Action Of ACh Research Articles

Muscarinic receptor M1and phosphodiesterase 1 are key determinants in pulmonary vascular dysfunction following perinatal hypoxia in mice

Perinatal adverse events such as limitation of nutrients or oxygen supply are associated with the occurrence of diseases in adulthood, like cardiovascular diseases and diabetes. We investigated the long-term effects of perinatal hypoxia on the lung circulation, with particular attention to the nitric oxide (NO)/cGMP pathway. Mice were placed under hypoxia in utero 5 days before delivery and for 5 days after birth. Pups were then bred in normoxia until adulthood. Adults born in hypoxia displayed an altered regulation of pulmonary vascular tone with higher right ventricular pressure in normoxia and increased sensitivity to acute hypoxia compared with controls. Perinatal hypoxia dramatically decreased endothelium-dependent relaxation induced by ACh in adult pulmonary arteries (PAs) but did not influence NO-mediated endothelium-independent relaxation. The M(3) muscarinic receptor was implicated in the relaxing action of ACh and M(1) muscarinic receptor (M(1)AChR) in its vasoconstrictive effects. Pirenzepine or telenzepine, two preferential inhibitors of M(1)AChR, abolished the adverse effects of perinatal hypoxia on ACh-induced relaxation. M(1)AChR mRNA expression was increased in lungs and PAs of mice born in hypoxia. The phosphodiesterase 1 (PDE1) inhibitor vinpocetine also reversed the decrease in ACh-induced relaxation following perinatal hypoxia, suggesting that M(1)AChR-mediated alteration of ACh-induced relaxation is due to the activation of calcium-dependent PDE1. Therefore, perinatal hypoxia leads to an altered pulmonary circulation in adulthood with vascular dysfunction characterized by impaired endothelium-dependent relaxation and M(1)AChR plays a predominant role. This raises the possibility that muscarinic receptors could be key determinants in pulmonary vascular diseases in relation to "perinatal imprinting."

Restoring balance at the neuromuscular junction

Argov and colleagues present the results of a Phase 1b trial of an oral antisense oligonucleotide, EN101 (Monarsen, Ester Neurosciences Ltd.), for treatment of myasthenia gravis (MG) in this issue of Neurology ®.1 The brief report and the authors' humble description of their study belie the remarkable story and potential for broad impact of their work. Why do I make this statement? 1) The science behind EN101 is fascinating and was rapidly translated to the clinic. 2) Of great surprise is that the agent is efficacious when given orally. 3) Human trials of antisense oligonucleotide technology have been disappointing, but this trial suggests a therapeutic benefit, which should bolster hope for this approach for drug development. 4) The work has broad implications beyond MG. It has long been appreciated that the AChE-synaptic (AChE-S) isoform is critical in terminating the action of ACh to prevent repeated activation of the acetylcholine receptor (AChR) (see figure). Supporting the importance of this critical function is the observation that mutations of the AChE-S gene lead to cholinesterase deficiency and a congenital myasthenic syndrome. The AChE-S is distributed through the primary and secondary synaptic clefts of …

Muscarinic potassium channels augment dynamic and static heart rate responses to vagal stimulation

Vagal control of heart rate (HR) is mediated by direct and indirect actions of ACh. Direct action of ACh activates the muscarinic K(+) (K(ACh)) channels, whereas indirect action inhibits adenylyl cyclase. The role of the K(ACh) channels in the overall picture of vagal HR control remains to be elucidated. We examined the role of the K(ACh) channels in the transfer characteristics of the HR response to vagal stimulation. In nine anesthetized sinoaortic-denerved and vagotomized rabbits, the vagal nerve was stimulated with a binary white-noise signal (0-10 Hz) for examination of the dynamic characteristic and in a step-wise manner (5, 10, 15, and 20 Hz/min) for examination of the static characteristic. The dynamic transfer function from vagal stimulation to HR approximated a first-order, low-pass filter with a lag time. Tertiapin, a selective K(ACh) channel blocker (30 nmol/kg iv), significantly decreased the dynamic gain from 5.0 +/- 1.2 to 2.0 +/- 0.6 (mean +/- SD) beats.min(-1).Hz(-1) (P < 0.01) and the corner frequency from 0.25 +/- 0.03 to 0.06 +/- 0.01 Hz (P < 0.01) without changing the lag time (0.37 +/- 0.04 vs. 0.39 +/- 0.05 s). Moreover, tertiapin significantly attenuated the vagal stimulation-induced HR decrease by 46 +/- 21, 58 +/- 18, 65 +/- 15, and 68 +/- 11% at stimulus frequencies of 5, 10, 15, and 20 Hz, respectively. We conclude that K(ACh) channels contribute to a rapid HR change and to a larger decrease in the steady-state HR in response to more potent tonic vagal stimulation.

