Synthesis of acetylcholine by endothelial cells isolated from rat brain cortex capillaries

Expression of the High-affinity Choline Transporter CHT1 in Rat and Human Arteries

The arterial vascular wall contains a non-neuronal intrinsic cholinergic system. The rate-limiting step in acetylcholine (ACh) synthesis is choline uptake. A high-affinity choline transporter, CHT1, has recently been cloned from neural tissue and has been identified in epithelial cholinergic cells. Here we investigated its presence in rat and human arteries and in primary cell cultures of rat vascular cells (endothelial cells, smooth muscle cells, fibroblasts). CHT1-mRNA was detected in the arterial wall and in all isolated cell types by RT-PCR using five different CHT1-specific primer pairs. Antisera raised against amino acids 29-40 of the rat sequence labeled a single band (50 kD) in Western blots of rat aorta, and an additional higher molecular weight band appeared in the hippocampus. Immunohistochemistry demonstrated CHT1 immunoreactivity in endothelial and smooth muscle cells in situ and in all cultured cell types. A high-affinity [3H]-choline uptake mechanism sharing characteristics with neuronal high-affinity choline uptake, i.e., sensitivity to hemicholinium-3 and dependence on sodium, was demonstrated in rat thoracic aortic segments by microimager autoradiography. Expression of the high-affinity choline transporter CHT1 is a novel component of the intrinsic non-neuronal cholinergic system of the arterial vascular wall, predominantly in the intimal and medial layers.

Modulation of cerebral microvascular permeability by endothelial nicotinic acetylcholine receptors

Nicotine increases the permeability of the blood-brain barrier in vivo. This implies a possible role for nicotinic acetylcholine receptors in the regulation of cerebral microvascular permeability. Expression of nicotinic acetylcholine receptor subunits in cerebral microvessels was investigated with immunofluorescence microscopy. Positive immunoreactivity was found for receptor subunits alpha3, alpha5, alpha7, and beta2, but not subunits alpha4, beta3, or beta4. Blood-brain barrier permeability was assessed via in situ brain perfusion with [14C]sucrose. Nicotine increased the rate of sucrose entry into the brain from 0.3 +/- 0.1 to 1.1 +/- 0.2 microl.g(-1).min(-1), as previously described. This nicotine-induced increase in blood-brain barrier permeability was significantly attenuated by both the blood-brain barrier-permeant nicotinic antagonist mecamylamine and the blood-brain barrier-impermeant nicotinic antagonist hexamethonium to 0.5 +/- 0.2 and 0.3 +/- 0.2 microl.g(-1).min(-1), respectively. These data suggest that nicotinic acetylcholine receptors expressed on the cerebral microvascular endothelium mediate nicotine-induced changes in blood-brain barrier permeability.

Expression and function of the non-neuronal cholinergic system in endothelial cells

Increasing evidence has shown the expression of the non-neuronal cholinergic system in endothelial cells. In the present experiments the expression of choline acetyltransferase (ChAT) was investigated in human endothelial cells by anti-ChAT immunohistochemistry and anti-ChAT immunofluorescence. Positive ChAT immunoreactivity was found in cultures of human umbilical endothelial cells (HUVEC) and a human angiosarcoma cell line (HAEND). In HUVEC and HAEND choline acetyltransferase activity and small amounts of acetylcholine were also detected. Positive ChAT-immunoreactivity was demonstrated in situ in endothelial cells of the human umbilical cord. In addition, in experiments with confocal laser scanning microscopy positive anti-ChAT immunoreactivity was found in situ in endothelial cells of human skin blood vessels. In the first functional experiments with HUVEC acetylcholine appeared to mediate a small facilitatory effect on the expression of intracellular adhesion molecule-1. The present experiments demonstrate the wide existence of ChAT in human endothelial cells. Further experiments are addressed to elucidate the biological role of acetylcholine in the endothelium and possible differences between the different subtypes of endothelial cells.

The Non-neuronal cholinergic system: an emerging drug target in the airways

The non-neuronal cholinergic system is widely expressed in human airways. Choline acetyltransferase (ChAT) and/or acetylcholine are demonstrated in more or less all epithelial surface cells (goblet cells, ciliated cells, basal cells), submucosal glands and airway smooth muscle fibres. Acetylcholine is also demonstrated in the effector cells of the immune system (lymphocytes, macrophages, mast cells). Epithelial, endothelial and immune cells express nicotinic and muscarinic receptors. Thus the cytomolecule acetylcholine can contribute to the regulation of basic cell functions via auto-/paracrine mechanisms (proliferation, differentiation, ciliary activity, secretion of water, ions and mucus, organization of the cytoskeleton, cell-cell contact). Acetylcholine also modulates immune functions (release of cytokines; proliferation, activation and inhibition of immune cells). Preliminary experimental evidence suggests that mucosal inflammation may be associated with raised acetylcholine levels, impairing cell and organ homeostasis. It should be considered that anti-muscarinic drugs which are applied for the treatment of chronic airway diseases antagonize the effect of both neuronal and non-neuronal acetylcholine. Non-neuronal acetylcholine, however, is still active, possibly directly within the cell cytosol and also via nicotinic receptors localized on various non-neuronal cells. It is an essential task to clarify the pathophysiological role of the non-neuronal cholinergic system in more detail to develop new drugs which can target the synthesis, release, inactivation and cellular activity of non-neuronal acetylcholine.

