Estradiol treatment in young postmenopausal women with self-reported cognitive complaints: Effects on cholinergic-mediated cognitive performance

OBJECTIVE

Older women are at increased risk of developing Alzheimer's disease compared to men. One proposed reason is that following menopause there is a decline in estrogens. Estrogens are important for cholinergic functioning and attenuate the impact of cholinergic antagonists on cognitive performance in postmenopausal women. Self-reported or subjective cognitive complaints in middle or older age may represent a harbinger of cognitive decline and those who endorse cognitive complaints appear more likely to develop future cognitive impairment. However, the response of individuals with cognitive complaints after menopause to estrogen and the relationship to cholinergic functioning has not been investigated. This study investigated the effect of estrogen treatment using 17β-estradiol on cognitive performance following anticholinergic blockade in postmenopausal women and the relationship of this interaction with the level of self-reported (subjective) postmenopausal cognitive complaints.

METHODS

Forty postmenopausal women (aged 50-60 years) completed a 3-month treatment regimen of either 1 mg oral estradiol or placebo. Participants then completed four challenge days in which they completed cognitive and behavioral tasks after one of four cholinergic antagonist drug conditions (oral mecamylamine (MECA), intravenous scopolamine, combined MECA and scopolamine, or PLC).

RESULTS

Compared to PLC, the estradiol treated group performed worse on attention tasks under cholinergic challenge including the choice reaction time task and the critical flicker fusion task. In addition, participants who endorsed greater cognitive complaints showed reduced performance on the N-back working memory task, regardless of whether they received estradiol treatment.

CONCLUSIONS

The findings of this study indicate that estradiol treatment was unable to mitigate anticholinergic blockade in postmenopausal women with subjective cognitive complaints, and worsened performance on attention tasks. Moreover, the present study suggests that greater levels of cognitive complaints following menopause may be associated with an underlying decline in cholinergic function that may manifest as an inability to compensate during working memory tasks.

Anticholinergic Amnesia is Mediated by Alterations in Human Network Connectivity Architecture

Disrupted cholinergic neurotransmission plays a central role in Alzheimer's disease, medication-induced memory impairment, and delirium. At the systems level, this suggests anticholinergic drugs may alter the activity and interplay of anatomically distributed neural networks critical for memory function. Using a network-sensitive imaging technique (functional connectivity MRI) and a double-blind, crossover design, we examined the consequences of anticholinergic drug administration on episodic memory and functional network architecture in a group of clinically normal elderly. We observed that low-dose scopolamine (0.2 mg IV) decreased episodic memory performance and selectively decreased connectivity strength in 3 of 7 cortical networks. Both memory and connectivity effects were independent of β-amyloid burden. Drug-induced connectivity changes within the Default and Salience networks, as well as reductions in the strength of anticorrelation between these 2 networks, were sufficient to fully statistically mediate the effects of scopolamine on memory performance. These results provide experimental support for the importance of the Default and Salience networks to memory performance and suggest scopolamine-induced amnesia is underpinned by disrupted connectivity within and between these 2 networks. More broadly, these results support the potential utility of fcMRI as tool examine the systems-level pharmacology of psychoactive drugs.

Attenuation in rats of impairments of memory by scopolamine, a muscarinic receptor antagonist, by mecamylamine, a nicotinic receptor antagonist

RATIONALE

Scopolamine, a muscarinic antagonist, impairs learning and memory for many tasks, supporting an important role for the cholinergic system in these cognitive functions. The findings are most often interpreted to indicate that a decrease in postsynaptic muscarinic receptor activation mediates the memory impairments. However, scopolamine also results in increased release of acetylcholine in the brain as a result of blocking presynaptic muscarinic receptors.

OBJECTIVES

The present experiments assess whether scopolamine-induced increases in acetylcholine release may impair memory by overstimulating postsynaptic cholinergic nicotinic receptors, i.e., by reaching the high end of a nicotinic receptor activation inverted-U dose-response function.

RESULTS

Rats tested in a spontaneous alternation task showed dose-dependent working memory deficits with systemic injections of mecamylamine and scopolamine. When an amnestic dose of scopolamine (0.15 mg/kg) was co-administered with a subamnestic dose of mecamylamine (0.25 mg/kg), this dose of mecamylamine significantly attenuated the scopolamine-induced memory impairments. We next assessed the levels of acetylcholine release in the hippocampus in the presence of scopolamine and mecamylamine. Mecamylamine injections resulted in decreased release of acetylcholine, while scopolamine administration caused a large increase in acetylcholine release.