Avaliação morfológica e dos mecanismos de mobilização de Ca2+ pela glicose e acetilcolina em células pancreáticas humanas

The proposal of this study was to analyze morphology of the organelles and cytoskeleton in human pancreatic cells cultured and the mobilization of the cytosolic calcium ([Ca2+]c) in response to glucose and ACh by fluorimetry method. The cells were plated on glass coverslips, fixed and stained with a combination of fluorophores: the nuclei were stained with DAPI and mitochondria with Mytotracker Red. It was used phalloidin and the secondary antibodies Alexa Fluor conjugated green and red-fluorescent (488 and 594) to identify the protein cell actin F and type M3 muscarinic receptor respectively. The cells also were loaded with fura-2/AM to study Ca2+ mobilization. The human pancreatic cells show characteristics morphologically preserved with great amount of mitochondria. In region major cell density was evidenced pseudo-islets and type M3 muscarinic receptors. Through increase of [Ca2+]c due to action of glucose and ACh were shown that the cells capacity to respond to these stimuli were conserved. The elevation of the [Ca2+]c depended on concentration by glucose-induced promoting sustained phase and ACh-induced a biphasic response. The morphologic characteristics of human pancreatic cells cultured were preserved. The Ca2+ mobilization in response to glucose and ACh confirmed its functionality. The expression of the M3 muscarinic receptors in human pancreatic cell cultured was demonstrated.

P2-063: Effects of Abeta1-42 mutants linked to familial Alzheimer’s disease on recombinant human neuronal nicotinic acetylcholine receptors (nAChRs)

It is thought that Aβ1–42 is the responsible agent for Alzhiemer disease progression, as its accumulation is toxic (Selkoe, 2001). However, the underlying mechanisms remain to be resolved. The inherited form of the disease, familial AD, accounts for only 10% of AD cases but has an earlier onset. In a subset of familial AD cases (Swedish, Dutch and Flemish families), genetic mutations in the amyloid precursor peptide (APP) lead to the over production of Aβ1–42 and result in disease pathology. Neurodegeneration and a loss of nicotinic acetylcholine receptors (nAChRs) are features of both forms of AD. It has been reported that Aβ1–42 binds with high–affinity to α7 and with low–affinity to α4β2 nAChRs and partially blocks the action of ACh on α7 nAChRs (Grassi et al., 2003; Pettit et al., 2001; Wang et al., 2000). The nature of the link between amyloid peptides, nAChRs and AD pathology is yet to be established. Determine the actions of Dutch, Swedish and Flemish variants of Aβ on recombinant nicotinic acetylcholine receptors. We used two–electrode voltage–clamp to study the effects of the Swedish, Dutch and Flemish mutant forms of Aβ1–42 (10 nM) on the ACh–evoked responses of recombinant human α7, α4β2 and α3β4 nAChRs expressed in Xenopus oocytes. We found that the Swedish peptide acted as a non–competitive antagonist of human α7 nAChRs and an allosteric enhancer of ACh–evoked currents mediated by α4β2 nAChRs, but had no effect on α3β4 nAChRs. These actions are identical to the actions of wild type Aβ1–42 on these human neuronal nicotinic receptors (Pym et al., 2005). In contrast to this, both the Flemish and Dutch mutant forms of Aβ1–42 caused an enhancement of ACh–evoked currents in all neuronal nicotinic receptors tested. This is the first demonstration that these mutations in Aβ have specific actions on nAChRs. These results may have implications for the subtle differences in the pathology of familial AD when compared to sporadic AD and highlights the sensitivity of nAChR subtypes to variants of Aβ1–42.

