Cholinergic Projection Neurons Research Articles

Postnatal lead exposure alters expression of forebrain p75 and TrkA nerve growth factor receptors

Lead is a potent developmental neurotoxicant that affects many aspects of cognition and behavior. The hippocampus and striatum are among the areas particularly sensitive to the effects of lead and cholinergic neurons in both regions depend upon nerve growth factor (NGF) for their survival and maturation. The present study examined the extent to which postnatal lead exposure may affect the survival and expression of neuroptrophin receptors of septo-hippocampal cholinergic projection neurons in the medial septum/vertical limb of the diagonal band of Broca (MS/VDB) and cholinergic neurons of the striatum. Weanling rats were fed chow containing lead acetate for 30 days and effects on cholinergic cell number and the number of cells expressing neurotrophin receptors p75 NGFR and trkA were assessed. A decrease in the number of cells expressing p75 NGFR and an increase in the number of cells expressing trkA receptor was observed in the MS/VDB of lead-exposed rats, without a loss of cholinergic cell number or alteration in cell size. Lead-exposure resulted in a significant decrease in trkA-expressing cells in the striatum but no change in the number or size of cholinergic neurons. These results suggest that a brief postnatal lead exposure does not result in loss of MS/VDB or striatal cholinergic neurons but does modify the expression of neurotrophin receptors in these regions. The significance of these effects on the septo-hippocampal and striatal functioning remains to be studied.

α7 Nicotinic Receptor Up-regulation in Cholinergic Basal Forebrain Neurons in Alzheimer Disease

Dysfunction of basocortical cholinergic projection neurons of the nucleus basalis (NB) correlates with cognitive deficits in Alzheimer disease (AD). Nucleus basalis neurons receive cholinergic inputs and express nicotinic acetylcholine receptors (nAChRs) and muscarinic AChRs (mAChRs), which may regulate NB neuron activity in AD. Although alterations in these AChRs occur in the AD cortex, there is little information detailing whether defects in nAChR and mAChR gene expression occur in cholinergic NB neurons during disease progression. To determine whether nAChR and mAChR gene expression is altered in cholinergic NB neurons during the progression of AD. Individual NB neurons from subjects diagnosed ante mortem as having no cognitive impairment (NCI), mild cognitive impairment (MCI), or mild to moderate AD were analyzed by single-cell AChR expression profiling via custom-designed microarrays. Academic research. Participants were members of the Rush Religious Orders Study cohort. Real-time quantitative polymerase chain reaction was performed to validate microarray findings. Cholinergic NB neurons displayed a statistically significant up-regulation of alpha7 nAChR messenger RNA expression in subjects with mild to moderate AD compared with those with NCI and MCI (P<.001). No differences were found for other nAChR and mAChR subtypes across the cohort. Expression levels of alpha7 nAChRs were inversely associated with Global Cognitive Score and with Mini-Mental State Examination performance. Up-regulation of alpha7 nAChRs may signal a compensatory response to maintain basocortical cholinergic activity during AD progression. Alternatively, putative competitive interactions of this receptor with beta-amyloid may provide a pathogenic mechanism for NB dysfunction. Increasing NB alpha7 nAChR expression may serve as a marker for the progression of AD.

Cholinergic Interneurons Control the Excitatory Input to the Striatum

How the extent and time course of presynaptic inhibition depend on the action potentials of the neuron controlling the terminals is unknown. We investigated this issue in the striatum using paired recordings from cholinergic interneurons and projection neurons. Glutamatergic EPSCs were evoked in projection neurons and cholinergic interneurons by stimulation of afferent fibers in the cortex and the striatum, respectively. A single spike in a cholinergic interneuron caused significant depression of the evoked glutamatergic EPSC in 34% of projection neurons located within 100 microm and 41% of cholinergic interneurons located within 200 microm. The time course of these effects was similar in the two cases, with EPSC inhibition peaking 20-30 ms after the spike and disappearing after 40-80 ms. Maximal depression of EPSC amplitude was up to 27% in projection neurons and to 19% in cholinergic interneurons. These effects were reversibly blocked by muscarinic receptor antagonists (atropine or methoctramine), which also significantly increased baseline EPSC (evoked without a preceding spike in the cholinergic interneuron), suggesting that some tonic cholinergic presynaptic inhibition was present. This was confirmed by the fact that lowering extracellular potassium, which silenced spontaneously active cholinergic interneurons, also increased baseline EPSC amplitude, and these effects were occluded by previous application of muscarinic receptor antagonists. Collectively, these results show that a single spike in a cholinergic interneuron exerts a fast and powerful inhibitory control over the glutamatergic input to striatal neurons.

