High affinity choline transport and acetylCoA production in brain and their roles in the regulation of acetylcholine synthesis

An electric lobe suppressor for a yeast choline transport mutation belongs to a new family of transporter-like proteins

Choline is an important metabolite in all cells due to the major contribution of phosphatidylcholine to the production of membranes, but it takes on an added role in cholinergic neurons where it participates in the synthesis of the neurotransmitter acetylcholine. We have cloned a suppressor for a yeast choline transport mutation from a Torpedo electric lobe yeast expression library by functional complementation. The full-length clone encodes a protein with 10 putative transmembrane domains, two of which contain transporter-like motifs, and whose expression increased high-affinity choline uptake in mutant yeast. The gene was called CTL1 for its choline transporter-like properties. The homologous rat gene, rCTL1, was isolated and found to be highly expressed as a 3. 5-kb transcript in the spinal cord and brain and as a 5-kb transcript in the colon. In situ hybridization showed strong expression of rCTL1 in motor neurons and oligodendrocytes and to a lesser extent in various neuronal populations throughout the rat brain. High levels of rCTL1 were also identified in the mucosal cell layer of the colon. Although the sequence of the CTL1 gene shows clear homology with a single gene in Caenorhabditis elegans, several homologous genes are found in mammals (CTL2-4). These results establish a new family of genes for transporter-like proteins in eukaryotes and suggest that one of its members, CTL1, is involved in supplying choline to certain cell types, including a specific subset of cholinergic neurons.

Time course for recovery of water maze performance and central cholinergic innervation after fluid percussion injury

This study further investigates the possible connection between postconcussive cognitive impairment and damage to forebrain cholinergic innervation. Moderate parasagittal fluid percussion injury was delivered to adult male rats. Water maze performance and synaptosomal choline uptake was measured at various times following injury. Water maze learning was severely impaired between 1 and 5 weeks, but recovered to normal by 10 weeks. Synaptosomal choline uptake was significantly decreased by 15-27% in the ipsilateral hippocampus and parietal cortex 3 and 7 days following injury, but not by 3 weeks or thereafter. Choline acetyltransferase was also significantly decreased in the ipsilateral cortex at 3 and 7 days with subsequent recovery. This study shows that parasagittal fluid percussion injury causes significant impairment in water maze learning and ipsilateral forebrain cholinergic innervation. Both of these parameters recover spontaneously, but with different time courses.

Neonatal chlorpyrifos exposure alters synaptic development and neuronal activity in cholinergic and catecholaminergic pathways

After routine home application of chlorpyrifos (CPF), infant and child exposures can exceed acceptable levels. We treated neonatal rats daily on postnatal days (PN) 1-4 (1 mg/kg) or days 11-14 (5 mg/kg), treatments that evoked no overt signs of toxicity. Effects on the development of cholinergic neuronal function were assessed using choline acetyltransferase (ChAT) activity and hemicholinium-3 (HC-3) binding as indices of synaptic proliferation and synaptic activity, respectively. In the forebrain, early CPF treatment caused a decrease in ChAT without affecting HC-3 binding; late treatment decreased HC-3 binding without affecting ChAT. In the brainstem, early treatment had no effect on either parameter but late treatment decreased both ChAT and HC-3 binding. Effects of CPF were not limited to development of cholinergic synapses but also involved catecholamine pathways. For norepinephrine or dopamine, either early or late CPF treatment evoked an increase in synaptic activity (transmitter turnover). The cerebellum, a region with sparse cholinergic innervation, was affected the most. Effects on catecholamine systems were unrelated to the magnitude or temporal pattern of cholinesterase inhibition. Our results suggest that CPF exposure during the postnatal period of synaptogenesis elicits widespread disruption of cholinergic and catecholaminergic pathways. As this is the period in which patterns of synaptic responsiveness is programmed by neural input, the period of developmental vulnerability to CPF is likely to extend into childhood.

Choline uptake by mouse brain capillary endothelial cells in culture

Choline, a precursor of the neurotransmitter acetylcholine, is synthesized in only small amounts in the brain, so the choline concentration in the brain may vary depending on the plasma concentration and the transport rate across the blood-brain barrier. To elucidate the transport mechanism of choline, we carried out uptake experiments with mouse brain capillary endothelial cells in culture (MBEC4). [3H]Choline uptake was linear for up to 5 min. An examination of the concentration dependence of [3H]choline uptake revealed the operation of both saturable (Jmax = 423+/-27 pmol min(-1) (mg protein)(-1) and Kt = 20.0+/-3.1 microM) and non-saturable (kd = 1.23+/-0.045 microL min(-1)(mgprotein)-1) processes. The saturable process was independent of Na+ and pH, but was dependent on membrane potential as a driving force. Various basic drugs and endogenous substances, including substrates and inhibitors of the organic cation transporter, significantly inhibited the [3H]choline uptake. These data suggest that choline was taken up into the endothelial cells via two routes and that a membrane potential-dependent carrier-mediated transport system may participate in choline transport across the blood-brain barrier.

Evidence that nerve growth factor influences recent memory through structural changes in septohippocampal cholinergic neurons

We compared, in 4- and 23-month-old Fischer-344 rats, the effects of nerve growth factor (NGF) on basal forebrain cholinergic neurons with behavioral performance in acetylcholine-dependent memory tasks (recent and reference memory). Noncholinergic monoamine markers in target fields of cholinergic neurons were also investigated. We found that NGF has contrasting effects on recent memory in the two age groups in causing improvement in aged rats and deterioration in young rats. In addition, NGF caused significant increase in the size of cholinergic perikarya in all sectors of the basal nucleus complex (BNC). Higher doses of NGF were required to produce hypertrophy in aged animals, a pattern consistent with a lower sensitivity to NGF of aged cholinergic neurons. Analysis of covariance showed that the behavioral effects of NGF were eliminated after covarying out the hypertrophy of cholinergic perikarya. Therefore, NGF causes hypertrophy of cholinergic perikarya regardless of age, and this neurobiological measure correlates with the effects of NGF on recent memory. Reference memory improved moderately only in old rats. This mild effect covaried with an increase in choline acetyltransferase activity in neocortex. Cortical terminal fields of noradrenergic and serotoninergic pathways were not affected by NGF. Taken together, our results indicate that NGF influences recent memory in an age- and transmitter-specific fashion. We postulate that the direct cause of the effects of NGF on memory is not perikaryal hypertrophy per se but rather an increased density of terminals, which always accompanies perikaryal hypertrophy. Although these results continue to support the use of NGF for the treatment of Alzheimer's disease, they raise questions regarding the therapeutic role of NGF for degeneration of BNC neurons occurring in young age.

