Seizures increase acetylcholine and choline concentrations in rat brain regions

Seizures induced by three convulsant treatment produced differential effects on the concentration of acetylcholine in rat brain. Status epilepticus induced by (i) coadministration of lithium and pilocarpine caused massive increases in the concentration of acetylcholine in the cerebral cortex and hippocampus, (ii) a high dose of pilocarpine did not cause an increase of acetylcholine, and (iii) kainate increased acetylcholine, but the magnitude was lower than with the lithium/pilocarpine model. The finding that the acetylcholine concentration increases in two models of status epilepticus in the cortex and hippocampus is in direct contrast with manyin vitro reports in which excessive stimulation causes depletion of acetylcholine. The concentration of choline increased during seizures with all three models. This is likely to be due to calcium- and agonist-induced activation of phospholipase C and/or D activity causing cleavage of choline-containing lipids. The excessive acetylcholine present during status epilepticus induced by lithium and pilocarpine was responsive to pharmacological manipulation. Atropine tended to decrease acetylcholine, similar to its effects in controls. The N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, reduced the excessive concentration of acetylcholine, especially in the cortex. Inhibition of choline uptake by hemicholinium-3 (HC-3) administered icv reduced the acetylcholine concentration in controls and when given to rats during status epilepticus. These results demonstrate that the rat brain concentrations of acetylcholine and choline can increase during status epilepticus. The accumulated acetylcholine was not in a static, inactive compartment, but was actively turning-over and was responsive to drug treatments. Excessive concentrations of acetylcholine and/or choline may play a role in seizure maintenance and in the neuronal damage and lethality associated with status epilepticus.     

Increased Acetylcholine Synthesis and Release in Brains of Cats with GM1 Gangliosidosis

Cholinergic processes were measured in motor cortex, hippocampus, and striatum of cats in the terminal stages of GM1 gangliosidosis and compared to those of control cats. The greatest difference observed was elevation in the rate of K+-stimulated release of acetylcholine (ACh) from brain slices prepared from affected cats. The K+-stimulated release of endogenous ACh was increased by 31-43% and of newly synthesized ACh by 19-80% in brain slices from different brain regions. All regions that were examined were affected but the greatest effects occurred in cortex. The rate of synthesis of ACh was elevated in cortical and hippocampal slices. Choline acetyltransferase activity in brain regions of cats with GM1 gangliosidosis was not significantly different from that in controls, whereas high-affinity choline transport in cortical synaptosomes was elevated. Muscarinic receptor binding sites were reduced in the cortex, hippocampus, and striatum of GM1 mutant cats, whereas the apparent affinity was not altered. These results indicate that there are major alterations of cholinergic function in the brains of cats with GM1 gangliosidosis.     

RELATIONSHIP BETWEEN CHOLINE AVAILABILITY AND ACETYLCHOLINE SYNTHESIS IN DISCRETE REGIONS OF RAT BRAIN

Abstract— The relationship between choline availability and the synthesis of acetylcholine in discrete brain regions was studied in animals treated with the organophosphorus cholinesterase inhibitor paraoxon. Administration of paraoxon (0.23 mg/kg) inhibited acetylcholinesterase activity by approx 90% in the striatum, hippocampus and cerebral cortex and increased acetylcholine levels to 149%, 124% and 152% of control values, respectively. Free choline levels were unaltered by paraoxon in the hippocampus and cerebral cortex, but were significantly decreased in the striatum to 74% of control. When animals were injected with choline chloride (60 mg/kg), 60 min prior to the administration of paraoxon, the paraoxon-induced choline depletion in the striatum was prevented and the paraoxon-induced acetylcholine increase was potentiated from 149% to 177% of control values. Choline pretreatment had no significant effect in either the hippocampus or cerebral cortex, brain regions that did not exhibit a decrease in free choline levels after paraoxon administration. Results indicate that choline administration, which had no significant effect on acetylcholine levels by itself, increased acetylcholine synthesis in the striatum in the presence of acetylcholinesterase inhibition. However, this effect was not apparent in either the hippocampus or the cerebral cortex at similar levels of enzyme inhibition. It appears that choline generated from the hydrolysis of acetylcholine may play a significant role in the regulation of neurotransmitter synthesis in the striatum, but not in the other brain areas studied. The evidence supports the concept that the regulatory mechanisms controlling the synthesis of acetylcholine in striatal interneurons may differ from those in other brain regions.     

