Peripheral Cholinergic Blockade Research Articles

Glucagon-like peptide-1 inhibits gastric emptying via vagal afferent-mediated central mechanisms.

Exogenous administration of glucagon-like peptide-1-(7-36) amide (GLP-1), an insulinotropic hormone, inhibits gastric emptying and acid secretion in humans. The role of GLP-1 as a regulator of gastric function is elusive. In gastric fistula rats, vagal afferent denervation and peripheral administration of the GLP-1 receptor antagonist exendin-(9-39) amide enhanced emptying of a glucose meal, whereas intracerebroventricular exendin was ineffective. The rate of saline emptying was attenuated by peripheral as well as by central administration of GLP-1, and pretreatment with exendin by the respective routes reversed the inhibition by GLP-1. Vagal afferent denervation abolished the central and peripheral action of GLP-1 on gastric emptying. Neither peripheral cholinergic nor adrenergic blockade altered the delay of methyl cellulose meal emptying by intracisternal GLP-1 injection. Acid secretion in conscious pylorus-ligated rats was inhibited by intracisternal GLP-1 administration, whereas systemic GLP-1 was ineffective. These results support the notion that GLP-1 receptors participate in the central and peripheral regulation of gastric function. Furthermore, vagal afferent nerves mediate the inhibitory action of GLP-1 on gastric motor function. GLP-1 may be a candidate brain-gut peptide that acts as a physiological modulator of gastric function.

Anticholinergic sensitivity in the aging rat septohippocampal system as assessed in a spatial memory task

The effects of central cholinergic blockade on spatial memory were tested in aged and basal forebrain-lesioned rats using the Morris Water Maze. In Experiment 1, aged rats (18–21 months old) were characterized as behaviorally impaired or nonimpaired based on water maze performance prior to an atropine sulfate challenge. In the atropine test (50 mg/kg, IP), both the impaired and the nonimpaired rats showed a severe disruption of their search behavior compared to young subjects. This effect was due to blockade of central receptors since peripheral cholinergic blockade using atropine methylbromide did not produce any impairments. Experiment 2 investigated effects of atropine on rats with septal lesions (SL), nucleus basalis lesions (NBL), and rats with both lesions combined (SL + NBL). Before drug treatment, the groups with septal lesions (SL and SL + NBL groups) displayed a moderate impairment in locating the platform site. However, similar to the aged rats, the septal-lesioned rats exhibited severe impairments in the water maze during the atropine treatment. This effect was not seen in the normal controls or in the NBL rats. Aged rats, either impaired or nonimpaired in a spatial memory task, showed a pronounced sensitivity to pharmacological blockade of central cholinergic neurotransmission which resulted in severe deficits in spatial navigation in the water maze. Since the same behavioral deficit was produced by cholinergic blockade in young rats with septal lesions, we concluded that the impaired water maze performance seen in the aged rats during cholinergic blockade resulted from impaired function in the septohippocampal system.

Separate controls for conditioned cephalic phases of acid secretion and drinking in the rat

Adult male Sprague-Dawley rats, surgically equipped with a stainless steel gastric cannula, were tested in a paradigm known to support conditioning of cephalic phase gastric acid secretion. Rats were tested first under (a) baseline conditions—prepared to sham feed, but no food was offered and none should have been expected; then (b) 5 conditioning trials—prepared and allowed to sham feed sweetened milk; and finally (c) extinction trials—prepared to sham feed milk, but milk was not offered. Combined antagonism of H 1 and H 2 histamine receptors (using intraperitoneal dexbrompheniramine plus cimetidine) prevented the acquisition of conditioned cephalic phase of acid secretion, but had no effect on acquisition of conditioned water intake. Such histaminergic antagonism also blocked the expression of an established conditioned acid secretory response without effect on conditioned water intake; peripheral cholinergic blockade using atropine methyl nitrate had virtually the same effect as did histaminergic antagonism. The converse was observed following blockade of angiotensin II synthesis using subcutaneous captopril: Established conditioned secretion was unaffected, but conditioned water intake was inhibited. These results dissociate the physiological controls for conditioned responses which occur simultaneously in rats anticipating eating: Peripheral histamine mediates the acquisition and expression of the conditioned cephalic phase of acid secretion, whereas angiotensin II appears to mediate at least the expression of conditioned water intake.