Effects of posttraining administration of insulin on retention of a habituation response in mice: participation of a central cholinergic mechanism

Male Swiss mice were allowed to explore a novel environment, provided by an open-field activity chamber for a 10-min period. The procedure was repeated twice within a 24-h interval. The difference in the exploratory activity between the first (training) and the second exposure (testing) to the chamber was taken as an index of retention of this habituation task. Posttraining intraperitoneal administration of insulin (8, 20, or 80 IU/kg) impaired retention in a dose-related manner, although only the dose of 20 IU/kg of insulin produced significant effects. Thus, the dose-response curve adopted a U-shaped form. Insulin (20 IU/kg) given to untrained mice did not modify their exploratory performance when recorded 24 h later. The effects of insulin on retention were time dependent, suggesting an action on memory storage. An ineffective dose (8 IU/kg) of insulin given together with an ineffective dose of a central acting muscarinic cholinergic antagonist atropine (0.5 mg/kg) or with a central acting nicotinic cholinergic antagonist mecamylamine (5 mg/kg) interacted to impair retention. In contrast, neither methylatropine (0.5 mg/kg), a peripherally acting muscarinic receptor blocker, nor hexamethonium (5 mg/kg), a peripherally acting nicotinic receptor blocker, interacted with the subeffective dose of insulin on retention. The impairing effects of insulin (20 IU/kg) on retention were reversed by the simultaneous administration of physostigmine (70 microg/kg) but not neostigmine (70 microg/kg). We suggest that insulin impairs memory storage of one form of learning elicited by stimuli repeatedly presented without reinforcement, probably through a decrement of brain acetylcholine synthesis.

Organic cation transporters in intestine, kidney, liver, and brain

This review focuses on sodium-independent transport systems for organic cations in small intestine, liver, kidney, and brain. The roles of P-glycoproteins (MDR) and anion transporters (OATP) in organic cation transport are reported, and two members of the new transporter family OCT are described. The OCT transporters belong to a superfamily that includes multidrug-resistance proteins, facilitative diffusion systems, and proton antiporters. They mediate electrogenic transport of small organic cations with different molecular structures, independently of sodium and proton gradients. The current knowledge of the distribution and functional properties of cloned cation transport systems and of cation transport measured in intact plasma membranes is used to postulate identical or homologous transporters in intestine, liver, kidney, and brain.

Kinetics of muscarinic reduction of IsAHP in hippocampal neurons: effects of acetylcholinesterase inhibitors

The present experiments were designed to elucidate the time frame in which an evoked cholinergic impulse decreases the Ca2+-dependent K+ current (IsAHP) in hippocampal CA1 neurons, and to determine to what extent acetylcholinesterase (AChE) inhibitors enhance the efficacy of the cholinergic impulse. Whole cell voltage-clamp recordings were performed on hippocampal CA1 neurons of rat brain slices and IsAHPs were evoked by constant depolarizing pulses. Cholinergic afferent fibers in stratum oriens were stimulated electrically and the time interval between the afferent stimulus and the depolarizing pulse was varied from 1 to 30 s. In slices perfused with the standard external medium, the afferent stimulus caused a profound decrease in the following IsAHP only when the stimulus preceded the depolarizing pulse by 1-2 s. The stimulus was without effects on the IsAHP when applied >/=5s before the depolarizing pulse. The effects of the afferent stimulus were greatly enhanced in CA1 neurons exposed to the catalytic AChE inhibitors neostigmine, physostigmine, or 9-amino-1,2,3, 4-tetrahydro-acridine. A substantial decrease in the IsAHP was observed even when the stimulus preceded the depolarizing pulse by >/=30 s. However applications of peripheral site AChE inhibitors decamethonium and propidium caused only minor or no enhancement of the IsAHP reduction after the afferent stimulus. We suggest in physiological conditions that muscarinic modulation of ionic conductances of CNS neurons has a limited time course after a cholinergic impulse and that the modulation is greatly enhanced and prolonged when catalytic activities of AChEs are suppressed pharmacologically.

Modulation of acetylcholine release in human cortical slices: possible implications for Alzheimer's disease

Superfused slices of human neocortex, prepared from surgically removed tissue (to gain access to subcortical tumors) and prelabelled with [3H]choline, were stimulated electrically to evoke action potential-induced, exocytotic [3H]acetylcholine release. For comparison, rat cortex slices were also used. [3H]ACh release decreased with the age of the patients and was modulated by muscarinic autoreceptors and by 5-hydroxytryptamine1F, neurokinin1, and kappa-opioid receptors located on cholinergic terminals. In addition, 5-hydroxytryptamine2 and delta-opioid receptors located on interneurons were also involved in the modulation of [3H]ACh release. The present findings might help to explain pathological conditions in Alzheimer's disease.

Selectivity of hemicholinium mustard, an affinity ligand, for the high-affinity choline transport system

The selectivity of the irreversible inhibition of high-affinity choline uptake (HACU) by hemicholinium mustard (HCM; 2,2'-(4,4'-biphenylene)bis[2-hydroxy-4-(2-bromoethyl)-morpholine] hydrochloride) with respect to other cholinergic proteins and other sodium-dependent transport systems was examined. Preincubation of rat forebrain membranes with HCM, followed by washing and measurement of [3H]-hemicholinium-3 binding to the high-affinity choline transporter, was shown to decrease binding capacity (Bmax) by 70% without affecting the apparent affinity of the ligand. However, a similar preincubation, wash and binding experiment using [3H]-NMS as a ligand for muscarinic receptors showed no HCM effect on binding parameters. To measure the effects of HCM on choline acetyltransferase (ChAT), synaptosomes were incubated in HCM, then washed. The synaptosomes were lysed and ChAT activity was measured. Treatment with 50 microM HCM, a concentration that inhibits 100% of synaptosomal HACU, results in a 24% decrease in ChAT activity. HCM demonstrates little residual inhibition of other sodium-dependent neurotransmitter transporter transporters: preincubation with 50 microM HCM results in a decrease of 12% in transport of [3H]-dopamine and a decrease of 6% in the transport of [3H]-GABA. The binding of HCM, like that of hemicholinium-3 is sodium-dependent. HCM preincubation in the presence of sodium results in inhibition of HACU to 32% of control; in the absence of sodium HACU is 65% of control. This represents a loss of 51% of the observed irreversible inhibition produced by HCM. Irreversible inhibition by HCM can also be prevented by co-incubation with hemicholinium-3. Co-incubation with hemicholinium-3 results in residual HACU inhibition that decreases from 51% (HCM alone) to 28% (HCM + hemicholinium-3). When atropine instead of hemicholinium-3 is co-incubated with HCM, HCM still inhibits 40% of transport, demonstrating the pharmacological specificity of the protective effect of hemicholinium-3. Experiments in the guinea-pig myenteric plexus preparation demonstrate a gradual recovery from the residual effects of HCM. Evoked ACh release decreases to 24% immediately following treatment with 1 microM HCM. After 2 hr of recovery, tissues have recovered to about 50% of control levels, after which recovery continues at a slower rate.

