Fols�ure gegen Hyperhomocystein�mie

There is growing evidence that chronic alcoholism is associated with a derangement in the sulfur amino acid metabolism. Excitatory aminoacids such as glutamate, aspartate, and homocysteine have been shown to be increased in patients with chronic alcoholism who underwent alcohol withdrawal. Furthermore, sustained hyperhomocysteinemia occurred in chronic alcoholics with active drinking pattern. Excitotoxicity can be induced by increased hormocysteine levels via rebound activation of NMDA receptor-mediated glutamatergic neurotransmission upon the removal of ethanol-evoked inhibition. Therefore, hyperhomocysteinemia may be responsible for the higher incidence of complications during alcohol withdrawal (e.g.stroke,convulsions). In addition, an association between brain atrophy and increased levels of homocysteine in chronic alcoholism was shown. This may have important implications for the pathogenesis of brain atrophy in alcoholics. Taking into account that high plasma homocysteine levels are helpful in the prediction of alcohol withdrawal seizures, early anti-convulsive therapy could prevent this severe complication. Supplementation of folate, a cofactor of the homocysteine metabolism, lowers raised homocysteine levels and therefore could be established as a new therapeutic strategy in alcohol withdrawal treatment. The results of various studies highlight the need for further research to prove whether alcoholics benefit from a reduced homocysteine level with respect to both, alcohol-related disorders and alcohol withdrawal symptoms.

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

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

Compensatory mechanisms enhance hippocampal acetylcholine release in transgenic mice expressing human acetylcholinesterase

Central cholinergic neurotransmission was studied in learning-impaired transgenic mice expressing human acetylcholinesterase (hAChE-Tg). Total catalytic activity of AChE was approximately twofold higher in synaptosomes from hippocampus, striatum and cortex of hAChE-Tg mice as compared with controls (FVB/N mice). Extracellular acetylcholine (ACh) levels in the hippocampus, monitored by microdialysis in the absence or presence of 10(-8)-10(-3) M neostigmine in the perfusion fluid, were indistinguishable in freely moving control and hAChE-Tg mice. Muscarinic receptor functions were unchanged as indicated by similar effects of scopolamine on ACh release and of carbachol on inositol phosphate formation. However, when the mice were anaesthetized with halothane (0.8 vol. %), hippocampal ACh reached significantly lower levels in AChE-Tg mice as compared with controls. Also, the high-affinity choline uptake (HACU) in hippocampal synaptosomes from awake hAChE-Tg mice was accelerated but was reduced by halothane anaesthesia. Moreover, hAChE-Tg mice displayed increased motor activity in novel but not in familiar environment and presented reduced anxiety in the elevated plus-maze test. Systemic application of a low dose of physostigmine (100 microgram/kg i.p.) normalized all of the enhanced parameters in hAChE-Tg mice: spontaneous motor activity, hippocampal ACh efflux and hippocampal HACU, attributing these parameters to the hypocholinergic state due to excessive AChE activity. We conclude that, in hAChE-Tg mice, hippocampal ACh release is up-regulated in response to external stimuli thereby facilitating cholinergic neurotransmission. Such compensatory phenomena most likely play important roles in counteracting functional deficits in mammals with central cholinergic dysfunctions.

Phospholipase D in rat myocardium: formation of lipid messengers and synergistic activation by G-protein and protein kinase C

Activation of phospholipase D (PLD) and phosphoinositide-specific phospholipase C (PI-PLC) by fluoride, to stimulate heterotrimeric G-proteins, and by phorbol esters, to stimulate protein kinase C (PKC), was studied in rat atria. Fluoride and 4beta-phorbol-12beta,13alpha-dibutyrate (PDB), in contrast to 4beta-phorbol-13alpha-acetate (PAc), activated PLD, catalyzing the formation of [3H]-phosphatidylethanol ([3H]-PETH), [3H]-phosphatidic acid ([3H]-PA), choline and sn-1,2-diacylglycerol (DAG). Basal PLD activity was resistant to drastic changes in Ca2+ and to Ro 31-8220, a PKC inhibitor, but was decreased by genistein, an inhibitor of tyrosine kinase, and increased by vanadate, a tyrosine phosphatase inhibitor; both effects were, however, very small. Fluoride-evoked PLD activity was resistant to Ro 31-8220 and to genistein, but was Ca2+-dependent. The rate of fluoride-induced PLD activation was maintained for at least 60 min. In contrast, PDB-mediated PLD activity was blocked by Ro 31-8220 and was resistant to extracellular Ca2+-depletion and desensitized within ca. 15 min. PDB markedly potentiated the fluoride-evoked generation of [3H]-phosphatidylethanol and of choline, but inhibited the formation of [3H]-inositol phosphates ([3H]-IP(1-3)). Ethanol (2%) blocked the PDB-evoked generation of both [3H]-phosphatidic acid and of sn-1,2-diacylglycerol, whereas fluoride-evoked responses were reduced only to approximately 50%. In conclusion, the trimeric G-protein-PLD pathway in heart tissue did not enclose PKC activation and was long-lasting and Ca2+-dependent; there was no evidence for an involvement of tyrosine phosphorylation. However, PKC activation modulated G-protein-coupled PLD and PI-PLC activities in opposite directions. PLD activity significantly contributed to the mass production of sn-1,2-diacylglycerol in the heart. The evidence for a pathophysiological role of PLD activation in cardiac hypertrophy and in ischemic preconditioning is discussed.

Brain choline has a typical precursor profile

Choline is product and precursor to both acetylcholine and membrane phospholipids, and, in the brain, is ultimately provided by the circulation. The brain is protected from excess choline and choline deprivation by a refined system of homeostatic mechanisms that maintain a level of extracellular choline that, for its role as precursor, meets saturation criteria under normal conditions. The kinetic and activity profiles of choline are typical for a biosynthetic precursor.

