Muscarinic cholinergic receptor subtypes in normal human brain and Alzheimer's presenile dementia

Basal forebrain cholinergic system in the dementias: Vulnerability, resilience, and resistance

The basal forebrain cholinergic neurons (BFCN) provide the primary source of cholinergic innervation of the human cerebral cortex. They are involved in the cognitive processes of learning, memory, and attention. These neurons are differentially vulnerable in various neuropathologic entities that cause dementia. This review summarizes the relevance to BFCN of neuropathologic markers associated with dementias, including the plaques and tangles of Alzheimer's disease (AD), the Lewy bodies of diffuse Lewy body disease, the tauopathy of frontotemporal lobar degeneration (FTLD-TAU) and the TDP-43 proteinopathy of FTLD-TDP. Each of these proteinopathies has a different relationship to BFCN and their corticofugal axons. Available evidence points to early and substantial degeneration of the BFCN in AD and diffuse Lewy body disease. In AD, the major neurodegenerative correlate is accumulation of phosphotau in neurofibrillary tangles. However, these neurons are less vulnerable to the tauopathy of FTLD. An intriguing finding is that the intracellular tau of AD causes destruction of the BFCN, whereas that of FTLD does not. This observation has profound implications for exploring the impact of different species of tauopathy on neuronal survival. The proteinopathy of FTLD-TDP shows virtually no abnormal inclusions within the BFCN. Thus, the BFCN are highly vulnerable to the neurodegenerative effects of tauopathy in AD, resilient to the neurodegenerative effect of tauopathy in FTLD and apparently resistant to the emergence of proteinopathy in FTLD-TDP and perhaps also in Pick's disease. Investigations are beginning to shed light on the potential mechanisms of this differential vulnerability and their implications for therapeutic intervention.

Cyto- and receptor architectonic mapping of the human brain

Mapping of the human brain is more than the generation of an atlas-based parcellation of brain regions using histologic or histochemical criteria. It is the attempt to provide a topographically informed model of the structural and functional organization of the brain. To achieve this goal a multimodal atlas of the detailed microscopic and neurochemical structure of the brain must be registered to a stereotaxic reference space or brain, which also serves as reference for topographic assignment of functional data, e.g., functional magnet resonance imaging, electroencephalography, or magnetoencephalography, as well as metabolic imaging, e.g., positron emission tomography. Although classic maps remain pioneering steps, they do not match recent concepts of the functional organization in many regions, and suffer from methodic drawbacks. This chapter provides a summary of the recent status of human brain mapping, which is based on multimodal approaches integrating results of quantitative cyto- and receptor architectonic studies with focus on the cerebral cortex in a widely used reference brain. Descriptions of the methods for observer-independent and statistically testable cytoarchitectonic parcellations, quantitative multireceptor mapping, and registration to the reference brain, including the concept of probability maps and a toolbox for using the maps in functional neuroimaging studies, are provided.

No change in cortical muscarinic M2, M3 receptors or [35S]GTPgammaS binding in schizophrenia

Muscarinic M1, but not M4, receptors have been shown to be decreased in Brodmann's area (BA) 9 obtained postmortem from subjects with schizophrenia. This study extends that data by measuring levels of muscarinic M2 and M3 receptor protein and mRNAs in BA 9 and BA 40 from the same cohorts of subjects used in the study of M1 and M4 receptors. In addition, the ability of carbachol to stimulate muscarinic receptors that signal through the Gi/o G-proteins was measured in BA 9 from the same cohorts of subjects. There were no changes in levels of muscarinic M2 or M3 protein or M3 mRNA with diagnosis in either CNS region. M2 receptor mRNA could not be detected in BA 9 or BA 40. Finally, carbachol-stimulated GTPgammaS binding did not differ between the diagnostic cohorts in BA 9 (p = 0.64). These data add considerable weight to the argument that the muscarinic M1 receptor is the muscarinic receptor predominantly affected in BA 9 by the pathology of schizophrenia. Given the widespread changes in muscarinic receptors identified in the CNS of subjects of schizophrenia using functional neuroimaging it remains possible that receptors other than the M1 receptor may be altered in different CNS regions.

