Memory modulation with peripherally acting cholinergic drugs

Glucose attenuates impairments in memory and CREB activation produced by an α4β2 but not an α7 nicotinic receptor antagonist

Glucose improves memory for a variety of tasks when administered to rats and mice near the time of training. Prior work indicates glucose may enhance memory by increasing the synthesis and release of the neurotransmitter acetylcholine in the brain. To investigate if specific acetylcholine receptor subtypes may mediate some of the memory-enhancing actions of glucose, we examined the effects of subtype-specific nicotinic acetylcholine receptor antagonists on memory in Fischer-344 rats and also examined the ability of glucose to reverse drug-induced impairments. Pre-training peripheral injections of methyllycaconitine (MLA) or dihydro-beta-erythroidine (DHβE), which are specific α7 and α4β2 nicotinic receptor antagonists, respectively, dose-dependently impaired retention latencies in an inhibitory avoidance task when tested 7-days but not 1 h after training. Immediate post-training glucose injections attenuated the impairments, but were more effective in attenuating the DHβE-induced impairments. Likewise, peripheral or direct intrahippocampal injections of MLA or DHβE dose-dependently impaired spatial working memory scores on a spontaneous alternation task. Concurrent administration of glucose reversed DHβE- but not MLA-induced impairments. CREB phosphorylation downstream of cholinergic signaling was assessed 30 min after spontaneous alternation testing and intrahippocampal drug infusions. Both MLA and DHβE impaired hippocampal CREB phosphorylation; glucose reversed DHβE- but not MLA-induced deficits. The effectiveness of glucose in reversing DHβE- but not MLA-induced impairments in behavioral performance and CREB phosphorylation suggests that activation of α7 receptors may play an important role in memory enhancement by glucose.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

The influence of mecamylamine on ethanol and sucrose self-administration

Neuronal nicotinic acetylcholine receptors (nAChRs) are believed to be critically involved in ethanol-related behaviors as well as in neurochemical responses to ethanol. However, discernment of nAChR contribution to ethanol reinforcement and consumption remains incomplete. The current studies examined the influence of the nAChR antagonist mecamylamine (MEC) on operant ethanol self-administration using a procedure that independently assessed appetitive and consumptive processes, and compared these findings to effects of MEC on sucrose self-administration. Male C57BL/6J (B6) mice were trained to respond for 30-min access to a retractable drinking tube containing either 10% v/v ethanol (10E) or 5% w/v sucrose (5S). Once trained, mice were habituated to saline injection and then treated with a series of MEC doses (0-8 mg/kg; i.p.) in a within-subject design. In a separate cohort, MEC was evaluated for its influence on locomotor activity. MEC dose-dependently reduced 10E and 5S self-administration. The suppression in ethanol intake was attributable to a reduction in bout frequency, whereas the attenuation in sucrose intake was due to a decrease in bout size. Doses of MEC (6-8 mg/kg) that altered drinking patterns were also found to impair locomotor activity. Although MEC non-selectively reduced 10E and 5S intakes in mice, there was some specificity in alterations of the underlying drinking pattern for each reinforcer. Assessment of drinking topography within an operant self-administration procedure may provide useful insights regarding the role of nAChR function in the regulation of ethanol consumption.

Petroleum ether extract of Cnidium monnieri ameliorated scopolamine-induced amnesia through adrenal gland-mediated mechanism in male rats

AIM

Our previous study indicated petroleum ether layer of Cnidium monnieri L. Cuss. (CM) and its ingredient osthole could alleviate scopolamine-induced amnesia in female rats.

MATERIALS AND METHODS

Hence, this study was desired to investigate the mechanism of the ameliorating effects of petroleum ether layer of CM on the performance impairment of inhibitory avoidance task and Morris water maze induced by scopolamine in male rats.

RESULTS

CM at 0.1-0.6g/kg orally administered 60 min before the training trial ameliorated the scopolamine-induced performance impairment on inhibitory avoidance learning and water maze in male rats. Only adrenalectomy but not peripheral cholinergic antagonist scopolamine methylbromide and catecholaminergic neurotoxin 6-hydroxydopamine blocked the ameliorating effects of CM on scopolamine-induced performance impairment in rats.

CONCLUSION

Therefore, we demonstrated that the ameliorating effects of CM on scopolamine-induced performance impairment may be related to activating the adrenal gland and central acetylcholingeric neuron, instead of peripheral nervous system.

