Parallel prefrontal pathways reach distinct excitatory and inhibitory systems in memory-related rhinal cortices

Neuroanatomical Substrates of Circuit-Specific Cholinergic Modulation across the Primate Anterior Cingulate Cortex

Acetylcholine is a robust neuromodulator of the limbic system and a critical regulator of arousal and emotions. The anterior cingulate cortex (ACC) and the amygdala (AMY) are key limbic structures that are both densely innervated by cholinergic afferents and interact with each other for emotional regulation. The ACC is composed of functionally distinct dorsal (A24), rostral (A32), and ventral (A25) areas that differ in their connections with the AMY. The structural substrates of cholinergic modulation of distinct ACC microcircuits and outputs to AMY are thought to depend on the laminar and subcellular localization of cholinergic receptors. The present study examines the distribution of muscarinic acetylcholine receptors, m1 and m2, on distinct excitatory and inhibitory neurons and on AMY-targeting projection neurons within ACC areas, via immunohistochemistry and injections of neural tracers into the basolateral AMY in adult rhesus monkeys of both sexes. We found that laminar densities of m1+ and m2+ expressing excitatory and inhibitory neurons depended on area and cell type. Among the ACC areas, ventral subgenual ACC A25 exhibited greater m2+ localization on presynaptic inhibitory axon terminals and greater density of m1+ and m2+ expressing AMY-targeting (tracer+) pyramidal neurons. These patterns suggest robust cholinergic disinhibition and potentiation of amygdalar outputs from the limbic ventral ACC, which may be linked to the hyperexcitability of this subgenual ACC area in depression. These findings reveal the anatomical substrate of diverse cholinergic modulation of specific ACC microcircuits and amygdalar outputs that mediate cognitive-emotional integration and dysfunctions underlying stress and affective disorders.

Pattern of ventral temporal lobe interconnections in rhesus macaques

The entorhinal cortex (EC, A28) is linked through reciprocal pathways with nearby perirhinal and visual, auditory, and multimodal association cortices in the temporal lobe, in pathways associated with the flow of information for memory processing. The density and laminar organization of these pathways is not well understood in primates. We studied interconnections within the ventral temporal lobe in young adult rhesus monkeys of both sexes with the aid of neural tracers injected in temporal areas (Ts1, Ts2, TE1, area 36, temporal polar area TPro, and area 28) to determine the density and laminar distribution of projection neurons within the temporal lobe. These temporal areas can be categorized into three different cortical types based on their laminar architecture: the sensory association areas Ts1, Ts2, and TE1 have six layers (eulaminate); the perirhinal limbic areas TPro and area 36 have an incipient layer IV (dysgranular); and area 28 lacks layer IV (agranular). We found that (1) temporal areas that are similar in laminar architecture by cortical type are strongly interconnected, and (2) the laminar pattern of connections is dependent on the difference in cortical laminar structure between linked areas. Thus, agranular A28 is more strongly connected with other agranular/dysgranular areas than with eulaminate cortices. Further, A28 predominantly projected via feedback-like pathways that originated in the deep layers, and received feedforward-like projections from areas of greater laminar differentiation, which emanated from the upper layers. Our results are consistent with the Structural Model, which relates the density and laminar distribution of connections to the relationship of the laminar structure between the linked areas. These connections were viewed in the context of the inhibitory microenvironment of A28, which is the key recipient of pathways from the cortex and of the output of hippocampus. Our findings revealed a higher population of calretinin (CR)-expressing neurons in EC, with a significantly higher density in its lateral division. Medial EC had a higher density of CR neurons in the deep layers, particularly in layer Va. In contrast, parvalbumin (PV) neurons were more densely distributed in the deep layers of the lateral subdivisions of rostral EC, especially in layer Va, whereas the densities of calbindin (CB) neurons in the medial and lateral EC were comparable in all layers, except for layer IIIa, in which medial EC had a higher CB population than the lateral. The pattern of connections in the inhibitory microenvironment of EC, which sends and receives input from the hippocampus, may shed light on signal propagation in this network associated with diverse aspects of memory, and disruptions in neurologic and psychiatric diseases that affect this region.

DARPP-32 cells and neuropil define striosomal system and isolated matrix cells in human striatum

The dorsal striatum forms a central node of the basal ganglia interconnecting the neocortex and thalamus with circuits modulating mood and movement. Striatal projection neurons (SPNs) include relatively intermixed populations expressing D1-type or D2-type dopamine receptors (dSPNs and iSPNs) that give rise to the direct (D1) and indirect (D2) output systems of the basal ganglia. Overlaid on this organization is a compartmental organization, in which a labyrinthine system of striosomes made up of sequestered SPNs is embedded within the larger striatal matrix. Striosomal SPNs also include D1-SPNs and D2-SPNs, but they can be distinguished from matrix SPNs by many neurochemical markers. In the rodent striatum the key signaling molecule, DARPP-32, is a exception to these compartmental expression patterns, thought to befit its functions through opposite actions in both D1- and D2-expressing SPNs. We demonstrate here, however, that in the dorsal human striatum, DARPP-32 is concentrated in the neuropil and SPNs of striosomes, especially in the caudate nucleus and dorsomedial putamen, relative to the matrix neuropil in these regions. The generally DARPP-32-poor matrix contains scattered DARPP-32-positive cells. DARPP-32 cell bodies in both compartments proved negative for conventional intraneuronal markers. These findings raise the potential for specialized DARPP-32 expression in the human striosomal system and in a set of DARPP-32-positive neurons in the matrix. If DARPP-32 immunohistochemical positivity predicts differential functional DARPP-32 activity, then the distributions demonstrated here could render striosomes and dispersed matrix cells susceptible to differential signaling through cAMP and other signaling systems in health and disease. DARPP-32 is highly concentrated in cells and neuropil of striosomes in post-mortem human brain tissue, particularly in the dorsal caudate nucleus. Scattered DARPP-32-positive cells are found in the human striatal matrix. Calbindin and DARPP-32 do not colocalize within every spiny projection neuron in the dorsal human caudate nucleus.

The brain of the tree pangolin (Manis tricuspis). X. The spinal cord

The spinal cord of the tree pangolin is known to be very short compared to the overall length of the body and tail. Here, we provide a description of the tree pangolin spinal cord to determine whether the short length contributes to specific structural, and potentially functional, differences. The short spinal cord of the adult tree pangolin, at around 13 cm, terminates at the midthoracic level. Within this shortened spinal cord, we could identify six regions, which from rostral to caudal include the prebrachial, brachial, interramal, crural, postcrural, and caudal regions, with both the brachial and crural regions showing distinct swellings. The chemoarchitecture of coronal sections through these regions confirmed regional assignation, being most readily delineated by the presence of cholinergic neurons forming the intermediolateral column in the interramal region and the sacral parasympathetic nucleus in the postcrural region. The 10 laminae of Rexed were observed throughout the spinal cord and presented with an anatomical organization similar to that observed in other mammals. Despite the shortened length of the tree pangolin spinal cord, the regional and laminar anatomical organization is very similar to that observed in other mammals. This indicates that the functional aspects of the short tree pangolin spinal cord can be inferred from what is known in other mammals.

