Potassium, but not atropine-stimulated cortical acetylcholine efflux, is reduced in aged rats

Changes in neuromuscular structure and functions of human colon during ageing are region-dependent

OBJECTIVE

To determine if human colonic neuromuscular functions decline with increasing age.

DESIGN

Looking for non-specific changes in neuromuscular function, a standard burst of electrical field stimulation (EFS) was used to evoke neuronally mediated (cholinergic/nitrergic) contractions/relaxations in ex vivomuscle strips of human ascending and descending colon, aged 35-91 years (macroscopically normal tissue; 239 patients undergoing cancer resection). Then, to understand mechanisms of change, numbers and phenotype of myenteric neurons (30 306 neurons stained with different markers), densities of intramuscular nerve fibres (51 patients in total) and pathways involved in functional changes were systematically investigated (by immunohistochemistry and use of pharmacological tools) in elderly (≥70 years) and adult (35-60 years) groups.

RESULTS

With increasing age, EFS was more likely to evoke muscle relaxation in ascending colon instead of contraction (linear regression: n=109, slope 0.49%±0.21%/year, 95% CI), generally uninfluenced by comorbidity or use of medications. Similar changes were absent in descending colon. In the elderly, overall numbers of myenteric and neuronal nitric oxide synthase-immunoreactive neurons and intramuscular nerve densities were unchanged in ascending and descending colon, compared with adults. In elderly ascending, not descending, colon numbers of cell bodies exhibiting choline acetyltransferase immunoreactivity increased compared with adults (5.0±0.6 vs 2.4±0.3 neurons/mm myenteric plexus, p=0.04). Cholinergically mediated contractions were smaller in elderly ascending colon compared with adults (2.1±0.4 and 4.1±1.1 g-tension/g-tissue during EFS; n=25/14; p=0.04); there were no changes in nitrergic function or in ability of the muscle to contract/relax. Similar changes were absent in descending colon.

CONCLUSION

In ascending not descending colon, ageing impairs cholinergic function.

Basal forebrain chemogenetic inhibition disrupts the superior complex movement control of goal-tracking rats

Sign- and goal-tracking behavior signifies the influence of opposed cognitive-motivational styles, with the former being characterized by a tendency for approaching and contacting reward cues, including a readiness for attending, bottom-up, to salient cues, and a relatively greater vulnerability for developing and maintaining addiction-like behaviors. We previously demonstrated that these styles also impact the cognitive-motor interactions that are taxed during traversal of dynamic surfaces, with goal-trackers (GTs) making less movement errors and falling less frequently than sign-trackers (STs). The present experiment tested the hypothesis that complex movement control in GTs, but not STs, depends on activation of the basal forebrain projection system to telencephalic regions. Chemogenetic inhibition of the basal forebrain increased movement errors and falls in GTs during traversal of a rotating zigzag rod but had no significant effect on the relatively lower performance of STs. Neurochemical evidence confirmed the efficacy of the inhibitory designer receptor exclusively activated by designer drug (DREADD). Administration of clozapine-N-oxide (CNO) had no significant effect in GTs not expressing the DREADD. These results indicate that GTs, but not STs, activate the basal forebrain projection system to mediate their relatively superior ability for complex movement control. STs may also serve as an animal model in research on the role of basal forebrain systems in aging- and Parkinson's disease-associated falls. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

In Vivo Chemical Monitoring at High Spatiotemporal Resolution Using Microfabricated Sampling Probes and Droplet-Based Microfluidics Coupled to Mass Spectrometry