Anti-dementia drugs and hippocampal-dependent memory in rodents

Abnormalities in hippocampal structure and function are characteristics of early Alzheimer's disease (AD). Behavioral tests measuring hippocampal-dependent memory in rodents are often used to evaluate novel treatments for AD and other dementias. In this study, we review the effects of drugs marketed for the treatment of AD, such as the acetylcholinesterase inhibitors, donepezil, rivastigmine, galantamine and the N-methyl-D-aspartic acid antagonist, memantine, in rodent models of memory impairment. We also briefly describe the effects of novel treatments for cognitive impairment in rodent models of memory impairment, and discuss issues concerning the selection of the animal model and behavioral tests. Suggestions for future research are offered.

The ameliorating effects of LiuWei Dihuang Wang on cycloheximide-induced impairment of passive avoidance performance in rats

The ameliorating effects of aqueous and ethanolic extracts of LiuWei Dihuang Wang (LDW(W) and LDW(E)) after single, 1-week or 2-week consecutive treatment on the cycloheximide-induced amnesia by using the passive avoidance task in rats were studied. After single treatment, LDW(W) and LDW(E) (1 and 2g/kg) significantly prolonged the shortened step-through latency induced by CXM and their potency was equal. LDW(W) at 1g/kg after 1-week consecutive treatment or at 0.1g/kg after 2-week consecutive treatment almost completely reversed CXM-induced amnesia. LDW(W) at any dose alone after single, 1-week or 2-week consecutive treatment did not influence the step-through latency in the training trial in rats. Furthermore, muscarinic antagonist scopolamine, peripheral cholinergic antagonist scopolamine methylbromide, serotonin precursor 5-hydroxytryptamine and serotonin releaser p-chloroamphetamine could block the ameliorating effects of LDW(W). GABA(A) receptor antagonist bicuculline and GABA(B) receptor agonist baclofen also blocked the ameliorating effects of LDW(W). These results suggest that the ameliorating effects of LDW whose potency were parallel to treatment duration might be related to activating peripheral cholinergic neuronal system and modulating the central nervous system.

Effects of luteolin on learning acquisition in rats: Involvement of the central cholinergic system

The study was conducted to investigate the ameliorating effects of luteolin on memory acquisition in rats. The effects of luteolin on scopolamine-induced impairment of passive avoidance response were evaluated primarily, as well as the role of the central nervous system through the use of central neurotoxins and central nervous antagonists. Luteolin was not reversed by scopolamine N-methylbromide (M-SCOP) but blocked the impairment of learning acquisition induced by cholinergic neurotoxin (ethylcholine aziridinium, AF64A) and muscarinic (scopolamine hydrobromide, SCOP) and nicotinic (mecamylamine, MECA) receptor antagonists. However, it did not block dopaminergic neurotoxin (6-hydroxydopamine, 6-OHDA)-induced and serotonergic neurotoxin (5,7-dihydroxytryptamine, 5,7-DHT)-induced impairments. From these results, we suggest that the attenuating effect of luteolin (10 mg/kg, i.p.) on the deficits of passive avoidance performance induced by SCOP may be related to the increases in the activities of central muscarinic and nicotinic receptors.

Dynorphin A (2-13) improves mecamylamine-induced learning impairment accompanied by reversal of reductions in acetylcholine release in rats

Accumulating evidence indicates that the endogenous opioid peptides dynorphin A (1-17) and synthetic dynorphin A (1-13) interact not only with opioid receptors but also with as yet poorly characterized non-opioid binding sites. Dynorphin A (1-13) improved impairments of learning and memory via not only kappa-opioid receptor-mediated, but also 'non-opioid' mechanisms. In the present study, the effects of des-tyrosine(1) dynorphin A (2-13) as a non-opioid metabolite of dynorphin A, and dynorphin A (1-13) on mecamylamine-induced impairment of the acquisition of learning in rats were investigated using a step-through type passive avoidance task. Further, hippocampal acetylcholine release was examined using in vivo microdialysis. Mecamylamine significantly shortened the step-through latency when given 30 min before the acquisition trial. Not only dynorphin A (1-13) but also dynorphin A (2-13) attenuated the mecamylamine-induced impairment of the acquisition of learning. The effect of dynorphin A (2-13) was not blocked by pre-treatment with nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist. Dynorphin A (2-13) completely abolished the decrease in the extracellular acetylcholine concentration induced by mecamylamine and this effect was not blocked by nor-BNI. Taken together with our previous findings, the present results may indicate that dynorphin A (2-13) improves impairment of learning and/or memory in 'non-opioid' mechanisms and dynorphin A (1-13) ameliorates impairment of the acquisition of learning via not only kappa-opioid receptor-mediated mechanisms but also 'non-opioid' mechanisms, by regulating the release of extracellular acetylcholine.

