Effects of choline and glucose on atropine-induced alterations of acetylcholine synthesis and content in the caudate nuclei of rats

Increasing dietary choline attenuates spatial memory deficits resulting from exposure to the chemotherapeutic agents cyclophosphamide and doxorubicin

BACKGROUND

Choline supplementation (+Ch) improves cognitive function in impaired animals and humans. Chemotherapy-related cognitive deficits (CRCDs) occur in cancer patients, and these deficits persist following treatment, adversely impacting quality of life. To date, there are no approved treatments for this condition.

AIM

Because +Ch improves impaired memory, it was of interest to determine whether +Ch can attenuate spatial memory deficits induced by the chemotherapeutic agents doxorubicin (DOX) and cyclophosphamide (CYP).

METHODS

Female BALB/C mice, 64 days of age, were trained in the Morris water maze and baseline performance determined on day 15. Following baseline assessment, mice were placed on +Ch diet (2.0% Ch) or remained on standard diet (0.12% Ch). Mice received intravenous injections of DOX (2.5 mg/kg) and CYP (25 mg/kg), or equivalent volumes of saline (0.9% NaCl), on days 16, 23, 30, and 37, and spatial memory was assessed weekly from day 22 to 71.

RESULTS

DOX and CYP produced a prolonged impairment in spatial memory as indicated by an increased latency to the correct zone (p < 0.05), and a decrease in time in the correct zone (p < 0.05), % of total swim distance in the correct zone (p < 0.05) and % entries to the correct zone (p < 0.05). These effects were attenuated by +Ch.

CONCLUSION

Although it remains to be determined whether this effect extends to other cognitive domains and whether +Ch is prophylactic or therapeutic, these findings suggest that +Ch may be an effective intervention for CRCDs.

Choline transporter 1 maintains cholinergic function in choline acetyltransferase haploinsufficiency

Choline acetyltransferase (ChAT), the enzyme that synthesizes the neurotransmitter acetylcholine (ACh), is thought to be present in kinetic excess in cholinergic neurons. The rate-limiting factor in ACh production is the provision of choline to ChAT. Cholinergic neurons are relatively unique in their expression of the choline transporter 1 (CHT1), which exhibits high-affinity for choline and catalyzes its uptake from the extracellular space to the neuron. Multiple lines of evidence indicate that the activity of CHT1 is a key determinant of choline supply for ACh synthesis. We examined the interaction of ChAT and ChT activity using mice heterozygous for a null mutation in the Chat gene (Chat+/-). In these mice, brain ChAT activity was reduced by 40-50% relative to the wild type, but brain ACh levels as well as ACh content and depolarization-evoked ACh release in hippocampal slices were normal. However, the amount of choline taken up by CHT1 and ACh synthesized de novo from choline transported by CHT1 in hippocampal slices, as well as levels of CHT1 mRNA in the septum and CHT1 protein in several regions of the CNS, were 50-100% higher in Chat+/- than in Chat+/+ mice. Thus, haploinsufficiency of ChAT leads to an increased expression of CHT1. Increased ChT activity may compensate for the reduced ChAT activity in Chat+/- mice, contributing to the maintenance of apparently normal cholinergic function as reflected by normal performance of these mice in several behavioral assays.

Glucose improvement of memory: a review

The memory-improving action of glucose has now been studied for almost 20 years and the study of this phenomenon has led to a number of important developments in the understanding of memory, brain physiology and pathological consequences of impaired glucose tolerance. Glucose improvement of memory appears to involve two optimal doses in animals (100 mg/kg and 2 g/kg) that may correspond to two physiological mechanisms underlying glucose effects on memory. In humans, there have been few dose-response studies so the existence of more than one effective dose in humans is uncertain. Many tasks are facilitated by glucose in humans but tasks that are difficult to master or involve divided attention are improved more readily that easier tasks. There are a number of hypotheses about the physiological bases of the memory-improving action of glucose. Peripheral glucose injections could alleviate localized deficits in extracellular glucose in the hippocampus. These localized deficits may be due to changes in glucose transporters in that structure. Because certain neurotransmitters such as acetylcholine are directly dependent on the glucose supply for their synthesis, glucose is thought to facilitate neurotransmitter synthesis under certain circumstances. However, these hypotheses cannot account for the specificity of the dose-response effect of glucose. A number of peripheral mechanisms have been proposed, including the possibility that glucose-sensitive neurons in the brain or in the periphery may serve as glucose sensors and eventually produce neural changes that would facilitate memory processing. These latter results could be of importance because the mechanisms they suggest appear to be dose-dependent, a crucial characteristic to explain the dose-dependent effects of glucose. There may be an advantage to develop hypotheses that include both peripheral and central actions of glucose. There is evidence that impaired glucose regulation is associated with impaired cognition, particularly episodic memory. This impairment is minimal in young people but increases in older people (65 years and over) where it may compound other aging processes leading to reduced brain function. A small number of studies showed that glucose improvement of memory is associated with poor glucose regulation although this may not be the case for diabetic patients. Results of a few studies also suggest that drug treatments that improve glucose regulation also produce cognitive improvement in diabetic patients.

Beta-amyloid and cholinergic neurons

It is generally accepted that the crucial events in the pathogeny of Alzheimer's disease (AD) are the increased accumulation of amyloidogenic peptides derived from amyloid precursor protein and the harmful actions of these peptides on neurons, which bring about neurodegeneration. The enhanced beta-amyloid accumulation is known to be caused by mutations of specific genes in patients who suffer from the familial (hereditary) form of AD but who represent just a minor group within the total population of AD patients. The reasons for beta-amyloid accumulation are not known in the much larger group of patients with the sporadic form of the disease. A biochemical feature common to either form of the disease is the preferential atrophy and degeneration of cholinergic neurons, which is probably responsible for much of the cognitive decline characteristic of the disease. We present an overview of recent investigations on the interactions between beta-amyloid and cholinergic neurons.