3,4-Methylenedioxymethamphetamine enhances the release of acetylcholine in the prefrontal cortex and dorsal hippocampus of the rat

The neurochemical effects produced by acute administration of 3,4-methylenedioxymethamphetamine (MDMA) on the monoaminergic systems in the brain are well documented; however, there has been little consideration of the potential effects of MDMA on other neurotransmitter systems. The present study was designed to investigate the acute effect of MDMA on cholinergic neurons by measuring acetylcholine (ACh) release in the medial prefrontal cortex (PFC) and dorsal hippocampus, terminal regions of cholinergic projection neurons originating in the basal forebrain. In vivo microdialysis and high-performance liquid chromatography with electrochemical detection (HPLC-ED) were used to assess the effects of MDMA on the extracellular concentration of ACh in the PFC and dorsal hippocampus of the rat. The systemic administration of MDMA (3-20 mg/kg, i.p.) resulted in an increased extracellular concentration of ACh in the PFC and dorsal hippocampus. Reverse dialysis of MDMA (100 microM) into the PFC and hippocampus also increased ACh release in these brain regions. Treatment with parachlorophenylalanine and alpha-methyl-para-tyrosine, inhibitors of serotonin (5-HT) and dopamine (DA) synthesis, respectively, significantly attenuated the release of ACh stimulated by MDMA in the PFC, but not in the dorsal hippocampus. MDMA exerts a stimulatory effect on the release of ACh in the PFC and dorsal hippocampus in vivo, possibly by mechanisms localized within these brain regions. In addition, these results suggest that the MDMA-induced release of ACh in the PFC involves both serotonergic and dopaminergic mechanisms.

A New Tool in the Battle Against Alzheimer's Disease and Aging: Ex Vivo Gene Therapy

Alzheimer's disease (AD) is the most common cause of severe dementia in the aging population and is caused by a loss of many different neural systems throughout the brain associated with memory. Amongst the many neural systems affected, large cholinergic projection neurons that innervate large regions of cortex are particularly vulnerable. Thus, boosting cholinergic neuronal function and survival has been a focus of the few drugs currently available for this disorder. Nerve growth factor (NGF) is the archetypical protein discovered in the 1960s that is able to both increase survival and functioning of cholinergic neurons. However, the blood-brain barrier does not allow penetration of this protein into the brain. A phase 1 clinical trial recently published in the journal Nature Medicine utilized a unique ex vivo gene therapy approach to deliver NGF directly to the basal forebrain of AD patients. Despite the need for further testing, their report illustrated a mild but significant therapeutic benefit of NGF for the treatment of AD and provided important data concerning the safety and efficacy of ex vivo gene therapy in humans.

Loss of forebrain cholinergic neurons and impairment in spatial learning and memory in LHX7‐deficient mice

The identification of the genetic determinants specifying neuronal networks in the mammalian brain is crucial for the understanding of the molecular and cellular mechanisms that ultimately control cognitive functions. Here we have generated a targeted allele of the LIM-homeodomain-encoding gene Lhx7 by replacing exons 3-5 with a LacZ reporter. In heterozygous animals, which are healthy, fertile and have no apparent cellular deficit in the forebrain, b-galactosidase activity reproduces the pattern of expression of the wild-type Lhx7 locus. However, homozygous mutant mice show severe deficits in forebrain cholinergic neurons (FCNs), while other classes of forebrain neurons appear unaffected. Using the LacZ reporter as a marker, we show that in LHX7-deficient mice FCN progenitors survive but fail to generate cholinergic interneurons in the striatum and cholinergic projection neurons in the basal forebrain. Analysis of behaviour in a series of spatial and non-spatial learning and memory tasks revealed that FCN ablation in Lhx7 mutants is associated with severe deficits in spatial but only mild impairment of non-spatial learning and memory. In addition, we found no deficit in long-term potentiation in mutant animals, suggesting that FCNs modulate hippocampal function independently of its capacity to store information. Overall our experiments demonstrate that Lhx7 expression is required for the specification or differentiation of cholinergic forebrain neurons involved in the processing of spatial information.

