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

α7 nicotinic acetylcholine receptors induce long-term synaptic enhancement in the dorsal but not ventral hippocampus

Agents that positively modulate the activity of α7nAChRs are used as cognitive enhancers and for the treatment of hippocampus-dependent functional decline. However, it is not known whether the expression and the effects of α7nAChRs apply to the entire longitudinal axis of the hippocampus equally. Given that cholinergic system-involving hippocampal functions are not equally distributed along the hippocampus, we comparatively examined the expression and the effects of α7nAChRs on excitatory synaptic transmission between the dorsal and the ventral hippocampal slices from adult rats. We found that α7nAChRs are equally expressed in the CA1 field of the two segments of the hippocampus. However, activation of α7nAChRs by their highly selective agonist PNU 282987 induced a gradually developing increase in field excitatory postsynaptic potential only in the dorsal hippocampus. This long-term potentiation was not reversed upon application of nonselective nicotinic receptor antagonist mecamylamine, but the induction of potentiation was prevented by prior blockade of α7nAChRs by their antagonist MG 624. In contrast to the long-term synaptic plasticity, we found that α7nAChRs did not modulate short-term synaptic plasticity in either the dorsal or the ventral hippocampus. These results may have implications for the role that α7nAChRs play in specifically modulating functions that depend on the normal function of the dorsal hippocampus. We propose that hippocampal functions that rely on a direct α7 nAChR-mediated persistent enhancement of glutamatergic synaptic transmission are preferably supported by dorsal but not ventral hippocampal synapses.

Neuroanatomical and psychological considerations in temporal lobe epilepsy

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

Three-dimensional synaptic organization of the human hippocampal CA1 field

The hippocampal CA1 field integrates a wide variety of subcortical and cortical inputs, but its synaptic organization in humans is still unknown due to the difficulties involved studying the human brain via electron microscope techniques. However, we have shown that the 3D reconstruction method using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) can be applied to study in detail the synaptic organization of the human brain obtained from autopsies, yielding excellent results. Using this technology, 24,752 synapses were fully reconstructed in CA1, revealing that most of them were excitatory, targeting dendritic spines and displaying a macular shape, regardless of the layer examined. However, remarkable differences were observed between layers. These data constitute the first extensive description of the synaptic organization of the neuropil of the human CA1 region.

Distribution of choline acetyltransferase (ChAT) immunoreactivity in the brain of the teleost Cyprinus carpio

Cholinergic systems play a role in basic cerebral functions and its dysfunction is associated with deficit in neurodegenerative disease. Mechanisms involved in human brain diseases, are often approached by using fish models, especially cyprinids, given basic similarities of the fish brain to that of mammals. In the present paper, the organization of central cholinergic systems have been described in the cyprinid Cyprinus carpio, the common carp, by using specific polyclonal antibodies against ChAT, the synthetic enzyme of acetylcholine, that is currently used as a specific marker for cholinergic neurons in all vertebrates.  In this work, serial transverse sections of the brain and the spinal cord were immunostained for ChAT. Results showed that positive neurons are present in several nuclei of the forebrain, the midbrain, the hindbrain and the spinal cord. Moreover, ChAT-positive neurons were detected in the synencephalon and in the cerebellum. In addition to neuronal bodies, afferent varicose fibers were stained for ChAT in the ventral telencephalon, the preoptic area, the hypothalamus and the posterior tuberculum. No neuronal cell bodies were present in the telencephalon. The comparison of cholinergic distribution pattern in the Cyprinus carpio central nervous system has revealed similarities but also some interesting differences with other cyprinids. Our results provide additional information on the cholinergic system from a phylogenetic point of view and may add new perspectives to physiological roles of cholinergic system during evolution and the neuroanatomical basis of neurological diseases.

Basal forebrain degeneration precedes and predicts the cortical spread of Alzheimer's pathology

There is considerable debate whether Alzheimer's disease (AD) originates in basal forebrain or entorhinal cortex. Here we examined whether longitudinal decreases in basal forebrain and entorhinal cortex grey matter volume were interdependent and sequential. In a large cohort of age-matched older adults ranging from cognitively normal to AD, we demonstrate that basal forebrain volume predicts longitudinal entorhinal degeneration. Models of parallel degeneration or entorhinal origin received negligible support. We then integrated volumetric measures with an amyloid biomarker sensitive to pre-symptomatic AD pathology. Comparison between cognitively matched normal adult subgroups, delineated according to the amyloid biomarker, revealed abnormal degeneration in basal forebrain, but not entorhinal cortex. Abnormal degeneration in both basal forebrain and entorhinal cortex was only observed among prodromal (mildly amnestic) individuals. We provide evidence that basal forebrain pathology precedes and predicts both entorhinal pathology and memory impairment, challenging the widely held belief that AD has a cortical origin.

Whether Alzheimer's disease originates in basal forebrain or entorhinal cortex remains highly debated. Here the authors use structural magnetic resonance data from a longitudinal sample of participants stratified by cerebrospinal biomarker and clinical diagnosis to show that tissue volume changes appear earlier in the basal forebrain than in the entorhinal cortex.

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.

Neuroanatomical organization of the cholinergic system in the central nervous system of a basal actinopterygian fish, the senegal bichir Polypterus senegalus

Polypterid bony fishes are believed to be basal to other living ray-finned fishes, and their brain organization is therefore critical in providing information as to primitive neural characters that existed in the earliest ray-finned fishes. The cholinergic system has been characterized in more advanced ray-finned fishes, but not in polypterids. In order to establish which cholinergic neural centers characterized the earliest ray-finned fishes, the distribution of choline acetyltransferase (ChAT) is described in Polypterus and compared with the distribution of this molecule in other ray-finned fishes. Cell groups immunoreactive for ChAT were observed in the hypothalamus, the habenula, the optic tectum, the isthmus, the cranial motor nuclei, and the spinal motor column. Cholinergic fibers were observed in both the telencephalic pallium and the subpallium, in the thalamus and pretectum, in the optic tectum and torus semicircularis, in the mesencephalic tegmentum, in the cerebellar crest, in the solitary nucleus, and in the dorsal column nuclei. Comparison of the data within a segmental neuromeric context indicates that the cholinergic system in polypterid fishes is generally similar to that in other ray-finned fishes, but cholinergic-positive neurons in the pallium and subpallium, and in the thalamus and cerebellum, of teleosts appear to have evolved following the separation of polypterids and other ray-finned fishes.

Cholinergic markers in the cortex and hippocampus of some animal species and their correlation to Alzheimer's disease

INTRODUCTION

The cholinergic system includes neurons located in the basal forebrain and their long axons that reach the cerebral cortex and the hippocampus. This system modulates cognitive function. In Alzheimer's disease (AD) and ageing, cognitive impairment is associated with progressive damage to cholinergic fibres, which leads us to the cholinergic hypothesis for AD.

DEVELOPMENT

The AD produces alterations in the expression and activity of acetyltransferase (ChAT) and acetyl cholinesterase (AChE), enzymes specifically related to cholinergic system function. Both proteins play a role in cholinergic transmission, which is altered in both the cerebral cortex and the hippocampus due to ageing and AD. Dementia disorders are associated with the severe destruction and disorganisation of the cholinergic projections extending to both structures. Specific markers, such as anti-ChAT and anti-AChE antibodies, have been used in light immunohistochemistry and electron microscopy assays to study this system in adult members of certain animal species.