Stimulatory and inhibitory effects of ethanol on hippocampal acetylcholine release

Using the microdialysis technique and sensitive HPLC procedures for the determination of acetylcholine (ACh) and ethanol, we investigated the release of ACh in rat hippocampus after acute ethanol administration. Systemic administration of ethanol (0.8 and 2.4 g/kg i.p.) led to peak ethanol concentrations of 21 and 42 mM in the hippocampus, respectively. The high dose caused a long-lasting inhibition of basal ACh release by up to 33%. Local infusion of scopolamine (1 microM) enhanced hippocampal ACh release up to eightfold in the presence of neostigmine (10 microM), and this stimulated release was also inhibited after systemic ethanol administration (by up to 45%). The low dose of ethanol (0.8 g/kg) led to a delayed stimulation of hippocampal ACh release. A stimulatory effect on ACh release was also observed when ethanol (50-100 mM) was infused directly into the hippocampus or into the septal area, i.e. to the origin of the cholinergic septohippocampal pathway; thus, the stimulatory effect may be mediated by a direct effect on cholinergic fibres. We conclude that ethanol exerts dual modulatory effects on the activity of the septohippocampal cholinergic fibres, depending on the dose and the site of administration. It is suggested that the inhibition of hippocampal ACh release by intoxicating doses of ethanol may contribute to the well-known cognitive and amnesic effects of ethanol intake.

Regulation of free choline in rat brain: dietary and pharmacological manipulations

The present study analyses, comparatively, the kinetics of free choline in the brain of rats during dietary and pharmacological manipulations. Low-choline diet halved the choline plasma level but did not cause significant changes of CSF choline. High-choline diet, hypoxia and treatment with nicotinamide increased brain choline availability through a central site of action and increased the CSF choline concentration. CSF choline concentrations were more effectively elevated by nicotinamide treatment (20-25 microM) than by acute choline administration (13-15 microM). Increases of CSF choline, due to brain choline mobilization, were consistently associated with a net release of choline from the brain as reflected by strongly negative arterio-venous differences (AVD) of brain choline. The balance between release and uptake of brain choline was controlled by the arterial plasma choline level in all treatment groups; however, the normal 'reversal point' of 15 microM--representing the plasma choline level where uptake and release of brain choline are balanced--was shifted to more than 40 microM by high-choline diet and nicotinamide. In conclusion, our data characterize the release of choline into the venous blood as an important component of brain choline homeostasis. Furthermore, we demonstrate that the concentration of brain choline (e.g. as a precursor of acetylcholine) can be enhanced more efficiently by manipulating choline homeostatic mechanisms than by acute choline administration.

Glutamatergic activation of hippocampal phospholipase D: postnatal fading and receptor desensitization

Phospholipase D (PLD) activity was determined in rat hippocampal slices between postnatal days 3 and 35. After birth, basal PLD activity was low and, within 2 weeks, increased to reach a plateau that was maintained up to the adult age. Likewise the response to glutamate developed postnatally to reach a maximum at day 8, but then faded rapidly and was almost absent at day 35. Activation of PLD by 4beta-phorbol 12beta,13alpha-dibutyrate (PDB) was independent of age, whereas the effect of aluminum fluoride (AlF4-) increased to a plateau within the first week. At day 8, PLD stimulation by glutamate via metabotropic receptors involved protein kinase C activation, but was independent of Ca2+ influx; the time course of PLD activation by PDB or AlF4- was linear throughout the experiment, whereas the response to glutamate or 1-aminocyclopentane-1,3-dicarboxylic acid followed a biphasic pattern: the rapid "first phase activation" desensitized within a few minutes and disclosed a small, but maintained "second phase." Pretreatment experiments confirmed desensitization of PLD activation by glutamate, but not by AlF4- or PDB. The biphasic pattern of glutamatergic PLD activation changed during development, i.e., the first phase activation faded and the second phase remained. These results were fully confirmed by the time courses of the PLD-mediated efflux of choline evoked by glutamate. In conclusion, postnatal glutamatergic activation of hippocampal PLD is composed of a pronounced and desensitizing first phase activation and a small, but nondesensitizing second phase. The first, but not the second, phase activation fades rapidly during development. The hypothesis is discussed that the glutamatergic activation of PLD occurs along different pathways in neonate and adult tissue.

Acetylcholine release and choline availability in rat hippocampus: effects of exogenous choline and nicotinamide

The influence of choline availability on acetylcholine (ACh) release in the hippocampus of the awake rat was investigated using the microdialysis procedure. Three treatments enhancing choline availability for basal and atropine-evoked ACh release were evaluated: acute administration of choline chloride (20 mg/kg i.p.); pretreatment of animals with nicotinamide (10 mmol/kg s.c.) 2 hr before atropine injection and dietary choline supplementation (5-fold increase of choline intake for 15-18 days). Although acute choline administration led to a short-lasting (15 min) increase of basal choline efflux by 25% and nicotinamide caused a long-lasting (5 hr) increase by 105%, neither one affected basal ACh release. However, basal release of choline (1.38 pmol/min) and of ACh (114 fmol/min) in the hippocampus was slightly increased in choline-supplemented animals (choline: 1.92 pmol/min; ACh: 140 fmol/min). In untreated animals, atropine administration caused a 3-fold increase of ACh efflux that lasted approximately 2.5 hr. All treatments, acute or chronic choline and nicotinamide, led to significant increases of the maximum and duration of atropine-evoked ACh release. Total atropine-evoked ACh efflux (area under the curve) was increased 2- to 3-fold, with the largest effect evoked by the combination of nicotinamide and choline. The results clearly demonstrate that, under stimulated conditions, hippocampal ACh release could be facilitated when the availability of choline for ACh synthesis was enhanced by dietary or pharmacological means. Under certain conditions, significant effects of increased choline availability on ACh release can be revealed in the absence of an overall increase of extracellular choline.

Modulation of hippocampal acetylcholine release after fimbria-fornix lesions and septal transplantation in rats

Female Long-Evans rats sustained electrolytic lesions of the fimbria and the dorsal fornix causing a partial lesion of the septohippocampal pathway. Two weeks later, the rats received intra-hippocampal grafts of fetal septal cell suspensions. Nine to twelve months later, the release of acetylcholine (ACh) in the hippocampus of sham-operated, lesion-only and grafted rats was measured by microdialysis. The extent of cholinergic (re)innervation was determined by acetylcholinesterase (AChE) staining and densitometry. In both lesion-only and grafted rats, the ratio of ACh release to AChE staining intensity was increased as compared to sham-operated rats, indicating a loss of endogenous inhibitory mechanisms. Scopolamine (0.5 mg/kg i.p.), a muscarinic antagonist, increased ACh release in all treatment groups. 8-OH-DPAT (0.5 mg/kg s.c.), an agonist at serotonergic 5HT1A-receptors, induced an increase of hippocampal ACh release in sham-operated rats. This effect was lost in lesion-only rats, but was fully restored by neuronal grafting. As 8-OH-DPAT influences hippocampal ACh release by a postsynaptic action, this finding indicates that the host brain exerts a serotonergic influence on the grafted cholinergic neurons.