Muscarinic1 and 2 receptor mRNA in the human caudate-putamen: no change in m1 mRNA in schizophrenia

Studies using tissue obtained at autopsy suggest that changes in cholinergic neurons could be important in the pathology of schizophrenia.1-4 We have previously reported a decrease in [3H]pirenzepine binding5 and [3H]AF-DX 384 binding6 to caudate-putamen (CP) from subjects who had schizophrenia. Under the conditions chosen, [3H]pirenzepine would predominately bind to muscarinic1 (M1) and muscarinic4 (M4) receptors,7whereas [3H]AF-DX 384 would mainly bind to muscarinic2 (M2) and M4 receptors.8 Given the relative concentrations of M1, M2 and M4 receptors in the human CP and the magnitude of the decreases in radioligand binding in schizophrenia, our results most likely reflected a change in the density of M1 and M2 receptors in the CP from the schizophrenic subjects. In situ hybridisation has now been used to determine levels of m1 and m2 mRNA in CP from 14 schizophrenic and 16 control subjects previously used for radioligand binding. m2 mRNA in the CP from the schizophrenic and control subjects was below the sensitivity of in situhybridisation. There was no difference in the levels of m1 mRNA in CP from schizophrenic and control subjects (mean +/- SEM: 103 +/- 16 vs106 +/- 17 fmol [35S]oligonucleotide probe g-1estimated tissue equivalents, P = 0.91). In conclusion, data from our radioligand binding studies show decreases in [3H]pirenzepine binding that are likely to reflect a decrease in the density of M1 receptors in CP from schizophrenic subjects. Our data in this study show the absence of a concomitant change in mRNA coding for that receptor.

Interaction of the novel antipsychotic drug amperozide and its metabolite FG5620 with central nervous system receptors and monoamine uptake sites: relation to behavioral and clinical effects

Behavioral, biochemical, and electrophysiological studies suggest that amperozide affects mesolimbic and mesocortical dopamine neurotransmission. The receptor binding profile of amperozide is discussed and related to behavioral and clinical, i.e., antipsychotic, effects of the drug. As previously reported, amperozide displayed high affinity for serotonin 5-HT2A receptors (Ki = 16 nmol/L), and moderate affinity for striatal dopamine D2 (Ki = 540 nmol/L) and cortical alpha 1-adrenergic receptors (Ki = 172 nmol/L). In the present study amperozide displayed low affinity for several serotonin receptor subtypes as well as for the dopamine D4 receptor transfected in COS7 cells (Ki D4.2 = 769 nmol/L and Ki D4.4 = 384 nmol/L). Amperozide was very weak or did not interact with several other receptor species including adrenergic, histaminergic, muscarinic, benzodiazepine, gamma-aminobutyric acid, amino acid, opiate, and Ca channels; however, amperozide was found to compete for [3H]paroxetine binding for the serotonin transporter in the nanomolar range (Ki = 49 nmol/L). In vitro and in vivo binding potency of amperozide correlates best with behavioral effects, indicating 5-HT2A antagonism, although serotonin uptake inhibition may contribute to the effects of amperozide on dopamine neurotransmission. The metabolite of amperozide, FG5620, displayed 5-10 times lower pharmacologic activity than amperozide. These properties of amperozide may suggest that the antipsychotic effects of amperozide are mediated by 5-HT2A receptors, although 5-HT uptake inhibition and alpha 1-adrenergic receptor-mediated effects may be considered, particularly at higher doses.

Autoradiographic distribution of M1, M2, M3, and M4 muscarinic receptor subtypes in Alzheimer's disease

We studied the autoradiographic densities of all pharmacologically characterised muscarinic receptors (MR) in frontal, temporal, and visual cortex, hippocampal formation, and striatum in autopsied brains from 19 histopathologically verified patients of Alzheimer's disease (AD) and in matched controls. Almost all (16 of 19) of the AD cases were severe. In AD brains, total MR, M1, and M3 MR subtypes were found to be significantly decreased in entorhinal cortex and in most hippocampal strata. Total MR and M1 receptors were also significantly reduced in visual area and in frontal cortex of AD brains, respectively. M2 receptors were significantly reduced over hippocampal formation but increased significantly in striatum of AD brains as compared with controls. M3 receptors in AD were in the range of controls in neocortex and striatum, whereas the M4 receptor subtype was also preserved in all brain regions in AD brains when compared with controls. This is the first autoradiographic study analysing the distribution of all MR subtypes in AD brains. These changes in MR densities concur with the general pattern of neuronal degeneration occurring in AD brains and partly explain the poor response of AD cognitive decline to present cholinergic supplementation therapies. Although M3 and M4 MR were labelled with nonselective approaches, the preservation of M4 and to a lesser degree M3 MR subtypes in AD brains could open an alternative way for the symptomatic therapy of AD dementia.