Choline increases serum insulin in rat when injected intraperitoneally and augments basal and stimulated aceylcholine release from the rat minced pancreas in vitro

Intraperitoneal injection of choline (30-90 mg.kg-1) produced a dose-dependent increase in serum insulin, glucose and choline levels in rats. The increase in serum insulin induced by choline (90 mg.kg-1) was blocked by pretreatment with the muscarinic acetylcholine receptor antagonists, atropine (2 mg.kg-1), pirenzepine (2 mg.kg-1) and 4-diphenylacetoxy-N-methylpiperidine (2 mg.kg-1) or the ganglionic nicotinic receptor antagonist, hexamethonium (15 mg.kg-1). The effect of choline on serum insulin and glucose was enhanced by oral glucose administration (3 g.kg-1). Choline administration was associated with a significant (P < 0.001) increase in the acetylcholine content of pancreatic tissue. Choline (10-130 microm) increased basal and stimulated acetylcholine release but failed to evoke insulin release from the minced pancreas at considerably higher concentrations (0.1-10 mm). Hemicholium-3, a choline uptake inhibitor, attenuated the increase in acetylcholine release induced by choline augmentation. Choline (1-32 mm) inhibited [3H]quinuclidinyl benzilate binding to the muscarinic receptors in the pancreatic homogenates. These data show that choline, a precursor of the neurotransmitter acetylcholine, increases serum insulin by indirectly stimulating peripheral acetylcholine receptors through the enhancement of acetylcholine synthesis and release.

Amyloid beta-peptide 25-35 reduces [3H]acetylcholine release in retinal neurons. Involvement of metabolic dysfunction

Cholinergic pathways serve important functions in learning and memory processes. The loss of basal forebrain cholinergic neurons and the presence of senile plaques composed by amyloid beta-peptide (A beta) are found in post-mortem brains of Alzheimer's disease (AD) patients. However, the role of A beta in the cholinergic dysfunction observed in AD is not yet clarified. In this study, we observed that the release of [3H]acetylcholine evoked by K(+)-depolarization was significantly lower in cells treated with A beta 25-35 peptide, than in untreated cells or in cells exposed to the reverse sequence peptide A beta 35-25. The levels of pyruvate, the substrate for pyruvate dehydrogenase, the enzyme involved in acetyl coenzyme A synthesis in the brain, which is rate-limiting for the synthesis of acetylcholine, were significantly decreased, about 40%, in A beta treated cells. A beta 25-35 did not affect choline acetyltransferase activity or [3H]choline uptake. 2-[3H]-deoxyglucose uptake was decreased when cells were exposed to A beta 25-35 or to A beta 1-40. Taken together these data suggest that an impairment of glycolysis, and the consequent decrease in pyruvate levels, may be responsible for the decrement of acetylcholine release observed in A beta treated cells, thus sustaining the hypothesis that the cholinergic dysfunction, observed in AD patients, might be associated with extracellular A beta accumulation.

Hyperglycemia induced by intracerebroventricular choline: involvement of the sympatho-adrenal system

Intracerebroventricular (i.c.v.) injection of choline (75-300 microg) produced a dose-dependent increase in blood glucose levels. Pre-treatment with the nicotinic acetylcholine receptor antagonist, mecamylamine (50 microg, i.c.v.) blocked the hyperglycemia induced by choline (150 microg, i.c.v.), but the response was not affected by pre-treatment with the muscarinic acetylcholine receptor antagonist, atropine (10 microg, i.c.v.). Pre-treatment with the neuronal choline uptake inhibitor, hemicholinium-3 (20 microg, i.c.v.), attenuated the hyperglycemia induced by choline. The hyperglycemic response to choline was associated increased plasma levels of adrenaline and noradrenaline. The hyperglycemia elicited by choline was greatly attenuated by bilateral adrenalectomy, and entirely blocked by either surgical transection of the splanchnic nerves or by pre-treatment with the alpha-adrenoceptor antagonist, phentolamine. These data show that choline, a precursor of acetylcholine, increases blood glucose and this effect is mediated by central nicotinic acetylcholine receptor activation. An increase in sympatho-adrenal activity appears to be involved in the hyperglycemic effect of choline.

The effects of citicoline and/or MK-801 on survival, neurological and behavioral outcome of mice exposed to transient hyperglycemia and oligemic hypoxia

The effects of citicoline and/or low dose of MK-801 (sufficient to prevent the development of seizures) on survival, neurological and behavioral recovery following transient hyperglycemic-oligemic-hypoxic insult have been evaluated in mice. Neurological recovery was assessed semi-quantitatively on the third and the 10th day after the insult, and behavioral tests evaluating spontaneous locomotor activity, motor coordination and spontaneous alternation performance were performed on day 10. Neither drug given alone did influence survival rate, but the combination of MK-801 and higher citicoline dose decreased mortality on day 10. Behavioral performance was markedly compromised by the insult. Citicoline, but not MK-801, slightly but significantly improved behavioral outcome in all three tests.

CONCLUSION

when brain ischemic insult is complicated with acute hyperglycemia, post-treatment with citicoline combined with MK-801 in low anti-convulsive dose improves survival and neurological recovery, and citicoline but not MK-801 enhances behavioral recovery.

Glucose plus choline improve passive avoidance behaviour and increase hippocampal acetylcholine release in mice

The present study tests the effects of glucose and choline, the biosynthetic precursors of acetylcholine, on passive avoidance behaviour and hippocampal acetylcholine release measured by microdialysis in awake mice. Glucose (10 and 30mg/kg) or choline chloride (6-60mg/kg), given by i.p. injection immediately after training, dose-dependently enhanced retention in an inhibitory avoidance task. Combinations of low doses of glucose (10mg/kg) and choline chloride (20mg/kg) which alone were submaximally effective significantly increased retention latencies in a synergistic manner, an effect which was sensitive to atropine (0.5mg/kg). This beneficial effect vanished when higher doses of glucose or choline were combined. Basal hippocampal acetylcholine release in mice habituated to their environment was not affected by administration of glucose and choline. However, when hippocampal acetylcholine release was stimulated either by infusion of scopolamine (0.3microM) or by transferring the mice into a novel environment, the combination of glucose plus choline further increased acetylcholine release to a significant extent. We conclude that low doses of glucose and choline act synergistically to improve memory storage, an effect which is due to facilitation of acetylcholine release. This finding reinforces the view that central cholinergic functions are influenced under certain conditions by dietary intake of precursors.