Role of Toraco-Lumbo (T12l2)-Sacral (S2S4) Centers from Puberty to Aging

In the context of the structures concerning the neurophysiology, the cardiovascular system of it may be important to revalue the role of the Toraco-Lumbo (T12-L2 = sympathetic efferent section)-Sacral (S2-S4 = vagal afferent section) centers with integrative nitrergic system, the vascular peptides, in the field of an active microcirculation vasomotor mechanism from puberty to aging. These are the only centres of the spinal cords to have a prominent neurovasodilatory action through the beta-stimulation and alpha-blockade starting with Ach action, but also through the vagal termination S2-S4 where neurotransmitting vasodilator reflex NO-cgmp- dependent is produced. Nitric-oxyde participates also in regularizing the control on peripheral tone in man. The synthesis of NO occurs in cardiac-sympathetic and vagal ganglia. The same can be verified during puberty in the Toraco-Lumbo-Sacral ganglia, therefore the vasal vascular tone becomes dynamic. Different factors published produce a bettering of microcirculation vasomotility. Therefore one should consider the structural parallelism explaining the relationship between the Toraco-Lumbo and Sacral selective system, the sexual circuit with the NO-vasodilator system prevailing in the young-adult period, while in older ages the vasoconstrictor endothelins-1 system prevails.

Estradiol-induced expression of N(+)-K(+)-ATPase catalytic isoforms in rat arteries: gender differences in activity mediated by nitric oxide donors.

We tested the hypothesis that previously demonstrated gender differences in ACh-induced vascular relaxation could involve diverse Na(+)-K(+)-ATPase functions. We determined Na(+)-K(+)-ATPase by measuring arterial ouabain-sensitive 86Rb uptake in response to ACh. We found a significant increase of Na+ pump activity only in aortic rings from female rats (control 206 +/- 11 vs. 367 +/- 29 nmol 86Rb/K.min(-1).g wt tissue(-1); P < 0.01). Ovariectomy eliminated sex differences in Na(+)-K(+)-ATPase function, and chronic in vivo hormone replacement with 17beta-estradiol restored the ACh effect on Na(+)-K(+)-ATPase. Because ACh acts by enhancing production of NO, we examined whether the NO donor sodium nitroprusside (SNP) mimics the action of ACh on Na(+)-K(+)-ATPase activity. SNP increased ouabain-sensitive 86Rb uptake in denuded female arteries (control 123 +/- 7 vs. 197 +/- 12 nmol 86Rb/K.min(-1).g wt tissue(-1); P < 0.05). Methylene blue (an inhibitor of guanylate cyclase) and KT-5823 (a cGMP-dependent kinase inhibitor) blocked the stimulatory action of SNP. Exposure of female thoracic aorta to the Na+/K+ pump inhibitor ouabain significantly decreased SNP-induced and ACh-mediated relaxation of aortic rings. At the molecular level, Western blot analysis of arterial tissue revealed significant gender differences in the relative abundance of catalytic isoforms of Na(+)-K(+)-ATPase. Female-derived aortas exhibited a greater proportion of alpha2-isoform (44%) compared with male-derived aortas. Furthermore, estradiol upregulated the expression of alpha2 mRNA in male arterial explants. Our results demonstrate that enhancement of ACh-induced relaxation observed in female rats may be in part explained by 1) NO-dependent increased Na(+)-K(+)-ATPase activity in female vascular tissue and 2) greater abundance of Na(+)-K(+)-ATPase alpha2-isoform in females.

Effects of ACh and adenosine mediated by Kir3.1 and Kir3.4 on ferret ventricular cells

The inotropic effects of ACh and adenosine on ferret ventricular cells were investigated with the action potential-clamp technique. Under current clamp, both agonists resulted in action potential shortening and a decrease in contraction. Under action potential clamp, both agonists failed to decrease contraction substantially. In the absence of agonist, application of the short action potential waveform (recorded previously in the presence of agonist) also resulted in a decrease in contraction. Under action potential clamp, application of ACh resulted in a Ba(2+)-sensitive outward current with the characteristics of muscarinic K+ current (I(K,ACh)); the presence of the muscarinic K+ channel was confirmed by PCR and immunocytochemistry. In the absence of agonist, on application of the short ACh action potential waveform, the decrease in contraction was accompanied by loss of the inward Na(+)/Ca(2+) exchange current (I(NaCa)). ACh also inhibited the background inward K+ current (I(K,1)). It is concluded that ACh activates I(K,ACh), inhibits I(K,1), and indirectly inhibits I(NaCa); this results in action potential shortening, decrease in contraction, and, as a result of the inhibition of I(K,1), minimum decrease in excitability.

