Effects of selection delays on radial maze performance: acquisition and effects of scopolamine

Assessing spatial learning and memory in mice: Classic radial maze versus a new animal-friendly automated radial maze allowing free access and not requiring food deprivation

The radial arm maze (RAM) is a common behavioral test to quantify spatial learning and memory in rodents. Prior attempts to refine the standard experimental setup have been insufficient. Previously, we demonstrated the feasibility of a fully automated, voluntary, and stress-free eight-arm RAM not requiring food or water deprivation. Here, we compared this newly developed refined RAM to a classic manual experimental setup using 24 female 10-12 weeks old C57BL/6J mice. We used a lipopolysaccharide (LPS)-induced model of systemic inflammation to examine long-term cognitive impairment for up to 13 weeks following LPS injection. Both mazes demonstrated robust spatial learning performance during the working memory paradigm. The refined RAM detected spatial learning and memory deficits among LPS-treated mice in the working memory paradigm, whereas the classic RAM detected spatial learning and memory deficits only in the combined working/reference memory paradigm. In addition, the refined RAM allowed for quantification of an animal's overall exploratory behavior and day/night activity pattern. While our study highlights important aspects of refinement of the new setup, our comparison of methods suggests that both RAMs have their respective merits depending on experimental requirements.

Automated radial 8-arm maze: A voluntary and stress-free behavior test to assess spatial learning and memory in mice

The radial arm maze (RAM) is a common behavioral test to assess spatial working and reference memory in mice. However, conventional RAM experiments require a substantial degree of manual handling and animals are usually subjected to prolonged periods of food or water deprivation to achieve sufficient learning motivation resulting in stress-induced confounding effects and unwanted intra- and inter-subject variation. In a proof-of-concept approach to improve reliability and repeatability of results by refining the conventional maze methodology, we developed a voluntary, fully automated 8-arm RAM and tested its feasibility and usability using both spatial working and combined working/reference memory paradigms in ten female C57BL/6J mice. We demonstrate that experimental procedures of up to 7 days duration could be conducted without any manual animal handling and that mice up to 18 months of age showed robust spatial learning performance without any food or water restrictions being applied. Therefore, a voluntary, automated 8-arm RAM can serve to minimize variation in experimental results by reducing an animal's distress, suffering, and pain, which, in turn, contributes to the comprehensive application of 3R principles.

Telencephalic neurocircuitry and synaptic plasticity in rodent spatial learning and memory

Spatial learning and memory in rodents represent close equivalents of human episodic declarative memory, which is especially sensitive to cerebral aging, neurodegeneration, and various neuropsychiatric disorders. Many tests and protocols are available for use in laboratory rodents, but Morris water maze and radial-arm maze remain the most widely used as well as the most valid and reliable spatial tests. Telencephalic neurocircuitry that plays functional roles in spatial learning and memory includes hippocampus, dorsal striatum and medial prefrontal cortex. Prefrontal-hippocampal circuitry comprises the major associative system in the rodent brain, and is critical for navigation in physical space, whereas interconnections between prefrontal cortex and dorsal striatum are probably more important for motivational or goal-directed aspects of spatial learning. Two major forms of synaptic plasticity, namely long-term potentiation, a lasting increase in synaptic strength between simultaneously activated neurons, and long-term depression, a decrease in synaptic strength, have been found to occur in hippocampus, dorsal striatum and medial prefrontal cortex. These and other phenomena of synaptic plasticity are probably crucial for the involvement of telencephalic neurocircuitry in spatial learning and memory. They also seem to play a role in the pathophysiology of two brain pathologies with episodic declarative memory impairments as core symptoms, namely Alzheimer's disease and schizophrenia. Further research emphasis on rodent telencephalic neurocircuitry could be relevant to more valid and reliable preclinical research on these most devastating brain disorders. This article is part of a Special Issue entitled SI: Brain and Memory.