CONCLUSIONS

This paper reviews the main immunomorphological studies of the cerebral cortex and hippocampus in some animal species with particular emphasis on the cholinergic system and its relationship with the AD.

Muscarinic receptors in amygdala control trace fear conditioning

Intelligent behavior requires transient memory, which entails the ability to retain information over short time periods. A newly-emerging hypothesis posits that endogenous persistent firing (EPF) is the neurophysiological foundation for aspects or types of transient memory. EPF is enabled by the activation of muscarinic acetylcholine receptors (mAChRs) and is triggered by suprathreshold stimulation. EPF occurs in several brain regions, including the lateral amygdala (LA). The present study examined the role of amygdalar mAChRs in trace fear conditioning, a paradigm that requires transient memory. If mAChR-dependent EPF selectively supports transient memory, then blocking amygdalar mAChRs should impair trace conditioning, while sparing delay and context conditioning, which presumably do not rely upon transient memory. To test the EPF hypothesis, LA was bilaterally infused, prior to trace or delay conditioning, with either a mAChR antagonist (scopolamine) or saline. Computerized video analysis quantified the amount of freezing elicited by the cue and by the training context. Scopolamine infusion profoundly reduced freezing in the trace conditioning group but had no significant effect on delay or context conditioning. This pattern of results was uniquely anticipated by the EPF hypothesis. The present findings are discussed in terms of a systems-level theory of how EPF in LA and several other brain regions might help support trace fear conditioning.

Dual effect of beta-amyloid on α7 and α4β2 nicotinic receptors controlling the release of glutamate, aspartate and GABA in rat hippocampus

Background

We previously showed that beta-amyloid (Aβ), a peptide considered as relevant to Alzheimer's Disease, is able to act as a neuromodulator affecting neurotransmitter release in absence of evident sign of neurotoxicity in two different rat brain areas. In this paper we focused on the hippocampus, a brain area which is sensitive to Alzheimer's Disease pathology, evaluating the effect of Aβ (at different concentrations) on the neurotransmitter release stimulated by the activation of pre-synaptic cholinergic nicotinic receptors (nAChRs, α4β2 and α7 subtypes). Particularly, we focused on some neurotransmitters that are usually involved in learning and memory: glutamate, aspartate and GABA.

Methodology/Findings

We used a dual approach: in vivo experiments (microdialysis technique on freely moving rats) in parallel to in vitro experiments (isolated nerve endings derived from rat hippocampus). Both in vivo and in vitro the administration of nicotine stimulated an overflow of aspartate, glutamate and GABA. This effect was greatly inhibited by the highest concentrations of Aβ considered (10 µM in vivo and 100 nM in vitro). In vivo administration of 100 nM Aβ (the lowest concentration considered) potentiated the GABA overflow evoked by nicotine. All these effects were specific for Aβ and for nicotinic secretory stimuli. The in vitro administration of either choline or 5-Iodo-A-85380 dihydrochloride (α7 and α4β2 nAChRs selective agonists, respectively) elicited the hippocampal release of aspartate, glutamate, and GABA. High Aβ concentrations (100 nM) inhibited the overflow of all three neurotransmitters evoked by both choline and 5-Iodo-A-85380 dihydrochloride. On the contrary, low Aβ concentrations (1 nM and 100 pM) selectively acted on α7 subtypes potentiating the choline-induced release of both aspartate and glutamate, but not the one of GABA.

Conclusions/Significance

The results reinforce the concept that Aβ has relevant neuromodulatory effects, which may span from facilitation to inhibition of stimulated release depending upon the concentration used.

TRPC channels underlie cholinergic plateau potentials and persistent activity in entorhinal cortex

Persistent neuronal activity lasting seconds to minutes has been proposed to allow for the transient storage of memory traces in entorhinal cortex and thus could play a major role in working memory. Nonsynaptic plateau potentials induced by acetylcholine account for persistent firing in many cortical and subcortical structures. The expression of these intrinsic properties in cortical neurons involves the recruitment of a non-selective cation conductance. Despite its functional importance, the identity of the cation channels remains unknown. Here we show that, in layer V of rat medial entorhinal cortex, muscarinic receptor-evoked plateau potentials and persistent firing induced by carbachol require phospholipase C activation, decrease of PIP(2) levels, and permissive intracellular Ca(2+) concentrations. Plateau potentials and persistent activity were suppressed by the generic nonselective cation channel blockers FFA (100 μM) and 2-APB (100 μM), as well as by the TRPC channel blocker SKF-96365 (50 μM). However, plateau potentials were not affected by the TRPV channel blocker ruthenium red (40 μM). The TRPC3/6/7 activator OAG did not induce or enhance persistent firing evoked by carbachol. Voltage clamp recordings revealed a carbachol-activated, nonselective cationic current with a heteromeric TRPC-like phenotype. Moreover, plateau potentials and persistent firing were inhibited by intracellular application of the peptide EQVTTRL that disrupts interactions between the C-terminal domain of TRPC4/5 subunits and associated PDZ proteins. Altogether, our data suggest that TRPC cation channels mediating persistent muscarinic currents significantly contribute to the firing and mnemonic properties of projection neurons in the entorhinal cortex.

Muscarinic receptor activation enables persistent firing in pyramidal neurons from superficial layers of dorsal perirhinal cortex

Persistent-firing neurons in the entorhinal cortex (EC) and the lateral nucleus of the amygdala (LA) continue to discharge long after the termination of the original, spike-initiating current. An emerging theory proposes that endogenous persistent firing helps support a transient memory system. This study demonstrated that persistent-firing neurons are also prevalent in rat perirhinal cortex (PR), which lies immediately adjacent to and is reciprocally connected with EC and LA. Several characteristics of persistent-firing neurons in PR were similar to those previously reported in LA and EC. Persistent firing in PR was enabled by the application of carbachol, a nonselective cholinergic agonist, and it was induced by injecting a suprathreshold current or by stimulating suprathreshold excitatory synaptic inputs to the neuron. Once induced, persistent firing lasted for seconds to minutes. Persistent firing could always be terminated by a sufficiently large and prolonged hyperpolarizing current; it was prevented by antagonists of muscarinic cholinergic receptors (mAChRs); and it was blocked by flufenamic acid. The latter has been suggested to inhibit a Ca(2+) -activated nonspecific cation conductance (G(CAN) ) that normally furnishes the sustained depolarization during persistent firing. In many PR neurons, the discharge rate during persistent firing was a graded function of depolarizing and/or hyperpolarizing inputs. Persistent firing was not prevented by blocking fast excitatory and inhibitory synaptic transmission, demonstrating that it can be generated endogenously. We suggest that persistent-firing neurons in PR, EC, LA, and certain other brain regions may cooperate in support of a transient-memory system.

Nicotinic mechanisms influencing synaptic plasticity in the hippocampus

Nicotinic acetylcholine receptors (nAChRs) are expressed throughout the hippocampus, and nicotinic signaling plays an important role in neuronal function. In the context of learning and memory related behaviors associated with hippocampal function, a potentially significant feature of nAChR activity is the impact it has on synaptic plasticity. Synaptic plasticity in hippocampal neurons has long been considered a contributing cellular mechanism of learning and memory. These same kinds of cellular mechanisms are a factor in the development of nicotine addiction. Nicotinic signaling has been demonstrated by in vitro studies to affect synaptic plasticity in hippocampal neurons via multiple steps, and the signaling has also been shown to evoke synaptic plasticity in vivo. This review focuses on the nAChRs subtypes that contribute to hippocampal synaptic plasticity at the cellular and circuit level. It also considers nicotinic influences over long-term changes in the hippocampus that may contribute to addiction.

