Blockade Of Cholinergic Muscarinic Receptors Research Articles

Loganin enhances long-term potentiation and recovers scopolamine-induced learning and memory impairments

Although the incidence rate of dementia is rapidly growing in the aged population, therapeutic and preventive reagents are still suboptimal. Various model systems are used for the development of such reagents in which scopolamine is one of the favorable pharmacological tools widely applied.Loganin is a major iridoid glycoside obtained from Corni fructus (Cornusofficinalis et Zucc) and demonstrated to have anti-inflammatory, anti-tumor and osteoporosis prevention effects. It has also been found to attenuate Aβ-induced inflammatory reactions and ameliorate memory deficits induced by scopolamine. However, there has been limited information available on how loganin affects learning and memory both electrophysiologically and behaviorally.To assess its effect on learning and memory, we investigated the influence of acute loganin administration on long-term potentiation (LTP) using organotypic cultured hippocampal tissues. In addition, we measured the effects of loganin on the behavior performance related to avoidance memory, short-term spatial navigation memory and long-term spatial learning and memory in the passive avoidance, Y-maze, and Morris water maze learning paradigms, respectively.Loganin dose-dependently increased the total activity of fEPSP after high frequency stimulation and attenuated scopolamine-induced blockade of fEPSP in the hippocampal CA1 area. In accordance with these findings, loganin behaviorally attenuated scopolamine-induced shortening of step-through latency in the passive avoidance test, reduced the percent alternation in the Y-maze, and increased memory retention in the Morris water maze test.These results indicate that loganin can effectively block cholinergic muscarinic receptor blockade -induced deterioration of LTP and memory related behavioral performance. Based on these findings, loganin may aid in the prevention and treatment of Alzheimer's disease and learning and memory-deficit disorders in the future.

Phosphodiesterase inhibition by sildenafil citrate attenuates the learning impairment induced by blockade of cholinergic muscarinic receptors in rats

We examined whether treatment with sildenafil citrate (the active compound of Viagra), a cyclic nucleotide phosphodiesterase type 5 inhibitor (PDE5), would reverse the learning impairment induced by cholinergic muscarinic (mACh) receptor blockade [0.75 mg/kg scopolamine HCl, intraperitoneal (i.p.) injections]. Rats were pretrained in a one-way active avoidance of foot shock in a straight runway and the next day received 15 training trials in a 14-unit T-maze. Performance in this maze paradigm requires accurate responding to avoid mild foot shock and has been shown to be sensitive to aging and to impairment in central cholinergic systems. Intraperitoneal (i.p.) injections of scopolamine or saline and sildenafil or vehicle were given 30 and 15 min before training, respectively. The combined treatment conditions were as follows: saline+vehicle (control), scopolamine (0.75 mg/kg)+vehicle, and scopolamine (0.75 mg/kg)+sildenafil (1.5, 3.0, or 4.5 mg/kg). Behavioral measures of performance included deviations from the correct pathway (errors), run time from start to goal, shock frequency, and duration. Statistical analysis revealed that scopolamine impaired maze performance and that sildenafil (3.0 mg/kg) significantly attenuated this impairment in a dose-dependent manner. These results suggest that sildenafil citrate may serve as a cognitive enhancer for therapeutic treatment of cholinergic dysfunction in age-related cognitive decline and Alzheimer's dementia (AD).

Control of skeletal muscle perfusion at the onset of dynamic exercise

At the onset of exercise there is a rapid increase in skeletal muscle vascular conductance and blood flow. Several mechanisms involved in the regulation of muscle perfusion have been proposed to initiate this hyperemic response, including neural, metabolic, endothelial, myogenic, and muscle pump mechanisms. Investigators utilizing pharmacological blockade of cholinergic muscarinic receptors and sympathectomy have concluded that neither sympathetic cholinergic nor adrenergic neural mechanisms are involved in the initial hyperemia. Studies have also shown that the time course for vasoactive metabolite release, diffusion, accumulation, and action is too long to account for the rapid increase in vascular conductance at the initiation of exercise. Furthermore, there is little or no evidence to support an endothelium or myogenic mechanism as the initiating factor in the muscle hyperemia. Thus, the rise in muscle blood flow does not appear to be explained by known neural, metabolic, endothelial, or myogenic influences. However, the initial hyperemia is consistent with the mechanical effects of the muscle pump to increase the arteriovenous pressure gradient across muscle. Because skeletal muscle blood flow is regulated by multiple and redundant mechanisms, it is likely that neural, metabolic, and possibly endothelial factors become important modulators of mechanically induced exercise hyperemia following the first 5-10 s of exercise.