The brain of the tree pangolin (Manis tricuspis). VIII. The subpallial telencephalon

The current study provides a detailed architectural analysis of the subpallial telencephalon of the tree pangolin. In the tree pangolin, the subpallial telencephalon was divided into septal and striatopallidal regions. The septal region contained the septal nuclear complex, diagonal band of Broca, and the bed nuclei of the stria terminalis. The striatopallidal region comprised of the dorsal (caudate, putamen, internal and external globus pallidus) and ventral (nucleus accumbens, olfactory tubercle, ventral pallidum, nucleus basalis, basal part of the substantia innominata, lateral stripe of the striatum, navicular nucleus, and the major island of Calleja) striatopallidal complexes. In the tree pangolin, the organization and numbers of nuclei forming these regions and complexes, their topographical relationships to each other, and the cyto‐, myelo‐, and chemoarchitecture, were found to be very similar to that observed in commonly studied mammals. Minor variations, such as less nuclear parcellation in the bed nuclei of the stria terminalis, may represent species‐specific variations, or may be the result of the limited range of stains used. Given the overall similarity across mammalian species, it appears that the subpallial telencephalon of the mammalian brain is highly conserved in terms of evolutionary changes detectable with the methods used. It is also likely that the functions associated with these nuclei in other mammals can be translated directly to the tree pangolin, albeit with the understanding that the stimuli that produce activity within these regions may be specific to the life history requirements of the tree pangolin.

The brain of the tree pangolin (Manis tricuspis). VII. The amygdaloid body

Here, we describe the cytoarchitecture and chemoarchitecture of the amygdaloid body of the tree pangolin. Our definition of the amygdaloid body includes the pallial portions of the amygdala, and the centromedial group that is a derivative of the subpallium and part of the extended amygdala. The remainder of the extended amygdala is not described herein. Within the amygdaloid body of the tree pangolin, we identified the basolateral group (composed of the lateral, basal, and accessory basal amygdaloid nuclei), the superficial, or cortical nuclei (the anterior and posterior cortical nuclei, the periamygdaloid cortex, and nuclei of the olfactory tract), the centromedial group (the central amygdaloid nucleus and the medial nuclear cluster), and other amygdaloid nuclei (the anterior amygdaloid area, the amygdalohippocampal area, the intramedullary group, and intercalated islands). The location within and relative to each other within the amygdaloid body and the internal subdivisions of these groups were very similar to that reported in other mammalian species, with no clearly derived features specific to the tree pangolin. The only variation was the lack of an insular appearance of the intercalated islands, which in the tree pangolin were observed as a continuous band of neurons located dorsomedial to the basolateral group similar in appearance to and almost continuous with the intramedullary group. In carnivores, the closest relatives of the pangolins, and laboratory rats, a similar appearance of portions of the intercalated islands has been noted.

The distribution, number, and certain neurochemical identities of infracortical white matter neurons in the brains of a southern lesser galago, a black-capped squirrel monkey, and a crested macaque

In the current study, we examined the number, distribution, and aspects of the neurochemical identities of infracortical white matter neurons, also termed white matter interstitial cells (WMICs), in the brains of a southern lesser galago (Galago moholi), a black-capped squirrel monkey (Saimiri boliviensis boliviensis), and a crested macaque (Macaca nigra). Staining for neuronal nuclear marker (NeuN) revealed WMICs throughout the infracortical white matter, these cells being most dense close to inner cortical border, decreasing in density with depth in the white matter. Stereological analysis of NeuN-immunopositive cells revealed estimates of approximately 1.1, 10.8, and 37.7 million WMICs within the infracortical white matter of the galago, squirrel monkey, and crested macaque, respectively. The total numbers of WMICs form a distinct negative allometric relationship with brain mass and white matter volume when examined in a larger sample of primates where similar measures have been obtained. In all three primates studied, the highest densities of WMICs were in the white matter of the frontal lobe, with the occipital lobe having the lowest. Immunostaining revealed significant subpopulations of WMICs containing neuronal nitric oxide synthase (nNOS) and calretinin, with very few WMICs containing parvalbumin, and none containing calbindin. The nNOS and calretinin immunopositive WMICs represent approximately 21% of the total WMIC population; however, variances in the proportions of these neurochemical phenotypes were noted. Our results indicate that both the squirrel monkey and crested macaque might be informative animal models for the study of WMICs in neurodegenerative and psychiatric disorders in humans.

The distribution, number, and certain neurochemical identities of infracortical white matter neurons in a chimpanzee (Pan troglodytes) brain

We examined the number, distribution, and immunoreactivity of the infracortical white matter neuronal population, also termed white matter interstitial cells (WMICs), throughout the telencephalic white matter of an adult female chimpanzee. Staining for neuronal nuclear marker (NeuN) revealed WMICs throughout the infracortical white matter, these cells being most numerous and dense close to the inner border of cortical layer VI, decreasing significantly in density with depth in the white matter. Stereological analysis of NeuN-immunopositive cells revealed an estimate of approximately 137.2 million WMICs within the infracortical white matter of the chimpanzee brain studied. Immunostaining revealed subpopulations of WMICs containing neuronal nitric oxide synthase (nNOS, approximately 14.4 million in number), calretinin (CR, approximately 16.7 million), very few WMICs containing parvalbumin (PV), and no calbindin-immunopositive neurons. The nNOS, CR, and PV immunopositive WMICs, possibly all inhibitory neurons, represent approximately 22.6% of the total WMIC population. As the white matter is affected in many cognitive conditions, such as schizophrenia, autism, epilepsy, and also in neurodegenerative diseases, understanding these neurons across species is important for the translation of findings of neural dysfunction in animal models to humans. Furthermore, studies of WMICs in species such as apes provide a crucial phylogenetic context for understanding the evolution of these cell types in the human brain.

Distribution and overlap of entorhinal, premotor, and amygdalar connections in the monkey anterior cingulate cortex

The anterior cingulate cortex (ACC) is important for decision-making as it integrates motor plans with affective and contextual limbic information. Disruptions in these networks have been observed in depression, bipolar disorder, and post-traumatic stress disorder. Yet, overlap of limbic and motor connections within subdivisions of the ACC is not well understood. Hence, we administered a combination of retrograde and anterograde tracers into structures important for contextual memories (entorhinal cortex), affective processing (amygdala), and motor planning (dorsal premotor cortex) to assess overlap of labeled projection neurons from (outputs) and axon terminals to (inputs) the ACC of adult rhesus monkeys (Macaca mulatta). Our data show that entorhinal and dorsal premotor cortical (dPMC) connections are segregated across ventral (A25, A24a) and dorsal (A24b,c) subregions of the ACC, while amygdalar connections are more evenly distributed across subregions. Among all areas, the rostral ACC (A32) had the lowest relative density of connections with all three regions. In the ventral ACC, entorhinal and amygdalar connections strongly overlap across all layers, especially in A25. In the dorsal ACC, outputs to dPMC and the amygdala strongly overlap in deep layers. However, dPMC input to the dorsal ACC was densest in deep layers, while amygdalar inputs predominantly localized in upper layers. These connection patterns are consistent with diverse roles of the dorsal ACC in motor evaluation and the ventral ACC in affective and contextual memory. Further, distinct laminar circuits suggest unique interactions within specific ACC compartments that are likely important for the temporal integration of motor and limbic information during flexible goal-directed behavior.