Interaction between the cholinergic system and CRH in the modulation of spatial discrimination learning in mice

Both cholinergic and CRH systems have been linked to cognitive processes such as learning and memory, and neuroanatomical as well as neurochemical evidence suggests important interactions between these two systems. Moreover, recent reports of pro-mnestic effects of CRH open the possibility that CRH could have beneficial effects in animals with cholinergic dysfunction. In a first experiment, spatial discrimination of C57BL/6 mice treated with various doses of scopolamine (0.5--2.0 mg/kg IP) was tested in a two-choice water maze task. Scopolamine, but not methylscopolamine, impaired accuracy and decreased responsivity. In contrast, similar doses of the nicotinic antagonist mecamylamine had no effect on choice accuracy but altered responsivity, as indicated by increased errors of omission and a reduction in swim speed during early experimental stages. ICV CRH (0.5--1.0 microg) also failed to significantly affect accuracy, but a strong tendency was observed to impair percentage correct responses. Measures of responsivity, such as errors of omission, choice latency and distance traveled, and of thigmotaxis were not significantly affected by CRH. However, initial swim speed was reduced by the peptide. Combined treatment with scopolamine (0.5 mg/kg IP) and CRH (0.5 microg ICV) had only mild, and primarily independent, effects, but overall suggested that concomitant blockade of muscarinic receptors and activation of the CRH system would rather act synergistically to disrupt spatial discrimination learning. Synergistic effects were also observed when animals receiving a combination of mecamylamine (2.0 mg/kg IP) and CRH (0.5 microg ICV) were tested, both in terms of responsivity and thigmotaxis, and there was limited evidence that part of these effects were potentiating. Thus, the cholinergic and CRH systems interact in the modulation of learning, but CRH, contrary to prediction, worsens the impairment caused by cholinergic blockade.

Dehydroevodiamine.HCl prevents impairment of learning and memory and neuronal loss in rat models of cognitive disturbance

We previously reported that dehydroevodiamine.HCl (DHED) has anticholinesterase and antiamnesic activities. To verify the effects of DHED on cognitive deficits further, we tested it on the scopolamine-induced amnesia model of the rat using the passive avoidance and eight-arm radial maze tests. A single (20 mg/kg p.o.) and repeated (10 mg/kg p.o.) administrations of DHED could significantly reverse the latency time shortened by scopolamine (1 mg/kg i.p.) to control level. The impaired spatial working memory induced by scopolamine (1 mg/kg i.p.) was also improved significantly by a single injection (6.25 mg/kg i.p.) and repeated administrations of DHED (10 mg/kg p.o.) in the eight-arm radial maze test. In addition, we examined the effects of DHED on the memory impairment and the histological changes of the brain after unilateral electrolytic lesion of the entorhinal cortex (EC) and middle cerebral artery occlusion in rats. The cognitive deficits caused by EC lesion and middle cerebral artery occlusion were improved significantly by repeated administrations of DHED (6.25 mg/kg i.p.) after EC lesion or ischemic insult once a day for 7 days in the passive avoidance test. Histological analysis showed that the neuronal loss in the DHED-treated group was notably reduced in the hippocampal area (CA1) of ischemic rats and in the dentate gyrus and hippocampal area (CA1 and CA3) of EC-lesioned rats compared with the nontreated group. The infarction area was decreased significantly by a single administration of DHED (6.25 mg/kg i.p.) 30 min before ischemic insult for 6 h. These results suggest that DHED might be an effective drug for not only the Alzheimer's disease type, but also the vascular type of dementia.