The neurochemistry of Alzheimer type, vascular type and mixed type dementias compared

We present the results of a meta-analysis of neurochemical changes in human post mortem brains of Alzheimer type (AD), vascular type (VD) and mixed type (MF) dementias, and matched controls based on 275 articles published between January 1980 and February 1994. Severity of degeneration between the different neurochemical systems is as follows, although ranking is difficult with regard to limited numbers of investigations in some neurochemical systems: Cholinergic system > serotonergic system > excitatory amino acids > GABAergic system > energy metabolism > NA > oxidative stress parameters > neuropeptides > DA. But, within a neurochemical system, degeneration is not evenly distributed. Spared parameters, e.g. muscarinic receptors and MAO-B, allow to make some suggestions for future therapeutic strategies.

Oral physostigmine in Alzheimer's disease: effects on norepinephrine and vasopressin in cerebrospinal fluid and plasma

Physostigmine is a cholinesterase inhibitor which enhances central and peripheral cholinergic activity. In this study, we explored in persons with Alzheimer's disease (AD) the effects of an acute dose of physostigmine in patients receiving chronic physostigmine treatment on the activity of the cholinergically regulated noradrenergic and arginine vasopressin (AVP) systems. Specifically, we estimated the effects of sustained release oral physostigmine on central and peripheral noradrenergic and AVP systems by measuring norepinephrine (NE) and AVP in cerebrospinal fluid (CSF) and plasma. Lumbar punctures were performed in both physostigmine and no drug treatment conditions. In some subjects the effects of physostigmine on the plasma AVP response to the osmolar stimulus of a hypertonic saline infusion also were measured. NE concentrations in both CSF and plasma were significantly lower in the physostigmine than in the no drug condition. AVP concentrations did not differ between conditions in either compartment, nor did physostigmine affect the AVP response to hypertonic saline. Physostigmine appears to decrease both central and peripheral noradrenergic activity in AD.

High-affinity choline uptake and muscarinic receptors in rat brain during aging

The aim of this study was to determine the effect of aging on the high-affinity choline uptake (HACU) and the muscarinic acetylcholine receptors (mAChR) in the brain of Wistar male rats and to define more precisely the steps of the brain cholinergic degeneration in the course of the whole animal life. In 24-month-old rats, a substantial decrease in HACU values in the hippocampus (to 65-75%) and in the density of mAChR in the cortex (to 76%) was found in comparison with 3-month-old controls. The interaction of muscarinic receptor antagonist pirenzepine with [(3)H]QNB indicated a decrease in low-affinity sites (M(2)) in 24-month-old rats. The first slight changes due to aging manifested themselves by the reduction in HACU values very early (between 6 and 12 months), the decrease of the muscarinic receptor density was observed in a later stage (19-month-old animals). Regression analysis indicated considerable dependence of the HACU values on age (the correlation coefficient r = -0.689, the slope b = -0.279 pmol/4 min per mg(prot) per month, P < 0.001) while the density of muscarinic receptors does not correlate with age so markedly (r = -0.415, b = -6.316 fmol/mg(prot) per month, P = 0.018).

Nicotinic and muscarinic subtypes in the human brain: changes with aging and dementia

Different effects of normal aging on muscarinic and nicotinic receptor subtypes were observed in postmortem brain tissue from different regions of the human brain. A significant decrease in M1 and M2 receptors was found in cerebral cortex, while the M1 and especially the M2 receptors increased with age in the thalamus. A similar pattern of changes was also observed when using (-)3H-nicotine as ligand for nicotinic receptors in the cortex and thalamus. No significant changes in nicotinic receptor binding were observed with age in the cortex or thalamus when using 3H-acetylcholine as ligand. Nicotinic and muscarinic receptors in the brain are not equally affected in dementia disorders. A marked loss of high affinity nicotinic receptors was observed in cortical tissue from patients with Alzheimer's disease and with multi-infarct dementia (MID). The muscarinic receptors were (both M1 and M2) increased in Alzheimer cortical tissue while they were decreased in MID.