The brain of the African wild dog. II. The olfactory system

Employing a range of neuroanatomical stains, we detail the organization of the main and accessory olfactory systems of the African wild dog. The organization of both these systems follows that typically observed in mammals, but variations of interest were noted. Within the main olfactory bulb, the size of the glomeruli, at approximately 350 μm in diameter, are on the larger end of the range observed across mammals. In addition, we estimate that approximately 3,500 glomeruli are present in each main olfactory bulb. This larger main olfactory bulb glomerular size and number of glomeruli indicates that enhanced peripheral processing of a broad range of odorants is occurring in the main olfactory bulb of the African wild dog. Within the accessory olfactory bulb, the glomeruli did not appear distinct, rather forming a homogenous syncytia-like arrangement as seen in the domestic dog. In addition, the laminar organization of the deeper layers of the accessory olfactory bulb was indistinct, perhaps as a consequence of the altered architecture of the glomeruli. This arrangement of glomeruli indicates that rather than parcellating the processing of semiochemicals peripherally, these odorants may be processed in a more nuanced and combinatorial manner in the periphery, allowing for more rapid and precise behavioral responses as required in the highly social group structure observed in the African wild dog. While having a similar organization to that of other mammals, the olfactory system of the African wild dog has certain features that appear to correlate to their environmental niche.

The brain of the African wild dog. III. The auditory system

The large external pinnae and extensive vocal repertoire of the African wild dog (Lycaon pictus) has led to the assumption that the auditory system of this unique canid may be specialized. Here, using cytoarchitecture, myeloarchitecture, and a range of immunohistochemical stains, we describe the systems-level anatomy of the auditory system of the African wild dog. We observed the cochlear nuclear complex, superior olivary nuclear complex, lateral lemniscus, inferior colliculus, medial geniculate body, and auditory cortex all being in their expected locations, and exhibiting the standard subdivisions of this system. While located in the ectosylvian gyri, the auditory cortex includes several areas, resembling the parcellation observed in cats and ferrets, although not all of the auditory areas known from these species could be identified in the African wild dog. These observations suggest that, broadly speaking, the systems-level anatomy of the auditory system, and by extension the processing of auditory information, within the brain of the African wild dog closely resembles that observed in other carnivores. Our findings indicate that it is likely that the extraction of the semantic content of the vocalizations of African wild dogs, and the behaviors generated, occurs beyond the classically defined auditory system, in limbic or association neocortical regions involved in cognitive functions. Thus, to obtain a deeper understanding of how auditory stimuli are processed, and how communication is achieved, in the African wild dog compared to other canids, cortical regions beyond the primary sensory areas will need to be examined in detail.

Distribution, number, and certain neurochemical identities of infracortical white matter neurons in the brains of three megachiropteran bat species

A large population of infracortical white matter neurons, or white matter interstitial cells (WMICs), are found within the subcortical white matter of the mammalian telencephalon. We examined WMICs in three species of megachiropterans, Megaloglossus woermanni, Casinycteris argynnis, and Rousettus aegyptiacus, using immunohistochemical and stereological techniques. Immunostaining for neuronal nuclear marker (NeuN) revealed substantial numbers of WMICs in each species-M. woermanni 124,496 WMICs, C. argynnis 138,458 WMICs, and the larger brained R. aegyptiacus having an estimated WMIC population of 360,503. To examine the range of inhibitory neurochemical types we used antibodies against parvalbumin, calbindin, calretinin, and neural nitric oxide synthase (nNOS). The calbindin and nNOS immunostained neurons were the most commonly observed, while those immunoreactive for calretinin and parvalbumin were sparse. The proportion of WMICs exhibiting inhibitory neurochemical profiles was ~26%, similar to that observed in previously studied primates. While for the most part the WMIC population in the megachiropterans studied was similar to that observed in other mammals, the one feature that differed was the high proportion of WMICs immunoreactive to calbindin, whereas in primates (macaque monkey, lar gibbon and human) the highest proportion of inhibitory WMICs contain calretinin. Interestingly, there appears to be an allometric scaling of WMIC numbers with brain mass. Further quantitative comparative work across more mammalian species will reveal the developmental and evolutionary trends associated with this infrequently studied neuronal population.

Differences in Noradrenaline Receptor Expression Across Different Neuronal Subtypes in Macaque Frontal Eye Field

Cognitive functions such as attention and working memory are modulated by noradrenaline receptors in the prefrontal cortex (PFC). The frontal eye field (FEF) has been shown to play an important role in visual spatial attention. However, little is known about the underlying circuitry. The aim of this study was to characterize the expression of noradrenaline receptors on different pyramidal neuron and inhibitory interneuron subtypes in macaque FEF. Using immunofluorescence, we found broad expression of noradrenaline receptors across all layers of the FEF. Differences in the expression of different noradrenaline receptors were observed across different inhibitory interneuron subtypes. No significant differences were observed in the expression of noradrenaline receptors across different pyramidal neuron subtypes. However, we found that putative long-range projecting pyramidal neurons expressed all noradrenaline receptor subtypes at a much higher proportion than any of the other neuronal subtypes. Nearly all long-range projecting pyramidal neurons expressed all types of noradrenaline receptor, suggesting that there is no receptor-specific machinery acting on these long-range projecting pyramidal neurons. This pattern of expression among long-range projecting pyramidal neurons suggests a mechanism by which noradrenergic modulation of FEF activity influences attention and working memory.

Tyrosine hydroxylase containing neurons in the thalamic reticular nucleus of male equids

Here we report the unusual presence of thalamic reticular neurons immunoreactive for tyrosine hydroxylase in equids. The diencephalons of one adult male of four equid species, domestic donkey (Equus africanus asinus), domestic horse (Equus caballus), Cape mountain zebra (Equus zebra zebra) and plains zebra (Equus quagga), were sectioned in a coronal plane with series of sections stained for Nissl substance, myelin, or immunostained for tyrosine hydroxylase, and the calcium-binding proteins parvalbumin, calbindin and calretinin. In all equid species studied the thalamic reticular nucleus was observed as a sheet of neurons surrounding the rostral, lateral and ventral portions of the nuclear mass of the dorsal thalamus. In addition, these thalamic reticular neurons were immunopositive for parvalbumin, but immunonegative for calbindin and calretinin. Moreover, the thalamic reticular neurons in the equids studied were also immunopositive for tyrosine hydroxylase. Throughout the grey matter of the dorsal thalamus a terminal network also immunoreactive for tyrosine hydroxylase was present. Thus, the equid thalamic reticular neurons appear to provide a direct and novel potentially catecholaminergic innervation of the thalamic relay neurons. This finding is discussed in relation to the function of the thalamic reticular nucleus and the possible effect of a potentially novel catecholaminergic pathway on the neural activity of the thalamocortical loop.