Recovery of deficient cholinergic calcium signaling by adenosine in cultured rat cortical astrocytes.

The regulation of the cholinergic calcium signaling in astroglial cells is thought to play a crucial role in the pathogenesis of Alzheimer's disease. We investigated the action of the cell modulator adenosine on acetylcholine (Ach)-mediated intracellular calcium ([Ca(2+)](i)) transients in cultured rat cortical astrocytes using the Ca(2+) imaging technique. The stable adenosine analog 2-chloroadenosine (2ClA) potentiated the [Ca(2+)](i) rise induced by activation of muscarinic Ach receptors by shifting approximately 30-fold the half-effective Ach concentration. This 2ClA effect was maintained upon removal of extracellular Ca(2+), indicating that Ach-induced [Ca(2+)](i) elevation was due mainly to Ca(2+) mobilization from intracellular stores. Pharmacological studies demonstrated that the 2ClA action was mediated by A1 receptors. Incubation with pertussis toxin abrogated the 2ClA effect but left unchanged the [Ca(2+)](i) rise produced by Ach alone. The [Ca(2+)](i) response elicited by Ach alone was abolished upon blockade of muscarinic receptor subtypes that stimulate phospholipase C, whereas the [Ca(2+)](i) elevation generated by the combined action of subthreshold Ach and 2ClA was not affected. Collectively, these results suggest that the impaired cholinergic signaling, the cardinal symptom of Alzheimer's disease, can be reinforced at the second messenger level by an alternative intracellular Ca(2+) mobilizing path, which can be brought into play by the concomitant activation of A1 purinoceptors and muscarinic receptors negatively coupled to adenylyl cyclase.

Influence of experimental diabetes on regulatory mechanisms of vascular response of rabbit carotid artery to acetylcholine

The purpose of this study was to analyse the influence of experimental diabetes on vascular response of rabbit carotid artery to acetylcholine (Ach). We compared the Ach-induced relaxant response of isolated arterial segments obtained from both control and diabetic animals. To assess the influence of the endothelium, this cell layer was mechanically removed in some of the arterial segments ("rubbed arteries") from each experimental group. Ach induced a concentration-related endothelium-mediated relaxation of carotid artery from control rabbits that was significantly higher with respect to that obtained in diabetic animals. Pre-treatment with N(G)-nitro-L-arginine (L-NA) induced a concentration-dependent inhibition of relaxant response to Ach, which was significantly higher in carotid arteries isolated from diabetic rabbits. Incubation of rubbed arteries with L-NA almost abolished the relaxant response to Ach in arterial segments from both control and diabetic animals. Indomethacin potentiated Ach-induced response of carotid arteries from control rabbits, without modifying that obtained in those from diabetic animals. Aminoguanidine did not significantly inhibit the relaxant action of Ach in arterial segments from either control or diabetic rabbits. These results suggest that diabetes impairs endothelial modulatory mechanisms of vascular response of rabbit carotid artery to Ach. This endothelial dysfunction is neither related with a lower release of nitric oxide (NO) or prostacyclin. Diabetes impairs the production of some arachidonic acid vasoconstrictor derivative. There has been observed an increased modulatory activity of NO, but this is not related with the expression of an inducible isoform of NO synthase.

Oita International Electrocardiology Symposium 2000 “Electrophysiology and Management of Lethal Arrhythmias in the New Millennium: From Genes to Bedside”

Recent clinical studies show that pulmonary veins (PVs) are important foci of atrial tachyarrhythmias. Autonomic nervous system was known to play a role in the genesis of atrial tachyarrhythmias. The present experiments aimed to investigate the actions of autonomic agents in dog PVs and to explore the arrhythmogenic activity in these tissues. Transmembrane action potentials were recorded with the conventional microelectrode techniques and tension was detected by a transducer. In PV driven electrically, isoproterenol (Isop, 1μM) increased while ACh (1-10μM) depressed twitch force but elevated resting tension. In PV active spontaneously, Isop increased the automatic rate and induced high frequency irregular rhythms while ACh suppressed them. The actions of Isop and ACh were antagonized by propranolol and atropine, respectively. Quite often a single microelectrode impalement recorded 2 and more types of action potentials with wide range of durations (APD), indicating presence of multiple cells with different electrophysiological properties and electrical uncoupling of neighboring cells in PV strips. Activation of β-adrenoceptor stimulated while cholinergic agonist suppressed spontaneous action potentials or high frequency irregular rhythms in PV.