Reactive plasticity in the dentate gyrus following bilateral entorhinal cortex lesions in cynomolgus monkeys

Hippocampal structural plasticity induced by entorhinal cortex (EC) lesions has been studied extensively in the rat, but little comparable research has been conducted in primates. In the current study we assessed the long-term effects of bilateral aspiration lesions of the EC on multiple markers of circuit organization in the hippocampal dentate gyrus of young adult monkeys (Macaca fascicularis). Alternate histological sections were processed for the visualization of somatostatin and vesicular acetylcholine transporter (VAChT) immunoreactivity and acetylcholinesterase histochemistry (AChE). The markers revealed the distinct laminar organization of dentate gyrus circuitry for stereology-based morphometric quantification. Consistent with findings in rats, the volume of the somatostatin-immunopositive outer molecular layer (OML), innervated by projections from the EC, was decreased by 42% relative to control values. The inner molecular layer (IML) displayed a corresponding volumetric expansion in response to denervation of the OML as measured by AChE staining, but not when visualized for quantification by VAChT immunoreactivity. Nonetheless, stereological estimation revealed a 36% increase in the total length of VAChT-positive cholinergic fibers in the IML after EC damage, along with no change in the OML. Together, these findings suggest that despite substantial species differences in the organization of hippocampal circuitry, the capacity for reactive plasticity following EC damage, previously documented in rats, is at least partly conserved in the primate dentate gyrus.

The length of hippocampal cholinergic fibers is reduced in the aging brain

Cholinergic deficits occur in the aged hippocampus and they are significant in Alzheimer's disease. Using stereological and biochemical approaches, we characterized the cholinergic septohippocampal pathway in old (24 months) and young adult (3 months) rats. The total length of choline acetyltransferase (ChAT)-positive fibers in the dorsal hippocampus was significantly decreased by 32% with aging (F((1,9))=20.94, p=0.0014), along with the levels of synaptophysin, a presynaptic marker. No significant changes were detected in ChAT activity or in the amounts of ChAT protein, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), tropomyosin related kinase receptor (Trk) A, TrkB, or p75 neurotrophin receptor (p75(NTR)) in the aged dorsal hippocampus. The number and size of ChAT-positive neurons and the levels of ChAT activity, NGF and BDNF were not statistically different in the septum of aged and young adult rats. This study suggests that substantial synaptic loss and cholinergic axonal degeneration occurs during aging and reinforces the importance of therapies that can protect axons and promote their growth in order to restore cholinergic neurotransmission.

Distribution of the dopamine innervation in the macaque and human thalamus

We recently defined the thalamic dopaminergic system in primates; it arises from numerous dopaminergic cell groups and selectively targets numerous thalamic nuclei. Given the central position of the thalamus in subcortical and cortical interplay, and the functional relevance of dopamine neuromodulation in the brain, detailing dopamine distribution in the thalamus should supply important information. To this end we performed immunohistochemistry for dopamine and the dopamine transporter in the thalamus of macaque monkeys and humans to generate maps, in the stereotaxic coronal plane, of the distribution of dopaminergic axons. The dopamine innervation of the thalamus follows the same pattern in both species and is most dense in midline limbic nuclei, the mediodorsal and lateral posterior association nuclei, and in the ventral lateral and ventral anterior motor nuclei. This distribution suggests that thalamic dopamine has a prominent role in emotion, attention, cognition and complex somatosensory and visual processing, as well as in motor control. Most thalamic dopaminergic axons are thin and varicose and target both the neuropil and small blood vessels, suggesting that, besides neuronal modulation, thalamic dopamine may have a direct influence on microcirculation. The maps provided here should be a useful reference in future experimental and neuroimaging studies aiming at clarifying the role of the thalamic dopaminergic system in health and in conditions involving brain dopamine, including Parkinson's disease, drug addiction and schizophrenia.

Chemical organization of the macaque monkey olfactory bulb: III. Distribution of cholinergic markers

The distribution patterns of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) were studied in the olfactory bulb (OB) of three species of macaque. AChE was detected by a histochemical method and ChAT immunoreactivity by immunocytochemistry. Similar results were observed in all species analyzed. With the exception of the olfactory nerve layer, all layers of the macaque monkey OB demonstrated a dense innervation of AChE- and ChAT-positive fibers. The distribution patterns of AChE- and ChAT-labeled fibers were similar for both cholinergic markers, although the number of AChE-labeled fibers was clearly higher than the number of ChAT-immunoreactive fibers. The highest density of AChE and ChAT-stained fibers was observed in the interface between the glomerular layer and the external plexiform layer and in the internal plexiform layer. Dense bundles of labeled fibers were observed in the caudal OB, coursing from the olfactory peduncle. All ChAT-immunopositive elements were identified as centrifugal fibers, derived from neurons caudal to the OB. Neither olfactory fibers nor intrinsic neurons were observed after ChAT immunocytochemistry. However, a few AChE-positive cells were observed in the glomerular layer and in both external and internal plexiform layers. These neurons were presumably identified as periglomerular cells, superficial short-axon cells, and/or external tufted cells and deep short-axon cells. Contrary to other neurotransmitters and neuroactive substances, the distribution patterns of ChAT and AChE activities in the macaque monkey OB closely resembled the patterns described in macrosmatic mammals and showed laminar differences with the distribution pattern observed in humans.

Vascular determinants of cholinergic deficits in Alzheimer disease and vascular dementia

Alzheimer's disease (AD) and vascular dementia (VaD) are widely accepted as the most common forms of dementia. Cerebrovascular lesions frequently coexist with AD, creating an overlap in the clinical and pathological features of VaD and AD. This review assembles evidence for a role for cholinergic mechanisms in the pathogenesis of VaD, as has been established for AD. We first consider the anatomy and vascularization of the basal forebrain cholinergic neuronal system, emphasizing its susceptibility to the effects of arterial hypertension, sustained hypoperfusion, and ischemic cerebrovascular disease. The impact of aging and consequences of disruption of the cholinergic system in cognition and in control of cerebral blood flow are further discussed. We also summarize preclinical and clinical evidence supporting cholinergic deficits and the use of cholinesterase inhibitors in patients with VaD. We postulate that vascular pathology likely plays a common role in initiating cholinergic neuronal abnormalities in VaD and AD.

Cytoarchitectonic organization of the entorhinal cortex of the canine brain

The present study describes the cytoarchitectonic and chemoarchitectonic organization of the canine entorhinal cortex (EC). We distinguished medial, laterodorsal, and latero-intermediate subdivisions based on the organization of cortical layers using Nissl and Timm staining and AChE histochemistry. The medial subdivision is located at the border of the parasubiculum and is characterized by a narrow cortex, wide layer II, and densely packed cells in layer V. At its caudal extent, distinct spherical groups of small cells are situated at the border of layer I/II. The laterodorsal subdivision is located along the rhinal sulcus and borders area 35 of the perirhinal cortex. Its cortex is wide and layers tend to merge. Layer II of the laterodorsal subdivision contains scattered "stellate" cells, which are not organized into islands. The latero-intermediate subdivision displays a complex layer organization. The most easily distinguished is layer II, which is comprised of two main cell populations; "stellate" neurons arranged into "islands" and small, round cells distributed within and below the stellate cells. Layer III contains sparse cells that are arranged into vertical clusters, whereas layer IV (lamina dissecans) is especially wide. Nine fields, named according to their rostral to caudal position, were distinguished based on further analyses of layer differentiation. The main features of the rostrocaudal differentiation are a gradual disappearance of "island" organization in layer II, increasing cortical thickness, and wider layers containing small and more densely packed cells. Cytoarchitectonic differentiation was determined by observation of specific histochemical patterns of AChE- and Timm-stained sections.