Serial Prefrontal Pathways Are Positioned to Balance Cognition and Emotion in Primates

The delicate balance among primate prefrontal networks is necessary for homeostasis and behavioral flexibility. Dorsolateral prefrontal cortex (dlPFC) is associated with cognition, while the most ventromedial subgenual cingulate area 25 (A25) is associated with emotion and emotional expression. Yet A25 is weakly connected with dlPFC, and it is unknown how the two regions communicate. In rhesus monkeys of both sexes, we investigated how these functionally distinct areas may interact through pregenual anterior cingulate area 32 (A32), which is strongly connected with both. We found that dlPFC innervated the deep layers of A32, while A32 innervated all layers of A25, mostly targeting spines of excitatory neurons. Approximately 20% of A32 terminations formed synapses on inhibitory neurons in A25, notably the powerful parvalbumin inhibitory neurons in the deep layers, and the disinhibitory calretinin neurons in the superficial layers. By innervating distinct inhibitory microenvironments in laminar compartments, A32 is positioned to tune activity in columns of A25. The circuitry of the sequential pathway indicates that when dlPFC is engaged, A32 can dampen A25 output through the parvalbumin inhibitory microsystem in the deep layers of A25. A32 thus may flexibly recruit or reduce activity in A25 to maintain emotional equilibrium, a process that is disrupted in depression. Moreover, pyramidal neurons in A25 had a heightened density of NMDARs, which are the targets of novel rapid-acting antidepressants. Pharmacologic antagonism of NMDARs in patients with depression may reduce excitability in A25, mimicking the effects of the neurotypical serial pathway identified here.SIGNIFICANCE STATEMENT The anterior cingulate is a critical hub in prefrontal networks through connections with functionally distinct areas. Dorsolateral and polar prefrontal areas that are associated with complex cognition are connected with the anterior cingulate in a pattern that allows them to indirectly control downstream activity from the anterior cingulate to the subgenual cingulate, which is associated with heightened activity and negative affect in depression. This set of pathways provides a circuit mechanism for emotional regulation, with the anterior cingulate playing a balancing role for integration of cognitive and emotional processes. Disruption of these pathways may perturb network function and the ability to regulate cognitive and affective processes based on context.

Imbalance of laminar-specific excitatory and inhibitory circuits of the orbitofrontal cortex in autism

BACKGROUND

The human orbitofrontal cortex (OFC) is involved in assessing the emotional significance of events and stimuli, emotion-based learning, allocation of attentional resources, and social cognition. Little is known about the structure, connectivity and excitatory/inhibitory circuit interactions underlying these diverse functions in human OFC, as well as how the circuit is disrupted in individuals with autism spectrum disorder (ASD).

METHODS

We used post-mortem brain tissue from neurotypical adults and individuals with ASD. We examined the morphology and distribution of myelinated axons across cortical layers in OFC, at the single axon level, as a proxy of excitatory pathways. In the same regions, we also examined the laminar distribution of all neurons and neurochemically- and functionally-distinct inhibitory neurons that express the calcium-binding proteins parvalbumin (PV), calbindin (CB), and calretinin (CR).

RESULTS

We found that the density of myelinated axons increased consistently towards layer 6, while the average axon diameter did not change significantly across layers in both groups. However, both the density and diameter of myelinated axons were significantly lower in the ASD group compared with the Control group. The distribution pattern and density of the three major types of inhibitory neurons was comparable between groups, but there was a significant reduction in the density of excitatory neurons across OFC layers in ASD.

LIMITATIONS

This study is limited by the availability of human post-mortem tissue optimally processed for high-resolution microscopy and immunolabeling, especially from individuals with ASD.

CONCLUSIONS

The balance between excitation and inhibition in OFC is at the core of its function, assessing and integrating emotional and social cues with internal states and external inputs. Our preliminary results provide evidence for laminar-specific changes in the ratio of excitation/inhibition in OFC of adults with ASD, with an overall weakening and likely disorganization of excitatory signals and a relative strengthening of local inhibition. These changes likely underlie pathology of major OFC communications with limbic or other cortices and the amygdala in individuals with ASD, and may provide the anatomic basis for disrupted transmission of signals for social interactions and emotions in autism.

The hippocampal formation of two carnivore species: The feliform banded mongoose and the caniform domestic ferret

Employing cyto-, myelo-, and chemoarchitectural staining techniques, we analyzed the structure of the hippocampal formation in the banded mongoose and domestic ferret, species belonging to the two carnivoran superfamilies, which have had independent evolutionary trajectories for the past 55 million years. Our observations indicate that, despite the time since sharing a last common ancestor, these species show extensive similarities. The four major portions of the hippocampal formation (cornu Ammonis, dentate gyrus, subicular complex, and entorhinal cortex) were readily observed, contained the same internal subdivisions, and maintained the topological relationships of these subdivisions that could be considered typically mammalian. In addition, adult hippocampal neurogenesis was observed in both species, occurring at a rate similar to that observed in other mammals. Despite the overall similarities, several differences to each other, and to other mammalian species, were observed. We could not find evidence for the presence of the CA2 and CA4 fields of the cornu Ammonis region. In the banded mongoose the dentate gyrus appears to be comprised of up to seven lamina, through the sublamination of the molecular and granule cell layers, which is not observed in the domestic ferret. In addition, numerous subtle variations in chemoarchitecture between the two species were observed. These differences may contribute to an overall variation in the functionality of the hippocampal formation between the species, and in comparison to other mammalian species. These similarities and variations are important to understanding to what extent phylogenetic affinities and constraints affect potential adaptive evolutionary plasticity of the hippocampal formation.

The diencephalon of two carnivore species: The feliform banded mongoose and the caniform domestic ferret

This study provides an analysis of the cytoarchitecture, myeloarchitecture, and chemoarchitecture of the diencephalon (dorsal thalamus, ventral thalamus, and epithalamus) of the banded mongoose (Mungos mungo) and domestic ferret (Mustela putorius furo). Using architectural and immunohistochemical stains, we observe that the nuclear organization of the diencephalon is very similar in the two species, and similar to that reported in other carnivores, such as the domestic cat and dog. The same complement of putatively homologous nuclei were identified in both species, with only one variance, that being the presence of the perireticular nucleus in the domestic ferret, that was not observed in the banded mongoose. The chemoarchitecture was also mostly consistent between species, although there were a number of minor variations across a range of nuclei in the density of structures expressing the calcium-binding proteins parvalbumin, calbindin, and calretinin. Thus, despite almost 53 million years since these two species of carnivores shared a common ancestor, strong phylogenetic constraints appear to limit the potential for adaptive evolutionary plasticity within the carnivore order. Apart from the presence of the perireticular nucleus, the most notable difference between the species studied was the physical inversion of the dorsal lateral geniculate nucleus, as well as the lateral posterior and pulvinar nuclei in the domestic ferret compared to the banded mongoose and other carnivores, although this inversion appears to be a feature of the Mustelidae family. While no functional sequelae are suggested, this inversion is likely to result from the altricial birth of Mustelidae species.

