Attenuation of scopolamine-induced spatial memory deficits in the rat by cholinomimetic and non-cholinomimetic drugs using a novel task in the 12-arm radial maze

Age Impacts the Burden That Reference Memory Imparts on an Increasing Working Memory Load and Modifies Relationships With Cholinergic Activity

Rodent aging research often utilizes spatial mazes, such as the water radial-arm-maze (WRAM), to evaluate cognition. The WRAM can simultaneously measure spatial working and reference memory, wherein these two memory types are often represented as orthogonal. There is evidence, however, that these two memory forms yield interference at a high working memory load. The current study systematically evaluated whether the presence of a reference memory component impacts handling of an increasing working memory load. Young and aged female rats were tested to assess whether aging impacts this relationship. Cholinergic projections from the basal forebrain to the hippocampus and cortex can affect cognitive outcomes, and are negatively impacted by aging. To evaluate whether age-related changes in working and reference memory profiles are associated with cholinergic functioning, we assessed choline acetyltransferase activity in these behaviorally-tested rats. Results showed that young rats outperformed aged rats on a task testing solely working memory. The addition of a reference memory component deteriorated the ability to handle an increasing working memory load, such that young rats performed similar to their aged counterparts. Aged rats also had challenges when reference memory was present, but in a different context. Specifically, aged rats had difficulty remembering which reference memory arms they had entered within a session, compared to young rats. Further, aged rats that excelled in reference memory also excelled in working memory when working memory demand was high, a relationship not seen in young rats. Relationships between cholinergic activity and maze performance differed by age in direction and brain region, reflecting the complex role that the cholinergic system plays in memory and attentional processes across the female lifespan. Overall, the addition of a reference memory requirement detrimentally impacted the ability to handle working memory information across young and aged timepoints, especially when the working memory challenge was high; these age-related deficits manifested differently with the addition of a reference memory component. This interplay between working and reference memory provides insight into the multiple domains necessary to solve complex cognitive tasks, potentially improving the understanding of complexities of age- and disease- related memory failures and optimizing their respective treatments.

Muscarinic receptor pharmacology and circuitry for the modulation of cognition

The muscarinic cholinergic system constitutes an important part of the neuronal circuitry that modulates normal cognition. Muscarinic receptor antagonists are well known to produce or exacerbate impairments in attention, learning, and memory. Conversely, both direct-acting muscarinic receptor agonists and indirect-acting muscarinic cholinergic agonists, such as acetylcholinesterase inhibitors, have shown cognition-enhancing properties, including improvements in normal cognitive function, reversal of cognitive deficits induced by muscarinic receptor antagonists, and attenuation of cognitive deficits in psychiatric and neurological disorders, such as Alzheimer's disease and schizophrenia. However, until recently, the lack of small molecule ligands that antagonize or activate specific muscarinic acetylcholine receptor (mAChR) subtypes with high selectivity has been a major obstacle in defining the relative contributions of individual mAChRs to different aspects of cognitive function and for the development of novel therapeutic agents. These limitations may be potentially overcome by the recent discovery of novel mAChR subtype-selective compounds, notably allosteric agonists and positive allosteric modulators, which exhibit greater selectivity for individual mAChR subtypes than previous mAChR orthosteric agonists. In preclinical studies, these novel ligands have shown promising efficacy in several models for the enhancement of cognition. In this chapter, we will review the muscarinic cholinergic circuitry and pharmacology of mAChR agonists and antagonists relevant to the modulation of different aspects of cognition in animals and clinical populations.

The role of nicotinic receptors in the amelioration of cholinesterase inhibitors in scopolamine-induced memory deficits

RATIONALE

Nicotine receptors in the brain are closely related with memory amelioration induced by cholinesterase inhibitors.

OBJECTIVE

The present study was undertaken to clarify the role of nicotinic receptors in the ameliorative effects of cholinesterase inhibitors on scopolamine-induced memory deficit.

METHOD

Drug effects were measured using an eight-arm radial maze with four arms baited. Hippocampal theta rhythm during the radial maze task was also recorded with a polygraph system using a telemetric technique.

RESULTS

Scopolamine (0.5 mg/kg, i.p.) caused a spatial memory deficit as well as an increase in hippocampal theta power during radial maze performance. Pilocarpine, nicotine, physostigmine, and donepezil antagonized the effects of scopolamine. The ameliorative effects of nicotine, physostigmine, and donepezil but not piocarpine on memory performance and hippocampal theta activity were reversed by mecamylamine.

CONCLUSION

These results indicate that nicotinic receptors have an essential role in the ameliorative effects of cholinesterase inhibitors in both scopolamine-induced memory deficit and the increase in hippocampal theta activity.