Inhibitory cholinergic effects on the autonomously contractile bulbus cordis branchialis of the cephalopod Sepia officinalis L.

In vitro experiments were performed on a standardized preparation of the autonomously contractile bulbus cordis branchialis of the branchial heart of Sepia officinalis to investigate its cholinergic neuroregulation. Apart from acetylcholine, nicotine and carbachol (nicotinic agonists), the muscarinic agonists muscarine, arecoline, pilocarpine, and oxotremorine also exerted concentration-dependent negative inotropic effects on the preparations. As both the muscarinic antagonist quinuclidinylbenzilate and the nicotinic antagonist α-bungarotoxin blocked the ACh action there might be a special, possibly mixed muscarinic/nicotinic ACh-receptor system in the myocytes of the bulbus cordis branchialis, which is different from the cholinergic receptor in the central part of the branchial heart.

Inhibitory interneurons in hippocampus.

In the hippocampus and DG, a small number of morphologically and physiologically diverse interneurons controls the neuronal activity of large numbers of the principal excitatory output cells. The inhibitory interneurons are themselves regulated by glutamatergic and GABA-ergic intrinsic hippocampus afferents, as well as by extrinsic afferents, including cholinergic and serotonergic projections from the basal forebrain and the brainstem, respectively. In addition to the slow modulatory effects of the neurotransmitters released from these extrinsic pathways (11), recent evidence has revealed rapid effects of ACh and 5-HT mediated by ligand-gated ion channel receptors for these neurotransmitters. The direct, rapid excitatory action of ACh and 5-HT on hippocampus interneurons can explain many of the effects of these neurotransmitters on neuronal activity in the hippocampus circuit. Because the hippocampus receives both serotonergic and cholinergic innervation, there is strong potential for fast cholinergic and serotonergic synaptic transmission between these fibers and hippocampus interneurons, such as has been reported in other brain regions (e.g., visual cortex) (36). Moreover, these receptors may play important roles in the cognitive functions of the hippocampus, and show impaired function in certain neurological disorders, such as neurodegeneration.

NO modulates autonomic effects on sinus discharge rate and AV nodal conduction in open-chest dogs.

The purpose of this study was to investigate the role of nitric oxide (NO) in mediating vagal and sympathetic modulation of spontaneous sinus cycle length (SCL) and atrioventricular (AV) nodal conduction time (A-H interval) in 62 open-chest mongrel dogs anesthetized with alpha-chloralose. Infusion of an NO synthase (NOS) inhibitor, NG-monomethyl-L-arginine (L-NMMA, 4 mg/ ml), into the sinus and AV nodal arteries attenuated significantly (P < 0.01) the negative chronotropic and dromotropic responses to vagal nerve stimulation (VS) and VS during ansae subclaviae stimulation (SS) or isoproterenol (Iso) infusion. Intravenous administration of L-arginine (100 mg/kg) reversed these responses toward control values, whereas D-arginine did not have a significant effect. L-NMMA significantly (P < 0.01) enhanced the effects of SS and Iso on SCL and A-H interval; L-arginine reversed these changes toward baseline. L-NMMA increased the minimum concentration of ACh needed to induce 50 or 100% prolongation of SCL or second-degree or complete AV block during concomitant Iso infusion. L-Arginine reversed these effects. NOS inhibition did not affect the direct cholinergic actions of ACh on SCL and A-H interval but enhanced adrenergic positive chronotropic and dromotropic effects. We conclude that NO plays a stimulatory role in mediating vagal neurotransmission and vagal modulation of sympathetic effects and an inhibitory role in mediating sympathetic neurotransmission.

Insulin resistance caused by hepatic cholinergic interruption and reversed by acetylcholine administration.