The amygdaloid body of two carnivore species: The feliform banded mongoose and the caniform domestic ferret

The current study provides an analysis of the cytoarchitecture, myeloarchitecture, and chemoarchitecture of the amygdaloid body of the banded mongoose (Mungos mungo) and domestic ferret (Mustela putorius furo). Using architectural and immunohistochemical stains, we observe that the organization of the nuclear and cortical portions of the amygdaloid complex is very similar in both species. The one major difference is the presence of a cortex-amygdala transition zone observed in the domestic ferret that is absent in the banded mongoose. In addition, the chemoarchitecture is, for the most part, quite similar in the two species, but several variances, such as differing densities of neurons expressing the calcium-binding proteins in specific nuclei are noted. Despite this, certain aspects of the chemoarchitecture, such as the cholinergic innervation of the magnocellular division of the basal nuclear cluster and the presence of doublecortin expressing neurons in the shell division of the accessory basal nuclear cluster, appear to be consistent features of the Eutherian mammal amygdala. The domestic ferret presented with an overall lower myelin density throughout the amygdaloid body than the banded mongoose, a feature that may reflect artificial selection in the process of domestication for increased juvenile-like behavior in the adult domestic ferret, such as a muted fear response. The shared, but temporally distant, ancestry of the banded mongoose and domestic ferret allows us to generate observations relevant to understanding the relative influence that phylogenetic constraints, adaptive evolutionary plasticity, and the domestication process may play in the organization and chemoarchitecture of the amygdaloid body.

The brain of the African wild dog. IV. The visual system

The variegated pelage and social complexity of the African wild dog (Lycaon pictus) hint at the possibility of specializations of the visual system. Here, using a range of architectural and immunohistochemical stains, we describe the systems-level organization of the image-forming, nonimage forming, oculomotor, and accessory optic, vision-associated systems in the brain of one representative individual of the African wild dog. For all of these systems, the organization, in terms of location, parcellation and topology (internal and external), is very similar to that reported in other carnivores. The image-forming visual system consists of the superior colliculus, visual dorsal thalamus (dorsal lateral geniculate nucleus, pulvinar and lateral posterior nucleus) and visual cortex (occipital, parietal, suprasylvian, temporal and splenial visual regions). The nonimage forming visual system comprises the suprachiasmatic nucleus, ventral lateral geniculate nucleus, pretectal nuclear complex and the Edinger-Westphal nucleus. The oculomotor system incorporates the oculomotor, trochlear and abducens cranial nerve nuclei as well as the parabigeminal nucleus, while the accessory optic system includes the dorsal, lateral and medial terminal nuclei. The extent of similarity to other carnivores in the systems-level organization of these systems indicates that the manner in which these systems process visual information is likely to be consistent with that found, for example, in the well-studied domestic cat. It would appear that the sociality of the African wild dog is dependent upon the processing of information extracted from the visual system in the higher-order cognitive and affective neural systems.

The brain of the tree pangolin (Manis tricuspis). IV. The hippocampal formation

Employing a range of standard and immunohistochemical stains we provide a description of the hippocampal formation in the brain of the tree pangolin. For the most part, the architecture, chemical neuroanatomy, and topological relationships of the component parts of the hippocampal formation of the tree pangolin were consistent with that observed in other mammalian species. Within the hippocampus proper fields CA1, 3, and 4 could be identified with certainty, while CA2 was tentatively identified as a small transitional zone between the CA1 and CA3 fields. Within the dentate gyrus evidence for adult hippocampal neurogenesis at a rate comparable to other mammals was observed. The subicular complex and entorhinal cortex also exhibited divisions typically observed in other mammalian species. In contrast to many other mammals, an architecturally and neurochemically distinct CA4 field was observed, supporting Lorente de Nó's proposed CA4 field, at least in some mammalian species. In addition, up to seven laminae were evident in the dentate gyrus. Calretinin immunostaining revealed the three sublamina of the molecular layer, while immunostaining for vesicular glutamate transporter 2 and neurofilament H indicate that the granule cell layer was composed of two sublamina. The similarities and differences observed in the tree pangolin indicate that the hippocampal formation is an anatomically and neurochemically conserved neural unit in mammalian evolution, but minor changes may relate to specific life history features and habits of species.

The distribution, number, and certain neurochemical identities of infracortical white matter neurons in a lar gibbon (Hylobates lar) brain

We examined the number, distribution, and immunoreactivity of the infracortical white matter neuronal population, also termed white matter interstitial cells (WMICs), in the brain of a lesser ape, the lar gibbon. Staining for neuronal nuclear marker (NeuN) revealed WMICs throughout the infracortical white matter, these cells being most numerous and dense close to cortical layer VI, decreasing significantly in density with depth in the white matter. Stereological analysis of NeuN-immunopositive cells revealed a global estimate of ~67.5 million WMICs within the infracortical white matter of the gibbon brain, indicating that the WMICs are a numerically significant population, ~2.5% of the total cortical gray matter neurons that would be estimated for a primate brain the mass of that of the lar gibbon. Immunostaining revealed subpopulations of WMICs containing neuronal nitric oxide synthase (nNOS, ~7 million in number, with both small and large soma volumes), calretinin (~8.6 million in number, all of similar soma volume), very few WMICs containing parvalbumin, and no calbindin-immunopositive neurons. These nNOS, calretinin, and parvalbumin immunopositive WMICs, presumably all inhibitory neurons, represent ~23.1% of the total WMIC population. As the white matter is affected in many cognitive conditions, such as schizophrenia, autism and also in neurodegenerative diseases, understanding these neurons across species is important for the translation of findings of neural dysfunction in animal models to humans. Furthermore, studies of WMICs in species such as apes provide a crucial phylogenetic context for understanding the evolution of these cell types in the human brain.

