Cholinergic innervation of the frontal cortex: Differences among humans, chimpanzees, and macaque monkeys

Noradrenergic and cholinergic systems take centre stage in neuropsychiatric diseases of ageing

Noradrenergic and cholinergic systems are among the most vulnerable brain systems in neuropsychiatric diseases of ageing, including Alzheimer's disease, Parkinson's disease, Lewy body dementia, and progressive supranuclear palsy. As these systems fail, they contribute directly to many of the characteristic cognitive and psychiatric symptoms. However, their contribution to symptoms is not sufficiently understood, and pharmacological interventions targeting noradrenergic and cholinergic systems have met with mixed success. Part of the challenge is the complex neurobiology of these systems, operating across multiple timescales, and with non-linear changes across the adult lifespan and disease course. We address these challenges in a detailed review of the noradrenergic and cholinergic systems, outlining their roles in cognition and behaviour, and how they influence neuropsychiatric symptoms in disease. By bridging across levels of analysis, we highlight opportunities for improving drug therapies and for pursuing personalised medicine strategies.

Epigenetic aging of the prefrontal cortex and cerebellum in humans and chimpanzees

Epigenetic age has emerged as an important biomarker of biological aging. It has revealed that some tissues age faster than others, which is vital to understanding the complex phenomenon of aging and developing effective interventions. Previous studies have demonstrated that humans exhibit heterogeneity in pace of epigenetic aging among brain structures that are consistent with differences in structural and microanatomical deterioration. Here, we add comparative data on epigenetic brain aging for chimpanzees, humans' closest relatives. Such comparisons can further our understanding of which aspects of human aging are evolutionarily conserved or specific to our species, especially given that humans are distinguished by a long lifespan, large brain, and, potentially, more severe neurodegeneration with age. Specifically, we investigated epigenetic aging of the dorsolateral prefrontal cortex and cerebellum, of humans and chimpanzees by generating genome-wide CpG methylation data and applying established epigenetic clock algorithms to produce estimates of biological age for these tissues. We found that both species exhibit relatively slow epigenetic aging in the brain relative to blood. Between brain structures, humans show a faster rate of epigenetic aging in the dorsolateral prefrontal cortex compared to the cerebellum, which is consistent with previous findings. Chimpanzees, in contrast, show comparable rates of epigenetic aging in the two brain structures. Greater epigenetic change in the human dorsolateral prefrontal cortex compared to the cerebellum may reflect both the protracted development of this structure in humans and its greater age-related vulnerability to neurodegenerative pathology.

Genetic Mechanisms Underlying the Evolution of Connectivity in the Human Cortex

One of the most salient features defining modern humans is our remarkable cognitive capacity, which is unrivaled by any other species. Although we still lack a complete understanding of how the human brain gives rise to these unique abilities, the past several decades have witnessed significant progress in uncovering some of the genetic, cellular, and molecular mechanisms shaping the development and function of the human brain. These features include an expansion of brain size and in particular cortical expansion, distinct physiological properties of human neurons, and modified synaptic development. Together they specify the human brain as a large primate brain with a unique underlying neuronal circuit architecture. Here, we review some of the known human-specific features of neuronal connectivity, and we outline how novel insights into the human genome led to the identification of human-specific genetic modifiers that played a role in the evolution of human brain development and function. Novel experimental paradigms are starting to provide a framework for understanding how the emergence of these human-specific genomic innovations shaped the structure and function of neuronal circuits in the human brain.

Evolution of prefrontal cortex

Subdivisions of the prefrontal cortex (PFC) evolved at different times. Agranular parts of the PFC emerged in early mammals, and rodents, primates, and other modern mammals share them by inheritance. These are limbic areas and include the agranular orbital cortex and agranular medial frontal cortex (areas 24, 32, and 25). Rodent research provides valuable insights into the structure, functions, and development of these shared areas, but it contributes less to parts of the PFC that are specific to primates, namely, the granular, isocortical PFC that dominates the frontal lobe in humans. The first granular PFC areas evolved either in early primates or in the last common ancestor of primates and tree shrews. Additional granular PFC areas emerged in the primate stem lineage, as represented by modern strepsirrhines. Other granular PFC areas evolved in simians, the group that includes apes, humans, and monkeys. In general, PFC accreted new areas along a roughly posterior to anterior trajectory during primate evolution. A major expansion of the granular PFC occurred in humans in concert with other association areas, with modifications of corticocortical connectivity and gene expression, although current evidence does not support the addition of a large number of new, human-specific PFC areas.

Distribution of cholinergic neurons in the brains of a lar gibbon and a chimpanzee

Using choline acetyltransferase immunohistochemistry, we describe the nuclear parcellation of the cholinergic system in the brains of two apes, a lar gibbon (Hylobates lar) and a chimpanzee (Pan troglodytes). The cholinergic nuclei observed in both apes studied are virtually identical to that observed in humans and show very strong similarity to the cholinergic nuclei observed in other primates and mammals more generally. One specific difference between humans and the two apes studied is that, with the specific choline acetyltransferase antibody used, the cholinergic pyramidal neurons observed in human cerebral cortex were not labeled. When comparing the two apes studied and humans to other primates, the presence of a greatly expanded cholinergic medullary tegmental field, and the presence of cholinergic neurons in the intermediate and dorsal horns of the cervical spinal cord are notable variations of the distribution of cholinergic neurons in apes compared to other primates. These neurons may play an important role in the modulation of ascending and descending neural transmissions through the spinal cord and caudal medulla, potentially related to the differing modes of locomotion in apes compared to other primates. Our observations also indicate that the average soma volume of the neurons forming the laterodorsal tegmental nucleus (LDT) is larger than those of the pedunculopontine nucleus (PPT) in both the lar gibbon and chimpanzee. This variability in soma volume appears to be related to the size of the adult derivatives of the alar and basal plate across mammalian species.