The Role of Nerve Growth Factor Receptors in Cholinergic Basal Forebrain Degeneration in Prodromal Alzheimer Disease

Dysfunction of nerve growth factor (NGF) and its high (TrkA) and low (p75NTR) affinity receptors has been suggested to underlie the selective degeneration of the nucleus basalis (NB) cholinergic cortical projection neurons in end stage Alzheimer disease (AD). Whether the NGF system is dysfunctional during the prodromal stages of AD has only recently been evaluated. Surprisingly, the number of choline acetyltransferase-containing neurons remains stable despite a significant reduction in NGF receptor-positive cells in people with mild cognitive impairment (MCI), suggesting a phenotypic NGF receptor downregulation but not a frank loss of NB neurons during prodromal AD. Moreover, there is a loss of cortical TrkA in the face of stable p75NTR and increased proNGF levels, the precursor molecule of mature NGF, in early AD. Depending upon the cellular context these changes may result in increased pro-apoptotic signaling, cell survival, or a defect in retrograde transport mechanisms. Alterations in NGF and its receptors within the cholinotrophic NB system in early AD suggest that NGF-mediated cell signaling is required for the longterm survival of these neurons. Therapeutic neurotrophic intervention might delay or prevent NB neuron degeneration and preserve cholinergic cortical function during prodromal AD.

Redistribution of CB 1 cannabinoid receptors during evolution of cholinergic basal forebrain territories and their cortical projection areas: A comparison between the gray mouse lemur ( Microcebus murinus, primates) and rat

Endocannabinoid signaling, mediated by presynaptic CB 1 cannabinoid receptors on neurons, is fundamental for the maintenance of synaptic plasticity by modulating neurotransmitter release from axon terminals. In the rodent basal forebrain, CB 1 cannabinoid receptor-like immunoreactivity is only harbored by a subpopulation of cholinergic projection neurons. However, endocannabinoid control of cholinergic output from the substantia innominata, coincident target innervation of cholinergic and CB 1 cannabinoid receptor-containing afferents, and cholinergic regulation of endocannabinoid synthesis in the hippocampus suggest a significant cholinergic–endocannabinergic interplay. Given the functional importance of the cholinergic modulation of endocannabinoid signaling, here we studied CB 1 cannabinoid receptor distribution in cholinergic basal forebrain territories and their cortical projection areas in a prosimian primate, the gray mouse lemur. Perisomatic CB 1 cannabinoid receptor immunoreactivity was unequivocally present in non-cholinergic neurons of the olfactory tubercule, and in cholecystokinin-containing interneurons in layers 2/3 of the neocortex. Significantly, CB 1 cannabinoid receptor-like immunoreactivity was localized to cholinergic perikarya in the magnocellular basal nucleus. However, cortical cholinergic terminals lacked detectable CB 1 cannabinoid receptor levels. A dichotomy of CB 1 cannabinoid receptor distribution in frontal (suprasylvian) and parietotemporal (subsylvian) cortices was apparent. In the frontal cortex, CB 1 cannabinoid receptor-containing axons concentrated in layers 2/3 and layer 6, while layer 4 and layer 5 were essentially devoid of CB 1 cannabinoid receptor immunoreactivity. In contrast, CB 1 cannabinoid receptors decorated axons in all layers of the parietotemporal cortex with peak densities in layer 2 and layer 4. In the hippocampus, CB 1 cannabinoid receptor-containing terminals concentrated around pyramidal cell somata and proximal dendrites in the CA1–CA3 areas, and granule cell dendrites in the molecular layer of the dentate gyrus. CB 1 cannabinoid receptors frequently localized to inhibitory GABAergic terminals while leaving glutamatergic boutons unlabeled. Aging did not affect either the density or layer-specific distribution of CB 1 cannabinoid receptor-immunoreactive processes. We concluded that organizing principles of CB 1 cannabinoid receptor-containing neurons and their terminal fields within the basal forebrain are evolutionarily conserved between rodents and prosimian primates. In contrast, the areal expansion and cytoarchitectonic differentiation of neocortical subfields in primates is associated with differential cortical patterning of CB 1 cannabinoid receptor-containing subcortical and intracortical afferents.