Blockade of cholinergic muscarinic receptors by pirenzepine and GHRH-induced GH secretion in the acute and recovery phase of anorexia nervosa and atypical eating disorders

In view of the important role played by the cholinergic system in the neural regulation of growth hormone (GH) secretion, the ability of pirenzepine, a selective antagonist of muscarinic cholinergic receptors, to blunt the GH response to GH-releasing hormone (GHRH) was studied in adolescent females with anorexia nervosa in the acute (AN-AP) and recovery (AN-RP) phases, and in patients with atypical eating disorders (AED). In five AN-AP patients, administration of GHRH 1–40 (1 μg/kg IV) evoked a significantly higher GH response than in controls at established intervals, whereas in eight AN-RP and seven AED patients it was higher than in controls at only one (150-min) and two (150-min, 180-min) time intervals, respectively. In the AN-AP patients, pretreatment with pirenzepine (0.6 mg/kg IV) only partially blocked the GH response to GHRH, whereas in the same AN-AP patients tested during recovery, and in AN-RP and AED patients, the drug completely suppressed the GH response to GHRH, as it did in controls. In view of pirenzepine's mechanism of action, these findings are best explained by the existence in the hypothalamus of AN-AP patients of a cholinergic hypertone and/or a diminished somatostatinergic function. Evaluation of the clinical and hormonal characteristics of the anorectic patients studied would indicate that factors other than undernutrition and its biological consequences, which subside in the recovery stage of the disease and are not present in AED patients, contribute to the anomalous GH response pattern of AN-AP patients.

Effect of Cholinergic Muscarinic Receptor Blockade on Human Growth Hormone (GH)-Releasing Hormone-(1–44)-Induced GH Secretion in Anorexia Nervosa

The presence of hypothalamic disturbances affecting GH secretion in anorexia nervosa has been suggested, although a normal GH response to GH-releasing hormone (GHRH) administration has been shown in these patients. The present study was performed to investigate the role of acetylcholine in regulating GH secretion by using pirenzepine, which selectively blocks muscarinic cholinergic receptors. Paired tests were performed in nine anorexia nervosa patients (age +/- SEM, 19.1 +/- 1.2 yr; percent ideal body weight, -32.7 +/- 2.2%) and in six normal controls (20.1 +/- 0.3 yr; -3.1 +/- 1.8%). GHRH-(1-44) (1 microgram/kg) was infused iv with and without pirenzepine pretreatment (0.6 mg/kg, iv). Basal levels of GH were not different in anorexia nervosa compared to normal controls, whereas, somatomedin-C levels were significantly lower in anorexia nervosa patients. However, after pirenzepine administration, the GHRH-stimulated GH responses were completely blocked in normal controls, but not in anorexia nervosa patients. These results suggested that altered muscarinic cholinergic mechanism are involved in the modulation of GH secretion in patients with anorexia nervosa.

Cholinergic muscarinic receptor blockade suppresses arginine- and exercise-induced growth hormone secretion in type I diabetic subjects.

Stimulatory cholinergic pathways participate in the regulation of GH release, and cholinergic receptor antagonists inhibit GH secretion in normal man. Whether similar mechanisms are active in subjects with insulin-dependent diabetes is not known, yet this is of potential importance since GH hypersecretion has been implicated in both the acute and chronic complications of diabetes mellitus. To address this question we studied the effects of cholinergic receptor blockade on stimulated GH release in 18 type I diabetic men. Paired tests were performed using 1 of 2 different stimuli (30 g arginine, iv, or physical exercise for 30 min) with or without prior administration of the selective cholinergic muscarinic receptor antagonist pirenzepine (30 mg, iv). Arginine elicited a mean peak serum GH level of 9.0 +/- 1.9 (+/- SEM) micrograms/L, which was completely suppressed by pirenzepine (1.5 +/- 0.4 micrograms/L; n = 8; P less than 0.01). Blood glucose rose after arginine infusion and was not affected by pirenzepine. Serum GH levels rose during physical exercise to a mean peak of 7.3 +/- 1.6 micrograms/L, which was abolished by pirenzepine (1.2 +/- 0.3 micrograms/L; n = 10; P less than 0.01). Blood glucose was not influenced by pirenzepine. Two subjects had no serum GH response to exercise. We conclude that GH secretion in subjects with insulin-dependent diabetes, as in normal subjects, is modulated by cholinergic pathways and is responsive to pharmacological suppression by muscarinic receptor blockade. This may have implications for therapeutic trials designed to lower elevated GH levels in subjects with diabetes mellitus.

Autonomic innervation of pulmonary artery: evidence for a nonadrenergic noncholinergic inhibitory system.