Expression of muscarinic receptor types in the primate ovary and evidence for nonneuronal acetylcholine synthesis

The presence of muscarinic receptors (MR) in the ovary of different species has been recognized, but the identity of these receptors as well as ovarian sources of their natural ligand, acetylcholine (ACh), have not been determined. Because luteinized human granulosa cells (GC) in culture express functional MR, we have determined whether the group of the related MR subtypes, M1R, M3R, and M5R, are present in vivo in human and rhesus monkey ovaries. To this end, ribonucleic acids (RNAs) of different human and monkey ovaries as well as RNAs from human GC and monkey oocytes were reverse transcribed and subjected to PCR amplification, followed by sequencing of the amplified complementary DNAs. Results obtained showed that M1R, M3R, and M5R messenger RNAs are present in adult human and monkey ovaries; oocytes express exclusively the M3R subtype, whereas GC express M1R and M5R. To determine the ovarian source(s) of the natural ligand of these ACh receptors, we attempted to localize the enzyme responsible for its synthesis with the help of a monoclonal antibody recognizing choline acetyltransferase for immunohistochemistry. In neither human nor monkey sections did we detect immunoreactive choline acetyltransferase-positive fibers or nerve cells, but, surprisingly, GC of antral follicles showed prominent staining. To determine whether GC can produce ACh, human cultured GC derived from preovulatory follicles were analyzed using a high pressure liquid chromatography technique. The results showed that these cells contained ACh in concentrations ranging from 4.2-11.5 pmol/10(6) cells. Samples of a rat granulosa cell line likewise contained ACh. Thus, the ovary contains multiple MR, and GC of antral follicles are able to synthesize ACh, the ligand of MR. We propose that ACh may serve as an as yet unrecognized factor involved in the complex regulation of ovarian function in the primate, e.g. regulation of cell proliferation or progesterone production.

The non-neuronal cholinergic system in the endothelium: evidence and possible pathobiological significance

An increasing body of knowledge indicates that the cholinergic system is not confined to the nervous system, but is practically ubiquitous. The present paper will address the question of the non-neuronal cholinergic system in vascular endothelial cells (EC). In tissue sections of human skin, immunohistochemical studies using confocal laser scanning microscopy showed ChAT (choline acetyltransferase) activity in the EC of dermal blood vessels. Positive ChAT immunoreactivity was also demonstrated in monolayer cultures of human umbilical vein EC (HUVEC) and a human angiosarcoma EC line (HAEND). That the synthesizing enzyme is not only present in EC, but also active was shown by measuring ChAT activity. Thus, in HUVEC cultures, ChAT activity amounted to 0.78 +/- 0.15 nmol x mg protein(-1) x h(-1) (n = 3), but was only partially (about 50%) inhibited by the ChAT inhibitor bromoacetylcholine (30 microM). In HPLC measurements, a concentration of 22 +/- 2 pmol acetylcholine (ACh) per 10(6) cells was found (n = 6). However, using a cholinesterase-packed analytical column to check the identity of the acetylcholine peak, the peak height was found to be reduced, although a significant peak still remained, indicating the existence of a compound closely related to ACh. Further immunocytochemical experiments indicated that EC in vitro also express the vesicular acetylcholine transporter (VAChT) system. Preliminary immunoelectron microscopic studies suggest a topographical association of VAChT with endothelial endocytotic vesicles. The presented experiments clearly demonstrate the existence of essential elements of the cholinergic system (ChAT, VAChT, ACh) in the human endothelium. The biological functions of ACh synthesized by endothelial cells are the focus of ongoing research activity.

Nitric oxide-mediated metabolic regulation during exercise: effects of training in health and cardiovascular disease

Accumulating data suggest that nitric oxide (NO) is important for both coronary and peripheral hemodynamic control and metabolic regulation during exercise. Although still controversial, NO of endothelial origin may potentiate exercise-induced hyperemia. Mechanisms of release include both acetylcholine derived from the neuromuscular junction and elevation in vascular shear stress. A splice variant of neuronal nitric oxide synthase (NOS), nNOSmu, is expressed in human skeletal muscle. In addition to being a potential modulator of blood flow, NO from skeletal muscle regulates muscle contraction and metabolism. In particular, recent human data indicate that NO plays a role in muscle glucose uptake during exercise independently of blood flow. Exercise training in healthy individuals elevates NO bioavailability through a variety of mechanisms including increased NOS enzyme expression and activity. Such adaptations likely contribute to increased exercise capacity and cardiovascular protection. Cardiovascular risk factors including hypercholesterolemia, hypertension, diabetes, and smoking as well as established disease are associated with impairment of the various NO systems. Given that NO is an important signaling mechanism during exercise, such impairment may contribute to limitations in exercise capacity through inadequate coronary or peripheral perfusion and via metabolic effects. Exercise training in individuals with elevated cardiovascular risk or established disease can increase NO bioavailability and may represent an important mechanism by which exercise training conveys benefit in the setting of secondary prevention.

