Modulation of rhythmical slow activity, long-term potentiation and memory by muscarinic receptor agonists

Neuropharmacology of Cevimeline and Muscarinic Drugs-Focus on Cognition and Neurodegeneration

At present, Alzheimer’s disease (AD) and related dementias cannot be cured. Therefore, scientists all over the world are trying to find a new approach to prolong an active life of patients with initial dementia. Both pharmacological and non-pharmacological pathways are investigated to improve the key symptom of the disease, memory loss. In this respect, influencing the neuromodulator acetylcholine via muscarinic receptors, such as cevimeline, might be one of the therapeutic alternatives. The purpose of this study is to explore the potential of cevimeline on the cognitive functions of AD patients. The methodology is based on a systematic literature review of available studies found in Web of Science, PubMed, Springer, and Scopus on the research topic. The findings indicate that cevimeline has shown an improvement in experimentally induced cognitive deficits in animal models. Furthermore, it has demonstrated to positively influence tau pathology and reduce the levels of amyloid-β (Aβ) peptide in the cerebral spinal fluid of Alzheimer’s patients. Although this drug has not been approved by the FDA for its use among AD patients and there is a lack of clinical studies confirming and extending this finding, cevimeline might represent a breakthrough in the treatment of AD.

Comparison of pro-amnesic efficacy of scopolamine, biperiden, and phencyclidine by using passive avoidance task in CD-1 mice

INTRODUCTION

Memory disorders accompany numerous diseases and therapies, and this is becoming a growing medical issue worldwide. Currently, various animal models of memory impairments are available; however, many of them require high financial outlay and/or are time-consuming. A simple way to achieve an efficient behavioral model of cognitive disorders is to inject defined drug that has pro-amnesic properties. Since the involvement of cholinergic and glutamatergic neurotransmission in cognition is well established, the utilization of a nonselective muscarinic receptor antagonist, scopolamine (SCOP), a selective M1 muscarinic receptor antagonist, biperiden (BIP), and a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, phencyclidine (PCP) seems to be reliable tools to induce amnesia. As the determination of their effective doses remains vague and the active doses vary significantly in laboratory settings and in mouse species being tested, the aim of this study was to compare these three models of amnesia in CD-1 mice.

METHODS

Male Swiss Albino mice were used in passive avoidance (PA) test. All the compounds were administered intraperitoneally (ip) at doses 1mg/kg, 5mg/kg, and 10mg/kg (SCOP and BIP), and 1mg/kg, 3mg/kg, and 6mg/kg (PCP).

RESULTS

In the retention trial of the PA task, SCOP and PCP led to the reduction of step-through latency at all the tested doses as compared to control, but BIP was effective only at the dose of 10mg/kg.

CONCLUSION

This study revealed the effectiveness of SCOP, PCP, and BIP as tools to induce amnesia, with the PCP model being the most efficacious and SCOP being the only model that demonstrates a clear dose-response relationship.

Behavioral tagging: A novel model for studying long-term memory

New information acquired by our brain is stored in the form of two types of memories: short term memory (STM) and long term memory (LTM). Initially, Synaptic and Capture hypothesis has been proposed to describe the synaptic changes that occur during memory formation. However, recently Behavioral Tagging hypothesis was proposed that relies on the setting of a learning tag and the synthesis of plasticity related proteins (PRPs). Behavioral Tagging has its roots in Synaptic and Capture hypothesis. It seeks to explain that how a learning tag produced as a result of weak training can be paired up with PRPs (formed as a result of novelty) and can lead to long lasting memories. We have focused on describing behavioral paradigms that have been used for establishing the model of "Behavioral Tagging" and the molecules which qualify for potential PRP candidature.

Effects of Pb2+ on muscarinic modulation of glutamatergic synaptic transmission in rat hippocampal CA1 area

Lead (Pb(2+)) is a pollutant commonly found in the environment. It causes a wide variety of detrimental effects on developing central nervous system. However, the mechanisms of its neurotoxicity remained to be elucidated. In hippocampus, the muscarinic cholinergic system modulates certain forms of synaptic transmission and plasticity, and plays an important role in learning and memory. In this study, the effects of Pb(2+) on muscarinic modulation of glutamatergic synaptic transmission in hippocampal CA1 area were investigated using the conventional whole-cell patch-clamp technique in rat hippocampal slices. In the presence of nicotinic antagonist mecamylamine, carbachol (CCh), a cholinergic agonist, concentration-dependently inhibited glutamatergic excitatory postsynaptic currents (EPSCs), enhanced paired-pulse facilitation (PPF) and the response to 10-Hz pulse-trains. The analysis of the spontaneous excitatory postsynaptic currents (sEPSCs) showed the activation of muscarinic receptors by CCh decreased the frequency, amplitude and decay time of sEPSCs. The 10 microM Pb(2+) depressed the inhibition of EPSCs by CCh, reduced the CCh-induced enhancement of PPF and the response to 10-Hz pulse-trains, and also affected the modulation of sEPSCs by CCh. The results suggested that the activation of muscarinic acetylcholine (ACh) receptors in hippocampus could modulate glutamatergic synaptic transmission, while Pb(2+) exposure would lead to an alteration of muscarinic modulation, which might be involved in the Pb(2+)-induced impairment of synaptic transmission and plasticity during learning and memory.