Selective 5-HT1A antagonists WAY 100635 and NAD-299 attenuate the impairment of passive avoidance caused by scopolamine in the rat

Systemic administration of the muscarinic-receptor antagonists atropine and scopolamine produces cognitive deficits in humans, nonhuman primates and rodents. In humans, these deficits resemble symptoms of dementia seen in Alzheimer's disease. The passive avoidance (PA) task has been one of the most frequently used animal models for studying cholinergic mechanisms in learning and memory. The present study examined the ability of two selective 5-HT(1A) receptor antagonists WAY 100635 and NAD-299 (robalzotan) and two acetylcholinesterase (AChE) inhibitors tacrine and donepezil to attenuate the impairment of PA retention caused by the nonselective muscarinic receptor antagonist scopolamine in the rat. Although demonstrating differences in their temporal kinetics, both WAY 100635 and NAD-299 attenuated the impairment of PA caused by scopolamine (0.3 mg/kg s.c.). Donepezil did not block the PA deficit caused by the 0.3 mg/kg dose of scopolamine, but it prevented the inhibitory effects of the 0.2 mg/kg dose of scopolamine. In contrast, tacrine was effective vs both the 0.2 and 0.3 mg/kg doses of scopolamine. These results indicate that (1). a functional 5-HT(1A) receptor antagonism can attenuate the anterograde amnesia produced by muscarinic-receptor blockade, and (2). the AChE inhibitors tacrine and donepezil differ in their ability to modify muscarinic-receptor-mediated function in vivo. These results suggest that 5-HT(1A) receptor antagonists may have a potential in the treatment of cognitive symptoms in psychopathologies characterized by reduced ACh transmission such as Alzheimer's disease.

The Effects of L-glucose on memory in mice are modulated by peripherally acting cholinergic drugs

D-Glucose improves memory in animals and humans and in subjects with memory pathologies. To date, the accepted conclusion drawn from animal research is that D-glucose improves memory via alterations in central cholinergic systems. However, recent evidence suggests that a sugar which does not cross the blood-brain barrier also facilitates memory (Talley, Arankowsky-Sandoval, McCarty, & Gold, 1999). The present study examined the effects of peripherally administered L-glucose, a stereoisomer of D-glucose, in male mice. Intraperitoneal administration of L-glucose (300 mg/kg) before testing enhanced place learning in the Morris water maze. Mice injected with L-glucose had significantly shorter escape latencies than mice injected with saline (1 ml/kg). Effects were observed on both reference memory and working memory tasks. L-Glucose did not facilitate performance on either task when it was simultaneously administered with cholinergic antagonists that are excluded from the central nervous system. Thus, simultaneous administration of either methyl-scopolamine (0.3 mg/kg), a peripherally acting muscarinic receptor blocker, or hexamethonium (1 mg/kg), a peripherally acting nicotinic receptor blocker, reversed the effect of L-glucose on memory. These findings suggest that the memory effects of l-glucose may be mediated by facilitated acetylcholine synthesis and/or release in the peripheral nervous system.

A peripheral mechanism of fever: differential sensitivity to the antipyretic action of methyl scopolamine

The organophosphate pesticide (OP) chlorpyrifos leads to an acute period of hypothermia followed by a delayed fever in the rat. Methyl scopolamine, a peripheral muscarinic antagonist, is thought to have little effect on body temperature of the rat because it does not cross the blood brain barrier. However, administration of methyl scopolamine (1 mg/kg, i.p.) during the period of chlorpyrifos-induced fever results in a rapid recovery of core temperature. This indicates a peripheral cholinergic pathway is operative in the febrile response to chlorpyrifos and possibly other modes of fever. In this study, we evaluated the possible antipyretic role of methyl scopolamine (i.p.) to a variety of stimuli that lead to fever-like responses in the rat: stress-induced (handling and cage switch), chlorpyrifos-induced (15 mg/kg, p.o.), nocturnal-induced, and lipopolysaccharide (LPS)-induced fever (50 microg/kg, i.p.). Methyl scopolamine led to marked reversal in the elevated core temperature caused by handling, cage switch, and during the nocturnal phase. It is of interest to note that all these elevations of core body temperature are prostaglandin mediated and are blocked with the antipyretic drug, sodium salicylate. However, LPS-induced fever, also a prostaglandin dependent fever, was unaffected by methyl scopolamine. Methyl scopolamine also lowered baseline core temperature when administered during the afternoon, but not during the morning in unstressed animals. It is proposed that a peripheral cholinergic pathway, possibly mediated through afferent vagal pathways, is operative in controlling core temperature during fevers associated with stress, nocturnal phase, and a pesticide. During recovery from exposure to a LPS, the fever appears to be mediated independently of peripheral cholinergic activation.

