Muscarinic Activation Research Articles

Functional and laminar dissociations between muscarinic and nicotinic cholinergic neuromodulation in the tree shrew primary visual cortex

Acetylcholine is an important neuromodulator involved in cognitive function. The impact of cholinergic neuromodulation on computations within the cortical microcircuit is not well understood. Here we investigate the effects of layer-specific cholinergic drug application in the tree shrew primary visual cortex during visual stimulation with drifting grating stimuli of varying contrast and orientation. We describe differences between muscarinic and nicotinic cholinergic effects in terms of both the layer of cortex and the attribute of visual representation. Nicotinic receptor activation enhanced the contrast response in the granular input layer of the cortex, while tending to reduce neural selectivity for orientation across all cortical layers. Muscarinic activation modestly enhanced the contrast response across cortical layers, and tended to improve orientation tuning. This resulted in highest orientation selectivity in the supra- and infragranular layers, where orientation selectivity was already greatest in the absence of pharmacological stimulation. Our results indicate that laminar position plays a crucial part in functional consequences of cholinergic stimulation, consistent with the differential distribution of cholinergic receptors. Nicotinic receptors function to enhance sensory representations arriving in the cortex, whereas muscarinic receptors act to boost the cortical computation of orientation tuning. Our findings suggest close homology between cholinergic mechanisms in tree shrew and primate visual cortices.

263 MODULATION OF UROTHELIAL ATP RELEASE AND ITS PARACRINE ACTION BY PURINERGIC AND MUSCARINIC NEUROTRANSMITTERS

You have accessJournal of UrologyBladder and Urethra: Anatomy, Physiology and Pharmacology I1 Apr 2012263 MODULATION OF UROTHELIAL ATP RELEASE AND ITS PARACRINE ACTION BY PURINERGIC AND MUSCARINIC NEUROTRANSMITTERS Guiping Sui, Chris Fry, Bruce Montgomery, and Changhao Wu Guiping SuiGuiping Sui Guildford, United Kingdom More articles by this author , Chris FryChris Fry Guildford, United Kingdom More articles by this author , Bruce MontgomeryBruce Montgomery Frimley, United Kingdom More articles by this author , and Changhao WuChanghao Wu Guildford, United Kingdom More articles by this author View All Author Informationhttps://doi.org/10.1016/j.juro.2012.02.320AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookTwitterLinked InEmail INTRODUCTION AND OBJECTIVES Urothelium has been recognised as an active sensory structure and pivotal to the signal transduction is ATP release from the urothelium. However, the functional pathways for ATP release mediated by the receptors identified in the urothelium are poorly understood and its paracrine action has never been characterised. This study used native urothelium to test the hypothesis that major urothelial receptors exert significant control over ATP release which further elicits a paracrine effect. METHODS Human bladder mucosal samples were obtained by cystoscopy with ethics approval and patient consent. Guinea-pig (2-3 month old) urothelial sheets and muscle strips were isolated by blunt dissection. Urothelial cells were freshly dissociated from the urothelial sheets. All preparations were superfused in a physiological saline. A tension transducer was used to record contractions. ATP was measured by sampling the superfusate adjacent to the urothelium and using a luciferin-luciferase assay. Intracellular Ca2+ was measured in urothelial cells with Fura-2. RESULTS Substantial intrinsic ATP release was observed in human (369±97 pmoles/g tissue, n=9 subjects) and guinea-pig urothelium (563±78 pmoles/g, n=46 bladders). P2Y specific agonist UTP increased ATP release (human: 308±34% of control, n=5, p<0.01; guinea-pig: 268±61%, n=11, p<0.01). P2X agonist α,β-methylene ATP had no effect. These effects were mirrored by UTP-evoked intracellular Ca2+ rise in urothelial cells. Muscarinic activator carbachol also generated ATP release (human: 512±121% of control, n=7, p<0.05; guinea-pig: 195±23%, n=38, p<0.01). This effect was mimicked by M2 preferential agonist oxotremorine and antagonised by methoctramine (M2), but not by 4-DAMP (M3). Carbachol produced little effect on intracellular Ca2+ but generated mucosal contractions that correlated with ATP release. Urothelium-attached but not urothelium-denuded smooth muscle exhibited intrinsic contractile activities that were augmented by sub-threshold concentrations of carbachol which had little effect on the smooth muscle; these activities were attenuated by desensitizing P2X receptors on the smooth muscle. CONCLUSIONS Purinergic and muscarinic neurotransmitters exert significant control over urothelial ATP release, mediated by P2Y and M2 receptors. Released ATP produces a paracrine effect on the underlying structures. Whilst intracellular Ca2+ rise may contribute to ATP release via purinergic receptors, mucosal contraction is more likely to underlie ATP release by muscarinic receptors. © 2012 by American Urological Association Education and Research, Inc.FiguresReferencesRelatedDetails Volume 187Issue 4SApril 2012Page: e107 Advertisement Copyright & Permissions© 2012 by American Urological Association Education and Research, Inc.MetricsAuthor Information Guiping Sui Guildford, United Kingdom More articles by this author Chris Fry Guildford, United Kingdom More articles by this author Bruce Montgomery Frimley, United Kingdom More articles by this author Changhao Wu Guildford, United Kingdom More articles by this author Expand All Advertisement Advertisement PDF DownloadLoading ...