An essential approach for in vivo chemical monitoring is to use sampling probes coupled with analytical methods; however, this method traditionally has limited spatial and temporal resolution. To address this problem, we developed an analytical system that combines microfabricated push-pull sampling probes with droplet-based microfluidics. The microfabricated probe provides spatial resolution approximately 1000-fold better than that of common microdialysis probes. Microfabrication also facilitated integration of an extra channel into the probe for microinjection. We created microfluidic devices and interfaces that allowed manipulation of nanoliter droplet samples collected from the microfabricated probe at intervals of a few seconds. Use of droplet-based microfluidics prevented broadening of collected zones, yielding 6 s temporal resolution at 100 nL/min perfusion rates. Resulting droplets were analyzed by direct infusion nanoelectrospray ionization (nESI) mass spectrometry for simultaneous determination of glutamine, glutamate, γ-aminobutyric acid, and acetylcholine. Use of low infusion rates that enabled nESI (50 nL/min) was critical to allowing detection in the complex samples. Addition of ¹³C-labeled internal standards to the droplet samples was used for improved quantification. Utility of the overall system was demonstrated by monitoring dynamic chemical changes evoked by microinjection of high potassium concentrations into the brain of live rats. The results showed stimulated neurochemical release with rise times of 15 s. This work demonstrates the potential of coupling microfabricated sampling probes to droplet-based mass spectrometric assays for studying chemical dynamics in a complex microenvironment at high spatiotemporal resolution.

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.

The effects of postnatal alcohol exposure and galantamine on the context pre-exposure facilitation effect and acetylcholine efflux using in vivo microdialysis

Fetal alcohol spectrum disorders (FASD) are characterized by damage to multiple brain regions, including the hippocampus, which is involved in learning and memory. The acetylcholine neurotransmitter system provides major input to the hippocampus and is a possible target of developmental alcohol exposure. Alcohol (3.0 g/kg/day) was administered via intubation to male rat pups (postnatal day [PD] 2-10; ethanol-treated [ET]). Controls received a sham intubation (IC) or no treatment (NC). Acetylcholine efflux was measured using in vivo microdialysis (PD 32-35). ET animals were not different at baseline, but had decreased K(+)/Ca(2+)-induced acetylcholine efflux compared to NC animals and an enhanced acetylcholine response to galantamine (acetylcholinesterase inhibitor; 2.0 mg/kg) compared to both control groups. A separate cohort of animals was tested in the context pre-exposure facilitation effect task (CPFE; PD 30-32) following postnatal alcohol exposure and administration of galantamine (2.0 mg/kg; PD 11-30). Neither chronic galantamine nor postnatal alcohol exposure influenced performance in the CPFE task. Using immunohistochemistry, we found that neither alcohol exposure nor behavioral testing significantly altered the density of vesicular acetylcholine transporter or alpha7 nicotinic acetylcholine receptor in the ventral hippocampus (CA1). In the medial septum, the average number of choline acetyltransferase (ChAT+) cells was increased in ET animals that displayed the context-shock association; there were no changes in IC and NC animals that learned the context-shock association or in any animals that were in the control task that entailed no learning. Taken together, these results indicate that the hippocampal acetylcholine system is significantly disrupted under conditions of pharmacological manipulations (e.g., galantamine) in alcohol-exposed animals. Furthermore, ChAT was up‑regulated in ET animals that learned the CPFE, which may account for their ability to perform this task. In sum, developmental alcohol exposure may disrupt learning and memory in adolescence via a cholinergic mechanism.

Behavioral-Cognitive Targets for Cholinergic Enhancement

Cholinergic mechanisms have long been considered a promising target for enhancing cognitive functions. Two distinct yet interacting components of cholinergic activity have been proposed to mediate specific cognitive functions. Transient spikes in cholinergic activity mediate the detection of cues in situations involving attentional mode shifts. More slowly changing cholinergic neuromodulation of cortical circuitry regulates task compliance specifically in response to performance challenges. Increases in cholinergic neuromodulation enhances the generation of cholinergic transients via stimulation of α4β2* nicotinic acetylcholine receptors. Stimulation of these receptors stabilizes attentional performance and increases cue detection rates. Adjunctive treatment with agonists or modulators at these receptors is predicted to benefit unstable attentional performance and low cue detection rates that are common to several brain disorders.