Integrated phylogeny of the human brain and pathobiology of Alzheimer's disease: A unifying hypothesis

The disproportionate evolutionary expansion of the human cerebral cortex with reinforcement of cholinergic innervations warranted a major rise in the functional and metabolic load of the conserved basal forebrain (BF) cholinergic system. Given that acetylcholine (ACh) regulates properties of the microtubule-associated protein (MAP) tau and promotes non-amyloidogenic processing of amyloid precursor protein (APP), growing neocortex predicts higher demands for ACh, while the emerging role of BF cholinergic projections in Aβ clearance infers greater exposure of source neurons and their innervation fields to amyloid pathology. The higher exposure of evolutionary most recent cortical areas to the amyloid pathology of Alzheimer's disease (AD) with synaptic impairments and atrophy, therefore, might involve attenuated homeostatic effects of BF cholinergic projections, in addition to fall-outs of inherent processes of expanding association areas. This unifying model, thus, views amyloid pathology and loss of cholinergic cells as a quid pro quo of the allometric evolution of the human brain, which in combination with increase in life expectancy overwhelm the fine homeostatic balance and trigger the disease process.

Chimpanzees Predict the Hedonic Outcome of Novel Taste Combinations: The Evolutionary Origins of Affective Forecasting

Affective forecasting–predicting the emotional outcome of never-before experienced situations–is pervasive in our lives. When facing novel situations, we can quickly integrate bits and pieces of prior experiences to envisage possible scenarios and their outcomes, and what these might feel like. Such affective glimpses of the future often steer the decisions we make. By enabling principled decision-making in novel situations, affective forecasting confers the important adaptive advantage of eluding the potentially costly consequences of tackling such situations by trial-and-error. Affective forecasting has been hypothesized as uniquely human, yet, in a recent study we found suggestive evidence of this ability in an orangutan. To test non-verbal subjects, we capitalized on culinary examples of affective forecasting and devised a behavioral test that required the subjects to make predictions about novel juice mixes produced from familiar ingredients. In the present study, we administered the same task to two chimpanzees and found that their performance was comparable to that of the previously tested orangutan and 10 humans, who served as a comparison group. To improve the comparability of human and animal performance, in the present study we also introduced a new approach to assessing if the subjects’ performance was indicative of affective forecasting, which relies exclusively on behavioral data. The results of the study open for the possibility that affective forecasting has evolved in the common ancestor of the great apes, providing Hominids with the adaptive advantage of e.g., quickly evaluating heterogeneous food patches using hedonic prediction.

Acetylcholine acts on songbird premotor circuitry to invigorate vocal output

Acetylcholine is well-understood to enhance cortical sensory responses and perceptual sensitivity in aroused or attentive states. Yet little is known about cholinergic influences on motor cortical regions. Here we use the quantifiable nature of birdsong to investigate how acetylcholine modulates the cortical (pallial) premotor nucleus HVC and shapes vocal output. We found that dialyzing the cholinergic agonist carbachol into HVC increased the pitch, amplitude, tempo and stereotypy of song, similar to the natural invigoration of song that occurs when males direct their songs to females. These carbachol-induced effects were associated with increased neural activity in HVC and occurred independently of basal ganglia circuitry. Moreover, we discovered that the normal invigoration of female-directed song was also accompanied by increased HVC activity and was attenuated by blocking muscarinic acetylcholine receptors. These results indicate that, analogous to its influence on sensory systems, acetylcholine can act directly on cortical premotor circuitry to adaptively shape behavior.

Diverse Spatiotemporal Scales of Cholinergic Signaling in the Neocortex

ACh is a signaling molecule in the mammalian CNS, with well-documented influence over cognition and behavior. However, the nature of cholinergic signaling in the brain remains controversial, with ongoing debates focused on the spatial and temporal resolution of ACh activity. Generally, opposing views have embraced a dichotomy between transmission as slow and volume-mediated versus fast and synaptic. Here, we provide the perspective that ACh, like most other neurotransmitters, exhibits both fast and slow modes that are strongly determined by the anatomy of cholinergic fibers, the distribution and the signaling mechanisms of receptor subtypes, and the dynamics of ACh hydrolysis. Current methodological approaches remain limited in their ability to provide detailed analyses of these underlying factors. However, we believe that the continued development of novel technologies in combination with a more nuanced view of cholinergic activity will open critical new avenues to a better understanding of ACh in the brain.Dual Perspectives Companion Paper: Forebrain Cholinergic Signaling: Wired and Phasic, Not Tonic, and Causing Behavior, by Martin Sarter and Cindy Lustig.

Life history changes accompany increased numbers of cortical neurons: A new framework for understanding human brain evolution

Narratives of human evolution have focused on cortical expansion and increases in brain size relative to body size, but considered that changes in life history, such as in age at sexual maturity and thus the extent of childhood and maternal dependence, or maximal longevity, are evolved features that appeared as consequences of selection for increased brain size, or increased cognitive abilities that decrease mortality rates, or due to selection for grandmotherly contribution to feeding the young. Here I build on my recent finding that slower life histories universally accompany increased numbers of cortical neurons across warm-blooded species to propose a simpler framework for human evolution: that slower development to sexual maturity and increased post-maturity longevity are features that do not require selection, but rather inevitably and immediately accompany evolutionary increases in numbers of cortical neurons, thus fostering human social interactions and cultural and technological evolution as generational overlap increases.

Structure and function of dual-source cholinergic modulation in early vision

Behavioral states such as arousal and attention have profound effects on sensory processing, determining how-even whether-a stimulus is perceived. This state-dependence is believed to arise, at least in part, in response to inputs from subcortical structures that release neuromodulators such as acetylcholine, often nonsynaptically. The mechanisms that underlie the interaction between these nonsynaptic signals and the more point-to-point synaptic cortical circuitry are not well understood. This review highlights the state of the field, with a focus on cholinergic action in early visual processing. Key anatomical and physiological features of both the cholinergic and the visual systems are discussed. Furthermore, presenting evidence of cholinergic modulation in visual thalamus and primary visual cortex, we explore potential functional roles of acetylcholine and its effects on the processing of visual input over the sleep-wake cycle, sensory gain control during wakefulness, and consider evidence for cholinergic support of visual attention.

