Fiber pathways of basal forebrain cholinergic neurons in monkeys

Functionally linked amygdala and prefrontal cortical regions are innervated by both single and double projecting cholinergic neurons

Cholinergic cells have been proposed to innervate simultaneously those cortical areas that are mutually interconnected with each other. To test this hypothesis, we investigated the cholinergic innervation of functionally linked amygdala and prefrontal cortical regions. First, using tracing experiments, we determined that cholinergic cells located in distinct basal forebrain (BF) areas projected to the different nuclei of the basolateral amygdala (BLA). Specifically, cholinergic cells in the ventral pallidum/substantia innominata (VP/SI) innervated the basal nucleus (BA), while the horizontal limb of the diagonal band of Broca (HDB) projected to its basomedial nucleus (BMA). In addition, cholinergic neurons in these two BF areas gave rise to overlapping innervation in the medial prefrontal cortex (mPFC), yet their axons segregated in the dorsal and ventral regions of the PFC. Using retrograde-anterograde viral tracing, we demonstrated that a portion of mPFC-projecting cholinergic neurons also innervated the BLA, especially the BA. By injecting retrograde tracers into the mPFC and BA, we found that 28% of retrogradely labeled cholinergic cells were double labeled, which typically located in the VP/SI. In addition, we found that vesicular glutamate transporter type 3 (VGLUT3)-expressing neurons within the VP/SI were also cholinergic and projected to the mPFC and BA, implicating that a part of the cholinergic afferents may release glutamate. In contrast, we uncovered that GABA is unlikely to be a co-transmitter molecule in HDB and VP/SI cholinergic neurons in adult mice. The dual innervation strategy, i.e., the existence of cholinergic cell populations with single as well as simultaneous projections to the BLA and mPFC, provides the possibility for both synchronous and independent control of the operation in these cortical areas, a structural arrangement that may maximize computational support for functionally linked regions. The presence of VGLUT3 in a portion of cholinergic afferents suggests more complex functional effects of cholinergic system in cortical structures.

Aspiration removal of orbitofrontal cortex disrupts cholinergic fibers of passage to anterior cingulate cortex in rhesus macaques

The study of anthropoid nonhuman primates has provided valuable insights into frontal cortex function in humans, as these primates share similar frontal anatomical subdivisions (Murray et al. 2011). Causal manipulation studies have been instrumental in advancing our understanding of this area. One puzzling finding is that macaques with bilateral aspiration removals of orbitofrontal cortex (OFC) are impaired on tests of cognitive flexibility and emotion regulation, whereas those with bilateral excitotoxic lesions of OFC are not (Rudebeck et al. 2013). This discrepancy is attributed to the inadvertent disruption of fibers of passage by aspiration lesions but not by excitotoxic lesions. Which fibers of passage are responsible for the impairments observed? One candidate is cholinergic fibers originating in the nucleus basalis magnocellularis (NBM) and passing nearby or through OFC on their way to other frontal cortex regions (Kitt et al. 1987). To investigate this possibility, we performed unilateral aspiration lesions of OFC in three macaques, and then compared cholinergic innervation of the anterior cingulate cortex (ACC) between hemispheres. Histological assessment revealed diminished cholinergic innervation in the ACC of hemispheres with OFC lesions relative to intact hemispheres. This finding indicates that aspiration lesions of the OFC disrupt cholinergic fibers of passage, and suggests the possibility that loss of cholinergic inputs to ACC contributes to the impairments in cognitive flexibility and emotion regulation observed after aspiration but not excitotoxic lesions of OFC.

Progression of regional cortical cholinergic denervation in Parkinson's disease

Cortical cholinergic deficits contribute to cognitive decline and other deficits in Parkinson's disease. Cross-sectional imaging studies suggest a stereotyped pattern of posterior-to-anterior cortical cholinergic denervation accompanying disease progression in Parkinson's disease. We used serial acetylcholinesterase PET ligand imaging to characterize the trajectory of regional cholinergic synapse deficits in Parkinson's disease, testing the hypothesis of posterior-to-anterior progression of cortical cholinergic deficits. The 16 Parkinson's disease subjects (4 females/12 males; mean age: 64.4 ± 6.7 years; disease duration: 5.5 ± 4.2 years; Hoehn & Yahr stage: 2.3 ± 0.6 at entry) completed serial 11C-methyl-4-piperidinyl propionate acetylcholinesterase PET scans over a 4-8 year period (median 5 years). Three-dimensional stereotactic cortical surface projections and volume-of-interest analyses were performed. Cholinergic synapse integrity was assessed by the magnitude, k 3, of acetylcholinesterase hydrolysis of 11C-methyl-4-piperidinyl propionate. Based on normative data, we generated Z-score maps for both the k 3 and the k 1 parameters, the latter as a proxy for regional cerebral blood flow. Compared with control subjects, baseline scans showed predominantly posterior cortical k 3 deficits in Parkinson's disease subjects. Interval change analyses showed evidence of posterior-to-anterior progression of cholinergic cortical deficits in the posterior cortices. In frontal cortices, an opposite gradient of anterior-to-posterior progression of cholinergic deficits was found. The topography of k 3 changes exhibited regionally specific disconnection from k 1 changes. Interval-change analysis based on k 3/k 1 ratio images (k 3 adjustment for regional cerebral blood flow changes) showed interval reductions (up to 20%) in ventral frontal, anterior cingulate and Brodmann area 6 cortices. In contrast, interval k 3 reductions in the posterior cortices, especially Brodmann areas 17-19, were largely proportional to k 1 changes. Our results partially support the hypothesis of progressive posterior-to-cortical cholinergic denervation in Parkinson's disease. This pattern appears characteristic of posterior cortices. In frontal cortices, an opposite pattern of anterior-to-posterior progression of cholinergic deficits was found. The progressive decline of posterior cortical acetylcholinesterase activity was largely proportional to declining regional cerebral blood flow, suggesting that posterior cortical cholinergic synapse deficits are part of a generalized loss of synapses. The disproportionate decline in regional frontal cortical acetylcholinesterase activity relative to regional cerebral blood flow suggests preferential loss or dysregulation of cholinergic synapses in these regions. Our observations suggest that cortical cholinergic synapse vulnerability in Parkinson's disease is mediated by both diffuse processes affecting cortical synapses and processes specific to subpopulations of cortical cholinergic afferents.

Common Neuroanatomical Substrate of Cholinergic Pathways and Language-Related Brain Regions as an Explanatory Framework for Evaluating the Efficacy of Cholinergic Pharmacotherapy in Post-Stroke Aphasia: A Review

Despite the relative scarcity of studies focusing on pharmacotherapy in aphasia, there is evidence in the literature indicating that remediation of language disorders via pharmaceutical agents could be a promising aphasia treatment option. Among the various agents used to treat chronic aphasic deficits, cholinergic drugs have provided meaningful results. In the current review, we focused on published reports investigating the impact of acetylcholine on language and other cognitive disturbances. It has been suggested that acetylcholine plays an important role in neuroplasticity and is related to several aspects of cognition, such as memory and attention. Moreover, cholinergic input is diffused to a wide network of cortical areas, which have been associated with language sub-processes. This could be a possible explanation for the positive reported outcomes of cholinergic drugs in aphasia recovery, and specifically in distinct language processes, such as naming and comprehension, as well as overall communication competence. However, evidence with regard to functional alterations in specific brain areas after pharmacotherapy is rather limited. Finally, despite the positive results derived from the relevant studies, cholinergic pharmacotherapy treatment in post-stroke aphasia has not been widely implemented. The present review aims to provide an overview of the existing literature in the common neuroanatomical substrate of cholinergic pathways and language related brain areas as a framework for interpreting the efficacy of cholinergic pharmacotherapy interventions in post-stroke aphasia, following an integrated approach by converging evidence from neuroanatomy, neurophysiology, and neuropsychology.

Aberrant Functional Connectivity of Basal Forebrain Subregions with Cholinergic System in Short-term and Chronic Insomnia Disorder

BACKGROUND

To systematically investigate structural and functional abnormalities in subregions of the basal forebrain (BF) using structural and resting-state fMRI, and to examine their clinical relevance in short-term and chronic insomnia disorder (ID).