Monitoring cholinergic activity during attentional performance in mice heterozygous for the choline transporter: a model of cholinergic capacity limits

Reductions in the capacity of the human choline transporter (SLC5A7, CHT) have been hypothesized to diminish cortical cholinergic neurotransmission, leading to risk for cognitive and mood disorders. To determine the acetylcholine (ACh) release capacity of cortical cholinergic projections in a mouse model of cholinergic hypofunction, the CHT+/- mouse, we assessed extracellular ACh levels while mice performed an operant sustained attention task (SAT). We found that whereas SAT-performance-associated increases in extracellular ACh levels of CHT+/- mice were significantly attenuated relative to wildtype littermates, performance on the SAT was normal. Tetrodotoxin-induced blockade of neuronal excitability reduced both dialysate ACh levels and SAT performance similarly in both genotypes. Likewise, lesions of cholinergic neurons abolished SAT performance in both genotypes. However, cholinergic activation remained more vulnerable to the reverse-dialyzed muscarinic antagonist atropine in CHT+/- mice. Additionally, CHT+/- mice displayed greater SAT-disrupting effects of reverse dialysis of the nAChR antagonist mecamylamine. Receptor binding assays revealed a higher density of α4β2* nAChRs in the cortex of CHT+/- mice compared to controls. These findings reveal compensatory mechanisms that, in the context of moderate cognitive challenges, can overcome the performance deficits expected from the significantly reduced ACh capacity of CHT+/- cholinergic terminals. Further analyses of molecular and functional compensations in the CHT+/- model may provide insights into both risk and resiliency factors involved in cognitive and mood disorders.

Neuroanatomical changes associated with cognitive aging

The literature on the neuroanatomical changes that occur during normal, non-demented aging is reviewed here with an emphasis on the improved accuracy of studies that use stereological techniques. Loss of neural tissue involved in cognition occurs during aging of humans as well as the other mammals that have been examined. There is considerable regional specificity within the cerebral cortex and the hippocampus in both the degree and cellular basis for loss. The anatomy of the prefrontal cortex is especially vulnerable to the effects of aging while the major subfields of the hippocampus are not. A loss of neurons, dendrites and synapses has been documented, as well as changes in neurotransmitter systems, in some regions of the cortex and hippocampus but not others. Species differences are also apparent in the cortical white matter and the corpus callosum where there are indications of loss of myelin in humans, but most evidence favors preservation in rats. The examination of whether the course of neuroanatomical aging is altered by hormone replacement in females is just beginning. When hormone replacement is started close to the time of cycle cessation, there are indications in humans and rats that replacement can preserve neural tissue but there is some variability due to the type of hormones and regimen of administration.

Galanthamine plus estradiol treatment enhances cognitive performance in aged ovariectomized rats

We hypothesize that beneficial effects of estradiol on cognitive performance diminish with age and time following menopause due to a progressive decline in basal forebrain cholinergic function. This study tested whether galanthamine, a cholinesterase inhibitor used to treat memory impairment associated with Alzheimer's disease, could enhance or restore estradiol effects on cognitive performance in aged rats that had been ovariectomized in middle-age. Rats were ovariectomized at 16-17 months of age. At 21-22 months of age rats began receiving daily injections of galanthamine (5mg/day) or vehicle. After one week, half of each group also received 17ß-estradiol administered subcutaneously. Rats were then trained on a delayed matching to position (DMP) T-maze task, followed by an operant stimulus discrimination/reversal learning task. Treatment with galanthamine+estradiol significantly enhanced the rate of DMP acquisition and improved short-term delay-dependent spatial memory performance. Treatment with galanthamine or estradiol alone was without significant effect. Effects were task-specific in that galanthamine+estradiol treatment did not significantly improve performance on the stimulus discrimination/reversal learning task. In fact, estradiol was associated with a significant increase in incorrect responses on this task after reversal of the stimulus contingency. In addition, treatments did not significantly affect hippocampal choline acetyltransferase activity or acetylcholine release. This may be an effect of age, or possibly is related to compensatory changes associated with long-term cholinesterase inhibitor treatment. The data suggest that treating with a cholinesterase inhibitor can enhance the effects of estradiol on acquisition of a DMP task by old rats following a long period of hormone deprivation. This could be of particular benefit to older women who have not used hormone therapy for many years and are beginning to show signs of mild cognitive impairment. Potential mechanisms for these effects are discussed.