Nerve growth factor attenuates cholinergic deficits following traumatic brain injury in rats

Traumatic brain injury (TBI) results in chronic derangements in central cholinergic neurotransmission that may contribute to posttraumatic memory deficits. Intraventricular cannula (IVC) nerve growth factor (NGF) infusion can reduce axotomy-induced spatial memory deficits and morphologic changes observed in medial septal cholinergic neurons immunostained for choline acetyltransferase (ChAT). We examined the efficacy of NGF to (1) ameliorate reduced posttraumatic spatial memory performance, (2) release of hippocampal acetylcholine (ACh), and (3) ChAT immunoreactivity in the rat medial septum. Rats (n = 36) were trained prior to TBI on the functional tasks and retested on Days 1-5 (motor) and on Day 7 (memory retention). Immediately following injury, an IVC and osmotic pump were implanted, and NGF or vehicle was infused for 7 days. While there were no differences in motor performance, the NGF-treated group had significantly better spatial memory retention (P < 0.05) than the vehicle-treated group. The IVC cannula was then removed on Day 7, and a microdialysis probe was placed into the dorsal hippocampus. After a 22-h equilibration period, samples were collected prior to and after administration of scopolamine (1 mg/kg), which evoked ACh release by blocking autoreceptors. The posttraumatic reduction in scopolamine-evoked ACh release was completely reversed with NGF. Injury produced a bilateral reduction in the number and cross-sectional area of ChAT immunopositive medial septal neurons that was reversed by NGF treatment. These data suggest that cognitive but not motor deficits following TBI are, in part, mediated by chronic deficits in cholinergic systems that can be modulated by neurotrophic factors such as NGF.

Deficits in water escape performance and alterations in hippocampal cholinergic mechanisms associated with neonatal monosodium glutamate treatment in mice

Mice treated neonatally with monosodium glutamate (MSG) were found to have learning and memory deficits in performing a non-spatial water escape task. Scopolamine impaired the water-escape performance of the control mice but not that of the MSG-treated mice. It was suggested that the water-escape performance deficit in the MSG-treated mice was a result of impaired central cholinergic mechanisms. As such, scopolamine was unable to further incapacitate an already impaired cholinergic system. This is strongly supported by the decreased affinity of the sodium-dependent high-affinity choline uptake observed in the hippocampus. D-Cycloserine, a partial agonist at the glycine site of the NMDA receptor, did not affect the water-escape performance of the MSG-treated and control mice; nor did it alter the effects of scopolamine. This lack of effect of D-Cycloserine may imply that the NMDA receptors are not involved in non-spatial learning, in contrast to their reported involvement in spatial learning.

Reversible hearing impairment induced by lithium in the guinea pig

Lithium salts remain one of the most widely used treatments for depressive illness. The mechanisms involved probably include reduction in free inositol. Visual perceptive disturbances can be a side effect of the treatment. We report here for the first time that chronic lithium treatment in the guinea pig induces a predominantly low frequency hearing loss and, in the longer term, loss of sensitivity is observed across the whole audiogram. The changes are reversed when treatment is arrested. The observations could be accounted for, at least partially, by a lithium-induced perturbation of the phosphoinositide cascade within the inner ear.

Evidence to suggest that extracellular acetate is accumulated by rat hippocampal cholinergic nerve terminals for acetylcholine formation and release

It is well established that extracellular choline is transported into central cholinergic nerve terminals by 'high' and 'low' affinity processes to form the neurotransmitter acetylcholine (ACh). The intent of the present investigation was to ascertain whether extracellular acetate might also be transported into central cholinergic nerve terminals to form ACh. To test this possibility, rat hippocampal tissue was incubated with varying concentrations of extracellular [1-(14)C]acetate (0.1-100 microM) and the uptake of [1-(14)C]acetate and the amount of [14C]ACh formed by the tissue determined. The results indicated that the uptake of extracellular [1-(14)C]acetate was temperature-dependent and saturable having an apparent Michaelis constant (Km) of 22 microM. The formation of [14C]ACh in the tissue as a function of extracellular [1-(14)C]acetate appeared to occur by both 'high' and 'low' affinity processes with apparent Km values of 0.5 and 19.6 microM, respectively. In other experiments, three inhibitors (lithium, allicin and sodium) of acetyl CoA synthetase (EC 6.2.1.1 acetate: CoA ligase), the enzyme which converts acetate to acetyl CoA when ATP and CoA are present, inhibited [1-(14)C]acetate uptake and the amount of [14C]ACh formed from that [1-(14)C]acetate. Additionally, vesamicol, an inhibitor of ACh transport into synaptic vesicles, blocked the filling of a synaptic vesicle-enriched fraction of hippocampal tissue with newly synthesized [14C]ACh formed from extracellular [1-(14)C]acetate. High K+ depolarization of hippocampal tissue loaded with extracellular [1-(14)C]acetate not only increased the synthesis but also the release of [14C]ACh. These results suggest that extracellular acetate is recycled by rat hippocampal cholinergic nerve terminals for the formation and release of ACh. They also suggest that the enzyme acetyl CoA synthetase mediates extracellular acetate uptake into hippocampal cholinergic nerve terminals by metabolizing it to acetyl CoA and thereby creating a diffusion gradient for it to follow.

Inhibitors of serine/threonine phosphatases increase membrane-bound choline acetyltransferase activity and enhance acetylcholine synthesis

The present investigation examines the effects of phosphatase inhibition on short-term regulation of cholinergic function, with particular emphasis on choline acetyltransferase, the enzyme which synthesizes acetylcholine. Rat hippocampal synaptosomes were treated with either okadaic acid (10 nM) or calyculin-A (50 nM) to inhibit protein phosphatases 1 and 2A for 20 min prior to subfractionation of nerve terminals and measurement of choline acetyltransferase activity, or quantification of high-affinity choline transport and acetylcholine synthesis. Inhibition of synaptosomal phosphatases did not alter total or salt-soluble choline acetyltransferase activity, but membrane-bound and water-soluble forms of the enzyme were selectively increased in okadaic acid-treated nerve terminals to 129 +/- 11% and 137 +/- 10% of control, respectively. High-affinity choline transport was reduced to 77 +/- 6% and 76 +/- 7% of control in calyculin-A- and okadaic acid-treated nerve terminals, respectively. Acetylcholine synthesis was reduced to 73 +/- 6% of control in calyculin-A-treated synaptosomes only; acetylcholine synthesis was at control levels in okadaic acid-treated cultures correlating with enhanced choline acetyltransferase activity in the water-soluble and nonionically membrane-bound fractions. These investigations indicate a role for phosphoprotein phosphatases in the regulation of acetylcholine synthesis in the cholinergic nerve terminal. The observed increases in choline acetyltransferase activity in two subcellular fractions appears to compensate for decreased choline precursor availability, allowing acetylcholine synthesis to be maintained at control levels. The uncoupling of choline transport and acetylcholine synthesis in this situation represents a unique functional role for a subfraction of choline acetyltransferase.

D2 dopamine receptor activation inhibits basal and forskolin-evoked acetylcholine release from dissociated striatal cholinergic interneurons

We tested whether D2 ligands inhibit basal and forskolin-stimulated [3H]ACh release from dissociated striata, as opposed to striatal slices. Quinpirole inhibited both basal (40% maximal inhibition; IC50 approximately 50 nM) and 10 microM forskolin-stimulated release (80% inhibition; IC50 approximately 25 nM quinpirole) and both actions were blocked by a D2 antagonist. Vesamicol prevented the quinpirole and forskolin actions. The ability of D2 agonists to inhibit basal and cyclase-stimulated acetylcholine release emanating from vesamicol-sensitive vesicles appears to be tonically suppressed by inhibitory elements within striatal circuitry.