IL-2 deficiency results in altered septal and hippocampal cytoarchitecture: relation to development and neurotrophins

We have found previously that brain IL-2 receptors are enriched in the hippocampal formation, and that loss of this cytokine results in cytoarchitectural alterations in the hippocampus and septum and related behavioral changes in IL-2 knockout (IL-2 KO) mice. These alterations included decreased cholinergic somata in the medial septum/vertical limb of the diagonal band of Broca (MS/vDB) and decreased distance across the infrapyramidal (IP) granule cell layer (GCL) of the dentate gyrus (DG). To extend our previous findings, several experiments were conducted comparing IL-2 KO mice and wild-type littermates to determine (1) whether the GABAergic projection neurons of IL-2 KO mice in this region were also affected; (2) if the reduction in septal cholinergic projection neurons found in adult IL-2 KO mice is present at weaning (and prior to the development of peripheral autoimmune disease); and (3) if loss of IL-2 may result in changes in the neurotrophins, brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), involved in maintenance of hippocampal neurons. No differences in GABAergic neurons in the MS/vDB were found in adult mice, and the reduction in cholinergic neurons seen in adult IL-2 KO mice was not found in animals at postnatal day 21. The number of neurons in the IP-GCL was also significantly reduced. Compared to wild-type mice, IL-2 KO mice had significantly reduced concentration of BDNF protein and increased concentrations of NGF. These data suggest that the septohippocampal neuronal loss in IL-2 KO mice is selective for the cholinergic neurons and appears to be due to a failure in neuronal maintenance/survival that may be, in part, associated with changes in neurotrophins.

Activity of Neurons in the Basal Magnocellular Nucleus During Performance of an Operant Task

Spike activity was studied in 95 neurons in the basal magnocellular nucleus in rabbits during spontaneous behavior and during performance of a conditioned operant response. Nearly half the neurons (48.4%) showed significant (p < 0.05) negative correlations between spontaneous discharges and the power of the frontal lobe EEG delta rhythm; most of these cells could be identified as cholinergic projection neurons. Neurons of this group had predominantly excitatory responses to the conditioned stimulus during performance of the operant task, while the responses to the conditioned stimulus of presumptively non-cholinergic neurons, not projecting to the cortex, were mainly inhibitory. The activatory responses of neurons in the basal magnocellular nucleus to the conditioned stimulus were markedly stronger while the animals performed the operant response as compared with performances in which there was no response to the conditioned stimulus. These results provide evidence that the basal magnocellular nucleus supports the level of waking and attending required for performance of operant conditioned reflex activity.

Synaptic connections of cholinergic antennal lobe relay neurons innervating the lateral horn neuropile in the brain of Drosophila melanogaster.