Muscarinic receptors activity in the perirhinal cortex and hippocampus has differential involvement in the formation of recognition memory

In this work we probed the effects of post-trial infusions of the muscarinic receptor antagonist scopolamine on object recognition memory formation. Scopolamine was infused bilaterally immediately after the sample phase in the perirhinal cortex or dorsal hippocampus and animals were tested for short-term (90min) or long-term (24h) memory. Results showed that scopolamine impaired short-term memory when injected in either the perirhinal cortex or hippocampus. Nevertheless, scopolamine disrupted long-term memory when administrated in the perirhinal cortex but not when applied in the hippocampus. Long-term memory was unaffected when scopolamine was infused 160min after the sample phase or 90min before test phase. Our data indicate that short-term recognition memory requires muscarinic receptors signaling in both the perirhinal cortex and hippocampus, whereas long-term recognition memory depends on muscarinic receptors in the perirhinal cortex but not hippocampus. These results support a differential involvement of muscarinic activity in these two medial temporal lobe structures in the formation of recognition memory.

Muscarinic properties of compounds related to arecaidine propargyl ester.

A series of new analogues of the arecaidine propargyl ester (CAS 35516-99-5), APE, 1a) with alcohols consisting of 4 or 5 carbon atoms were investigated at muscarinic receptor subtypes. The muscarinic activity of the quaternary and tertiary salts of the APE-related compounds were assayed on the isolated guinea-pig ileum (M3 receptor subtype) and guinea-pig left atria (M2 receptor subtype) as well as on rabbit isolated vas deferens (M1 receptor subtype). The structural variations made in the APE molecule, replacing the triple bond in the ester side chain with structures such as double bond, an allene moiety, a single bond, a cyclopropyl group or two triple bonds should alter the selectivity and potency in favour of the M2 subtype. Enhanced, though modest, selectivity for M2 receptors was achieved with the 2-butynyl ester 2a. The other structural variations resulted in a loss of potency, but not necessarily of efficacy.

Effects of combined estrogen and progesterone replacement treatment on detrusor contractility and histology in oophorectomized rats

The aim of this study was to investigate the effects of estrogen plus progesterone treatment administered after surgical menopause on morphological and muscarinic receptor sensitivity in detrusor muscle bands of rats. A total of 40 female Albino-Wistar rats were assigned into two groups to undergo surgical menopause with sham operation (n = 10) and bilateral oophorectomy (n = 30). Thirty oophorectomized rats were assigned into three groups for different treatments administered for 8 weeks beginning from the 7th postoperative day. At the end of the 8th postoperative week, laparotomy was performed in all of them and the urinary bladders were excised and investigated with light microscope. All statistical analysis and graphs were performed using GraphPad Prism version 4. P < 0.05 was considered to be statistically significant. It was determined that average bodyweights were increased to some extent in oophorectomized and sham groups after treatment and while it was more marked in the group receiving estrogen therapy, bodyweights were decreased in the group receiving estrogen plus progesterone therapy. However, this was not statistically significant. No statistically significant difference was determined between the sham, placebo and 17-β estradiol treatment groups regarding contractile response of urinary bladder tapes to carbachol (P > 0.05). Contractile responses of urinary bladder tapes of the estrogen plus progesterone treatment group were found to be statistically lower than the placebo group (P < 0.05). Our in vitro findings demonstrate that treatment with estrogen plus progesterone decreases muscarinic activity in oophorectomized rats, whereas estrogen-only treatment does not. Our study establishes the basis for further studies to answer whether combination of estrogen and progesterone treatment can restore detrusor overactivity associated with sex hormone defects seen at menopause.