Specificity of Primate Amygdalar Pathways to Hippocampus

The amygdala projects to hippocampus in pathways through which affective or social stimuli may influence learning and memory. We investigated the still unknown amygdalar termination patterns and their postsynaptic targets in hippocampus from system to synapse in rhesus monkeys of both sexes. The amygdala robustly innervated the stratum lacunosum-moleculare layer of cornu ammonis fields and uncus anteriorly. Sparser terminations in posterior hippocampus innervated the radiatum and pyramidal layers at the prosubicular/CA1 juncture. The terminations, which were larger than other afferents in the surrounding neuropil, position the amygdala to influence hippocampal input anteriorly, and its output posteriorly. Most amygdalar boutons (76-80%) innervated spines of excitatory hippocampal neurons, and most of the remaining innervated presumed inhibitory neurons, identified by morphology and label with parvalbumin or calretinin, which distinguished nonoverlapping neurochemical classes of hippocampal inhibitory neurons. In CA1, amygdalar axons innervated some calretinin neurons, which disinhibit pyramidal neurons. By contrast, in CA3 the amygdala innervated both calretinin and parvalbumin neurons; the latter strongly inhibit nearby excitatory neurons. In CA3, amygdalar pathways also made closely spaced dual synapses on excitatory neurons. The strong excitatory synapses in CA3 may facilitate affective context representations and trigger sharp-wave ripples associated with memory consolidation. When the amygdala is excessively activated during traumatic events, the specialized innervation of excitatory neurons and the powerful parvalbumin inhibitory neurons in CA3 may allow the suppression of activity of nearby neurons that receive weaker nonamygdalar input, leading to biased passage of highly charged affective stimuli and generalized fear.SIGNIFICANCE STATEMENT Strong pathways from the amygdala targeted the anterior hippocampus, and more weakly its posterior sectors, positioned to influence a variety of emotional and cognitive functions. In hippocampal field CA1, the amygdala innervated some calretinin neurons, which disinhibit excitatory neurons. By contrast, in CA3 the amygdala innervated calretinin as well as some of the powerful parvalbumin inhibitory neurons and may help balance the activity of neural ensembles to allow social interactions, learning, and memory. These results suggest that when the amygdala is hyperactive during emotional upheaval, it strongly activates excitatory hippocampal neurons and parvalbumin inhibitory neurons in CA3, which can suppress nearby neurons that receive weaker input from other sources, biasing the passage of stimuli with high emotional import and leading to generalized fear.

Parallel trends in cortical gray and white matter architecture and connections in primates allow fine study of pathways in humans and reveal network disruptions in autism

Noninvasive imaging and tractography methods have yielded information on broad communication networks but lack resolution to delineate intralaminar cortical and subcortical pathways in humans. An important unanswered question is whether we can use the wealth of precise information on pathways from monkeys to understand connections in humans. We addressed this question within a theoretical framework of systematic cortical variation and used identical high-resolution methods to compare the architecture of cortical gray matter and the white matter beneath, which gives rise to short- and long-distance pathways in humans and rhesus monkeys. We used the prefrontal cortex as a model system because of its key role in attention, emotions, and executive function, which are processes often affected in brain diseases. We found striking parallels and consistent trends in the gray and white matter architecture in humans and monkeys and between the architecture and actual connections mapped with neural tracers in rhesus monkeys and, by extension, in humans. Using the novel architectonic portrait as a base, we found significant changes in pathways between nearby prefrontal and distant areas in autism. Our findings reveal that a theoretical framework allows study of normal neural communication in humans at high resolution and specific disruptions in diverse psychiatric and neurodegenerative diseases.

Cortical Connections Position Primate Area 25 as a Keystone for Interoception, Emotion, and Memory

The structural and functional integrity of subgenual cingulate area 25 (A25) is crucial for emotional expression and equilibrium. A25 has a key role in affective networks, and its disruption has been linked to mood disorders, but its cortical connections have yet to be systematically or fully studied. Using neural tracers in rhesus monkeys, we found that A25 was densely connected with other ventromedial and posterior orbitofrontal areas associated with emotions and homeostasis. A moderate pathway linked A25 with frontopolar area 10, an area associated with complex cognition, which may regulate emotions and dampen negative affect. Beyond the frontal lobe, A25 was connected with auditory association areas and memory-related medial temporal cortices, and with the interoceptive-related anterior insula. A25 mostly targeted the superficial cortical layers of other areas, where broadly dispersed terminations comingled with modulatory inhibitory or disinhibitory microsystems, suggesting a dominant excitatory effect. The architecture and connections suggest that A25 is the consummate feedback system in the PFC. Conversely, in the entorhinal cortex, A25 pathways terminated in the middle-deep layers amid a strong local inhibitory microenvironment, suggesting gating of hippocampal output to other cortices and memory storage. The graded cortical architecture and associated laminar patterns of connections suggest how areas, layers, and functionally distinct classes of inhibitory neurons can be recruited dynamically to meet task demands. The complement of cortical connections of A25 with areas associated with memory, emotion, and somatic homeostasis provide the circuit basis to understand its vulnerability in psychiatric and neurologic disorders.SIGNIFICANCE STATEMENT Integrity of the prefrontal subgenual cingulate cortex is crucial for healthy emotional function. Subgenual area 25 (A25) is mostly linked with other prefrontal areas associated with emotion in a dense network positioned to recruit large fields of cortex. In healthy states, A25 is associated with internal states, autonomic function, and transient negative affect. Constant hyperactivity in A25 is a biomarker for depression in humans and may trigger extensive activation in its dominant connections with areas associated with emotions and internal balance. A pathway between A25 and frontopolar area 10 may provide a critical link to regulate emotions and dampen persistent negative affect, which may be explored for therapeutic intervention in depression.

Anterior Cingulate Pathways May Affect Emotions Through Orbitofrontal Cortex

The anterior cingulate cortex (ACC) and posterior orbitofrontal cortex (pOFC) are associated with emotional regulation. These regions are old in phylogeny and have widespread connections with eulaminate neocortices, intricately linking areas associated with emotion and cognition. The ACC and pOFC have distinct cortical and subcortical connections and are also interlinked, but the pattern of their connections-which may be used to infer the flow of information between them-is not well understood. Here we found that pathways from ACC area 32 innervated all pOFC areas with a significant proportion of large and efficient terminals, seen at the level of the system and the synapse. The pathway from area 32 targeted overwhelmingly elements of excitatory neurons in pOFC, with few postsynaptic sites found on presumed inhibitory neurons. Moreover, pathways from area 32 originated mostly in the upper layers and innervated preferentially the middle-deep layers of the least differentiated pOFC areas, in a pattern reminiscent of feedforward communication. Pathway terminations from area 32 overlapped in the deep layers of pOFC with output pathways that project to the thalamus and the amygdala, and may have cascading downstream effects on emotional and cognitive processes and their disruption in psychiatric disorders.

