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

Up-regulation of nicotinic acetylcholine receptors following chronic exposure of rats to mainstream cigarette smoke or alpha 4 beta 2 receptors to nicotine

Smokers are reported to have a higher density of central nicotinic acetylcholine receptors (nAChRs) that non-smokers at autopsy. Whether this increased receptor density is a response to smoking or a result of genetic variability is not known. While sub-chronic treatment of rats and mice with nicotine results in upregulation of central nAChRs, changes in receptor density in response to cigarette smoke have not been studied previously. In this study, male Sprague-Dawley rats were exposed nose-only for 13 weeks to mainstream cigarette smoke followed by assessment of [3H]nicotine binding in five brain regions of smoke- and sham-exposed animals. In smoke-exposed animals, there was a significant increase in nAChR density in the cortex, striatum, and cerebellum (35, 25, and 31% increases, respectively), while there was no significant change in receptor density in the thalamus and hippocampus. Smoke exposure did not alter markedly the affinity of the receptor for nicotine in these brain regions. Furthermore, up-regulation of nAChRs did not alter the biphasic binding properties by which nicotine binds to its receptor. There were no changes in the association (fast phase) or isomerization (slow phase) rate constants, and the percent contribution of slow and fast phase binding to nAChRs was not altered in the up-regulated receptor population compared with control. Similar results were observed following chronic nicotine exposure of cultured cortical cells from fetal rat brain or cells transfected with the alpha 4 beta 2 nAChR subtype. These results show that the up-regulation following smoke exposure in the rat is phenomenologically similar to that observed in vitro. These data provide preliminary evidence for a relationship between cigarette smoking and nAChR up-regulation in vivo and suggest that similar mechanisms of upregulation may underlie chronic smoke exposure of live animals and nicotine exposure of artificially expressed alpha 4 beta 2 receptors in vitro.

Nicotine enhances the learning and memory of aged rats

The cognitive performance of young adult (2-3-month-old) and aged (22-24 month-old) rats was characterized in one of three different behavioral tasks, and the ability of daily nicotine treatment to alleviate age-related learning and memory deficits was evaluated. Aged rats received an IP injection of either nicotine (0.2 mg/kg) or saline vehicle 15 min prior to daily testing; young control rats received saline vehicle injections. Compared to young controls, aged control rats were severely impaired in the acquisition of one-way active avoidance pole jumping over 12 days of testing. Nicotine pretreatment of similar aged rats markedly improved overall learning and the rate of learning compared to aged controls. In Lashley III maze performance, aged control rats made substantially more alternation errors than young controls; however, aged rats pretreated with nicotine made significantly fewer errors over the entire 20-day test period compared to aged controls. During 30 days of 17-arm radial maze testing, aged control animals were severely impaired in general learning and reference (long-term) memory, but only mildly impaired in working (short-term) memory. Nicotine pretreatment of similar aged rats induced a substantial enhancement in overall learning and reference memory, but did not affect working memory. These results indicate that chronic nicotine administration can improve the impaired learning/memory abilities of aged rats in several tasks, and suggest that stimulation of central nicotinic receptors may be of considerable therapeutic value to treat age-related memory impairment.

Nicotine enhances Morris water maze performance of young and aged rats

We have recently demonstrated that nicotine administration improves the acquisition and/or memory retention of aged rats in 17-arm radial maze, Lashley III maze, and one-way active avoidance testing. The present study extends our evaluation of nicotine's cognition-enhancing potential by determining the effect of nicotine on acquisition and retention of the Morris water maze in young adult (2 to 3 months old) and aged (25 to 26 months old) Sprague-Dawley rats. For 3 days prior to the onset of testing, and 15 min prior to daily testing, rats were treated IP with 0.2 mg/kg nicotine or saline vehicle. Compared to the performance of young adults, vehicle-treated aged rats were impaired in water maze acquisition. Nicotine substantially enhanced the acquisition of aged rats. Furthermore, nicotine significantly improved the memory retention of young adult rats. These cognitive improvements may involve a nicotine-receptor induced increase in generalized alertness and/or a facilitation of higher integrative function. The results suggest that nicotine and/or nicotinic agonists may be useful in treating age-associated memory impairment and/or Alzheimer's disease.