Estrogen therapy and cognition: a review of the cholinergic hypothesis

The pros and cons of estrogen therapy for use in postmenopausal women continue to be a major topic of debate in women's health. Much of this debate focuses on the potential benefits vs. harm of estrogen therapy on the brain and the risks for cognitive impairment associated with aging and Alzheimer's disease. Many animal and human studies suggest that estrogens can have significant beneficial effects on brain aging and cognition and reduce the risk of Alzheimer's-related dementia; however, others disagree. Important discoveries have been made, and hypotheses have emerged that may explain some of the inconsistencies. This review focuses on the cholinergic hypothesis, specifically on evidence that beneficial effects of estrogens on brain aging and cognition are related to interactions with cholinergic projections emanating from the basal forebrain. These cholinergic projections play an important role in learning and attentional processes, and their function is known to decline with advanced age and in association with Alzheimer's disease. Evidence suggests that many of the effects of estrogens on neuronal plasticity and function and cognitive performance are related to or rely upon interactions with these cholinergic projections; however, studies also suggest that the effectiveness of estrogen therapy decreases with age and time after loss of ovarian function. We propose a model in which deficits in basal forebrain cholinergic function contribute to age-related changes in the response to estrogen therapy. Based on this model, we propose that cholinergic-enhancing drugs, used in combination with an appropriate estrogen-containing drug regimen, may be a viable therapeutic strategy for use in older postmenopausal women with early evidence of mild cognitive decline.

Regulation of cortical acetylcholine release: insights from in vivo microdialysis studies

Acetylcholine release links the activity of presynaptic neurons with their postsynaptic targets and thus represents the intercellular correlate of cholinergic neurotransmission. Here, we review the regulation and functional significance of acetylcholine release in the mammalian cerebral cortex, with a particular emphasis on information derived from in vivo microdialysis studies over the past three decades. This information is integrated with anatomical and behavioral data to derive conclusions regarding the role of cortical cholinergic transmission in normal behavioral and how its dysregulation may contribute to cognitive correlates of several neuropsychiatric conditions. Some unresolved issues regarding the regulation and significance of cortical acetylcholine release and the promise of new methodology for advancing our knowledge in this area are also briefly discussed.

Donepezil treatment restores the ability of estradiol to enhance cognitive performance in aged rats: evidence for the cholinergic basis of the critical period hypothesis

Recent studies suggest that the ability of estradiol to enhance cognitive performance diminishes with age and/or time following loss of ovarian function. We hypothesize that this is due, in part, to a decrease in basal forebrain cholinergic function. This study tested whether donepezil, a cholinesterase inhibitor, could restore estradiol effects on cognitive performance in aged rats that had been ovariectomized as young adults. Rats were ovariectomized at 3 months of age, and then trained on a delayed matching to position (DMP) T-maze task, followed by a configural association (CA) operant condition task, beginning at 12-17 or 22-27 months of age. Three weeks prior to testing, rats started to receive either donepezil or vehicle. After one week, half of each group also began receiving estradiol. Acclimation and testing began seven days later and treatment continued throughout testing. Estradiol alone significantly enhanced DMP acquisition in middle-aged rats, but not in aged rats. Donepezil alone had no effect on DMP acquisition in either age group; however, donepezil treatment restored the ability of estradiol to enhance DMP acquisition in aged rats. This effect was due largely to a reduction in the predisposition to adopt a persistent turn strategy during acquisition. These same treatments did not affect acquisition of the CA task in middle-aged rats, but did have small but significant effects on response time in aged rats. The data are consistent with the idea that estrogen effects on cognitive performance are task specific, and that deficits in basal forebrain cholinergic function are responsible for the loss of estradiol effect on DMP acquisition in aged ovariectomized rats. In addition, the data suggest that enhancing cholinergic function pharmacologically can restore the ability of estradiol to enhance acquisition of the DMP task in very old rats following long periods of hormone deprivation. Whether donepezil has similar restorative effects on other estrogen-sensitive tasks needs to be explored.