Is There a Canonical Cortical Circuit for the Cholinergic System? Anatomical Differences Across Common Model Systems

Acetylcholine (ACh) is believed to act as a neuromodulator in cortical circuits that support cognition, specifically in processes including learning, memory consolidation, vigilance, arousal and attention. The cholinergic modulation of cortical processes is studied in many model systems including rodents, cats and primates. Further, these studies are performed in cortical areas ranging from the primary visual cortex to the prefrontal cortex and using diverse methodologies. The results of these studies have been combined into singular models of function-a practice based on an implicit assumption that the various model systems are equivalent and interchangeable. However, comparative anatomy both within and across species reveals important differences in the structure of the cholinergic system. Here, we will review anatomical data including innervation patterns, receptor expression, synthesis and release compared across species and cortical area with a focus on rodents and primates. We argue that these data suggest no canonical cortical model system exists for the cholinergic system. Further, we will argue that as a result, care must be taken both in combining data from studies across cortical areas and species, and in choosing the best model systems to improve our understanding and support of human health.

Axonal components of nerves innervating the human arm

OBJECTIVE

Axons traveling within the brachial plexus are responsible for the dexterous control of human arm and hand movements. Despite comprehensive knowledge on the topographical anatomy of nerves innervating the human upper limbs, the definite quantity of sensory and motor axons within this neural network remains elusive. Our aim was to perform a quantitative analysis of the axonal components of human upper limb nerves based on highly specific molecular features from spinal cord level to the terminal nerves at wrist level.

METHODS

Nerve specimen harvest at predefined harvesting sites (plexus roots and cords as well as major nerves originating from the brachial plexus innervating the arm and hand) was performed in 9 human heart-beating organ donors. Double immunofluorescence staining using antibodies against choline-acetyltransferase and neurofilament was performed to differentiate motor and sensory axons on nerve cross sections.

RESULTS

Three hundred fifty thousand axons emerge from the spinal cord to innervate the human upper limb, of which 10% are motor neurons. In all nerves studied, sensory axons outnumber motor axons by a ratio of at least 9:1. The sensory axon contribution increases when moving distally, whereas only 1,700 motor axons reach the hand to innervate the intrinsic musculature.

INTERPRETATION

Our results suggest that upper limb motor execution, and particularly dexterous coordination of hand movement, require an unexpectedly low number of motor neurons, with a large convergence of afferent input for feedback control. Ann Neurol 2017;82:396-408.

Evolution of the Human Nervous System Function, Structure, and Development

The nervous system-in particular, the brain and its cognitive abilities-is among humans' most distinctive and impressive attributes. How the nervous system has changed in the human lineage and how it differs from that of closely related primates is not well understood. Here, we consider recent comparative analyses of extant species that are uncovering new evidence for evolutionary changes in the size and the number of neurons in the human nervous system, as well as the cellular and molecular reorganization of its neural circuits. We also discuss the developmental mechanisms and underlying genetic and molecular changes that generate these structural and functional differences. As relevant new information and tools materialize at an unprecedented pace, the field is now ripe for systematic and functionally relevant studies of the development and evolution of human nervous system specializations.

Ultrastructural analysis of parvalbumin synapses in human dorsolateral prefrontal cortex

Coordinated activity of neural circuitry in the primate dorsolateral prefrontal cortex (DLPFC) supports a range of cognitive functions. Altered DLPFC activation is implicated in a number of human psychiatric and neurological illnesses. Proper DLPFC activity is, in part, maintained by two populations of neurons containing the calcium-binding protein parvalbumin (PV): local inhibitory interneurons that form Type II synapses, and long-range glutamatergic inputs from the thalamus that form Type I synapses. Understanding the contributions of each PV neuronal population to human DLPFC function requires a detailed examination of their anatomical properties. Consequently, we performed an electron microscopic analysis of (1) the distribution of PV immunoreactivity within the neuropil, (2) the properties of dendritic shafts of PV-IR interneurons, (3) Type II PV-IR synapses from PV interneurons, and (4) Type I PV-IR synapses from long-range projections, within the superficial and middle laminar zones of the human DLPFC. In both laminar zones, Type II PV-IR synapses from interneurons comprised ∼60% of all PV-IR synapses, and Type I PV-IR synapses from putative thalamocortical terminals comprised the remaining ∼40% of PV-IR synapses. Thus, the present study suggests that innervation from PV-containing thalamic nuclei extends across superficial and middle layers of the human DLPFC. These findings contrast with previous ultrastructural studies in monkey DLPFC where Type I PV-IR synapses were not identified in the superficial laminar zone. The presumptive added modulation of DLPFC circuitry by the thalamus in human may contribute to species-specific, higher-order functions.