Differential effects of central and peripheral cholinergic blockade on water consumption by rats

Atropine (5 mg/kg, s.c., twice daily) had no significant effect on 24-h water consumption on day 1 of treatment; on subsequent days the rats showed a significant increase. Procyclidine (5 mg/kg, s.c., twice daily) had a similar effect, except that the increase in daily water consumption began on the third day of treatment. Methylatropine (5 mg/kg, s.c., twice daily) markedly depressed water consumption on day 1; from the second day on no significant effects on 24-h water consumption were seen. The results suggest that the dipsogenic actions of cholinergic blocking agents on 24-h water consumption involve central rather than purely peripheral actions.

Time-Related Facilitation and Suppression of Drinking by Muscarinic Anticholinergic Drugs in Water Non-Deprived Rats

Effects of tertiary anticholinergic drugs, atropine (1.3, 2.5, 5 and 10 mg/ kg, s.c.) and scopolamine (0.13, 0.25, 0.5 and 1 mg/kg, s.c), and a quarternary anticholinergic drug, methylatropine (1.3, 2.5, 5 and 10 mg/kg, s.c), on the drinking behavior were investigated in water non-deprived rats that were housed in a 12-hr light-dark situation (light period: 6:00-1 8:00) with a free access to food. Atropine, 5 mg/kg, and scopolamine, 0.13 and 0.25 mg/kg, administered at 12:00, significantly increased the drinking during the 12:00-18:00 period. Furthermore, lower to medium doses of atropine increased the drinking during the 18:00-6:00 period. In contrast, the drinking did not change during the 12:00-18:00 period, but decreased during the 18:00-6:00 period in a dose-dependent manner after administration of methylatropine at 12:00, whereas the drinking during the 18:00-24:00 period decreased in a dose-dependent manner when both the tertiary and quarternary drugs were administered at 18:00. However, the drinking during the 24:00-6:00 period increased in the rats that were administered atropine at the dose of 5 or 10 mg/kg at 18:00, while the drinking still decreased after methylatropine at the same time. The present results suggest that in water non-deprived rats, central muscarinic cholinergic blockade is effective for both increasing and decreasing drinking behavior, depending on the doses, when the drug is administered, and time span between the drug administration and the behavior observation. It is also suggested that peripheral cholinergic blockade monotonously suppresses the drinking behavior.

Cholinergic blockade effects on spatial integration versus cue discrimination performance.

A within-subjects investigation was conducted to determine the effects of central versus peripheral cholinergic blockade in animals tested either on a spatial integration task in which the possibility of rule learning was also available or on a visual discrimination task in which the daily location of food was marked by a distinctive visual stimulus pattern. All testing was conducted on the Maier three-table apparatus. It was found that the only effect of the peripheral cholinergic blockade on the performance of either task group was to produce a decrease in exploratory behavior. In contrast, central cholinergic blockade markedly impaired spatial integration performance; however, it did not impair the ability of animals in rule learning or visual discrimination learning. It was also found that central cholinergic blockade impaired the animal's tendency to enter all tables before reentering a given table during the exploratory phase of the daily session. This finding was interpreted as reflecting an impairment of working memory for spatial information, rather than a general impairment in working memory, and this interpretation was applied to the explanation of the deficit in the spatial integration performance.

Sympathetic nervous system mediation of acute cardiovascular actions of gamma 2-melanocyte-stimulating hormone.

Peptides of the pro-opiocortin class produce pronounced cardiovascular and natriuretic actions. We have investigated the acute cardiovascular effects of one of the most potent members of this class, gamma 2-melanocyte stimulating hormone (gamma 2-MSH), in rats. Pressor actions of gamma 2-MSH administered systemically were eliminated by ganglionic blockade with chlorisondamine. Peripheral cholinergic blockade failed to affect either the pressor or cardioaccelerator responses to gamma 2-MSH. Administration of gamma 2-MSH (2.0-10.0 micrograms) produced vasoconstriction primarily in the mesenteric and hindlimb vascular beds, while the renal bed showed little response. Infusions of phenylephrine produced pressor responses similar to those found with gamma 2-MSH, which were accompanied by a decrease in heart rate and vasoconstriction in the mesenteric and renal vascular beds. Hemodynamic changes produced by gamma 2-MSH and phenylephrine were blocked or attenuated by alpha 1-adrenergic receptor blockade with prazosin. Direct injection of gamma 2-MSH into the renal artery produced an acute renal vasoconstriction that was not attenuated by alpha 1-adrenergic or ganglionic blockade. These findings and the results of previous publications are consistent with the hypothesis that gamma 2-MSH may produce a centrally mediated activation of the sympathetic nervous system, have direct vasoconstriction actions on the renal vasculature, and inhibit baroreceptor function to produce an increase in blood pressure without an accompanying bradycardia.

