, and gene expression

Cortical patterns of gene expression provide a new approach to long standing issues of lamination, and area identity and formation. In this review, we summarize recent findings where molecular biological techniques have revealed a small number of area-specific genes in the nonhuman primate cortex. One of these (occ1) is strongly expressed in primary visual cortex and is associated with thalamocortical connections. Another gene, RBP, is more strongly expressed in association areas. It is not clear whether RBP might be linked with any particular connectional system, but several possibilities are raised. We also discuss possible roles of area-specific genes in postnatal development, and conclude with a brief sketch of future directions.

Differential connections of the perirhinal and parahippocampal cortex with the orbital and medial prefrontal networks in macaque monkeys

Previous anatomical studies indicate that the orbital and medial prefrontal cortex (OMPFC) of monkeys is organized into an "orbital" network, which appears to be related to feeding and reward, and a "medial" network, related to visceral control and emotion. In this study, we examined the connections of the orbital and medial prefrontal networks with the perirhinal (areas 35 and 36) and parahippocampal (areas TF and TH) cortex with anterograde and retrograde axonal tracers. The perirhinal cortex is reciprocally connected with orbital network areas Iapm, Iam, Ial, 13m, 13l, 12r, and 11l. In contrast, the parahippocampal cortex is reciprocally connected with the medial network, especially areas around the corpus callosum (areas 24a/b, caudal 32, and 25), and with area 11m. Projections from the parahippocampal cortex also extend to areas 10m, 10o, Iai, and rostral area 32, as well as to dorsolateral areas 9 and 46. In addition, both the perirhinal and parahippocampal cortex are reciprocally connected with areas that are intermediate between the orbital and medial networks (areas 13a, 13b, and 14c) and with the supracallosal area 24a'/b'. Outside the frontal cortex, the perirhinal cortex and the orbital prefrontal network are both interconnected with the ventral part of the temporal pole (TG), area TE and the ventral bank and fundus of the superior temporal sulcus (STS), and the dysgranular insula. In contrast, the parahippocampal cortex and the medial prefrontal network are connected with the dorsal TG, the rostral superior temporal gyrus (STG) and dorsal bank of STS, and the retrosplenial cortex.

Distinctive morphology of hippocampal CA1 terminations in orbital and medial frontal cortex in macaque monkeys

The fine morphology of hippocampal connections to the orbital and medial frontal cortex (OMFC) was investigated by placing injections of anterograde tracers in the CA1 in two monkeys. The axons terminated mainly in layers 2 and 3, of areas 11, 13, 14c, 25, and 32, and were widely divergent in these layers, traversing 2-4 mm. Boutons were scattered along the main axon, but also occurred as distinctive small, spherical clusters of terminations ("mini-clusters"; diameter <50 microm). Occasional larger terminal arbors were observed in layer 3 and these were often unusually tortuous or convoluted. These features may imply a specialized microcircuitry of hippocampal-OMFC connections, including an origin from a particular subpopulation of CA1 projection neurons.