METHODS

Thirty-four patients with short-term ID, 41 patients with chronic ID, and 46 healthy controls (HCs) were recruited. Grey matter volume and seed-based resting-state functional connectivity (RSFC) in each BF subregion (Ch1,2,3 and 4) were computed and compared among the three groups. Spearman correlation was used to estimate the relationships between MRI-based alterations and clinical variables.

RESULTS

The short-term group exhibited lower RSFC with the bilateral striatum and bilateral Ch_4 than HCs and the chronic group. In the left Ch_4, subjects in the chronic group exhibited lower RSFC with the left middle cingulate cortex than HCs and the short-term group. The short-term group exhibited lower RSFC with the left parahippocampal gyrus (PHG) than HCs and the chronic group. The chronic group exhibited the highest RSFC with the left middle frontal gyrus (MFG), followed by HCs and the short-term group. In the right Ch_4, the chronic group exhibited the lowest RSFC with the right superior temporal gyrus, followed by HCs and the short-term group. Moreover, in the short-term group, negative correlations were found between the left Ch_4 and left MFG RSFC and Epworth Sleepiness Scale scores.

CONCLUSIONS

These findings suggest that the Ch_4 may be a key node for establishing diagnostic and categorical biomarkers of ID, which could be useful in developing more effective treatment strategies for insomnia.

Dichotomous organization of amygdala/temporal-prefrontal bundles in both humans and monkeys

The interactions of anterior temporal structures, and especially the amygdala, with the prefrontal cortex are pivotal to learning, decision-making, and socio-emotional regulation. A clear anatomical description of the organization and dissociation of fiber bundles linking anterior temporal cortex/amygdala and prefrontal cortex in humans is still lacking. Using diffusion imaging techniques, we reconstructed fiber bundles between these anatomical regions in human and macaque brains. First, by studying macaques, we assessed which aspects of connectivity known from tracer studies could be identified with diffusion imaging. Second, by comparing diffusion imaging results in humans and macaques, we estimated the patterns of fibers coursing between human amygdala and prefrontal cortex and compared them with those in the monkey. In posterior prefrontal cortex, we observed a prominent and well-preserved bifurcation of bundles into primarily two fiber systems-an amygdalofugal path and an uncinate path-in both species. This dissociation fades away in more rostral prefrontal regions.

The corticotopic organization of the human basal forebrain as revealed by regionally selective functional connectivity profiles

The cholinergic basal forebrain (CBF), comprising different groups of cortically projecting cholinergic neurons, plays a crucial role in higher cognitive processes and has been implicated in diverse neuropsychiatric disorders. A distinct corticotopic organization of CBF projections has been revealed in animal studies, but little is known about their organization in the human brain. We explored regional differences in functional connectivity (FC) profiles within the human CBF by applying a clustering approach to resting-state functional magnetic resonance imaging (rs-fMRI) data of healthy adult individuals (N = 85; 19-85 years). We further examined effects of age on FC of the identified CBF clusters and assessed the reproducibility of cluster-specific FC profiles in independent data from healthy older individuals (N = 25; 65-89 years). Results showed that the human CBF is functionally organized into distinct anterior-medial and posterior-lateral subdivisions that largely follow anatomically defined boundaries of the medial septum/diagonal band and nucleus basalis Meynert. The anterior-medial CBF subdivision was characterized by connectivity with the hippocampus and interconnected nodes of an extended medial cortical memory network, whereas the posterior-lateral subdivision was specifically connected to anterior insula and dorsal anterior cingulate components of a salience/attention network. FC of both CBF subdivisions declined with increasing age, but the overall topography of subregion-specific FC profiles was reproduced in independent rs-fMRI data of healthy older individuals acquired in a typical clinical setting. Rs-fMRI-based assessments of subregion-specific CBF function may complement established volumetric approaches for the in vivo study of CBF involvement in neuropsychiatric disorders.

The cingulum bundle: Anatomy, function, and dysfunction

The cingulum bundle is a prominent white matter tract that interconnects frontal, parietal, and medial temporal sites, while also linking subcortical nuclei to the cingulate gyrus. Despite its apparent continuity, the cingulum's composition continually changes as fibres join and leave the bundle. To help understand its complex structure, this review begins with detailed, comparative descriptions of the multiple connections comprising the cingulum bundle. Next, the impact of cingulum bundle damage in rats, monkeys, and humans is analysed. Despite causing extensive anatomical disconnections, cingulum bundle lesions typically produce only mild deficits, highlighting the importance of parallel pathways and the distributed nature of its various functions. Meanwhile, non-invasive imaging implicates the cingulum bundle in executive control, emotion, pain (dorsal cingulum), and episodic memory (parahippocampal cingulum), while clinical studies reveal cingulum abnormalities in numerous conditions, including schizophrenia, depression, post-traumatic stress disorder, obsessive compulsive disorder, autism spectrum disorder, Mild Cognitive Impairment, and Alzheimer's disease. Understanding the seemingly diverse contributions of the cingulum will require better ways of isolating pathways within this highly complex tract.

Bridging the gap between rodents and humans: The role of non-human primates in oxytocin research

Oxytocin (OT), a neuropeptide that acts in the brain as a neuromodulator, has been long known to shape maternal physiology and behavior in mammals, however its role in regulating social cognition and behavior in primates has come to the forefront only in the recent decade. Many of the current perspectives on the role of OT in modulating social behavior emerged first from studies in rodents, where invasive techniques with a high degree of precision have permitted the mechanistic dissection of OT-related behaviors, as well as their underlying neural circuits in exquisite detail. In parallel, behavioral and imaging studies in humans have suggested that brain OT may similarly influence human social behavior and neural activity. These studies in rodents and humans have spurred interest in the therapeutic potential of targeting the OT system to remedy deficits in social cognition and behavior that are present across numerous psychiatric disorders. Yet there remains a tremendous gap in our mechanistic understanding of the influence of brain OT on social neural circuitry between rodents and man. In fact, very little is known regarding the neural mechanisms by which exogenous or endogenous OT influences human social cognition, limiting its therapeutic potential. Here we discuss how non-human primates (NHPs) are uniquely positioned to now bridge the gaps in knowledge provided by the precise circuit-level approaches widely used in rodent models and the behavioral, imaging, and clinical studies in humans. This review provides a perspective on what has been achieved, and what can be expected from exploring the role of OT in shaping social behaviors in NHPs in the coming years.

The Basal Forebrain Regulates Global Resting-State fMRI Fluctuations

Patterns of spontaneous brain activity, typically measured in humans at rest with fMRI, are used routinely to assess the brain's functional organization. The mechanisms that generate and coordinate the underlying neural fluctuations are largely unknown. Here we investigate the hypothesis that the nucleus basalis of Meynert (NBM), the principal source of widespread cholinergic and GABAergic projections to the cortex, contributes critically to such activity. We reversibly inactivated two distinct sites of the NBM in macaques while measuring fMRI activity across the brain. We found that inactivation led to strong, regionalized suppression of shared or "global" signal components of cortical fluctuations ipsilateral to the injection. At the same time, the commonly studied resting-state networks retained their spatial structure under this suppression. The results indicate that the NBM contributes selectively to the global component of functional connectivity but plays little if any role in the specific correlations that define resting-state networks.