The primate thalamus is a key target for brain dopamine

The thalamus relays information to the cerebral cortex from subcortical centers or other cortices; in addition, it projects to the striatum and amygdala. The thalamic relay function is subject to modulation, so the flow of information to the target regions may change depending on behavioral demands. Modulation of thalamic relay by dopamine is not currently acknowledged, perhaps because dopamine innervation is reportedly scant in the rodent thalamus. We show that dopaminergic axons profusely target the human and macaque monkey thalamus using immunolabeling with three markers of the dopaminergic phenotype (tyrosine hydroxylase, dopamine, and the dopamine transporter). The dopamine innervation is especially prominent in specific association, limbic, and motor thalamic nuclei, where the densities of dopaminergic axons are as high as or higher than in the cortical area with the densest dopamine innervation. We also identified the dopaminergic neurons projecting to the macaque thalamus using retrograde tract-tracing combined with immunohistochemistry. The origin of thalamic dopamine is multiple, and thus more complex, than in any other dopaminergic system defined to date: dopaminergic neurons of the hypothalamus, periaqueductal gray matter, ventral mesencephalon, and the lateral parabrachial nucleus project bilaterally to the monkey thalamus. We propose a novel dopaminergic system that targets the primate thalamus and is independent from the previously defined nigrostriatal, mesocortical, and mesolimbic dopaminergic systems. Investigating this "thalamic dopaminergic system" should further our understanding of higher brain functions and conditions such as Parkinson's disease, schizophrenia, and drug addiction.

Cholinergic dysfunction in vascular dementia

Vascular dementia (VaD) is the second most common type of dementia in the elderly after Alzheimer's disease (AD). Evidence is presented indicating the occurrence of cholinergic dysfunction in VaD, independent from AD. Controlled clinical trials of cholinesterase inhibitors (ChEIs) in VaD and in patients with AD plus cerebrovascular disease are reviewed. Compared with placebo, ChEI treatment improves cognition, behavior, and activities of daily living. Cholinergic deficits in patients with VaD may result from ischemia of basal forebrain cholinergic nuclei that are irrigated by penetrating arteries that are highly susceptible to arterial hypertension, or from ischemic lesions in basal ganglia or white matter that sever the extensive cholinergic cortical projections. Cholinergic stimulation produces increases in cortical cerebral blood flow that may be relevant to the therapeutic effect of ChEIs.

Spike Patterning by Ca2+-Dependent Regulation of a Muscarinic Cation Current in Entorhinal Cortex Layer II Neurons

In entorhinal cortex layer II neurons, muscarinic receptor activation promotes depolarization via activation of a nonspecific cation current ( INCM). Under muscarinic influence, these neurons also develop changes in excitability that result in activity-dependent induction of delayed firing and bursting activity. To identify the membrane processes underlying these phenomena, we examined whether INCM may undergo activity-dependent regulation. Our voltage-clamp experiments revealed that appropriate depolarizing protocols increased the basal level of inward current activated during muscarinic stimulation and suggested that this effect was due to INCM upregulation. In the presence of low buffering for intracellular Ca2+, this upregulation was transient, and its decay could be followed by a phase of INCM downregulation. Both up- and downregulation were elicited by depolarizing stimuli able to activate voltage-gated Ca2+ channels (VGCC); both were sensitive to increasing concentrations of intracellular Ca2+-chelating agents with downregulation being abolished at lower Ca2+-buffering capacities; both were reduced or suppressed by VGCC block or in the absence of extracellular Ca2+. These data indicate that relatively small increases in [Ca2+]i driven by firing activity can induce upregulation of a basal muscarinic depolarizing-current level, whereas more pronounced [Ca2+]i elevations can result in INCM downregulation. We propose that the interaction of activity-dependent positive and negative feedback mechanisms on INCM allows entorhinal cortex layer II neurons to exhibit emergent properties, such as delayed firing and enhanced or suppressed responses to repeated stimuli, that may be of importance in the memory functions of the temporal lobe and in the pathophysiology of epilepsy.

Reduction in hippocampal cholinergic innervation is unrelated to recognition memory impairment in aged rhesus monkeys

Alterations in the basal forebrain cholinergic system have been widely studied in brain aging and Alzheimer's disease, but the magnitude of decline and relationship to cognitive impairment are still a matter of debate. The rhesus monkey (Macaca mulatta) provides a compelling model to study age-related memory decline, as the pattern of impairment closely parallels that observed in humans. Here, we used antibodies against the vesicular acetylcholine transporter and a new stereological technique to estimate total cholinergic fiber length in hippocampal subregions of behaviorally characterized young and aged rhesus monkeys. The analysis revealed an age-related decline in the length of cholinergic fibers of 22%, which was similar across the hippocampal subregions studied (dentate gyrus granule cell and molecular layers, CA2/3-hilus, and CA1), and across the rostral-caudal extent of the hippocampus. This effect, however, was unrelated to performance on the delayed nonmatching-to-sample task, a test of recognition memory sensitive to hippocampal system dysfunction and cognitive aging in monkeys. These findings indicate that a decline in cholinergic input fails to account for the influence of normal aging on memory supported by the primate hippocampal region.

Cholinergic elements in the zebrafish central nervous system: Histochemical and immunohistochemical analysis

Recently, the zebrafish has been extensively used for studying the development of the central nervous system (CNS). However, the zebrafish CNS has been poorly analyzed in the adult. The cholinergic/cholinoceptive system of the zebrafish CNS was analyzed by using choline acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) histochemistry in the brain, retina, and spinal cord. AChE labeling was more abundant and more widely distributed than ChAT immunoreactivity. In the telencephalon, ChAT-immunoreactive (ChAT-ir) cells were absent, whereas AChE-positive neurons were observed in both the olfactory bulb and the telencephalic hemispheres. The diencephalon was the region with the lowest density of AChE-positive cells, mainly located in the pretectum, whereas ChAT-ir cells were exclusively located in the preoptic region. ChAT-ir cells were restricted to the periventricular stratum of the optic tectum, but AChE-positive neurons were observed throughout the whole extension of the lamination except in the marginal stratum. Although ChAT immunoreactivity was restricted to the rostral tegmental, oculomotor, and trochlear nuclei within the mesencephalic tegmentum, a widespread distribution of AChE reactivity was observed in this region. The isthmic region showed abundant AChE-positive and ChAT-ir cells in the isthmic, secondary gustatory and superior reticular nucleus and in the nucleus lateralis valvulae. ChAT immunoreactivity was absent in the cerebellum, although AChE staining was observed in Purkinje and granule cells. The medulla oblongata showed a widespread distribution of AChE-positive cells in all main subdivisions, including the octavolateral area, reticular formation, and motor nuclei of the cranial nerves. ChAT-ir elements in this area were restricted to the descending octaval nucleus, the octaval efferent nucleus and the motor nuclei of the cranial nerves. Additionally, spinal cord motoneurons appeared positive to both markers. Substantial differences in the ChAT and AChE distribution between zebrafish and other fish species were observed, which could be important because zebrafish is widely used as a genetic or developmental animal model.