Neural grafting as a tool for the study and reversal of neurobehavioral birth defects

The transplantation of fetal neurons has gained notoriety in recent years for its perceived potential to reverse neurological deficits caused by loss of one or another neuronal population. The present paper describes a neural grafting approach employed by our laboratory to gain more insight into the drug-induced neurobehavioral teratogenicity. Mice were exposed prenatally to phenobarbital by feeding the barbiturate to the pregnant dam on gestation days 9-18. Heroin exposure was accomplished by injecting dams during the same gestational period. At maturity, the drug-exposed offspring displayed profound deficits in specific behavioral tasks, suggesting alterations in the septohippocampal cholinergic pathway. Biochemically, we observed increased presynaptic activity in the pathway, which was not accompanied by a corresponding reduction in postsynaptic activity. Rather, there was a general hyperactivation along the different postsynaptic phases. In contrast, we noted a desensitization of protein kinase C activity in response to the exposure of a cholinergic agonist to the drug-exposed offspring. Subsequent transplantation of embryonic cholinergic cells from normal mice to the impaired hippocampus reversed the behavioral deficits, whereas sham-operated controls exhibited no improvement. Concomitantly, all the biochemical alterations studied, both presynaptic and postsynaptic, were either partially or completely reversed following grafting.

Adolescent development influences functional responsiveness of noradrenergic projections to the hypothalamus in male rats

Hypothalamic noradrenergic utilization in vivo and the in vitro depolarized release of norepinephrine (NE) were measured at 28, 42, and 70 days of age in male rats to determine the impact of adolescent development on the functional responsiveness of this transmitter system. At each age, function was determined in control rats and rats challenged by restraint. NE utilization in vivo was estimated by measuring the decrease in NE levels following administration of a synthesis inhibitor, alpha-methyl-p-tyrosine methyl ester (alpha-MT, 250 mg/kg). The half-life of approach to a new steady-state was determined. To measure depolarized release of NE, the hypothalamus was incubated in vitro in a high potassium (50 mM) medium and the percent of endogenous NE released into the medium was determined. The in vivo results indicated that hypothalamic NE utilization in control animals decreases as animals mature. Additionally, the in vitro results indicated that the percent NE released upon depolarization also decreased with maturation in control animals. Restraint shifted the NE decay curve measured in vivo to the right at all ages. Overall, however, restraint tended to increase NE utilization at 70 days, have little effect at 42 days, and retard utilization at 28 days. Furthermore, restraint markedly reduced the depolarized release of NE at 28 days, had no effect at 42 days and slightly, but significantly, increased release at 70 days. Restraint significantly increased plasma corticosterone at all ages. Hypothalamic NE projections are important to an organism's regulatory responses, and changes that take place over adolescence in this system may be important for the emergence of adult-typical responses as well as render adolescents vulnerable to specific dysfunctions.

Effects of intraventricular transplantation of NGF-secreting cells on cholinergic basal forebrain neurons after partial immunolesion

The aim of the present study was to examine the effects of nerve growth factor on brain cholinergic function after a partial immunolesion to the rat cholinergic basal forebrain neurons (CBFNs) by 192 IgG-saporin. Two weeks after intraventricular injections of 1.3 micrograms of 192 IgG-saporin, about 50% of CBFNs were lost which was associated with 40-60% reductions of choline acetyltransferase (ChAT) and high-affinity choline uptake (HACU) activities throughout the basal forebrain cholinergic system. Two groups of lesioned animals received intraventricular transplantations of mouse 3T3 fibroblasts retrovirally transfected with either the rat NGF gene (3T3NGF+) or the retrovirus alone (3T3NGF-) and were sacrificed eight weeks later. In vivo production of NGF by 3T3NGF+ cells was confirmed by NGF immunohistochemistry on the grafts and NGF immunoassay on cerebrospinal fluid (CSF) samples. Both ChAT and HACU activities returned to normal control levels in the basal forebrain and cortex after 3T3NGF+ transplants, whereas no recovery was observed in 3T3NGF- transplanted animals. There was a 25% increase in the size of remaining CBFNs and an increased staining intensity for NGF immunoreactivity in these cells after NGF treatments. Acetylcholinesterase (AChE) histochemistry revealed that the optical density of AChE-positive fibers in the cerebral cortex and hippocampus were reduced by about 60% in immunolesioned rats which were completely restored by 3T3NGF+ grafts. In addition, decreases in growth-associated protein (GAP)-43 immunoreactivity after immunolesion and increases in synaptophysin immunoreactivity after 3T3NGF+ grafts were observed in the hippocampus. Our results further confirm the notion that transfected NGF-secreting cells are useful in long-term in vivo NGF treatment and NGF can upregulate CBFN function. They also highly suggest that NGF induces terminal sprouting from remaining CBFNs.

Evidence to suggest that cytosolic acetylcholine in rat hippocampal nerve terminals is not directly transferred into synaptic vesicles for release

Rat hippocampal minces were loaded with [acetyl 1-14C]acetylcholine ([14C]ACh) in the presence of the "poorly penetrating" acetylcholinesterase (EC 3.1.1.7; AChE) inhibitor echothiophate and the effect of high K+ depolarization determined on the subcellular storage and release of [14C]ACh and its metabolites. Results indicated that high K+ did not augment the release of [14C]ACh. Rather, it increased the release of [14C]acetate while simultaneously reducing the level of [14C]ACh in the cytosolic (S3) fraction. When the identical experiment was performed with paraoxon, a "penetrating" AChE inhibitor, high K+ still did not increase the release of [14C]ACh. However, paraoxon prevented the K(+)-induced loss of [14C]ACh from the cytosolic fraction as well as the K(+)-induced gain of [14C]acetate in the release medium. When minces were loaded with [14C]ACh in the presence of echothiophate and subsequently subjected to high K+ depolarization in the absence or presence of vesamicol (AH5183; (-)-trans-2-[4-phenylpiperidino] cyclohexanol), a drug which blocks the refilling of synaptic vesicles with ACh, the amount of endogenous ACh released was reduced approximately 50%. Conversely, the amount of [14C]ACh released was not reduced at all. These results suggest that cytosolic ACh is not directly transported into synaptic vesicles for release when hippocampal nerve terminals are depolarized. Rather, its hydrolysis is accelerated in response to depolarization. A working hypothesis explaining the importance of the depolarization-induced breakdown of cytosolic ACh to central ACh metabolism is presented.