The tyrosine kinase receptor Tyro3 enhances lifespan and neuropeptide Y (Npy) neuron survival in the mouse anorexia (anx) mutation

Severe appetite and weight loss define the eating disorder anorexia nervosa, and can also accompany the progression of some neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS). Although acute loss of hypothalamic neurons that produce appetite-stimulating neuropeptide Y (Npy) and agouti-related peptide (Agrp) in adult mice or in mice homozygous for the anorexia (anx) mutation causes aphagia, our understanding of the factors that help maintain appetite regulatory circuitry is limited. Here we identify a mutation (C19T) that converts an arginine to a tryptophan (R7W) in the TYRO3 protein tyrosine kinase 3 (Tyro3) gene, which resides within the anx critical interval, as contributing to the severity of anx phenotypes. Our observation that, like Tyro3-/- mice, anx/anx mice exhibit abnormal secondary platelet aggregation suggested that the C19T Tyro3 variant might have functional consequences. Tyro3 is expressed in the hypothalamus and other brain regions affected by the anx mutation, and its mRNA localization appeared abnormal in anx/anx brains by postnatal day 19 (P19). The presence of wild-type Tyro3 transgenes, but not an R7W-Tyro3 transgene, doubled the weight and lifespans of anx/anx mice and near-normal numbers of hypothalamic Npy-expressing neurons were present in Tyro3-transgenic anx/anx mice at P19. Although no differences in R7W-Tyro3 signal sequence function or protein localization were discernible in vitro, distribution of R7W-Tyro3 protein differed from that of Tyro3 protein in the cerebellum of transgenic wild-type mice. Thus, R7W-Tyro3 protein localization deficits are only detectable in vivo Further analyses revealed that the C19T Tyro3 mutation is present in a few other mouse strains, and hence is not the causative anx mutation, but rather an anx modifier. Our work shows that Tyro3 has prosurvival roles in the appetite regulatory circuitry and could also provide useful insights towards the development of interventions targeting detrimental weight loss.

Oxytocin enhances gaze-following responses to videos of natural social behavior in adult male rhesus monkeys

Gaze following is a basic building block of social behavior that has been observed in multiple species, including primates. The absence of gaze following is associated with abnormal development of social cognition, such as in autism spectrum disorders (ASD). Some social deficits in ASD, including the failure to look at eyes and the inability to recognize facial expressions, are ameliorated by intranasal administration of oxytocin (IN-OT). Here we tested the hypothesis that IN-OT might enhance social processes that require active engagement with a social partner, such as gaze following. Alternatively, IN-OT may only enhance the perceptual salience of the eyes, and may not modify behavioral responses to social signals. To test this hypothesis, we presented four monkeys with videos of conspecifics displaying natural behaviors. Each video was viewed multiple times before and after the monkeys received intranasally either 50 IU of OT or saline. We found that despite a gradual decrease in attention to the repeated viewing of the same videos (habituation), IN-OT consistently increased the frequency of gaze following saccades. Further analysis confirmed that these behaviors did not occur randomly, but rather predictably in response to the same segments of the videos. These findings suggest that in response to more naturalistic social stimuli IN-OT enhances the propensity to interact with a social partner rather than merely elevating the perceptual salience of the eyes. In light of these findings, gaze following may serve as a metric for pro-social effects of oxytocin that target social action more than social perception.

Cholinergic basal forebrain structure influences the reconfiguration of white matter connections to support residual memory in mild cognitive impairment

The fornix and hippocampus are critical to recollection in the healthy human brain. Fornix degeneration is a feature of aging and Alzheimer's disease. In the presence of fornix damage in mild cognitive impairment (MCI), a recognized prodrome of Alzheimer's disease, recall shows greater dependence on other tracts, notably the parahippocampal cingulum (PHC). The current aims were to determine whether this shift is adaptive and to probe its relationship to cholinergic signaling, which is also compromised in Alzheimer's disease. Twenty-five human participants with MCI and 20 matched healthy volunteers underwent diffusion MRI, behavioral assessment, and volumetric measurement of the basal forebrain. In a regression model for recall, there was a significant group × fornix interaction, indicating that the association between recall and fornix structure was weaker in patients. The opposite trend was present for the left PHC. To further investigate this pattern, two regression models were generated to account for recall performance: one based on fornix microstructure and the other on both fornix and left PHC. The realignment to PHC was positively correlated with free recall but not non-memory measures, implying a reconfiguration that is beneficial to residual memory. There was a positive relationship between realignment to PHC and basal forebrain gray matter volume despite this region demonstrating atrophy at a group level, i.e., the cognitive realignment to left PHC was most apparent when cholinergic areas were relatively spared. Therefore, cholinergic systems appear to enable adaptation to injury even as they degenerate, which has implications for functional restoration.

The orbitofrontal oracle: cortical mechanisms for the prediction and evaluation of specific behavioral outcomes

The orbitofrontal cortex (OFC) has long been associated with the flexible control of behavior and concepts such as behavioral inhibition, self-control, and emotional regulation. These ideas emphasize the suppression of behaviors and emotions, but OFC's affirmative functions have remained enigmatic. Here we review recent work that has advanced our understanding of this prefrontal area and how its functions are shaped through interaction with subcortical structures such as the amygdala. Recent findings have overturned theories emphasizing behavioral inhibition as OFC's fundamental function. Instead, new findings indicate that OFC provides predictions about specific outcomes associated with stimuli, choices, and actions, especially their moment-to-moment value based on current internal states. OFC function thereby encompasses a broad representation or model of an individual's sensory milieu and potential actions, along with their relationship to likely behavioral outcomes.

Prefrontal mechanisms of behavioral flexibility, emotion regulation and value updating

Two ideas have dominated the neuropsychology of the orbitofrontal cortex (OFC). One holds that OFC regulates emotion and enhances behavioral flexibility through inhibitory control. The other ascribes to OFC a role in updating valuations based on current motivational states. Neuroimaging, neurophysiological and clinical observations are consistent with either or both hypotheses. Although these hypotheses are compatible in principle, the present results support the latter view of OFC function and argue against the former. We show that excitotoxic, fibersparing lesions confined to OFC in monkeys do not alter either behavioral flexibility, as measured by object reversal learning, or emotion regulation, as assessed by snake fear. A follow-up experiment indicates that previous reports of a loss of inhibitory control resulted from damage to nearby fiber tracts and not from OFC dysfunction. Thus, OFC plays a more specialized role in reward-guided behavior and emotion than currently thought, a function that includes value updating.

Distinct subdivisions of the cingulum bundle revealed by diffusion MRI fibre tracking: implications for neuropsychological investigations

The cingulum is a prominent white matter tract that supports prefrontal, parietal, and temporal lobe interactions. Despite being composed of both short and long association fibres, many MRI-based reconstructions (tractography) of the cingulum depict an essentially uniform tract that almost encircles the corpus callosum. The present study tested the validity of dividing this tract into subdivisions corresponding to the ‘parahippocampal’, ‘retrosplenial’, and ‘subgenual’ portions of the cingulum. These three cingulum subdivisions occupied different medial–lateral locations, producing a topographic arrangement of cingulum fibres. Other comparisons based on these different reconstructions indicate that only a small proportion of the total white matter in the cingulum traverses the length of the tract. In addition, both the radial diffusivity and fractional anisotropy of the subgenual subdivision differed from that of the retrosplenial subdivision which, in turn, differed from that of the parahippocampal subdivision. The extent to which the radial diffusivity scores and the fractional anisotropy scores correlated between the various cingulum subdivisions proved variable, illustrating how one subdivision may not act as a proxy for other cingulum subdivisions. Attempts to relate the status of the cingulum, as measured by MRI-based fibre tracking, with cognitive or affective measures will, therefore, depend greatly on how and where the cingulum is reconstructed. The present study provides a new framework for subdividing the cingulum, based both on its known connectivity and MRI-based properties.

Multiple anatomical systems embedded within the primate medial temporal lobe: implications for hippocampal function

A review of medial temporal lobe connections reveals three distinct groupings of hippocampal efferents. These efferent systems and their putative memory functions are: (1) The 'extended-hippocampal system' for episodic memory, which involves the anterior thalamic nuclei, mammillary bodies and retrosplenial cortex, originates in the subicular cortices, and has a largely laminar organisation; (2) The 'rostral hippocampal system' for affective and social learning, which involves prefrontal cortex, amygdala and nucleus accumbens, has a columnar organisation, and originates from rostral CA1 and subiculum; (3) The 'reciprocal hippocampal-parahippocampal system' for sensory processing and integration, which originates from the length of CA1 and the subiculum, and is characterised by columnar, connections with reciprocal topographies. A fourth system, the 'parahippocampal-prefrontal system' that supports familiarity signalling and retrieval processing, has more widespread prefrontal connections than those of the hippocampus, along with different thalamic inputs. Despite many interactions between these four systems, they may retain different roles in memory which when combined explain the importance of the medial temporal lobe for the formation of declarative memories.