Nitric oxide as a metabolic regulator during exercise: effects of training in health and disease

1. Accumulating animal and human data suggest that nitric oxide (NO) is important for both coronary and peripheral haemodynamic control and metabolic regulation during performance of exercise. 2. While still controversial, NO of endothelial origin is thought to potentiate exercise-induced hyperaemia, both in the peripheral and coronary circulations. The mechanism of release may include both acetylcholine derived from the neuromuscular junction and vascular shear stress. 3. A splice variant of neuronal nitric oxide synthase (NOS), nNOSmicro, incorporating an extra 34 amino acids, is expressed in human skeletal muscle. In addition to being a potential modulator of blood flow, skeletal muscle-derived NO is an important regulator of muscle contraction and metabolism. In particular, recent human data indicate that NO modulates muscle glucose uptake during exercise, independently of blood flow. 4. Exercise training in healthy individuals promotes adaptations in the various NO systems, which can increase NO bioavailability through a variety of mechanisms, including increased NOS enzyme expression and activity. Such adaptations likely contribute to increased exercise capacity and protection from cardiovascular events. 5. Cardiovascular risk factors, including hypercholesterolaemia, hypertension, diabetes and smoking, as well as established disease, are associated with impairment of the various NO systems. Given that NO is an important signalling mechanism during exercise, such impairment may contribute to limitations in exercise capacity through inadequate coronary or peripheral blood delivery and via metabolic effects. 6. Exercise training in individuals with elevated cardiovascular risk or established disease can increase NO bioavailability and may represent an important mechanism by which exercise training provides benefit in the setting of secondary prevention.

The cholinergic 'pitfall': acetylcholine, a universal cell molecule in biological systems, including humans

1. Acetylcholine (ACh) represents one of the most exemplary neurotransmitters. In addition to its presence in neuronal tissue, there is increasing experimental evidence that ACh is widely expressed in pro- and eukaryotic non-neuronal cells. Thus, ACh has been detected in bacteria, algae, protozoa, tubellariae and primitive plants, suggesting an extremely early appearance of ACh in the evolutionary process. 2. In humans, ACh and/or the synthesizing enzyme, choline acetyltransferase, has been demonstrated in epithelial cells (airways, alimentary tract, urogenital tract, epidermis), mesothelial (pleura, pericardium) and endothelial and muscle cells. In addition, immune cells express the non-neuronal cholinergic system (i.e. the synthesis of ACh can be detected in human leucocytes (granulocytes, lymphocytes and macrophages)), as well as in rat microglia in vitro. 3. The widespread expression of non-neuronal ACh is accompanied by the ubiquitous expression of cholinesterase activity, which prevents ACh from acting as a classical hormone. 4. Non-neuronal ACh mediates its cellular actions in an auto- and paracrine manner via the activation of the widely expressed nicotinic and muscarinic acetylcholine receptors, which can interfere with virtually all cellular signalling pathways (ion channels and key enzymes). 5. Non-neuronal ACh appears to be involved in the regulation of basic cell functions, such as mitosis, cell differentiation, organization of the cytoskeleton, cell-cell contact, secretion and absorption. Non-neuronal ACh also plays a role in the regulation of immune functions. All these qualities together may mediate the so-called 'trophic property' of ACh. 6. Future experiments should be designed to analyse the cellular effects of ACh in greater detail. The involvement of the non-neuronal cholinergic system in the pathogenesis of chronic inflammatory diseases should be investigated to open up new therapeutic strategies.

Choline acetyltransferase (ChAT) immunoreactivity in paraffin sections of normal and diseased intestines

There is increasing interest in localizing nerves in the intestine, especially specific populations of nerves. At present, the usual histochemical marker for cholinergic nerves in tissue sections is acetylcholinesterase activity. However, such techniques are applicable only to frozen sections and have uncertain specificity. Choline acetyltransferase (ChAT) is also present in cholinergic nerves, and we therefore aimed to establish a paraffin section immunocytochemical technique using an anti-ChAT antibody. Monoclonal anti-choline acetyltransferase (1.B3.9B3) and a biotin-streptavidin detection system were used to study the distribution of ChAT immunoreactivity (ChAT IR) in paraffin-embedded normal and diseased gastrointestinal tracts from both rats and humans. Optimal staining was seen after 6-24 hr of fixation in neutral buffered formalin and overnight incubation in 1 microgram/ml of 1.B3.9B3, with a similar distribution to that seen in frozen sections. In the rat diaphragm (used as a positive control), axons and motor endplates were ChAT IR. Proportions of ganglion cells and nerve fibers in the intramural plexi of both human and rat gastrointestinal tracts were also ChAT IR, as well as extrinsic nerve bundles in aganglionic segments of Hirschsprung's disease. Mucosal cholinergic nerves, however, were not visualized. In addition, non-neuronal cells such as endothelium, epithelium, and inflammatory cells were ChAT IR. We were able to localize ChAT to nerves in formalin-fixed, paraffin-embedded sections. The presence of ChAT IR in non-neuronal cells indicates that this method should be used in conjunction with other antibodies. Nevertheless, it proves to be a useful technique for studying cholinergic neuronal distinction in normal tissues and pathological disorders.

Non-neuronal acetylcholine, a locally acting molecule, widely distributed in biological systems: expression and function in humans

Acetylcholine acts as a neurotransmitter in the central and peripheral nervous systems in humans. However, recent experiments demonstrate a widespread expression of the cholinergic system in non-neuronal cells in humans. The synthesizing enzyme choline acetyltransferase, the signalling molecule acetylcholine, and the respective receptors (nicotinic or muscarinic) are expressed in epithelial cells (human airways, alimentary tract, epidermis). Acetylcholine is also found in mesothelial, endothelial, glial, and circulating blood cells (platelets, mononuclear cells), as well as in alveolar macrophages. The existence of non-neuronal acetylcholine explains the widespread expression of muscarinic and nicotinic receptors in cells not innervated by cholinergic neurons. Non-neuronal acetylcholine appears to be involved in the regulation of important cell functions, such as mitosis, trophic functions, automaticity, locomotion, ciliary activity, cell-cell contact, cytoskeleton, as well as barrier and immune functions. The most important tasks for the future will be to clarify the multiple biological roles of non-neuronal acetylcholine in detail and to identify pathological conditions in which this system is up- or down-regulated. This could provide the basis for the development of new therapeutic strategies to target the non-neuronal cholinergic system.