Meal initiation controlled by learned cues: Effects of peripheral cholinergic blockade and cholecystokinin

Sated animals can be induced to initiate meals by exposing them to external stimuli which they have learned, via Pavlovian conditioning, to associate with food. This study examined physiological properties of this control of feeding. The initial hypothesis examined was that conditioned feeding depended on the elaboration of cholinergic cephalic phase responses (e.g., anticipatory insulin secretion). This idea was evaluated by comparing feeding responses to presentation of conditioned cues following an injection of either a peripheral cholinergic blocker, atropine methyl nitrate, or a control substance, physiological saline. Peripheral cholinergic blockade had no effect on the meal initiated by presentation of conditioned cues even though the dose of atropine methyl nitrate used was demonstrated to be sufficient to completely suppress cholinergic cephalic phase responses. These results indicate that cholinergic anticipatory digestive secretions do not contribute to feeding in this preparation. The effects of exogenously administered cholecystokinin on feeding controlled by learned cues were also studied. Cholecystokinin suppressed the size of the meal induced by presentation of conditioned stimuli but did not influence the latency, or initial rate of eating. The implications of these results to a conclusion that cholecystokinin is a satiety factor are discussed.

Preabsorptive pregastric vagally mediated histaminergic component of drinking elicited by eating in the rat.

Preabsorptive stimulation by food was confined to the pregastric (oropharynx and esophagus) segment of the gastrointestinal tract by having male Sprague-Dawley rats sham feed liquid food which then drained out a gastric cannula. This procedure provided a paradigm for studying the effect of preabsorptive pregastric food-contingent stimulation on drinking behavior. Sham feeding elicited drinking that was (a) attenuated by complete bilateral subdiaphragmatic vagotomy (with hepatic branch intact), (b) attenuated by peripheral cholinergic blockade with intraperitoneal atropine methyl nitrate (0.25 mg/kg), and (c) abolished by combined antagonism of H1 and H2 histamine receptors with the use of intraperitoneal dexbrompheniramine (1 mg/kg) and cimetidine (16 mg/kg). These results provide evidence for a preabsorptive pregastric vagally mediated histaminergic component of drinking elicited by eating in the rat.

Alteration of leukotriene D4 hypotension by thyrotropin releasing hormone.

Leukotriene D4 (LTD4), a component of slow reacting substance of anaphylaxis (SRS-A)1, produces marked hypotension in laboratory animals2,3, implicating it as a potential mediator of anaphylactic shock. It has been demonstrated that naloxone reverses the hypotension associated with endotoxaemia4-6, hypovolaemia7,8 and spinal injury9,10, presumably through blockade of endogenous opioid systems11. More recently, it has also been shown that thyrotropin releasing hormone (TRH) improves experimental shock12 and spinal injury13, and it has been postulated that TRH acts by 'physiological' antagonism of endogenous opioid systems in these conditions. Here we report that TRH both blocked and reversed leukotriene-induced hypotension in the unanaesthetized guinea pig, whereas naloxone had no effect. LTD4 hypotension was also reversed by intracerebroventricular (i.c.v.) administration of TRH at a dose that had no effect when given systemically. LTD4 administration was associated with sympatho-adrenomedullary activation, and TRH further augmented this response. Peripheral cholinergic blockade with methylatropine did not alter the leukotriene hypotension. These data demonstrate the first dissociation of TRH and naloxone in experimental shock and suggest that the beneficial effects of TRH in this model result from central interactions which are not mediated by endogenous opioids. The findings further provide a potential link between the physiological (that is, cardiovascular) effects of a peptide (TRH) and those of a leukotriene; this may have implications with regard to the pathophysiology and therapy of anaphylaxis.