We have investigated the presence and significance of the nonadrenergic noncholinergic relaxant system in isolated strips of guinea pig and cat pulmonary artery (PA), using transmural electrical field stimulation (EFS). Changes in PA strip tension were measured isometrically, in response to EFS with 50-70 V at 20 Hz and pulse durations of 1 or 2 ms. We examined the influence on these responses of alpha- and beta-adrenergic and cholinergic muscarinic receptor blockade, the neurotoxin tetrodotoxin, and the local anesthetic lidocaine. EFS induced an initial brief contraction followed by a slow prolonged relaxation. Phenoxybenzamine, 1.5 X 10(-5) M, greatly reduced the contraction (74.1-87.6% in guinea pig, 65.7-67.4% in cat) and enhanced the subsequent relaxation (23.9-35.5% in guinea pig, 66.8-67.8% in cat). The relaxation was moderately reduced by propranolol, 7.7 X 10(-6) M (30.5-35.0% in guinea pigs, 6.0-21.2% in cat), and by atropine, 1.4 X 10(-6) M (23.7-32.1% in guinea pig, 24.9-40.6% in cat). The major portion of the relaxation, however, persisted in the presence of these blockers. The neurogenic origin of this relaxation was confirmed by its loss with either tetrodotoxin or lidocaine. We conclude that in guinea pig and cat lungs (1) neurogenic pulmonary vascular contraction is principally mediated by the alpha-adrenergic system, (2) the beta-adrenergic and cholinergic components have a moderate relaxant influence, but (3) the dominant part of neurogenic relaxation is mediated by a nonadrenergic noncholinergic mechanism.

Cholinergic blockade in the rat impairs strategy selection but not learning and retention of nonspatial visual discrimination problems in a swimming pool.

A procedure was developed to study black versus white and horizontal versus vertical pattern visual discriminations in a swimming pool. The effects of central cholinergic muscarinic receptor blockade by atropine sulfate was then evaluated. The drug treatment impaired acquisition but not retention. Behavioral observations showed that the control rats used a number of strategies during the process of problem solving that facilitated acquisition of the discrimination. Through modifications of training procedures, the processes of strategy selection and discrimination learning were dissociated. Cholinergic blockade was found to impair strategy selection but not discrimination learning. The results question the widely held view that cholinergic systems are involved in learning and memory and suggest instead that cholinergic systems are involved in the selection of the movements or strategies that are prerequisite for learning.

Effect of Cholinergic Muscarinic Receptor Blockade on Human Growth Hormone(GH)-Releasing Hormone-(l-44)-Induced GH Secretion in Acromegaly and Type I Diabetes Mellitus

In 10 patients with type I diabetes mellitus, 9 acromegalic patients, and 8 normal subjects, serum GH levels after an iv injection of 1 microgram/kg BW human GH-releasing hormone-(1-44) (GHRH) with and without cholinergic muscarinic receptor blockade (1 mg atropine, im, 15 min before GHRH) were measured. The GH increase after GHRH treatment in type I diabetic patients did not differ significantly from that in normal subjects. Pretreatment with atropine almost completely blocked the GH response to GHRH in patients with type I diabetes and normal subjects, but did not suppress the GH response to GHRH in acromegalic patients. These data indicate that cholinergic muscarinic receptors modulate GH secretion in normal man as well as in patients with type I diabetes mellitus, whereas acromegalic patients have a defect of cholinergic control of GH secretion.

Generation of cortical event-related slow potentials in the rat involves nucleus basalis cholinergic innervation

These experiments were conducted to gather information regarding the role of cholinergic innervation to the cortex in the generation of event-related slow potentials. The effects of unilateral drug treatments or lesions on ipsilateral and contralateral frontal cortex slow potential (SP) responses were examined in rats. The SP responses were recorded with silver-silver chloride electrodes and were generated by a 2 sec light cue which preceded medial forebrain bundle stimulation. The following approaches were used: (1) microinjection of GABA, procaine or saline into the nucleus basalis magnocellularis; (2) microinjection of atropine or saline subdurally in the SP recording area; (3) electrolytic lesion of the nucleus basalis area; and (4) kainic acid lesion of the nucleus basalis area. The following bilateral measurements were obtained in the lesion studies: choline acetyltransferase (ChAT) in cortex and hippocampus; serotonin in cortex, hippocampus, striatum and nucleus accumbens; norepinephrine in cortex and hippocampus; dopamine in striatum and nucleus accumbens; and metabolites of serotonin, norepinephrine and dopamine in these areas. The cortical SP responses were reduced on the side ipsilateral to the injections of GABA and procaine into the nucleus basalis, and on the side of the subdural atropine injection. With either type of lesion, the SP responses on the lesioned side were significantly reduced as compared to the non-lesioned side. Reductions in cortical ChAT and other measures were observed ipsilateral to the electrolytic lesion, but only cortical ChAT activity was reduced in the kainic acid-lesioned animals. Thus, pharmacological depression of nucleus basalis neurons, blockade of cholinergic muscarinic receptors in the cortex, and nucleus basalis lesions that reduce cortical choline acetyltransferase activity depress event-related slow potentials in the rat frontal cortex. These results provide evidence that cortical slow potential responses in the rat are dependent upon cholinergic innervation from the nucleus basalis.