Posterior Orbitofrontal and Anterior Cingulate Pathways to the Amygdala Target Inhibitory and Excitatory Systems with Opposite Functions

The bidirectional dialogue of the primate posterior orbitofrontal cortex (pOFC) with the amygdala is essential in cognitive-emotional functions. The pOFC also sends a uniquely one-way excitatory pathway to the amygdalar inhibitory intercalated masses (IM), which inhibit the medial part of the central amygdalar nucleus (CeM). Inhibition of IM has the opposite effect, allowing amygdalar activation of autonomic structures and emotional arousal. Using multiple labeling approaches to identify pathways and their postsynaptic sites in the amygdala in rhesus monkeys, we found that the anterior cingulate cortex innervated mostly the basolateral and CeM amygdalar nuclei, poised to activate CeM for autonomic arousal. By contrast, a pathway from pOFC to IM exceeded all other pathways to the amygdala by density and size and proportion of large and efficient terminals. Moreover, whereas pOFC terminals in IM innervated each of the three distinct classes of inhibitory neurons, most targeted neurons expressing dopamine- and cAMP-regulated phosphoprotein (DARPP-32+), known to be modulated by dopamine. The predominant pOFC innervation of DARPP-32+ neurons suggests activation of IM and inhibition of CeM, resulting in modulated autonomic function. By contrast, inhibition of DARPP-32 neurons in IM by high dopamine levels disinhibits CeM and triggers autonomic arousal. The findings provide a mechanism to help explain how a strong pOFC pathway, which is poised to moderate activity of CeM, through IM, can be undermined by the high level of dopamine during stress, resulting in collapse of potent inhibitory mechanisms in the amygdala and heightened autonomic drive, as seen in chronic anxiety disorders.SIGNIFICANCE STATEMENT The dialogue between prefrontal cortex and amygdala allows thoughts and emotions to influence actions. The posterior orbitofrontal cortex sends a powerful pathway that targets a special class of amygdalar intercalated mass (IM) inhibitory neurons, whose wiring may help modulate autonomic function. By contrast, the anterior cingulate cortex innervates other amygdalar parts, activating circuits to help avoid danger. Most IM neurons in primates label for the protein DARPP-32, known to be activated or inhibited based on the level of dopamine. Stress markedly increases dopamine release and inhibits IM neurons, compromises prefrontal control of the amygdala, and sets off a general alarm system as seen in affective disorders, such as chronic anxiety and post-traumatic stress disorder.

Prefrontal-hippocampal pathways underlying inhibitory control over memory

A key function of the prefrontal cortex is to support inhibitory control over behavior. It is widely believed that this function extends to stopping cognitive processes as well. Consistent with this, mounting evidence establishes the role of the right lateral prefrontal cortex in a clear case of cognitive control: retrieval suppression. Retrieval suppression refers to the ability to intentionally stop the retrieval process that arises when a reminder to a memory appears. Functional imaging data indicate that retrieval suppression involves top-down modulation of hippocampal activity by the dorsolateral prefrontal cortex, but the anatomical pathways supporting this inhibitory modulation remain unclear. Here we bridge this gap by integrating key findings about retrieval suppression observed through functional imaging with a detailed consideration of relevant anatomical pathways observed in non-human primates. Focusing selectively on the potential role of the anterior cingulate cortex, we develop two hypotheses about the pathways mediating interactions between lateral prefrontal cortex and the medial temporal lobes during suppression, and their cellular targets: the entorhinal gating hypothesis, and thalamo-hippocampal modulation via the nucleus reuniens. We hypothesize that whereas entorhinal gating is well situated to stop retrieval proactively, thalamo-hippocampal modulation may interrupt an ongoing act of retrieval reactively. Isolating the pathways that underlie retrieval suppression holds the potential to advance our understanding of a range of psychiatric disorders characterized by persistent intrusive thoughts. More broadly, an anatomical account of retrieval suppression would provide a key model system for understanding inhibitory control over cognition.

Late postnatal shifts of parvalbumin and nitric oxide synthase expression within the GABAergic and glutamatergic phenotypes of inferior colliculus neurons

The inferior colliculus (IC) is partitioned into three subdivisions: the dorsal and lateral cortices (DC and LC) and the central nucleus (ICC), and serves as an integration center of auditory information. Recent studies indicate that a certain population of IC neurons may represent the non-GABAergic phenotype, while they express well-established cortical/hippocampal GABAergic neuron markers. In this study we used the optical disector to investigate the phenotype of IC neurons expressing parvalbumin (PV) and/or nitric oxide synthase (NOS) in C57BL/6J mice during the late postnatal period. Four major types of IC neurons were defined by the presence (+) or absence (-) of PV, NOS, and glutamic acid decarboxylase 67 (GAD67): PV⁺ /NOS^- /GAD67⁺ , PV⁺ /NOS⁺ /GAD67⁺ , PV⁺ /NOS^- /GAD67^- , and PV^- /NOS⁺ /GAD67^- . Fluorescent in situ hybridization for vesicular glutamate transporter 2 mRNA indicated that almost all GAD67^- IC neurons represented the glutamatergic phenotype. The numerical densities (NDs) of total GAD67⁺ IC neurons remained unchanged in all subdivisions. The NDs of PV⁺ /NOS^- /GAD67⁺ neurons and PV^- /NOS⁺ /GAD67^- neurons were reduced with age in the ICC, while they remained unchanged in the DC and LC. By contrast, the NDs of PV⁺ /NOS⁺ /GAD67⁺ neurons and PV⁺ /NOS^- /GAD67^- neurons were increased with age in the ICC, although there were no changes in the DC and LC. The cell body size of GAD67⁺ IC neurons did not vary according to the expression of PV with or without NOS. The present findings indicate that the expression of PV and NOS may shift with age within the GABAergic and glutamatergic phenotypes of IC neurons during the late postnatal period. J. Comp. Neurol. 525:868-884, 2017. © 2016 Wiley Periodicals, Inc.

Organization of the connections between claustrum and cortex in the mouse

The connections between the claustrum and the cortex in mouse are systematically investigated with adeno-associated virus (AAV), an anterograde viral tracer. We first define the boundary and the three-dimensional structure of the claustrum based on a variety of molecular and anatomical data. From AAV injections into 42 neocortical and allocortical areas, we conclude that most cortical areas send bilateral projections to the claustrum, the majority being denser on the ipsilateral side. This includes prelimbic, infralimbic, medial, ventrolateral and lateral orbital, ventral retrosplenial, dorsal and posterior agranular insular, visceral, temporal association, dorsal and ventral auditory, ectorhinal, perirhinal, lateral entorhinal, and anteromedial, posteromedial, lateroposterior, laterointermediate, and postrhinal visual areas. In contrast, the cingulate and the secondary motor areas send denser projections to the contralateral claustrum than to the ipsilateral one. The gustatory, primary auditory, primary visual, rostrolateral visual, and medial entorhinal cortices send projections only to the ipsilateral claustrum. Primary motor, primary somatosensory and subicular areas barely send projections to either ipsi- or contralateral claustrum. Corticoclaustral projections are organized in a rough topographic manner, with variable projection strengths. We find that the claustrum, in turn, sends widespread projections preferentially to ipsilateral cortical areas with different projection strengths and laminar distribution patterns and to certain contralateral cortical areas. Our quantitative results show that the claustrum has strong reciprocal and bilateral connections with prefrontal and cingulate areas as well as strong reciprocal connections with the ipsilateral temporal and retrohippocampal areas, suggesting that it may play a crucial role in a variety of cognitive processes. J. Comp. Neurol. 525:1317-1346, 2017. © 2016 Wiley Periodicals, Inc.