Regional Acetylcholine Metabolism in Brain During Acute Hypoglycemia and Recovery

Insulin-induced hypoglycemia in normothermic rats caused progressive neurological depression and differentially altered regional cerebral acetylcholine metabolism. Reductions of plasma glucose from 7.7 mM (control) to 2.5-1.7 mM (moderate hypoglycemia associated with decreased motor activity) or 1.5 mM (severe hypoglycemia with lethargy progressing to stupor) decreased glucose concentrations in the cerebral cortex, striatum, and hippocampus to less than 10% of control. Moderate hypoglycemia diminished acetylcholine concentrations in cortex and striatum (21% and 45%, respectively) and reduced [1-2H2, 2-2H2]choline incorporation into acetylcholine (62% and 41%, respectively). Severe hypoglycemia did not reduce the acetylcholine concentration or synthesis in cortex and striatum further. The concentrations of choline rose in the cortex (+53%) and striatum (+130%) of animals that became stuporous but a similar rise in [1-2H2, 2-2H2]choline left the specific activities of choline in these structures unchanged. Even severe hypoglycemia did not alter the hippocampal cholinergic system. In rats that developed hypoglycemic stupor and were then treated with glucose, the animals recovered apparently normal behavior, and the concentrations of acetylcholine and the incorporation of [1-2H2, 2-2H2]-choline into acetylcholine returned to control values in the striatum but not in the cerebral cortex. Thus, impaired acetylcholine metabolism in selected regions of the brain may contribute to the early symptoms of neurological dysfunction in hypoglycemia.     

Chronic lithium treatment and status epilepticus induced by lithium and pilocarpine cause selective changes of amino acid concentrations in rat brain regions

We measured the effects of four weeks of dietary lithium treatment and of status epilepticus induced by administration of pilocarpine to lithium-treated rats on the concentrations of amino acids in four regions of rat brain: cerebral cortex, hippocampus, striatum, and substantia nigra. To ensure accurate quantitation of the amino acids, animals were sacrificed by focussed beam microwave irradiation and amino acids were measured using a fully validated triple-column ion-exchanged amino acid analyzer with post-column o-phthalaldehyde derivatization and fluorometric detection. The concentrations of four amino acids, threonine, methionine, lysine and tyrosine, were increased significantly in two to four brain regions by chronic lithium treatment. Their concentrations remained elevated, or were further increased, during status epilepticus. The concentrations of eight amino acids and ammonia were not altered by lithium treatment but increased in concentration during status epilepticus in some brain regions. Glycine, serine, arginine and citrulline were decreased by chronic lithium treatment. Status epilepticus increased the concentrations of these four amino acids above that found in the lithium-treated samples in some of the brain regions that were examined. Six amino acids and glutathione were generally unaltered by both treatments. These results are related to the effects of lithium treatment and are compared with changes reported by others following treatment with a variety of convulsive stimuli.     

Effects of Two Convulsant β-Carboline Derivatives, DMCM and β-CCM, on Regional Neurotransmitter Amino Acid Levels and on In Vitro D-[3H]Aspartate Release in Rodents

Clonic seizures were induced in Swiss or DBA/2 mice by methyl-6-7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), 0.048 mmol/kg i.p., or by methyl-beta-carboline-3-carboxylate (beta-CCM), 0.044 mmol/kg i.p. Measurement of regional brain (cortex, hippocampus, striatum, and cerebellum) amino acid levels after 15 min of seizure activity showed increases in gamma-aminobutyric acid (GABA) (in all regions after beta-CCM, and in cortex and hippocampus after DMCM), and an increase in glycine in the striatum after beta-CCM. Aspartate levels fell (in cortex and hippocampus) after DMCM, but were unchanged in all regions after beta-CCM. Glutamate levels fell in cortex after beta-CCM and in striatum after DMCM. Pretreatment with the excitatory amino acid receptor antagonist, 2-amino-7-phosphonoheptanoic acid, 0.5 mmol/kg i.p., 45 min prior to the beta-carboline, significantly increased the ED50 for DMCM-induced clonic seizures (4.68 mumol/kg vs. 9.39 mumol/kg). Similar pretreatment did not significantly alter the ED50 for beta-CCM (4.22 mumol/kg vs. 6.6 mumol/kg). Pretreatment with 2-amino-7-phosphonoheptanoic acid, 1.0 mmol/kg, blocked the increase in GABA content produced by DMCM but not the fall in cortical aspartate content. Potassium-induced release of preloaded D-[3H]aspartate from rat cortical or hippocampal minislices was enhanced in the presence of DMCM (100 microM). In contrast, stimulated release of D-[3H]aspartate (from cortex or hippocampus) was not altered in the presence of beta-CCM (100 microM). Although DMCM and beta-CCM are both considered to induce convulsion by acting at the GABA--benzodiazepine receptor complex, the convulsions differ in several pharmacological and biochemical respects. It is suggested that enhanced release of excitatory amino acid neurotransmitters plays a more important role in seizures induced by DMCM.     