Phospholipid breakdown and choline release under hypoxic conditions: inhibition by bilobalide, a constituent of Ginkgo biloba

A marked increase of choline release from rat hippocampal slices was observed when the slices were superfused with oxygen-free buffer, indicating hypoxia-induced hydrolysis of choline-containing phospholipids. This increase of choline release was suppressed by bilobalide, an ingredient of Ginkgo biloba, but not by a mixture of ginkgolides. The EC50 value for bilobalide was 0.38 microM. In ex vivo experiments, bilobalide also inhibited hypoxia-induced choline release when given p.o. in doses of 2-20 mg/kg 1 h prior to slice preparation. The half-maximum effect was observed with 6 mg/kg bilobalide. A similar effect was noted after p.o. administration of 200 mg/kg EGb 761, a ginkgo extract containing approximately 3% of bilobalide. We conclude that ginkgo extracts can suppress hypoxia-induced membrane breakdown in the brain, and that bilobalide is the active constituent for this effect.

Ontogenetic and pharmacological studies on metabotropic glutamate receptors coupled to phospholipase D activation

The present study was aimed at characterizing the metabotropic receptor subtype which is involved in the activation of phospholipase D (PLD) by glutamate in rat hippocampal slices. We first observed that the ontogenetic profile of glutamate-induced hydrolysis of phosphoinositides and of phosphatidylcholine was strikingly similar. Both pathways were significantly activated by glutamate in tissue taken from 3-, 8- and 15-day old rats, but not in adult rats. PLD activation was strongest in slices taken from 8-day old rats. At this age, quisqualate had a higher potency for PLD activation (EC50: 0.6 microM) than 1S,3R-ACPD (EC50: 16 microM) and DHPG, a specific activator of group I mGluR, was a full agonist at PLD activation (EC50: 3.5 microM) indicating an involvement of a group I mGluR (mGluR1 and 5). MCPG and AIDA, two putative antagonists at mGluR1 receptors, caused a small but (in the case of MCPG) significant inhibition. DCG-IV, an activator of group II mGluR, was a weak partial agonist at PLD activation (EC50: 22 nM) while L-AP 4, an activator at group III mGluR, was totally inactive. Likewise, forskolin, a stimulant of cyclic AMP formation, was inactive either alone, or in combination with glutamatergic agonists. Pretreatment of the slices with pertussis toxin did not affect PLD activation. In summary, the glutamate-mediated activation of hippocampal PLD, which occurs transiently during postnatal development, is mediated by a group I mGluR, possibly involving mGluR5.

Adrenergic activation of phospholipase D in primary rat astrocytes

Phospholipase D (PLD) activity was investigated in astrocytes prepared from newborn rat cerebral cortex using the transphosphatidylation assay. Basal PLD activity was measurable and was found to be enhanced by ATP, carbachol and noradrenaline. The activation by noradrenaline (EC50, 0.68 microM) was mimicked by methoxamine (EC50, 65 microM), an alpha 1-specific adrenergic agonist, and was inhibited by prazosine, an alpha 1-specific adrenergic antagonist. Clonidin, an alpha 2-adrenergic agonist, slightly lowered PLD activity whereas beta-adrenergic drugs were without effect. Experiments with mitogens indicate that PLD activation in astrocytes may be involved in the control of astrocytic cell proliferation.

Effects of nicotinamide on central cholinergic transmission and on spatial learning in rats

High-dose nicotinamide (1000 mg/kg) leads to a minor increase of plasma choline but to a major increase of the choline concentrations in the intra- and extracellular spaces of the brain. In the hippocampus, the nicotinamide-induced increase in choline was associated with an increase in the release of acetylcholine under stimulated conditions. In young rats, nicotinamide in doses between 10 and 1000 mg/kg did not influence spatial learning, as tested in the Morris water maze. In old rats, low doses of nicotinamide were ineffective whereas the high dose of 1000 mg/kg even impaired spatial learning. The combined administration of choline and nicotinamide had a synergistic effect on brain choline levels but had similar effects as nicotinamide given alone in the behavioral experiments. Additional tests for spontaneous behaviour and locomotion revealed procholinergic and sedative effects of the compound. We conclude that the ineffectiveness of the putative cognition enhancer nicotinamide in the learning task may be due to the observed sedative effect. Therefore, the development of nonsedative nicotinamide derivatives is recommended.

Phospholipase C and phospholipase D are independently activated in rat hippocampal slices

In order to investigate a possible G-protein-mediated activation of phospholipase D (PLD) and its relationship to the activation of phosphoinositide-specific phospholipase C (PI-PLC), we measured the effects of aluminium fluoride and carbachol on choline release, the PLD-specific transphosphatidylation reaction (generation of phosphatidylpropanol) and the formation of inositol phosphates in rat hippocampal slices. Aluminium fluoride markedly enhanced the formation of choline and phosphatidylpropanol but failed to increase the formation of inositol phosphates. In contrast, the muscarinic agonist carbachol strongly stimulated PI-PLC but failed to activate PLD. We conclude that PLD in hippocampal slices is activated by a G-protein independently of phosphoinositide hydrolysis.

Release of choline from rat brain under hypoxia: contribution from phospholipase A2 but not from phospholipase D

Moderate hypoxia induced in rats by inhalation of 10% oxygen led to an increase of the concentration of free choline in the brain and caused a large net-release of choline from the brain into the venous blood as determined by the measurement of the arterio-venous difference. In hippocampal slices from rat brain, hypoxia increased the release of choline into the superfusion medium. The activity of phospholipase D, as measured by the formation of phosphatidylpropanol in the presence of propanol, was not stimulated under these conditions. However, the mobilization of choline was completely depressed by lowering extracellular calcium and by 0.1 mM mepacrine. We conclude that hypoxia leads to a selective activation of phospholipase A2 in the brain and, consequently, to a net loss of choline-containing phospholipids and membrane structures.