Muscarinic cholinoceptive neurons in the frontal cortex in Alzheimer's disease

The cellular distribution of muscarinic acetylcholine receptor protein in the frontal cortex of Alzheimer (AD) patients, age-matched and middle-aged controls was assessed quantitatively by means of immunohistochemistry using the monoclonal antibody M35. As shown previously in biopsy cortices, mainly layer II/III and V pyramidal neurons were immunolabeled. Neither distribution nor numbers of labeled cells displayed significant differences between the groups investigated. This is in accordance with the results of ligand binding studies that mostly failed to reveal different binding characteristics in AD compared to controls. Muscarinic and nicotinic receptor proteins have been shown to be colocalized in many cholinoceptive pyramidal neurons. Since nicotinic receptors--in contrast to muscarinic receptor proteins--are severely reduced in AD, this indicates a selective impairment of nicotinic receptor expression and not a significant death of cholinoceptive neurons per se.

Scopolamine intoxication as a model of transient global amnesia

In Colombia (South America) during recent decades the administration of scopolamine, extracted from plants belonging to the Datura or Brugmansia genus, has become an important neurologic and toxicologic phenomenon. These extracts have been popularly known as "Burundanga." Chemical characteristics and clinical features of scopolamine intoxication are described. Anterograde amnesia and submissive behavior found in patients intoxicated with scopolamine are analyzed. Burundanga intoxication is related to other toxic phenomena found in different countries and similitudes with transient global amnesia are emphasized.

Cholinergic receptors in human brain: effects of aging and Alzheimer disease

A general review of cholinergic receptors in human brain is presented. The paper focuses upon changes in normal aging brain and in Alzheimer disease. Studies from five different approaches are reported: 1) molecular biology; 2) receptor binding studies; 3) studies with specific neurotoxins; 4) immunocytochemistry; and 5) PET scan. These studies document profound and characteristic differences between the normal aging and the pathological Alzheimer brain with regard to cholinergic receptor localization, distribution, and function.

Cholinergic nicotinic and muscarinic receptors in dementia of Alzheimer, Parkinson and Lewy body types

Cholinergic nicotinic and muscarinic receptor binding were measured in post mortem human brain tissue, using low (nM) concentrations of (3H)-nicotine to detect predominately the high affinity nicotinic site and (3H)-N-methylscopolamine in the presence and absence of 3 x 10(-4) M carbachol to measure both the low and high affinity agonist subtypes of the muscarinic receptor group. Consistent with most previous reports, the nicotinic but not muscarinic binding was reduced in the different forms of dementia associated with cortical cholinergic deficits, including Alzheimer's and Parkinson's disease, senile dementia of Lewy body type (SDLT) and Down's syndrome (over 50 years). Analysis of (3H)-nicotine binding displaced by a range of carbachol concentrations (10(-9)-10(-3) M) indicated 2 binding sites for nicotine and that the high affinity rather than low affinity site was reduced in Alzheimer's disease. In all 3 cortical areas investigated (temporal, parietal and occipital) there were increases in the low affinity muscarinic site in Parkinson's disease and SDLT but not Alzheimer's disease or middle-aged Down's syndrome. This observation raised the question of whether the presence of neurofibrillary tangles (evident in the latter but not former 2 disorders) is incompatible with denervation-induced muscarinic supersensitivity in cholinoceptive neurons which include cortical pyramids generally affeted by tangle formation.

Human M1-, M2- and M3-muscarinic cholinergic receptors: binding characteristics of agonists and antagonists

The muscarinic acetylcholine receptors were identified in membrane preparations from human tissues by the specific binding of 1-[benzilic-4,4'-3H] quinuclidinyl benzilate. Saturation binding isotherms of this radioligand yielded a total amount of receptors of 435 +/- 208, 159 +/- 65 and 913 +/- 89 fmol/mg protein, respectively, in the hippocampus, pons and submandibular gland. Non linear least squares analysis of competition binding studies with the antagonists pirenzepine and AF-DX 116 indicates that the majority of receptors are of the M1-type in the hippocampus (83%, high affinity for pirenzepine, intermediate affinity for AF-DX 116), the M2-type in the pons (low affinity for pirenzepine and high affinity for AF-DX 116), and the M3-type in the submandibular gland (low affinity for pirenzepine and AF-DX 116). Competition binding parameters of the agonists carbachol, arecoline, oxotremorine, pilocarpine and MCN-A-343 were compared for M1, M2 and M3 receptors in the human hippocampus, pons and submandibular gland. GTP caused a shift to the right and a steepening of the shallow agonist competition curves in the 3 tissues but did not affect the initially steep ones. This effect is explained by a GTP-mediated conversion of high- to low-agonist affinity sites. The extent of the nucleotide shift was much greater for M2 receptors as compared with M1 and M3 receptors. The GTP effect was impaired by the sulphydryl reagent N-ethylmaleimide, probably due to alkylation of GTP-binding proteins. Moreover, the reagent provoked also an increase of the agonist affinity for the uncoupled muscarinic receptors. For all agonists, this increase was more pronounced for the M2 receptors than for the M1 and M3 receptors. These findings suggest structural differences between the agonist binding sites of M1 and M3 receptors versus the M2 receptors.