Neuroactive Amino Acids in Focally Epileptic Human Brain: A Review

Studies of neuroactive amino acids and their regulatory enzymes in surgically excised focally epileptic human brain are reviewed. Concentrations of glutamate, aspartate and glycine are significantly increased in epileptogenic cerebral cortex. The activities of the enzymes, glutamate dehydrogenase and aspartate aminotransferase, involved in glutamate and aspartate metabolism are also increased. Polyamine synthesis is enhanced in epileptogenic cortex and may contribute to the activation of N-methyl-D-aspartate (NMDA) receptors. Nuclear magnetic resonance spectroscopy (NMRS) reveals that patients with poorly controlled complex partial seizures have a significant diminution in occipital lobe gamma aminobutyric acid (GABA) concentration. The activity of the enzyme GABA-aminotransaminase (GABA-T) which catalyzes GABA degredation is not altered in epileptogenic cortex. NMRS studies show that vigabatrin, a GABA-T inhibitor and effective antiepileptic, significantly increases brain GABA. Glutamate decarboxylase (GAD), responsible for GABA synthesis, is diminished in interneurons in discrete regions of epileptogenic cortex and hippocampus. In vivo microdialysis performed in epilepsy surgery patients provides measurements of extracellular amino acid levels during spontaneous seizures. Glutamate concentrations are higher in epileptic hippocampi and increase before seizure onset reaching potentially excitotoxic levels. Frontal or temporal cortical epileptogenic foci also release aspartate, glutamate and serine particularly during intense seizures or status epilepticus. GABA in contrast, exhibits a delayed and feeble rise in the epileptic hippocampus possibly due to a reduction in the number and/or efficiency of GABA transporters.     

Effect of Chronic Nicotine Treatment on Nicotinic Autoreceptor Function and N-[3H]Methylcarbamylcholine Binding Sites in the Rat Brain

It has been reported that N-methylcarbamylcholine (MCC), a nicotinic agonist, binds to central nicotinic receptors and causes an increase of acetylcholine (ACh) release from certain central cholinergic nerve terminals. The present experiments determine whether these two phenomena change in response to the chronic administration of nicotine, a procedure known to result in an increase in nicotinic binding sites. Chronic nicotine caused a brain region-specific up-regulation of [3H]MCC sites; binding increased in the frontal cortex, parietal cortex, striatum, and hippocampus, but not in the occipital cortex or cerebellum. The effect of nicotine was selective to nicotinic binding sites, because muscarinic sites, both M1 ([ 3H]pirenzepine) and M2 ([3H]ACh), were unaffected by chronic nicotine treatment. MCC increased the release of ACh from the frontal cortex and hippocampus by a calcium-dependent mechanism; MCC did not alter ACh release from striatum or occipital cortex of control animals. The MCC-induced increase in ACh release was not apparent in those animals which had been treated with nicotine. There was a partial recovery of nicotinic autoreceptor function when animals were allowed to recover (4 days) following chronic nicotine treatment, but the density of binding sites remained increased compared to control. Chronic nicotine did not change the potassium-evoked release of ACh from the frontal cortex or hippocampus, but decreased this measure from striatum. It also decreased the ACh content of the striatum, but not that of the cortex or the hippocampus; the activity of choline acetyltransferase was not altered in any of the regions tested.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regional CNS Levels of Acetylcholine and Choline During Hypoglycemic Stupor and Recovery

During insulin stupor in mice, acetylcholine levels in cerebral cortex, cerebellum. brainstem, striatum, and hippocampus were unchanged from control values despite brain glucose concentrations 3-10% of normal, whereas choline levels rose 2.4-3.6-fold in all five CNS regions. Brain acetylcholine and choline levels did not change during recovery following glucose injection. The data suggest that. in hypoglycemic stupor, (1) overall rates of acetylcholine synthesis and degradation remain balanced within each of the CNS regions studied: (2) the biochemical mechanism that elevates brain choline levels is unlikely to be related only to cholinergic synaptic processes: and (3) brain choline levels need not rise for stupor to occur.     