Electrophysiological and neurochemical characterization of the effect of repeated treatment with milnacipran on the rat serotonergic and noradrenergic systems

The present study was undertaken to elucidate the effects of repeated treatment with milnacipran, a serotonin (5-HT) and noradrenaline (NA) reuptake inhibitor (SNRI), on the synaptic plasticity in the hippocampal CA1 field, focusing on the interaction between the serotonergic and noradrenergic system. Repeated treatment with milnacipran (30 mg/kg, i.p. after 30 mg/kg, p.o. x 14 days) completely restored the suppression of the long-term potentiation (LTP) induced by single milnacipran treatment (30 mg/kg, i.p.). Single and repeated milnacipran increased to a similar extent extracellular NA in the hippocampus. Single milnacipran increased extracellular 5-HT and this effect tended to be enhanced by repeated treatment. The restoration of LTP and facilitation of the 5-HT level were not shown after repeated treatment with a selective 5-HT reuptake inhibitor (SSRI) fluvoxamine (30 mg/kg, p.o. x 14 days). These results suggest that milnacipran-induced restoration of LTP suppression is responsible for the enhancement of 5-HT neurotransmission, which appears to be associated with noradrenergic neuronal activity. In addition, the 5-HT1A receptor agonist tandospirone-induced suppression of LTP was completely blocked by repeated treatment with milnacipran, indicating the possibility that this reversal effect is due to the functional changes in postsynaptic 5-HT1A receptors. Taken together, the present data suggest that the interaction between the serotonergic and noradrenergic mechanism play an important role in the modulation of synaptic plasticity caused by repeated treatment with milnacipran, which may be implicated in the therapeutic effects of SNRI on psychiatric disorders.

Serotonergic modulation of psychological stress-induced alteration in synaptic plasticity in the rat hippocampal CA1 field

In order to elucidate possible involvement of the serotonergic neuronal system in the stress-induced alteration in synaptic plasticity, the effects of contextual fear conditioning (CFC) on long-term potentiation (LTP) in the hippocampal CA1 field were examined in 5-HT-depleted rats by pretreatment with 5,7-dihydroxytryptamine (5,7-DHT, 200 microg/rat, i.c.v.). LTP induction was suppressed by footshock (FS) stimulation in 5-HT-lesioned rats and vehicle-treated controls. When rats were exposed to CFC, which was received 24 h after FS stimulation, LTP was also blocked in both-treated groups. CFC-induced impairment of LTP, however, significantly attenuated in 5-HT-lesioned rats when compared with that in controls. Fear-related freezing behavior after FS stimulation occurred similarly in both treated groups, whereas the behavior observed during exposure to CFC significantly reduced in 5-HT-lesioned rats. These results suggest that the serotonergic mechanism is involved in the psychological stress-induced alteration in synaptic plasticity, which appears to be associated with fear-related behavior.

Acetylcholine release in hippocampus and striatum during testing on a rewarded spontaneous alternation task

The present experiment tested male Sprague-Dawley rats for spontaneous alternation performance in a food-rewarded Y-shaped maze. Microdialysis samples, later assessed for acetylcholine concentration, were collected from the hippocampus and striatum of each rat prior to and during testing; testing sessions lasted 20 min. Early in testing, rats alternated at a rate of 72%. Alternation scores increased throughout the 20-min testing session and reached 93% during the last 5 min. The behavioral findings suggest that, during testing, rats changed the basis for their performance from a spatial working memory strategy to a persistent turning strategy. ACh release in both hippocampus and striatum increased at the onset of testing. Increases in ACh release in the striatum began at 18% above baseline during the first 5 min of testing and steadily increased reaching 58% above baseline during the final 5 min. The progressive rise of striatum ACh release during testing occurred at about the time rats adopted a persistent turning strategy. In contrast, ACh release in the hippocampus increased by 50% with the onset of testing and remained at this level until declining slightly during the last 5 min of testing. The relative changes in ACh release in the striatum and hippocampus resulted in a close negative relationship between the ratio of ACh release in the hippocampus/striatum and alternation scores.