Cholinergic Potentiation and Audiovisual Repetition-Imitation Therapy Improve Speech Production and Communication Deficits in a Person with Crossed Aphasia by Inducing Structural Plasticity in White Matter Tracts

Donepezil (DP), a cognitive-enhancing drug targeting the cholinergic system, combined with massed sentence repetition training augmented and speeded up recovery of speech production deficits in patients with chronic conduction aphasia and extensive left hemisphere infarctions (Berthier et al., 2014). Nevertheless, a still unsettled question is whether such improvements correlate with restorative structural changes in gray matter and white matter pathways mediating speech production. In the present study, we used pharmacological magnetic resonance imaging to study treatment-induced brain changes in gray matter and white matter tracts in a right-handed male with chronic conduction aphasia and a right subcortical lesion (crossed aphasia). A single-patient, open-label multiple-baseline design incorporating two different treatments and two post-treatment evaluations was used. The patient received an initial dose of DP (5 mg/day) which was maintained during 4 weeks and then titrated up to 10 mg/day and administered alone (without aphasia therapy) during 8 weeks (Endpoint 1). Thereafter, the drug was combined with an audiovisual repetition-imitation therapy (Look-Listen-Repeat, LLR) during 3 months (Endpoint 2). Language evaluations, diffusion weighted imaging (DWI), and voxel-based morphometry (VBM) were performed at baseline and at both endpoints in JAM and once in 21 healthy control males. Treatment with DP alone and combined with LLR therapy induced marked improvement in aphasia and communication deficits as well as in selected measures of connected speech production, and phrase repetition. The obtained gains in speech production remained well-above baseline scores even 4 months after ending combined therapy. Longitudinal DWI showed structural plasticity in the right frontal aslant tract and direct segment of the arcuate fasciculus with both interventions. VBM revealed no structural changes in other white matter tracts nor in cortical areas linked by these tracts. In conclusion, cholinergic potentiation alone and combined with a model-based aphasia therapy improved language deficits by promoting structural plastic changes in right white matter tracts.

Cell-specific modulation of plasticity and cortical state by cholinergic inputs to the visual cortex

Acetylcholine (ACh) modulates diverse vital brain functions. Cholinergic neurons from the basal forebrain innervate a wide range of cortical areas, including the primary visual cortex (V1), and multiple cortical cell types have been found to be responsive to ACh. Here we review how different cell types contribute to different cortical functions modulated by ACh. We specifically focus on two major cortical functions: plasticity and cortical state. In layer II/III of V1, ACh acting on astrocytes and somatostatin-expressing inhibitory neurons plays critical roles in these functions. Cell type specificity of cholinergic modulation points towards the growing understanding that even diffuse neurotransmitter systems can mediate specific functions through specific cell classes and receptors.

Cholinergic profiles in the Goettingen miniature pig (Sus scrofa domesticus) brain

Central cholinergic structures within the brain of the even-toed hoofed Goettingen miniature domestic pig (Sus scrofa domesticus) were evaluated by immunohistochemical visualization of choline acetyltransferase (ChAT) and the low-affinity neurotrophin receptor, p75^NTR . ChAT-immunoreactive (-ir) perikarya were seen in the olfactory tubercle, striatum, medial septal nucleus, vertical and horizontal limbs of the diagonal band of Broca, and the nucleus basalis of Meynert, medial habenular nucleus, zona incerta, neurosecretory arcuate nucleus, cranial motor nuclei III and IV, Edinger-Westphal nucleus, parabigeminal nucleus, pedunculopontine nucleus, and laterodorsal tegmental nucleus. Cholinergic ChAT-ir neurons were also found within transitional cortical areas (insular, cingulate, and piriform cortices) and hippocampus proper. ChAT-ir fibers were seen throughout the dentate gyrus and hippocampus, in the mediodorsal, laterodorsal, anteroventral, and parateanial thalamic nuclei, the fasciculus retroflexus of Meynert, basolateral and basomedial amygdaloid nuclei, anterior pretectal and interpeduncular nuclei, as well as select laminae of the superior colliculus. Double immunofluorescence demonstrated that virtually all ChAT-ir basal forebrain neurons were also p75^NTR -positive. The present findings indicate that the central cholinergic system in the miniature pig is similar to other mammalian species. Therefore, the miniature pig may be an appropriate animal model for preclinical studies of neurodegenerative diseases where the cholinergic system is compromised. J. Comp. Neurol. 525:553-573, 2017. © 2016 Wiley Periodicals, Inc.

Modulatory compartments in cortex and local regulation of cholinergic tone

Neuromodulatory signaling is generally considered broad in its impact across cortex. However, variations in the characteristics of cortical circuits may introduce regionally-specific responses to diffuse modulatory signals. Features such as patterns of axonal innervation, tissue tortuosity and molecular diffusion, effectiveness of degradation pathways, subcellular receptor localization, and patterns of receptor expression can lead to local modification of modulatory inputs. We propose that modulatory compartments exist in cortex and can be defined by variation in structural features of local circuits. Further, we argue that these compartments are responsible for local regulation of neuromodulatory tone. For the cholinergic system, these modulatory compartments are regions of cortical tissue within which signaling conditions for acetylcholine are relatively uniform, but between which signaling can vary profoundly. In the visual system, evidence for the existence of compartments indicates that cholinergic modulation likely differs across the visual pathway. We argue that the existence of these compartments calls for thinking about cholinergic modulation in terms of finer-grained control of local cortical circuits than is implied by the traditional view of this system as a diffuse modulator. Further, an understanding of modulatory compartments provides an opportunity to better understand and perhaps correct signal modifications that lead to pathological states.

Human-specific increase of dopaminergic innervation in a striatal region associated with speech and language: A comparative analysis of the primate basal ganglia

The dopaminergic innervation of the striatum has been implicated in learning processes and in the development of human speech and language. Several lines of evidence suggest that evolutionary changes in dopaminergic afferents of the striatum may be associated with uniquely human cognitive and behavioral abilities, including the association of the human-specific sequence of the FOXP2 gene with decreased dopamine in the dorsomedial striatum of mice. To examine this possibility, we quantified the density of tyrosine hydroxylase-immunoreactive axons as a measure of dopaminergic innervation within five basal ganglia regions in humans, great apes, and New and Old World monkeys. Our results indicate that humans differ from nonhuman primate species in having a significant increase in dopaminergic innervation selectively localized to the medial caudate nucleus. This region of the striatum is highly interconnected, receiving afferents from multiple neocortical regions, and supports behavioral and cognitive flexibility. The medial caudate nucleus also shows hyperactivity in humans lacking a functional FOXP2 allele and exhibits altered dopamine concentrations in humanized Foxp2 mice. Additionally, striatal dopaminergic input was not altered in chimpanzees that used socially learned attention-getting sounds versus those that did not. This evidence indicates that the increase in dopamine innervation of the medial caudate nucleus in humans is a species-typical characteristic not associated with experience-dependent plasticity. The specificity of this increase may be related to the degree of convergence from cortical areas within this region of the striatum and may also be involved in human speech and language. J. Comp. Neurol. 524:2117-2129, 2016. © 2015 Wiley Periodicals, Inc.