Cognitive performance and its relationship with postprandial metabolic changes after ingestion of different macronutrients in the morning

The effect of carbohydrate, protein and fat ingestion on simple as well as complex cognitive functions and the relationship between the respective postprandial metabolic changes and changes in cognitive performance were studied in fifteen healthy male students. Subjects were tested in three sessions, separated by 1 week, for short-term changes in blood variables, indirect calorimetry, subjective performance and different objective performance tasks using a repeated-measures counterbalanced cross-over design. Measurements were made after an overnight fast before and hourly during 3 h after test meal ingestion. Test meals consisted of either pure carbohydrates, protein or fat and were served as isoenergetic (1670 kJ) spoonable creams with similar sensory properties. Most aspects of subjective performance did not differ between test meals. For all objective tasks, however, postprandial cognitive performance was best after fat ingestion concomitant with an almost constant glucose metabolism and constant metabolic activation state measured by glucagon:insulin (G:I). In contrast, carbohydrate as well as protein ingestion resulted in lower overall cognitive performance, both together with partly marked changes in glucose metabolism and metabolic activation. They also differently affected specific cognitive functions in relation to their specific effect on metabolism. Carbohydrate ingestion resulted in relatively better short-term memory and accuracy of tasks concomitant with low metabolic activation, whereas protein ingestion resulted in better attention and efficiency of tasks concomitant with higher metabolic activation. Our findings support the concept that good and stable cognitive performance is related to a balanced glucose metabolism and metabolic activation state.

Mitochondrial genome lesions in the pathogenesis of sporadic Alzheimer's disease

BACKGROUND

The recent, magnificent results of molecular biology concerning beta-amyloid (betaA) metabolism in early onset Alzheimer's disease (AD) have generated a series of new findings and, in turn, a new etiological concept. Attention on the early events in the pathogenesis of AD has been shifted from the chromosomal abnormalities in the nucleus of nerve cells onto genetic changes in the mitochondrial genome. This offers a new pathogenetic approach which also opens new pharmacological challenges particularly for the episodic forms of AD.

OBJECTIVE

Alterations occurring at the mitochondrial genome result in major consequences of oxidative phosphorylation and, if a specific threshold is exceeded, they may constitute important causative events in the apoptosis of selected nerve cells. The fact that the main source of mitochondrial metabolism is its glucose turnover allows monitoring brain changes in glucose metabolism by 18F-2 deoxyglucose positron emission tomography. In the demented brain, a low glucose turnover causes a cholinergic deficit by decreasing the synthetic rate of acetyl coenzyme A (AcCoA). AcCoA represents the key substrate for the acetylation of choline to acetylcholine by choline acetyltransferase. The consistent energy need for AcCoA synthesis appears obvious when considering that 1 molecule of glucose generates just 2 molecules of AcCoA, but 38 molecules of ATP. In the brain, AcCoA is exclusively synthesized in the glycolitic pathway. Generation of betaA is increased if the synthetic rate of ATP drops below a critical threshold: under these conditions, the betaA precursor protein (betaAPP) is inserted only in part into synaptic membranes which have the highest betaAPP turnover. In conditions of short ATP supply, betaAPP is not split at the beta region by an ATP-activated protease and this results in a substantial increase in uncleaved betaA molecules.

CONCLUSION

Peroxidative alterations in mitochondrial DNA are of importance in degenerative diseases of postmitotic tissues, particularly in degenerative diseases. This offers a new pharmacological approach for the treatment of AD. Neurotrophic factors and estrogen seem to be the first pharmacological leads.

The absence of effect of glucose on memory is associated with low susceptibility to the amnestic effects of scopolamine in a strain of mice

In this series of experiments, we examined the ability of post-training glucose injections to improve memory of the Balb/cAnNCrlBR strain of mice for a bar-pressing task. We could not replicate this effect which has been demonstrated in many other strains of mice including Balb/cbyJ, a related strain. We found that the Balb/cAnNCrlBR strain of mice is also much less sensitive to the disrupting effects produced by pre- or post-training injections of the competitive cholinergic antagonist scopolamine. This strain also shows altered glucoregulation compared to the Balb/cbyJ strain. The absence of glucose effects on memory in Balb/cAnNCrlBR mice appears to be associated with decreased sensitivity to cholinergic antagonists. These results can be contrasted with previous ones obtained in a related strain, the Balb/cbyJ, in which glucose was shown to improve memory while scopolamine could easily disrupt memory processes. Taken together, these data provide additional indirect support for the hypothesis that glucose improvement of memory is closely linked to a functional interaction with central cholinergic systems. The comparison of these two strains could be the basis for a useful animal model to investigate the relationship between age-related changes in memory and central cholinergic function.

Intraseptal infusions of muscimol impair spontaneous alternation performance: infusions of glucose into the hippocampus, but not the medial septum, reverse the deficit

As observed with intraseptal injections of opioid receptor agonists, direct infusions of GABAergic receptor agonists into the medial septum impair performance on several tasks that involve spatial or working memory processes in rats. Because the effects of opioid-induced impairments can be reliably reversed by concomitant intraseptal infusions of glucose, the experiments reported here determined whether impairments produced by GABAergic agonists would similarly be reversed by glucose. The findings of Experiment 1 showed, in male Sprague-Dawley rats, that intraseptal infusions of the GABA agonist muscimol (1 or 3 nmol/0.5 microliter) impaired spontaneous alternation performance. The results of Experiment 2 indicated that intraseptal infusions of glucose (8, 17, or 33 nmol) or glutamate (15 or 30 nmol) did not attenuate the muscimol-induced deficit on spontaneous alternation performance, whereas infusions of the GABAergic antagonist bicuculline methiodide (0.1 nmol) did. However, the findings of Experiment 3 indicated that glucose injections (50 nmol/0.5 microliter) into the hippocampus did reverse the impairing effect of the intraseptal muscimol infusions. Combined, these findings suggest that the neurochemical regulation of learning and memory may involve hierarchical interactions between particular neurotransmitter and neuroanatomical systems. Specifically, medial septal GABAergic effects on spontaneous alternation prevail over those of glucose or glutamate in the medial septum, but are overridden by the effects of glucose in the hippocampus.