Presumed cholinergic projection neurons (PNs) in the brain of the fruit fly Drosophila melanogaster, immunoreactive to choline acetyltransferase (ChAT), convey olfactory information between the primary sensory antennal lobe neuropile and the mushroom body calyces, and finally terminate in the lateral horn (LH) neuropile. The texture and synaptic connections of ChAT PNs in the LH and, comparatively, in the smaller mushroom body calyces were investigated by immuno light and electron microscopy. The ChAT PN fibers of the massive inner antennocerebral tract (iACT) extend into all portions of the LH, distributing in a nonrandom fashion. Immunoreactive boutons accumulate in the lateral margins of the LH, whereas the more proximal LH exhibits less intense immunolabeling. Boutons with divergent presynaptic sites, unlabeled as well as ChAT-immunoreactive, appear to be the preponderant mode of synaptic input throughout the LH. Synapses of ChAT-labeled fibers appear predominantly as divergent synaptic boutons (diameters 1-3 microm), connected to unlabeled postsynaptic profiles, or alternatively as a minority of tiny postsynaptic spines (diameters 0.05-0.5 microm) among unlabeled profiles. Together these spines encircle unidentified presynaptic boutons of interneurons which occupy large areas of the LH. Thus, synaptic circuits in the LH differ profoundly from those of the PNs in the mushroom body calyx, where ChAT spines have not been encountered. Synaptic contacts between LH ChAT elements were not observed. The synaptic LH neuropile may serve as an output area for terminals of the ChAT PNs, their presynaptic boutons providing input to noncholinergic relay neurons. The significance of the postsynaptic neurites of the ChAT PNs is discussed; either local or other interneurons might connect the ChAT PNs within the LH, or PNs might receive inputs arising from outside the LH.

Cognitive function of the brainstem cholinergic system

A major population of cholinergic projection neurons is located in the pedunculopontine and laterodorsal tegmental nuclei in the midbrain. Although much work has been performed on their role in the regulation of sleep and wakefulness, their function in the awake state is not well understood. We made single unit recordings in the pedunculopontine tegmental nucleus (PPTN) of macaque monkeys during visually guided saccade tasks. Three major types of neuron activities were observed. The first was tonic or phasic increase in activity around the time of fixation point onset and closely correlated with the performance of the monkey in the coming saccade task. The second was the activity related to preparation and execution of saccades. The third was the activity related to reward delivery. A subpopulation of PPTN neurons exhibited more than two types of activities. We conclude from these findings that the PPTN may serve as an integrative interface between various signals required for performance of purposive behaviors.

Rabbit forebrain cholinergic system: morphological characterization of nuclei and distribution of cholinergic terminals in the cerebral cortex and hippocampus.

Although the rabbit brain, in particular the basal forebrain cholinergic system, has become a common model for neuropathological changes associated with Alzheimer's disease, detailed neuroanatomical studies on the morphological organization of basal forebrain cholinergic nuclei and on their output pathways are still awaited. Therefore, we performed quantitative choline acetyltransferase (ChAT) immunocytochemistry to localize major cholinergic nuclei and to determine the number of respective cholinergic neurons in the rabbit forebrain. The density of ChAT-immunoreactive terminals in layer V of distinct neocortical territories and in hippocampal subfields was also measured. Another cholinergic marker, the low-affinity neurotrophin receptor (p75(NTR)), was also employed to identify subsets of cholinergic neurons. Double-immunofluorescence labeling of ChAT and p75(NTR), calbindin D-28k (CB), parvalbumin, calretinin, neuronal nitric oxide synthase (nNOS), tyrosine hydroxylase, or substance P was used to elucidate the neuroanatomical borders of cholinergic nuclei and to analyze the neurochemical complexity of cholinergic cell populations. Cholinergic projection neurons with heterogeneous densities were found in the medial septum, vertical and horizontal diagonal bands of Broca, ventral pallidum, and magnocellular nucleus basalis (MBN)/substantia innominata (SI) complex; cholinergic interneurons were observed in the caudate nucleus, putamen, accumbens nucleus, and olfactory tubercule, whereas the globus pallidus was devoid of cholinergic nerve cells. Cholinergic interneurons were frequently present in the hippocampus and to a lesser extent in cerebral cortex. Cholinergic projection neurons, except those localized in SI, abundantly expressed p75(NTR), and a subset of cholinergic neurons in posterior MBN was immunoreactive for CB and nNOS. A strict laminar distribution pattern of cholinergic terminals was recorded both in the cerebral cortex and in CA1-CA3 and dentate gyrus of the hippocampus. In summary, the structural organization and chemoarchitecture of rabbit basal forebrain may be considered as a transition between that of rodents and that of primates.