A dual comparative approach: integrating lines of evidence from human evolutionary neuroanatomy and neurodevelopmental disorders

The evolution of the human brain has been marked by a nearly 3-fold increase in size since our divergence from the last common ancestor shared with chimpanzees and bonobos. Despite increased interest in comparative neuroanatomy and phylogenetic methods, relatively little is known regarding the effects that this enlargement has had on its internal organization, and how certain areas of the brain have differentially expanded over evolutionary time. Analyses of the microstructure of several regions of the human cortex and subcortical structures have demonstrated subtle changes at the cellular and molecular level, suggesting that the human brain is more than simply a 'scaled-up' primate brain. Ongoing research in comparative neuroanatomy has much to offer regarding our understanding of human brain evolution. Through analysis of the neuroanatomical phenotype at the level of reorganization in cytoarchitecture and cellular morphology, new data continue to highlight changes in cell density and organization associated with volumetric changes in discrete regions. An understanding of the functional significance of variation in neural circuitry can further be approached through studies of atypical human development. Many neurodevelopmental disorders cause disruption in systems associated with uniquely human features of cognition, including language and social cognition. Understanding the genetic and developmental mechanisms that underlie variation in the human cognitive phenotype can help to clarify the functional significance of interspecific variation. By uniting approaches from comparative neuroanatomy and neuropathology, insights can be gained that clarify trends in human evolution. Here, we explore these lines of evidence and their significance for understanding functional variation between species as well as within neuropathological variation in the human brain.

Evidence for evolutionary specialization in human limbic structures

Increasingly, functional and evolutionary research has highlighted the important contribution emotion processing makes to complex human social cognition. As such, it may be asked whether neural structures involved in emotion processing, commonly referred to as limbic structures, have been impacted in human brain evolution. To address this question, we performed an extensive evolutionary analysis of multiple limbic structures using modern phylogenetic tools. For this analysis, we combined new volumetric data for the hominoid (human and ape) amygdala and 4 amygdaloid nuclei, hippocampus, and striatum, collected using stereological methods in complete histological series, with previously published datasets on the amygdala, orbital and medial frontal cortex, and insula, as well as a non-limbic structure, the dorsal frontal cortex, for contrast. We performed a parallel analysis using large published datasets including many anthropoid species (human, ape, and monkey), but fewer hominoids, for the amygdala and 2 amygdaloid subdivisions, hippocampus, schizocortex, striatum, and septal nuclei. To address evolutionary change, we compared observed human values to values predicted from regressions run through (a) non-human hominoids and (b) non-human anthropoids, assessing phylogenetic influence using phylogenetic generalized least squares regression. Compared with other hominoids, the volumes of the hippocampus, the lateral nucleus of the amygdala, and the orbital frontal cortex were, respectively, 50, 37, and 11% greater in humans than predicted for an ape of human hemisphere volume, while the medial and dorsal frontal cortex were, respectively, 26 and 29% significantly smaller. Compared with other anthropoids, only human values for the striatum fell significantly below predicted values. Overall, the data present support for the idea that regions involved in emotion processing are not necessarily conserved or regressive, but may even be enhanced in recent human evolution.

Complete morphologies of basal forebrain cholinergic neurons in the mouse

The basal forebrain cholinergic system modulates neuronal excitability and vascular tone throughout the cerebral cortex and hippocampus. This system is severely affected in Alzheimer's disease (AD), and drug treatment to enhance cholinergic signaling is widely used as symptomatic therapy in AD. Defining the full morphologies of individual basal forebrain cholinergic neurons has, until now, been technically beyond reach due to their large axon arbor sizes. Using genetically-directed sparse labeling, we have characterized the complete morphologies of basal forebrain cholinergic neurons in the mouse. Individual arbors were observed to span multiple cortical columns, and to have >1000 branch points and total axon lengths up to 50 cm. In an AD model, cholinergic axons were slowly lost and there was an accumulation of axon-derived material in discrete puncta. Calculations based on published morphometric data indicate that basal forebrain cholinergic neurons in humans have a mean axon length of ∼100 meters.

DOI:http://dx.doi.org/10.7554/eLife.02444.001

The human brain is made up of roughly 80 to 100 billion neurons, organized into extensive networks. Each neuron consists of a number of components: a cell body, which contains the nucleus; numerous short protrusions from the cell body called dendrites; and a long thin structure called an axon that carries the electrical signals generated in the cell body and the dendrites to the next neuron in the network.

One of the most studied networks in the human brain is the basal forebrain network, which is made up of large neurons that communicate with one another using a chemical transmitter called acetylcholine. This network has a key role in cognition, and its neurons are among the first to degenerate in Alzheimer's disease. However, relatively little is known about the structure of these ‘cholinergic’ neurons because their large size makes them difficult to study using standard techniques.

Now, Wu et al. have visualized, for the first time, the complete 3D structure of cholinergic neurons in the mouse forebrain. The mice in question had been genetically modified so that only ten or so of their many thousands of cholinergic neurons expressed a distinctive ‘marker’ protein. This made it possible to distinguish these neurons from surrounding brain tissue in order to visualize their structures. The resulting pictures clearly illustrate the neurons' complexity, with individual axons in adult mice displaying up to 1000 branches.