Glucose regulation and cognitive functions: relation to Alzheimer's disease and diabetes

Glucose has been found to improve memory in animals and humans. Animal research has revealed that glucose may improve memory through a facilitation of acetylcholine (ACh) synthesis and release in the brain. This glucose-related memory improvement has prompted research in elderly humans. These studies have shown that the memory-improving action of glucose depends on each individuals' blood glucose regulation. Based on these data, researchers have evaluated the effect of glucose on memory in patients with Alzheimer's disease (AD). Results demonstrated that glucose could improve memory in a subset of patients that had abnormalities in their blood glucose regulation. Interestingly, these alterations in blood glucose regulation were believed to depend on the severity of the disease process. Another line of investigation has focused on alterations in brain glucose metabolism. Both animal models and studies with Type II diabetic elderly patients have shown that altered glucose regulation impairs learning and memory processes. It is possible that in AD patients, hyperglycemia exerts a deleterious effect by potentiating the neuronal death produced by other pathological processes taking place such as amyloid deposition. Based on these data, it appears important to find the prevalence of altered glucoregulation at various stages of AD. Secondly, it may be of interest to determine prospectively whether altered glucoregulation is linked to a faster progression of the disease. Finally, if such a relationship is observed, the next logical step would be to determine whether AD patients could benefit from treatments aimed at normalizing blood glucose regulation and improving insulin sensitivity.

The significance of glucose turnover in the brain in the pathogenetic mechanisms of Alzheimer's disease

This paper presents a comprehensive survey of the pathogenesis and pathophysiology of Alzheimer's disease (AD). Two mechanisms are of etiological importance in the development of a degenerative dementing brain disease: 1. Lesions in the mitochondrial genome that are caused by free radicals. Primary degenerative AD is characterized by a tendency to acquire random lesions within mitochondrial DNA that are produced by free radicals. The consequence of these lesions is a decrease in glucose turnover and a decline in oxidative phosphorylation. Point mutations on chromosome 21 are hypothesized to increase the susceptibility of mitochondrial DNA to lesions created by free radicals. 2. Ischemic brain lesions as well as traumatic brain damage cause an increase in the release of excitotoxic amino acids (glutamate, aspartate, etc.). These neurotransmitters increase CA(+2) influx into the nerve cell and significantly lower energy production. From a pathogenetic point of view, AD is characterized by a decrease in glucose turnover in the brain. The progression of AD can be monitored by F18- deoxyglucose PET studies. This technique also allows the recognition of patients who are prone to develop AD. The actual development of a cognitive deficit is a threshold phenomenon that occurs if glucose turnover in the hippocampus or temporoparietal cortex drops below a critical level of about 40% of the level of age-matched controls. The low glucose turnover in AD causes a cholinergic deficit by decreasing the synthesis of AcCoA, which is used by choline acetyltransferase in the acetylation of choline to acetylcholine. The decrease in glucose turnover also reduces oxidative phosphorylation. The resulting decrease in ATP triggers the hyperphosphorylation of tau protein by activating protein kinase 40erk. The hyperphosphorylation leads to the development of paired helical filaments. The generation of beta amyloid and the loss of neuronal synapses are also caused by a decrease in oxidative phosphorylation, since beta amyloid precursor proteins are not inserted into the membranes of nerve cells in the absence of a sufficient amount of ATP. This results in the generation of intact beta amyloid molecules and leads to amyloidosis in the brains of patients with Alzheimer's disease.

Glucose attenuation of atropine-induced deficits in paradoxical sleep and memory

When administered systemically, glucose attenuates deficits in memory produced by several classes of drugs, including cholinergic antagonists and opiate agonists. Glucose also enhances memory in aged rats, mice, and humans. In addition, glucose ameliorates age-related reductions in paradoxical sleep. Because deficits in paradoxical sleep are most marked in those individual aged rats that also have deficits in memory, treatments which improve one of these functions may similarly improve the other. The present experiments show that glucose attenuates deficits in paradoxical sleep and memory after atropine administration, with similar dose-response curves for both actions. In the first experiment, rats received saline, atropine (1 mg/kg), glucose (100 mg/kg) or combinations of atropine + glucose (10, 100, 250, and 500 mg/kg) 30 min before assessment on a spontaneous alternation task. In the second experiment, 3-h EEGs were assessed for spontaneous daytime sleep in rats administered saline, atropine (1 mg/kg), glucose (100 mg/kg) or combinations of atropine + glucose (10, 100 and 250 mg/kg). In both experiments, glucose significantly attenuated deficits at an optimal dose of 100 mg/kg. A third experiment assessed blood glucose levels after injections of atropine + glucose (100 mg/kg) and determined that blood glucose levels were similar to those produced by other treatments which enhance memory. These results are consistent with the view that paradoxical sleep and at least one test of memory are similarly influenced by atropine and glucose.

Glucose injections into the medial septum reverse the effects of intraseptal morphine infusions on hippocampal acetylcholine output and memory

Morphine infusions into the medial septum produce memory deficits which can be attenuated by concurrent intraseptal injections of glucose. The mnemonic deficits following intraseptal morphine injections may be due, in part, to opioid inhibition of cholinergic neurons projecting to the hippocampus, with glucose reducing the effect. The present experiment determined whether glucose injections into the medial septum attenuate the effects of intraseptal morphine injections on hippocampal acetylcholine release and on memory. Samples of extracellular acetylcholine levels were assessed at 12 min intervals using in vitro microdialysis with high-performance liquid chromatography with electrochemical detection. Intraseptal morphine injections (4.0 nmol) reduced acetylcholine output starting at 12 min and lasting up to 72 min post-injection. Glucose (18.3 nmol) injected concomitantly with morphine reversed the drug infusions in the septum 20 min prior to spontaneous alternation testing. Intraseptal morphine infusions reduced alternation scores; this behavioral effect was reversed by concurrent glucose infusions. The effect of drugs infused into the septal area on spontaneous alternation performance and acetylcholine output were positively correlated. These findings suggest that memory deficits induced by intraseptal morphine injections may result, at least partially, from a decrease in the activity of cholinergic neurons and that this effect is reversed by glucose.

Glucose enhancement of scopolamine-induced increase of hippocampal high-affinity choline uptake in mice: relation to plasma glucose levels

The administration of glucose has been shown to improve memory for various learning tasks in rodents. In humans, glucose also increases declarative memory performance in elderly people and in some patients with mild Alzheimer's disease. One of the possible physiological bases for the effect of glucose on memory processes is a facilitation of cholinergic function through increased synthesis. In support of this hypothesis, glucose was shown to attenuate the amnesia induced by scopolamine and, in similar conditions, glucose increased extracellular levels of acetylcholine following a scopolamine injection. To further examine the interaction between glucose and cholinergic function, the present experiment measured the effects of combined injections of glucose and scopolamine on hippocampal sodium-dependent high-affinity choline uptake, an indirect index of cholinergic activity. Results showed that the injection of 3 g/kg glucose enhanced the increase in high affinity choline uptake in hippocampal synaptosomes produced by scopolamine. A regression analysis revealed the existence of a positive correlation between plasma blood glucose level and hippocampal choline uptake particularly in the animals receiving a combined injection of scopolamine and glucose. These data further support the hypothesis that glucose administration can facilitate acetylcholine synthesis under certain conditions and that this action could explain how glucose attenuates scopolamine-induced amnesia.