Maintenance of constant blood acetylcholine content before and after feeding in young chimpanzees

We have shown that acetylcholine (ACh) is present in the blood of various species of mammals using a specific, sensitive radioimmunoassay. In the present study, the effect on blood and plasma ACh levels of feeding after overnight fasting was studied in one male and five female 4- to 7-year-old chimpanzees. The mean basal ACh concentrations of the blood and plasma were 3143 +/- 380 and 184 +/- 10 pg/ml (+/-SEM, n = 6), respectively. Feeding each chimpanzee 500 g boiled sweet potatoes as breakfast at 1000 h and tap water given ad libitum did not affect the ACh content of the blood and plasma, and constant values of the blood and plasma ACh contents were observed for 4 h after the feeding. Hematocrit and plasma acetylcholinesterase (AChE) activity were also insensitive to feeding. No correlation was observed between plasma AChE activity and either blood or plasma ACh content. The results of the present study indicate that the blood ACh of chimpanzees is distributed mainly in the blood cell fraction, and that the blood ACh content is not regulated directly by cholinergic nerve activity or by plasma AChE activity.

Oral administration of KW-5092, a novel gastroprokinetic agent with acetylcholinesterase inhibitory and acetylcholine release enhancing activities, causes a dose-dependent increase in the blood acetylcholine content of beagle dogs

Acetylcholine (ACh) was detected in the blood and plasma of beagle dogs using a specific, sensitive radioimmunoassay. The mean basal ACh contents in the blood and plasma of beagle dogs were 451 +/- 65 and 83.5 +/- 12.3 pg/ml (+/- SEM, n = 7), respectively, and were lower than the contents in humans reported previously by our laboratory. Oral administration of KW-5092 (10-30 mg/kg), a gastroprokinetic agent with acetylcholinesterase (AChE) inhibitory and ACh release enhancing activities, caused a dose-dependent increase in the ACh content of both the blood and plasma, as well as several behavioral side effects due to peripheral cholinergic stimulation. The size of the increase in the plasma ACh content at each dose of KW-5092 was greater than that in the blood, indicating that KW-5092 caused the increase in the blood ACh content through elevation of the plasma ACh content, by inhibition of AChE and facilitation of ACh release. These results demonstrate that the blood ACh of beagle dogs is present mainly in the blood cells and to a lesser degree in the plasma, and that KW-5092 increased the blood ACh content mainly by increasing the plasma ACh concentration.

Localization and synthesis of acetylcholine in human leukemic T cell lines

In order to clarify the origin of acetylcholine (ACh) in human blood, we measured the content and synthesis activity of ACh in several human leukemic cell lines. The intracellular ACh content determined by a specific and sensitive radioimmunoassay in the human leukemic T cell lines, HSB-2, MOLT-3, and CEM, was 79.6, 36.2, and 9.5 pmol/10(6) cells, respectively. These values were 9-70-fold higher than those of other cell lines, including a helper T cell line, Jurkat. Stimulation of HSB-2 and MOLT-3 by phytohemagglutinin (PHA) increased both the intracellular content and release of ACh into the culture medium, but did not influence the intracellular content and release of ACh in CEM. ACh synthesis activity was found in all the T cell lines tested. Bromoacetylcholine (100 microM), a choline acetyltransferase (ChAT) inhibitor, and bromoacetyl-L-carnitine (100 microM), a carnitine acetyltransferase (CarAT) inhibitor, decreased ACh-synthesizing activity in MOLT-3, and HSB-2 and CEM, by about 50% and 30%, respectively, indicating that both ChAT, and to a lesser extent CarAt, are involved in ACh synthesis in T cells. These results suggest that T lymphocytes have the potential to synthesize and release ACh, which may play a role in regulating T cell-dependent immune responses.

Microvessels isolated from brain: localization of muscarinic sites by radioligand binding and immunofluorescent techniques

The present investigation was carried out to determine the extent to which muscarinic acetylcholine receptors (mAChRs) in vascular and perivascular structures were colocalized with glial fibrillary acidic protein (GFAP)-positive structures. To this aim, an immunocytochemical approach on free-floating cryosections and isolated microvessels obtained from rat brain was performed to study the possible colocalization of immunostaining with the anti-mAChR protein antibody (M35) and an anti-GFAP antibody. Double-labeling experiments were carried out by fluorescent techniques. Confocal microscopic observations of GFAP and M35 immunoreactivities on free-floating sections showed a high degree of colocalization on astrocyte processes associated with large vessels or capillaries. This pattern suggests that muscarinic receptors are associated with astrocytic endfeet. Confocal microscopic observations of immunoreactivity from isolated cerebral microvessels strengthen this conclusion since double-labeling of M35 and GFAP showed that perivascular astrocytic structures remained attached to the isolated microvessels and were present on vascular segments showing M35 immunoreactivity. In another set of experiments, the specific binding of [3H]quinuclidinylbenzylate ([3H]QNB) to isolated microvessel membrane preparations from cerebral cortex, caudate nucleus, thalamus, and cerebellum showed that a constant binding yield (20% in bovine and 40% in rat) was observed for microvessels compared with the corresponding brain region. According to our immunocytochemical results, the astrocytic membrane remaining attached to microvessels may account for the majority of the muscarinic binding to isolated microvessels. [3H]QNB binding values found in isolated microvessels cannot therefore be considered as artifacts without any link with vascular function. Taken together, the present study strengthens the idea that the muscarinic receptors may be implicated in the functional relationship between glial and vascular structures.