Cholinergic mechanisms of episodic memory: what specific behavioural tasks can tell us about specific neural mechanisms

Understanding the neural basis of episodic memory is crucial for understanding how to treat memory loss in normal ageing as well as in disorders such as Alzheimer's disease. However, it is only recently that episodic memory has been able to be reliably modelled in animals allowing the biological basis to be fully explored. Here we review studies on the role of the cholinergic basal forebrain on episodic memory, and highlight differences in findings from studies in monkeys and rats. The results highlight the importance of choosing appropriate behavioural models of cognitive processes in order to understand the neural basis of the processes accurately.

What does the anatomical organization of the entorhinal cortex tell us?

The entorhinal cortex is commonly perceived as a major input and output structure of the hippocampal formation, entertaining the role of the nodal point of cortico-hippocampal circuits. Superficial layers receive convergent cortical information, which is relayed to structures in the hippocampus, and hippocampal output reaches deep layers of entorhinal cortex, that project back to the cortex. The finding of the grid cells in all layers and reports on interactions between deep and superficial layers indicate that this rather simplistic perception may be at fault. Therefore, an integrative approach on the entorhinal cortex, that takes into account recent additions to our knowledge database on entorhinal connectivity, is timely. We argue that layers in entorhinal cortex show different functional characteristics most likely not on the basis of strikingly different inputs or outputs, but much more likely on the basis of differences in intrinsic organization, combined with very specific sets of inputs. Here, we aim to summarize recent anatomical data supporting the notion that the traditional description of the entorhinal cortex as a layered input-output structure for the hippocampal formation does not give the deserved credit to what this structure might be contributing to the overall functions of cortico-hippocampal networks.

Specific mechanism for blood inflow stimulation in brain area prone to Alzheimer's disease lesions

The present study describes the specific two-stage mechanism that intensifies blood supply to the brain area comprising amygdala, hippocampus, olfactory bulb, entorhinal cortex, and neocortex (AHBC). Cholinergic neurons from the nuclei of basal forebrain induce vasodilatory effect through release of acetylcholine. In physiological aging the efficacy of this neuronal system declines, while intensive formation of amyloidogenic peptides starts. These peptides at low, picomolar concentrations activate alpha7 nicotinic acetylcholine receptors, thus enhancing angiogenesis and in so doing restoring blood supply to the AHBC area.

Effects of beta-amyloid protein on M1 and M2 subtypes of muscarinic acetylcholine receptors in the medial septum-diagonal band complex of the rat: relationship with cholinergic, GABAergic, and calcium-binding protein perikarya

Cortical cholinergic dysfunction has been correlated with the expression and processing of beta-amyloid precursor protein. However, it remains unclear as to how cholinergic dysfunction and beta-amyloid (Abeta) formation and deposition might be related to one another. Since the M1- and M2 subtypes of muscarinic acetylcholine receptors (mAChRs) are considered key molecules that transduce the cholinergic message, the purpose of the present study was to assess the effects of the injected Abeta peptide on the number of M1mAchR- and M2mAChR-immunoreactive cells in the medial septum-diagonal band (MS-nDBB) complex of the rat. Injections of Abeta protein into the retrosplenial cortex resulted in a decrease in M1mAChR and M2mAChR immunoreactivity in the MS-nDBB complex. Quantitative analysis revealed a significant reduction in the number of M1mAChR- and M2mAChR-immunoreactive cells in the medial septum nucleus (MS) and in the horizontal nucleus of the diagonal band of Broca (HDB) as compared to the corresponding hemisphere in control animals and with that seen in the contralateral hemisphere, which corresponds to the PBS-injected side. Co-localization studies showed that the M1mAChR protein is localized in GABA-immunoreactive cells of the MS-nDBB complex, in particular those of the MS nucleus, while M2mAChR protein is localized in both the cholinergic and GABAergic cells. Moreover, GABAergic cells containing M2mAChR are mainly localized in the MS nucleus, while cholinergic cells containing M2mAChR are localized in the MS and the HDB nuclei. Our findings suggest that Abeta induces a reduction in M1mAChR- and M2mAChR-containing cells, which may contribute to impairments of cholinergic and GABAergic transmission in the MS-nDBB complex.

Sex difference in the 24-h acetylcholine release profile in the premotor/supplementary motor area of behaving rats

The sex differences in various motor functions suggest a sex-specific neural basis in the nonprimary or primary motor area. To examine the sex difference in the 24-h profile of acetylcholine (ACh) release in the rostral frontal cortex area 2 (rFr2), which is equivalent to the premotor/supplementary motor area in primates, we performed an in vivo microdialysis study in both sexes of rats fed pelleted or powdered diet. The dialysate was automatically collected from the rFr2 for 24 h under freely moving conditions. Moreover, the number of cholinergic neurons in the nucleus basalis magnocellularis (NBM) was examined. Further, to confirm the relation between ACh release in the rFr2 and motor function, the spontaneous locomotor activity was monitored for 24 h. Both sexes showed a distinct 24-h rhythm of ACh release, which was high during the dark phase and low during the light phase. Female rats, however, showed a greater ACh release and more cholinergic neurons in the NBM than male rats. Similarly, spontaneous locomotor activity also showed a 24-h rhythm, which paralleled the changes in ACh release in both sexes, and these changes were again greater in female rats than in male rats. In addition, feeding with powdered diet significantly increased the ACh release and spontaneous locomotor activity. The present study is the first to report the sex difference in the 24-h profile of ACh release in the rFr2 in rats. The sex specific ACh release in the rFr2 may partly contribute to the sex difference in motor function in rats.

Vascular dementia and the cholinergic pathways

Vascular cognitive impairment/vascular dementia have been the subject of a large number of studies, due to their high prevalence and broad preventive and compensatory therapeutic potential. The knowledge of the cerebral anatomy correlated to the vascular territories of irrigation enables understanding of clinical manifestations, as well as classification into the several types of syndromic presentations. The central cholinergic system exercises important neuromodulatory functions on cerebral circuits related to cognitive and behavioral integration, as well as on vasomotor control related to cerebral blood flow adjustments. The acquisition of data on the anatomy of the cholinergic pathways, including the localization of the nuclei of the basal prosencephalon and the routes of their projections, established an important milestone. The knowledge of the vascular distribution and of the trajectories of the cholinergic pathways allows identification of the strategic points where a vascular lesion can cause interruption. The ensuing denervation leads to cholinergic hypofunction in the involved territories. This information proves important to better evaluate the sites of vascular lesions, emphasizing their strategic localizations in relation to the cholinergic pathways, and offering more robust foundations for treatment aiming at enhancing cholinergic activity.

Role of substantia innominata in cerebral blood flow autoregulation

Ascending projections from the substantia innominata (SI) may have an important role in the regulation of cerebral blood flow (CBF). However, several reports have suggested that unilateral lesion of the SI does not affect CBF autoregulation. On the other hand, it is also reported that several cortical and subcortical functions may be regulated not only by ipsilateral SI, but also by contralateral SI. Thus, the objective of this study is to test the hypothesis that bilateral lesions of the SI affect CBF autoregulation. Experiments were performed on anesthetized male Sprague-Dawley rats. Ibotenic acid or physiological saline was microinjected into bilateral SI. Rats were classified into four groups as follows: bilateral SI lesion rats (ibotenic acid was injected bilaterally), left or right SI lesion rats (ibotenic acid was injected into the unilateral SI and saline into the contralateral SI), and control rats (saline was injected bilaterally). Ten days after injection, CBF in the left frontal cortex was measured by laser-Doppler flowmetry during stepwise controlled hemorrhagic hypotension. In bilateral SI lesion rats, CBF was started to decrease significantly at 80 mm Hg (p<0.01). In the other three groups, CBF was well maintained until 50 mm Hg. Changes in CBF through stepwise hypotension in bilateral SI lesion rats were significantly different from the other groups (p<0.01). These results suggest that bilateral SI regulates cortical vasodilator mechanisms during hemorrhagic hypotension. Under unilateral SI lesion, some compensatory effects from the contralateral SI may maintain CBF autoregulation.