Comparative ontogenic profile of cholinergic markers, including nicotinic and muscarinic receptors, in the rat brain

The ontogenic profiles of several cholinergic markers were assessed in the rat brain by using quantitative in vitro receptor autoradiography. Brain sections from animals at different stages of development were processed with [3H]AH5183 (vesamicol; vesicular acetylcholine transport sites), [3H]N-methylcarbamylcholine (alpha(4)beta(2) nicotinic receptor sites), [3H]hemicholinium-3 (high-affinity choline uptake sites), [3H]3-quinuclidinyl benzilate (total population of muscarinic receptor sites), [3H]4-DAMP (muscarinic M1/M3 receptor sites), [3H]pirenzepine (muscarinic M1 receptor sites), and [3H]AF-DX 116 and [3H]AF-DX 384 (muscarinic M2 receptor sites) as radiolabeled probes. The results revealed that, by the end of the prenatal period (embryonic day 20), the densities of nicotinic receptor and vesicular acetylcholine transport sites already represented a considerable proportion of those observed in adulthood (postnatal day 60) in different laminae of the frontal, parietal, and occipital cortices, in the layers of Ammon's horn fields and the dentate gyrus of the hippocampal formation, as well as in the amygdaloid body, the olfactory tubercle, and the striatum. In contrast, at that stage, the densities of total muscarinic, M1/M3, M1, and possibly M2 receptor and high-affinity choline uptake sites represent only a small proportion of levels seen in the adult. Differences were also observed in the postnatal ontogenic profiles of nicotinic, muscarinic, vesamicol, and high-affinity choline uptake sites. For example, between postnatal weeks 3 and 5, the levels of M1/M3 and M1 sites were at least as high as in the adult, whereas M2 and high-affinity choline uptake site densities appeared to be delayed and to reach adult values only after postnatal week 5. With regard to cholinergic innervation in the developing rat brain, the present findings suggest a temporal establishment of several components of the cholinergic systems. The first components are the vesicular acetylcholine transporter and nicotinic sites; these are followed by M1/M3 and M1 sites and, finally, by M2 and high-affinity choline uptake sites.

Regulation of acetylcholine synthesis in the presence of hemicholinium mustard

High affinity choline uptake (HACU) is a critical element in the synthetic pathway for acetylcholine (ACh), and is known to demonstrate activity-dependent regulation in vivo and in vitro. However, little is known about this important sodium-dependent transport protein at the biochemical level, and about the nature of its interaction with the ACh synthetic enzyme ChAT. Hemicholinium mustard (HCM), an irreversibly binding analog of hemicholinium-3 (HC3), was used to create a preparation with HACU that is completely inhibited in order to investigate the immediate source of Ch for ACh synthesis. Rat brain synaptosomes were pre-incubated with HCM and washed before transport incubations of increasing length (0-6 min) were carried out. The contribution of endogenous and extracellular (tracer) Ch to the ACh level was measured at each time point using a gas chromatography mass spectrometry (GCMS) system that allows quantitative measurement of endogenous (unlabelled; [2Ho]) Ch as well as tracer (deuterium-labelled; [2H4]) Ch. The hypothesis was that if an endogenous intraterminal Ch pool can be used for ACh synthesis, an increase in unlabelled ACh across time would be observed. In neither HCM-treated nor control synaptosomes was an increase observed in intraterminal (pellet) unlabelled ACh. To test the effects of high tissue demand, in other experiments synaptosomes were depolarized with addition of 40 mM KCl to the buffer after HCM treatment; again, no significant increase in intraterminal unlabelled ACh was observed across time. These experiments demonstrate that endogenous unlabelled Ch does not contribute to ACh synthesis, even when HACU is inactivated, and under conditions of high demand.

Identification and partial characterization of the high-affinity choline carrier from rat brain striatum

[3H]Choline mustard aziridinium ion binds irreversibly to the sodium-coupled high-affinity choline transport protein in a sodium-dependent and hemicholinium-sensitive manner, and thus is a useful affinity ligand. In rat striatal synaptosomal membranes, it radiolabels two polypeptides with apparent molecular masses of 58 and 35 kDa. Based upon the use of two different experimental approaches, it appears that neither of these polypeptides is glycosylated.

Inhibition by halothane of potassium-stimulated acetylcholine release from rat cortical slices

1. Cholinergic neurones in the basal forebrain are linked to cortical activation and arousal. 2. The present study was designed to examine the hypothesis that clinically relevant doses of halothane (0.1 to 5%) would significantly reduce depolarization-evoked acetylcholine (ACh) release from rat cortical slices. 3. ACh release was measured from rat cortical slices by a chemiluminescent technique. 4. Depolarization-evoked ACh release was inhibited significantly by halothane with an IC50 of 0.38%. This value equates to 0.3 MAC (the minimum alveolar concentration at which no movement occurs to a standard surgical stimulus in 50% of subjects) for the rat. 5. The potent effect of halothane on ACh release suggests that this mechanism may be a target for the action of volatile anaesthetic agents. This in vitro effect on ACh release is consistent with effects of halothane reported in vivo.

Reduced densities of sodium-dependent [3H] hemicholinium-3 binding sites in hippocampus of developmental rats following perinatal low-level lead exposure

We investigated the effect of perinatal, low-level lead exposure on [3H]hemicholinium-3 (HC-3) binding in the hippocampus of postnatal rat. Rat pups were maternally lead-exposed from gestational day 16 through postnatal day 28 (PN28). In control animals, the [3H]HC-3 binding sites increased from 7 fmol/mg protein at postnatal day 1 (PN1) to 14 and 35 fmol/mg protein at PN7 and PN14, respectively, and reached adult values of 50 fmol/mg protein, at PN21 and PN28. In lead-exposed litters, the [3H]HC-3 binding was reduced by 30-40% throughout the early postnatal development and remained 40% below control values in PN60 animals, one month after termination of lead exposure. The Pb-induced reduction in HC-3 binding was associated with a similar decrease in ChAT activity and was comparable to the effect of localized lesion of medial septum. Septal cell counts in the lead-exposed PN21 rats indicated a 22% reduction in the number of ChAT-immunoreactive cells in the medial septum/vertical diagonal band (MS/vDB) complex although cell numbers in the horizontal limb of the diagonal band (hDB) were not altered. These results suggest that perinatal, low-level lead exposure results in a reduced density of cholinergic nerve terminals in the hippocampus, either due to impaired development or degeneration of the cholinergic projection neurons in the MS/vDB complex.

(3H)hemicholinium-3 binding sites in postmortem brains of human patients with Alzheimer's disease and multi-infarct dementia

(3H)Hemicholinium-3 ((3H)HCh-3), a potent, selective, and competitive inhibitor of the high-affinity choline uptake process was used for the detection of high-affinity choline carriers in the hippocampus (gyrus parahippocampalis), neocortex (gyrus frontalis medius), and cerebellum (lobulus semilunaris inferior) in autopsy samples of people with Alzheimer's disease, multi-infarct dementia and from other psychiatric and nonpsychiatric patients. The effect of postmortem delay was eliminated by means of the cerebellum used as an individual standard. The density of (3H)HCh-3 binding sites was decreased in the hippocampus and neocortex from individuals with multi-infarct dementia and unchanged in the brain tissue from people with Alzheimer's disease in comparison with control patients. No changes in dissociation constants were found. In Alzheimer's disease, high-affinity choline transport appears to be reduced by a dysfunction of cholinergic neuronal membrane rather than by a significant decrease in the number of presynaptic cholinergic nerve terminals. Results provide evidence of a decrease in the number of nerve endings in people with multi-infarct dementia and suggest different vulnerability of particular brain areas to vascular disorders.