Vasodilation in porcine ophthalmic artery: peptide interaction with acetylcholine and endothelial dependence

Co-activation of cranial perivascular sensory and parasympathetic fibres in vivo induces simultaneous release of several vasodilatory substances with neurotransmitter or neuromodulatory roles. The role of the endothelium and possible interactions between such substances are poorly understood. The objective of this study was therefore to investigate these aspects with the sensory dilator calcitonin gene-related peptide (CGRP) and the parasympathetic dilators acetylcholine (ACh) and vasoactive intestinal peptide (VIP) in isolated porcine ophthalmic artery. Whilst ACh induced relatively rapid, endothelium-dependent dilation, CGRP and VIP induced slower dilations. Both CGRP and VIP were found to have partial endothelium-dependence in this artery. The simultaneous addition of ACh with CGRP potentiated the relaxation induced by CGRP, as has already been shown for substance P. ACh did not potentiate VIP relaxation, but the results generally indicate a potential role for ACh in initiating rapid dilation prior to strong, sustained relaxation by CGRP or VIP. The potential role of the endothelium and of substances like ACh or substance P in enhancing the rate of dilation of neuropeptides inducing strong and sustained relaxation is discussed.

Cholinergic and vasoactive intestinal polypeptidergic innervation of the cerebral arteries

Acetylcholine and vasoactive intestinal polypeptide are not only two vasoactive agonists that predominantly induce a vasodilatation of the cerebral arteries, but also correspond to neurotransmitters that innervate the various anatomical segments of the cerebral vasculature. The distinct patterns of the cerebrovascular cholinergic and vasoactive intestinal polypeptidergic innervation, their neurochemistry, in vitro and in vivo pharmacology, as well as the putative pathophysiological implications of these neurotransmission systems are critically summarized on the basis of the most recently published literature.

Architectonic subdivisions of the motor thalamus of owl monkeys: Nissl, acetylcholinesterase, and cytochrome oxidase patterns

As the first part of an investigation of the motor thalamus and its cortical connections in the owl monkey, a New World anthropoid primate, we studied thalamic architecture by using stains for Nissl, acetylcholinesterase (AChE), and cytochrome oxidase (CO), in order to identify subdivisions of the ventrolateral thalamic region as well as other nuclei with motor connections. Material was obtained from brains cut in the frontal, horizontal, and parasagittal planes. Our results indicate that the ventrolateral thalamic region (VL) of owl monkeys is a heterogeneous structure composed of several architectonic subdivisions that resemble divisions that have been described in macaques and other Old World anthropoids. All of these subdivisions are more readily distinguished in AChE than in Nissl or CO preparations. The anterior part of VL, VLa (VLo of Olszewski), is characterized by clusters of medium-sized, darkly stained neurons. VLa is also distinguished by AChE-positive cells embedded in a matrix of neurites as well as by a characteristic dark, irregular net of blood vessels. The posterior part of VL is rather uniform cytoarchitectonically and contains large, darkly stained, and sparsely distributed neurons. However, we were able to distinguish three subdivisions of posterior VL that closely correspond to structures described by Olszewski in macaques: a principal segment, VLp (VPLo of Olszewski), a medial segment, VLx ("area X" of Olszewski), and a dorsal segment, VLd (VLc and VLps of Olszewski). In AChE, VLd is much darker than the other divisions. The distinction between VLp and VLx, which together make up the largest part of VL, is less marked, although VLp is somewhat darker and more irregular in appearance in AChE than is VLx.

Nicotinic cholinergic receptors associated with mammalian cerebral vessels

Current evidence suggests that the cerebral vasculature may be modulated by cholinergic nerves. We used ligand binding methods to examine the presence of nicotinic cholinergic receptors in brain vasculature. We found carbachol-displaceable [3H]acetylcholine (ACh) and [3H]nicotine (NIC) binding sites in preparations of intraparenchymal cerebral microvessels (CMV) and larger pial vessels from human and pig brains. Specific binding sites for [3H]ACh and [3H]NIC in cerebral microvessels were saturable and comparable in density to those in cerebral cortex. The Kds for the two ligands ranged 3-18 nM whereas the Bmaxs were 25-45 fmol/mg protein. In contrast, the binding of [3H]pirenzipine or [3H]quinuclidinyl benzilate, index for muscarinic receptors, was low (9-15% of cortex) in microvessels compared to the cerebral cortex. Our observations suggest the association of cholinergic nicotinic receptors with cerebral microvessels, which may be involved in the modulation of the cerebral circulation by cholinergic neurons.

Phorbol ester stimulates acetylcholine synthesis in cultured endothelial cells isolated from porcine cerebral microvessels

Acetylcholine (ACh) is one of the factor which induces vasodilation through the release of endothelium-derived relaxing factor. The aim of this study was to clarify whether endothelial cells can synthesize ACh and the types of substance which regulate the synthesis of ACh in endothelial cells. We determined the ACh content of endothelial cells isolated from porcine cerebral microvessels and of the culture medium. ACh was detected in the medium after 12 h incubation in the presence of diisopropylfluorophosphate, a non-specific cholinesterase inhibitor, and increased linearly up to 24 h. Phorbol 12-myristate 13-acetate (PMA, 10(-7) M) increased the ACh content of the medium in a dose-dependent manner. The effect of PMA was most apparent between 12 and 24 h after treatment, and was inhibited by cycloheximide. Calphostin C, a specific inhibitor of protein kinase C (PKC), did not inhibit the effect of PMA. Dioctanoyl glycerol, a specific activator of PKC, did not increase the intracellular ACh content or the amount released into the culture medium. ACh synthesis was not inhibited by bromoacetylcholine, a specific inhibitor of choline acetyltransferase (ChAT). PMA treatment did not affect the specific activity of ACh synthesis in endothelial cells. These data show that endothelial cells are able to synthesize ACh, and that ACh synthesis is up-regulated by PMA through the PKC independent mechanism via protein induction. The enzyme which synthesizes ACh in endothelial cells is not ChAT. The increase in ACh synthesis induced by PMA may not be due to induction of the ACh synthetic enzyme.