Recognition memory in rats--I. Concepts and classification

Recognition is the process by which a subject is aware that a stimulus has been previously experienced. It requires that the characteristics of events are perceived, discriminated, identified and then compared (matched) against a memory of the characteristics of previously experienced events. Understanding recognition memory, its underlying neuronal mechanisms, its dysfunction and alleviation of the latter by putative cognition enhancing drugs is a major research target and has triggered a wealth of animal studies. One of the most widely used animals for this purpose is the rat, and it is the rat's recognition memory which is the focus of this review. In this first part, concepts of recognition memory, stages of mnemonic processing and paradigms for the measurement of the rat's recognition memory will be discussed. In two subsequent articles (parts II and III) we will focus on the neuronal mechanisms underlying recognition memory in rats. Three major points arise from the comparison of paradigms that have in the past been used to assess recognition memory in rats. First, it should be realized that some tasks which, at face value, can all be considered to measure recognition memory in rats, may not assess recognition memory at all but may, for example, be based on recall rather than recognition. Second, it is evident that different types of recognition memory can be distinguished and that tasks differ in the type of recognition memory taxed. Some paradigms, for example, measure familiarity, whereas others assess recency. Furthermore, paradigms differ as to whether spatial stimuli or items are employed. Third, different processes, ranging from stimulus-response learning to the formation of concepts, may be involved to varying extent in different tasks. These are important considerations and question the predictive validity of the results obtained from studies examining, for example, the effects of putative cognition enhancing drugs.

Recognition memory in rats--III. Neurochemical substrates

In the first part of three overviews on recognition memory in the rat, we discussed the tasks employed to study recognition memory. In the second part, we discussed the neuroanatomical systems thought to be of importance for the mediation of recognition memory in the rat. In particular, we delineated two parallel-distributed neuronal networks, one that is essential for the processing of non-spatial/item recognition memory processes and incorporates the cortical association areas such as TE1, TE2 and TE3, the rhinal cortices, the mediodorsal thalamic nucleus and prefrontal cortical areas (Network 1), the other comprising of the hippocampus, mamillary bodies, anterior thalamic nuclei and medial prefrontal areas (Network 2), suggested to be pivotal for the processing of spatial recognition memory. The next step will progress to the level of the neurotransmitters thought to be involved. Current data suggest that the majority of drugs have non-specific, i.e. delay-independent effects in tasks measuring recognition memory. This may be due to attentional, motivational or motoric changes. Alternatively, delay-independent effects may result from altered acquisition/encoding rather than from altered retention. Furthermore, the neurotransmitter systems affected by these drugs could be important as modulators rather than as mediators of recognition memory per se. It could, of course, also be the case that systemic treatment induces non-specific effects which overshadow any specific, delay-dependent, effect. This possibility receives support from lesion experiments (for example, of the septohippocampal cholinergic system) or studies employing local intracerebral infusion techniques. However, it is evident that those delay-dependent effects are relatively subtle and more readily seen in delayed response paradigms, which tax spatial recognition memory. One interpretation of these results could be that some neurotransmitter systems are more involved in spatial than in item recognition memory processes. However, performance in delayed response tasks can be aided by mediating strategies. Drugs or lesions can alter those strategies, which could equally explain some of the (delay-dependent) drug effects on delayed responding. Thus, it is evident that neither of the neurotransmitter systems reviewed (glutamate, GABA, acetylcholine, serotonin, dopamine and noradrenaline) can be viewed as being directly and exclusively concerned with storage/retention. Rather, our model of recognition memory suggests that information about previously encountered items is differentially processed by distinct neural networks and is not mediated by a single neurotransmitter type.

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.

AF150(S): a new functionally selective M1 agonist improves cognitive performance in rats

This study was aimed at evaluating the ability of a new functionally selective partial M1 agonist, AF150(S), to reverse cognitive impairments in rats. A memory deficits-induced animal model was used that involved AF64A (3 nmol/2 microliters/side) bilaterally injected ICV. AF150(S) was administered PO. The pharmacodynamic profile of the compound was established and its general toxicity was evaluated. Animals were tested on three behavioral tasks: step-through passive avoidance, Morris water maze reference memory paradigm, and radial arm maze working memory paradigm. The sign-free dose of AF150(S) was > 40 mg/kg whereas the LD50 was > 500 mg/kg. In comparison, the effective dose in reversing performance impairments on the various tasks was much lower (0.5-5 mg/kg). The data suggest that AF150(S) possesses potential cognitive enhancement abilities, probably due to a specific increase of cholinergic function.