Depletion of cholinergic neurons in the nucleus of the medial septum and the vertical limb of the diagonal band in dementia with Lewy bodies

The cholinergic basal forebrain is divided into four subregions (Ch1-4), and cholinergic neuronal loss in the nucleus basalis of Meynert (Ch4) has been correlated with cognitive impairments in both Alzheimer's disease (AD) and dementia with Lewy bodies (DLB). However, the Ch1-2 regions, which provide the major cholinergic innervation to the hippocampus, have not been investigated in DLB. The purpose of this study was to reveal the cholinergic neuronal changes in the medial septum (Ch1) and the nucleus of the vertical limb of the diagonal band (Ch2) of DLB brains. Using choline acetyltransferase (ChAT) immunohistochemistry, we showed that the number of ChAT-immunoreactive neurons in DLB brains was significantly lower than the numbers in AD and non-demented (control) brains. No significant difference in the number of ChAT-immunoreactive neurons was found between the AD and control brains. Moreover, the size of the ChAT-immunoreactive neurons was significantly smaller in the AD and DLB brains than in the control brains. These results show that cholinergic neurons of the Ch1-2 regions are more severely affected in DLB than in AD. Our DLB cases did not fulfill the neuropathologic criteria for definite AD. Furthermore, some Lewy bodies were observed in the Ch1-2 regions. Thus, cholinergic neuronal loss in the Ch1-2 regions might be specific to the pathology of DLB. Taking the distribution of cholinergic fibers in the hippocampus into consideration, this study suggests a possibility that hippocampal cholinergic projection is involved in Lewy-related neurites in the CA2-3 regions, the origin of which remains unclear.

Human cholinergic basal forebrain: chemoanatomy and neurologic dysfunction

The human cholinergic basal forebrain (CBF) is comprised of magnocellular hyperchromic neurons within the septal/diagonal band complex and nucleus basalis (NB) of Meynert. CBF neurons provide the major cholinergic innervation to the hippocampus, amygdala and neocortex. They play a role in cognition and attentional behaviors, and are dysfunctional in Alzheimer's disease (AD). The human CBF displays a continuum of large cells that contain various cholinergic markers, nerve growth factor (NGF) and its cognate receptors, calbindin, glutamate receptors, and the estrogen receptors, ERalpha and ERbeta. Admixed with these cholinergic neuronal phenotypes are smaller interneurons containing the m2 muscarinic acetylcholine receptor (mAChRs), NADPH-diaphorase, GABA, calcium binding proteins and several inhibitory neuropeptides including galanin (GAL), which is over expressed in AD. Studies using human autopsy material indicate an age-related dissociation of calbindin and the glutamate receptor GluR2 within CBF neurons, suggesting that these molecules act synergistically to induce excitotoxic cell death during aging, and possibly during AD. Choline acetyltrasnferease (ChAT) activity and CBF neuron number is preserved in the cholinergic basocortical system and up regulated in the septohippocampal system during prodromal as compared with end stage AD. In contrast, the number of CBF neurons containing NGF receptors is reduced early in the disease process suggesting a phenotypic silence and not a frank loss of neurons. In end stage AD, there is a selective reduction in trkA mRNA but not p75(NTR) in single CBF cells suggesting a neurotrophic defect throughout the progression of AD. These observations indicate the complexity of the chemoanatomy of the human CBF and suggest that multiple factors play different roles in its dysfunction in aging and AD.

Crossed unilateral lesions of temporal lobe structures and cholinergic cell bodies impair visual conditional and object discrimination learning in monkeys

Monkeys with excitotoxic lesions of the CA1/subiculum region in the right hemisphere and with immunotoxic lesions of the cholinergic cells of the diagonal band in the left hemisphere were impaired on a visual conditional task. In this task, correct choice of one of two objects depends on which of two background fields both objects are presented against, irrespective of the spatial positions of the objects. They were not impaired on simple object or shape discrimination tasks. The pattern of impairments is the same as that seen after bilateral excitotoxic lesions of CA1/subiculum, implying that the diagonal band lesion disables the ipsilateral CA1/subiculum. It also argues that CA1/subiculum, sustained by its cholinergic input, is necessary for some forms of nonspatial conditional learning. Addition of an inferotemporal (IT) cortical ablation to the left hemisphere did not affect simple visual discrimination learning, although all the monkeys then failed to learn a new visual conditional task. This demonstrates that intact IT cortex in only one hemisphere is sufficient to sustain simple visual discrimination learning but implies that the cholinergic input and the inferotemporal cortical input to the hippocampus both contribute to visual conditional learning. The subsequent addition of an immunotoxic lesion of the basal nucleus of Meynert in the right hemisphere resulted in an additional impairment on a difficult shape discrimination. This argues that it is the cholinergic projection to the inferotemporal cortex, rather than to the rest of the cortex, which contributes to visual discrimination learning and memory.

What is a memory system? Horel's critique revisited

J.A. Horel's critique of what he termed "the hippocampal memory hypothesis" turns out, 23 years later, to have been remarkably discerning and prophetic. There is now an overwhelming weight of evidence to confirm his four key proposals: that selective destruction of the hippocampus or fornix does not produce dense global amnesia; that the effects of hippocampal or fornix lesions are not primarily a memory impairment, but an impairment in processing spatial information; that damage to the anterior temporal stem is part of the explanation of dense temporal lobe amnesia; and that the interaction of temporal cortex with prefrontal cortex is essential in memory. This review summarizes the modern evidence that reinforces each of these four proposals. A final section argues that, not only in the case of the hippocampus but also in the case of other temporal and frontal cortical areas that are involved in normal memory, the concept of a "memory system" is harmful.

Crossed unilateral lesions of medial forebrain bundle and either inferior temporal or frontal cortex impair object recognition memory in Rhesus monkeys

In monkeys, section of the fornix, amygdala and anterior temporal stem results in a severe anterograde amnesia. Immunolesions of the cholinergic cells of the basal forebrain suggest that this amnesia is a result of isolating the inferior temporal cortex and medial temporal lobe from their cholinergic basal forebrain afferents. In this experiment, six monkeys were trained in a delayed match-to-sample task and then received a section of the medial forebrain bundle in one hemisphere and an ablation of either the frontal or inferior temporal cortex in the opposite hemisphere. All the animals were severely impaired in the performance of this task following this surgery, and the severity of the impairment was independent of the cortical area from which the medial forebrain bundle was disconnected. These results support a model of fronto-temporal interaction via the basal forebrain in new learning, in which midbrain sites related to reward modulate the cholinergic basal forebrain activity.

Dense amnesia in the monkey after transection of fornix, amygdala and anterior temporal stem

The traditional explanation of dense amnesia after medial temporal lesions is that the amnesia is caused by damage to the hippocampus and related structures. An alternative view is that dense amnesia after medial temporal lesions is caused by the interruption of afferents to the temporal cortex from the basal forebrain. These afferents travel to the temporal cortex through three pathways, namely the anterior temporal stem, the amygdala and the fornix-fimbria, and all these three pathways are damaged in dense medial temporal amnesia. In four experiments using different memory tasks, we tested the effects on memory of sectioning some or all of these three pathways in macaque monkeys. In a test of scene-specific memory for objects, which is analogous in some ways to human episodic memory, section of fornix alone, or section of amygdala and anterior temporal stem sparing the fornix, each produced a significant but mild impairment. When fornix section was added to the section of anterior temporal stem and amygdala in this task, however, a very severe impairment resulted. In an object recognition memory task (delayed matching-to-sample) a severe impairment was seen after section of anterior temporal stem and amygdala alone, with or without the addition of fornix section; this impairment was significantly more severe than that which was seen in the same task after amygdalectomy leaving the temporal stem intact, with or without fornix section. Animals with combined section of anterior temporal stem, amygdala and fornix were also impaired in object-reward association learning. However, the retention of pre-operatively acquired object-reward associations was at a high level. These results show that the pattern of impairments after section of anterior temporal stem, amygdala and fornix in the monkey, leaving hippocampus intact, resembles human dense amnesia and is different from the effects of hippocampal lesions in the monkey.