The primate connectome in context: Principles of connections of the cortical visual system

Which principles determine the organization of the intricate network formed by nerve fibers that link the primate cerebral cortex? We addressed this issue for the connections of primate visual cortices by systematically analyzing how the existence or absence of connections, their density as well as laminar patterns of projection origins and terminations are correlated with distance, similarity in cortical type as well as neuronal density or the thickness of cortical areas. Analyses were based on four extensive compilations of qualitative as well as quantitative data for connections of the primate visual cortical system in macaque monkeys (Felleman and Van Essen 1991; Barbas 1986; Barbas and Rempel-Clower 1997; Barone et al. 2000; Markov et al. 2014). Distance and thickness similarity were not consistently correlated with connection features, but similarity of cortical type, determined by qualitative features of laminar differentiation, or measured quantitatively as the areas' overall neuronal density, was a reliable predictor for the existence of connections between areas. Cortical type similarity was also consistently and closely correlated with characteristic laminar connection profiles: structurally dissimilar areas had origin and termination patterns that were biased to the upper or deep cortical layers, while similar areas showed more bilaminar origins and terminations. These results suggest that patterns of corticocortical connections of primate visual cortices are closely linked to the stratified architecture of the cerebral cortex. In particular, the regularity of laminar projection origins and terminations arises from the structural differences between cortical areas. The observed integration of projections with the intrinsic cortical architecture provides a structural basis for advanced theories of cortical organization and function.

Organization of the sleep-related neural systems in the brain of the harbour porpoise (Phocoena phocoena)

The present study provides the first systematic immunohistochemical neuroanatomical investigation of the systems involved in the control and regulation of sleep in an odontocete cetacean, the harbor porpoise (Phocoena phocoena). The odontocete cetaceans show an unusual form of mammalian sleep, with unihemispheric slow waves, suppressed REM sleep, and continuous bodily movement. All the neural elements involved in sleep regulation and control found in bihemispheric sleeping mammals were present in the harbor porpoise, with no specific nuclei being absent, and no novel nuclei being present. This qualitative similarity of nuclear organization relates to the cholinergic, noradrenergic, serotonergic, and orexinergic systems and is extended to the γ-aminobutyric acid (GABA)ergic elements involved with these nuclei. Quantitative analysis of the cholinergic and noradrenergic nuclei of the pontine region revealed that in comparison with other mammals, the numbers of pontine cholinergic (126,776) and noradrenergic (122,878) neurons are markedly higher than in other large-brained bihemispheric sleeping mammals. The diminutive telencephalic commissures (anterior commissure, corpus callosum, and hippocampal commissure) along with an enlarged posterior commissure and supernumerary pontine cholinergic and noradrenergic neurons indicate that the control of unihemispheric slow-wave sleep is likely to be a function of interpontine competition, facilitated through the posterior commissure, in response to unilateral telencephalic input related to the drive for sleep. In addition, an expanded peripheral division of the dorsal raphe nuclear complex appears likely to play a role in the suppression of REM sleep in odontocete cetaceans. Thus, the current study provides several clues to the understanding of the neural control of the unusual sleep phenomenology present in odontocete cetaceans. J. Comp. Neurol. 524:1999-2017, 2016. © 2016 Wiley Periodicals, Inc.

Molecular features distinguish ten neuronal types in the mouse superficial superior colliculus

The superior colliculus (SC) is a midbrain center involved in controlling head and eye movements in response to inputs from multiple sensory modalities. Visual inputs arise from both the retina and visual cortex and converge onto the superficial layer of the SC (sSC). Neurons in the sSC send information to deeper layers of the SC and to thalamic nuclei that modulate visually guided behaviors. Presently, our understanding of sSC neurons is impeded by a lack of molecular markers that define specific cell types. To better understand the identity and organization of sSC neurons, we took a systematic approach to investigate gene expression within four molecular families: transcription factors, cell adhesion molecules, neuropeptides, and calcium binding proteins. Our analysis revealed 12 molecules with distinct expression patterns in mouse sSC: cadherin 7, contactin 3, netrin G2, cadherin 6, protocadherin 20, retinoid-related orphan receptor β, brain-specific homeobox/POU domain protein 3b, Ets variant gene 1, substance P, somatostatin, vasoactive intestinal polypeptide, and parvalbumin. Double labeling experiments, by either in situ hybridization or immunostaining, demonstrated that the 12 molecular markers collectively define 10 different sSC neuronal types. The characteristic positions of these cell types divide the sSC into four distinct layers. The 12 markers identified here will serve as valuable tools to examine molecular mechanisms that regulate development of sSC neuronal types. These markers could also be used to examine the connections between specific cell types that form retinocollicular, corticocollicular, or colliculothalamic pathways. J. Comp. Neurol. 524:2300-2321, 2016. © 2016 Wiley Periodicals, Inc.

Representation of memories in the cortical-hippocampal system: Results from the application of population similarity analyses

Here we consider the value of neural population analysis as an approach to understanding how information is represented in the hippocampus and cortical areas and how these areas might interact as a brain system to support memory. We argue that models based on sparse coding of different individual features by single neurons in these areas (e.g., place cells, grid cells) are inadequate to capture the complexity of experience represented within this system. By contrast, population analyses of neurons with denser coding and mixed selectivity reveal new and important insights into the organization of memories. Furthermore, comparisons of the organization of information in interconnected areas suggest a model of hippocampal-cortical interactions that mediates the fundamental features of memory.

Blockade of glutamatergic transmission in perirhinal cortex impairs object recognition memory in macaques

The perirhinal cortex (PRc) is essential for visual recognition memory, as shown by electrophysiological recordings and lesion studies in a variety of species. However, relatively little is known about the functional contributions of perirhinal subregions. Here we used a systematic mapping approach to identify the critical subregions of PRc through transient, focal blockade of glutamate receptors by intracerebral infusion of kynurenic acid. Nine macaques were tested for visual recognition memory using the delayed nonmatch-to-sample task. We found that inactivation of medial PRc (consisting of Area 35 together with the medial portion of Area 36), but not lateral PRc (the lateral portion of Area 36), resulted in a significant delay-dependent impairment. Significant impairment was observed with 30 and 60 s delays but not with 10 s delays. The magnitude of impairment fell within the range previously reported after PRc lesions. Furthermore, we identified a restricted area located within the most anterior part of medial PRc as critical for this effect. Moreover, we found that focal blockade of either NMDA receptors by the receptor-specific antagonist AP-7 or AMPA receptors by the receptor-specific antagonist NBQX was sufficient to disrupt object recognition memory. The present study expands the knowledge of the role of PRc in recognition memory by identifying a subregion within this area that is critical for this function. Our results also indicate that, like in the rodent, both NMDA and AMPA-mediated transmission contributes to object recognition memory.