Chronic treatment with milnacipran reverses the impairment of synaptic plasticity induced by conditioned fear stress

RATIONALE

Recent studies have focused on neural plasticity at the cellular and molecular levels in the etiology and treatment of stress-related disorders; however, there are no reports concerning modulation of synaptic plasticity in the hippocampus underlying therapeutic effects of antidepressants and/or anxiolytics.

OBJECTIVES

To elucidate the functional interaction between the stress-induced alteration of synaptic plasticity and therapeutic effects, we examined the anxiolytic mechanism(s) of milnacipran, focusing on modulation of long-term potentiation (LTP) in the hippocampal CA1 field.

METHODS

Rats that received footshock stimulation five times (intensity, 0.5 mA; duration, 2 s; shock interval, 30 s) for 5 days were treated with milnacipran (30 mg kg(-1), p.o.) or vehicle for 14 days. On the 15th day, rats were subjected to conditioned fear stress (CFS) to evaluate freezing behavior. Separate from the behavioral study, electrophysiological approach was performed to evaluate the synaptic efficacy under anesthesia.

RESULTS

Exposure to CFS suppressed LTP in the CA1 field. Chronic treatment with milnacipran (30 mg kg(-1), i.p. after 30 mg kg(-1) day(-1), p.o. x14 days), but not acute treatment (30 mg kg(-1), i.p. after vehicle 5 ml kg(-1) day(-1), p.o. x14 days), reduced freezing behavior and reversed the impairment of LTP induced by CFS.

CONCLUSION

The present data suggest that a correspondence exists between fear-related behavior and synaptic plasticity in the hippocampus. In other words, anxiolytic mechanism(s) of chronic treatment with milnacipran may be explained by reversal effects on the psychological stress-induced impairment of synaptic plasticity.

Early postnatal stress alters the 5-HTergic modulation to emotional stress at postadolescent periods of rats

Several lines of evidence have shown that traumatic events during the early postnatal stage might precipitate long-lasting alteration in the functional properties underlying emotional expression. The aim of the present study was to examine whether the early postnatal stress alters the 5-HTergic mechanism underlying regulation of emotional stress, focusing on the 5-HT(1A) receptor-mediated synaptic responses in adult rats. Pups were exposed to aversive stimulus, footshock (FS) at the postnatal period of the second (2W) and the third week (3W). At postadolescent period (10-12-week-old), electrophysiological and behavioral studies were performed. The 5-HT(1A) receptor agonist tandospirone (10 mg/kg body wt, i.p.) blocked the long-term potentiation (LTP) in the hippocampal CA1 field of 3W-FS, as well as non-FS control, whereas this inhibition was not observed in 2W-FS group. Fear-related freezing behavior observed during exposure to contextual fear conditioning markedly attenuated in 2W-FS, but not in 3W-FS. These data suggest that aversive stress exposed at 2W is attributable to changes in the 5-HT(1A) receptor-mediated synaptic plasticity, which may be responsible for the attenuation of freezing behavior. Thus, 5-HT(1A) receptors appear to play a key role in the 5-HTergic mechanism underlying regulation of emotional stress on the postnatal development of the brain. In other words, the second postnatal week may be the "critical period" for establishing proper behavioral responses to emotional stress in adult rats. (c) 2005 Wiley-Liss, Inc.

Facilitation of memory for extinction of drug-induced conditioned reward: role of amygdala and acetylcholine

These experiments examined the effects of posttrial peripheral and intra-amygdala injections of the cholinergic muscarinic receptor agonist oxotremorine on memory consolidation underlying extinction of amphetamine conditioned place preference (CPP) behavior. Male Long-Evans rats were initially trained and tested for an amphetamine (2 mg/kg) CPP. Rats were subsequently given limited extinction training, followed by immediate posttrial peripheral or intrabasolateral amygdala injections of oxotremorine. A second CPP test was then administered, and the amount of time spent in the previously amphetamine-paired and saline-paired apparatus compartments was recorded. Peripheral (0.07 or 0.01 mg/kg) or intra-amygdala (10 etag/0.5 microL) postextinction trial injections of oxotremorine facilitated CPP extinction. Oxotremorine injections that were delayed 2 h posttrial training did not enhance CPP extinction, indicating a time-dependent effect of the drug on memory consolidation processes. The findings indicate that memory consolidation for extinction of approach behavior to environmental stimuli previously paired with drug reward can be facilitated by posttrial peripheral or intrabasolateral amygdala administration of a cholinergic agonist.