Overexpression of the high affinity choline transporter in cortical regions affected by Alzheimer's disease. Evidence from rapid autopsy studies

Cholinergic deficits in Alzheimer's disease are typically assessed by choline acetyltransferase, the enzyme that synthesizes acetylcholine. However, the determining step in acetylcholine formation is choline uptake via a high affinity transporter in nerve terminal membranes. Evaluating uptake is difficult because regulatory changes in transporter function decay rapidly postmortem. To overcome this problem, brain regions from patients with or without Alzheimer's disease were frozen within 4 h of death and examined for both choline acetyltransferase activity and for binding of [3H]-hemicholinium-3 to the choline transporter. Consistent with the loss of cholinergic projections, cerebral cortical areas exhibited marked decreases in enzyme activity whereas the putamen, a region not involved in Alzheimer's disease, was unaffected. However, [3H]hemicholinium-3 binding was significantly enhanced in the cortical regions. In the frontal cortex, the increase in [3H]hemicholinium-3 binding far exceeded the loss of choline acetyltransferase, indicating transporter overexpression beyond that necessary to offset loss of synaptic terminals. These results suggest that, in Alzheimer's disease, the loss of cholinergic function is not dictated simply by destruction of nerve terminals, but rather involves additional alterations in choline utilization; interventions aimed at increasing the activity of cholinergic neurons may thus accelerate neurodegeneration.

'Choline/orphan V8-2-1/creatine transporter' mRNA is expressed in nervous, renal and gastrointestinal systems

Several cDNAs with substantial sequence homologies to members of the neurotransmitter transporter gene family currently remain 'orphan' transporters, without clearly-identified substrates. We were concerned that a cDNA 'V8-2-1' isolated from a ventral midbrain cDNA library in this laboratory and a virtually-identical cDNA 'CHOT1' reported by Mayser et al. [J. Neurochem., 20 (1973) 581-593] might represent such an orphan. Despite initial reports that it could mediate some choline uptake; neither CHOT1 nor V8-2-1 was demonstrated to confer pharmacologically appropriate choline uptake not already present in either Xenopus oocytes or COS cells. Determination of the regional and tissue-specific distribution of mRNA hybridizing with V8-2-1 cDNA was undertaken to aid in identifying its function. Examination of the distribution of V8-2-1 expression reveals several novel features of this transporter gene family member's distribution, including several features that add to current evidence suggesting that the clone may not encode the classical pharmacologically-defined, hemicholinium-3 sensitive high affinity transporter of cholinergic neurons. These data fit with and extend recent data that suggest that this cDNA represents creatine transporter, and provide initial documentation of its regional distribution in brain.

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.

Striatal extracellular choline and acetylcholine in choline-free plasma rats

Choline (Ch)-free plasma rats were induced successfully by intravenous (IV) injection of choline oxidase (ChO) (16). However, brain acetylcholine (ACh) levels were not affected by ChO treatment, maintaining the same levels as those in controls, although brain Ch levels were significantly decreased. To clarify the reasons for this, in vivo microdialysis was carried out in the treated rats' striata. The ChO treatment induced not only a 70% decrease of extracellular Ch levels but also a 40% decrease of extracellular ACh levels, reflecting the amount of ACh released from cholinergic terminals. In addition, plasma-bound Ch levels and choline acetyltransferase (CAT) and acetylcholinesterase (AChE) activities in the brain were examined in the rats receiving ChO treatment. No significant differences from controls were observed in these levels. The results suggest that: approximately 70% of striatal extracellular Ch is physiologically supplied from circulating plasma-free Ch; the inhibition of ACh release is related to the maintenance of tissue (intraneuronal) ACh levels under the condition of halting of the supply of free Ch from blood to the brain; if there is a compensative supply of free Ch from de novo synthesis, autocannibalism, or plasma-bound Ch, this may be supplied within neuronal cells, because the level of extracellular-free Ch maintained its depressed level even 11-14 h after ChO treatment.

Acute neurochemical changes in mouse brain following cerebral ischemia

Acute changes in neurochemical levels induced by ischemia were studied in the mouse brain. Contents of neurochemicals in the frontal, parietal and occipital cortices and hippocampus were determined immediately after 15 min of ischemia (0), and then at 15, 30, 90 and 180 min after recirculation following ischemia. These data were compared with those for sham-operated control mice. Choline (Ch) contents in ischemic animals increased by 530-630% from control levels immediately after ischemia, and returned to control levels by 90 min. Decreases in levels of norepinephrine (NE) and serotonin (5-HT) were observed during 30 min after recirculation. There were no significant changes in levels of acetylcholine (ACh) or dopamine (DA), throughout recirculation. On the other hand, DA and 5-HT metabolites (3,4-dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid) significantly increased. Thus, comprehensively investigating the various neurotransmitters will provide meaningful information regarding the disturbance of central nervous system induced by cerebrovascular disease.

AF64A (ethylcholine mustard aziridinium) impairs acquisition and performance of a spatial, but not a cued water maze task: relation to cholinergic hypofunction

The cholinergic neurotoxin AF64A (ethylcholine mustard aziridinium) produced alterations in a spatial but not a nonspatial cognitive task following ICV injection. AF64A impaired acquisition and performance in the standard Morris water maze task, evidenced by significantly longer latencies to find the submerged platform. However, the AF64A group exhibited shorter latencies and more direct paths to the target at the end of training, which suggests acquisition of efficient search strategies and a sparing of procedural memory. However, the AF64A group spent significantly less time in the target quadrant during the probe trial than the CSF group. This suggests a failure to learn the specific location of the target and impaired declarative memory processes. In contrast, AF64A did not affect performance of a cued MWM task that did not involve spatial memory processing, demonstrating the absence of motoric, sensory, or motivational impairments. The AF64A-induced behavioral deficits were associated with a) a significant decrease in high affinity choline transport (HAChT), b) reduced concentrations of 5-HT and 5-HIAA, and c) an increased ratio of 5-HIAA/5HT, in the HPC. There were no changes in choline uptake in the gustatory cortex, the amygdala, or the striatum. Percent time in the target quadrant during the probe trial was significantly correlated with HAChT in the HPC. There were no correlations between any of the behavioral measures and HAChT in the striatum, gustatory cortex, or the amygdala, or between serotonergic or noradrenergic parameters in the HPC. These data suggest that AF64A produces cognitive deficits in spatial tasks that are correlated with the cholinergic hypofunction induced by the compound.

Phosphorylation of rat brain choline acetyltransferase and its relationship to enzyme activity

Choline acetyltransferase catalyzes the formation of acetylcholine from choline and acetyl-CoA in cholinergic neurons. The present study examined conditions for modulation of kinase-mediated phosphorylation of this enzyme. By using a monospecific polyclonal rabbit anti-human choline acetyltransferase antibody to immunoprecipitate cytosolic and membrane-associated subcellular pools of enzyme from rat hippocampal synaptosomes, we determined that only the cytosolic fraction of the enzyme (67,000 +/- 730 daltons) was phosphorylated under basal, unstimulated conditions. The quantity of this endogenous phosphoprotein was dependent, in part, upon the level of intracellular calcium, with 32Pi incorporation into the enzyme in nerve terminals incubated in nominally calcium-free medium only 43 +/- 7% of control. The corresponding enzymatic activity of cytosolic choline acetyltransferase did not appear to be altered by lowered cytosolic calcium, whereas membrane-associated choline acetyltransferase activity was decreased to 58 +/- 11% of control. Depolarization of synaptosomes with 50 microM veratridine neither altered the extent of phosphorylation or specific activity of cytosolic choline acetyltransferase, nor induced detectable phosphorylation of membrane-associated choline acetyltransferase, although the specific activity of the membrane-associated enzyme was increased to 132 +/- 5% of control. In summary, phosphorylation of choline acetyltransferase does not appear to regulate cholinergic neurotransmission by a direct action on catalytic activity of the enzyme.