The blood-brain barrier in vitro: the second decade

Ever since the discovery of Paul Ehrlich (1885 Das Sauerstoff-bedürfnis des Organismus: Hirschwald, Berlin) about the restricted material exchange, existing between the blood and the brain, the ultimate goal of subsequent studies has been mainly directed towards the elucidation of relative importance of different cellular compartments in the peculiar penetration barrier consisting the structural basis of the blood-brain barrier (BBB). It is now generally agreed that, in most vertebrates, the endothelial cells of the central nervous system (CNS) are responsible for the unique penetration barrier, which restricts the free passage of nutrients, hormones, immunologically relevant molecules and drugs to the brain. After an era of studying with endogenous or exogenous tracers the unique permeability properties of cerebral endothelial cells in vivo, the next generation, i.e. the in vitro blood-brain barrier model system was introduced in 1973. Recent advances in our knowledge of the BBB have in part been made by studying the properties and function of cerebral endothelial cells (CEC) with this in vitro approach. This review summarizes the results obtained on isolated brain microvessels in the second decade of its advent.

Carnitine acetyltransferase activity in the human brain and its microvessels is decreased in Alzheimer's disease

L-Carnitine and acetyl-L-carnitine facilitate mitochondrial beta-oxidation of fatty acids. In the brain, they may also have a role in acetylcholine synthesis. Carnitine acetyltransferase catalyzes the interchange between L-carnitine and acetyl-L-carnitine. Recently, acetyl-L-carnitine was reported to have a beneficial effect in patients with Alzheimer's disease. We therefore assessed carnitine acetyl-transferase activity in selected brain regions and in isolated cerebral microvessels obtained at autopsy from patients with Alzheimer's disease and from age-matched control subjects. We found a 25 to 40% decrease in carnitine acetyltransferase activity in patients with Alzheimer's disease, which attained statistical significance in most brain regions and in cerebral microvessels. These findings document another neurochemical abnormality in patients with Alzheimer's disease and provide a rationale for the use of acetyl-L-carnitine in the treatment of patients with Alzheimer's disease.

Glial cells in coculture can increase the acetylcholinesterase activity in human brain endothelial cells

The elements of the cholinergic system (acetylcholinesterase and choline acetyltransferase) and butyrylcholinesterase were studied in human cortical capillary samples, brain-derived endothelial cell cultures and glial cell cultures. It was shown that the elements of the cholinergic system are present in the microvessels, but the choline acetyltransferase activity may be due to contamination with cholinergic nerve terminals since no choline acetyltransferase could be demonstrated in endothelial cell cultures. The present results revealed that the activity of acetylcholinesterase is reduced in the cortical endothelial cell cultures after longer culture times, while butyrylcholinesterase activity is not altered. In a system where endothelial cells were cocultured with embryonic human brain astroglial cells for 12 days in vitro, the acetylcholinesterase activity was increased 2-fold. These results support a glial influence on the enzyme activity of the cerebral endothelium.

Periendothelial acetylcholine synthesis and release in bovine cerebral cortex capillaries

Choline acetyltransferase (ChAT) activity is present in isolated cerebral capillaries, where it has been considered to be a marker for perivascular cholinergic nerve terminals. However, ChAT-like immunoreactivity has been visualized in endothelial cells. This finding raised the possibility that at least part of the biochemically detected ChAT has a nonneuronal origin. To evaluate the relative contribution of endothelial cells and nerve fibers to the total acetylcholine (ACh)-synthesizing capacity of cerebral capillaries, ChAT activity and ACh release were measured in capillaries and in purified endothelial cells isolated from bovine cerebral cortex. Isolated capillaries showed ChAT activity, which was inhibited by 2-benzoylethyl trimethylammonium to the same extent as cerebral ChAT. When preincubated with [3H]choline, these capillaries presented a calcium-dependent enhancement in tritium release upon electrical field stimulation. Purified endothelial cells had minor ChAT activity and lacked the ability to release tritium in response to electrical stimulation, although the endothelial markers alkaline phosphatase, gamma-glutamyltranspeptidase, and 1,1'-dioctadecyl-1,3,3',3'-tetramethyl-iodocarbocyanide perchlorate-labeled acetylated low-density lipoprotein uptake were fully preserved. These data indicate that, within isolated cerebral capillaries, ACh is synthesized and released by a periendothelial structure. The fact that ACh release is provoked by electrical stimulation and by a calcium-dependent mechanism strongly suggests that cerebrovascular ACh has a neuronal origin.