Increased μ-opioid receptor labeling is found on inner molecular layer terminals of the dentate gyrus following seizures

The hippocampal formation is a brain region sensitive to seizure development, a phenomenon thought to be mediated in part by μ-opioid receptor (MOR) activation. Previous studies have found a delayed increase in MOR immunoreactivity (IR) in the inner molecular layer (IML) of the dentate gyrus after experimentally induced seizures. However, whether these increases in MOR-IR are restricted to certain cell types or cellular compartments (i.e., presynaptic, postsynaptic, or glial profiles) has not been determined. Thus, the present study examined which subcellular profiles demonstrate changes in MOR-IR after kainic acid (KA)-induced seizures. Light microscopic (LM) analysis demonstrated seizure-induced increases in MOR-IR at three points of the IML (dorsal blade, ventral blade, and crest) at three levels of section (septal, mid-septotemporal, and temporal). Electron microscopic analysis of the IML revealed that MOR-IR was present in the same types of cellular profiles in both control and KA-treated rats. However, a significant increase in the number of MOR-labeled terminal profiles was revealed in KA-treated rats compared to controls. Additionally, some MOR-labeled terminals in KA-treated rats possessed excitatory-type morphology and contained enkephalin or dynorphin, peptides found in mossy fiber terminals. These data suggest that most of the seizure-induced increases in MOR expression in the IML are associated with terminals originating from several different neuronal populations, including granule cells, and possibly, surviving GABAergic interneurons, septal cholinergic, and/or supramamillary projection neurons.

Sleep-wake mechanisms and basal forebrain circuitry.

The seminal studies by von Economo in humans (1) and by Nauta (2) in rats implicated specific basal forebrain areas at the preoptic level as important in sleep regulation. In the last two decades, studies employing recording of single neurons and monitoring of sleep parameters with subsequent chemical and electron microscopic identification of the synaptic input-output relations of these recorded neurons, provided an increasingly detailed understanding of the function of specific neurotransmitters and corresponding chemically specific neuronal circuits in the forebrain in relation to sleep-wake states. In this review, first the electrophysiology of cholinergic and parvalbumin-containing GABAergic basalo-cortical projection neurons is described, followed by an examination of possible functional interconnections between basal forebrain neuropeptide Y- (NPY) and somatostatin-containing putative interneurons and cholinergic projection neurons. A survey of various inputs to basal forebrain neurons that show state-related changes is then discussed in relation to their possible effects via basal forebrain circuitry on cortical activity. This treatise suggests that cholinergic and GABAergic projection neurons of the basal forebrain are anatomically in a unique position to enable the channeling of specific cellular and homeostatic states from different subcortical systems to the cortical mantle to modulate behavioral adaptation and cognitive functions.

β-Amyloid Peptide Activates α7 Nicotinic Acetylcholine Receptors Expressed in Xenopus Oocytes

Abstract The α7 nicotinic acetylcholine receptor is highly expressed in hippocampus and in cholinergic projection neurons from the basal forebrain, structures that are particularly vulnerable to the ravages of Alzheimer's disease. Previous work suggests that β-amyloid peptide can interact with α7 nicotinic acetylcholine receptors, although the nature of this interaction has not been well characterized. To test whether β-amyloid peptide can activate α7 nicotinic acetylcholine receptors, we expressed these receptors inXenopus oocytes and performed two-electrode voltage clamp recordings, characterizing the response to β-amyloid peptide 1–42 applied at concentrations ranging from 1 pm to 100 nm. In α7-expressing oocytes, β-amyloid peptide 1–42 elicits inward currents at low concentrations (1–100 pm), whereas at higher concentrations (nm), less effective receptor activation is observed, indicative of receptor desensitization. Preincubation with the α7-selective agents, the antagonist methyllycaconatine, and the agonist 4-OH-GTS-21 blocked β-amyloid peptide-induced receptor activation. β-amyloid peptide 1–42 at low concentrations was able to activate the L250T mutant α7 receptor. The endogenous Ca2+-activated chloride current inXenopus oocytes is recruited upon receptor activation since replacing Ca2+ with Ba2+ in the recording solution reduced current amplitude. Thus, when β-amyloid peptide activation of α7 receptors occurs, these currents are comprised, at least in part, of Ca2+.