The role of hepatic parasympathetic nerves in insulin effectiveness was evaluated in fed anesthetized rats. Insulin sensitivity was assessed using a modified euglycemic clamp to quantitate the amount of glucose required to maintain euglycemia over 60 min after administration of insulin (50 mU/kg). With normal innervation, intraportal infusions of acetylcholine (ACh, 2.5 micrograms.kg-1.min-1) did not alter insulin sensitivity, but intraportal venous atropine (3 mg/kg) reduced insulin sensitivity (49.4 +/- 5.8%, P < 0.001, n = 7). Liver denervation resulted in reduction of insulin responsiveness by 70.8 +/- 5.8% (P < 0.001, n = 8), which was fully (96.8 +/- 12.5%) reversed by ACh. ACh into the portal vein reversed insulin resistance produced by denervation, but intravenous ACh was without effect, thus showing that the liver was the site of ACh action. Regression analysis suggests that some component of insulin response is not dependent on hepatic cholinergic nerve effects but that virtually all of the variability in response to insulin in normal fed rats tested under these conditions could be accounted for by variability in the hepatic parasympathetic-dependent insulin sensitivity. These data suggest a major role for hepatic parasympathetic nerves in regulation of whole body clearance of glucose in response to insulin.

Regional differences in the negative inotropic effect of acetylcholine within the canine ventricle.

1. Regional differences in the effects of ACh on sub-epicardial, mid-wall and sub-endocardial cells of the dog left ventricle have been studied. 2. ACh produced a dose-dependent, atropine-sensitive negative inotropic effect that was greatest in sub-epicardial cells and small or absent in sub-endocardial cells. 3. In sub-epicardial (but not sub-endocardial) cells, ACh also resulted in a dose-dependent decrease in action potential duration. The inotropic effect of ACh on sub-epicardial cells was primarily the result of the decrease of action potential duration, because during trains of voltage clamp pulses the inotropic effect of ACh was reduced or abolished. At a holding potential of -80 mV, 10(-5)M ACh decreased L-type Ca2+ current by approximately 8% and this is thought to be responsible for the small inotropic effect during trains of pulses. 4. Although 4-AP, a blocker of the transient outward current (I(to)), abolished the "spike and dome' morphology of the sub-epicardial action potential, it had little or no effect on the actions of ACh on sub-epicardial cells. ACh had no effect on I(to) in sub-epicardial cells in voltage clamp experiments. 5. ACh activated a Ba(2+)-sensitive outward current (IK,ACh) in sub-epicardial cells, but little or no such current in sub-endocardial cells. In sub-epicardial cells, ACh also inhibited the inward rectifier current, IK,1. 6. It is concluded that in left ventricular sub-epicardial cells, ACh activates IK,ACh. This results in a shortening of the action potential and, therefore, a negative inotropic effect. In subendocardial cells, ACh activates little or no IK,ACh and, therefore, it has little or no negative inotropic effect. This may result from a regional variation in the expression of the muscarinic K+ channel.

Muscarinic antagonists are anxiogenic in rats tested in the black-white box

Central cholinergic (ACh) projections have been shown to modulate stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis and are integral to the expression of electrophysiological correlates of arousal, namely hippocampal theta rhythm. The degree to which these actions of ACh are behaviorally relevant has received comparatively less attention, and we sought to investigate if manipulations of ACh systems might also affect behaviors related to stress and arousal. We chose to examine indices of anxiety as revealed by changes in behavior elicited by the black-white box test, a relatively novel and recently validated model of rodent anxiety. Groups of rats were injected with either scopolamine hydrobromide (SCOP; 0, 0.05, and 0.10 mg/kg IP) or the peripherally acting scopolamine methyl bromide (methyl-SCOP; 0, 0.05, and 0.10 mg/kg IP) to compare and contrast the effects of central and peripheral ACh blockade on measures of anxiety. SCOP pretreatment significantly lowered latencies for rats to escape from the white to black compartment, while methyl-SCOP elevated latencies to reenter the white chamber from the black. Both drugs increased the amount of time rats spent in the black compartment and also suppressed exploration as revealed by decreased episodes of intercompartmental locomotion. Neither drug deleteriously affected locomotor activity, however; in fact, SCOP significantly increased locomotion in the white chamber. In the absence of motor disturbances to account for any group differences, we contend that both central and peripheral ACh blockade may affect measures of anxiety, perhaps by directly or indirectly affecting HPA activity. Central ACh systems may underlie sensory filtering whereby irrelevant stimuli are excluded from sensory processing. Antagonism of ACh transmission may render an animal incapable of correctly processing sensory information leading to hyperresponsiveness, which can manifest itself as enhanced anxiety and fear.