Cholinergic markers in aged cognitively impaired Long-Evans rats

Aged Long-Evans rats (24-25 months old) were classified into cognitively impaired or unimpaired subgroups based on their performances in the Morris Swim Maze task compared to young controls. Using quantitative in vitro receptor autoradiography, we investigated the status of various cholinergic markers in these two groups and in young adults (six months) animals. The apparent density of [3H]pirenzepine (muscarinic M1) sites was similar in the three groups of rats in various cortical areas, subfields of the hippocampus, medial septum and striatum. Similarly, choline acetyltransferase activity and the density of [3H]hemicholinium-3 (high-affinity choline uptake) and [3H]cytisine (nicotinic) binding sites were also unchanged in the brain regions studied between the aged cognitively impaired, unimpaired and young adult rats. In contrast, significant increases in [3H]AF-DX 384 (muscarinic M2) binding density were observed in various cortical areas and in the molecular layer of the dentate gyrus of aged cognitively impaired versus unimpaired rats and in few cortical regions of old as compared to young animals. Therefore, a selective alteration in the regulation of putative M2 receptor sites is apparent, particularly in the aged cognitively impaired rats. Increases in M2 binding sites could lead to a decrease in the capacity to release acetylcholine, as some of the M2 receptors are believed to act as negative autoreceptors. This could influence cognitive functions as selective M2 blockers have recently been reported to facilitate spatial memory in aged impaired rats [Doods et al. (1993) Life Sci. 52, 497-503: Quirion et al. (1995) J. Neurosci. 15, 1455-1462.

Nutrition and its relationship to aging

The aging process alone has no significant adverse consequences for the caloric intake and the nutritional status of healthy elderly individuals. Epidemiological data suggest that in humans, in contrast to rodents, undernutrition reduces the life span. In the Western World, malnutrition in old age has become uncommon and is, for the most part, the result of physical illness and/or of psychological and socio-economic factors, such as depressive disorders, social isolation, smoking, alcohol abuse, and poverty. Body weight shows a U- or J-shaped relationship to mortality risk with the highest survival rates found at normal to moderate overweight. However, studies that have controlled for disease already present, smoking status, serum cholesterol level, or hypertension, suggest an increased mortality risk for lower and upper extremes of body weight, only. Populations with healthy lifestyles have significantly greater life expectancy that the average normal population. Even in the very old, exercise has been shown to improve muscle strength and function. The studies suggest that nutritional intake and nutritional status in old age is multifactorial and dependent not only on appetite and availability of diverse food, but also on physical activity, body mass, education, and an involved social lifestyle.

Attenuation of muscarinic receptor-G-protein interaction in Alzheimer disease

Cortical M1 muscarinic receptor-G-protein coupling, high-affinity, guanine nucleotide-sensitive agonist binding (Flynn et al., 1991; Warpman et al., 1993) and muscarinic receptor-stimulated [3H]PIP2 hydrolysis (Ferrari-DiLeo and Flynn, 1993) are known to be defective in Alzheimer disease. Whether this defect reflects an alteration in the M1 muscarinic receptor, its respective guanine nucleotide binding (G) protein or both is not known. This study compares the number and both basal and muscarinic receptor-mediated function of G-proteins in synaptosomal membranes from cerebral cortical samples of age-matched control subjects and Alzheimer disease patients. Immunoblotting with anti-G alpha q/11 and anti-G beta antibodies demonstrated no alteration in the number of these G-protein subunits in Alzheimer disease. Basal [35S]GTP gamma S binding and hydrolysis of [gamma-32P]GTP by high-affinity GTPase also were not significantly altered in Alzheimer disease compared to control membrane samples. However, muscarinic agonist-stimulated GTP gamma S binding and GTP hydrolysis were significantly reduced (80-100%) in Alzheimer disease cortical samples. Diminished agonist-stimulated GTP gamma S binding and GTP hydrolysis correlated with the loss of guanine nucleotide-sensitive, high-affinity agonist binding (KL/KH) ratio) to the M1 receptor subtype. These data provide further evidence for the loss of muscarinic receptor-G protein coupling in Alzheimer disease and support the hypothesis that muscarinic receptor-mediated cortical activation may be compromised in Alzheimer disease.