Ca2+-independent muscarinic excitation of rat medial entorhinal cortex layer V neurons

Cholinergic activation of entorhinal cortex (EC) layer V neurons plays a crucial role in the medial temporal lobe memory system and in the pathophysiology of temporal lobe epilepsy. Here, we demonstrate that muscarinic activation by focal application of carbachol depolarizes EC layer V neurons and induces epileptiform activity in rat brain slices. These seizure-like bursts are associated with a somatic [Ca2+]i increase of 293 +/- 82 nm and are blocked by the glutamate receptor antagonists CNQX and APV. Muscarinic activation did not directly evoke a [Ca2+]i increase, but subthreshold and suprathreshold depolarization did. Functional axon mapping revealed local axon branching as well as axon collaterals ascending to layers II and III. During blockade of ionotropic glutamatergic AMPA and NMDA receptors, carbachol depolarized layer V neurons by +7.5 +/- 3.4 mV. This direct muscarinic depolarization was associated with a conductance increase of 35 +/- 10.3% (+4.3 +/- 1.25 nS). Intracellular buffering of [Ca2+]i changes did not block this depolarization, but prolonged action potential duration and reduced adaptation of action potential firing. The muscarinic depolarization was neither blocked by combining intracellular Ca2+-buffering (EGTA or BAPTA) with non-specific Ca2+-channel inhibition by Ni+ (1 mm), nor by Ba2+ (1 mm) nor during inhibition of the h-current by 2 mm Cs+. In whole-cell patch-clamp recording, reversal of the muscarinic current occurred at about -45 mV and -5 mV with complete substitution of intrapipette K+ with Cs+. Thus, muscarinic depolarization of EC layer V neurons appears to be primarily mediated by Ca2+-independent activation of non-specific cation channels that conduct K+ about three times as well as Na+.

A cytoarchitectonic study of the hippocampal formation of the tree shrew (Tupaia belangeri)

Tree shrews constitute an interesting animal model to study the impact of stress or aging on the hippocampal formation, a brain structure known to be affected under such environmental or internal influences. To perform detailed investigations of the hippocampal formation, adequate knowledge of its anatomy should be present. Until now, the hippocampal formation of the tree shrew has not yet been studied extensively. The main objective of this study, therefore, was to describe the subfield boundaries in various levels of the dorsoventral hippocampal axis of the tree shrew (Tupaia belangeri) in detail. The secondary aim was to clarify whether a separate CA2 field can actually be distinguished in the tree shrew hippocampus, a fact that was denied in former reports. In addition, we aimed at investigating whether or not a CA4 subfield can be identified in the tree shrew's hippocampus. The immunocytochemical distribution of microtubule-associated protein 2 and the calcium-binding proteins, parvalbumin and calbindin, and the characteristics of Nissl staining in adjacent sections were compared. Because of the rather dorsoventral orientation of the long hippocampal axis in tree shrews, staining patterns were analyzed mainly in horizontal sections. The subiculum and the hippocampal CA1 and CA3 areas were easily identified. Moreover, we were able to demonstrate the existence of a distinct CA2 subfield in the tree shrew's Ammon's horn, contrary to previous reports. However, our results indicate that a CA4 field in the tree shrew hippocampal formation cannot be identified with the methods that we used. Therefore, supposed CA4 pyramidal neurons should be included into the CA3 field.

Modulation of presynaptic store calcium induces release of glutamate and postsynaptic firing

Action potential-independent transmitter release is random and produces small depolarizations in the postsynaptic neuron. This process is, therefore, not thought to play a significant role in impulse propagation across synapses. Here we show that calcium flux through presynaptic neuronal nicotinic receptors leads to mobilization of store calcium by calcium-induced calcium release. Recruitment of store calcium induces vesicular release of glutamate in a manner consistent with synchronization across multiple active zones in the CA3 region of the rat hippocampus. This modulation of action potential-independent release of glutamate is sufficient to drive the postsynaptic pyramidal cell above its firing threshold, thus providing a mechanism for impulse propagation.

Localization of [3H]nicotine, [3H]cytisine, [3H]epibatidine, and [125I]alpha-bungarotoxin binding sites in the brain of Macaca mulatta

We determined the localization of [(3)H]nicotine, [(3)H]cytisine, [(3)H]epibatidine, and [(125)I]alpha-bungarotoxin binding sites in the brain of rhesus monkey by means of receptor autoradiography. The labelings by [(3)H]nicotine, [(3)H]cytisine, and [(3)H]epibatidine were highly concordant, except for epibatidine. Layer IV of some cortical areas, most thalamic nuclei, and presubiculum displayed high levels of labeling for the three ligands. Moderate levels of binding were detected in the subiculum, the septum, and the mesencephalon. Low levels were present in layers I-II and VI of the cortex, the cornu Ammonis, the dentate gyrus, and the amygdala. In addition, the level of epibatidine labeling was very high in the epithalamic nuclei and the interpeduncular nucleus, whereas labeling by nicotine and cytisine was very weak in the same regions. The distribution of [(125)I]alpha-bungarotoxin binding differed from the binding of the three agonists. The labeling was dense in layer I of most cortical areas, dentate gyrus, stratum lacunosum-moleculare of CA1 field, several thalamic nuclei, and medial habenula. A moderate labeling was found in layers V and VI of the prefrontal and frontal cortices, layer IV of primary visual cortex, amygdala, septum, hypothalamus, and some mesencenphalic nuclei. A weak signal was also detected in subiculum, claustrum, stratum oriens, and stratum lucidum of cornu Ammonis and also in some mesencephalic nuclei. The distribution of nicotine, cytisine, and epibatidine bindings corresponds broadly to the patterns observed in rodents, with the marked exception of the epithalamus. However, in monkey, those distributions match the distribution of alpha2 messenger RNA, rather than that of alpha4 transcripts as it exists in rodent brains. The distribution of the binding sites for alpha-bungarotoxin is larger in the brain of rhesus monkeys than in rodent brain, suggesting a more important role of alpha7 receptors in primates.

Graded persistent activity in entorhinal cortex neurons

Working memory represents the ability of the brain to hold externally or internally driven information for relatively short periods of time. Persistent neuronal activity is the elementary process underlying working memory but its cellular basis remains unknown. The most widely accepted hypothesis is that persistent activity is based on synaptic reverberations in recurrent circuits. The entorhinal cortex in the parahippocampal region is crucially involved in the acquisition, consolidation and retrieval of long-term memory traces for which working memory operations are essential. Here we show that individual neurons from layer V of the entorhinal cortex-which link the hippocampus to extensive cortical regions-respond to consecutive stimuli with graded changes in firing frequency that remain stable after each stimulus presentation. In addition, the sustained levels of firing frequency can be either increased or decreased in an input-specific manner. This firing behaviour displays robustness to distractors; it is linked to cholinergic muscarinic receptor activation, and relies on activity-dependent changes of a Ca2+-sensitive cationic current. Such an intrinsic neuronal ability to generate graded persistent activity constitutes an elementary mechanism for working memory.