Visual discrimination learning impairments produced by combined transections of the anterior temporal stem, amygdala and fornix in marmoset monkeys

Marmoset monkeys (Callithrix jacchus) with bilateral transections of the anterior temporal stem, amygdala and fornix were unable to relearn a 2-choice object discrimination first learnt prior to surgery, and were very severely impaired at relearning a concurrent object discrimination task which they had learnt and relearnt prior to surgery, indicating that they had a dense retrograde amnesia. They also had difficulty learning new visual object discriminations but were only mildly impaired on spatial learning. When tested on new learning of concurrent discriminations 8 to 10 weeks after surgery, three operated monkeys were unable to reach criterion in 400 trials while the remaining two operated monkeys performed within the normal range. The operated monkeys were subsequently shown to be impaired on acquisition of shape discriminations using black objects. These anterograde effects suggest that the impairment runs mainly in the domain of visual analysis. The monkeys also exhibited many of the features of the Klüver-Bucy syndrome. Histological analysis indicated that in addition to cutting some of the subcortical temporal lobe efferent pathways, the surgical procedures had cut the cholinergic afferents to the temporal neocortex, entorhinal cortex, and hippocampus. In a second experiment we found that treatment with the cholinergic agonist pilocarpine, which is effective in monkeys with specific cholinergic lesions, was unable to remediate the lesion-induced impairments. This suggests that transection of the non-cholinergic afferents, or the temporal lobe subcortical efferents, contributed to the behavioural syndrome and the learning and retention deficits seen in these monkeys.

Comparison of perirhinal cortex ablation and crossed unilateral lesions of the medial forebrain bundle from the inferior temporal cortex in the rhesus monkey: effects on learning and retrieval

Seven monkeys learned new object-reward associations and scene problems and were overtrained on 100 problems of each type. Four monkeys received crossed lesions of the medial forebrain bundle (MFB) and inferior temporal cortex, with the later addition of a fornix section ipsilateral to the MFB lesion. The remaining 3 monkeys received bilateral perirhinal cortex ablation. Disconnection of the MFB from the inferior temporal cortex impaired postoperative new learning, but the retrieval of problems overtrained preoperatively was relatively preserved. Subjects with perirhinal cortex ablation were severely impaired in new learning and at the retrieval of scene problems, but retention of object-reward associations was relatively well preserved. The results support the hypothesis that isolation of the inferior temporal cortex from basal forebrain and midbrain afferents results in dense anterograde amnesia, whereas the role of the perirhinal cortex in learning is dependent on the perceptual difficulty of the task.

Ovarian hormones differentially influence immunoreactivity for dopamine beta- hydroxylase, choline acetyltransferase, and serotonin in the dorsolateral prefrontal cortex of adult rhesus monkeys

Recent studies have shown that ovariectomy reduces, and subsequent hormone replacement restores the density of axons immunoreactive for tyrosine hydroxylase in the dorsolateral prefrontal cortex of adult female rhesus monkeys. The present study indicates that three additional extrathalamic frontal lobe afferents are also sensitive to changes in the ovarian hormone environment. Specifically, the combination of hormone manipulation with qualitative and quantitative analysis of immunocytochemistry for dopamine beta-hydroxylase, choline acetyltransferase, and serotonin in the primate prefrontal cortex revealed quantitative responses in both cholinergic and monoaminergic axons to changing ovarian hormone levels. However, whereas ovariectomy produced a modest net decrease in the density of fibers immunoreactive for choline acetyltransferase, this same treatment markedly increased the density of axons immunoreactive for dopamine beta-hydroxylase and for serotonin. Further, the effects of ovariectomy on these afferent systems were differentially attenuated by estrogen verses estrogen plus progesterone hormone replacement. Estrogen was as effective as estrogen plus progesterone in stimulating normal prefrontal immunoreactivity for choline acetyltransferase and dopamine beta-hydroxylase. The dual replacement of estrogen plus progesterone, however, was a much more potent influence than estrogen alone for serotonin immunoreactivity. Thus, ovarian hormones appear to provide stimulation that differentially affects each of four chemically identified extrathalamic prefrontal afferent systems examined to date, and may have roles in maintaining the normal balance and functional interactions between these neurotransmitter systems.

The role of the central cholinergic projections in cognition: implications of the effects of scopolamine on discrimination learning by monkeys

In humans, administration of the cholinergic antagonist scopolamine impairs the encoding of information into long-term memory and has effects on other cognitive processes. It has been supposed that it is inhibition of the rising cholinergic projections from the basal forebrain, specifically from the basal nucleus of Meynert (NBM) to the neocortex and from the medial septum/vertical limb of the diagonal band of Broca (MS/VDB) to the hippocampus, that results in these cognitive impairments. In this paper, we describe the effects of scopolamine treatment in monkeys on learning different sorts of visual discrimination and visuospatial conditional tasks and compare these results to the effects of lesions of the rising cholinergic projections. Experiments in rodents in which these projections have been selectively destroyed have failed to produce a consensus view of the functions of these two areas. In particular, highly specific immunotoxic lesions of the NBM have largely failed to produce changes in task performance that can be interpreted as resulting from a cognitive impairment. In monkeys, lesions of the NBM produce modest or short-lasting, impairments in visual discrimination learning, retention, and reversal, whereas lesions of the MS/VDB produce large and permanent impairments of certain types of conditional learning. Similar impairments produced by scopolamine in monkeys and additive effects of lesions of the NBM or MS/VDB with scopolamine suggest that scopolamine has these effects by acting on the rising cholinergic pathways rather than on other cholinergic systems in the brain. It is argued that the rising cholinergic projections sustain the functions of the target areas; in the case of the hippocampus in humans, the function is usually regarded as being the analysis of information in a way that is pertinent to the formation of episodic memories and in the case of the neocortex, is the analysis of information in a manner that is relevant to the cognitive processing of on-going events and the acquisition of semantic knowledge.

Cell loss in the nucleus basalis is related to regional cortical atrophy in Alzheimer's disease

Cortical atrophy and cell loss in the cholinergic nucleus basalis is a well-established characteristic of Alzheimer's disease; however, previous studies not have analysed cholinergic cell loss and cortical atrophy in concert. In autopsy brains from eight patients with Alzheimer's disease and 12 control subjects, the numbers of nucleus basalis neurons were determined from 50-microm serial Nissl-stained sections. Volumes of the cerebrum, cortical gray matter (total, lobar and subregional), white matter and deep gray structures were computed by point counting on black and white photographs of gapless 3-mm coronal slices of formalin-fixed brains. Cell loss in the nucleus basalis was found to range between 89% and 42% in Alzheimer's disease compared with controls. White matter volume was unchanged in absolute terms in Alzheimer's disease patients compared with controls, while cortical volume was significantly reduced. Gray matter atrophy was most prominent in temporal and frontal cortices. A highly significant linear relationship was found between cortical volume and nucleus basalis cell number in controls and Alzheimer's disease patients, with values for both groups on a single regression line. Whole brain and cerebral volumes were also highly correlated to nucleus basalis cell numbers in both groups. A quantitative analysis of plaque and tangle burden in cortical target areas of the nucleus basalis was performed. In contrast to the relationship with cortical volume, nucleus basalis cell number and neurofibrillary tangle number were not significantly correlated to the density of cortical histopathology. These results suggest that the volume of cortical gray matter is coupled to the number of nucleus basalis neurons. Compromised viability of nucleus basalis neurons may precede cortical volume loss as large numbers of neurofibrillary tangles, detected with nickel peroxidase staining, were found in this nucleus in all Alzheimer's disease cases, including those with minimal cell loss.