The effect of intracisternally administered bombesin on cysteamine-induced duodenal ulcers in rats

Cysteamine (420 mg/kg s.c.) increased gastric acid secretion in rats 3–6 h post-injection; duodenal pH fell from 6.0±0.2 (S.E.M.) at 0 time to 2.2.±0.3 at 12 h. A high (60–80%) incidence of duodenal ulcerations was observed 18 h after cysteamine injection. Intracisternal (i.c.) administration of bombesin (1 μg) significantly inhibited both cysteamine-induced increases in gastric acid secretion and the development of duodenal ulcers. The effect of bombesin was dose-dependent and appears to be relatively specific since i.c. neurotensin (1 or 30 μg) was not cytoprotective. Peripheral cholinergic (muscarinic) blockade with atropine methylbromide (10 mg/kg i.p.) was as effective as i.c. bombesin (1 μg) in inhibiting cysteamine-induced gastric hypersecretion and duodenal ulcers.

Changes in schedule-controlled response and schedule-induced drinking after cholinergic blockers in rats.

The effects of the tertiary cholinergic blockers atropine sulfate (AT) and scopolamine hydrobromide (SCOP), and the quaternary cholinergic blockers atropine methylnitrate (AT-Mt) and scopolamine butylbromide (SCOP-Bt), on operant response (lever-pressing) and adjunctive drinking in the rat developed under a fixed interval (FI) 1.5 min food reinforcement situation were investigated. The tertiary compounds, particularly SCOP, increased total responses, and produced a dose-related decrease in the index of curvature (IC) for local rates of response during the FI. The quaternary compounds, however, did not produce changes in the total responses and the IC for local rates of response. AT, AT-Mt and SCOP suppressed adjunctive drinking, while SCOP-Bt did not. AT and SCOP, but not AT-Mt and SCOP-Bt, decreased the IC for local rates of drinking. AT-Mt suppressed drinking at a uniform rate throughout the FI. In addition, several doses of AT and SCOP tended to increase licking counts at the drinking spout per 1 ml water intake. The present results suggest that central cholinergic blockade increases the response and suppress adjunctive drinking, and that the peripheral cholinergic blockade suppresses only adjunctive drinking. Moreover, central cholinergic blockade may produce changes in the patterns of response and drinking during the FI, and in the manner of drinking at the drinking spout.

Effects of cholinergic stimulation on pituitary hormone release

Physostigmine was infused into human volunteers to assess the effect of central cholinergic stimulation on memory and on neuroendocrine function. Methscopolamine bromide, a peripheral anticholinergic agent, was given simultaneously. The lower dose of physostigmine (1.0 mg) produced no change in AVP, cortisol, melatonin, GH or LH in those subjects without unpleasant cholinergic side effects. Larger doses of physostigmine usually produced nausea, and were associated with marked elevations of AVP, cortisol and prolactin, but no change in GH, LH or melatonin. Thus, cholinergic agents easily induce a stress response, but the GH component of this response can be suppressed by peripheral cholinergic blockade.

Cholinergic receptor blockade impairs spatial localization by use of distal cues in the rat.

Spatial localization was studied in the Morris water maze. The task required rats to escape from cool water (made opaque by milk) by finding a submerged, invisible platform located at a fixed place within the room. The start point was varied randomly, and there was no local cues to indicate the position of the platform. After training, the platform was moved. Rats subjected to central cholinergic receptor blockade with atropine sulfate were compared with normal rats and rats receiving peripheral cholinergic blockade with atropine methylnitrate. The controls for the use of search strategies, as compared with spatial mapping, were a group of blind rats and a group of rats for which the platform was moved from trial to trial. Measures of escape latency, swim distance, initial heading error, posttrial rearing on the platform, and search strategy after platform displacement revealed that the control rats and the atropine methylnitrate rats used a spatial mapping strategy to locate the platform. The atropine sulfate treated rats adopted a search strategy like that of the blind rats and the rats for which the platform was randomly moved: Their escape latency and swimming distance decreased across trials, including reversal trials, but their initial heading errors remained unchanged. The results support the idea that central cholinergic systems are important for spatial mapping, which demands the use of distal visual cues, but not for spatial localization, which requires other search strategies and possibly the use of proximal tactile, kinesthetic, and visual cues. Consistent with this idea, certain features of the atropine sulfate treated rats' behavior also suggested a novel explantation for some aspects of atropine stereotypies.

Survival under hypoxia. Age dependence and effect of cholinergic drugs.