Ang II and Ang IV: unraveling the mechanism of action on synaptic plasticity, memory, and epilepsy

The central angiotensin system plays a crucial role in cardiovascular regulation. More recently, angiotensin peptides have been implicated in stress, anxiety, depression, cognition, and epilepsy. Angiotensin II (Ang II) exerts its actions through AT(1) and AT(2) receptors, while most actions of its metabolite Ang IV were believed to be independent of AT(1) or AT(2) receptor activation. A specific binding site with high affinity for Ang IV was discovered and denominated "AT(4) receptor". The beneficiary effects of AT(4) ligands in animal models for cognitive impairment and epileptic seizures initiated the search for their mechanism of action. This proved to be a challenging task, and after 20 years of research, the nature of the "AT(4) receptor" remains controversial. Insulin-regulated aminopeptidase (IRAP) was first identified as the high-affinity binding site for AT(4) ligands. Recently, the hepatocyte growth factor receptor c-MET was also proposed as a receptor for AT(4) ligands. The present review focuses on the effects of Ang II and Ang IV on synaptic transmission and plasticity, learning, memory, and epileptic seizure activity. Possible interactions of Ang IV with the classical AT(1) and AT(2) receptor subtypes are evaluated, and other potential mechanisms by which AT(4) ligands may exert their effects are discussed. Identification of these mechanisms may provide a valuable target in the development in novel drugs for the treatment of cognitive disorders and epilepsy.

Increasing hippocampal acetylcholine levels enhance behavioral performance in an animal model of diencephalic amnesia

Pyrithiamine-induced thiamine deficiency (PTD) was used to produce a rodent model of Wernicke-Korsakoff syndrome that results in acute neurological disturbances, thalamic lesions, and learning and memory impairments. There is also cholinergic septohippocampal dysfunction in the PTD model. Systemic (Experiment 1) and intrahippocampal (Experiment 2) injections of the acetylcholinesterase inhibitor physostigmine were administered to determine if increasing acetylcholine levels would eliminate the behavioral impairment produced by PTD. Prior to spontaneous alternation testing, rats received injections of either physostigmine (systemic=0.075 mg/kg; intrahippocampal=20, 40 ng/muL) or saline. In Experiment 2, intrahippocampal injections of physostigmine significantly enhanced alternation rates in the PTD-treated rats. In addition, although intrahippocampal infusions of 40 ng of physostigmine increased the available amount of ACh in both pair-fed (PF) and PTD rats, it did so to a greater extent in PF rats. The increase in ACh levels induced by the direct hippocampal application of physostigmine in the PTD model likely increased activation of the extended limbic system, which was dysfunctional, and therefore led to recovery of function on the spontaneous alternation task. In contrast, the lack of behavioral improvement by intrahippocampal physostigmine infusion in the PF rats, despite a greater rise in hippocampal ACh levels, supports the theory that there is an optimal range of cholinergic tone for optimal behavioral and hippocampal function.

Rats with scopolamine- or MK-801-induced spatial discrimination deficits in the cone field task: animal models for impaired spatial orientation performance

Spatial cognition appears to be compromised in elderly and in patients suffering from dementia. These deficits are believed to be modelled, at least partly, by the administration of scopolamine or MK-801 in normal adult animals. In order to establish an animal model suited for the evaluation of putative cognition enhancers, we assessed the effects of scopolamine (0.3, 0.5, 0.7 mg kg(-1), i.p.) and MK-801 (0.07, 0.08, 0.09 mg kg(-1), s.c.) in rats trained in the cone field. This task allows the simultaneous investigation of working memory (WM), reference memory (RM) and search strategies. Scopolamine and MK-801 reliably induced spatial cognition deficits in the cone field without inducing behavioural side effects. This task appears to be suited for assessing the effects of putative cognition-enhancing compounds on spatial cognition.

Comparative studies on the memory-enhancing actions of captopril and losartan in mice using inhibitory shock avoidance paradigm

Renin angiotensin system (RAS) in the central nervous system participates in the processing of sensory information, learning and memory processes. Inhibitors of RAS, particularly angiotensin converting enzyme (ACE) inhibitors and angiotensin II (Ang II) receptor antagonists are reported to have potential nootropic effects in various learning and memory paradigms. The neurochemical basis underlying nootropic effect of ACE inhibitors are unclear due to wide range of substrate for this enzyme. In this study, we compared the effect of ACE inhibitor captopril and a selective AT(1)receptor antagonist losartan in a step-up shock avoidance (active avoidance) task. Captopril (5-10 mg/kg) but not losartan (5-10 mg/kg) improved learning in the second trial of the acquisition test. However, both these drugs were equally effective in enhancing retention of memory when administered prior to training. Retention enhancing effect of captopril and losartan were reversed by post-acquisition test administration of L-NAME (15 mg/kg), dizocilpine (0.05 mg/kg) and scopolamine (0.1 mg/kg). On the basis of above observations, it is concluded that decrease in endogenous Ang II activity in the brain might result in improved cognitive performance by enhancing cGMP pathways. However facilitation of acquisition only by captopril may be due to other putative mechanisms.