Toward a neuro-cognitive animal model of the cognitive symptoms of schizophrenia: disruption of cortical cholinergic neurotransmission following repeated amphetamine exposure in attentional task-performing, but not non-performing, rats

Impairments in attentional functions and capacities represent core aspects of the cognitive symptoms of schizophrenia. Attentional performance has been demonstrated to depend on the integrity and activity of cortical cholinergic inputs. The neurobiological, behavioral, and cognitive effects of repeated exposure to psychostimulants model important aspects of schizophrenia. In the present experiment, prefrontal acetylcholine (ACh) release was measured in attentional task-performing and non-performing rats pretreated with an escalating dosing regimen of amphetamine (AMPH) and following challenges with AMPH. In non-performing rats, pretreatment with AMPH did not affect the increases in ACh release produced by AMPH-challenges. In contrast, attentional task performance-associated increases in ACh release were attenuated in AMPH-pretreated and AMPH-challenged rats. This effect of repeated AMPH exposure on ACh release was already present before task-onset, suggesting that the loss of cognitive control that characterized these animals' performance was a result of cholinergic dysregulation. The findings indicate that the demonstration of repeated AMPH-induced dysregulation of the prefrontal cholinergic input system depends on interactions between the effects of repeated AMPH exposure and cognitive performance-associated recruitment of this neuronal system. Repeated AMPH-induced disruption of prefrontal cholinergic activity and attentional performance represents a useful model to investigate the cholinergic mechanisms contributing to the cognitive impairments of schizophrenia.

Microdialysis measures of functional increases in ACh release in the hippocampus with and without inclusion of acetylcholinesterase inhibitors in the perfusate

Because brain extracellular acetylcholine (ACh) levels are near detection limits in microdialysis samples, an acetylcholinesterase (AChE) inhibitor such as neostigmine is often added to microdialysis perfusates to increase ACh levels in the dialysate, a practice that raises concerns that the inhibitor might alter the results. Two experiments compared functional differences in ACh release with and without neostigmine. In the first experiment, 30-60% increases in extracellular ACh concentrations in the hippocampus were evident during food-rewarded T-maze training with 20-500 nm neostigmine in the perfusate but no increases were seen without neostigmine. In the second experiment, 78% increases in ACh release in the hippocampus were seen after injections of the GABA(A) receptor antagonist, bicuculline, into medial septum only if neostigmine (50 nm) was included in the perfusate. These findings suggest that, in the hippocampus, endogenous brain AChEs are very efficient at removing extracellular ACh, obscuring differences in ACh release in these experiments. Therefore, inclusion of AChE inhibitors in the microdialysis perfusate may be necessary under some conditions for observations of functional changes in release of ACh in the hippocampus.

MEG spectral profile in Alzheimer's disease and mild cognitive impairment

OBJECTIVE

In this study, we applied a novel procedure to calculate the mean frequency from the Magnetoencephalography (MEG) signals of 22 patients with Alzheimer's Disease (AD), 22 patients with mild cognitive impairment (MCI), and 21 healthy controls. A significant mean frequency decrease was expected in pathological groups. MCI subjects are expected to show intermediate mean frequency values between AD patients and controls.

METHODS

MEG signal was obtained from a whole-head 148 channels magnetometer in a resting condition. We estimated the power spectral density from the MEG signal by means of the Fourier transform of the autocorrelation function. Then, we computed the mean frequency for each subject.

RESULTS

Mean frequency was higher in controls (12.46 +/- 2.00 Hz, mean +/- SD) than in MCI subjects (10.82 +/- 2.21 Hz) with significant differences (P < 0.05). Moreover, mean frequency values in MCI subjects were higher than in AD patients (9.06 +/- 2.48 Hz, P < 0.05). We also detected a decrease of 0.17 Hz per year in mean frequency from normal subjects' MEG (P < 0.05)

CONCLUSIONS

Results demonstrated that the approach adopted for the calculation of a mean frequency score seems to be adequate and sensitive to detect differences between normal aging, cognitive deterioration and AD. In addition, data may contribute to the theoretical discussion on the nature of mild cognitive impairment and its similitudes with Alzheimer's disease.

SIGNIFICANCE

This paper may be considered a first step to obtain a reliable measure which summarizes spectral information, and might be of a potential clinical interest.