Stimulation of choline acetyl transferase activity by l- and d-carnitine in brain areas of neonate rats

Acetyl-CoA supply to the cytosol and its regulatory influence on acetylcholine biosynthesis is still an unsolved question. Acetylcarnitine through the carnitine acetyl transferase (CarAT) system has been proposed to be the acetyl donor in this process. Carnitine isomers were injected into rat developing brains every day for 14 days after birth. Results showed that carnitine and its associated forms produced a choline acetyl transferase (ChAT) activity increase in the striatum and the hippocampus. Carnitine acetyl transferase activity was stimulated by the treatment of l-carnitine in the hippocampus but it remained unchanged in the striatum and the cerebral cortex. These results suggest that ChAT and CarAT activities might be modulated by Acetyl-CoA derived preferentially from acetylcarnitine. It is suggested that ChAT activity enhancement depends on intrinsic and extrinsic cholinergic afferents to these brain areas.

Glucose attenuates a morphine-induced decrease in hippocampal acetylcholine output: an in vivo microdialysis study in rats

Systemic injections of morphine impair performance in memory tests. Glucose administration ameliorates memory deficits produced by morphine treatment. The memory impairments induced by morphine may be related to opioid inhibition of acetylcholine release with reversal of this effect by glucose. The present experiment determined whether: (1) systemic morphine treatment decreases acetylcholine output in the hippocampal formation; and (2) systemic glucose administration attenuates the effect of morphine treatment. Employing microdialysis, samples were collected at 12-min intervals and assayed for acetylcholine using HPLC with electrochemical detection. Morphine (10 mg/kg)/saline injections resulted in an immediate decrease in acetylcholine output (20-35%) that was observed up to the third postinjection sample (36 min). Glucose (100 mg/kg) administered concurrently with morphine attenuated the reduction in acetylcholine output in the second and third samples. These findings suggest that glucose may attenuate morphine-induced memory impairments by reversing a decrease in acetylcholine output produced by morphine.

Glucose attenuates the effect of combined muscarinic-nicotinic receptor blockade on spontaneous alternation

Glucose administration reverses the effects of both muscarinic and nicotinic cholinergic receptor antagonists on memory and other measures. In experiment 1, we found that glucose attenuated impairments on spontaneous alternation after muscarinic (scopolamine, 0.5 mg/kg) or nicotinic (mecamylamine, 5.0 mg/kg) receptor blockade. In experiment 2, we examined whether glucose could reverse the spontaneous alternation impairments produced by combined muscarinic-nicotinic receptor blockade. Scopolamine (0.1 mg/kg) and mecamylamine (2.5 mg/kg) when administered separately did not modify alternation performance, but when coadministered they decreased spontaneous alternation scores. This decrease was attenuated by glucose at 100, 300, 500 and 3000 mg/kg. These findings suggest that glucose may attenuate the behavioral impairment by enhancing cholinergic activity and/or other neurotransmitter systems.

Effects of supplemental choline on extracellular acetylcholine in the nucleus accumbens during normal behavior and pharmacological acetylcholine depletion

Brain microdialysis was used to determine whether systemic or local application of choline would modify the extracellular concentration of acetylcholine (ACh) in the nucleus accumbens (NAc) of freely moving rats. Supplemental choline given intraperitoneally or into the NAc of normal rats did not increase extracellular ACh. When local ACh interneurons in the NAc were treated pharmacologically to deplete the intracellular stores of ACh, then systemic choline (80 mg/kg) was an effective treatment. Specifically, 1) blockade of the high-affinity choline transporter with hemicholinium-3 (HC-3) to reduce ACh synthesis caused a decrease in extracellular ACh, but choline supplementation restored ACh toward its normal level in the NAc. 2) Local bicuculline treatment released ACh to the point of depletion, but systemic choline or locally infused choline helped maintain normal ACh levels. These results suggest that choline supplementation might be useful in preventing depletion of ACh in the nucleus accumbens during pathological conditions.

Biphasic striatal acetylcholine release during and after transient cerebral ischemia in gerbils

Acetylcholine (ACh) release into the extracellular space was measured by HPLC with electrochemical detection after in vivo intracerebral microdialysis in the striatum of gerbils subjected to 15 min of bilateral carotid artery occlusion followed by 5 h of recirculation. Tissue ACh and choline (Ch) contents were also determined during ischemia and after 5, 30, 60, and 120 min of reflow. Fifteen minutes of ischemia led to a significant transient increase in extracellular ACh concentration (threefold after 7.5 min of ischemia) concomitant with a reduced endogenous ACh level (-62%) and increased tissue Ch content (ninefold). Recirculation significantly reduced the ACh release during the early period of reflow (-50% vs. basal level), followed by a significant increase in ACh release between 1 and 3 h of reflow (45-55% vs. basal level) and subsequent normalization. Simultaneously, a "rebound" of tissue ACh level occurred in the early period of reflow (fourfold vs. ischemic value), followed by gradual normalization after 2 h of reperfusion, whereas a rapid decrease in tissue Ch levels was found after 30 min of reflow. These findings represent the first demonstration of a biphasic release of ACh during ischemia and reperfusion, as assessed by intracerebral microdialysis in gerbils.