GABAergic neurons in the rat dentate gyrus are innervated by subcortical calretinin-containing afferents

Fibers of supramammillary origin establish putatively excitatory asymmetric synaptic connections with dentate granule cells. The present study was designed to determine whether hippocampal gamma-aminobutyric acid (GABA)-ergic nonprincipal cells are also targets of these calretinin (CR)-containing subcortical afferents. Light and electron microscopic double immunostaining for CR and parvalbumin (PA) or calbindin (CB) were performed in the rat dentate gyrus ipsilateral and contralateral to a unilateral fimbria-fornix transection. GABA-postembedding immunostaining was performed on ultrathin sections of this double-labeled material. Contralateral to the transection, CR-immunoreactive fibers formed multiple large boutons in the inner molecular layer. These fibers also impinged on PA-containing basket cells located adjacent to the granular layer and on CB-immunoreactive hilar neurons. Ipsilateral to the transection, CR-containing fibers in the inner molecular layer and boutons impinging on PA-containing or CB-immunoreactive neurons were absent. Parent cell bodies of extrinsic CR-containing afferents were traced using wheat germ agglutinin-conjugated horseradish peroxidase. Additional CR immunostaining of the subcortical region unveiled retrogradely labeled neurons that were also immunostained for CR only in the supramammillary area and the nucleus reuniens. The latter projection, however, terminates in CA1 and not in the dentate gyrus. Subcortical afferents impinging on dentate nonprincipal cells formed exclusively asymmetric synapses. Postembedding immunostaining demonstrated that CB-containing cells contain GABA, whereas CR-positive axon terminals forming asymmetric synapses are devoid of this labeling. These data indicate that dentate inhibitory neurons receive a putative excitatory input originating from the supramammillary nucleus. Thus, the supramamillo-hippocampal pathway may exert a powerful feed-forward inhibitory control of the signal flow in the rat dentate gyrus.

Effects of lesions of different parts of the septo-hippocampal system in primates on learning and retention of information acquired before or after surgery

Data from a large series of experiments on marmosets with lesions of the septal/diagonal band area (DB), fornix or CA1 area of the hippocampus are analysed in terms of retention of information learned before surgery, acquisition of new information and retention of information acquired after surgery. It is shown that although all three lesions impair acquisition of a specific type of new information, lesions of CA1 result in a severe retrograde amnesia but no forgetting of that type of information adequately acquired after surgery, whereas lesions of the DB do not cause retrograde amnesia but do result in significant forgetting. Monkeys with fornix transection occupied an intermediate position in their pattern of learning impairments; some animals showed evidence of forgetting, whereas the great majority showed retrograde amnesia. These data may be relevant to an understanding of the different extent of amnesia in patients with different pathology within the medial temporal lobe and associated subcortical structures.

Calretinin immunoreactive structures in the human hippocampal formation

The calcium-binding protein calretinin is present in an intrinsic GABAergic and an extrinsic non-GABAergic system in the rat and monkey hippocampal formation. Important species differences have been noted in hippocampal cell types immunostained for calretinin and the termination pattern of calretinin containing hypothalamic afferents in the hippocampus. In the present study, calretinin-containing neurons were visualized using immunocytochemistry in the human hippocampal formation of individuals which showed no significant neuropathological alterations. Calretinin-immunoreactivity was present exclusively in non-granule cells of the dentate gyrus and in non-pyramidal cells of Ammon's horn. Calretinin-positive neurons were found most frequently in the hilus of the fascia dentata and in strata radiatum and lacunosum-moleculare of CA1, whereas neurons in CA2 and CA3 were rarely immunostained. The majority of calretinin-immunoreactive neurons were small, bipolar or fusiform neurons. The dendritic trees of the calretinin-positive neurons were, for the most part, parallel to the dendrites of the principal cells. In the hilus, however, we observed cells with dendrites restricted to the hilar area. These dendrites were parallel to the granule cell layer. In the stratum lacunosum-moleculare, neurons with dendrites oriented parallel to the hippocampal fissure were frequently detected. In general, dendrites were smooth or sparsely spiny, displaying small conventional spines. The axons usually emerged from the proximal dendrite and could be followed over long distances. Axons were thin, had small varicosities and displayed only few collaterals which branched relatively far away from the cell body. Distinct bands of darkly stained calretinin-positive fibers occupied the innermost portion of the dentate molecular layer and the pyramidal cell layer of CA2. This distribution of calretinin-immunoreactive structures in the human hippocampus is similar to that observed in other primates but differs from that described in lower mammals, i.e., the rat. Our findings suggest that primates may share a common hippocampal calretinin-containing system, presumably both the intrinsic GABAergic and the extrinsic hypothalamic non-GABAergic components.

Cholinergic innervation of the primate hippocampal formation. I. Distribution of choline acetyltransferase immunoreactivity in the Macaca fascicularis and Macaca mulatta monkeys

The cholinergic innervation of the hippocampal formation of Macaca fascicularis (cynomolgus) and Macaca mulatta (rhesus) monkeys was investigated by immunohistochemical procedures using a monoclonal antibody directed against choline acetyltransferase. The distribution of choline acetyltransferase in the monkey demonstrated both similarities and differences with the staining patterns observed in the rat or with acetylcholinesterase in the monkey. While both of these latter preparations demonstrated labeled cells, for example, no choline acetyltransferase labeled neurons were observed in the monkey hippocampal formation. Choline acetyltransferase activity was restricted to fibers which varied in thickness and number of varicosities and in their regional and laminar distribution. The highest densities of labeled fibers were observed in the uncal portion of the hippocampus, in the parasubiculum, and in the entorhinal cortex; the lowest densities of labeled fibers were observed in CA1 and in midrostrocaudal levels of the dentate gyrus. In the dentate gyrus, immunoreactive fibers were densely distributed in the molecular layer and in an infragranular plexus. One of the few striking noticeable interspecies differences was observed in the dentate gyrus. In the rhesus monkey, labeled fibers in the molecular layer were divided into a superficial denser and an inner lighter lamina, whereas in M. fascicularis, the cholinergic fibers were distributed more homogeneously throughout the molecular layer. In the hippocampus proper, there was a progressive decrease in the density of ChAT-immunoreactive fibers from CA3/CA2 into CA1. The subiculum also demonstrated modest labeling which was nonetheless higher than in CA1; the border of these fields demonstrated increased fiber labeling. The density of choline acetyltransferase staining was high in the presubiculum and parasubiculum. In the entorhinal cortex, a relatively clear boundary was observed between the more heavily stained superficial layers (I, II, and III) and the more weakly labeled deep layers (V and VI), especially in the intermediate and caudal fields. A transverse decreasing gradient was observed with the densest plexus of cholinergic fibers found in the medially situated olfactory field of the entorhinal cortex and the lowest density in the laterally located caudal and lateral fields.

Substance P-containing hypothalamic afferents to the monkey hippocampus: an immunocytochemical, tracing, and coexistence study

In order to identify the synaptic connections of substance P-containing afferents within the hypothalamo-hippocampal projection of the monkey, we performed a combined light and electron microscopic, immunocytochemical study, made lesions of the fimbriafornix, and employed retrograde tracing using WGA-HRP. Furthermore, coexistence studies for substance P and GAD were performed to identify the putative transmitters of these hypothalamic projection neurons. A plexus of large substance P-immunoreactive terminals was identified in both the innermost portion of the molecular layer and in CA2. Axon terminals in both plexuses established exclusively asymmetric synapses with spines and dendritic shafts. Substance P-immunoreactive boutons were degenerating 5 days after lesioning, and had disappeared 10 days after ipsilateral fimbria-fornix transection. Thus, these terminals were of extrinsic origin. In contrast, immunoreactive fibers in the outer third of the dentate molecular layer remained unaffected by the lesion. Retrograde tracing combined with immunostaining for substance P revealed the parent cell bodies of the extrinsic substance P-containing afferents in the supramammillary nucleus. Colocalization studies employing a consecutive semi-thin sections technique indicate that these large substance P-containing projection neurons lack GABA as an inhibitory transmitter. These results suggest that hypothalamic afferents of the monkey hippocampus contain substance P. Because these afferents lack GABA as an inhibitory transmitter and establish exclusively asymmetric synapses, this projection may excite hippocampal target neurons.