Prefrontal cortical modulation of acetylcholine release in posterior parietal cortex

Attentional processing is a crucial early stage in cognition and is subject to "top-down" regulation by prefrontal cortex (PFC). Top-down regulation involves modification of input processing in cortical and subcortical areas, including the posterior parietal cortex (PPC). Cortical cholinergic inputs, originating from the basal forebrain cholinergic system, have been demonstrated to mediate important aspects of attentional processing. The present study investigated the ability of cholinergic and glutamatergic transmission within PFC to regulate acetylcholine (ACh) release in PPC. The first set of experiments demonstrated increases in ACh efflux in PPC following AMPA administration into the PFC. These increases were antagonized by co-administration of the AMPA receptor antagonist DNQX into the PFC. The second set of experiments demonstrated that administration of carbachol, but not nicotine, into the PFC also increased ACh efflux in PPC. The effects of carbachol were attenuated by co-administration (into PFC) of a muscarinic antagonist (atropine) and partially attenuated by the nicotine antagonist mecamylamine and DNQX. Perfusion of carbachol, nicotine, or AMPA into the PPC did not affect PFC ACh efflux, suggesting that these cortical interactions are not bi-directional. These studies demonstrate the capacity of the PFC to regulate ACh release in the PPC via glutamatergic and cholinergic prefrontal mechanisms. Prefrontal regulation of ACh release elsewhere in the cortex is hypothesized to contribute to the cognitive optimization of input processing.

Aging and subcellular localization of m2 muscarinic autoreceptor in basalocortical neurons in vivo

By using immunohistochemical approaches at the light and electron microscopic levels, we have shown that aging modifies the subcellular distribution of the m2 muscarinic autoreceptor (m2R) differentially at somato-dendritic postsynaptic sites and at axonal presynaptic sites in cholinergic basalocortical neurons, in vivo. In cholinergic perikarya and dendrites of the nucleus basalis magnocellularis (NBM), aging is associated with a decrease of the density of m2R at the plasma membrane and in the cytoplasm, suggesting a decrease of the total number of m2R in the somato-dendritic field. In contrast, the number of substance P receptors per somato-dendritic surface was not affected. In the frontal cortex (FC), we have shown a decrease of cytoplasmic m2R density also leading to a decrease of the number of m2R per surface of varicosities but with no change of the density of m2R at the membrane. Our results suggest that the decrease of m2R in the somato-dendritic field of the NBM, but not a modification of the number of presynaptic m2 autoreceptors at the plasma membrane in the FC, could contribute to the decrease of the efficacy of cholinergic transmission observed with aging in the rat.

The cholinergic hypothesis of age and Alzheimer's disease-related cognitive deficits: recent challenges and their implications for novel drug development

The cholinergic hypothesis was initially presented over 20 years ago and suggests that a dysfunction of acetylcholine containing neurons in the brain contributes substantially to the cognitive decline observed in those with advanced age and Alzheimer's disease (AD). This premise has since served as the basis for the majority of treatment strategies and drug development approaches for AD to date. Recent studies of the brains of patients who had mild cognitive impairment or early stage AD in which choline acetyltransferase and/or acetylcholinesterase activity was unaffected (or even up-regulated) have, however, led some to challenge the validity of the hypothesis as well as the rationale for using cholinomimetics to treat the disorder, particularly in the earlier stages. These challenges, primarily based on assays of post mortem enzyme activity, should be taken in perspective and evaluated within the wide range of cholinergic abnormalities known to exist in both aging and AD. The results of both post mortem and antemortem studies in aged humans and AD patients, as well as animal experiments suggest that a host of cholinergic abnormalities including alterations in choline transport, acetylcholine release, nicotinic and muscarinic receptor expression, neurotrophin support, and perhaps axonal transport may all contribute to cognitive abnormalities in aging and AD. Cholinergic abnormalities may also contribute to noncognitive behavioral abnormalities as well as the deposition of toxic neuritic plaques in AD. Therefore, cholinergic-based strategies will likely remain valid as one approach to rational drug development for the treatment of AD other forms of dementia.

Effects of ageing and long-term hormone replacement on cholinergic neurones in the medial septum and nucleus basalis magnocellularis of ovariectomized rats

Ovariectomized aged rats, some of which received long-term hormone replacement with oestrogen or oestrogen plus progesterone, were evaluated for the number and size of basal forebrain cholinergic neurones, as well as relative levels of choline acetyltransferase (ChAT) and trkA mRNA, in order to determine whether effects on basal forebrain cholinergic cell survival and function correspond with differences in cognitive performance previously described. The results show that ageing combined with long-term loss of ovarian function produced substantial reductions in the levels of ChAT and trkA mRNA in the medial septum and nucleus basalis magnocellularis, relative to much younger ovariectomized controls. In contrast, no significant effects on the number or size of the cholinergic cells were detected, indicating that loss of ovarian function does not cause a loss of cholinergic neurones with age. Long-term hormone replacement had no apparent effect on the number of ChAT-positive neurones detected, and did not prevent the reductions in ChAT and trkA mRNA associated with ovariectomy and ageing. Collectively, the data suggest that ageing combined with long-term loss of ovarian function has a severe negative impact on basal forebrain cholinergic function, but not on cholinergic cell survival per se.