Cholinergic modulation on spike timing-dependent plasticity in hippocampal CA1 network

Cholinergic inputs from the medial septum are projected to pyramidal neurons in the hippocampal CA1 region and release acetylcholine (ACh) from their terminals. The cholinergic inputs are considered to be integrated with sensory inputs and to play a crucial role in learning and memory. Meanwhile, it has been reported that the relative timing between pre- and post-synaptic spiking determines the direction and extent of synaptic changes in a critical temporal window, a process known as spike timing-dependent plasticity (STDP). Positive timing where excitatory postsynaptic potential (EPSP) precedes the postsynaptic action potential induces long-term potentiation (LTP) while negative timing where EPSP follows the action potential induces long-term depression (LTD). To investigate the influence of muscarinic activation by cholinergic inputs on synaptic plasticity, STDP-inducing stimuli were applied during the muscarinic induction of a slow EPSP followed by repetitive stimulation in the stratum oriens. As a result, LTP was facilitated and LTD was abolished by the muscarinic activation. Furthermore, interestingly, LTP was also facilitated and LTD was switched to LTP with an increase in ACh concentration following application of the cholinesterase inhibitor eserine. These results indicate that the orientation of plasticity was shifted for potentiation by muscarinic activation. On the other hand, the application of excess ACh concentration completely suppressed STDP, LTP and LTD. In addition, STDP was suppressed in the presence of atropine, a muscarinic ACh receptor antagonist. Taken together, the findings suggest that synaptic plasticity modulation depends on the amount of cholinergic inputs. The modulation of synaptic plasticity by muscarinic activation might be an important stage in the integration of top-down and bottom-up information in hippocampal CA1 neurons.

The importance of stereoisomerism in muscarinic activity.

Abstract The muscarinic activities and rates of hydrolysis by acetylcholinesterase of the optical isomers of the acetyl-α- and acetyl-β-methylcholines of known configuration have been determined. The results have been correlated and the possible characteristics of muscarinic receptors outlined.

Actions of the muscle relaxant chandonium iodide on guinea-pig ileum and vas deferens preparations.

The effects of the newly synthesized neuromuscular blocking agent, chandonium iodide (17a-methyl-3beta-pyrrolidino-17a-aza-D-homo-5-androstene dimethiodide) have been investigated on guinea-pig isolated ileum and vas deferens preparations. On the ileum, chandonium (0-1-10-0 mug ml(-1); 1-6 X 10(-7) M-1-6 X 10(-5) M) had weak muscarinic receptor blocking action (pA2 is 5-7), but no antihistamine properties at the concentration tested. No evidence for anticholinesterase actions was found. On the vas deferens, chandonium (10-50 mug ml(-1); 1-6-8-1 X 10(-5) M) potentiated responses to exogenous noradrenaline; responses to electrical stimulation were potentiated only in the presence of 50 mug ml(-1) chandonium. No adrenoceptor or adrenergic neuron blockade was found. The results provide evidence that chandonium acts selectively at acetylcholine receptors and that it is more active as a nicotinic receptor antagonist than as a muscarinic receptor antagonist.

Investigation of the antimuscarinic and other actions of proadifen in-vitro.

The possibility that proadifen (SKF 525A) antagonizes endothelium-dependent relaxations to acetylcholine (ACh) in isolated blood vessel preparations via a muscarinic receptor blocking action has been investigated. In phenylephrine-contracted rat isolated aortic ring preparations (with endothelium), proadifen (10-100 microM) shifts ACh relaxant curves to the right without affecting the maximal response, yet endothelium-dependent relaxations to ATP are unaffected. At lower concentrations, proadifen (1-10 microM) antagonizes negative inotropic responses to ACh and ATP in guinea-pig left atria, antagonizes contractile responses to ACh and elevated [K+] in guinea-pig ileal preparations, displaces (-)-[3H]quinuclidinyl benzilate from muscarinic binding sites in membrane homogenates of guinea-pig ileal longitudinal muscle and reduces contractile responses to elevated [K+] in rat aortic ring preparations. It is concluded that proadifen may possess complex interactions with muscarinic receptors and Ca2+ entry blocking properties in concentrations 10-100 times lower than those reported to inhibit cytochrome P450-catalysed reactions.