Glutamate activates phospholipase D in hippocampal slices of newborn and adult rats

Phospholipase D (PLD) is activated by many neurotransmitters in a novel signal transduction pathway. In the present work, PLD activity was studied comparatively in hippocampal slices of newborn and adult rats. Basal PLD activity in adult rats was almost three times higher than in newborn rats. In newborn rats, L-glutamate and 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) time- and concentration-dependently enhanced the formation of [3H]phosphatidylpropanol ([3H]PP) and of [3H]phosphatidic acid in the presence of 2% propanol. N-Methyl-D-aspartate and kainate (both 1 mM) caused small, but significant increases (approximately 50%), whereas alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (100 microM) was ineffective. Maximally effective concentrations of glutamate (1 mM) and of 1S,3R-ACPD (300 microM) increased the PLD activity to almost 300% of basal activity; the EC50 values were 199 and 47 microM, respectively. Glutamate receptor antagonists, such as DL-2-amino-3-phosphonopropionic acid (AP3), DL-2-amino-5-phosphonovaleric acid, and kynurenate (all 1 mM) did not inhibit the glutamate-evoked increase of PP formation. In slices of adult rats, the response to 1S,3R-ACPD was significant, but small, whereas glutamate was effective only in the presence of the glutamate uptake inhibitor L-aspartate-beta-hydroxamate. It is concluded that glutamate activates PLD in rat hippocampus through an AP3-resistant metabotropic receptor. This effect is subject to ontogenetic development, with one important factor being glutamate uptake.

Synergistic effect of nicotinamide and choline administration on extracellular choline levels in the brain

Experimental studies indicate that the availability of free choline is a rate-limiting step for acetylcholine synthesis in central cholinergic neurons, especially when the release of acetylcholine is increased. In the present study we applied the microdialysis technique to measure the concentration of extracellular choline in the rat hippocampus. The i.p. injection of 6, 20 and 60 mg/kg of choline chloride led to short-lasting elevations of the basal choline efflux (1.78 pmol/min) by 14, 26 and 131%. N-Methylnicotinamide, a metabolite of nicotinamide, has been reported to inhibit the outward transport of choline from the cerebrospinal fluid to the blood. The s.c. injection of 5 and 10 mmol/kg of nicotinamide caused increases of extracellular choline by 54 and 113%, respectively, and choline levels remained elevated for several hr. Moreover, the administration of 10 mmol/kg of nicotinamide dramatically potentiated the effects of exogenous choline administration on choline availability in the central nervous system. The effects of 6 and 20 mg/kg of choline chloride were increased by a factor of more than 10-fold when determined as area under the curve. Additional experiments demonstrated that neither nicotinamide nor N-methylnicotinamide (100 microM) have an influence on the uptake, metabolism or release of choline in the hippocampal slice preparation. It is likely, therefore, that nicotinamide, after metabolic conversion in the brain to N-methylnicotinamide, leads to a blockade of choline clearance from the brain. The combined administration of choline and of a choline transport blocker analogous to nicotinamide may be of potential use in central cholinergic dysfunction.

Free choline and choline metabolites in rat brain and body fluids: sensitive determination and implications for choline supply to the brain

In the central nervous system, choline is an essential precursor of choline-containing phospholipids in neurons and glial cells and of acetylcholine in cholinergic neurons. In order to study choline transport and metabolism in the brain, we developed a comprehensive methodical procedure for the analysis of choline and its major metabolites which involves a separation step, selective hydrolysis and subsequent determination of free choline by HPLC and electrochemical detection. In the present paper, we report the levels of choline, acetylcholine, phosphocholine, glycerophosphocholine and choline-containing phospholipids in brain tissue, cerebrospinal fluid and blood plasma of the untreated rat. The levels of free choline in blood plasma (11.4 microM), CSF (6.7 microM) and brain intracellular space (64.0 microM) were sufficiently similar to be compatible with an exchange of choline between these compartments. In contrast, the intracellular levels of glycerophosphocholine (1.15 mM) and phosphocholine (0.59 mM) in the brain were considerably higher than their CSF concentrations of 2.83 and 1.70 microM, respectively. In blood plasma, glycerophosphocholine was present in a concentration of 4.58 microM while phosphocholine levels were very low or absent (< 0.1 microM). The levels of phosphatidylcholine and lyso-phosphatidylcholine were high in blood plasma (1267 and 268 microM) but very low in cerebrospinal fluid (< 10 microM). We concluded that the transport of free choline is the only likely mechanism which contributes to the supply of choline to the brain under physiological conditions.

Phospholipase D in heart: basal activity and stimulation by phorbol esters and aluminum fluoride

Evidence for a general role of phospholipase D in signal transduction is accumulating. In the present study, the activity of the enzyme was investigated in heart tissue under basal conditions and after addition of phorbol esters or aluminum fluoride (AlF-4; 10 mM NaF plus 10 microM AlCl3). Atria of rats and chickens were incubated with [3H]-myristic acid in order to label preferentially phosphatidylcholine. Under basal conditions, the tissues generated choline and phosphatidic acid (PtdOH), the primary catalytic products of phospholipase D. When 0.5 or 2.0% ethanol was present, [3H]-phosphatidylethanol (PETH) was rapidly formed at the expense of [3H]-PtdOH. This transphosphatidylation reaction is specific for phospholipase D activity. The basal formation of PETH was not inhibited by a Ca(2+)-free, EGTA-containing medium. The phorbol ester 4 beta-phorbol-12 beta, 13 alpha-dibutyrate (PDB), which is known to activate protein kinase C, enhanced the net formation of choline, whereas the inactive 4 beta-phorbol-13 alpha-acetate (PAc) was ineffective. PDB (0.2 microM), in contrast to PAc, also increased the formation of [3H]-PtdOH and, in the presence of ethanol, of [3H]-PETH. The PDB-evoked formation of PETH occurred again at the expense of PtdOH. Treshold and maximum effective concentrations of PDB were 10 nM and 0.2-0.6 microM, respectively. The effects of PDB on either choline efflux and generation of PETH showed the same Ca(2+)-dependency, i.e., both effects were blocked by a Ca(2+)-free, EGTA-containing medium, but not by a Ca(2+)-free medium without EGTA.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory and excitatory muscarinic receptors modulating the release of acetylcholine from the postganglionic parasympathetic neuron of the chicken heart