Nicotinic cholinergic receptors associated with mammalian cerebral vessels

Current evidence suggests that the cerebral vasculature may be modulated by cholinergic nerves. We used ligand binding methods to examine the presence of nicotinic cholinergic receptors in brain vasculature. We found carbachol-displaceable [3H]acetylcholine (ACh) and [3H]nicotine (NIC) binding sites in preparations of intraparenchymal cerebral microvessels (CMV) and larger pial vessels from human and pig brains. Specific binding sites for [3H]ACh and [3H]NIC in cerebral microvessels were saturable and comparable in density to those in cerebral cortex. The Kds for the two ligands ranged 3-18 nM whereas the Bmaxs were 25-45 fmol/mg protein. In contrast, the binding of [3H]pirenzipine or [3H]quinuclidinyl benzilate, index for muscarinic receptors, was low (9-15% of cortex) in microvessels compared to the cerebral cortex. Our observations suggest the association of cholinergic nicotinic receptors with cerebral microvessels, which may be involved in the modulation of the cerebral circulation by cholinergic neurons.

Cellular distribution of nicotinic acetylcholine receptor subunit mRNAs in the human cerebral cortex as revealed by non-isotopic in situ hybridization

The pharmacology of telencephalic nicotinic acetylcholine receptors (nAChRs) has become an important issue in recent years. While in the human brain a direct pharmacological assessment is difficult to achieve the visualization of nAChRs has been enabled by histochemical techniques providing an ever increasing and improving resolution. Receptor autoradiography was used to visualize binding sites on the level of cortical layers whereas immunohistochemistry has allowed for the cell type-specific and ultrastructural localization of receptor protein. Further investigations have to elucidate the cellular sites of NAChR biosynthesis by visualizing subunit-specific transcripts. Using autopsy samples of the human precentral cortex (Area 4) as a paradigm we have applied digoxigenin-labeled cRNA probes to localize transcripts for the alpha 3- and alpha 4-1-subunits of the nAChR. In accordance with findings in the monkey cortex, the alpha 3-subunit seems to be expressed mainly in pyramidal neurons of layers III-VI of the human cerebral cortex. Transcripts for the alpha 4-1-subunit, by contrast, appear to be present in a large number of neurons throughout all layers of the cerebral cortex, consonant with its ubiquitous distribution in the rodent brain. The present findings show that also in human autopsy brains the cell type-specific detection of nAChR transcripts is possible. For the future, this technique will enable to investigate the expression of receptor transcripts in diseased human brains as compared to controls.

Characterization of muscarinic receptor subtypes in Alzheimer and control brain cortices by selective muscarinic antagonists

Subtypes of muscarinic receptors were characterized in the frontal cortices of control and Alzheimer brains, with labelled quinuclidinyl benzilate [3H]QNB and the unlabelled muscarinic antagonists pirenzepine, AF-DX 116, hexahydro-sila-diphenidol (HHSiD), para-fluoro-hexahydro-sila-diphenidol (p-F-HHSiD) and himbacine. High and low affinity sites were observed for both pirenzepine and AF-DX 116 in human control frontal cortices. The majority (76%) of the pirenzepine binding sites showed high affinity to the muscarinic receptors (M1), while the rest of the binding sites had an affinity that was 40 times less. AF-DX 116 displayed two sets of binding sites where the high affinity AF-DX 116 (M2) sites constituted 27%, while the low affinity AF-DX 116 (non-M2 site) was 73%. A single class of binding sites was observed for HHSiD, p-F-HHSiD and himbacine in human frontal cortices. HHSiD showed an affinity in the frontal cortices that was comparable to that of the pirenzepine high affinity binding (M1) sites. The affinity of p-F-HHSiD was three times lower than that of HHSiD but similar to himbacine. A significant increase in the affinity (+ 40%) as well as in the Bmax (+ 99%) value was observed for the pirenzepine high affinity binding sites (M1) in the frontal cortices of Alzheimer brains compared to controls. Similarly, a significant increase was observed in the Bmax value (+ 60%) for the AF-DX 116 low affinity binding sites (non-M2), while no change was found for the high affinity binding sites (M2).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the concentration of messenger RNA encoding four muscarinic receptor subtypes in control and Alzheimer brains

We determined the concentration of the messenger RNA species which encode four (m1-m4) of the five cloned muscarinic receptors in brains of Alzheimer's disease patients as compared to age-matched controls. Assays were performed using the quantitative method of DNA-excess solution hybridization in the cerebral cortex (frontal, temporal and occipital), hippocampus, nucleus basalis of Meynert and brainstem. The results suggest a statistically significant decrease in the m1 muscarinic receptor message in the temporal and occipital cortex, with no change in other regions. There was no change in the level of mRNA encoding the m2, m3 or m4 receptors in any of the brain regions studied.