Electroencephalographic activation by tacrine, deprenyl, and quipazine: cholinergic vs. non-cholinergic contributions

Drugs that stimulate central cholinergic transmission can induce activated, high frequency electroencephalographic (EEG) activity in rats. Monoaminergic enhancement also produces EEG activation, either by a direct stimulation of monoaminergic transmission in cortex, or a transsynaptic excitation of cholinergic projection neurons receiving excitatory monoaminergic afferents. We examined the degree of cholinergic involvement in EEG activation produced by monoaminergic and cholinergic drugs in rats. All animals were pretreated with 10 mg/kg reserpine and either 1 or 5 mg/kg scopolamine to abolish EEG activation. The acetylcholinesterase inhibitor tacrine (5–20 mg/kg) restored EEG activation in the low dose scopolamine group, but was less effective against the high scopolamine dose. The monoamine oxidase inhibitor deprenyl and the serotonergic receptor agonist quipazine restored EEG activation, an effect that was largely unaffected by different scopolamine doses. These results confirm that tacrine produces EEG activation by means of cholinergic stimulation. In contrast, activation produced by deprenyl or quipazine does not appear to be mediated by a transsynaptic excitation of cholinergic neurons and likely depends on a direct enhancement of cortical monoaminergic neurotransmission.

Neuropathology of Mice Carrying Mutant APPswe and/or PS1M146L Transgenes: Alterations in the p75NTR Cholinergic Basal Forebrain Septohippocampal Pathway

Cholinergic basal forebrain (CBF) projection systems are defective in late Alzheimer's disease (AD). We examined the brains of 12-month-old singly and doubly transgenic mice overexpressing mutant amyloid precursor protein (APPswe) and/or presenilin-1 (PS1M146L) to investigate the effects of these AD-related genes on plaque and tangle pathology, astrocytic expression, and the CBF projection system. Two types of β-amyloid (Aβ)-immunoreactive (ir) plaques were observed: type 1 were darkly stained oval and elongated deposits of Aβ, and type 2 were diffuse plaques containing amyloid fibrils. APPswe and PS1M146L mouse brains contained some type 1 plaques, while the doubly transgenic (APPswe/PS1M146L) mice displayed a greater abundance of types 1 and 2 plaques. Sections immunostained for the p75 NGF receptor (p75NTR) revealed circular patches scattered throughout the cortex and hippocampus of the APPswe/PS1M146L mice that contained Aβ, were innervated by p75NTR-ir neurites, but displayed virtually no immunopositive neurons. Tau pathology was not seen in any transgenic genotype, although a massive glial response occurred in the APPswe/PS1M146L mice associated with amyloid plaques. Stereology revealed a significant increase in p75NTR-ir medial septal neurons in the APPswe and PS1M146L singly transgenic mice compared to the APPswe/PS1M146L mice. No differences in size or optical density of p75NTR-ir neurons were observed in these three mutants. p75NTR-ir fibers in hippocampus and cortex were more pronounced in the APPswe and PS1M146L mice, while the APPswe/PS1M146L mice showed the least p75NTR-ir fiber staining. These findings suggest a neurotrophic role for mutant APP and PS1 upon cholinergic hippocampal projection neurons at 12 months of age.

β-Amyloid(1-42)-Induced Cholinergic Lesions in Rat Nucleus Basalis Bidirectionally Modulate Serotonergic Innervation of the Basal Forebrain and Cerebral Cortex