Receptor binding properties of amperozide

The receptor pharmacology of amperozide was investigated with in vitro radioligand binding technique. Amperozide possessed a high affinity to the 5-HT2 receptors (Ki = 16.5 +/- 2.1 nM) and a moderate affinity to alpha 1-adrenergic receptors of rat cerebral cortical membranes (Ki = 172 +/- 14 nM). The affinity of amperozide for striatal and limbic dopamine D2 receptors was low and not significantly different (Ki +/- S.E.M. = 540 +/- 59 nM vs 403 +/- 42 nM; p less than 0.11, n = 4). The affinity for striatal and limbic 5-HT2 receptors was measured as well and found to be very close to the affinity to the cerebral cortical 5-HT2 receptor. The drug affinity for D2 and 5-HT2 receptors seems thus not to be influenced by the location of the receptor moiety. The affinity for several other rat brain receptors such as 5-HT1A, alpha 2-adrenergic, dopamine D1, muscarinic M1 and M2, opiate sigma and beta 2-adrenergic was low. The pseudo-Hill coefficient of the amperozide competition binding curve was consistently higher than one indicating antagonistic and complex interactions with the 5-HT2 receptor or with alpha 1-adrenergic and dopamine D2 receptors. The antagonistic properties of amperozide were investigated by its ability to antagonize the serotonin-induced formation of inositol-1-phosphate in human blood platelets. Amperozide inhibited this 5-HT2 receptor-mediated intracellular response with similar potency as ketanserin. These results suggest that amperozide is a selective 5-HT2 receptor antagonist.

Monoaminergic neurotransmitters in Alzheimer's disease. An HPLC study comparing presenile familial and sporadic senile cases

Norepinephrine, epinephrine, dopamine, serotonin and their major metabolites were measured in 20 regions of the left hemisphere in 4 presenile familial cases of Alzheimer-type dementia and 4 sporadic senile cases. Both groups were compared to values in normal brains obtained in our laboratory. Quantitative determination of the monoamines was performed by HPLC with electrochemical detection. The clinical diagnosis of Alzheimer-type dementia was confirmed by histological examination of the right hemisphere and brain stem. The serotonergic system was dramatically affected in the familial cases with very low or undetectable serotonin concentrations in most cortical and subcortical areas studied and an important cell loss in the nucleus raphe dorsalis, origin of the main ascending serotonergic system. In the senile demented patients the serotonergic deficit is less important but still clearly present. The noradrenergic, adrenergic and dopaminergic systems were less affected by the disease process in senile sporadic as well as in the presenile familial type of Alzheimer's disease.

D2 dopamine receptors in the human brain: heterogeneity based on differences in guanine nucleotide effect on agonist binding, and their presence on corticostriatal nerve terminals

In human brain, regulation of agonist binding to the D2 dopamine receptors by guanine nucleotides is different between several regions. In membranes of the anterior pituitary lobe, agonist binding is fully sensitive to GTP or Gpp(NH)p, whereas it is resistant in membranes of globus pallidus. Both guanine nucleotide-sensitive (G-S) and -insensitive (G-I) receptors are found in membranes of the striatum. The G-S and G-I type receptors display similar affinities for antipsychotic drugs of different classes, suggesting that they only differ in their effector-coupling system. We investigated the D2 receptors in striatal membranes of postmortem human brains in which corticostriatal pathways were unilaterally destroyed by an infarction. Compared to the intact side, D2 receptor densities in striata from the lesioned side were reduced by 46-65%, whereas the densities of the muscarinic cholinergic receptors, dopamine uptake sites, and D1 dopamine receptors were unchanged. In the deafferented striata all G-I, but also a substantial number of G-S receptors were lost, suggesting that both receptor subtypes are present on corticostriatal nerve terminals.