Effect of agonists and antagonists of cholinergic neurotransmission on growth hormone release in the dog.

In unanaesthetized dogs iv administration of the cholinesterase inhibitor eserine (0.5 mg) or the cholinergic muscarinic receptor agonist oxotremorine (0.25 mg) induced a clear-cut rise in plasma canine growth hormone (cGH) levels. Complete suppression of the GH-releasing effect of eserine and oxotremorine was induced by blockade of cholinergic muscarinic receptors by atropine (80 or 20 micrograms/kg, 30 min before) but not by scopolamine-N-butyl bromide (0.8 mg/dog, 30 min before), an anticholinergic drug which does not cross the blood brain barrier (BBB). In contrast, activation of cholinergic nicotinic receptors by nicotine (6 mg) failed to alter resting cGH concentrations, and pre-treatment with the nicotinic receptor blocker mecamylamine (5 mg, 30 min before) did not counteract the GH-releasing effect of eserine. Other cholinomimetic drugs, e.g. pilocarpine, 4-aminopyridine, carbachol and bethanechol failed to induce a rise in plasma cGH concentrations. These data indicate that: 1) cholinergic muscarinic but not nicotinic receptors located in the central nervous system (CNS) inside the BBB play a facilitatory role in tonic cGH release; 2) pharmacologically distinct muscarinic receptors may exist in the CNS.

Neural control of the intestinal migrating myoelectric complex. A pharmacological analysis.

A pharmacological analysis of the participation of autonomic nerves in the control of the intestinal migrating myoelectric complex (MMC) was conducted on six conscious fasting dogs with implanted bipolar electrodes. All dogs exhibited regularly recurring MMC's. Hexamethonium (2-10 mg/kg i.v.) or atropine (25-100 micrograms/kg i.v.) immediately suppressed all spiking activity, stopped the progression of the ongoing complexes, and prevented the initiation of new complexes for 2.5 to 5 h. Phentolamine (2-4 mg/kg i.v.) and propranolol (2 mg/kg i.v.) given separately or in combination had no observable effect on any of the different phases of the complex, its progression, or its frequency. Phenylephrine (0.5-1 mg/kg) inhibited the spiking phases of the complex. Isopropylnoradrenaline had no effect on the complex at doses (up to 30 micrograms/kg) which produced maximal cardiac chronotropic effect. The effects of these agonists were totally blocked by the doses of respective adrenergic antagonists used. Phenoxybenzamine given in a dose of 10-15 mg/kg i.v. produced effects similar to atropine apparently owing to blockade of cholinergic muscarinic receptors. It is concluded that (a) the regular spiking phases of the migrating complex result from cyclical and sequential activation of preganglionic fibers forming nicotinic synapses on postganglionic cholinergic excitatory neurones; and (b) the function of adrenergic nerves is not normally required for the MMC.

Growth hormone-releasing effect of an enkephalin analog in the dog: evidence for cholinergic mediation.

The GH-releasing effect of a potent enkephalin analog, [D-Ala2, MePhe4, Met(o)5-ol]enkephalin (DAMME) and its possible mechanism(s) of action were investigated in unanesthetized dogs. DAMME injected iv into 14 male and female beagles (34 tests) at doses of 2.0, 4.0, 8.0, and 16.0 μg/kg induced clear-cut and dose-related rises in canine GH (cGH) levels. Peak cGH levels occurred 15–30 min post injection and cGH levels were still elevated at 120–180 min. Prior administration of the opiate antagonist naloxone (0.5 mg/kg iv) completely suppressed the hormonal response to DAMME (8 μg/kg) without significantly altering baseline cGH levels. Blockade of serotonin receptors by metergoline (1 mg/kg, orally 60 min before) induced a slight increase in cGH levels but did not modify the GH-releasing effect of DAMME. Subchronic administration of pimozide (1 mg, orally three times daily for 2 days and 3.0 mg orally 120 min before), a blocker of dopamine receptors, significantly reduced the peak cGH rise induced by DAMME. Complete suppression of the GH-releasing effect of DAMME was induced by blockade of cholinergic muscarinic receptors by atropine (80 μg/kg, sc 30 min before) but not by butyl-scopolamine (0.5 mg/dog, iv 30 min before), an anticholinergic drug which does not cross the bloodbrain barrier. Pretreatment with eserine (0.5 mg/dog, iv 15 min before), a reversible inhibitor of cholinesterase, was unable to potentiate the GH-releasing effect of a threshold dose of DAMME but induced a rise in plasma cGH levels per se. These results indicate that enkephalins are potent GH releasers in the dog and that cholinergic mediation plays an essential role in this effect. © 1980 by The Endocrine Society.