Attenuation of scopolamine-induced spatial learning impairments by an angiotensin IV analog

Recently, a receptor for the angiotensin II(3-8) (Ang IV) hexapeptide, was discovered in the hippocampus, suggesting a possible role in learning. The present study utilized intracerebroventricularly (icv) infused scopolamine hydrobromide (scop) to disrupt spatial learning in the circular water maze, followed by the Ang IV analog norleucine1-Ang IV (Nle1-Ang IV), to restore normal performance. Rats were icv pretreated with either scop or artificial cerebrospinal fluid (aCSF) followed by either icv injected Nle1-Ang IV or aCSF, and then behaviourally tested. During acquisition training, each animal's latency to locate the platform, path distance, speed, and efficiency ratios were measured. A probe trial was conducted on the final day of training and the time spent in the target quadrant and the number of crossings over the former location of the platform (annulus crossings) were observed. The results indicate that those animals treated with scop followed by aCSF performed poorly during acquisition training as compared with controls. In contrast, those animals that received scop followed by Nle1-Ang IV attained equivalent latencies, distances, and efficiency ratios to find the platform as those achieved by controls. There were no observed differences in swimming speed, thus arguing against drug-induced motor impairment. During the probe trial, animals treated with scop followed by aCSF spent less time in the target quadrant and made fewer annulus crossings as compared to controls, while the scop, Nle'-Ang IV treated animals performed equivalently to controls. These results suggest that Nle1-Ang IV acts to counteract the disruption of spatial learning induced by scopolamine.

The effect of several putative cognition enhancers on a water maze acquisition deficit produced by pCPA + scopolamine combination treatment

A combined treatment of a 3-day regimen of pCPA and low-dose scopolamine produced a significant deficit in the acquisition of a water maze task, which has been suggested as a model for the cognitive deficits of Alzheimer's disease. The putative cognition enhancers oxotremorine, captopril, ondansetron, and tacrine were used in attempts to alleviate the water maze impairment. The effects of oxotremorine were difficult to determine due to nonspecific motor effects causing alterations in swimming speed. No evidence for cognition-enhancing properties of captopril was found. Ondansetron showed a cognition-enhancing effect on one of 4 days, but only at a relatively high dose (1 mg/kg i.p.). Tacrine, however, alleviated the pCPA + scopolamine-induced cognitive deficit. This study may thus provide evidence for the usefulness of tacrine in treating spatial deficits in dementia.

Effects of the selective angiotensin II receptor antagonists losartan and PD123177 in animal models of anxiety and memory

There is increasing interest in the potential functional role of the octapeptide angiotensin II (AII) in psychiatric and cognitive disorders. The novel angiotensin II (AII) receptor antagonists, losartan and PD123177, selective for the AT1 and AT2 receptor subtypes respectively, constitute important pharmacological tools for the assessment of the behavioural consequences of modulation of AII function. The present series of studies investigated the effects of each compound in two animal models of anxiety, the rat elevated zero-maze and mouse light/dark box, and two models of working memory in the rat, the operant delayed matching to position (DMTP) task and the spatial reinforced alternation test in the T-maze. Our data indicate that both compounds (0.01-10 mg/kg s.c.) were without significant effect in any of the behavioural assays. Using the present methods and strains of laboratory rodents, these findings provide no support for the involvement of AII receptor function in the mediation of anxiety of working memory.

Delta 9-tetrahydrocannabinol impairs spatial memory through a cannabinoid receptor mechanism

The purpose of the present study was to investigate whether the cannabinoid and cholinergic systems impair working memory through a common mechanism. This hypothesis was tested by examining whether the cannabinoid antagonist SR141716A would ameliorate radial-arm performance deficits caused by either the naturally occurring cannabinoid, delta 9-THC, or scopolamine, a muscarinic antagonist. In addition, we evaluated whether the cholinesterase inhibitor, physostigmine, would prevent delta 9-THC-induced impairment of spatial memory. Finally, because the locomotor suppressive effects of cannabinoids may decrease radial arm choice accuracy independent of a direct effect on memory, we examined the impact of increasing the intertrial error on radial arm choice accuracy. As previously reported, delta 9-THC impaired maze performance (ED50 = 3.0 mg/kg). Increasing the intertrial interval from 5 s to 30 s resulted in a three-fold increase in the amount of time required to complete the maze without affecting choice accuracy. Importantly, SR141716A prevented delta 9-THC-induced deficits in radial-arm choice accuracy in a dose-dependent manner (AD50 = 2.4 mg/kg); however, the cannabinoid antagonist failed to improve the disruptive effects of scopolamine. Conversely, physostigmine failed to improve performance deficits produced by delta 9-THC. These data provide strong evidence that delta 9-THC impairs working memory through direct action at cannabinoid receptors. Moreover, these results suggest that scopolamine and delta 9-THC do not impair spatial memory in a common serial pathway, though they may converge on a third neurochemical system.