Effects of tetrodotoxin, Ca2+ absence, d-tubocurarine and vesamicol on spontaneous acetylcholine release from rat muscle

1. Rat hemidiaphragms were incubated in a physiological low-K+ medium without stimulation and the amount of acetylcholine (ACh) released was measured radioenzymatically. Cholinesterases were inhibited by paraoxon. 2. In the presence of 1 microM tetrodotoxin (TTX), the amount of ACh released during a 2 h incubation was lowered by 40%. A similar decrease was observed in the absence of Ca2+ and in the presence of 10 microM-d-tubocurarine (dTC). The effects of TTX combined with Ca2+ removal, and of TTX combined with dTC were no greater than those of TTX, dTC or Ca2+ removal alone. TTX and dTC had no effect on the release of ACh from diaphragms 4 days after denervation. 3. The reduction of spontaneous ACh release observed in the presence of TTX or dTC or in the absence of Ca2+ is best interpreted on the assumption that about 40% of the ACh release was due to the impulse activity known to be generated in intramuscular motor nerve branches by the ACh which accumulates after the inhibition of cholinesterases. 4. In the presence of 1 and 10 microM vesamicol (AH5183, 2-(4-phenylpiperidino)-cyclohexanol), the release of ACh was also diminished by approximately 40%. Vesamicol did not augment the inhibition of release produced by TTX or by the omission of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcarnitine, carnitine and glucose diminish the effect of muscarinic antagonist quinuclidinyl benzilate on striatal acetylcholine content

The content of acetylcholine (ACh) in the striatum, brain cortex and hippocampus of rats was lowered 20-180 min after intraperitoneal injection of the muscarinic antagonist quinuclidinyl benzilate (QNB). The depletion of ACh content in the striatum was diminished in animals treated with a single dose of acetyl-L-carnitine, L- or D,L-carnitine, or D-glucose. It is likely that QNB stimulates ACh release by blocking presynaptic muscarinic autoreceptors and that acetylcarnitine, carnitine and glucose support the resynthesis of ACh by increasing the availability of acetylcoenzyme A. They do not have the same consistent effect in the brain cortex and hippocampus; this difference may be related to the lower turnover rate of ACh and to the difference in the anatomical arrangement of cholinergic structures in these parts of the brain.

Glucose attenuation of paradoxical sleep deficits in old rats

Glucose administration enhances memory in several amnestic populations, including old humans and rodents. The present experiment demonstrates that glucose also enhances measures of sleep in old rats. Three-hour day-time sleep EEGs were assessed in 3- and 24-month-old rats. The animals received injections of saline or glucose (100, 500, and 1000 mg/kg) on different days in a counter-balanced order. At doses of 100 and 500 mg/kg, glucose augmented the duration of paradoxical sleep bouts and total paradoxical sleep time in old, but not young, rats. Within 2 weeks after the sleep tests, measures of several brain neurotransmitter functions were obtained. Glucose was more effective in enhancing paradoxical sleep in those individual aged rats with high levels of hippocampal choline acetyltransferase and occipital cortex serotonin concentrations than in aged rats with lower levels on these neurochemical measures. The findings suggest that glucose attenuates selective age-related sleep deficits in old rats. More generally, these results add to a growing body of evidence indicating that moderate doses of peripheral glucose can influence a variety of CNS measures.

Acetylcholine formation from glucose following acute choline supplementation

The effects of choline administration on acetylcholine metabolism in the central nervous system are controversial. Although choline supplementation may elevate acetylcholine (ACh) content in brain, turnover studies with labelled choline precursors suggest that systemic choline administration either has no effect or actually diminishes brain ACh synthesis. Since choline supplementation elevates brain choline levels, the apparent decreases in previous turnover studies may reflect dilution of the labelled choline precursor pool rather than altered ACh formation. Therefore, brain ACh formation from [U-14C]glucose was determined after choline supplementation. A two to three fold elevation of brain choline did not alter ACh levels or [U-14C]glucose incorporation into ACh in the cortex, hippocampus or striatum. Although atropine stimulated ACh formation from [U-14C]glucose in hippocampus, two to three fold increases in brain choline did not augment ACh synthesis or content in atropine pretreated animals. Atropine depressed brain regional glucose utilization and this effect was not reversed by choline treatment. These results suggest that short-term elevation of brain choline does not enhance ACh formation from [U-14C]glucose, and argue against enhanced presynaptic cholinergic function after acute, systemic choline administration.

Positive and negative effects of tacrine (tetrahydroaminoacridine) and methoxytacrine on the metabolism of acetylcholine in brain cortical prisms incubated under "resting" conditions

The effects of tacrine (1,2,3,4-tetrahydro-9-aminoacridine) and 7-methoxytacrine on the metabolism of acetylcholine were investigated in experiments on prisms of rat cerebral cortex incubated in vitro in low-potassium (3 mmol/L K+) media; cholinesterases were inactivated by paraoxon to avoid any action of tacrine and methoxytacrine via their inhibition. Under "resting" conditions, tacrine and methoxytacrine increased the synthesis of unlabeled acetylcholine in the prisms; at the same time, they inhibited the uptake of [14C]choline from the medium and the synthesis of [14C]acetylcholine. The concentration of free choline was not increased by tacrine or methoxytacrine in either the tissue or the medium. The contradiction between the increased synthesis of unlabeled and the diminished synthesis of labeled acetylcholine indicates that the utilization of intracellular choline (which is presumably mobilized from intracellular choline esters) for the synthesis of acetylcholine is increased by tacrine and methoxytacrine. This conclusion is supported by the observation that the inhibition of acetylcholine synthesis during incubation with hemicholinium-3 (an inhibitor of choline transport into cholinergic nerve terminals) was overcome when tacrine was present simultaneously with hemicholinium-3. When the prisms were preincubated with [14C]choline and incubated with tacrine or methoxytacrine only after this, the amount of [14C]acetylcholine recovered in the tissue plus the medium was higher at the end of incubation with tacrine or methoxytacrine than without them, again suggesting that the drugs were able to increase the utilization of intracellular [14C]choline or its esters for acetylcholine synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose and physostigmine effects on morphine- and amphetamine-induced increases in locomotor activity in mice

Recent findings indicate that glucose antagonizes several behavioral effects of cholinergic antagonists and augments those of cholinergic agonists. For example, scopolamine elicits increased locomotor activity, an action which is attenuated by glucose and by combined treatment with glucose and physostigmine at doses which are individually without effect. Opiate and catecholamine agonists, such as morphine and amphetamine, also elicit hyperactivity. The present study examined interactions of glucose and physostigmine with morphine- and amphetamine-induced hyperactivity. Mice received saline, morphine (10 mg/kg), or amphetamine (1 mg/kg) 50 min prior to testing, followed by saline, physostigmine (0.01, 0.05, 0.1, or 0.2 mg/kg), or glucose (10, 50, 100, or 500 mg/kg) administered 20 min prior to activity testing in an open field. Physostigmine significantly attenuated both morphine- and amphetamine-induced increases in activity, but higher doses were required to attenuate the effects of amphetamine. Like physostigmine, glucose significantly attenuated morphine-induced activity levels, but unlike physostigmine, glucose did not attenuate amphetamine-induced activity. Thus, the behavioral effects of morphine were more susceptible to modification by physostigmine and glucose than were the effects of amphetamine. The attenuation of morphine-induced hyperactivity demonstrates a similarity between glucose and cholinergic agonists, and also indicates that glucose may inhibit, directly or indirectly, opiate functions. More generally, these findings add to the evidence that circulating glucose levels selectively influence a growing list of behavioral and neurobiological functions.