Effect of central and peripheral cholinergic antagonists on chlorpyrifos-induced changes in body temperature in the rat

Exposure to the organophosphate (OP)-based pesticide chlorpyrifos (CHP) in the rat results in an initial period of hypothermia lasting < 24 h, followed by a fever lasting 48-72 h. The purpose of this study was to determine how cholinergic pathways participate in the mediation of the thermoregulatory effects of CHP. The corn oil (CO) vehicle or CHP (25 mg/kg; p.o.) was administered to female rats while core temperature (Tc) and motor activity (MA) were monitored by radiotelemetry. The peripheral muscarinic antagonist, methyl scopolamine (MS) and central antagonist, scopolamine (S) were administered during the period of CHP-induced hypothermia and fever. The hypothermia was attenuated by scopolamine but not by methyl scopolamine. The delayed fever was augmented by scopolamine but blocked by methyl scopolamine. The results indicate that CHP-induced hypothermia is mediated by cholinergic stimulation of heat loss pathways in CNS thermoregulatory centers. Peripheral cholinergic pathways appear to have a minimal role in mediating chlorpyrifos-induced hypothermia. On the other hand, the chlorpyrifos-induced fever appears to be mediated by a peripheral pathway that is blocked by methyl scopolamine. The data provides a possible explanation for the persistent fever in humans exposed to OP pesticides and treated with atropine. Methyl atropine or methyl scopolamine may be a more effective therapy in the treatment of the fever.

Effects of Jen-San-Yaung-Jung-Tang on scopolamine-induced amnesia in rats

The effect of Jen-San-Yaung-Jung-Tang (YJT) on scopolamine (SCOP)-induced amnesia was investigated in a step-through passive avoidance task in rats. It was observed that YJT (0.5 and 1.0 g/kg) significantly improved SCOP-induced amnesia and did not change the horizontal activity and pain threshold. YJT at 0.5 and 1.0 g/kg also did not change SCOP-treated horizontal activity and pain threshold. Furthermore, the antiamnesic effect of YJT at 1.0 g/kg on the SCOP-induced amnesia was augmented by physostigmine, but was not altered by neostigmine or scopolamine N-methylbromide. These results suggest that the antiamnesic effect of YJT could only be related to the memory-related process, and to an increase in central cholinergic neuronal activity.

Learning/memory processes under stress conditions

Using mice, changes in the learning/memory processes under various stress conditions were investigated in one-trial step-through type passive avoidance learning task. Pre-, post-training and pre-test foot shock (FS)-stress induced long-lasting, at least 96 h, facilitation of test trial latencies. Pre-training psychological (PSY)-stress induces facilitation and pre-test swimming (SW)-stress provokes impairment of test trial latencies. These effects of FS-, PSY- and SW-stress are all dependent on the timing of their exposure and due to their acute effect. Intraperitoneal administration of 1 mg/kg scopolamine 30 min pre-training caused impairment of test latencies in naive and pre-test FS-stressed animals but failed to affect both pre- and post-training FS-induced enhancement. Taken all these data together, it seems that cholinergic mechanism is partly involved in the FS-stress induced facilitation of test latencies, though the research on the changes in ACh levels at the action site in brain after FS-stress exposure is necessary for a definite conclusion.

The neurosteroid pregnenolone sulfate reduces learning deficits induced by scopolamine and has promnestic effects in mice performing an appetitive learning task

The effects of the neurosteroid pregnenolone sulfate (PS) on learning as well as on scopolamine-induced learning deficits were studied in Swiss mice using an appetitively reinforced Go-No Go visual discrimination task. Subcutaneous (SC) administration of scopolamine (0.3-3 mg/kg) after the first session of training dose-dependently impairs learning during the following sessions in this task. Moreover, intracerebroventricular (ICV) administration of PS (0.01-10 nmol) dose-dependently blocks learning deficits induced by scopolamine (3 mg/kg), with the most potent effects at the dose of 0.5 nmol PS. In addition to antagonizing the amnestic effects of scopolamine, PS (0.5 nmol ICV) has a memory-enhancing effect, when administered alone after the first training session. Scopolamine (3 mg/kg SC) also produced substantial deficits on retrieval performance in the Go-No Go visual discrimination task, and caused motor disturbances, when administered 15 min before testing. PS (0.5 nmol ICV) also reduced scopolamine-induced deficits on retrieval but had no effect on scopolamine-induced motor impairments in the traction reflex test. Such a rapid effect of PS on memory processes may be mediated via NMDA and/or GABAA receptors.