Survival under hypoxia (5% O2; 95% N2) was tested in mice 1 day to 50-weeks-old. Survival Rate (ratio of number of animals that survived 30 min under 5% O2 to total number of animals exposed) and the time from onset of exposure until the last gasp (Survival Time) were maximum in newborn animals and decreased as a function of age. Survival Rate and Survival Time were strongly influenced by drugs affecting cholinergic transmission. Atropine decreased the high survival under hypoxia of 1-week-old mice in a dose-related manner. Physostigmine increased survival under hypoxia in mice 3 to 50-weeks-old. This effect was not related to a peripheral action of the drug since it was not mimicked by neostigmine, a cholinesterase inhibitor without central actions. Moreover, peripheral cholinergic blockade with glycopyrrolate, a quaternary cholinergic blocker, did not prevent the protective effect of physostigmine. Since atropine impairs the ability of very young mice to survive hypoxia and physostigmine improves survival at later ages, it is concluded that a central cholinergic mechanism, possibly related to blood flow regulation, plays a significant role in the acute adaptation to hypoxia.

A Benzodiazepine—Anticholinergic Drug Synergism in the Prevention of Stress‐Induced Gastric Mucosal Erosion in Mice

Chlordiazepoxide and clidinium each, as a function of dose, prevent stress-induced gastric mucosal erosion in mice. Clidinium was 2.5 times more potent than chlordiazepoxide. When used in a combination of 2 parts chlordiazepoxide and 1 part clidinium, the protective effect was nearly five times greater than that produced by clidinium alone. Furthermore, the combination dosing proved nearly three times more potent than the potency that was predicted from simple additivity of the individual drug effects. This potentiation appears related to the number of ways in which the combination treatment can decrease autonomic input to the gastric mucosa. Thus, the peripheral cholinergic blockade by clidinium may be potentiated by a central chlordiazepoxide suppression of both sympathetic and parasympathetic activities. Therefore, the combined use of these drugs in the therapy of stress-induced gastric disorder appears to have a rational pharmacologic basis.

Physostigmine, a highly potent antidote for acute experimental diazepam intoxication

Rabbits, cats and rats with permanent intracerebral recording electrodes were acutely poisoned with lethal, or close-to-lethal, doses of diazepam (80–120 mg/kg i.p. or i.v.) and the protective effect of physostigmine, given at the peak of the diazepam coma, was studied by using three parameters of observation: changes in EEG activity, gross behaviour and mortality rate. Physostigmine (0.25–1.50 mg/kg i.v. or i.p.) exerted a surprisingly strong antidote effect in all the three species investigated. It restored pathological EEG alterations and normalized behaviour within a few minutes. Mortality rate was also reduced to a minimum. Peripheral cholinergic blockade by intraperitoneal methylatropine (10–20 mg/kg) given concomitantly with, or prior to, physostigmine application prevented the peripheral parasympathetic excitation due to physostigmine but left the antidote effect unaltered.

The effects of muscarinic cholinergic blockade upon shock-elicited aggression

Graded dosages of atropine sulfate, atropine methyl nitrate, scopolamine hydrochloride, and scopolamine methyl nitrate were administered to rats and shock-elicited fighting frequencies determined. Central cholinergic blockade decreased fighting at appropriate dosages, but peripheral cholinergic blockade had little or no effect upon shock-elicited aggression. These result suggest that shock-elicited aggression is similar to other kinds of agonistic behavior (e.g., isolation-induced fighting and muricide) in that a central cholinergic system is apparently involved in its mediation.

Influence of Ganglionic Blocking Agents on Survival Time in Irradiated Mice

Peripheral cholinergic blockade was shown by Larkin (1) and Haley and Rhodes (2) to modify survival time in irradiated mice. However, the time interval between the day on which 50 % of the animals were dead and the one on which all had died was relatively short, and thus such drugs could not be considered very beneficial in radiation injury. On the other hand, this peripheral approach to the cholinergic branch of the autonomic nervous system did not take into consideration the possible benefits which might be derived by blocking nervous transmission at the ganglia in both the adrenergic and cholinergic branches of the system. To answer this question, the following ganglionic blocking drugs were tested in irradiated mice under the same conditions used previously (2): Banthine, B-diethylaminoethyl-9xanthene carboxylate methochloride; nicotine; Prantal, N. N.-4-dimethylpiperidylidine-diphenylmethane methyl sulfate; tetraethyl-ammonium chloride; Su-1194, tri (B-diethylamine) ethylamine hydrochloride; and Pendiomide, pentamethyldiethyl-3-azapentane-1,5 diammonium-dibromide.