EFFECTS OF LITHIUM TREATMENT IN VITRO AND IN VIVO ON ACETYLCHOLINE METABOLISM IN RAT BRAIN

Abstract— The effects of LiCl on cholinergic function in rat brain in vitro and in vivo have been investigated. The high affinity transport of choline and the synthesis of acetylcholine in synaptosomes were reduced when part (25-75%) of the NaCl in the buffer was replaced with LiCl or sucrose. This appeared to be due to lack of Na⁺ rather than to Li⁺, as addition of LiCl to normal buffer had little effect. Following an injection of LiCl (10mmol/kg, i.p.) into rats the concentration of a pulsed dose of [²H₄]choline (20 μmol/kg, i.v., 1 min) and its conversion to [²H₄]acetylcholine, and the concentrations of [²H₂]acetylcholine and [²H₀]choline were measured in the striatum, cortex, hippocampus and cerebellum. The [²H₄]choline and [²H₄]acetylcholine were initially (15 min after LiCl) reduced (to ?30% in the cortex) and later (24 h after LiCl) increased (to + 50% in the striatum). There was a corresponding initial increase (to +50% in the cerebellum) and later decrease (to ?30% in the hippocampus) of the endogenous acetylcholine and choline. These results indicate an initial decrease and later increase in the utilization of acetylcholine after acute treatment with LiCl. Following 10 days of treatment with LiCl there was an increased rate of synthesis of [²H₄]acetylcholine from pulsed [²H₄]choline in the striatum, hippocampus and cortex (P < 0.05). The high affinity transport of [²H₄]choline and its conversion to [²H₄]acetylcholine was activated (131% of control; P < 0.01) in synaptosomes isolated from brains of 10-day treated rats. Investigation of synaptosomes isolated from striatum, hippocampus and cortex revealed that only striatal [²H₄]acetylcholine synthesis was significantly stimulated. Kinetic analysis demonstrated that the apparent K_T for choline was decreased by 30% in striatal synaptosomes isolated from rats treated for 10 days with LiCl. Striatal synaptosomes from 10-day treated rats compared to striatal synaptosomes from untreated rats also released acetylcholine at a stimulated rate in a medium containing 35 mM-KCl. These results indicate that LiCl treatment stimulates cholinergic activity in certain brain regions and this may play a significant role in the therapeutic effect of LiCl in neuropsychiatric disorders.     

Effect of electroconvulsive shock on the content of thyrotropin-releasing hormone in rat brain

Five grand-mal seizures were electrically induced in rats on alternate days. Forty-eight hours following the last seizure, TRH was quantitated in extracts of anterior cortex, hippocampus, striatum, thalamus plus midbrain, and hypothalamus. When compared to sham treated controls, TRH was found to be elevated 5-fold in the hippocampus and 2-fold in the striatum with no changes observed in the remaining regions. Since the time chosen for analysis excludes acute post-ictal effects, these results draw attention to a prolonged alteration of TRH levels in specific brain regions in an animal model of electroconvulsive treatment.     

Effects of MK-771, A TRH analog,on pentobarbital-induced alterations of cholinergic parameters in discrete regions of rat brain

MK-771 (l-pyro-2-aminoadipyl-histidyl-thiazolidine-4-carboxamide) was administered intraventricularly to conscious and pentobarbital-narcotized rats. In the conscious rats MK-771 did not affect the regional levels of acetylcholine (ACh) or the rate of sodium-dependent high-affinity choline uptake (HACU). MK-771 was found to antagonize pentobarbital-induced elevations of ACh levels in the cortex, hippocampus and striatum. MK-771 also reversed the depressant effects of pentobarbital on the HACU of the cortex and hippocampus. Striatal HACU was unaltered by the administration of pentobarbital or the combination of pentobarbital and MK-771.     

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.     

beta-Endorphin administration increases hippocampal acetylcholine levels.