Task difficulty determines the differential memory-impairing effects of EAA antagonists in gerbils

Excitatory amino acid antagonists (EAAAs) have been shown to disrupt learning and memory in a variety of cognitive tasks. EAAAs have been reported to produce differential effects on working memory (WM) and reference memory (RM) or to have no effect at all. Apparent selective effects of EAAAs on WM and/or RM may have been due to differences between the effects of competitive and noncompetitive EAAAs, dose selection, or to different task requirements for the WM and RM components. In the present experiments, we assessed the effects of a noncompetitive EAAA (MK-801), a competitive EAAA (CPP), and the muscarinic antagonist scopolamine in two cognitive tasks, the split-stem T-maze and the eight-arm radial maze. In these two tasks, the WM and the RM components differed in their relative degree of difficulty. Gerbils were trained on either the T-maze, where WM was more difficult than RM, or on the radial arm maze, where RM was more difficult than WM. In the T-maze, MK-801 (0.1 mg/kg, IP, 30 min prior), CPP (30.0 mg/kg, IP, 2 h prior) and scopolamine (0.3 mg/kg, IP, 30 min prior) impaired both WM and RM, but the magnitude of the impairing effect was statistically greater for the WM component, the more difficult of the two components. Lower doses of these three compounds produced either selective effects on WM or no effect at all. In the radial arm maze all three drugs impaired both components, but the magnitude of the impairing effect was statistically greater for the RM component, the more difficult of the two components.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 3,3'-iminodipropionitrile on acquisition and performance of spatial tasks in rats

3,3'-Iminodipropionitrile (IDPN) has been reported to disrupt learning and memory in rats (24). The present work addressed the effects of IDPN on tasks requiring the use of spatial information. Separate groups of male rats were dosed with IDPN (IP, in 1 ml/kg saline) for 3 consecutive days and tested in the following procedures: (a) step-through passive avoidance conditioning (0, 100, 150, and 200 mg/kg/day); (b) Morris water maze (MWM) acquisition and retention (0, 125, 150, 175, and 200 mg/kg/day); (c) radial arm maze (RAM) acquisition (0, 100, 200, and 400 mg/kg/day); (d) RAM steady-state performance (0, 200, and 400 mg/kg/day); (e) repeated acquisition in the RAM (0, and 200 mg/kg/day). The vestibular toxicity of IDPN resulted in alterations in spontaneous behavior or swimming deficits in 5 of 8 rats treated with 175 mg/kg/day and in all the animals dosed with 200 or 400 mg/kg/day. IDPN increased step-through PA latencies at 200 mg/kg/day but not at lower doses. In the MWM, no performance deficits were observed at the dose levels preserving the swimming ability of the animals. In both the acquisition and the steady-state RAM tasks, IDPN (400 mg/kg/day) induced an increase in both choice errors and perseverative errors. In the RAM repeated acquisition paradigm, IDPN (200 mg/kg/day) induced performance deficits that included a decreased rate of within-session reduction in errors. The present data show that IDPN disrupts performance of tasks requiring spatial learning and memory and indicate that these deficits can be in part caused by an acquisition deficit.

Negative and positive effects of intracerebroventricular scopolamine on memory in mice undergoing passive avoidance and escape tests

The effects of intracerebroventricular administration of scopolamine on memory and learning in the conscious, freely moving mouse were evaluated using step-down passive avoidance and water maze tests. A new technique was used that allows convenient injection into the cerebral ventricles without disturbing the animal's behavior. No significant changes in locomotor activity were observed after low doses of scopolamine (0.1 and 1.0 microgram). However, 10 micrograms produced an increase in locomotor activity, while 100 micrograms caused an initial decrease followed by an increase in activity. In the passive avoidance test, scopolamine significantly impaired memory acquisition at doses higher than 1.0 microgram, consolidation at a dose of 100 micrograms, and retrieval at doses of 10 and 100 micrograms. In contrast, a dose of 0.1 microgram significantly improved consolidation and retrieval. In the water maze with a bridge, scopolamine either impaired memory acquisition, consolidation, and retrieval, or had no significant effect in the dose range tested. These results suggest that there are differences in the process of memory formation in the passive avoidance and escape tests.