Muscarinic activation of a cation current and associated current noise in entorhinal-cortex layer-II neurons

The effects of muscarinic stimulation on the membrane potential and current of in situ rat entorhinal-cortex layer-II principal neurons were analyzed using the whole cell, patch-clamp technique. In current-clamp experiments, application of carbachol (CCh) induced a slowly developing, prolonged depolarization initially accompanied by a slight decrease or no significant change in input resistance. By contrast, in a later phase of the depolarization input resistance appeared consistently increased. To elucidate the ionic bases of these effects, voltage-clamp experiments were then carried out. In recordings performed in nearly physiological ionic conditions at the holding potential of -60 mV, CCh application promoted the slow development of an inward current deflection consistently associated with a prominent increase in current noise. Similarly to voltage responses to CCh, this inward-current induction was abolished by the muscarinic antagonist, atropine. Current-voltage relationships derived by applying ramp voltage protocols during the different phases of the CCh-induced inward-current deflection revealed the early induction of an inward current that manifested a linear current/voltage relationship in the subthreshold range and the longer-lasting block of an outward K(+) current. The latter current could be blocked by 1 mM extracellular Ba(2+), which allowed us to study the CCh-induced inward current (I(CCh)) in isolation. The extrapolated reversal potential of the isolated I(CCh) was approximately 0 mV and was not modified by complete substitution of intrapipette K(+) with Cs(+). Moreover, the extrapolated I(CCh) reversal shifted to approximately -20 mV on removal of 50% extracellular Na(+). These results are consistent with I(CCh) being a nonspecific cation current. Finally, noise analysis of I(CCh) returned an estimated conductance of the underlying channels of approximately 13.5 pS. We conclude that the depolarizing effect of muscarinic stimuli on entorhinal-cortex layer-II principal neurons depends on both the block of a K(+) conductance and the activation of a "noisy" nonspecific cation current. We suggest that the membrane current fluctuations brought about by I(CCh) channel noise may facilitate the "theta" oscillatory dynamics of these neurons and enhance firing reliability and synchronization.

Cholinergic systems and schizophrenia: primary pathology or epiphenomena?

Post mortem schizophrenia research has been driven first by the dopamine and then the glutamate hypotheses. These hypotheses posit primary pathology in pathways dependent upon dopamine or glutamate neurotransmission. Although the dopamine and glutamate hypotheses retain considerable theoretical strength, neurobiological findings of altered dopamine or glutamate activity in schizophrenia do not explain all features of this disorder. A more synthetic approach would suggest that focal pathological change in either the prefrontal cortex or mesial temporal lobe leads to neurochemical changes in multiple neurotransmitter systems. Despite the limited experimental evidence for abnormal cholinergic neurotransmission in psychiatric disorders, increased understanding of the role of acetylcholine in the human brain and its relationship to other neurotransmitter systems has led to a rapidly growing interest in the cholinergic system in schizophrenia. This review focuses on the basic anatomy of the mammalian cholinergic system, and its possible involvement in the neurobiology of schizophrenia. Summaries of cholinergic cell groups, projection pathways, and receptor systems, in the primate and human brain, are followed by a brief discussion of the functional correlations between aberrant cholinergic neurotransmission and the signs and symptoms of schizophrenia.

Distribution of choline acetyltransferase-immunoreactive structures in the lamprey brain

The distribution of cholinergic neurons and fibers was studied immunohistochemically in the brain of two species of lampreys (Petromyzon marinus and Lampetra fluviatilis), by using an antiserum against choline acetyltransferase (ChAT). The results obtained in both species were similar, but there appeared some interspecies differences. In the forebrain, cholinergic cells were present in the striatum, preoptic region, paraventricular nucleus, pineal and parapineal organs, habenula, and pretectum. The cranial nerve motoneurons (III, IV, V, VI, VII, IX, and X), the first and second spino-occipital nerves (so), and the ventral horn of the spinal cord showed a strong ChAT immunoreactivity. Additional cholinergic neurons were observed: the mesencephalic M5 nucleus of Schober, two different cell populations in the isthmic region, the efferent component of the eighth nerve, putative preganglionic parasympathetic cells, cells in the solitary tract nucleus, and the rhombencephalic reticular formation. Cholinergic fibers were widely distributed in the brain. Comparison with previous studies in other vertebrates suggests that major cholinergic pathways, like tectal innervation from the isthmic region, are also present in lampreys. Of particular interest was the prominent projection to the neurohypophysis from cholinergic neurons in the preoptic region and paraventricular nucleus. Present data were analyzed within the segmental paradigm, as was previously done in other vertebrates. Our results reveal that the organization of many cholinergic systems in the lamprey as, for example, in the striatal, preoptic, and isthmic regions, comprises features of the anamniote brain that remain common to all living amniotes studied so far, thus being conservative to a surprisingly high degree. Therefore, the distribution of ChAT-immunoreactive structures in the lamprey brain is, in general, comparable to that previously described in other vertebrate species.

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

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

Nicotinic receptors on hippocampal cultures can increase synaptic glutamate currents while decreasing the NMDA-receptor component

Activation of presynaptic nicotinic acetylcholine receptors (nAChRs) can enhance the release of glutamate from synapses in hippocampal slices and cultures. In hippocampal cultures making autaptic connections, rapid application of a high concentration of nicotine activated presynaptic, postsynaptic, and somatic nAChRs, which consequently enhanced the amplitude of evoked excitatory postsynaptic currents (eEPSCs) mediated by glutamate receptors. The increased eEPSC amplitudes arose from enhanced glutamate release caused by presynaptic nAChRs (Radcliffe and Dani, 1998, Journal of Neuroscience 18, 7075). The same whole-cell nicotine applications that enhanced non-NMDA eEPSCs often decreased the NMDA-receptor component of the eEPSCs. Furthermore, whole-cell activation of nAChRs by nicotine selectively reduced the amplitude of the whole-cell NMDA-receptor currents without affecting the non-NMDA receptor currents. The inhibition by nicotine was prevented by the alpha7-specific antagonist, methyllycaconitine, and required the presence of extracellular Ca(2+). The calmodulin antagonist fluphenazine prevented inhibition of the NMDA-receptor current by nAChR activity, suggesting that a Ca(2+)-calmodulin-dependent process mediated the effect of nicotine. Our results indicate that activation of nAChRs can modulate glutamatergic synapses in several ways. Presynaptic nAChR activity enhances synaptic transmission by increasing transmitter release. Additionally, somatic or postsynaptic nAChRs can initiate a Ca(2+) signal that can act via calmodulin to reduce the responsiveness of NMDA receptors.

Cholinergic modulation of synaptic transmission and plasticity in entorhinal cortex and hippocampus of the rat

Effects of cholinergic agents on synaptic transmission and plasticity were examined in entorhinal cortex and hippocampus. Bath application of carbachol (0.25-0.75 microM) induced transient depression of field potential responses in all cases tested (24/24 in layer III of medial entorhinal cortex slices and 24/24 in CA1 of hippocampal slices; 11.0+/-1.9% and 7.8+/-2.5%, respectively) and long-lasting potentiation in some cases (4/24 in entorhinal cortex and 12/24 in hippocampus; 33.7+/-3.7% and 32.1+/-9.9%, respectively, in successful cases). Carbachol (0.5 microM) induced transient depression, but not long-lasting potentiation, of N-methyl-D-aspartate receptor-mediated responses in entorhinal cortex. At 5 microM, carbachol induced transient depression only (55. 9+/-4.7% in entorhinal cortex and 41.4+/-2.9% in hippocampus), which was blocked by atropine. Paired-pulse facilitation was not altered during carbachol-induced potentiation but enhanced during carbachol-induced depression. These results suggest that the underlying mechanisms of carbachol-induced depression and potentiation are decreased transmitter release and selective enhancement of non-N-methyl-D-aspartate receptor-mediated responses, respectively. Long-term potentiation could be induced in the presence of 10 microM atropine by theta burst stimulation. The magnitude was significantly lower (15.2+/-5.2%, n=9) compared with control (37.2+/-6.1%, n=8) in entorhinal cortex, however. These results demonstrate similar, but not identical, cholinergic modulation of synaptic transmission and plasticity in entorhinal cortex and hippocampus.