Measurements showed that each cholinergic axon in the mouse brain is roughly 30 centimeters long, even though the brain itself is less than 2 centimeters from front to back. Based on measurements by other researchers, Wu et al. calculated that the axons of single cholinergic neurons in the human brain are about 100 meters long on average.

The extreme length and complex branching structure of cholinergic forebrain neurons helps to explain why each neuron is able to modulate the activity of many others in the network. It could also explain their vulnerability to degeneration, as the need to transport materials over such long distances may limit the ability of these neurons to respond to damage.

DOI:http://dx.doi.org/10.7554/eLife.02444.002

Maternal choline supplementation differentially alters the basal forebrain cholinergic system of young-adult Ts65Dn and disomic mice

Down syndrome (DS), trisomy 21, is a multifaceted condition marked by intellectual disability and early presentation of Alzheimer's disease (AD) neuropathological lesions including degeneration of the basal forebrain cholinergic neuron (BFCN) system. Although DS is diagnosable during gestation, there is no treatment option for expectant mothers or DS individuals. Using the Ts65Dn mouse model of DS that displays age-related degeneration of the BFCN system, we investigated the effects of maternal choline supplementation on the BFCN system in adult Ts65Dn mice and disomic (2N) littermates at 4.3-7.5 months of age. Ts65Dn dams were maintained on a choline-supplemented diet (5.1 g/kg choline chloride) or a control, unsupplemented diet with adequate amounts of choline (1 g/kg choline chloride) from conception until weaning of offspring; post weaning, offspring were fed the control diet. Mice were transcardially perfused with paraformaldehyde, and brains were sectioned and immunolabeled for choline acetyltransferase (ChAT) or p75-neurotrophin receptor (p75(NTR) ). BFCN number and size, the area of the regions, and the intensity of hippocampal labeling were determined. Ts65Dn-unsupplemented mice displayed region- and immunolabel-dependent increased BFCN number, larger areas, smaller BFCNs, and overall increased hippocampal ChAT intensity compared with 2N unsupplemented mice. These effects were partially normalized by maternal choline supplementation. Taken together, the results suggest a developmental imbalance in the Ts65Dn BFCN system. Early maternal-diet choline supplementation attenuates some of the genotype-dependent alterations in the BFCN system, suggesting this naturally occurring nutrient as a treatment option for pregnant mothers with knowledge that their offspring is trisomy 21.

Humans and great apes share increased neocortical neuropeptide Y innervation compared to other haplorhine primates

Neuropeptide Y (NPY) plays a role in a variety of basic physiological functions and has also been implicated in regulating cognition, including learning and memory. A decrease in neocortical NPY has been reported for Alzheimer's disease, schizophrenia, bipolar disorder, and depression, potentially contributing to associated cognitive deficits. The goal of the present analysis was to examine variation in neocortical NPY-immunoreactive axon and varicosity density among haplorhine primates (monkeys, apes, and humans). Stereologic methods were used to measure the ratios of NPY-expressing axon length density to total neuron density (ALv/Nv) and NPY-immunoreactive varicosity density to neuron density (Vv/Nv), as well as the mean varicosity spacing in neocortical areas 10, 24, 44, and 22 (Tpt) of humans, African great apes, New World monkeys, and Old World monkeys. Humans and great apes showed increased cortical NPY innervation relative to monkey species for ALv/Nv and Vv/Nv. Furthermore, humans and great apes displayed a conserved pattern of varicosity spacing across cortical areas and layers, with no differences between cortical layers or among cortical areas. These phylogenetic differences may be related to shared life history variables and may reflect specific cognitive abilities.

Alzheimer's disease pathology in the neocortex and hippocampus of the western lowland gorilla (Gorilla gorilla gorilla)

The two major histopathologic hallmarks of Alzheimer's disease (AD) are amyloid beta protein (Aβ) plaques and neurofibrillary tangles (NFT). Aβ pathology is a common feature in the aged nonhuman primate brain, whereas NFT are found almost exclusively in humans. Few studies have examined AD-related pathology in great apes, which are the closest phylogenetic relatives of humans. In the present study, we examined Aβ and tau-like lesions in the neocortex and hippocampus of aged male and female western lowland gorillas using immunohistochemistry and histochemistry. Analysis revealed an age-related increase in Aβ-immunoreactive plaques and vasculature in the gorilla brain. Aβ plaques were more abundant in the neocortex and hippocampus of females, whereas Aβ-positive blood vessels were more widespread in male gorillas. Plaques were also Aβ40-, Aβ42-, and Aβ oligomer-immunoreactive, but only weakly thioflavine S- or 6-CN-PiB-positive in both sexes, indicative of the less fibrillar (diffuse) nature of Aβ plaques in gorillas. Although phosphorylated neurofilament immunostaining revealed a few dystrophic neurites and neurons, choline acetyltransferase-immunoreactive fibers were not dystrophic. Neurons stained for the tau marker Alz50 were found in the neocortex and hippocampus of gorillas at all ages. Occasional Alz50-, MC1-, and AT8-immunoreactive astrocyte and oligodendrocyte coiled bodies and neuritic clusters were seen in the neocortex and hippocampus of the oldest gorillas. This study demonstrates the spontaneous presence of both Aβ plaques and tau-like lesions in the neocortex and hippocampus in old male and female western lowland gorillas, placing this species at relevance in the context of AD research.