DHA-enriched phospholipid diets modulate age-related alterations in rat hippocampus

Our previous work on rat hippocampus showed that a loss of docosahexaenoic acid (DHA) occurs in the fatty acid composition of phosphatidylethanolamine (PE), plasmenylethanolamine (PmE) and phosphatidylserine (PS) with increasing age. The present study investigated whether a DHA-enriched phospholipid dietary supplement could restore DHA levels and cholinergic activity. Male rats were fed a balanced diet containing both linoleic and alpha-linolenic acids until the age of 2, 18 and 21 months. From 18 to 21 months, one subgroup received a diet supplemented with DHA-enriched phospholipids from egg yolk (E-PL), and another a diet with DHA-enriched phospholipids from pig brain (B-PL). Compared to the control diet, the E-PL diet restored the proportion of polyunsaturated fatty acids (PUFAs: 22:6n-3 and 20:4n-6) in PE and PmE, while enhancing spontaneous and evoked-acetylcholine (Ach) release. The B-PL diet had no effect on PUFAs, but increased basal extracellular levels of Ach in 21-month-old rats as compared to the age-matched control. Our results show that supplementation with DHA-enriched egg PL can enhance Ach release and correct PUFA composition.

Differential cortical acetylcholine release in rats performing a sustained attention task versus behavioral control tasks that do not explicitly tax attention

The present study used microdialysis techniques to compare acetylcholine release in the frontoparietal cortex of rats performing in a task requiring sustained attention with that of rats performing in two control procedures. The two control procedures were a fixed-interval 9-s schedule of reinforcement assessing primarily the effects of operant responding and comparable reward rates, and an operant procedure designed to test the effects of lever extension to prompt responding. These two control procedures involved comparable sensory-motor and motivational variables to those of the sustained attention task, but did not explicitly tax attentional processes. Performance of the sustained attention task was associated with a significant increase in cortical acetylcholine efflux, reaching a maximum of nearly 140%. Performance of the two control procedures was associated with significantly smaller (approximately 50%) increases in cortical acetylcholine release. This robust dissociation between attentional and control performance-associated increases in cortical acetylcholine release resulted, in part, from the elimination of the pre-task transfer of the animals into the operant chambers and the associated increases in acetylcholine release observed in previous studies. The present results support the hypothesis that demands on attentional performance, as opposed to the frequency of lever pressing, reward delivery and other task-related variables, selectively activate the basal forebrain corticopetal cholinergic system.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

Age-related attenuation of stimulated cortical acetylcholine release in basal forebrain-lesioned rats

In vivo microdialysis was used to measure the effects of partial deafferentation of cortical cholinergic inputs on acetylcholine efflux in young (four to seven months) and aged (24-28 months) male F344/BNNIA rats. Partial deafferentation was produced by bilateral infusions of the immunotoxin 192 immunoglobulin G-saporin (0.56 microg/1.0 microl) or its vehicle solution into the ventral pallidum/substantia innominata region of the basal forebrain. The lesion produced comparable (65%) decreases in basal cortical acetylcholine efflux in young and aged rats. Presentation of a complex environmental stimulus (exposure to darkness/palatable food), in conjunction with the systemic administration of the benzodiazepine receptor weak inverse agonist ZK 93 426, increased cortical acetylcholine efflux in young shams, aged shams and young lesioned rats, but not in aged lesioned rats. Administration of the benzodiazepine receptor partial inverse agonist FG 7142, in the absence of the environmental stimulus, comparably stimulated cortical acetylcholine efflux in young and aged sham rats. FG 7142-induced increases in acetylcholine efflux were attenuated by approximately 50% following partial deafferentation in both young and aged rats. These results suggests that, under certain conditions, ageing potently interacts with the integrity of the cortical cholinergic afferent system. The effects of ageing on cortical cholinergic function may be most potently revealed by experiments assessing age-related limitations in the responsiveness of a partially deafferented cholinergic system to certain behavioral and/or pharmacological stimuli.