Comparison of the effects of aging in vivo and of oxygen free radicals in vitro on high-affinity choline uptake and hemicholinium-3 binding in the rat brain

The effects of aging in vivo (Wistar rats aged 3-26 months) and of an oxygen free-radical generating system in vitro (Fe(2+)/ascorbic acid) on high-affinity choline uptake in the hippocampus and on (3H)hemicholinium-3 binding sites in the cortex and hippocampus are compared. The high-affinity choline transport system was found to be more damaged than the low-affinity system during aging (Na(+)-dependent part of the uptake drops to 76%: Na(+)-independent part increases to 120%). The decrease in high-affinity choline uptake values is probably more influenced by the impairment of correct function of carriers (the fall in the turnover rate of each carrier) than by a decrease in the number of transport sites (no change of the density of the carriers in the hippocampus and cortex). The causes of the defect in high-affinity choline transport during aging are discussed.

Modulation of high-affinity choline carrier activity following incubation of rat hippocampal synaptosomes with hemicholinium-3

Membrane carriers display structural and functional asymmetry with a substrate binding site which can be oriented alternately, but not simultaneously, to the extracellular and intracellular environment. Hemicholinium-3 is an inhibitor of the high-affinity choline carrier in cholinergic nerve terminals which binds to the transporter at the outer membrane surface but is not taken up into the cell. In the present study, we investigated the decline in choline transport which occurs during the first few minutes cholinergic nerve terminals are incubated in physiological salt solutions. Following incubation of rat hippocampal synaptosomes with hemicholinium-3, samples were washed free of the inhibitor and high-affinity choline uptake was measured. Choline uptake into hemicholinium-treated nerve terminals was significantly greater than control (132 +/- 4%). This effect appeared not to be due to an increase in uptake of choline above initial values in the hemicholinium-treated synaptosomes, but to a decrease in choline carrier activity in control samples by more than 25% during the first few minutes of incubation. Addition of hemicholinium-3 to samples after the preincubation induced decrease in choline uptake, followed by a wash period to remove the inhibitor resulted in elevation of choline uptake levels to initial levels. The effect of hemicholinium-3 was concentration-dependent, requiring near saturating concentrations of the inhibitor to elicit the effect. Measurement of acetylcholine content of synaptosomes at different points during the incubation procedure revealed that there was a trend for transmitter levels to vary inversely compared to choline uptake activity, but the differences were not statistically significant during treatments when significant changes in transport activity were measured.

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.

Recovery of [3H]acetylcholine synthesis after AF64A-treatment in primary, neuron-enriched, rat septal-hippocampal and striatal cultures

Neuron-enriched cultures prepared from several different rat brain regions were incubated with 10 microM or 30 microM monoethylcholine mustard aziridinium ion (AF64A) under conditions (1 h, 37 degrees C in Krebs Ringer buffer) that reduced acetylcholine (ACh) synthesis coupled to high-affinity choline uptake, without affecting choline acetyltransferase activity. Co-cultures of septum-hippocampus and cultures of striatum were similarly sensitive to the AF64A-induced inhibition of ACh synthesis. However, ACh-synthesis recovered more rapidly in the striatal cultures than in septal-hippocampal co-cultures after AF64A washout. In septal-hippocampal co-cultures, neither tunicamycin (20 micrograms/ml) nor cycloheximide (0.5 microgram/ml) had any effect on the basal synthesis of ACh or its recovery within 24 h following 10 microM AF64A washout. However, the recovery of ACh synthesis in these co-cultures after 30 microM AF64A-washout was blocked by either tunicamycin or cyclohexamide. Neither tunicamycin nor cyclohexamide interfered with ACh-synthesis recovery after washout of 30 microM AF64A in striatal cultures. These studies suggest that the turnover of high-affinity choline transporters can be modulated in a brain-region specific manner in intact primary neuronal cultures.

Basal synthesis of acetylcholine in hippocampal synaptosomes is not dependent upon membrane-bound choline acetyltransferase activity

Choline acetyltransferase, the enzyme which catalyses the formation of acetylcholine within cholinergic nerve terminals, exists in both cytosolic and membrane-associated subcellular pools. In the present study, alteration in nerve terminal Cl- homeostasis was used as an experimental tool to elucidate the role of membrane-bound choline acetyltransferase in regulation of the biosynthesis of acetylcholine in rat hippocampal synaptosomes under basal or resting conditions. Reduction of extracellular Cl- concentration from 131 to 48 mM through iso-osmotic replacement with isethionate ions produced a selective decrease, to approximately 50% of control, of nerve terminal membrane-associated choline acetyltransferase activity. Under these experimental conditions, there were no changes in the activity of cytosolic enzyme or high-affinity choline uptake, or in acetylcholine synthesis. Replacement of medium Cl- with Br- supported maintenance of synaptosomal membrane-bound choline acetyltransferase activity better than did I- or isethionate ions; high-affinity choline uptake activity and acetylcholine synthesis were affected similarly. Incubation of synaptosomes with low concentrations of the Cl- channel blockers 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (50 microM) and niflumic acid (100 microM) selectively decreased activity of the membrane-bound enzyme, with no effect on cytosolic choline acetyltransferase or high-affinity choline uptake activities. Acetylcholine synthesis was unchanged, even though membrane-bound choline acetyltransferase activity was decreased in some samples (250 microM 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) to about 10% of control. Experimental manipulations designed to alter neuronal Cl- homeostasis resulted in selective changes in membrane-bound choline acetyltransferase activity, thereby allowing the first direct examination of its physiological role in regulation of acetylcholine synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between the effects of basal forebrain lesions and chronic treatment with MDL 26,479 on learning and markers of cholinergic transmission

The effects of ibotenic acid-induced basal forebrain lesions and treatment with the triazole MDL 26,479 on the acquisition of an operant visual conditional discrimination task and on [3H]hemicholinium-3 and [3H]vesamicol binding were examined. Lesioned animals required more training sessions to acquire the stimulus-response rules of this task. They also showed longer response latencies throughout the experiment. The effects of the treatment with MDL 26,479 (5 mg/kg; i.p. 60 min before each training session) interacted with the effects of the lesion, producing a decrease in the number of sessions required to perform above chance-level in lesioned but not in control animals. MDL 26,479 did not seem to produce immediate performance effects but interacted with the learning process. The lesions destroyed the cell bodies in the area of the substantia innominata, basal nucleus of Meynert, and the globus pallidus. The number of frontocortical cholinergic terminals as primarily indicated by hemicholinium-3 binding was reduced in lesioned animals; however, another measure of cholinergic terminals, vesamicol binding, was unchanged. Behavioral performance of animals correlated significantly with hemicholinium binding in the frontal cortex of the right hemisphere. The fact that the lesion delayed but did not block the acquisition of the task may have been a result of compensatory mechanisms in remaining cholinergic terminals as indicated by stable vesamicol binding. These data allow assumptions about the conditions for the demonstration of beneficial behavioral effects of MDL 26,479. They also suggest that the long-term effects of basal forebrain lesions on cortical cholinergic transmission remain unsettled.