The contents of acetylcholine and choline were determined in rat cortex, striatum, and hippocampus following intraventricular injection of β-endorphin or D-Ala²-enkephalinamide, a synthetic enkephalin analog, in doses known to produce analgesia in experimental animals. These opiate polypeptides produced significant increases in acetylcholine levels in the hippocampus, a subcortical structure rich in cholinergic terminals. The acetylcholine content of the hippocampus (but not the cortex or striatum) was significantly elevated 15, 30, and 60 minutes after a single intraventricular injection of β-endorphin (10 μg/brain) or D-Ala²-enkephalinamide (10 μg/brain). Peak alterations in regional acetylcholine concentrations and in analgetic effectiveness both occurred 30 minutes after peptide administration. Choline concentrations were unchanged by any of the experimental treatments. Naloxone hydrochloride (1 mg/kg, subcutaneously) affected neither brain acetylcholine concentrations, nor the response latencies of rats placed on a hot-plate; it did, however, antagonize the changes in these parameters caused by β-endorphin or D-Ala²-enkephalinamide. These data suggest that endorphins may normally regulate the physiologic activity of some cholinergic neurons.     

Increase in Exogenous Choline Fails to Elevate the Content or Turnover Rate of Cortical, Striatal, or Hippocampal Acetylcholine

Abstract: The present experiments were designed to test whether increasing the availability of choline to rat brain increases the rate of acetylcholine synthesis in that organ. The content of choline and acetylcholine and the turnover rate of acetylcholine in striatum, hippocampus, and cerebral cortex were measured following changes in dietary choline, intraperitoneal choline, or intravenous infusion of choline. Increasing plasma choline caused some increase in tissue choline but did not increase acetylcholine levels nor acetylcholine turn-over rate in any of the areas of brain studied. Indeed, in hippocampus, choline decreased the turnover rate of acetylcholine.     

Effect of nicotine on extracellular levels of neurotransmitters assessed by microdialysis in various brain regions: Role of glutamic acid

We studied the effect of local administration of nicotine on the release of monoamines in striatum, substantia nigra, cerebellum, hippocampus, cortex (frontal, cingulate), and pontine nucleus and on the release of glutamic acid in striatum of rats in vivo, using microdialysis for nicotine administration and for measuring extracellular amine and glutamic acid levels. Following nicotine administration the extracellular concentration of dopamine, increased in all regions except cerebellum; serotonin increased in cingulate and frontal cortex; and norepinephrine increased in substantia nigra, cingulate cortex, and pontine nucleus. Cotinine, the major nicotine metabolite, had no effect at similar concentrations. The cholinergic antagonists mecamylamine and atropine, the dopaminergic antagonists haloperidol and sulpiride, and the excitatory amino acid antagonist kynurenic acid all inhibited the nicotine-induced increase of extracellular dopamine in the striatum. The fact that kynurenic acid almost completely prevented the effects of nicotine, and nicotine at this concentration produced a 6-fold increase of glutamic acid release, suggests that the effect of nicotine is mainly mediated via glutamic acid release.     

Characterization of N-[3H]Methylcarbamylcholine Binding Sites and Effect of N-Methylcarbamylcholine on Acetylcholine Release in Rat Brain

The present experiments show that N-[3H]-methylcarbamylcholine ([3H]MCC) binds specifically and with high affinity to rat hippocampus, frontal cortex, and striatum. The highest maximal density of binding sites was apparent in frontal cortex and the lowest in hippocampus. [3H]MCC binding was potently inhibited by nicotinic, but not muscarinic, agonists and by the nicotinic antagonist dihydro-beta-erythroidine in all three brain regions studied. The effect of unlabeled MCC on acetylcholine (ACh) release from slices of rat brain was tested. The drug significantly enhanced spontaneous ACh release from slices of hippocampus and frontal cortex, but not from striatal slices. This effect of MCC to increase ACh release from rat hippocampus and frontal cortex was antagonized by the nicotinic antagonists dihydro-beta-erythroidine and d-tubocurarine, but not by alpha-bungarotoxin or by the muscarinic antagonist atropine. The MCC-induced increase in spontaneous ACh release from hippocampal and frontal cortical slices was not affected by tetrodotoxin. The results suggest that MCC might alter cholinergic transmission in rat brain by a direct activation of presynaptic nicotinic receptors on the cholinergic terminals. That this alteration of ACh release is apparent in hippocampus and frontal cortex, but not in striatum, suggests that there may be a regional specificity in the regulation of ACh by nicotinic receptors in rat brain.     