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

The effects of cholinomimetic and non-cholinomimetic agents on spatial memory using a novel task in the 12-arm radial maze were investigated. The task was designed to reduce the tendency to use non-spatial strategies. Animals were repeatedly trained to retrieve food rewards from three arms, until a criterion level of performance was reached. Scopolamine (0.03 and 0.1 mg/kg SC), but not N-methylscopolamine (0.1 mg/kg SC) disrupted performance of this task. Physostigmine (0.3 mg/kg SC) and pilocarpine (30 mg/kg SC) completely reversed the deficit of performance produced by scopolamine. Furthermore, the ACE inhibitor Hoe 288 (10 nmol ICV) and the angiotensin AT1 receptor antagonist losartan (10 mg/kg SC) also significantly attenuated the scopolamine-induced deficit. These results show that this novel task in the radial maze is sensitive to the disruptive effects of scopolamine and can identify cognitive enhancing effects of both cholinomimetic and non-cholinomimetic drugs. Thus, this maze task provides a useful model for the evaluation of novel cognitive enhancing agents.

Behavioral microanalysis of spatial delayed alternation performance: rehearsal through overt behavior, and effects of scopolamine and chlordiazepoxide

Rats were trained in an operant spatial delayed alternation task utilizing retention intervals from 2 to 32 s. In addition to response accuracy, operations of the levers during the retention intervals were recorded and analyzed. Animals were tested following the administration of the muscarinic antagonists scopolamine hydrobromide and methylbromide, and the benzodiazepine receptor agonist chlordiazepoxide. In vehicle-treated animals, the relative number of correct responses and correct rehearsal operations (operation of the forthcoming correct lever during retention intervals) varied with the length of the retention intervals, and these measures were correlated. The response rate for rehearsal operations increased with the length of the retention intervals. It is speculated that the delay-dependent increase in response rate reflects an effect of delayed reward that was also associated with a delay-dependent increase in the tendency to alternate between levers. The effects of delay on the accuracy of rehearsal operations may have contributed to the delay-dependent correct responding. Scopolamine hydrobromide (0.01, 0.03, 0.1, 0.3 mg/kg) and methylbromide (0.1, 0.3 mg/kg) impaired correct responding, but did not seem to interfere with the relative number of correct rehearsal operations. As only the presentation of the panel light indicated trial onset, it is speculated that the cholinergic receptor blockade resulted in an increase in the probability of a repositioning response that was triggered by light onset. Chlordiazepoxide (1, 3, 5, 10 mg/kg) did not affect behavioral performance. These results suggest that in tasks that allow the development of rehearsal operations, delay-dependent response accuracy does not represent a sufficient condition for conclusions on task demands on memory.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of physostigmine and scopolamine on rats' performances in object-recognition and radial-maze tests

The effects of physostigmine and scopolamine were evaluated on working memory of rats in object recognition and radial-maze tests. Three doses of physostigmine hemi-sulfate (Phys: 0.05, 0.10 and 0.20 mg/kg), five doses of scopolamine hydrobromide (Scop: 0.125, 0.25, 0.5, 1.0 and 2.0 mg/kg), and one dose of scopolamine methylbromide (Mscop: 2.0 mg/kg) were used. In object recognition test, rats were submitted to three or four intertrial delay conditions (1-min, 15-min and either 60-min or 24-h). The higher doses of Scop (1.0 and 2.0 mg/kg) in 1-min and 15-min delay and of Phys (0.20 mg/kg) in 1-min delay impaired discrimination between new and familiar objects. Mscop impaired discrimination between objects in 60-min but not in 1-min and 15-min delay. This effect may be state dependent. Radial-maze learning was impaired by the lower doses of scopolamine (0.25 and 0.50 mg/kg) which had no effect in object recognition test. These results show that in our conditions, object recognition is less sensitive than radial-maze test to cholinergic drugs.