High-affinity choline transport sites: use of [3H]hemicholinium-3 as a quantitative marker

High-affinity choline transport (HAChT), the rate-limiting and regulatory step in acetylcholine (ACh) synthesis, is selectively localized to cholinergic neurons. Hemicholinium-3 (HC3), a potent and selective inhibitor of HAChT, has been used as a specific radioligand to quantify HAChT sites in membrane binding and autoradiographic studies. Because both HAChT velocity and [3H]HC3 binding change as in vivo activity of cholinergic neurons is altered, these markers are also useful measures of cholinergic neuronal activity. Evidence that [3H]HC3 is a specific ligand for HAChT sites on cholinergic terminals is reviewed. The ion requirements of HAChT and [3H]HC3 binding indicate that sodium and chloride are required for recognition of both choline and [3H]HC3. A common recognition site is also indicated by the close correspondence of the potency of HC3 and choline analogues for inhibiting both HAChT and [3H]HC3 binding. The parallel regional distributions of both markers in adult brain, during development and after specific lesions, all indicate specific cholinergic localization. The close association of HAChT and [3H]HC3 binding sites is also supported by parallel regulatory changes occurring after in vivo drug treatments and in vitro depolarization. Overall, the data indicate a close association between HAChT and [3H]HC3 binding and are consistent with the sites being identical. Methodologic considerations in using [3H]HC3 as a ligand and considerations in interpretation of results are also discussed.

Direct cytotoxicity of ethylcholine mustard aziridinium in cerebral microvascular endothelial cells

The choline analogue ethylcholine mustard aziridinium (AF64A) is a potent and irreversible inhibitor of choline uptake in brain synaptosomes and is used as a neurotoxin to produce animal models of cholinergic hypofunction. However, previous studies have shown that intraocular administration of AF64A in rats not only reduced the number of cholinergic neurons in the retina, but also induced ultrastructural alterations in the microvasculature. The purpose of this study was to investigate whether AF64A has a direct cytotoxic effect on endothelial cells. As revealed by the measurement of lactate dehydrogenase activity in the culture medium, AF64A produced similar concentration-dependent cellular damage in cultures of bovine cerebral endothelial cells and in the human cholinergic neuroblastoma cell line SK-N-MC, but not in bovine cerebral smooth muscle cells. The toxic effect of AF64A correlated well with the affinity of the choline transport system detected in each cell type. The effect of the toxin on endothelial cells was mediated by its interaction with the endothelial cell choline carrier, as demonstrated by the following observations: (a) AF64A inhibited [3H]choline uptake in a concentration-dependent manner in both cultured and freshly isolated cerebral endothelial cells, and (b) the addition of choline or hemicholinium-3 to the culture medium prevented the AF64A-induced toxicity in endothelial cell cultures.

Actions of a monoclonal antibody Tor 23 on rat brain presynaptic cholinergic processes

Tor 23 is a monoclonal antibody, generated against cholinergic terminals of the Torpedo californica, that has been found to bind to the extracellular surface of cholinergic neurons in a variety of tissues. This study shows that Tor 23 inhibits: 1) high affinity [3H]hemicholinium-3 binding to detergent-solubilized membranes prepared from rat neocortices; 2) high affinity [3H]choline uptake in rat neocortical and striatal P2 preparations; and 3) [3H]acetylcholine synthesis in isolated nerve terminals. Tor 23 does not appear to affect low affinity [3H]choline uptake or [3H]acetylcholine release. These results are consistent with the hypothesis that Tor 23 may bind to nerve terminal high affinity choline transporters in the rat brain.

Hippocampal choline acetyltransferase activity correlates with spatial learning in aged rats

Age-related cognitive deficits in both humans and experimental animals appear to relate to dysfunction of basal forebrain cholinergic neuron systems. The present study assessed spatial learning performance in a water maze task as a function of choline acetyltransferase and high-affinity choline uptake specific activity (the two phenotypic markers for cholinergic neurons) in frontal cortex, hippocampus and striatum of aged male Fischer-344 rats. We observed that increased hippocampal choline acetyltransferase activity was related to better performance on the water maze task, and that, of the individual measures, hippocampal choline acetyltransferase activity was the best predictor of behavioral performance in the spatial learning task.

Effects of choline-free plasma induced by choline oxidase on regional levels of choline and acetylcholine in rat brain

Choline-free plasma (CFP) was induced in rats by intravenous (IV) injection of 56.0 x 10(2) units kg-1 of choline oxidase (ChO) which completely metabolized the free Ch circulating in the plasma for at least 15.0 h and caused subsequent significant decrease in the concentration of free Ch in the three brain regions examined, the striatum, hippocampus, and cortex. However, the treatment did not affect concentrations of acetylcholine (ACh) in these regions. By contrast, intraperitoneal (IP) injection of 1.0 mmol kg-1 Ch chloride resulted in a maximum concentration of free Ch in plasma in 5 min, after which tissue Ch in all regions examined increased (p < 0.001). Concomitant increases were observed in cortical and hippocampal ACh (p < 0.05) 20 min after the injection. It is thus suggested that the brain may possess compensative mechanisms to prevent the supply of free Ch from circulating to the brain during synthesis of ACh in the brain. It is also suggested that the CFP rat would be a useful and readily available animal model for future study.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of a partially purified phosphodimethylethanolamine methyltransferase from rat brain cytosol

The presence of cytosolic S-adenosylmethionine-dependent N-methyltransferase(s) activity(ies) capable of converting phosphoethanolamine into phosphocholine has been recently demonstrated in the rat brain. At least two enzymes are involved in the methylation of phosphoethanolamine to phosphocholine and these are separable by ammonium sulphate fractionation. The enzyme catalysing the last step of this methylation process is present in the 50-80% ammonium sulphate fraction. A 220-fold purified enzyme has been obtained with sequentially employed Q-Sepharose fast flow and octyl-Sepharose CL4B column chromatography. The maximum enzyme activity was at pH 9.5. The Km values for S-adenosylmethionine, the methyl donor, and phosphodimethylethanolamine, the methyl acceptor, were 125 microM and 750 microM respectively. This phosphodimethylethanolamine N-methyltransferase was found to be calcium-dependent, with a 4-fold increase in activity at 0.5 mM-CaCl2. S-Adenosylhomocysteine at 0.5 mM caused 100% inhibition of the activity. The effects of various structural analogues on the phosphodimethylethanolamine N-methyltransferase activity were also investigated and these results suggest that the enzyme is specific to the substrate. These results provide evidence for the existence of the pathway for the methylation of phosphoethanolamine to phosphocholine in rat brain cytosol.