Cortical biopsy in Alzheimer's disease: Diagnostic accuracy and neurochemical, neuropathological, and cognitive correlations

Neurochemical assessments were performed on biopsy samples taken from the right frontal lobe of patients diagnosed with Alzheimer's disease (AD), before the implantation of a ventricular catheter and pump assembly for the infusion of bethanechol chloride as an experimental therapy. The pathologically diagnosed patients with AD (n = 35; mean age, 67 +/- 1.5 yr) were compared with a group of samples from normal age-equivalent autopsied controls (n = 22; mean age, 68 +/- 2 yr) and autopsied AD brains (n = 11; mean age, 73 +/- 2 yr). Samples were assayed for choline acetyltransferase (ChAT), acetylcholinesterase, binding to [3H]quinuclidinyl benzilate as an index of total muscarinic cholinergic binding, and [3H]pirenzepine binding as an index of M1 cholinergic receptor subtype binding. Mean levels of ChAT activity were decreased in the biopsied patients to 36% of age-matched autopsied controls. The loss of ChAT activity correlated significantly with the Mini-Mental State Examination, an index of global cognitive function. Mean ChAT activity in autopsied AD cortex was further decreased compared with controls, indicating continuous decline through the course of the disease. Acetylcholinesterase followed a similar, less dramatic decline. No differences were found in [3H]quinuclidinyl benzilate binding or [3H]pirenzepine binding between biopsied and autopsied controls. Neuritic plaque counts did not correlate with either the Mini-Mental State Examination or ChAT activity in the biopsy specimens.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic distribution of [3H]nicotine binding in human cortex: relative abundance in subicular complex

Distinct patterns of [3H]nicotine (3 nM) binding were apparent in various regions of adult human neo- and archicortex. Receptor binding was greatest in the subicular complex--particularly presubiculum--and entorhinal cortex, where it was prominent in the characteristic parvo- and magnocellular islands of these regions and in middle layers of entorhinal cortex. In somatosensory cortex (Brodmann areas 3, 1 and 2) and occipital (area 17) cortex binding was highest in the upper and lower layers, and relatively sparse in the sensory input, layer IV. In primary motor (area 4) and temporal (area 21) cortex, binding in the outer half of the cortical ribbon was denser than that in the inner half and a distinct band was apparent in temporal and cingulate (area 32) in the lower portion of layer III. In prefrontal association cortex the pattern of binding was less distinct although slightly higher in the lower architectonic layers. There was generally little binding in the hippocampus (areas CA1-4) and dentate gyrus with the exception of the stratum lacunosum moleculare in CA2-3 and, to a lesser extent, supra- and subgranule zones of the dentate. These patterns of reactivity, which are distinct from that of the major cortical cholinergic innervation, suggest that the nicotinic receptor, detected using nanomolar concentrations of [3H]nicotine, may primarily be associated with intracortical circuitry in the neocortex. The relatively high density in entorhinal and subicular regions may be related to the extensive phylogenetic development of these regions which has occurred in conjunction with the development of multimodal association circuitry in the human cortex.

Acute nicotinic blockade produces cognitive impairment in normal humans

Single oral doses of the central and peripheral nicotinic antagonist mecamylamine were administered to healthy young normal males in doses of 5, 10, and 20 mg in a placebo-controlled, double-blind study. The 20 mg dose caused a significant increase in errors in the learning condition of the Repeated Acquisition task, producing a slower acquisition curve. The lower doses produced less errors, but more than in the placebo condition. There was no effect of drug on the performance component (retrieval of previously learned information). On the recognition memory task, dose-related increases in false-alarms during the delay period were seen, with little effect on misses or hits. Reaction time measures suggested a dose-related slowing of RT on several tasks. Behavioral effects were minimal and physiologic measures were consistent with dose-related ganglionic blockade. We interpret these results to indicate that acute blockade of nicotinic receptor function can produce measurable and significant cognitive impairment, even in non-smoking normals.