The effects of scopolamine and cues to forget on pigeons' memory for time

Pigeons were trained with a 0-s delayed symbolic matching-to-sample procedure to indicate whether a houselight sample stimulus was short (2 s) or long (8 s) by pecking a red or a green comparison stimulus. In Experiment 1, the pigeons received injections of scopolamine hydrobromide (0.015 mg/kg), or saline, and the delay interval was manipulated (0, 1, 3, and 9 s). Memory for time was significantly poorer following scopolamine injections than following saline injections. A significant choose-short bias was observed under scopolamine at delays as brief as 3 s, but not under saline. In Experiment 2, a brief postsample cue (a vertical or horizontal line) signaled whether the comparison stimuli would be presented or omitted on each trial. During training, comparison stimuli were always presented following the remember (R) cue, but never following the forget (F) cue. During testing, memory for time was significantly poorer on F-cue trials than on R-cue trials. A significant choose-short bias was observed on F-cue trials at the 5- and 10-s delays, but not on R-cue trials. The results suggest that anticholinergic blockade accelerates the rate at which memory for temporal events is foreshortened in working memory. This effect is similar to that produced by an explicit cue to forget the temporal sample.

Biochemical and behavioral characterization of a novel cholinergic agonist, SR 95639

Selective M1 cholinergic agonists may be useful in treating dementias due to cholinergic hypofunction. SR 95639 has recently been described as such a compound. We found the compound to have affinity for M1 sites (Ki = 2.1 microM) which was approximately 3-fold higher than its affinity for M2 sites. Functional partial agonism was suggested by an inconsistent increase in phosphoinositide (PI) turnover in rat hippocampal slices, combined with blockade of carbachol-stimulated PI turnover. In vivo M2-mediated effects were absent. Scopolamine-induced hyperactivity was attenuated by SR 95639 and scopolamine-impaired inhibitory avoidance and radial maze performance were improved. The compound appears to be a weakly selective M1 partial agonist with potential advantages over existing compounds.

Utility of an elevated plus-maze for dissociation of amnesic and behavioral effects of drugs in mice

Learning and memory were previously evaluated by using the elevated plus-maze test in mice. We investigated whether this method could be used for the evaluation of amnesic properties of drugs, including those which alter behavior on the first (training) trial. Six drugs of different types, scopolamine, MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate), diazepam, butylscopolamine, methamphetamine and haloperidol were administered before training. The transfer latency of vehicle-treated mice on retention testing was significantly shorter than that on training. The transfer latencies in mice given scopolamine (1 and 3 mg/kg s.c.), butylscopolamine (6 mg/kg s.c.), methamphetamine (2 and 4 mg/kg i.p.), or haloperidol (0.4 mg/kg i.p.) were significantly prolonged on training compared with those of the corresponding vehicle groups. However, significant prolongation of the transfer latency in the retention test, compared to the vehicle groups, was observed only in mice given scopolamine (3 mg/kg s.c.), MK-801 (0.1 and 0.15 mg/kg i.v.), diazepam (4 mg/kg i.p.), or methamphetamine (4 mg/kg i.p.). These results suggested that the prolongation of the transfer latency on retention testing in the plus-maze method might be used as an indicator for impairment of learning and memory induced by the drugs which have amnesic properties, and is not related to the change in transfer latency on training.

Reversal of age-related cognitive impairments by an M1 cholinergic agonist, AF102B

This study examined the effect of a specific M1 cholinergic agonist, AF102B, on place learning of aged and young rats. Spatial reference memory was tested in the Morris Water Maze task, while spatial working memory was tested on an 8-arm radial maze. Both memory functions were impaired in aged rats compared to young animals. However, the administration of AF102B significantly reduced the age-related cognitive impairments observed in both tasks. This data supports the assertion of the "cholinergic hypothesis," namely that specific enhancement of cholinergic function may reverse geriatric cognitive deficits.