Ample experimental evidence suggests that β-amyloid (Aβ), when injected into the rat magnocellular nucleus basalis (MBN), impels excitotoxic injury of cholinergic projection neurons. Whereas learning and memory dysfunction is a hallmark of Aβ-induced cholinergic deficits, anxiety, or hypoactivity under novel conditions cannot be attributed to the loss of cholinergic MBN neurons. As mood-related behavioral parameters are primarily influenced by the central serotonergic system, in the present study we investigated whether Aβ(1-42) toxicity in the rat MBN leads to an altered serotonergic innervation pattern in the rat basal forebrain and cerebral cortex 7 days postsurgery. Aβ infusion into the MBN elicited significant anxiety in the elevated plus maze. Aβ toxicity on cholinergic MBN neurons, expressed as the loss of acetylcholinesterase-positive cortical projections, was accompanied by sprouting of serotonergic projection fibers in the MBN. In contrast, the loss of serotonin-positive fiber projections, decreased concentrations of both serotonin and 5-hydroxyindoleacetic acid, and decline of cortical 5-HT1A receptor binding sites indicated reduced serotonergic activity in the somatosensory cortex. In conclusion, the Aβ-induced primary cholinergic deficit in the MBN and subsequent cortical cholinergic denervation bidirectionally modulate serotonergic parameters in the rat basal forebrain and cerebral cortex. We assume that enhanced serotonin immunoreactivity in the damaged MBN indicates intrinsic processes facilitating neuronal recovery and cellular repair mechanisms, while diminished cortical serotonergic activity correlates with the loss of the subcortical cholinergic input, thereby maintaining the balance of neurotransmitter concentrations in the cerebral cortex.

Increased amyloid precursor protein expression and serotonergic sprouting following excitotoxic lesion of the rat magnocellular nucleus basalis: neuroprotection by Ca 2+ antagonist nimodipine

In the present study plastic neural responses to N-methyl- d-aspartate-induced excitotoxic lesions and the neuroprotective effects of the L-type voltage-dependent Ca 2+ channel antagonist nimodipine were investigated in the rat magnocellular nucleus basalis. Assessment of spontaneous behaviour in the elevated plus maze and small open-field paradigms on day 5 and day 14 post-surgery indicated anxiety and persistent hypoactivity of N-methyl- d-aspartate-lesioned rats, as compared with sham-operated controls. Nimodipine administration significantly alleviated the behavioural deficits. Quantitative histochemical analysis of acetylcholinesterase-positive fibre innervation of the somatosensory cortex and determination of the numbers of choline-acetyltransferase-positive proximal fibre branches of cholinergic projection neurons in the magnocellular nucleus basalis demonstrated a severe cholinergic deficit as a consequence of the excitotoxic lesion 14 days post-surgery. Nimodipine pre-treatment significantly attenuated the loss of cortical cholinergic innervation and preserved the functional integrity of cholinergic projection neurons in the magnocellular nucleus basalis. Double-labelling immunocytochemistry demonstrated increased amyloid precursor protein expression in shrinking and presumably apoptotic choline-acetyltransferase-positive neurons, whereas surviving cholinergic nerve cells were devoid of excessive amyloid precursor protein immunoreactivity. Moreover, as a consequence of N-methyl- d-aspartate infusion, rim-like accumulation of amyloid precursor protein-positive astrocytes was visualized in a penumbra-like zone of the excitotoxic injury. Furthermore, abundant sprouting of serotonergic projection fibres invading the damaged magnocellular nucleus basalis subdivision was demonstrated. Pharmacological blockade by the Ca 2+ antagonist nimodipine significantly attenuated both neuronal and glial amyloid precursor protein immunoreactivity and serotonergic fibre sprouting following N-methyl- d-aspartate infusion. The present data characterize plastic endogenous glial and neuronal responses in the magnocellular nucleus basalis model of acute excitotoxic brain damage. The increased amyloid precursor protein expression may indicate effective means of intrinsic neuroprotection, as secreted amyloid precursor protein isoforms are suggested to play a role in neuronal rescue following excitotoxic injury. From a pharmacological point of view, extensive sprouting of serotonergic projections in the damaged magnocellular nucleus basalis may also counteract N-methyl- d-aspartate excitotoxicity via serotonin-induced inhibition of Ca 2+ currents and membrane hyperpolarization. Hence, lesion-induced changes in spontaneous animal behaviour, such as anxiety and novelty-induced hypoactivity, may well be attributed to the considerable re-distribution of serotonergic projections in the basal forebrain. In conclusion, our present data emphasize a role of neuron-glia and neurotransmitter-system interactions in functional recovery after acute excitotoxic brain injury, and the efficacy of L-type Ca 2+ channel blockade by the selective 1,4-dihydropyridine antagonist nimodipine.