Blockade by 4-Aminopyridine of the Muscarinic-receptor-mediated Responses of Guinea-pig Atria and Trachea

The potassium channel blocker 4-aminopyridine (4-AP) has been shown to antagonize the negative inotropic effects of muscarinic receptor agonists on atrial preparations. This is consistent with muscarinic agonists mediating their negative inotropy through the opening of potassium channels. In the present study, the possibility of a direct antagonism of the muscarinic receptor was examined by comparing the effects of 4-AP upon the responses to carbachol of isolated left atria (negative inotropy) and tracheal spirals (contraction) from reserpine pretreated guinea-pigs. The latter response is K+ channel-independent. The concentration-response curve for carbachol on the paced left atria was displaced 520-fold to the right by 4-AP (10 mM). 4-AP alone caused dose-related contractions of the tracheal spirals. Carbachol-induced contractions were, however, superimposed upon the raised tone and there were substantial rightwards shifts of the concentration-response curves of 4.7- and 31.4-fold by 1 and 10 mM of 4-AP, respectively. Thus 4-AP appears to have muscarinic receptor antagonistic blocking properties. The blockade of the atrial responses was, however, substantially greater and could be attributed to an additional blockade of muscarinic receptor-linked potassium channels. The negative inotropic responses of the A1-adenosine receptor agonist L-phenylisopropyladenosine (L-PIA) were also antagonized by 4-AP, but to a lesser extent than for carbachol. After allowing for the muscarinic receptor blocking activity of 4-AP, carbachol was still antagonized more effectively than L-PIA. This could be due to the presence of a K+ channel-independent component in the response to L-PIA which is not susceptible to 4-AP.

The pharmacology of propionyl atropine methyl nitrate.

Abstract A series of esters of atropine and hyoscine, both in the tertiary and quaternary form, have been examined for their muscarinic and ganglionic blocking and neuromuscular transmission inhibiting activity. In addition the effect on the gastric secretion has been studied by the Shay rat method. Figures are also given for toxicities. In general, the pharmacology of the quaternary derivatives is qualitatively and quantitatively the same as the parent substances. The quaternary compounds retain a considerable amount of their tertiary activity but in addition become relatively strong ganglionic blocking agents. Most of the compounds reduce the volume and acidity of the gastric juice.

Some actions of 4,17a-dimethyl-4,17a-diaza-D-homo-5alpha-androstane dimethiodide (HS-342), a new neuromuscular blocking drug.

Abstract The effects of the newly-synthesized bisquaternary steroid 4,17a-dimethyl-4,17a-diaza-D-homo-5α-androstane dimethiodide (HS-342) have been investigated on the rat isolated phrenic nerve-hemidiaphragm, chick biventer cervicis and intact cat tibialis anterior muscle preparations, on the guinea-pig isolated ileum and on the rat isolated vas deferens preparations. The anticholinesterase activity of HS-342 was also measured. In the skeletal muscle preparations HS-342 exhibited properties of a post-junctionally active neuromuscular blocking agent of the non-depolarizing class. In the anaesthetized cat HS-342 exhibited approximately equal activity to that of tubocurarine, but its duration of action was about one-third of that of tubocurarine. HS-342 exhibited low anticholinesterase activity, low muscarinic blocking activity and apparently had no effect on the adrenergic neuroeffector junction studied.

The effects of tremorine and oxotremorine in hyperthyroid mice.

Abstract Hyperthyroid mice exhibit only transient low-level tremor and hypothermia in response to tremorine and oxotremorine, yet the peripheral effects of these two agents are similar in intensity and duration to those elicited in control animals. These findings support the theory that tremor and hypothermia are both central effects and separate from the peripheral muscarinic actions of tremorine and oxotremorine. Of several possible explanations for the reduction in tremor and hypothermia, the most likely appears to be a potentiation of central adrenergic mechanisms in the hyperthyroid mice.