Neuropil distribution in the cerebral cortex differs between humans and chimpanzees

Increased connectivity of high-order association regions in the neocortex has been proposed as a defining feature of human brain evolution. At present, however, there are limited comparative data to examine this claim fully. We tested the hypothesis that the distribution of neuropil across areas of the neocortex of humans differs from that of one of our closest living relatives, the common chimpanzee. The neuropil provides a proxy measure of total connectivity within a local region because it is composed mostly of dendrites, axons, and synapses. Using image analysis techniques, we quantified the neuropil fraction from both hemispheres in six cytoarchitectonically defined regions including frontopolar cortex (area 10), Broca's area (area 45), frontoinsular cortex (area FI), primary motor cortex (area 4), primary auditory cortex (area 41/42), and the planum temporale (area 22). Our results demonstrate that humans exhibit a unique distribution of neuropil in the neocortex compared to chimpanzees. In particular, the human frontopolar cortex and the frontoinsular cortex had a significantly higher neuropil fraction than the other areas. In chimpanzees these prefrontal regions did not display significantly more neuropil, but the primary auditory cortex had a lower neuropil fraction than other areas. Our results support the conclusion that enhanced connectivity in the prefrontal cortex accompanied the evolution of the human brain. These species differences in neuropil distribution may offer insight into the neural basis of human cognition, reflecting enhancement of the integrative capacity of the prefrontal cortex.

Human Evolution and the Chimpanzee Referential Doctrine

Chimpanzees are our closest living genomic relatives, but they lack the bipedal locomotion, markedly enlarged brains, and advanced communication skills of humans. This has led many to view them as “primitive” and to presume that their behavior and anatomy are also primitive. If true, they could serve as models of our last common ancestor (LCA), i.e., a territorially aggressive knuckle walker, reliant on vertical climbing and below-branch suspension to access the high canopy as a ripe-fruit frugivore. Ardipithecus now provides abundant information that the LCA differed substantially from chimpanzees (as well as bonobos and gorillas), both anatomically and behaviorally, and exhibited many characters that are more similar to those of modern humans than to any living ape. This major extension of the hominoid fossil record contravenes strict referential modeling based on the extant chimpanzee and greatly improves our ability to reconstruct the LCA more accurately, but only when viewed within the broader context of evolutionary ecology.

Neuropeptide Y-immunoreactive neurons in the cerebral cortex of humans and other haplorrhine primates

We examined the distribution of neurons immunoreactive for neuropeptide Y (NPY) in the posterior part of the superior temporal cortex (Brodmann's area 22 or area Tpt) of humans and nonhuman haplorrhine primates. NPY has been implicated in learning and memory and the density of NPY-expressing cortical neurons and axons is reduced in depression, bipolar disorder, schizophrenia, and Alzheimer's disease. Due to the role that NPY plays in both cognition and neurodegenerative diseases, we tested the hypothesis that the density of cortical and interstitial neurons expressing NPY was increased in humans relative to other primate species. The study sample included great apes (chimpanzee and gorilla), Old World monkeys (pigtailed macaque, moor macaque, and baboon) and New World monkeys (squirrel monkey and capuchin). Stereologic methods were used to estimate the density of NPY-immunoreactive (-ir) neurons in layers I-VI of area Tpt and the subjacent white matter. Adjacent Nissl-stained sections were used to calculate local densities of all neurons. The ratio of NPY-ir neurons to total neurons within area Tpt and the total density of NPY-ir neurons within the white matter were compared among species. Overall, NPY-ir neurons represented only an average of 0.006% of the total neuron population. While there were significant differences among species, phylogenetic trends in NPY-ir neuron distributions were not observed and humans did not differ from other primates. However, variation among species warrants further investigation into the distribution of this neuromodulator system.

Human brain evolution writ large and small

Human evolution was marked by an extraordinary increase in total brain size relative to body size. While it is certain that increased encephalization is an important factor contributing to the origin of our species-specific cognitive abilities, it is difficult to disentangle which aspects of human neural structure and function are correlated by-products of brain size expansion from those that are specifically related to particular psychological specializations, such as language and enhanced "mentalizing" abilities. In this chapter, we review evidence from allometric scaling studies demonstrating that much of human neocortical organization can be understood as a product of brain enlargement. Defining extra-allometric specializations in humans is often hampered by a severe lack of comparative data from the same neuroanatomical variables across a broad range of primates. When possible, we highlight evidence for features of human neocortical architecture and function that cannot be easily explained as correlates of brain size and, hence, might be more directly associated with the evolution of uniquely human cognitive capacities.

The human brain: rewired and running hot

The past two decades have witnessed tremendous advances in noninvasive and postmortem neuroscientific techniques, advances that have made it possible, for the first time, to compare in detail the organization of the human brain to that of other primates. Studies comparing humans to chimpanzees and other great apes reveal that human brain evolution was not merely a matter of enlargement, but involved changes at all levels of organization that have been examined. These include the cellular and laminar organization of cortical areas; the higher order organization of the cortex, as reflected in the expansion of association cortex (in absolute terms, as well as relative to primary areas); the distribution of long-distance cortical connections; and hemispheric asymmetry. Additionally, genetic differences between humans and other primates have proven to be more extensive than previously thought, raising the possibility that human brain evolution involved significant modifications of neurophysiology and cerebral energy metabolism.

Layer-specific activity of tissue non-specific alkaline phosphatase in the human neocortex

The ectoenzyme tissue non-specific alkaline phosphatase (TNAP) is mostly known for its role in bone mineralization. However, in the severe form of hypophosphatasia, TNAP deficiency also results in epileptic seizures, suggesting a role of this enzyme in brain functions. Accordingly, TNAP activity was shown in the neuropil of the cerebral cortex in diverse mammalian species. However in spite of its clinical significance, the neuronal localization of TNAP has not been investigated in the human brain. By using enzyme histochemistry, we found an unprecedented pattern of TNAP activity appearing as an uninterrupted layer across diverse occipital-, frontal- and temporal lobe areas of the human cerebral cortex. This marked TNAP-active band was localized infragranulary in layer 5 as defined by quantitative comparisons on parallel sections stained by various techniques to reveal the laminar pattern. On the contrary, TNAP activity was localized in layer 4 of the primary visual and somatosensory cortices, which is consistent with earlier observations on other species. This result suggests that the expression of TNAP in the thalamo-recipient granular layer is an evolutionary conserved feature of the sensory cortex. The observations of the present study also suggest that diverse neurocognitive functions share a common cerebral cortical mechanism depending on TNAP activity in layer 5. In summary, the present data point on the distinctive role of layer 5 in cortical computation and neurological disorders caused by TNAP dysfunctions in the human brain.