Inhibition and disinhibition of pyramidal neurons by activation of nicotinic receptors on hippocampal interneurons

Nicotinic acetylcholine receptors (nAChRs) are expressed in the hippocampus, and their functional roles are beginning to be delineated. The effect of nAChR activation on the activity of both interneurons and pyramidal neurons in the CA1 region was studied in rat hippocampal slices. In CA1 stratum radiatum with muscarinic receptors inhibited, local pressure application of acetylcholine (ACh) elicited a nicotinic current in 82% of the neurons. The majority of the ACh-induced currents were sensitive to methyllycaconitine, which is a specific inhibitor of alpha7-containing nAChRs. Methyllycaconitine-insensitive nicotinic currents also were present as detected by a nonspecific nAChR inhibitor. The ACh-sensitive neurons in the s. radiatum were identified as GABAergic interneurons by their electrophysiological properties. Pressure application of ACh induced firing of action potentials in approximately 70% of the interneurons. The ACh-induced excitation of interneurons could induce either inhibition or disinhibition of pyramidal neurons. The inhibition was recorded from the pyramidal neuron as a burst of GABAergic synaptic activity. That synaptic activity was sensitive to bicuculline, indicating that GABA(A) receptors mediated the ACh-induced synaptic currents. The disinhibition was recorded from the pyramidal neuron as a reduction of spontaneous GABAergic synaptic activity when ACh was delivered onto an interneuron. Both the inhibition and disinhibition were sensitive to either methyllycaconitine or mecamylamine, indicating that activation of nicotinic receptors on interneurons was necessary for the effects. These results show that nAChRs are capable of regulating hippocampal circuits by exciting interneurons and, subsequently, inhibiting or disinhibiting pyramidal neurons.

Distributions of nicotinic acetylcholine receptor alpha7 and beta2 subunits on cultured hippocampal neurons

The hippocampus receives cholinergic afferents and expresses neuronal nicotinic acetylcholine receptors. In particular, the alpha7 and beta2 nicotinic subunits are highly expressed in the hippocampus. There has been controversy about the location, distribution and roles of neuronal nicotinic acetylcholine receptors [Role L. W. and Berg D. K. (1996) Neuron 16, 1077-1085; Wonnacott S. (1997) Trends Neurosci. 20, 92-98]. Using immunocytochemistry and patch-clamp techniques, we examined the density and distribution of nicotinic receptors on rat hippocampal neurons in primary tissue culture. The density and distribution of alpha7 subunits change with days in culture. Before 10 days in culture, alpha7 expression, monitored immunocytochemically, is low and nicotinic currents are small or absent. In older cultures, about two-thirds of the neurons express nicotinic currents, and alpha7 appears in small patches on the soma and out along the neuronal processes. These patches of alpha7 subunits on the surface of the neuronal processes often co-localize with the presynaptic marker, synaptotagmin. The other most common nicotinic subunit, beta2, stays confined mainly to the soma and proximal processes, and beta2 is distributed more uniformly and is not specifically localized at presynaptic areas. The two subunits, alpha7 and beta2, have different expression patterns on the surface of the cultured hippocampal neurons. Taken together with previous physiological studies, the results indicate that alpha7 subunits can be found at presynaptic terminals, and at these locations, these calcium-permeable channels may influence transmitter release.

Nicotinic stimulation produces multiple forms of increased glutamatergic synaptic transmission

Synaptic modulation and long-term synaptic changes are thought to be the cellular correlates for learning and memory (Madison et al., 1991; Aiba et al., 1994, Goda and Stevens, 1996). The hippocampus is a center for learning and memory that receives abundant cholinergic innervation and has a high density of nicotinic acetylcholine receptors (nAChRs) (Wada et al., 1989; Woolf, 1991). We report that stro ng, brief stimulation of nAChRs enhanced hippocampal glutamatergic synaptic transmission on two independent time scales and altered the relationship between consecutively evoked synaptic currents. The nicotinic synaptic enhancement required extracellular calcium and was produced by the activation of presynaptic alpha7-containing nAChRs. Although one form of glutamatergic enhancement lasted only for seconds, another form lasted for minutes after the nicotinic stimulation had ceased and the nicotinic agonist had been washed away. The synaptic enhancement lasting minutes suggests that nAChR activity can initiate calcium-dependent mechanisms that are known to induce glutamatergic synaptic plasticity. The results with evoked synaptic currents showed that nAChR activity can alter the relationship between the incoming presynaptic activity and outgoing postsynaptic signaling along glutamatergic fibers. Thus, the same information arriving along the same glutamatergic afferents will be processed differently when properly timed nicotinic activity converges onto the glutamatergic presynaptic terminals. Influencing information processing at glutamatergic synapses may be one way in which nicotinic cholinergic activity influences cognitive processes. Disruption of these nicotinic cholinergic mechanisms may contribute to the deficits associated with the degeneration of cholinergic functions during Alzheimer's disease.

Mice deficient in the alpha7 neuronal nicotinic acetylcholine receptor lack alpha-bungarotoxin binding sites and hippocampal fast nicotinic currents

The alpha7 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) is abundantly expressed in hippocampus and is implicated in modulating neurotransmitter release and in binding alpha-bungarotoxin (alpha-BGT). A null mutation for the alpha7 subunit was prepared by deleting the last three exons of the gene. Mice homozygous for the null mutation lack detectable mRNA, but the mice are viable and anatomically normal. Neuropathological examination of the brain revealed normal structure and cell layering, including normal cortical barrel fields; histochemical assessment of the hippocampus was also normal. Autoradiography with [3H]nicotine revealed no detectable abnormalities of high-affinity nicotine binding sites, but there was an absence of high-affinity [125I]alpha-BGT sites. Null mice also lack rapidly desensitizing, methyllycaconitine-sensitive, nicotinic currents that are present in hippocampal neurons. The results of this study indicate that the alpha-BGT binding sites are equivalent to the alpha7-containing nAChRs that mediate fast, desensitizing nicotinic currents in the hippocampus. These mice demonstrate that the alpha7 subunit is not essential for normal development or for apparently normal neurological function, but the mice may prove to have subtle phenotypic abnormalities and will be valuable in defining the functional role of this gene product in vivo.

Neurotransmitter modulation of gap junctional communication in the rat hippocampus

Increasing experimental evidence indicates that gap junctions can be modulated by neurotransmitters, in particular dopamine. To examine possible modulation of gap junctional communication in the rat hippocampus by neurotransmitters, we studied dye coupling and electrotonic transmission in the CA1 area in the presence of carbachol, a cholinergic agonist, and dopamine agonists. Carbachol markedly reduced dye coupling and the frequency of electrotonic potentials (spikelets). Spikelet amplitudes were decreased in the presence of carbachol. These effects were reversed by the cholinergic antagonist atropine, suggesting a muscarinic action of carbachol on gap junctional function. The non-specific dopamine agonist apomorphine, and the specific D1 receptor agonist SKF 38393, reduced dye coupling between pyramidal cells. Spikelet frequency was also decreased in the presence of dopamine agonists, but less than with carbachol. The specific D1 receptor antagonist, SCH 23390, reversed the effects of both dopamine agonists. These observations indicate that cholinergic and dopaminergic transmission can affect electrical and chemical (dye coupling) communication through gap junctions, and could therefore alter properties of neuronal assemblies, in addition to their effects on intrinsic membrane properties.