CONCLUSIONS

These findings indicate that a nicotinic antagonist can attenuate impairments in memory produced by a muscarinic antagonist. The nicotinic antagonist may block excessive activation of nicotinic receptors postsynaptically or attenuate increases in acetylcholine release presynaptically. Either effect of a nicotinic antagonist-to decrease scopolamine-induced increases in acetylcholine output or to decrease postsynaptic acetylcholine receptor activation-may mediate the negative effects on memory of muscarinic antagonists.

Neurochemical changes observed by in vivo proton magnetic resonance spectroscopy in the mouse brain postadministration of scopolamine

RATIONALE AND OBJECTIVES

This study is aimed at investigating neurochemical changes in scopolamine (SCP)-induced memory impairment using spatially localized in vivo magnetic resonance spectroscopy (MRS) of the hippocampus.

MATERIALS AND METHODS

Four groups of mice (eight mice per group) were scanned after the injection of different SCP doses: 0, 1, 3, and 5 mg/kg (intraperitoneally). All the animals received (1)H MRS of their hippocampus at two time intervals: 30 minutes and 72 hours after SCP injection.

RESULTS

This work demonstrated that the doses of 3 mg/kg SCP or higher reduce the concentration of total choline-containing compounds, and these levels returned to baseline after 72 hours. These results are consistent with observations made by others using more invasive brain dialysis approaches. The levels of glutamate and glutamic compounds (glutamate + glutamine) were slightly changed at 3 and 5 mg/kg SCP dose, but the differences were not statistically significant (P > .05). These findings suggest that SCP produces transient, in vivo measurable alterations in the cholinergic system in the hippocampus.

CONCLUSIONS

On this basis, we conclude that in vivo MRS is a feasible noninvasive method to probe aspects of the alterations induced by SCP in the cholinergic neurotransmission pathways in both animal models and human studies of memory impairment.

Additive effects of a cholinesterase inhibitor and a histamine inverse agonist on scopolamine deficits in humans

RATIONALE

Enhancement of histaminergic neurotransmission or histaminergic plus cholinergic neurotransmission may represent novel strategies for improving cognition in Alzheimer's disease.

OBJECTIVE

To evaluate the effects of a novel histamine H3 receptor inverse agonist (MK-3134), an acetylcholinesterase inhibitor (donepezil), and their combination in attenuating the cognitive impairment associated with scopolamine.

METHODS

Thirty-one subjects were randomized, and 28 completed this double-blind, placebo-controlled, five-period crossover study. Cognition was assessed using the Groton Maze Learning Task (GMLT) as the primary outcome measure. The two primary hypotheses were that donepezil 10 mg and MK-3134 25 mg, respectively, would attenuate scopolamine (0.5 mg)-induced impairment as measured by the GMLT over the first 12 h after scopolamine administration (AUC(1-12)  (h)). A secondary hypothesis was that the combination of donepezil and MK-3134 would attenuate scopolamine-induced cognitive impairment to a greater extent than either agent alone as measured by the GMLT AUC(1-12 h).

RESULTS

The primary and secondary hypotheses were not met. Upon examining the time course of the scopolamine effects (an exploratory objective), peak effects were generally observed around 2 h after scopolamine administration. Administration of MK-3134 or donepezil improved performance on the GMLT at the 2-h time point, rather than AUC(1-12 h), compared with scopolamine alone. Moreover, it appeared that the combination of MK-3134 and donepezil blunted the scopolamine effect to a greater extent than either drug alone.

CONCLUSIONS

Exploratory analyses provide evidence for cognitive improvement through inverse agonism of the H3 histamine receptor and for cooperation between human cholinergic and histaminergic neurotransmitter systems. (ClinicalTrials.gov trial registration number: NCT01181310).

Cholinergic receptor subtypes and their role in cognition, emotion, and vigilance control: an overview of preclinical and clinical findings

RATIONALE

The cholinergic system has long been linked to cognitive processes. Two main classes of acetylcholine (ACh) receptors exist in the human brain, namely muscarinic and nicotinic receptors, of which several subtypes occur.

OBJECTIVES

This review seeks to provide an overview of previous findings on the influence of cholinergic receptor manipulations on cognition in animals and humans, with particular emphasis on the role of selected cholinergic receptor subtypes. Furthermore, the involvement of these receptor subtypes in the regulation of emotion and brain electrical activity as measured by electroencephalography (EEG) shall be addressed since these domains are considered to be important modulators of cognitive functioning.