Changes in acetylcholine extracellular levels during cognitive processes

Measuring the changes in neurotransmitter extracellular levels in discrete brain areas is considered a tool for identifying the neuronal systems involved in specific behavioral responses or cognitive processes. Acetylcholine (ACh) is the first neurotransmitter whose diffusion from the central nervous system was investigated and whose extracellular levels variations were correlated to changes in neuronal activity. This was done initially by means of the cup technique and then by the microdialysis technique. The latter, notwithstanding some technical limitations, makes it possible to detect variations in extracellular levels of ACh in unrestrained, behaving animals. This review summarizes and discusses the results obtained investigating the changes in ACh release during performance of operant tasks, exposition to novel stimuli, locomotor activity, and the performance of spatial memory tasks, working memory, and place preference memory tasks. Activation of the forebrain cholinergic system has been demonstrated in many tasks and conditions in which the environment requires the animal to analyze novel stimuli that may represent a threat or offer a reward. The sustained cholinergic activation, demonstrated by high levels of extracellular ACh observed during the behavioral paradigms, indicates that many behaviors occur within or require the facilitation provided by the cholinergic system to the operation of pertinent neuronal pathways.

Involvement of beta-adrenergic receptors in protein synthesis-dependent late long-term potentiation (LTP) in the dentate gyrus of freely moving rats: the critical role of the LTP induction strength

We have investigated the requirement of beta-adrenergic receptor activation and protein synthesis for the induction and specifically for the maintenance of long-term potentiation (LTP) in the dentate gyrus of freely moving rats in dependency on different LTP-induction procedures. Three tetanization paradigms were used: a relatively weak protocol A (10 bursts of 15 biphasic pulses at 200 Hz; 10-s interburst interval; 0.2-ms pulse width per phase), a stronger protocol B (as protocol A but 20 bursts and 0.25-ms pulse width) and, as the strongest condition, protocol C (2 times protocol B; inter-tetanus interval: 5 min). All protocols led to robust late-LTP in control animals. Late- but not early-LTP was protein synthesis-dependent under all tetanization conditions as indicated by the absence of long-lasting LTP when the protein synthesis inhibitor anisomycin was applied before tetanization. Application of the beta-adrenergic receptor antagonist propranolol before LTP induction prevented late-LTP when either protocol A or B but not when protocol C was used. Thus, repeated strong tetanization can compensate for the loss of beta-adrenergic receptor activation. We suggest that the results could provide a link to cellular mechanisms of memory consolidation in respect to the strength and relevance of the incoming sensory information during learning.

Age-related decreases in the response of aquaporin-5 to acetylcholine in rat parotid glands

Aquaporin-5 (AQP5) is important in salivary fluid secretion in response to cholinergic and adrenergic stimuli in rat parotid glands. We hypothesized that expression and function of AQP5 might change with age. Acetylcholine and epinephrine induced increases in AQP5 levels in the apical plasma membranes of both young adult and senescent rats. The stimulatory effect of acetylcholine, but not that of epinephrine, on AQP5 levels in the apical plasma membranes of the cells decreased markedly during aging. The quinuclidine derivative, SNI-2011, induced a persistent increase in AQP5 levels in the apical plasma membrane in the cells of both these rats. The amounts of M(3)-muscarinic receptor and Gq proteins did not decrease during aging. The age-related alteration in the responsiveness of AQP5 in the cells to these stimuli might account for the concomitant changes in nitric oxide synthase activity. These results suggest that SNI-2011 might have therapeutic benefit for the treatment of age-related xerostomia.

Fluvoxamine suppresses the long-term potentiation in the hippocampal CA1 field of anesthetized rats: an effect mediated via 5-HT1A receptors

A selective 5-HT reuptake inhibitor, fluvoxamine (10 and 30 mg/kg, i.p.) suppressed long-term potentiation (LTP) in the hippocampal CA1 field of anesthetized rats. Fluvoxamine (30 mg/kg, i.p.)-induced suppression of LTP was completely reversed by the 5-HT(1A) receptor antagonist NAN-190 (0.5 mg/kg, i.p), but not by the 5-HT(4) receptor antagonist GR 113808 (20 microg/rat, i.c.v.) and the 5-HT(7) receptor antagonist DR 4004 (10 microg/rat, i.c.v.). These data suggest that the inhibitory effect of fluvoxamine on LTP induction is mediated via 5-HT(1A) receptors.