Effects of atropine on the release of newly synthesized acetylcholine from rat striatal slices at various concentrations of calcium ions

The release of acetylcholine (ACh) from brain tissue is known to be inhibited by muscarinic autoreceptors on cholinergic nerve terminals but the mechanism of the inhibition is not understood. Atropine brings about an increase of ACh release by removing the inhibitory action of autoreceptors. We investigated whether the effect of atropine on the release of [14C]ACh newly synthesized during incubations from [U-14C] glucose depends on the concentration of Ca2+ in the medium. In rat striatal slices incubated in the presence of an inhibitor of cholinesterases and of 30 mmol/l K+, significant increases in the release of [14C]ACh elicited by atropine were only observed during incubations with very low concentrations of Ca2+. This finding supports the view that the activation of presynaptic muscarinic autoreceptors in the brain affects the release of ACh by reducing the availability of Ca2+ that is required for transmitter liberation.

Effects of soman and sarin on high affinity choline uptake by rat brain synaptosomes

Synaptosomes were incubated at various time intervals following injection of 120 micrograms/kg SC of soman or sarin or with various concentrations (10(-8) to 10(-2) M) of soman or sarin in vitro. Total cholinesterase (ChE) activities in each brain region were also measured. Following soman injection, sodium-dependent, high affinity choline uptake (SDHACU) was decreased from 1 to 4 hr in the cortex and from 1 to 2 hr in the hippocampus, but increased from 2 to 24 hr in the striatum. Similarly, following sarin injection SDHACU was decreased at 0.5 hr in the cortex and from 1 to 4 hr in the hippocampus, but increased at 1 hr in the striatum. Injection of soman severely inhibited (83-99%) total ChE activity in the cortex, hippocampus and striatum from 1 to 24 hr. In contrast, sarin did not severely inhibit ChE activity in these regions and maximal inhibition (40-60%) did not occur until 24 hr after injection. With both compounds, by 168 hr ChE activity in all regions had partially recovered. Incubation of synaptosomes with soman or sarin in vitro at concentrations below 10(-4) M did not affect SDHACU in any of the brain regions. These data demonstrated that acute soman and sarin injection produced similar effects upon SDHACU in different brain regions, although the time-course of these effects was different for the two compounds. These effects were probably neither due to a direct action of these compounds on the uptake process nor dependent on ChE inhibition.

Acute choline supplementation in vivo enhances acetylcholine synthesis in vitro when neurotransmitter release is increased by potassium

The main objective of these studies was to determine whether the acute administration of choline to rats provides supplemental precursor that can be used to support acetylcholine synthesis when the demand for choline is increased by increasing neurotransmitter release. For these experiments, hippocampal and striatal slices were prepared form rats that had received saline or an acute injection of choline. Slices were incubated in a choline-free buffer containing 4.74-35 mM KCl, and acetylcholine synthesis and release and choline production were measured. The initial tissue contents of acetylcholine and choline did not differ between experimental groups for either brain region. When hippocampal slices from the controls were incubated for 10 min with depolarizing concentrations of KCl, acetylcholine release increased and the tissue content decreased in a concentration-dependent fashion; no net synthesis of acetylcholine occurred. In contrast, hippocampal slices from the choline-injected animals maintained their tissue content in the presence of high concentrations of KCl, despite an increase in acetylcholine release that was similar in magnitude to that of the controls; positive net synthesis of acetylcholine resulted. Although the molar concentration of choline achieved in the incubation media at the end of the 10-min period did not differ between groups, the mobilization of free choline from bound stores was significantly greater in hippocampal slices from the choline-injected group than the controls. In addition, the synthesis of acetylcholine by hippocampal slices from the choline-injected group was prevented by the presence of hemicholinium-3 (1 microM) in the media.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of dietary choline availability and neuronal demand on acetylcholine synthesis by rat brain

The main objective of this study was to test the hypothesis that the chronic administration of choline supplements a bound pool of choline from which free choline can be mobilized and used to support acetylcholine synthesis when the demand for precursor is increased. For these experiments, brain slices from rats fed diets containing different amounts of choline were incubated in a choline-free buffer and acetylcholine synthesis was measured under resting conditions and in the presence of K+-induced increases in acetylcholine synthesis and release. Rats fed the choline-supplemented diet had circulating choline levels that were 52% greater than the controls, and striatal and cerebral cortical slices from this group produced significantly more free choline during the incubation than slices from the controls. However, the synthesis and release of acetylcholine by these tissues did not differ from those by controls, during either resting or K+-evoked conditions. In contrast, acetylcholine synthesis and release by striatal and hippocampal slices from choline-deficient rats, animals that had circulating choline levels that were 80% of control values, decreased significantly; the production of free choline by these tissues was also depressed. Results indicate that, despite an increased production of free choline by brain slices from choline-supplemented rats, the synthesis of acetylcholine was unaltered, even in the presence of an increased neuronal demand. In contrast, the choline-deficient diet led to a decreased release of free choline from bound stores and an impaired ability of brain to synthesize acetylcholine.

Choline as an agonist: determination of its agonistic potency on cholinergic receptors

These experiments examined the potency of choline as a cholinergic agonist at both muscarinic and nicotinic receptors in rat brain and peripheral tissues. Choline stimulated the contraction of isolated smooth muscle preparations of the stomach fundus, urinary bladder and trachea and reduced the frequency of spontaneous contractions of the right atrium at high micromolar and low millimolar concentrations. The potency of choline to elicit a biological response varied markedly among these tissues; EC50 values ranged between 0.41 mM in the fundus to 14.45 mM in the atrium. Choline also displaced [3H]quinuclidinyl benzilate binding in a concentration-dependent manner although, again, its potency varied among different brain regions (Ki = 1.2 to 3.5 mM) and peripheral tissues (Ki = 0.28 to 3.00 mM). Choline exhibited a comparable affinity for nicotinic receptors. It stimulated catecholamine release from the vascularly perfused adrenal gland (EC50 = 1.3 mM) and displaced L-[3H]nicotine binding to membrane preparations of brain and peripheral tissues (Ki = 0.38 to 1.17 mM). However, the concentration of choline required to bind to cholinergic receptors in most tissues was considerably higher than serum levels either in controls (8-13 microM) or following the administration of choline chloride (200 microM). These results clearly demonstrate that choline is a weak cholinergic agonist. Its potency is too low to account for the central nervous system effects produced by choline administration, although the direct activation of cholinergic receptors in several peripheral tissues may explain some of its side effects.