RESULTS

In regard to cognition, the muscarinic receptor subtypes have been implicated mainly in memory functions, but have also been linked to attentional processes. The nicotinic α7 receptor subtype is involved in working memory, whereas the α4β2* subtype has been linked to tests of attention. Both muscarinic and nicotinic cholinergic mechanisms play a role in modulating brain electrical activity. Nicotinic receptors have been strongly associated with the modulation of depression and anxiety.

CONCLUSIONS

Cholinergic receptor manipulations have an effect on cognition, emotion, and brain electrical activity as measured by EEG. Changes in cognition can result from direct cholinergic receptor manipulation or from cholinergically induced changes in vigilance or affective state.

Modes and models of forebrain cholinergic neuromodulation of cognition

As indicated by the profound cognitive impairments caused by cholinergic receptor antagonists, cholinergic neurotransmission has a vital role in cognitive function, specifically attention and memory encoding. Abnormally regulated cholinergic neurotransmission has been hypothesized to contribute to the cognitive symptoms of neuropsychiatric disorders. Loss of cholinergic neurons enhances the severity of the symptoms of dementia. Cholinergic receptor agonists and acetylcholinesterase inhibitors have been investigated for the treatment of cognitive dysfunction. Evidence from experiments using new techniques for measuring rapid changes in cholinergic neurotransmission provides a novel perspective on the cholinergic regulation of cognitive processes. This evidence indicates that changes in cholinergic modulation on a timescale of seconds is triggered by sensory input cues and serves to facilitate cue detection and attentional performance. Furthermore, the evidence indicates cholinergic induction of evoked intrinsic, persistent spiking mechanisms for active maintenance of sensory input, and planned responses. Models have been developed to describe the neuronal mechanisms underlying the transient modulation of cortical target circuits by cholinergic activity. These models postulate specific locations and roles of nicotinic and muscarinic acetylcholine receptors and that cholinergic neurotransmission is controlled in part by (cortical) target circuits. The available evidence and these models point to new principles governing the development of the next generation of cholinergic treatments for cognitive disorders.

Muscarinic agonists in Alzheimer's disease

As therapeutic agents, M1 agonists in the short-term may palliate symptoms of AD and improve memory function. In the long-term, M1 agonists have the potential to modify the underlying pathophysiology of AD, and thereby prevent or retard the course of dementia.

Age-related decline in central cholinergic function demonstrated with scopolamine

Scopolamine hydrobromide was administered intravenously to 23 normal subjects (40-89 years) in doses of 0.1 mg, 0.25 mg, and 0.5 mg, in a double-blind. Placebo-controlled, random-order fashion. The effects of scopolamine, as compared to placebo, were assessed using a comprehensive cognitive test battery, as well as behavioral and physiological measures. Scopolamine produced the expected dose-dependent impairments in most of the cognitive functions assessed. Behavioral and physiological measures were also affected, but only minimally. More importantly, there was a significant overall correlation between age and scopolamine-impaired performances on psychomotor speed, short-term recall, visual tracking speed, visuo-motor coordination, and sequencing ability. There was, however, some inter-individual variability in this phenomenon. The results provide further evidence that cholinergically mediated cognitive functions show an increased sensitivity to scopolamine with age, albeit with heterogeneity that bears further investigation.

Reduced evoked release of acetylcholine in the rodent hippocampus following traumatic brain injury

The chronic effects of traumatic brain injury on acetylcholine release were evaluated by using in vivo microdialysis. Acetylcholine release was measured in the hippocampus of anesthetized rats 2 weeks after lateral controlled cortical impact (n = 10) or sham surgery (n = 10). Prior to microdialysis, behavioral assessments of motor and spatial memory were performed. Cortical impact (6 meter/s, 2 mm deformation) produced beam balance deficits that persisted for 1 day and beam walking deficits that persisted for 3 days after injury. In addition, spatial memory, as measured by swim latencies in a Morris water maze, was compromised between 10-14 days after injury. Immediately following behavioral testing, the animals were anesthetized with halothane, and a microdialysis probe was placed into the dorsal hippocampus. After a 160 min equilibration period, extracellular levels of acetylcholine were measured prior to and after an intraperitoneal administration of scopolamine (1 mg/kg), which evokes acetylcholine release by blocking autoreceptors. Prior to scopolamine administration, there were no differences in extracellular levels of acetylcholine between injured and sham animals. However, there was a significant reduction of hippocampal acetylcholine release evoked by scopolamine in injured animals as compared to sham controls. In separate control groups, saline administration alone did not change hippocampal acetylcholine release in injured (n = 5) or sham (n = 5) animals. This study represents the first application of in vivo microdialysis to evaluate chronic neurotransmission deficits following TBI. The present study demonstrates that a magnitude of traumatic brain injury (TBI) sufficient to produce spatial memory deficits can result in a reduction in scopolamine-evoked release of acetylcholine within the hippocampus. The data further suggest that presynaptic mechanisms mediating release of acetylcholine could play a significant role in cholinergic neurotransmission deficits following TBI.