Effects of local cholinesterase inhibition on acetylcholine release assessed simultaneously in prefrontal and frontoparietal cortex

To investigate whether acetylcholine is released in a similar fashion in different regions of the cortex, in vivo microdialysis was used to measure acetylcholine efflux simultaneously in the medial prefrontal and the frontoparietal cortex, under both basal conditions and following tactile stimulation. Additionally, the effects of including two different concentrations (0.05 microM and 0.5 microM) of a cholinesterase inhibitor (neostigmine) in the perfusion fluid were assessed. Basal levels of acetylcholine (i.e. during non-stimulated sessions) were similar in medial prefrontal and frontoparietal areas. Tactile stimulation reliably increased acetylcholine efflux in a similar fashion (up to 140% increase above baseline) in both cortical areas studied. Predictably, the higher concentration of neostigmine (0.5 microM) increased basal acetylcholine efflux by about 150% from levels observed with the lower neostigmine concentration (0.05 microM), but the concentration of local neostigmine had no effect on either the magnitude or the duration of the increased acetylcholine efflux following tactile stimulation. These results suggest that the pattern of acetylcholine release may be comparable in different areas of the cortex, supporting the idea that cholinergic projections from the basal forebrain to the cortex represent a globally regulated system. Furthermore, while the inclusion of neostigmine in perfusion fluid must be taken into account when interpreting acetylcholine efflux data, it appears that concentrations of up to 0.5 microM do not interfere fundamentally with the lability of cortical acetylcholine efflux in response to behavioural stimulation.

Operant performance and cortical acetylcholine release: role of response rate, reward density, and non-contingent stimuli

The relationship between acetylcholine (ACh) efflux in medial prefrontal cortex (mPFC) and performance in a visual discrimination task and a variable interval (VI) schedule of reinforcement was studied in rats. Animals were pretrained in one of the two tasks and then unilaterally implanted with microdialysis guide cannula into the mPFC. Animals were then dialyzed, during 12 min collection intervals, in the operant chambers prior to task onset and during and after task performance. Each animal was dialyzed for a total of four sessions: two standard task sessions, one session in which a houselight was flashed at 0.5 Hz during the third 12 min block, and an extinction session (always the last session) in which reinforcement was withheld during the final three blocks. Response accuracy in the discrimination task was very high (> 95% correct) and stable across the four blocks with a progressive increase in omissions. The flashing houselight did not affect performance whereas the loss of reinforcement led to an increase in omissions. VI performance was associated with a high number of lever presses and a high reward rate that declined over the four blocks. Again, the flashing houselight did not affect VI performance whereas lever pressing declined markedly during the extinction session. ACh efflux did not change, relative to baseline, during performance in either task, or with the presentation of the flashing houselight or the loss of reinforcement. These data contrast with the changes in cortical ACh efflux observed in situations characterized by the presentation of novel stimuli or changing demands on attentional processing and, therefore, assist in the specification of hypotheses on the cognitive functions of cortical ACh.

Acetylcholine, aging, and Alzheimer's disease

A substantial body of literature has suggested that the memory and learning deficits associated with Alzheimer's disease and aging are attributable to degeneration of the cholinergic magnocellular neurons of the nucleus basalis of Meynert (nbM). Subsequently, lesion-induced damage to the cholinergic projections from the nbM to the neocortex has been utilized extensively as an animal model of dementia. In addition, the effect of the normal aging process on deterioration of these neurons and on cognitive function has also been examined. Such studies have revealed, for example, that many of the learning and memory impairments traditionally attributed to the cholinergic corticopetal system are not due to degeneration of the cholinergic neurons of the nbM, but instead may be due to damage of more rostral elements of the cholinergic basal forebrain system. This review will examine the contribution of behavioural animal and human studies to out understanding of the role of the basal forebrain cholinergic neurons in age-related cognitive impairments.