Scientific Meetings: Some Aspects of Pharmacological Chemistry

Abstract A Summary of Three University Lectures given in the Department of Pharmacology, University College, London. January-February, 1949. Only the symptomatic drugs were considered in this series. The first lecture dealt with the significance of total chemical constitution and the theories of modes of drug action1. Examples indicating constitution-action-parallelism were quoted, but it was emphasised that much more data were required to produce a more secure basis for a rational approach. In the next lecture the work on the analgesic substances, particularly on morphine, was discussed2. On the third occasion the chemistry of the parasympathomimetics and parasympatholytics was considered. These drugs are based on acetylcholine, which is not strange to the organism. Acetylcholine has three pharmacological actions, muscarinic, nicotinic and under certain conditions, curare-like. As an ester, it is hydrolysed by the enzymes, cholinesterases of blood cells and plasma and transformed into the weakly active choline. Its re-synthesis may be due to the action of an acetylase, the co-enzyme of which may consist partly of pantothenic acid. Its existence was demonstrated by Feldberg and Mann in brain and by Bülbring and Burn in rabbit heart auricles. There exists also an inactive acetylcholine complex, the formation of which may partly explain the disappearance of the neurohormone. That it also occurs in plants was shown by Feldberg’s discovery of acetylcholine in the stinging nettle. The instability and amphotropic properties of acetylcholine stimulated the pharmacological chemist to synthesise substances with more sustained and clear-cut action. When considering substances which, like acetylcholine, possess parasympathomimetic action, Pfeiffer’s theory of prosthetic3 distances comes to mind, postulating an optimum distance between the N-methyl group and the two oxygen atoms. Thus it can be understood why three natural drugs–muscarine, pilocarpine, and arecoline–show, on the whole, lower activities and have never gained clinical importance. Virtual changes of the acetylcholine molecule itself have produced evidence that the free aminoalkanols are very much weaker and that, for the existence of full muscarinic action, the alcohol group must be esterified and the nitrogen carry at least two methyl groups. When the chain of the alcohol or the acid is elongated, activity falls considerably. There is one exception, and that is when the aminoalkanol chain is branched, as in mecholyl which shows strong muscarinic effect. Acetic acid has been exchanged against other acidic residues, such as carbamic acid in carbachol, thioacetic acid, etc. Transformation of the choline ester into choline ethers produces more stable but less active compounds. An ether-like product was made by Fourneau (2268 F) and found to be very potent indeed. Another ether-like compound is Esmodil which contains a double bond like arecoline and croton betaine methyl ester. The latter, carrying the nitrogen on the acidic side, is very much like acetylcholine though weaker. While choline itself shows very little activity, 3-hydroxymethylpyridine, the alcohol corresponding to nicotinic acid, possesses interesting parasympathomimetic properties. Other aromatic compounds with phenolic groups in place of alcoholic hydroxy groups, show remarkable properties, especially when esterified with carbamic acid.

Conditions affecting the interaction of cholinergic agents on the longitudinal muscle of the guinea-pig ileum--unexpected effects with hexamethonium.

Abstract Affinity constants were determined for atropine and N-methyl atropine, using both acetylbetamethylcholine and carbaminoylcholine as agonists, on plexus-containing and plexus-free preparations of the guinea-pig ileum. Hexamethonium (1 times 10−4M) decreased the apparent affinity constant of atropine on the plexus-free preparation when carbaminoylcholine but not acetylbetamethylcholine was the agonist. With the latter agonist both antiacetylcholine drugs were equiactive. Hexamethonium’s selective action was associated with a significant shift to the left of the log dose-response curve to carbaminoylcholine on the plexus-free preparation. While others have observed that hexamethonium may be atropine-like, samples of hexamethonium have been reported as being contaminated with a “depolarizing compound”, which may account for the results presented in this study. Nevertheless, the present results are consistent with an allosteric rather than a direct binding mechanism in the blocking action of atropine. The observation that the agonist, acetylbetamethylcholine, was not similarly affected by hexamethonium (or any possible contaminant) suggests that agents with primarily muscarinic activity may not be so affected. This study is another example emphasizing the care needed in interpreting data obtained with the aid of “pharmacological tools”.

Synthesis and pharmacology of 1-(2-thioxopyrrolidino)-4-pyrrolidino-2-butyne, the thiolactam analogue of oxotremorine.

Abstract The thiolactam analogue of oxotremorine (I) has been prepared and examined for muscarinic activity in mice and on guinea-pig ileal preparations. It is inactive in itself, but is transformed in vivo to oxotremorine (II), which has been identified in the urine of mice. The urine also contains 1-(2-oxopyrrolidino)-4-pyrrolidino-2-butanone (III) which is formed non-enzymatically from I under acidic conditions, but does not display any acetylcholine-like properties in vivo or in vitro. Oxotremorine is not formed from I in vitro.