Behavioral assessment of the ability of intracerebral embryonic neural tissue grafts to ameliorate the effects of brain damage in marmosets

The transplantation of neuronal tissue into the brains of patients with Parkinson's disease is already being assessed as an experimental treatment for the symptoms of this disease, and the possibility of using similar graft tissue to ameliorate the symptoms of other neurodegenerative diseases is being considered. In this context, a small number of transplant experiments have been carried out in monkeys with lesions of the central dopamine and cholinergic systems. These experiments make it possible to determine the optimum methods of transplantation in an animal whose brain is structurally more closely related to the human than that of the rat and to assess the behavioral consequences of transplantation on symptoms that either resemble very closely the symptoms seen in patients, or are of a complex cognitive nature and are therefore more difficult to measure in the rat. It is intended that these experiments will contribute to the development of better treatments for the neurodegenerative diseases, either by the use of transplantation as a clinical treatment, or by contributing to a better understanding of the mechanisms that normally maintain neuronal function and that fail in these diseases.

Primate striatonigral projections: a comparison of the sensorimotor-related striatum and the ventral striatum

The striatum receives topographic cortical inputs with the limbic lobe terminating in the ventral striatum and sensorimotor cortical regions terminating in the dorsolateral striatum. The organization of striatonigral projections originating from these different striatal territories was examined in primate by using several anterograde tracers. The ventral striatum innervates a large area of the substantia nigra, including the medial pars reticulata and much of the pars compacta. Moreover, projections from separate areas of the ventral striatum overlap considerably in the substantia nigra. No mediolateral or rostrocaudal topographic order is apparent, and the area of the substantia nigra associated with the ventral striatum is extensive. In contrast, the sensorimotor-related striatum innervates a limited region of the ventrolateral substantia nigra. Similar to ventral striatonigral projections, projections originating from different areas of the sensorimotor-related striatum send converging inputs to the substantia nigra. Sensorimotor-related striatonigral projections avoid the region of the dopaminergic neurons in the dorsal pars compacta. Striatonigral projections from the sensorimotor-related and ventral striatum do not overlap in the substantia nigra. Examination of the outputs of discrete striatal loci indicates that the organization of striatonigral projections is more related to corticostriatal inputs than to a simple rostrocaudal, dorsoventral, or mediolateral topography of the striatum. Striatal projections that originate from different striatal territories are distinct and nonoverlapping, thus supporting the concept of segregated striatonigral circuits. However, areas of the striatum that receive common cortical inputs send converging inputs to the substantia nigra. This suggests that the substantia nigra is also an important link for integrating information between functionally related (sub)circuits.

Cholinergic innervation in the human hippocampal formation including the entorhinal cortex

The cholinergic innervation of the hippocampal formation is thought to play an important role in memory processes, but its organization in humans has not been described in detail. We studied the cholinergic innervation of the human hippocampal formation by means of immunohistochemistry with polyclonal antisera directed against acetylcholinesterase (AChE), choline acetyltransferase (ChAT), and the low-affinity (p75) nerve growth factor receptor (NGFR). The density of ChAT-like immunoreactive (ChAT-li) fibers differed substantially among the various regions, in general paralleling the pattern of AChE-li staining. One notable exception was the presence of AChE-li cell bodies. In contrast, ChAT immunoreactivity was associated only with fibers and terminals. NGFR-li staining corresponded closely to the ChAT-li fiber pattern. ChAT-li fibers in the CA fields diffusely filled the stratum pyramidale and extended into the stratum oriens and radiatum as well. The highest density was consistently observed in CA4 and CA3 subfields. Staining decreased from CA4 to CA1 and was substantially less dense in the subicular complex. In the entorhinal cortex, the ChAT- and NGFR-li fiber innervation displayed a laminar pattern, most intense over the nests of cells in layer II. There was a trend towards an age-related reduction in the density of ChAT- and AChE-li fibers and terminals. Nonetheless, we also found a surprisingly conserved NGFR-li innervation and the presence of occasional NGFR-li pyramidal cells, providing evidence of a plastic response in the brains of the elderly patients.

Nerve growth factor and Alzheimer's disease

Nerve growth factor (NGF) is a well-characterized protein that exerts pharmacological effects on a group of cholinergic neurons known to atrophy in Alzheimer's disease (AD). Considerable evidence from animal studies suggests that NGF may be useful in reversing, halting, or at least slowing the progression of AD-related cholinergic basal forebrain atrophy, perhaps even attenuating the cognitive deficit associated with the disorder. However, many questions remain concerning the role of NGF in AD. Levels of the low-affinity receptor for NGF appear to be at least stable in AD basal forebrain, and the recent finding of AD-related increases in cortical NGF brings into question whether endogenous NGF levels are related to the observed cholinergic atrophy and whether additional NGF will be useful in treating this disorder. Evidence regarding the localization of NGF within the central nervous system and its presumed role in maintaining basal forebrain cholinergic neurons is summarized, followed by a synopsis of the relevant aspects of AD neuropathology. The available data regarding levels of NGF and its receptor in the AD brain, as well as potential roles for NGF in the pathogenesis and treatment of AD, are also reviewed. NGF and its low affinity receptor are abundantly present within the AD brain, although this does not rule out an NGF-related mechanism in the degeneration of basal forebrain neurons, nor does it eliminate the possibility that exogenous NGF may be successfully used to treat AD. Further studies of the degree and distribution of NGF within the human brain in normal aging and in AD, and of the possible relationship between target NGF levels and the status of basal forebrain neurons in vivo, are necessary before engaging in clinical trials.

Neocortical infarction in subhuman primates leads to restricted morphological damage of the cholinergic neurons in the nucleus basalis of Meynert

The aim of the present study was to investigate the long-term effect of cortical infarction on the subhuman primate (Cercopithecus aethiops) basal forebrain. The lesion, carried out by cauterizing the pial blood vessels supplying the left fronto-parieto-temporal neocortex, induced retrograde degenerative processes within the ipsilateral nucleus basalis of Meynert. The morphometrical analysis revealed that significant shrinkage of cholinergic neurons and loss of neuritic processes were localized within the intermediate regions of the nucleus basalis. The average cross-sectional areas of choline acetyltransferase-immunoreactive neurons in the intermedio-ventral (Ch4iv) and intermedio-dorsal (Ch4id) nucleus basalis were decreased to 62.5 +/- 9.5 and 58.0 +/- 8.6%, respectively, of the sham-operated values. Although an apparent loss of Nissl-stained magnocellular neurons in Ch4iv and Ch4id was found by applying a quantitative analysis based on a perikaryal-size criterion, data obtained by the quantification of immunostained material failed to reveal any significant decrease of cholinergic cell density. Results are discussed in view of future application of this ischemic model to study processes of retrograde degeneration following cortical target removal and to assess potential neurotrophic and neuroprotective properties of pharmacologic agents.

Cholinergic innervation of mouse forebrain structures

Using choline acetyltransferase (ChAT) immunocytochemistry and acetylcholinesterase (AChE) histochemistry, we investigated regional and laminar differences in cholinergic innervation in the cerebral cortex, hippocampus, amygdala, and thalamus of mice. In mice, unlike rats, the patterns of ChAT-immunostained and AChE-positive fibers are virtually identical in the cortex and are organized in a trilaminar pattern with cholinergic processes prominent in layers I and IV and within the lower portion of layer V and upper segment of layer VI. ChAT-immunoreactive cells were not seen in cortex. In the amygdala, the basolateral nucleus showed the highest density of cholinergic processes. In the hippocampus, a thin, dense band of ChAT-labeled processes was present in the inner segment of the molecular layer of the dentate gyrus and within the stratum oriens of CA1-3, adjacent to the basal aspect of pyramidal cells. Within the thalamus, anteroventral, mediodorsal (lateral portion), intralaminar, and reticular nuclei showed high densities of cholinergic processes. The results of this study provide the basis for examining the effects of transgenes and age on forebrain cholinergic systems.