Ability of angiotensin II to modulate striatal dopamine release via the AT1 receptor in vitro and in vivo

1. The ability of angiotensin II to modulate dopamine release from rat striatal slices in vitro and in the intact rat striatum in vivo was assessed by the microdialysis technique. 2. In slices of rat striatum, angiotensin II (0.1-1.0 microM) induced a concentration-related increase in endogenous dopamine release which was maximal (approximately 250% above basal levels) within the first 2-4 min of agonist application and subsequently declined to near basal values. The angiotensin II-induced increase in dopamine release was Ca(2+)-dependent and was completely antagonized by the selective AT1 receptor antagonist, losartan (1.0 microM). In contrast, the AT2 receptor antagonist, PD123177 (1.0 microM) failed to modify the angiotensin II-induced response. Neither antagonist alone modified basal dopamine release from striatal slices. 3. In freely moving rats, angiotensin II (1.0-10 microM; administered via the microdialysis probe) induced a concentration-related increase in extracellular levels of dopamine which was maximal (approximately 150% above basal levels) within 20-40 min of agonist application and subsequently declined. The angiotensin II (10 microM)-induced increase in extracellular levels of dopamine was completely antagonized by the AT1 receptor antagonist, losartan (0.1-1.0 microM; administered via the microdialysis probe) but not by the AT2 receptor antagonist, PD123177 (1.0 microM; administered via the microdialysis probe). Neither antagonist alone modified basal extracellular levels of dopamine. 4. Homogenate radioligand binding studies with [125I]-angiotensin II (0.1 nm) identified relatively low levels of specific binding sites in rat striatal homogenates compared to homogenates of pyriform cortex (51.3 +/- 9.2 and 651.3 +/- 55.1 fmol g-1 wet weight, respectively, mean +/- s.e.mean, n = 3; non-specific binding defined by unlabelled angiotensin II). The majority of the specific [125I]-angiotensin II (0.1 nM) binding in the striatal and pyriform cortex homogenates was sensitive to the selective AT1 receptor antagonist, losartan (1.0 microM). 5. In conclusions the present study provides direct evidence that angiotensin II acting via the AT1 receptor subtype facilitates the release of dopamine in the rat striatum in vitro and in vivo. This receptor-mediated response may account for the modulation of dopamine-mediated behavioural responses by antagonists of the AT1 receptor and inhibitors of angiotensin converting enzyme.

An inverted U-shaped curve for heptylphysostigmine on radial maze performance in rats: comparison with other cholinesterase inhibitors

The potential of heptylphysostigmine tartrate (pyrrolo [2,3b] indol-5-ol, 3,3a,8,8a-hexahydro-1,3a,8-trimethylheptylcarbamate [ester, (3aS-cis)]) (MF201), a new second-generation cholinesterase inhibitor, to antagonize scopolamine-induced amnesia in rats was assessed in an 8-arm radial maze. Upon completing the training session, the rats were orally administered increasing doses of MF201 (2, 3, 4, 6 and 8 mg/kg) 60 min prior to a s.c. injection of scopolamine (0.25 mg/kg). 9-Amino-1,2,3,4-tetrahydroamino-acridine hydrochloride hydrate (tacrine) (0.25, 0.37, 0.5, 1 and 2 mg/kg), 1-benzil-4-[(5,6-dimethoxy-1-indanon)-2-yl]-methyl piperidine (E2020) (0.125, 0.18, 0.25 and 0.5 mg/kg) and physostigmine (0.15, 0.25, 0.5 and 1 mg/kg) were orally administered and rats were tested in the same task. As previously described, scopolamine induced an impairment in radial maze performance, measured in terms of total number of errors, total time taken to complete the task and the percentage of amnesic animals. The reversal of scopolamine-induced impairment was characterized by the presence of an inverted U-shaped dose-response curve. A significant antagonistic effect was achieved with a dose (mg/kg) of 0.25 for E2020, 0.5 for tacrine and physostigmine and 3, 4 and 6 for MF201, the latter manifesting a broader spectrum of activity (3-6 mg/kg). While the maximal active doses restored the scopolamine-induced modified pattern of arm entry, they were ineffective in reducing hypermotility, suggesting the drugs have a specific effect on cognitive function.