Central muscarinic activation elicits compulsive drinking behaviour in the rat

Injection of bethanechol into the lateral cerebral ventricle of the rat induces a marked increase in drinking, within 30 min from administration. The response is dose-related, maximal water intake (6.1 +/- 0.55 mL; mean +/- s.e.) occurring at 10 micrograms of bethanechol. Peripheral administration of the agonist (up to 3 mg kg-1 i.p.) fails to elicit drinking. Among several specific antagonists only antimuscarinic drugs produced a significant inhibition of the response, suggesting that the compulsive drinking behaviour in the rat is caused by activation of central muscarinic receptors. The drinking behaviour emerges as a reliable test to assess central muscarinic activity of both agonists and antagonists.

A comparison of the contractile properties of smooth muscle from pig urethra and internal anal sphincter

Smooth muscles from the urethra and internal anal sphincter (IAS) play an essential role in the maintenance of urinary and fecal continence. Any damage in these muscles may cause serious problems. The aim of this study was to directly compare the contractile properties of pig urethra and IAS taken from the same animal. Smooth muscle strips of urethra and IAS dissected from the same pig were transferred to organ baths superfused with Krebs' solution, loaded with 1 g tension and equilibrated for 1 hr. Carbachol and phenylephrine response curves and EFS responses were elicited in the absence and presence of inhibitors. Both tissues developed tone during the 1 hr equilibration period. Carbachol (3 × 10(-6)-10(-3) M) contracted urethra whilst relaxing IAS. Guanethidine (10(-6) M) inhibited the carbachol responses in both tissues. L-NOARG (10(-4) M) decreased carbachol responses in IAS, but not in urethra. Phenylephrine (3 × 10(-6)-10(-2) M) contracted both tissues. EFS (1-40 Hz) induced a contractile response in urethra which was decreased with guanethidine (10(-6) M) and further blocked by atropine (10(-6) M). In the presence of both, a relaxation response was observed that is sensitive to NOS inhibitors especially at low frequencies. EFS induced a relaxation followed by a contraction in IAS strips. This contraction was blocked by guanethidine but not by atropine, and the remaining relaxation at 20 Hz was decreased with L-NOARG and increased with L-arginine. There are differences between urethra and IAS in terms of muscarinic activation and neural innervation, relevant for pharmacotherapy.

ChemInform Abstract: Synthesis and Muscarinic Activities of 1,2,4-Thiadiazoles.

A series of novel 1,2,4-thiadiazoles bearing a mono- or bicyclic amine at C5 were prepared. Quinuclidine and 1-azabicyclo[2.2.1]heptane derivatives were synthesized by reaction of the lithium enolate of the 3-methoxycarbonyl compounds followed by ester hydrolysis and decarboxylation. The receptor-binding affinity and efficacy of these compounds as muscarinic ligands was assessed by radioligand binding assays using [3H]-N-methylscopolamine and [3H]oxotremorine-M. Optimal agonist affinity was observed for 3'-methyl compounds. Smaller substituents (H) retained efficacy with reduced affinity while larger groups led to substantially lower efficacy. The observed binding affinity was influenced both by the conformational energy of rotation around the C3-C5' bond and the steric requirement of the mono- or bicyclic amine.

Carbachol-Induced Long-Term Synaptic Depression Is Enhanced During Senescence at Hippocampal CA3–CA1 Synapses

Dysregulation of the cholinergic transmitter system is a hallmark of Alzheimer's disease and contributes to an age-associated decline in memory performance. The current study examined the influence of carbachol, a cholinergic receptor agonist, on synaptic transmission over the course of aging. Extracellular excitatory postsynaptic field potentials were recorded from CA3-CA1 synapses in acute hippocampal slices obtained from young adult (5-8 mo) and aged (22-24 mo) male Fischer 344 rats. Bath application of carbachol elicited a transient depression of synaptic transmission, which was followed by a long-lasting depression (CCh-LTD) observed 90 min after carbachol cessation in both age groups. However, the magnitude of CCh-LTD was significantly larger in senescent animals and was attenuated by N-methyl-D-aspartate receptor blockade in aged animals. Blockade of L-type Ca(2+) channels inhibited CCh-LTD to a greater extent in aged animals compared to young adults. Finally, the expression of CCh-LTD was dependent on protein synthesis. The results indicate that altered Ca(2+) homeostasis or muscarinic activation of Ca(2+) signaling contribute to the enhanced CCh-LTD during senescence.