Time course of increased vulnerability of cholinergic neurotransmission following traumatic brain injury in the rat

We have previously shown that spatial memory changes following experimental traumatic brain injury (TBI) include long-term changes that are (1) 'overt': detected by routine behavioral assessments, or (2) 'covert': undetected in the absence of a secondary pharmacological challenge, such as by the cholinergic antagonist, scopolamine. Our objective in this study was to extend this finding by characterizing the time course of recovery of overt and covert spatial memory performance following two magnitudes of experimental TBI. The Morris water maze was used to assess cognitive performance. Rats received either moderate magnitude (6 m/s, 1.77 mm deformation) or low magnitude (6 m/s, 1 mm deformation) impacts through a lateral craniectomy under isoflurane anesthesia. Sham rats underwent identical surgical procedures but were not injured. To avoid motor deficits, water maze testing started two weeks post-injury. Rats were given four trials per day for seven consecutive days. For each trial, latency to find a hidden platform was timed. On the sixth, rats were injected (i.p.) with scopolamine (1 mg/kg) 15 min prior to maze testing. The next day, rats were retested. This testing regimen was repeated, beginning 4, 6, and 10 weeks post-TBI. Results showed that, while the low-magnitude injury produced no overt spatial memory deficits, the moderate-magnitude group exhibited overt deficits during the first test regimen. Also, while both injury magnitudes produced an enhanced sensitivity to spatial memory impairment by scopolamine at two weeks post-TBI, this covert deficit persisted only in the severe group at 4, 6, and 10 weeks post-TBI. Qualitative light microscopy showed that both injury groups had graded cortical necrosis. However, underlying subcortical structures such as the hippocampus appeared intact, with no overt cellular or parenchymal damage to the neuropil. These data suggest three distinct stages of functional recovery: (1) the initial period when overt deficits are present, (2) a period following recovery from overt deficits within which covert deficits can be reinstated by a pharmacological challenge, and (3) a period following recovery from both overt and covert deficits. Covert deficits can persist long after the recovery of overt deficits and, like other neurological deficits, the rate of recovery is dependent on the magnitude of TBI. Finally, spatial memory deficits can occur in the absence of light microscopic evidence of cell death in the hippocampus.

Differential cholinergic regulation in Alzheimer's patients compared to controls following chronic blockade with scopolamine: a SPECT study

The effects of low-dose chronic scopolamine on measures of cerebral perfusion and muscarinic receptors were tested in eight Alzheimer's disease (AD) subjects and eight elderly controls. Single photon emission computed tomography (SPECT) scans using technetium-labelled hexamethypropylene amine oxide (99mTc-HMPAO) to measure cerebral perfusion before and after chronic scopolamine revealed a significant 12% increase in the normal controls (P < 0.01) while the AD subjects showed no significant change. In contrast, the controls showed decreased muscarinic binding as evidenced by 123I-quinuclidinyl-4-iodobenzilate (123I-QNB) labelling after chronic drug (-10%, P < 0.01) whereas the AD subjects showed increased 123I-QNB labelling (+8%, P < 0.05). The difference between AD and control subjects was even more marked when the ratio of I-QNB to HMPAO uptake was compared, pointing to a double dissociation in the SPECT results. These data cannot be explained by group differences in cerebral perfusion alone and suggest a differential sensitivity between AD and elderly controls to chronic cholinergic blockade.