Some effects of CNS cholinergic neurons on memory

The aim of this study is to observe the relationship between the impairment in passive avoidance task induced in rats by the i.p. administration of muscarinic antagonists, scopolamine and methyl-scopolamine, and the change in acetylcholine (ACh) output induced by these drugs. Initially we studied the effects of these drugs on the animals' performance of a step-through passive avoidance task. We then measured the change in ACh levels after administration of these drugs using an in vivo brain dialysis technique. Scopolamine was effective in impairing the performance of the passive avoidance task, while methyl-scopolamine did not have clear effects on the performance of the task. With regard to ACh output, scopolamine increased ACh dose-dependently and methyl-scopolamine also affected ACh release. These data suggest that the accumulation of ACh in the synaptic cleft may be involved in the memory deficit induced by scopolamine.

Scopolamine and methylscopolamine differentially affect fixed-consecutive-number performance of male and female Wistar rats

Male and female Wistar rats were trained on a fixed-consecutive-number schedule in which a response on a food lever was followed by the presentation of reinforcement when at least three, but not more than seven responses had been completed on a work lever. Subjects were treated with different doses of the centrally acting cholinergic antagonist scopolamine hydrobromide or the more peripherally active cholinergic antagonist scopolamine methylbromide (0.08, 0.16 or 0.32 mg/ml/kg) once behavior had stabilized. Scopolamine hydrobromide and scopolamine methylbromide dose-dependently decreased response rates in males and females. Scopolamine methylbromide decreased response rates more than equivalent doses of scopolamine hydrobromide and the rate-suppressant effects of both drugs were more marked in males than in females. Scopolamine hydrobromide dose-dependently decreased response accuracy, but differences between males and females were not observed. Response accuracy also decreased after scopolamine methylbromide, but did not vary as a function of the dose of the drug. The decrease in response accuracy induced by both drugs was attributable to an increase in the percentage of trials with a premature switch from the work lever to the food lever. Both scopolamine hydrobromide and scopolamine methylbromide dose-dependently increased the number of premature switches. Differences between males and females were not observed. Administration of scopolamine hydrobromide and scopolamine methylbromide also decreased the number of obtained reinforcers in a dose-dependent manner. Females obtained significantly fewer reinforcers than males, while scopolamine methylbromide affected the number of obtained reinforcers to a larger extent than scopolamine hydrobromide.

The effects of scopolamine and methylscopolamine on visual and auditory discriminations in male and female Wistar rats

The present experiment was designed to investigate whether or not the administration of scopolamine hydrobromide would differentially disrupt auditory or visual discrimination performance in male and female Wistar rats. Two groups of male and female Wistar rats were trained to discriminate between a continuous and intermittent visual stimulus, while two other groups were trained to discriminate between a continuous or intermittent auditory stimulus in a discrete-trial discrimination procedure. Once discrimination performance had stabilized, subjects were treated with different doses (0.125, 0.25, 0.50 or 1.0) of scopolamine hydrobromide or scopolamine methylbromide. Treatment effects were assessed with respect to discrimination performance, as well as with respect to the number of trials which were not completed. Scopolamine hydrobromide, but not scopolamine methylbromide, disrupted visual and auditory discrimination performance. The auditory discrimination was more seriously disrupted. However, both the administration of scopolamine hydrobromide and of scopolamine methylbromide increased the number of trials which were not completed suggesting that the accuracy of visual and auditory discriminations after drug treatment may have been influenced by other variables than drug effects on memory processes. Sex differences were not observed, neither with respect to discrimination performance, nor with respect to the number of trials which were not completed.

Psychopharmacological effects in the radial-arm maze

The radial-arm maze (RAM) has become a very widely used method for assessing spatial memory in rodents. It has proven to be quite useful in the investigation of the effects of a variety of pharmacological manipulations on spatial memory. The cholinergic system has been found to be crucial for accurate RAM performance. Blockade of either muscarinic or nicotinic receptors impairs performance. Other transmitter systems such as dopamine and the opiates have also been found to be involved with the maintenance of accurate RAM performance. This test has been found to be sensitive to the effects of a variety of toxicants given either in adulthood or during development. These findings provide a background for the assessment of the effects of novel substances on RAM performance as well as the basis for the further understanding of the neural substrates of memory.