A comparative perspective on minicolumns and inhibitory GABAergic interneurons in the neocortex

Neocortical columns are functional and morphological units whose architecture may have been under selective evolutionary pressure in different mammalian lineages in response to encephalization and specializations of cognitive abilities. Inhibitory interneurons make a substantial contribution to the morphology and distribution of minicolumns within the cortex. In this context, we review differences in minicolumns and GABAergic interneurons among species and discuss possible implications for signaling among and within minicolumns. Furthermore, we discuss how abnormalities of both minicolumn disposition and inhibitory interneurons might be associated with neuropathological processes, such as Alzheimer's disease, autism, and schizophrenia. Specifically, we explore the possibility that phylogenetic variability in calcium-binding protein-expressing interneuron subtypes is directly related to differences in minicolumn morphology among species and might contribute to neuropathological susceptibility in humans.

Inhibitory interneurons of the human prefrontal cortex display conserved evolution of the phenotype and related genes

Inhibitory interneurons participate in local processing circuits, playing a central role in executive cognitive functions of the prefrontal cortex. Although humans differ from other primates in a number of cognitive domains, it is not currently known whether the interneuron system has changed in the course of primate evolution leading to our species. In this study, we examined the distribution of different interneuron subtypes in the prefrontal cortex of anthropoid primates as revealed by immunohistochemistry against the calcium-binding proteins calbindin, calretinin and parvalbumin. In addition, we tested whether genes involved in the specification, differentiation and migration of interneurons show evidence of positive selection in the evolution of humans. Our findings demonstrate that cellular distributions of interneuron subtypes in human prefrontal cortex are similar to other anthropoid primates and can be explained by general scaling rules. Furthermore, genes underlying interneuron development are highly conserved at the amino acid level in primate evolution. Taken together, these results suggest that the prefrontal cortex in humans retains a similar inhibitory circuitry to that in closely related primates, even though it performs functional operations that are unique to our species. Thus, it is likely that other significant modifications to the connectivity and molecular biology of the prefrontal cortex were overlaid on this conserved interneuron architecture in the course of human evolution.

Phasic acetylcholine release and the volume transmission hypothesis: time to move on

Traditional descriptions of the cortical cholinergic input system focused on the diffuse organization of cholinergic projections and the hypothesis that slowly changing levels of extracellular acetylcholine (ACh) mediate different arousal states. The ability of ACh to reach the extrasynaptic space (volume neurotransmission), as opposed to remaining confined to the synaptic cleft (wired neurotransmission), has been considered an integral component of this conceptualization. Recent studies demonstrated that phasic release of ACh, at the scale of seconds, mediates precisely defined cognitive operations. This characteristic of cholinergic neurotransmission is proposed to be of primary importance for understanding cholinergic function and developing treatments for cognitive disorders that result from abnormal cholinergic neurotransmission.

Cortical dopaminergic innervation among humans, chimpanzees, and macaque monkeys: a comparative study

In this study, we assessed the possibility that humans differ from other primate species in the supply of dopamine to the frontal cortex. To this end, quantitative comparative analyses were performed among humans, chimpanzees, and macaques using immunohistochemical methods to visualize tyrosine hydroxylase-immunoreactive axons within the cerebral cortex. Axon densities and neuron densities were quantified using computer-assisted stereology. Prefrontal areas 9 and 32 were chosen for evaluation due to their roles in higher-order executive functions and theory of mind, respectively. Primary motor cortex (area 4) was also evaluated because it is not directly associated with cognition. We did not find an overt quantitative increase in cortical dopaminergic innervation in humans relative to the other primates examined. However, several differences in cortical dopaminergic innervation were observed among species which may have functional implications. Specifically, humans exhibited a sublaminar pattern of innervation in layer I of areas 9 and 32 that differed from that of macaques and chimpanzees. Analysis of axon length density to neuron density among species revealed that humans and chimpanzees together deviated from macaques in having increased dopaminergic afferents in layers III and V/VI of areas 9 and 32, but there were no phylogenetic differences in area 4. Finally, morphological specializations of axon coils that may be indicative of cortical plasticity events were observed in humans and chimpanzees, but not macaques. Our findings suggest significant modifications of dopamine's role in cortical organization occurred in the evolution of the apes, with further changes in the descent of humans.

A natural history of the human mind: tracing evolutionary changes in brain and cognition

Since the last common ancestor shared by modern humans, chimpanzees and bonobos, the lineage leading to Homo sapiens has undergone a substantial change in brain size and organization. As a result, modern humans display striking differences from the living apes in the realm of cognition and linguistic expression. In this article, we review the evolutionary changes that occurred in the descent of Homo sapiens by reconstructing the neural and cognitive traits that would have characterized the last common ancestor and comparing these with the modern human condition. The last common ancestor can be reconstructed to have had a brain of approximately 300-400 g that displayed several unique phylogenetic specializations of development, anatomical organization, and biochemical function. These neuroanatomical substrates contributed to the enhancement of behavioral flexibility and social cognition. With this evolutionary history as precursor, the modern human mind may be conceived as a mosaic of traits inherited from a common ancestry with our close relatives, along with the addition of evolutionary specializations within particular domains. These modern human-specific cognitive and linguistic adaptations appear to be correlated with enlargement of the neocortex and related structures. Accompanying this general neocortical expansion, certain higher-order unimodal and multimodal cortical areas have grown disproportionately relative to primary cortical areas. Anatomical and molecular changes have also been identified that might relate to the greater metabolic demand and enhanced synaptic plasticity of modern human brain's. Finally, the unique brain growth trajectory of modern humans has made a significant contribution to our species' cognitive and linguistic abilities.