Increased anticholinergic sensitivity following closed skull impact and controlled cortical impact traumatic brain injury in the rat

Evidence suggests that prolonged memory deficits in several neurodegenerative diseases are attributable to deficits in central cholinergic neurotransmission. In traumatic brain injury (TBI), such cholinergic deficits also may contribute to prolonged memory disturbances. This study determined whether moderate magnitudes of TBI produced by controlled cortical impact and mild magnitudes of experimental TBI produced by a new closed head impact technique in rats would produce an enhanced vulnerability to the memory disruptive effects of scopolamine, a muscarinic cholinergic receptor antagonist. Water maze performance was used to determine changes in cholinergic hippocampal function following TBI. In the first experiment, rats received a moderate level of TBI by means of a controlled cortical impact. A Morris water maze task assessed spatial memory function on days 30-34 postinjury. During the 5 day assessment period, statistical analyses showed a group main effect for swim latency. Subsequent post hoc analyses indicated that injured rats had significantly longer latencies on days 30 and 31 (p < 0.05, injury vs sham controls). By days 32-35, injured rats showed no statistically significant deficits in spatial memory performance. On day 35, scopolamine (1 mg/kg, IP) was injected into injured rats and sham-injured rats 15 min prior to being retested in the maze. Results showed that although the scopolamine had no effects on the performance of the sham-injured rats, the same dose significantly (p < 0.05) increased the latency to find the hidden platform in the injured group. In the second experiment, rats received a mild concussive closed head impact. Water maze performance was assessed on days 8-12 postinjury. No significant water maze performance deficits were observed. On day 13, injured and uninjured rats were pharmacologically challenged with scopolamine (1 mg/kg) and retested. Similar to the first experiment, injured rats manifested a significantly greater (p < 0.05) sensitivity to scopolamine than sham controls. The results from both experiments suggest that concussive and more severe levels of TBI can produce an enhanced vulnerability to disruption of cholinergically mediated memory function, even when memory function appears normal in the absence of secondary challenges. These data demonstrate that covert deficits can persist after the recovery of normal function. These deficits may be attributable to a decrease in the ability of cholinergic neurons to function properly. These data also provide important insights into features of receptor-coupled disturbances that could contribute to the maintenance of enduring cognitive deficits following TBI.

Neuronal mechanisms of the attentional dysfunctions in senile dementia and schizophrenia: two sides of the same coin?

Deficits in early stages of information processing, specifically the inability to "disattend" irrelevant stimuli and to selectively allocate processing resources (i.e., hyperattention), have been associated with the development of psychotic symptoms. Opposite deficits, i.e., the failure to attend and select stimuli, and to divide attention (i.e., hypoattention), represent a major variable in the development of dementia. The hypothesis that hyperattention and hypoattention are mediated via cortical cholinergic hyperactivity and hypoactivity, respectively, is discussed. Several lines of evidence support the role of cholinergic hyperactivity in the development of psychotic symptoms, including the therapeutic effects of anticholinergic drugs in schizophrenic patients, the psychotic effects of chronic exposure to irreversible cholinesterase inhibitors, and the worsening of psychotic symptoms as a result of the treatment with cholinomimetic compounds. The potent impairments of attentional abilities as a result of the administration of muscarinic antagonists in intact subjects, and the attentional effects of cholinomimetic compounds in demented patients are two examples of the evidence that supports the role of cholinergic hypofunction in the cognitive impairments of dementia. A neuronal model of dopamine-GABAergic modulation of cortical acetylcholine is proposed on the basis of evidence indicating that nucleus accumbens dopamine, via a GABAergic pathway to the substantia innominata of the basal forebrain, modulates cortical acetylcholine release. The available evidence confirms several predictions derived from this model, including the dopaminergic regulation of cortical acetylcholine (ACh) release, the bidirectional modulation of this release by benzodiazepine receptor (BZR) agonists and inverse agonists, and the antipsychotic effects of BZR agonists. Bidirectional deviations in the activity of cortical cholinergic inputs are hypothesized to represent a major neuronal substrate of the attentional dysfunctions associated with, or even underlying, the development of psychotic symptoms and dementia.

Aniracetam improves behavioural responses and facilitates signal transduction in the rat brain

The effect of aniracetam (10, 50, 100 mg/kg i.p. daily for 15 days) on both behavioural and biochemical parameters was investigated in the adult rat. Animals given aniracetam (50 mg/kg 1 h before the trial) showed a significant increase in the percentage of conditioned active avoidance responses and a reduction of latency times. Aniracetam significantly counteracted the scopolamine-induced memory failure at the passive avoidance (step down) test, while it did not modify the locomotion of the animals. In purified frontocortical and hippocampal synaptic membranes of rats treated with aniracetam (50 mg/kg i.p. daily for 15 days) a potentiation of basal, carbamylcholine-, dopamine- and norepinephrine-stimulated adenylyl cyclase activity was observed, while forskolin-stimulated enzyme activity was not modified. With regard to inositol phosphate production measured in fronto-cortical synaptoneurosomes, aniracetam potentiated the stimulation by angiotensin II, while the stimulation by carbamylcholine, not affected by 10 and 50 mg/kg aniracetam, was notably, although not significantly, decreased by 100 mg/kg aniracetam. Furthermore, in synaptosomes derived from hippocampus, aniracetam (50 mg/kg i.p. daily for 15 days) caused an increase of both basal and K(+)-stimulated intrasynaptosomal Ca(2+) concentration. In conclusion, a correlation between the improvement of behavioural performance and the modulation of transducing systems by aniracetam seems to take place in brain areas, such as frontal cortex and hippocampus, known to play a major role in the control of cognitive functions.