The effects of muscarinic receptor antagonists on ACh release were studied in the absence or presence of cholinesterase (ChE) inhibition using the isolated perfused chicken heart. Presynaptic inhibitory muscarinic autoreceptor were characterized by determining the potency of various antagonists to enhance [3H]-ACh release evoked by field stimulation (3 Hz, 1 min). The order of potencies was: (+/-)-telenzepine > atropine > 4-DAMP > silahexocyclium > pirenzepine > hexahydro-siladifenid-ol > AF-DX 116. The comparison with known pA2 values for M1-, M2- and M3-receptors revealed that the presynaptic autoreceptor meets the criteria of an M1-receptor. Basal, not electrically evoked overflow of unlabelled ACh into the perfusate was caused by 'leakage' release (non-exocytotic), as it was independent of extracellular Ca2+. Muscarinic receptor antagonists failed to enhance basel overflow. In contrast, when ChE activity was inhibited by 10(-6) M tacrine or pretreatment with 10(-4) M DFP, the ACh overflow was partially Ca(2+)-dependent and was reduced by tetrodotoxine. Moreover, block of the inhibitory muscarinic autoreceptors by (+/-)-telenzepine or pirenzepine caused a several-fold enhancement of the ACh release. The potencies of these antagonists were identical to those found for the electrically evoked [3H]-ACh release. The rate of ACh release enhanced by ChE inhibition plus telenzepine corresponds to about 12% of the total ACh pool per min, which is about the maximum amount of ACh that is available for any kind of stimuli. The release was dependent on the presence of exogenous choline. Hence elevation of ACh release led to a correspondingly enhanced ACh synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake and metabolism of choline by rat brain after acute choline administration

The present study is concerned with the uptake and metabolism of choline by the rat brain. Intraperitoneal administration of choline chloride (4-60 mg/kg) caused a dose-dependent elevation of the plasma choline concentration from 11.8 to up to 165.2 microM within 10 min and the reversal of the negative arteriovenous difference (AVD) of choline across the brain to positive values at plasma choline levels of greater than 23 microM. Net choline release and uptake were linearly dependent on the plasma choline level in the physiological range of 10-50 microM, whereas the CSF choline level was significantly increased only at plasma choline levels of greater than 50 microM. The bolus injection of 60 mg/kg of [3H]choline chloride caused the net uptake of greater than 500 nmol/g of choline by the brain as calculated from the AVD, which was reflected in a minor increase of free choline level and a long-lasting increase of brain phosphorylcholine content, which paralleled the uptake curve. Loss of label from phosphorylcholine 30 min to 24 h after choline administration was accompanied by an increase of label in phosphatidylcholine, an indication of a delayed transfer of newly taken-up choline into membrane choline pools. In conclusion, homeostasis of brain choline is maintained by a complex system that interrelates choline net movements into and out of the brain and choline incorporation into and release from phospholipids.

Uptake and storage of choline by rat brain: influence of dietary choline supplementation

In order to elucidate the regulation of the levels of free choline in the brain, we investigated the influence of chronic and acute choline administration on choline levels in blood, CSF, and brain of the rat and on net movements of choline into and out of the brain as calculated from the arteriovenous differences of choline across the brain. Dietary choline supplementation led to an increase in plasma choline levels of 50% and to an increase in the net release of choline from the brain as compared to a matched group of animals which were kept on a standard diet and exhibited identical arterial plasma levels. Moreover, the choline concentration in the CSF and brain tissue was doubled. In the same rats, the injection of 60 mg/kg choline chloride did not lead to an additional increase of the brain choline levels, whereas in control animals choline injection caused a significant increase; however, this increase in no case surpassed the levels caused by chronic choline supplementation. The net uptake of choline after acute choline administration was strongly reduced in the high-choline group (from 418 to 158 nmol/g). Both diet groups metabolized the bulk (greater than 96%) of newly taken up choline rapidly. The results indicate that choline supplementation markedly attenuates the rise of free choline in the brain that is observed after acute choline administration. The rapid metabolic choline clearance was not reduced by dietary choline load. We conclude that the brain is protected from excess choline by rapid metabolism, as well as by adaptive, diet-induced changes of the net uptake and release of choline.

The effect of tacrine on acetylcholine overflow in the heart

Tacrine, 10(-6) M, enhanced the acetylcholine (ACh) overflow evoked in perfused chicken hearts by field stimulation (5 Hz, 1 min) from 183 to 346 pmol g-1 min-1. Increase to the same level were observed after pretreatment with diisopropylfluorophosphate (DFP) as well as after DFP plus 10(-6) M tacrine. Tacrine, 10(-5) M, caused further enhancement with or without DFP up to 851 pmol g-1 min-1. It was concluded that 10(-6) M tacrine enhanced the ACh overflow by choline esterase inhibition, whereas 10(-5) M tacrine caused, in addition, an increase of neuronal ACh release.

Small rises in plasma choline reverse the negative arteriovenous difference of brain choline

The concentrations of free choline in blood plasma from a peripheral artery and from the transverse sinus, in the CSF, and in total brain homogenate, have been measured in untreated rats and in rats after acute intraperitoneal administration of choline chloride. In untreated rats, the arteriovenous difference of brain choline was related to the arterial choline level. At low arterial blood levels (less than 10 microM) as observed under fasting conditions, the arteriovenous difference was negative (about -2 microM), indicating a net release of choline from the brain of about 1.6 nmol/g/min. In rats with spontaneously high arterial blood levels (greater than 15 microM), the arteriovenous difference was positive, implying a marked net uptake of choline by the brain (3.1 nmol/g/min). The CSF choline concentration, which reflects changes in the extracellular choline concentration, also increased with increasing plasma levels and closely paralleled the gradually rising net uptake. Acute administration of 6, 20, or 60 mg of choline chloride/kg caused, in a dose-dependent manner, a sharp rise of the arterial blood levels and the CSF choline, and reversed the arteriovenous difference of choline to markedly positive values. The total free choline in the brain rose only initially and to a quantitatively negligible extent. Thus, the amount of choline taken up by the brain within 30 min was stored almost completely in a metabolized form and was sufficient to sustain the release of choline from the brain as long as the plasma level remained low. We conclude that the extracellular choline concentration of the brain closely parallels fluctuations in the plasma level of choline.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of phorbol esters on choline phospholipid hydrolysis in heart and brain

The efflux of choline was determined in rat striatal slices, incubated chicken atria and perfused chicken hearts. 4 beta-Phorbol-12 beta,13 alpha-dibutyrate (PDB) and 4 beta-phorbol-12 beta-myristate, 13 alpha-acetate (PMA) were used to stimulate protein kinase C. The other phorbol esters, 4 beta-phorbol-13 alpha-acetate (PAc) and 4 alpha-phorbol-12 beta,13 alpha-didecanoate (4 alpha PDD), known to be inactive, were tested to evaluate the specificity of the responses. PDB markedly enhanced the efflux of choline in all of the three preparations. The PDB-evoked efflux of choline in incubated chicken atria was equal to the net production of choline and, therefore, was not caused by translocation of intracellular free choline. After inhibition of the cholinesterase activity, PDB linearly increased the efflux of choline in rat striatal slices, but failed to alter the spontaneous efflux of acetylcholine. Thus acetylcholine did not serve as the source of the PDB-evoked efflux of choline. PMA was as effective as PDB, whereas PAc and 4 alpha PDD failed to alter the choline efflux in the perfused heart. Both infusion of a Ca2(+)-free EGTA-containing Tyrode solution and mepacrine reduced the spontaneous efflux of choline by about 40% and blocked the PDB-evoked efflux of choline. In contrast, a Ca2(+)-free solution without EGTA failed to alter the spontaneous and the PDB-evoked choline efflux. It is concluded that phorbol esters stimulate the hydrolysis of choline-containing phospholipids in heart and brain via activation of protein kinase C.