Double dissociation between the effects of muscarinic antagonists and benzodiazepine receptor agonists on the acquisition and retention of passive avoidance

Both muscarinic antagonists, such as scopolamine, and benzodiazepine receptor (BZR) agonists, such as diazepam, produce a reliable impairment in the performance of one trial passive avoidance. Such deficits are frequently interpreted as drug-induced amnesia. However, these deficits could also result from a learning impairment. The present experiments compared the effects of two BZR agonists, lorazepam (0, 0.125, 0.25, and 0.375 mg/kg, IP) and diazepam (0, 0.78, 1.56, and 3.13 mg/kg, IP) with the effects of two muscarinic antagonists, scopolamine (0, 0.6, 0.8 and 1.0 mg/kg, SC) and atropine (0, 15, 30 and 60 mg/kg, IP) on a multiple trial passive avoidance task. In this procedure, the rats were trained with a 5-min inter-trial interval until a learning criterion was achieved. Retention was assessed 24 h later. This enabled the effects of the drugs on the acquisition and the retention of a passive avoidance response to be dissociated. Both atropine and scopolamine produced a marked impairment in the acquisition of the passive avoidance response, but did not impair retention. In contrast, diazepam and lorazepam did not alter the acquisition of a passive avoidance response, but did produce a dose-dependent impairment of retention. These results therefore demonstrate a double dissociation between the effects of muscarinic antagonists and BZR agonists on the acquisition and retention of passive avoidance.

Effects of several amnesic drugs on the habituation of exploratory activity in mice as indexed by horizontal and vertical activities

1. The effects of several drugs on the habituation of exploratory activity in mice were investigated by using animal movement analyzing systems which could detect horizontal and vertical activities. 2. Habituation was regarded as the difference between exploratory activity measured first (session 1) and that measured second (session 2). 3. Scopolamine (1, 3 and/or 6 mg/kg), pirenzepine (20 and 40 micrograms) injected immediately after session 1, cycloheximide (30 mg/kg), diazepam (1, 2 and/or 4 mg/kg) and dizocilpine (0.05, 0.1 and/or 0.2 mg/kg), unlike butylscopolamine (1, 3 and 6 mg/kg) or haloperidol (0.2 and 0.4 mg/kg), significantly increased horizontal activity but not always vertical activity in session 2. 4. From the results obtained, it appears that the increase in horizontal activity in session 2 is more appropriate as an indicator of the impairments of memory processes than that in vertical activity.

Effects of central nicotinic cholinergic receptor blockade produced by chlorisondamine on learning and memory performance in rats

The effects of chronic nicotinic receptor blockade on the performance of learning and memory tasks were determined using chlorisondamine, a compound which produces central nicotinic cholinergic receptor blockade that lasts for several weeks after a single icv administration. Chlorisondamine treatment did not affect the acquisition of spatial information in the Morris water maze or in the radial arm maze, tasks in which performance is reportedly disrupted by acute administration of the nicotinic antagonist, mecamylamine. Chlorisondamine also did not affect performance in the inhibitory avoidance task and did not alter the memory enhancement found in this task after post-training administration of nicotine. Mecamylamine, however, completely blocked the memory-enhancing effects of nicotine. In contrast to the differential ability to chlorisondamine and mecamylamine to block nicotine's memory-enhancing effects, these antagonists produced comparable blockade of nicotine's effects on open field behavior. It is unlikely that the different effects of systemically administered mecamylamine and centrally administered chlorisondamine on nicotine-induced memory enhancement are due to mecamylamine's peripheral effects, since hexamethonium, a peripherally active nicotinic antagonist, did not block nicotine-induced memory enhancement. The different pattern of effects of mecamylamine and chlorisondamine may be related to compensatory mechanisms being selectively induced by chronic blockade produced by chlorisondamine and not by acute blockade produced by mecamylamine. Alternatively, different effects of these two nicotinic cholinergic antagonists on the performance of learning and memory tasks might be related to selective actions of these compounds at nicotinic receptor subtypes or at nonnicotinic receptors.