Toward modeling age-related changes of attentional abilities in rats: Simple and choice reaction time tasks and vigilance

Fischer-344 rats aged 4, 12, or 18 months were trained in a simple or choice reaction time task (SRTT; CRTT). Animals were required to detect a brief (50 ms), rarely, and unpredictably occurring signal that was presented either at the central panel light (SRTT) or above one of the two levers (CRTT). Animals reported detection by pressing either lever (SRTT) or the cued lever (CRTT) within 3 s. False alarm rates were obtained from a nonsignal 3-s bin. In comparison to younger animals, 18-month-old animals showed a reduced signal detectability, and this effect did not interact with practice. These results suggest that age affected vigilance and practice did not attenuate this effect. The benzodiazepine receptor agonist chlordiazepoxide (at subsedative doses; 1, 3, and 5 mg/kg) and the beta-carboline ZK 93 426 (1, 3, and 5 mg/kg) failed to affect signal detectability. Scopolamine HBr and MBr impaired detectability and responsivity to a similar extent. However, scopolamine MBr, unlike the tertiary compound, failed to affect response accuracy in the CRTT. It is speculated that the failure of chlordiazepoxide to affect performance was related to low processing demands of both tasks. Although these behavioral models show good face validity, they do not allow determination of the major components of attentional processes (perceptual sensitivity, response criterion, processing capacity). Animal behavioral paradigms that allow determination of such components are required for the investigation of the neuronal basis of age-related changes in attentional abilities.

Behavioral screening for cognition enhancers: from indiscriminate to valid testing: Part I

Preclinical efforts to detect and characterize potential cognition enhancers appear to have been dominated by a strategy of demonstrating a wide variety of apparently beneficial behavioral effects with little attention given to the specific psychological mechanisms underlying behavioral enhancement. In particular, the question of whether or not behavioral facilitation is based on relevant mnemonic mechanisms and is independent of the stimulus properties and/or the motivational and attentional components of a task is not often considered. As a result, an overwhelming number of compounds have failed to produce the clinical effects predicted for them on the basis of preclinical research. The available data suggest that a more successful approach requires deductive research strategies rather than the indiscriminate accumulation of apparently beneficial effects in a variety of behavioral tasks and animal models. The first step towards such an approach is a systematic and rigorous evaluation of the different aspects of validity for the models most frequently used in preclinical research. It is concluded that a combination of good construct validity and good face validity represents a necessary condition for screening tests with predictive validity, and that the most popular paradigms fail to fulfil these criteria. Future screening programs for cognition enhancers will probably be characterized by a depreciation of "fast and dirty tests" in favor of approaches focussing on the validity of the effects of potential cognition enhancers.

The potential for muscarinic receptor subtype-specific pharmacotherapy for Alzheimer's disease

In several neurodegenerative disorders, including Alzheimer's disease, a loss of the cholinergic projections of the basal forebrain to the cerebral cortex and hippocampus occurs. Studies of the anatomic and physiologic characteristics of these ascending cholinergic systems suggest that they are important in processing information and in memory function. Muscarinic receptors are situated at various critical control points in these pathways. Activation of postsynaptic muscarinic receptors often increases the excitability of neurons; thus, the signal-to-noise ratio for sensory processing is enhanced. In addition, muscarinic receptors negatively control cholinergic tone at presynaptic sites. Molecular biologic methods have disclosed the existence of five muscarinic receptors, which are coupled to different second messenger systems. The evidence reviewed suggests that at least four of the five muscarinic receptor genes are expressed as functional receptor proteins in the neocortex and hippocampal formation. On the basis of the current information about their pharmacologic properties and coupling mechanisms in nervous tissue, drugs that selectively affect subtypes of muscarinic receptors could enhance cortical cholinergic function and thereby ameliorate certain cognitive impairments in Alzheimer's disease.