On the mechanism of muscarinic hydrolysis of choline phospholipids in the heart

In the heart, choline phospholipids were by far the largest fraction (about 50%) of phospholipids, much larger than that of inositol phospholipids (less than 6%) and phosphatidic acid (0.3%). The choline phospholipids (11 mumol/g) maintained a constant efflux of choline of about 1.5 nmol g-1 min-1 into the perfusate. Carbachol (10 microM) rapidly enhanced the choline efflux by a muscarinic mechanism, that was independent of mepacrine, an inhibitor of phospholipase A2, as well as of extracellular Ca2+; the maximum acceleration was reached within 2 min. In contrast, the accumulation of inositol phosphates by carbachol was blocked in the presence of a Ca2+-free perfusion medium. Similar to the carbachol-evoked choline efflux, the increase in tissue content of phosphatidic acid by carbachol was unaffected by infusion of a Ca2+-free, EGTA-containing solution. Sodium oleate (20 microM), an activator of phospholipase D, imitated the effects of carbachol on choline and phosphatidic acid, whereas NaF (5 mM), which has been reported to inhibit phospholipase D, blocked carbachol-evoked efflux of choline. In conclusion, muscarinic receptor stimulation enhanced the hydrolysis of choline phospholipids presumably via activation of phospholipase D. The immediate formation of choline, phosphatidic acid and presumably diacylglycerol is discussed including its possible physiological importance.

Subtypes of muscarinic receptor on cholinergic nerves and atrial cells of chicken and guinea-pig hearts

1. Electrically driven chicken and guinea-pig atria were used to investigate the negative inotropic effects of the muscarinic agonists methacholine and acetylcholine (ACh). The release of ACh from isolated hearts into the perfusate in response to (preganglionic) vagal or (pre- and postganglionic) field stimulation was bioassayed on the guinea-pig ileum or determined by labelling with [3H]-choline. 2. Concentration-response curves for the negative inotropic effect of methacholine were shifted to the right by pirenzepine in various concentrations (0.03 to 10 mumol l-1). The pA2 values were 7.76 in chicken atria and 6.53 in guinea-pig atria. Pirenzepine and atropine antagonized the negative inotropic response to 0.3 mumol l-1 ACh. The half-maximally effective concentrations (IC50) of pirenzepine (Pz) and atropine were 40 and 5.4 nmol l-1 in chicken atria and 330 and 3.5 nmol l-1, respectively, in guinea-pig atria. Thus, the respective potency ratios (IC50Pz/IC50atropine) were 7.4 and 94.3 in the two species. 3. Pirenzepine in low concentrations increased the release of unlabelled and 3H-labelled ACh from isolated hearts evoked by vagal and field stimulation only in chicken, but not in guinea-pigs. The half-maximally-effective concentration of pirenzepine was about 30 nmol l-1 in the chicken heart, whereas, in the guinea-pig heart, an increased release was observed at 300 nmol l-1. 4. (+)-Tubocurarine [(+)-Tc; 100 mumol l-1] reduced the release of ACh evoked by (preganglionic) vagal stimulation to a (+)-Tc-resistant release of about 30%. The time-course of the neuronal release of [3H]-ACh was markedly altered: the onset was delayed and the termination was extended beyond the period of stimulation (1 min or 5s) by several seconds. The (+)-Tc-resistant release was nearly abolished by 30 nmol l-1 pirenzepine. 5. In conclusion, the pre- and post-synaptic muscarinic receptors of the parasympathetic neuroeffector junction of the heart both belong to the M1-subtype in the chicken and to an M2-subtype in the guinea-pig. Block of the nicotinic ganglionic transmission in the chicken heart by (+)-Tc unmasked a muscarinic transmission, which presumably was mediated through M1-receptors stimulating a low and prolonged postganglionic release of ACh.

Muscarinic mobilization of choline in rat brain in vivo as shown by the cerebral arterio-venous difference of choline

In anesthetized rats, the choline levels of cerebrospinal fluid and plasma obtained from blood collected from peripheral vessels (carotid artery, cardiac vessels) and from the transverse sinus were determined with a radioenzymatic assay. Cortical release of choline was studied using the "cup technique." The plasma choline level of the peripheral blood (11.5 mumol/L) was lower than that of the sinus blood. The resulting cerebral arterio-venous difference of choline was negative (3.2 mumol/L) and reflected the net release of choline from the whole brain. The plasma choline levels were not different irrespective of whether the rats were anesthetized with ether, urethane, or pentobarbital. However, the choline level of the cerebrospinal fluid, which normally was lower than the plasma choline levels, was increased by urethane anesthesia to a level between the arterial and venous plasma concentrations of the brain. In old rats (24 months), the choline level of the cerebrospinal fluid was significantly lowered, when compared with the results obtained with younger rats (2-4 months). In rats kept on a low-choline diet for 2 weeks, the plasma choline level of the peripheral blood was reduced to 51% of the control. The effect on the choline level of the sinus blood was smaller; the cerebral arterio-venous difference of choline was not reduced (it was even slightly enhanced). Likewise, the choline level of the cerebrospinal fluid and the cortical release of choline were not altered. Intraperitoneal administration of oxotremorine in pentobarbital-anesthetized rats kept on a low-choline diet increased the plasma levels of choline.(ABSTRACT TRUNCATED AT 250 WORDS)

The release of choline from phospholipids mediated by beta-adrenoceptor activation in isolated hearts