Endothelial cells from human fetal brain microvessels may be cholinoceptive, but do not synthesize acetylcholine

Brain homogenate, cerebral microvessels, and endothelial cells (ECs) were prepared from 15-18-week-old human fetuses and analyzed biochemically for the presence of elements of the cholinergic system [acetylcholinesterase (AChE), choline acetyltransferase (ChAT), and butyrylcholinesterase]. The ECs were cultured, and their purity was checked by light microscopic immunohistochemistry with the application of anti-human factor VIII and glial fibrillary acidic protein. The highest activity of ChAT was found in the brain homogenate and the lowest in the microvessel fraction. No ChAT activity could be detected in the cultured ECs, despite the presence of high AChE activity. It is suggested that human brain ECs may be under the control of acetylcholine released from cholinergic nerve terminals but that the cells do not produce the transmitter itself. In coculture experiments, when ECs were plated on the upper surface of a polycarbonate filter and glial cells were seeded on the lower surface, the electric resistance was measured. During the culture period, the resistance first increased up to 5 days in vitro (297 +/- 17 ohm.cm2) but later gradually declined. These results demonstrate that human ECs cocultured with glial cells provide a useful model for study of the function of the blood-brain barrier in vitro.

Effect of acetylcholine on the electrical and secretory activities of frog pituitary melanotrophs

The activity of melanotroph cells of the amphibian pars intermedia is regulated by multiple factors including classical neurotransmitters and neuropeptides. In this study, we have examined the possible involvement of acetylcholine (ACh) in the regulation of electrical and secretory activities of frog pituitary melanotrophs. Electrophysiological recordings were conducted on cultured cells by using the patch-clamp technique in the whole-cell configuration. In parallel, alpha-MSH release from acutely dispersed pars intermedia cells was studied by means of the perifusion technique. In all cells tested in the current-clamp mode, superfusion with ACh (10(-6) M) gave rise to a depolarization associated with an enhanced frequency of action potentials. Administration of ACh (10(-6) M) to perifused cells also induced stimulation of alpha-MSH release. These results indicate that the neurotransmitter ACh exerts a direct stimulatory effect on pituitary melanotrophs. The action of ACh on electrical and secretory activities was mimicked by muscarine (10(-5) M), while ACh-induced alpha-MSH secretion was completely abolished by the muscarinic antagonist atropine (10(-6) M). The depolarizing effect of muscarine was suppressed by the specific M1 muscarinic antagonist pirenzepine (10(-5) M), indicating the existence of a M1 subtype muscarinic receptor in frog pars intermedia cells. In addition, using a monoclonal antibody against calf muscarinic receptors, we have visualized, by the immunofluorescence technique, the presence of muscarinic receptor-like immunoreactivity in cultured intermediate lobe cells. Electrophysiological recordings showed that nicotine (10(-5) M) induces membrane depolarization associated with an increase of the frequency of action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of endothelial L-arginine pathway in resistance arteries. Effect of age and hypertension

In conduit arteries, nitric oxide is formed from L-arginine in the endothelium and released after stimulation with acetylcholine. The contribution of the L-arginine pathway and the effects of age and hypertension on endothelium-dependent vascular regulation were studied, using a video dimension analyzer, in pressurized and perfused mesenteric resistance arteries of 8- and 16-20-week-old Wistar-Kyoto and spontaneously hypertensive rats. Norepinephrine and phenylephrine caused contractions, which were similarly augmented after removal of the endothelium. NG-Monomethyl-L-arginine, an inhibitor of nitric oxide formation, augmented the contraction, but less than endothelial removal. Acetylcholine caused endothelium-dependent relaxations that were much more pronounced with intraluminal than with extraluminal application. NG-Monomethyl-L-arginine, methylene blue, and hemoglobin only partially inhibited the response. With aging, the endothelium-dependent inhibition of the response to norepinephrine decreased in Wistar-Kyoto rats; in spontaneously hypertensive rats this inhibition was smaller as compared with age-matched Wistar-Kyoto rats. In Wistar-Kyoto rats, the difference between intraluminal and extraluminal activation became more pronounced in adult rats. In the adult but not the young spontaneously hypertensive rats, the response to intraluminal but not extraluminal acetylcholine was reduced as compared with Wistar-Kyoto rats. Thus, in mesenteric resistance arteries of the rat, nitric oxide is released from L-arginine under basal conditions and after stimulation with acetylcholine but only in part accounts for endothelium-dependent responses. With aging and hypertension, the inhibitory effects of the endothelium against norepinephrine-induced contractions decrease. In hypertension, the intraluminal but not extraluminal activation of the release of endothelium-derived relaxing factors is impaired.

Small pial vessels, but not choroid plexus, exhibit specific biochemical correlates of functional cholinergic innervation

In an attempt to provide the biochemical foundations for a putative cholinergic innervation of small pial vessels and choroid plexus, we have assessed their ability to specifically accumulate choline, synthesize and release acetylcholine (ACh) in response to depolarization. Our results show that both small pial vessels and choroid plexus avidly accumulate choline via a sodium-dependent mechanism which could be inhibited by hemicholinium-3 (IC50 in pial vessels = 47.8 microM). Light microscopic examination of radioautographs from vessels incubated with [3H]choline revealed two distinct sites of accumulation in the vessel wall. One site probably corresponded to nerve terminals and the other was closely associated with the endothelial cells. In small pial vessels, a major proportion (60%-70%) of the choline acetyltransferase (ChAT) activity could be inhibited by 4-naphthylvinylpyridine (4-NVP), a potent inhibitor of neuronal ChAT; and, following either K+ or veratridine depolarization, a Ca2(+)-dependent release of authentic [3H]ACh could be measured. In contrast, the choroid plexus exhibited a rather low ChAT activity which was not inhibited by 4-NVP and no release of ACh could be detected in this tissue following depolarization. Altogether, the results of the present study show that (1) small pial vessels exhibit all the most selective biochemical markers that are characteristic of cholinergic nerves; (2) [3H]choline in pial vessels can be accumulated in non-neuronal elements which probably correspond to the endothelial cells; and (3) the choroid plexus failed to exhibit convincing biochemical markers that would attest in favor of a functional cholinergic innervation.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular cholinesterases and choline uptake in isolated rat forebrain microvessels: a possible link