Cholinergic mechanisms in the neurocontrol of the cephalic aorta of the cephalopod Sepia officinalis L.

In dopamine (DA)-precontracted, ring-shaped muscle preparations of the cephalic aorta of Sepia officinalis L. acetylcholine (ACh) and several cholinomimetics showed vasodilaroty effects. The pD 2-values evaluated from the concentration-response curves reveal a vasodilatory potency range of: carbachol ( pD 2 = 6.69) > ACh ( pD 2 = 5.09) > muscarine ( pD 2 = 4.09). The reduced action of nicotine results in a biphasic concentration-response curve that indicates a significantly lower intrinsic activity of this agonist. Under the ACh-antagonists applied, only tetraethylammonium (TEA) had a blocking effect ( pA 2 = 5.46 TEA/ACh; pA 2 = 5.74 TEA/carbachol), whereas D-tubocurarine, α-bungaroxitin (α-BTX), atropine and pirenzepine enhanced or did not block the vasodilatory action of ACH and carbachol. The results suggest the presence of an “M-like” receptor in the cepahlic aorta of Sepia. The putative neurotransmitter FMRFamide which mimicked the vasodilatory effects of the cholinomimetics showed the highest potency ( pD 2 = 8.24). Its action could not be blocked by TEA and seems to be mediated by another receptor mechanism.

Chapter 34: Central cholinergic mechanisms and function

Central actions of ACh, liberated at cholinergic synapses, are mediated by both muscarinic and nicotinic receptors. So far, much the most prominent have been muscarinic actions. Activation of muscarinic receptors can result in either excitation (most often, especially in cortical regions) or inhibition. These excitatory or inhibitory effects are mediated by a variety of mechanisms, as described in this chapter. In contrast to the (nicotinic) paradigmal fast excitatory transmitter actions of ACh at the neuromuscular junction and sympathetic ganglia and of glutamate at central synapses, both of which are produced by the opening of membrane conductances that generate an inward current, muscarinic excitations are produced by the suppression of on-going outward currents. Although indirect and slower, this can be a powerful effect, because most central neurons are subject to the continual stabilizing action of endogenous, more-or-less voltage-dependent K-currents, as well as synaptically activated C1 channels. Both of these muscarinic actions are discussed. Two other excitatory effects listed in the chapter, enhanced NMDA-receptor mediated synaptic responses and increased ephaptic (direct electrical) interactions between neighboring neurons, are also discussed in the final section of this chapter.

Effects of "non-quantal" acetylcholine on postsynaptic membrane senstitivity: Effects of ouabain, and mimicry by exogenous acetylcholine

The development of postsynaptic potentiation (PSP) and desensitization due to "non-quantal" acetylcholine that occurs when acetylcholinesterase (AChE) is inhibited was studied using the Na,K-ATPase inhibitor, ouabain, to alter (initially increasing, then decreasing) the level of non-quantal acetylcholine secretion, and exogenous acetylcholine. When ouabain increased non-quantal secretion the time constant (τ) of the miniature end-plate current (MEPC) decay increased, i.e., PSP developed. The later the application of ouabain relative to inhibition of AChE, the greater the degree of PSP. During the next phase when non-quantal secretion was inhibited the MEPC time course shortened more rapidly than in the controls, i.e., desensitization occurred. If ouabain abolished non-quantal secretion before AChE had been inhibited τ did not change, and neither PSP nor desensitization developed. When AChE was not inhibited ouabain had no effect on τ. When ACh was continuously applied at 20 nmol·liter−1, similar to the nonquantal concentration, the shortening of τ slowed down, and the signal amplitude declined more rapidly than in controls. Addition of exogenous ACh (50 nmol·liter−1) after acceleration of MEPC decay had developed caused τ to increase to its initial value. The combined appearance of PSP and desensitization during the action of non-quantal ACh, and the sustained desensitization after removal of released ACh from the synaptic cleft are discussed.