The resting efflux of choline into the perfusate (Tyrode's solution) of isolated hearts was equal to the rate, at which choline was liberated from phospholipid degradation (Lindmar et al. 1986). Infusion of isoprenaline (2 X 10(-7) mol/l), forskolin (1-3 X 10(-6) mol/l) or 3-isobutyl-1-methylxanthine (IBMX; 3 X 10(-4) mol/l) for 40 min markedly enhanced the efflux of choline. The increase was linear during the experimental period and, in the case of isoprenaline, was blocked by 3 X 10(-7) mol/l atenolol. In the guinea-pig heart, IBMX at a threshold concentration of 10(-4) mol/l shifted the concentration-response curve for the effect of forskolin on the efflux of choline to the left by one log unit. Forskolin (10(-6) mol/l) increased also the tissue content of cyclic AMP. This effect and the increase of choline efflux evoked by forskolin were blocked by 2 X 10(-7) mol/l carbachol. Likewise, inhibition of cholinesterase activity caused by diisopropylfluorophosphate antagonized the forskolin-evoked acceleration of choline efflux indicating a response to endogenous acetylcholine. The muscarinic inhibition of the enhanced choline efflux was reversed by 3 X 10(-7) mol/l atropine. The phospholipase A2 inhibitor mepacrine as well as infusion of a low Ca2+-Tyrode's solution (0.2 instead of 1.8 mmol/l) blocked the effect of forskolin on choline efflux, whereas the generation of cyclic AMP by forskolin was unaffected by low Ca2+-solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Ouabain enhances release of acetylcholine in the heart evoked by unilateral vagal stimulation

The aim of the study was to elucidate peripheral effects of ouabain on the parasympathetic innervation of the heart, effects that could contribute to the experimentally and clinically well established "vagal effect of cardiac glycosides". The experiments were carried out with ouabain concentrations of 3 X 10(-7) and 10(-6) mol/l, which were considered "therapeutic", as they increased force of contraction and did not elicit arrhythmias in incubated chicken atria. In atrial preparations of chickens and guinea-pigs the negative chronotropic and inotropic effects of acetylcholine (ACh) were not altered by 3 X 10(-7) mol/l ouabain. Resting efflux of ACh from perfused chicken hearts was increased by ouabain from 10 to a maximum of 30 pmol/g min, whereas release of ACh evoked by bilateral vagal stimulation at 3 or 20 Hz for 1 min was unchanged (resting release subtracted). In contrast, release of ACh caused by unilateral vagal stimulation was augmented by ouabain up to 200% of the control. Release by unilateral stimulation (80 pmol/g; 20 Hz) was calculated for each experiment by averaging the releases evoked by consecutive stimulation of the right and left nerves. Ouabain infused for 90 min did not alter the tissue content of ACh (5.5 nmol/g). Within 2 days after unilateral (left) vagal transsection (denervation of cardiac ganglia) the release of ACh evoked by stimulation of the intact nerve (20 Hz) increased from about 80 to 200 pmol/g, whereas the release from the lesioned nerve markedly declined.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of choline efflux from the perfused heart at rest and after muscarine receptor activation

The resting efflux of choline from perfused chicken hearts varied from 0.4 to 2.6 nmol/g min, but was constant for at least 80 min in the individual experiments. The rate of choline efflux was found to be equal to the rate of choline formation in the heart, which, from the following reasons, was essentially due to hydrolysis of choline phospholipids. Cardiac content of choline phospholipids (7,200 nmol/g) was much higher than that of acetylcholine (5.5 nmol/g). Resting release of acetylcholine was 0.016 nmol/g min and, after inhibition of cholinesterase, only about 0.1 nmol/g min. Resting efflux of choline was reduced by mepacrine, a phospholipase A2 inhibitor, by perfusion with a Ca2+-free Tyrode's solution containing EGTA and by the combination mepacrine plus Ca2+-free/EGTA solution. In all experiments the reduced choline efflux levelled off within 10 min at about 50%. Omission or elevation of Mg2+ from 1.05 to 10.5 mmol/l had no effect. Resting efflux was increased to 150% by oleic acid (as sodium salt; 2 X 10(-5) mol/l) which is known to activate phospholipase D. Likewise muscarinic agonists (carbachol and acetylcholine) caused facilitation of the efflux of endogenous choline that was blocked by 3 X 10(-7) mol/l atropine. This effect was not reduced, but even slightly enhanced, by mepacrine and by infusion of EGTA in a modified Tyrode's solution (Ca2+-free, 10.5 mmol/l Mg2+). It is concluded that the resting efflux of choline from the heart is essentially due to hydrolysis of choline phospholipids, that half of the efflux is insensitive to mepacrine and is Ca2+-independent (excluding an involvement of phospholipase A2).(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic control of respiration as shown by effects of 4-aminopyridine

The dopaminergic control of respiration in conscious and urethane-anaesthetized rabbits, was studied by comparing the respiratory effects of 4-aminopyridine alone (4-AP; 1 mg/kg i.v.) and those after the administration of dopamine antagonists (domperidone and haloperidol; 1 mg/kg each). The respiratory rate in conscious rabbits was increased by 4-AP. After domperidone this increase was reduced and preceded by a transient decrease. In spontaneously breathing, anesthetized rabbits there was a transient reduction after which the respiratory rate was increased by 4-AP; tidal volume was affected in an inverse manner. After domperidone, the excitatory effect of 4-AP on respiratory rate and the inhibitory effect on tidal volume were blocked. The effects of 4-AP on respiratory rate were prevented by vagotomy. In anesthetized, vagotomized, paralyzed and artificially ventilated rabbits (VPV animals) the peak amplitude of the integrated phrenic nerve activity ("phrenic activity') was increased by 4-AP. After pretreatment with haloperidol this effect of 4-AP on phrenic activity was reduced while the respiratory rate was now increased. In VPV animals with denervated carotid bodies the excitatory effect of 4-AP on phrenic activity was strongly enhanced and respiratory rate was increased. These effects were slightly reduced but not blocked by haloperidol. It is concluded that endogenous dopamine is involved in the control of respiration through effects on peripheral mechanisms (inhibition of inspiratory activity and enhancement of respiratory rate) as well as on central mechanisms (stimulation of inspiratory activity and reduction of respiratory rate).