Cholinergic modulation of synaptic physiology in deep layer entorhinal cortex of the rat

We have recently shown that cholinergic effects on synaptic transmission and plasticity in the superficial (II/III) layers of the rat medial entorhinal cortex (EC) are similar, but not identical, to those in the hippocampus (Yun et al. [2000] Neuroscience 97:671-676). Because the superficial and deep layers of the EC preferentially convey afferent and efferent hippocampal projections, respectively, it is of interest to compare cholinergic effects between the two regions. We therefore investigated the physiological effects of cholinergic agents in the layer V of medial EC slices under experimental conditions identical to those in the previous study. Bath application of carbachol (0.5 microM) induced transient depression of field potential responses in all cases tested (30 of 30; 18.5% +/- 2.3%) and rarely induced long-lasting potentiation (only 3 of 30; 20.4% +/- 3.2% in successful cases). At 5 microM, carbachol induced transient depression only (20 of 20, 48.9% +/- 2.8%), which was blocked by atropine (10 microM). Paired-pulse facilitation was enhanced during carbachol-induced depression, suggesting presynaptic action of carbachol. Long-term potentiation (LTP) could be induced in the presence of 10 microM atropine by theta burst stimulation, but its magnitude was significantly lower (9.1% +/- 4.7%, n = 15) compared to LTP in control slices (22.4% +/- 3.9%, n = 20). These results, combined with our previous findings, demonstrate remarkably similar cholinergic modulation of synaptic transmission and plasticity across the superficial and deep layers of EC.

Neuronal hypersynchronization, creativity and endogenous psychoses

I have investigated a neuronal hypersynchronism, currently included under the general subject of epilepsy, and termed interictal activity. I suggest that it is a physiological activity of the mammalian brain and propose it be termed Hyperia. After a thorough study of the extraordinary psychic manifestations of this neuronal hypersynchronism shown by mystics and artists, I have reviewed several scientific publications bearing on my hypothesis. I conclude by elaborating on a variety of cerebral hypersynchronous functions whose cause I consider to be physiological. Such behaviour is a common basis for extraordinary psychic manifestations found not only in mystics and artists, but also in patients suffering from endogenous psychoses, especially Bipolar Disorder.

Cholinergic modulation of synaptic transmission and plasticity in entorhinal cortex and hippocampus of the rat

Effects of cholinergic agents on synaptic transmission and plasticity were examined in entorhinal cortex and hippocampus. Bath application of carbachol (0.25-0.75 microM) induced transient depression of field potential responses in all cases tested (24/24 in layer III of medial entorhinal cortex slices and 24/24 in CA1 of hippocampal slices; 11.0+/-1.9% and 7.8+/-2.5%, respectively) and long-lasting potentiation in some cases (4/24 in entorhinal cortex and 12/24 in hippocampus; 33.7+/-3.7% and 32.1+/-9.9%, respectively, in successful cases). Carbachol (0.5 microM) induced transient depression, but not long-lasting potentiation, of N-methyl-D-aspartate receptor-mediated responses in entorhinal cortex. At 5 microM, carbachol induced transient depression only (55. 9+/-4.7% in entorhinal cortex and 41.4+/-2.9% in hippocampus), which was blocked by atropine. Paired-pulse facilitation was not altered during carbachol-induced potentiation but enhanced during carbachol-induced depression. These results suggest that the underlying mechanisms of carbachol-induced depression and potentiation are decreased transmitter release and selective enhancement of non-N-methyl-D-aspartate receptor-mediated responses, respectively. Long-term potentiation could be induced in the presence of 10 microM atropine by theta burst stimulation. The magnitude was significantly lower (15.2+/-5.2%, n=9) compared with control (37.2+/-6.1%, n=8) in entorhinal cortex, however. These results demonstrate similar, but not identical, cholinergic modulation of synaptic transmission and plasticity in entorhinal cortex and hippocampus.

Persistent increase in the amount of aquaporin-5 in the apical plasma membrane of rat parotid acinar cells induced by a muscarinic agonist SNI-2011

SNI-2011 induces the long-lasting increase in the amount of aquaporin-5 (AQP5) in apical plasma membranes (APMs) of rat parotid acini in a concentration-dependent manner. This induction was inhibited by p-F-HHSiD, U73122, TMB-8, or dantrolene but not by bisindolmaleimide or H-7, indicating that SNI-2011 acting at M(3) muscarinic receptors induced translocation of AQP5 via [Ca(2+)](i) elevation but not via the activation of protein kinase C. In contrast, acetylcholine induced a transient translocation of AQP5 to APMs. SNI-2011 induces long-lasting oscillations of [Ca(2+)](i) in the presence of extracellular Ca(2+). Thus, SNI-2011 induces a long-lasting translocation of AQP5 to APMs coupled with persistent [Ca(2+)](i) oscillations.