Effect of specific serotonergic lesions on cholinergic neurons in the hippocampus,cortex and striatum

Local injection of 5, 7-dihydroxytryptamine into the median raphe nucleus of rats pretreated with desipramine decreases the serotonin content of the hippocampus and cortex. The turnover of acetylcholine, as measured by the rate of decline of acetylcholine content after hemicholinium-3, the rate of decline of acetylcholine content after hemicholinium-3, is not affected in the hippocampus or the striatum, but is increased in the cortex by such treatment. Local injection of 5, 7-dihydroxytryptamine into the dorsal raphe nucleus of desipramine-treated rats decreases the serotonin content of the hippocampus, cortex, and striatum. The turnover of acetylcholine is increased in the hippocampus and cortex, but not affected in the striatum. Thus, serotonergic neurons from the median raphe nucleus appear to tonically inhibit cholinergic neurons in the cortex, and serotonergic neurons from the dorsal raphe nucleus appear to tonically inhibit cholinergic neurons in the hippocampus and cortex. These serotonergic neurons do not appear to act tonically on striatal cholinergic neurons.     

Exogenous Choline Enhances the Synthesis of Acetylcholine Only Under Conditions of Increased Cholinergic Neuronal Activity

Abstract: The effect of choline (60 mg/kg, i.p.) on fluphenazine- and pentylenetetrazol-induced alterations in the concentration of acetylcholine (ACh) and/or the rate of sodium-dependent high-affinity choline uptake (HACU) in rat striatum and hippocampus was studied. Systemic administration of the dopamine receptor blocking agent fluphenazine hydrochloride (0.5 mg/kg, i.p.) decreased the concentration of ACh in the striatum; this effect was prevented by the prior administration of choline. The central nervous system stimulant pentylenetetrazol (30 mg/kg, i.p.) reduced the concentration of ACh in both striatum and hippocampus and increased the velocity of HACU in the hippocampus. Pretreatment with choline totally prevented the depletion of ACh induced by pentylenetetrazol in the striatum. In the hippocampus, prior administration of choline prevented the pentylenetetrazol-induced increase in the rate of HACU and attenuated the effect of pentylenetetrazol on the levels of ACh. Results indicate that the acute administration of choline antagonizes pharmacologically induced alterations in cholinergic activity as assessed by the rate of HACU and the steady-state concentration of ACh. Furthermore, data support the hypothesis that the administration of choline increases the ability of central cholinergic neurons to synthesize ACh under conditions of increased neuronal activity.     

Amyloid β-Peptide Inhibits High-Affinity Choline Uptake and Acetylcholine Release in Rat Hippocampal Slices

Abstract: The characteristic pathological features of the postmortem brain of Alzheimer's disease (AD) patients include, among other features, the presence of neuritic plaques composed of amyloid β-peptide (Aβ) and the loss of basal forebrain cholinergic neurons, which innervate the hippocampus and the cortex. Studies of the pathological changes that characterize AD and several other lines of evidence indicate that Aβ accumulation in vivo may initiate and/or contribute to the process of neurodegeneration and thereby the development of AD. However, the mechanisms by which Aβ peptide influences/causes degeneration of the basal forebrain cholinergic neurons and/or the cognitive impairment characteristic of AD remain obscure. Using in vitro slice preparations, we have recently reported that Aβ-related peptides, under acute conditions, potently inhibit K⁺-evoked endogenous acetylcholine (ACh) release from hippocampus and cortex but not from striatum. In the present study, we have further characterized Aβ-mediated inhibition of ACh release and also measured the effects of these peptides on choline acetyltransferase (ChAT) activity and high-affinity choline uptake (HACU) in hippocampal, cortical, and striatal regions of the rat brain. Aβ_1–40 (10^?8M) potently inhibited veratridine-evoked endogenous ACh release from rat hippocampal slices and also decreased the K⁺-evoked release potentiated by the nitric oxide-generating agent, sodium nitroprusside (SNP). It is interesting that the endogenous cyclic GMP level induced by SNP was found to be unaltered in the presence of Aβ_1–40. The activity of the enzyme ChAT was not altered by Aβ peptides in hippocampus, cortex, or striatum. HACU was reduced significantly by various Aβ peptides (10^?14 to 10^?6M) in hippocampal and cortical synaptosomes. However, the uptake of choline by striatal synaptosomes was altered only at high concentration of Aβ (10^?6M). Taken together, these results indicate that Aβ peptides, under acute conditions, can decrease endogenous ACh release and the uptake of choline but exhibit no effect on ChAT activity. In addition, the evidence that Aβ peptides target primarily the hippocampus and cortex provides a potential mechanistic framework suggesting that the preferential vulnerability of basal forebrain cholinergic neurons and their projections in AD could relate, at least in part, to their sensitivity to Aβ peptides.