Muscarinic mobilization of choline in rat cerebral cortex does not involve alterations of blood-brain barrier

Efflux of choline from the rat cerebral cortex in vivo was investigated using the cup technique. After removal of the dura mater, the cup was placed on the cortex. Transmission and scanning electron microscopy revealed that the cortex was separated from the cup solution (100-300 microliter) by basal lamina, pia mater, arachnoid (with discrete defects) and remainders of the subdural neurothelium. Two kinds of experiments were carried out to determine: efflux of unlabelled choline into the cup solution; and translocation of radioactivity from the plasma into the cup solution (via blood-brain barrier and leptomeningeal layers) during i.v. infusion of [3H]choline or [14C]inulin. The former process was highly temperature-sensitive in contrast to the latter. Penicillin-G-sodium, which is known to damage the blood-brain barrier, was added to the cup solution, enhanced efflux of unlabelled choline, and caused a 5-fold increase in the rates of translocation of radioactivity during infusion of either labelled choline or inulin. In contrast, physostigmine (3 X 10(-4) M, added to cup solution) failed to enhance 3H-translocation, but markedly facilitated the efflux of unlabelled choline; this effect was highly temperature-sensitive and was blocked by atropine. It is concluded that activation of muscarinic receptors enhanced the choline efflux from cortical tissue. This effect was caused by cellular mobilization of choline presumably through an action on the metabolism of phosphatidylcholine. The effect was not due to alterations in the translocation of choline from the plasma to the cup solution, i.e. through permeability changes in the blood-brain barrier and in the leptomeningeal 'barrier'.(ABSTRACT TRUNCATED AT 250 WORDS)

The parasympathetic neuroeffector junction of the heart

This review has directed attention to the anatomical, biochemical, physiological, and pharmacological features of cardiac parasympathetic neuroeffector transmission. The relationships among these properties have permitted a synthesis of the operation of the parasympathetic nervous system at the subcellular, cellular, tissue, and organ level in the heart. However, the attempt to obtain a comprehensive model of parasympathetic neuroeffector transmission in the heart has not only unmasked many features of junctional activity that are poorly understood, but also indicated significant gaps between in vitro and in vivo experimental situations.

Hydrolysis, synthesis, and release of acetylcholine in the isolated heart

The occurrence of unhydrolyzed acetylcholine (ACh) in the cardiac perfusate during vagal stimulation in the absence of cholinesterase inhibition has been demonstrated by several methods. Because some ACh was found unhydrolyzed in the extracellular space for several seconds after vagal stimulation (half-time of decay 2.5 s), it appears that the prolonged time course of the cardiac responses to bursts of vagal activity is determined by a slow rate of transmitter inactivation (diffusion plus hydrolysis) in addition to slowly operating postsynaptic mechanisms mediated by activation of the muscarinic receptor. The neuronal uptake of choline in isolated heart preparations was found to be Na+ dependent, sensitive to hemicholinium 3, and activated by vagal stimulation. Activation occurred after a delay of 1 or 2 min and slowly faded within 5 min after stimulation. Resting release of ACh was insensitive to extracellular Ca2+ and to muscarinic feedback inhibition, in contrast to the evoked transmitter release. Inasmuch as atropine increased ACh release by vagal and field stimulation to the same extent, muscarinic feedback inhibition is likely to occur at postganglionic parasympathetic neurons. Adrenergic agonists and propranolol did not significantly change the release of ACh.

Mobilization of cellular choline by stimulation of muscarine receptors in isolated chicken heart and rat cortex in vivo

The effects of cholinesterase inhibitors and muscarinic agonists on efflux of choline were studied in isolated perfused chicken heart and rat cortex in vivo. In the heart, the phospholipase A2 inhibitor mepacrine (10(-4) M) reduced the choline efflux (1.1 nmol g-1 min-1) by 51 +/- 5% (N = 3), whereas several cholinesterase inhibitors (physostigmine, neostigmine and diisopropylfluorophosphate) and muscarinic agonists (acetylcholine, oxotremorine and bethanechol) caused an increase. The muscarinic increase in choline efflux appears to be specific, as the increase caused by 10(-6) M physostigmine (+113%), by 3 X 10(-7) M acetylcholine (+89%) or by 5 X 10(-4) M bethanechol (+29%) was blocked by atropine. Cholinesterase inhibitors and muscarinic agonists also caused a decrease in heart rate by about 50%. Papaverine (10(-5) M) blocked the physostigmine- or bethanechol-evoked increase in choline efflux, but left the decrease in heart rate unchanged. Choline efflux from rat cortex in vivo was studied using the "cup technique." During the experimental period (3 hr), resting efflux declined from 60 to 15 pmol cm-2 min-1. Again choline efflux was increased by physostigmine (+48%) or by bethanechol (+48%) added to the cup solution from 80 to 160 min, whereas a decrease was observed after atropine plus physostigmine (-36%) or atropine plus bethanechol (-26%). In conclusion, stimulation of muscarine receptors increased extracellular choline by mobilization of cellular choline presumably through an effect on phospholipid metabolism. The hypothesis is discussed that synthesis of acetylcholine in the brain may be supported by an autoregulation of precursor availability.

Evidence for bilateral vagal innervation of postganglionic parasympathetic neurons in chicken heart

Stimulation of the cervical vagus nerves caused an output of acetylcholine (ACh) from the isolated chicken heart, which almost exclusively was released from the postganglionic neurons: (+)-tubocurarine (3 X 10(-14) M) reduced the output to 12 +/- 6% (n = 7) of the control. Stimulation of the two nerve trunks ws equally effective in releasing ACh.--Evidence that a large number of postganglionic neurons receives bilateral innervation was based on two experimental series. (1). The sum of the ACh outputs evoked by unilateral (separate) nerve stimulation of the right and the left vagus was higher than the bilaterally evoked output (100%) and increased with increasing frequencies (10, 20 and 40 Hz) from 115 +/- 13% to 131 +/- 9% (n = 13). In the presence of 10(-4) M 4-aminopyridine, unilaterally evoked output (40 Hz) was further increased from 131 to 176 +/- 5% (n = 21).--(2.) In the presence of 4-aminopyridine plus hemicholinium-3 (2 X 10(-5) M), unilateral nerve stimulation at 40 Hz evoked an output of ACh that decreased from 477 to 79 pmol g-1 min-1 during a 20 min-period of stimulation due to transmitter depletion. Thereafter output of ACh evoked by stimulation of the contralateral nerve was reduced by 73% as compared to the control value (475 pmol g-1 min-1; output without the preceding 20 min-stimulation).--It is concluded that a large number of parasympathetic postganglionic neurons of the chicken heart receives a dual excitatory input from both right and left vagus nerve.