Acetylcholine and choline in rat adrenals and brain cortex prisms incubated at elevated concentrations of choline in the medium

Experiments were performed with rat adrenals and brain cortex prisms incubated in vitro in order to clarify whether it is possible to increase their acetylcholine (ACh) content by adding a high concentration of choline to the medium and whether the additional ACh formed can be released by subsequent depolarization. After 60 min incubation with 0.5 mmol/l choline, the concentration of ACh in the adrenals was increased by 116% (compared to the incubation without added choline), while in cortical prisms the observed increase (by 37%) was statistically non-significant. The content of ACh in both tissues was raised by paraoxon during incubations without added choline, but paraoxon did not augment the increased concentration of ACh in tissues incubated with added 0.5 mmol/l choline. The ACh that accumulated in the adrenals during 60 min preincubations with added choline could be released during subsequent depolarizing incubations; the release was Ca2+ independent. In contrast to brain cortex prisms and to the adrenals preincubated without choline, no resynthesis of ACh occurred during the period of depolarization in the adrenals preincubated with 0.5 mmol/l choline. Large amounts of choline accumulated in both tissues during incubations with 0.5 mmol/l choline and the accumulated choline could be released by depolarization; the release of choline from the adrenals was Ca2+ independent. Free choline was produced in the adrenals (presumably from choline esters) during the periods of depolarization. The reason for differences between the effects of increased concentrations of choline on ACh in the adrenals and in brain cortex is not known.(ABSTRACT TRUNCATED AT 250 WORDS)

Increases in the concentration of brain alpha-bungarotoxin binding sites induced by dietary choline are age-dependent

We have previously reported that a diet supplemented with choline induces an increase in the concentration of a brain nicotinic-like receptor, as measured by alpha-bungarotoxin (BuTX) binding. Here we report the effects of choline administered in the drinking water on BuTX binding in the cortex, midbrain and brainstem of rats at 3 ages. In comparison with animals fed a choline-free diet, choline supplementation produced increases averaging 50% in 23-day-old rats and increases of approximately 30% in 60-day-old rats. Increases were also found in 6-month-old animals (averaging 16%), but the differences were generally not statistically significant. The mechanism responsible for the increase in the concentration of BuTX binding sites following the administration of dietary choline is not known, but the results are discussed in terms of choline as a precursor for the biosynthesis of acetylcholine and the biosynthesis of phospholipids. These data indicate that the administration of dietary choline is not likely to be effective in reversing cholinergic deficits by increasing the concentration of nicotinic-like receptors in aging rats.

Diisopropylfluorophosphate inhibits choline efflux from the perfused rat hemidiaphragm

Acetylcholine (ACh) synthesis by motor nerve terminals requires an adequate supply of its precursor, choline. The results reported here show that diisopropylfluorophosphate (DFP), an acetylcholinesterase inhibitor commonly used in ACh release studies, reduces the rate of endogenous choline efflux from the perfused rat hemidiaphragm. Perfusion of the isolated hemidiaphragm with 10 microM or 100 microM DFP reduced choline efflux by 39% and 69% respectively. DFP administration to rats (6 mg/kg) also lowered the in vitro release of choline by 33%. The rate of ACh release from hemidiaphragm preparations perfused with DFP was significantly lower than the rate of release from preparations perfused with physostigmine, an acetylcholinesterase inhibitor which had no effect on choline efflux. The addition of choline (10-30 micron) to the perfusion medium restored the rate of ACh release from DFP-treated hemidiaphragms but did not further elevated ACh release from physostigmine-treated preparations. These results demonstrate that DFP inhibits choline efflux from the isolated hemidiaphragm and further suggest that, by limiting the availability of choline for ACh synthesis, DFP reduces the rate of ACh release.

Interaction of choline and scopolamine in human memory

In a previous study (1), 8 gm. of oral choline given in a single dose were found to partially reverse the amnestic and subjective effects of 0.43 mg of scopolamine. The present study determined whether similar effects could be produced by 14 gm of choline given in 4 divided doses over 24 hrs and whether choline's effects in either study were related to plasma choline concentration. Ten normal subjects were given memory tests on three separate days, once after receiving 14 gm choline and 0.35 mg scopolamine (Ch-Sc), once following placebo and scopolamine (Pl-Sc), and once following placebo and placebo (Pl-Pl). Scopolamine markedly impaired memory performance but there was no difference between the Ch-Sc and Pl-Sc conditions. Plasma choline levels were significantly elevated in the Ch-Sc condition of both the single dose and divided dose studies. There was no difference in levels between studies nor were differences in memory performance correlated with elevations in plasma choline. These results indicate that any enhancement of cholinergic activity due to increased dietary choline is transient and of small magnitude.

Activation of muscarinic receptors stimulates the release of choline from brain slices

The effect of several agents known to interact with muscarinic acetylcholine receptors on the release of choline from slices of rat corpus striatum into the incubation medium has been investigated. The amount of released choline was increased if choline, acetylcholine, or oxotremorine had been added to the incubation medium. Atropine blocked the effects of acetylcholine and oxotremorine; it was not tested with choline. It is proposed that the increased release of choline is due to an increased hydrolysis of phosphatidylcholine, brought about by the activation of muscarinic acetylcholine receptors.

Choline and cholinergic neurons

Mammalian neurons can synthesize choline by methylating phosphatidylethanolamine and hydrolyzing the resulting phosphatidylcholine. This process is stimulated by catecholamines. The phosphatidylethanolamine is synthesized in part from phosphatidylserine; hence the amino acids methionine (acting after conversion to S-adenosylmethionine) and serine can be the ultimate precursors of choline. Brain choline concentrations are generally higher than plasma concentrations, but depend on plasma concentrations because of the kinetic characteristics of the blood-brain-barrier transport system. When cholinergic neurons are activated, acetylcholine release can be enhanced by treatments that increase plasma choline (for example, consumption of certain foods).