The two parameters of the active [methyl-3H]choline uptake into isolated rat forebrain microvessels, Km and Vmax, were determined for 1-, 3-, 10-, and 24-month-old Charles River male rats and compared with the activities of the enzymes choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and butyrylcholinesterase (BuChE) in these microvessels over the same time course. The value of Km remained constant over the entire period, but that of Vmax increased from 8.5 +/- 1.0 to 80.6 +/- 16.4 nmol g-1 (mean +/- SEM) over the first 3 months of life. Over the same period, the increase in ChAT activity, from an initial value of 7.1 +/- 1.6 to 10.2 +/- 0.3 nmol g-1 min-1, was not proportional to that of choline uptake. Levels of BuChE activity (0.9-1.3 mumol g-1 min-1) were almost unchanged throughout the entire 24-month period, but those of AChE showed a steady and significant increase from 1 to 24 months, remaining relatively high at senescence (4.7 mumol g-1 min-1), when choline uptake had decreased to one-third of its optimal value. Selective inhibition of AChE with 1,5-bis(4-allyldimethylammonium-phenyl)pentan-3-one dibromide (0.5 microM) in unruptured capillaries from 3-month-old rats resulted in a decrease in Vmax of choline uptake from approximately 81 to 59 nmol g-1 min-1 or with 9-amino-1,2,3,4-tetrahydroacridine (10 microM) in capillaries from 2-month-old rats from approximately 30 to 15 nmol g-1 min-1. Selective inhibition of BuChE with tetraisopropyl pyrophosphoramide (100 microM) resulted in an increase in Vmax from approximately 81 to 96 nmol g-1 min-1. It is possible that the two vascular enzyme systems are coupled to a hypothetical endothelial choline transporter, but with an action opposite to each other.

Kinetics of choline uptake into isolated rat forebrain microvessels: evidence of endocrine modulation

The active uptake of [methyl-3H]choline into isolated rat brain microvessel suspension was studied as a likely guide to the transport of choline across the blood-brain barrier. The method consisted primarily of incubation of the suspension with a fixed concentration of labeled choline in the presence of increasing concentrations of unlabeled choline or any other inhibitor (I) of active uptake, defined as the difference in uptake at 37 degrees and 0 degrees C. From the linear regression of (1/V) against [I], the following values of Vmax (nmol g-1 min-1) and Km (microM) were obtained for choline: 2-month-old males, 10.6 +/- 3.8 and 6.1 +/- 0.9; 3-month old random females, 28.4 +/- 5.9 and 12.6 +/- 4.0; females at metaestrus, 17.8 +/- 10.3 and 8.3 +/- 5.0; at diestrus, 31.1 +/- 9.3 and 13.0 +/- 2.6; at proestrus, 54.9 +/- 2.2 and 14.0 +/- 1.5; at estrus, 19.2 +/- 2.2 and 2.6 +/- 1.7. The differences between males and random females (p less than 0.018) and between females at proestrus and estrus (p less than 0.005) are significant. It is suggested that these inter- and intrasex variations in choline uptake reflect a dynamic adjustment of supply in accordance with brain demand for choline at the time of assay. Hemicholinium-3 was an effective inhibitor of choline uptake, Ki = 14.0 +/- 8.5 microM; dimethylaminoethanol was much less effective; and imipramine had no measurable effect.

The role of basal forebrain in the primary cholinergic vasodilation in rat neocortex produced by systemic administration of cismethrin

The pyrethroid insecticide cismethrin (9 mumol/kg) causes a large blood flow increase in cerebral cortex, without a parallel increase in metabolism. A unilateral lesion of the basal forebrain attenuated the blood flow increase in the cortex ipsilateral to the lesion but augmented that in the contralateral cortex. Cortical choline acetyltransferase was similarly affected. Atropine sulphate substantially reduced the flow increase and was additive to the lesion effects. Systemic cismethrin is thus capable of activating a cholinergic vasodilation in the cortex and, in the parietal cortex at least, a substantial proportion of the flow increase is mediated by extrinsic projections from the basal forebrain.

Choline acetyltransferase activity in the rat brain cortex homogenate, synaptosomes, and capillaries after lesioning the nucleus basalis magnocellularis

Stereotaxic lesions of the nucleus basalis magnocellularis were made unilaterally in male Wistar rats with either kainic or ibotenic acid, using the contralateral side as control. Differences in behavior, body weight, and survival were observed between the kainic and ibotenic acid-treated rats. One week after surgery, the rats were sacrificed and the effect of the lesions on choline acetyltransferase activity was measured in brain cortex homogenate, synaptosomes, and capillaries. In kainic acid-lesioned rats, choline acetyltransferase activity decreased in homogenate and synaptosomes of the ipsilateral side with respect to that of the contralateral side; but the ibotenic acid lesion, which also reduced the ipsilateral choline acetyltransferase activity in homogenate, showed a rather different effect on the enzymatic activity of the synaptosomes. There were also differences between the effect of kainic and ibotenic acid lesions on choline acetyltransferase activity in the capillaries of the ipsilateral side with respect to that of the contralateral one. However, capillary choline acetyltransferase activity of the treated rats was in both sides three times higher than that of unoperated rats.