Intracerebral grafts promote recovery of the cholinergic innervation of the hippocampal formation in rats withdrawn from chronic alcohol intake. An immunocytochemical study

We have previously found that alcohol withdrawal aggravates the neuronal cell loss induced by chronic alcohol consumption in the rat hippocampal formation. We have also shown that intracerebral grafts of immature hippocampal tissue could reverse the progressive degeneration that occurs during this withdrawal. Furthermore, we have shown that chronic alcohol consumption reduces the areal density of choline acetyltransferase-immunoreactive neurons and the density of choline acetyltransferase-immunoreactive fibres in the hippocampal formation. Thus, we thought it would be of interest to investigate the effects of alcohol withdrawal in the hippocampal cholinergic innervation and to determine whether the intracerebral grafting of immature hippocampal tissue would have beneficial effects upon the cholinergic system in this condition. Choline acetyltransferase-immunoreactive fibres and perikarya were analysed in 14-month-old control, alcohol-fed, withdrawal and withdrawal-grafted groups of rats. The areal density of choline acetyltransferase-immunoreactive neurons was reduced in all experimental groups when compared to controls. The density of choline acetyltransferase-immunoreactive fibres was lower in the alcohol-fed and withdrawal groups than in the control and withdrawal-grafted groups. We conclude that the grafted tissue probably produced neurotrophic factors which allowed a recovery of the hippocampal cholinergic fibre network. This recovery might be of importance to reverse the cognitive dysfunction described after chronic alcohol consumption and withdrawal.

Distribution of choline acetyltransferase immunoreactivity in the brain of anuran (Rana perezi, Xenopus laevis) and urodele (Pleurodeles waltl) amphibians

Because our knowledge of cholinergic systems in the brains of amphibians is limited, the present study aimed to provide detailed information on the distribution of cholinergic cell bodies and fibers as revealed by immunohistochemistry with antibodies directed against the enzyme choline acetyltransferase (ChAT). To determine general and derived features of the cholinergic systems within the class of Amphibia, both anuran (Rana perezi, Xenopus laevis) and urodele (Pleurodeles waltl) amphibians were studied. Distinct groups of ChAT-immunoreactive cell bodies were observed in the basal telencephalon, hypothalamus, habenula, isthmic nucleus, isthmic reticular formation, cranial nerve motor nuclei, and spinal cord. Prominent plexuses of cholinergic fibers were found in the olfactory bulb, pallium, basal telencephalon, ventral thalamus, tectum, and nucleus interpeduncularis. Comparison of these results with those obtained in other vertebrates, including a segmental approach to correlate cell populations, reveals that the cholinergic systems in amphibians share many features with amniotes. Thus, cholinergic pedunculopontine and laterodorsal tegmental nuclei could be identified in the amphibian brain. The finding of weakly immunoreactive cells in the striatum of Rana, which is in contrast with the condition found in Xenopus, Pleurodeles, and other anamniotes studied so far, has revived the notion that basal ganglia organization is more preserved during evolution than previously thought.

H. M.'s medial temporal lobe lesion: findings from magnetic resonance imaging

Although neuropsychological studies of the amnesic patient H. M. provide compelling evidence that normal memory function depends on the medial temporal lobe, the full extent of his surgical resection has not been elucidated. We conducted magnetic resonance imaging studies to specify precisely the extent of his bilateral resection and to document any other brain abnormalities. The MRI studies indicated that the lesion was bilaterally symmetrical and included the medial temporal polar cortex, most of the amygdaloid complex, most or all of the entorhinal cortex, and approximately half of the rostrocaudal extent of the intraventricular portion of the hippocampal formation (dentate gyrus, hippocampus, and subicular complex). The collateral sulcus was visible throughout much of the temporal lobe, indicating that portions of the ventral perirhinal cortex, located on the banks of the sulcus, were spared; the parahippocampal cortex (areas TF and TH) was largely intact. The rostrocaudal extent of the ablation was approximately 5.4 cm (left) and 5.1 cm (right). The caudal 2 cm, approximately, of the hippocampus body (normal length, approximately 4 cm) was intact, although atrophic. The temporal stem was intact. Outside the temporal lobes, the cerebellum demonstrated marked atrophy, and the mammillary nuclei were shrunken. The lateral temporal, frontal, parietal, and occipital lobe cortices appeared normal for age 66 years. The mediodorsal thalamic nuclei showed no obvious radiological changes. These findings reinforce the view that lesions of the hippocampal formation and adjacent cortical structures can produce global and enduring amnesia and can exacerbate amnesia beyond that seen after more selective hippocampal lesions.

Muscarinic modulation of the oscillatory and repetitive firing properties of entorhinal cortex layer II neurons

Neurons in layer II of the entorhinal cortex (EC) are key elements in the temporal lobe memory system because they integrate and transfer into the hippocampal formation convergent sensory input from the entire cortical mantle. EC layer II also receives a profuse cholinergic innervation from the basal forebrain that promotes oscillatory dynamics in the EC network and may also implement memory function. To understand the cellular basis of cholinergic actions in EC, we investigated by intracellular recording in an in vitro rat brain slice preparation the muscarinic modulation of the electroresponsive properties of the two distinct classes of medial EC layer II projection neurons, the stellate cells (SCs) and non-SCs. In both SCs and non-SCs, muscarinic receptor activation with carbachol (CCh, 10-50 microM) caused atropine-sensitive (300 nM) membrane depolarization. In SCs, the CCh-induced membrane depolarization was associated with subthreshold membrane potential oscillations and "spike cluster" discharge, which are typically expressed by these cells on depolarization. CCh, however, caused a decrease of the dominant frequency of the membrane potential oscillations from 9.2 +/- 1.1 (SD) Hz to 6.3 +/- 1.1 Hz, as well as a decrease of the intracluster firing frequency from 18.1 +/- 1.7 Hz to 13.6 +/- 1.3 Hz. In addition, spike cluster discharge was less robust, and the cells tended to shift into tonic firing during CCh. In contrast to SCs, in non-SCs, CCh drastically affected firing behavior by promoting the development of voltage-dependent, long-duration (1-5 s) slow bursts of action potentials that could repeat rhythmically at slow frequencies (0.2-0.5 Hz). Concomitantly, the slow afterhyperpolarization (sAHP) was replaced by long-lasting plateau postdepolarizations. In both SCs and non-SCs, CCh also produced conspicuous changes on the action potential waveform and its afterpotentials. Notably, CCh significantly decreased spike amplitude and rate of rise, which suggests muscarinic modulation of a voltage-dependent Na+ conductance. Finally, we also observed that whereas CCh abolished the sAHP in both SCs and non-SCs, the membrane-permeant analogues of adenosine 3',5'-cyclic monophosphate, 8-(4-chlorophenylthio)-adenosine-cyclic monophosphate and 8-bromo-adenosine-cyclic-monophosphate, abolished the sAHP in SCs but not in non-SCs. The data demonstrate that cholinergic modulation further differentiates the intrinsic electroresponsiveness of SCs and non-SCs, and add support to the presence of two parallel processing systems in medial EC layer II that could thereby differentially influence their hippocampal targets. The results also indicate an important role for the cholinergic system in tuning the oscillatory dynamics of entorhinal neurons.