Modelling working and reference memory in rats: effects of scopolamine on delayed matching-to-position(1,2)

A model of working and reference memory in rats is described, based on a discrete-trial operant procedure with concurrent components of spatial matching (for working memory) and nonspatial discrimination (for reference memory). On each trial in the matching component, rats received food for pressing one of two retractable levers after a delay if that lever had been presented in the prior sample phase of the trial. On each trial in the discrimination component, food was delivered if the rat pressed a lever illuminated by a cue light after the delay interval. The model was tested with scopolamine (0.10 to 0.56 mg/kg, ip), which reduced matching accuracy in a dose-related manner. Linear slope and intercept estimates of retention gradients showed that intercepts declined and slopes remained unchanged with increasing scopolamine dose. In contrast, scopolamine had no significant effect on nonspatial discrimination accuracy, indicating a relative insensitivity of reference memory to cholinergic blockade. Because the matching component involved spatial cues and the discrimination component did not, a second group of rats was trained to discriminate between the spatial locations of two levers, to compare the effects of scopolamine on spatial and nonspatial discriminations. Scopolamine at the same doses caused a small, consistent decrease in spatial discrimination accuracy, suggesting that spatial discrimination was more sensitive to disruption by scopolamine than was nonspatial discrimination. The combined delayed matching-to-position/nonspatial discrimination procedure appears to provide a useful technique for characterizing mnemonic effects of drugs and toxicants in rats.     

Effect of destruction of the 5-hydroxytryptaminergic pathways on the acquisition of temporal discrimination and memory for duration in a delayed conditional discrimination task

This experiment examined the effect of destruction of the ascending 5-hydroxytryptaminergic (5HTergic) pathways on the acquistion of a temporal discrimination and on memory for duration, using a delayed conditional discrimination task. In phase I, rats that had received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei, and shamlesioned control rats, were trained in a series of discrete trials to press lever A following a 2-s presentation of a light stimulus, and lever B following an 8-s presentation of the same stimulus. Following stimulus offset, a response on a panel placed midway between the two levers was required in order to intiate lever presentation; a single response on either lever resulted in withdrawal of both levers and, in the case of a correct response, reinforcer delivery. Both groups gradually acquired accurate discrimination, achieving >90% correct choices within 20–30 sessions; the lesioned group acquired accurate performance significantly faster than the control group. In phase II, delays were interposed between stimulus offset and lever presentation in 50% of the trials (2, 4, 8, 16 and 32 s; 10% of trials in each case). Accuracy declined as a function of post-stimulus delay in both groups, and there was no significant difference between the performances of the two groups. Both groups showed an increasing tendency to respond on lever A following longer post-stimulus delays (choose-short effect); this effect was somewhat enhanced in the lesioned group. The levels of 5HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered.     

Effects of scopolamine and unilateral lesions of the basal forebrain on T-maze spatial discrimination and alternation in rats

Cholinergic systems are thought to play a role in memory. It has been suggested that cholinergic neurons, possibly the cortically projecting cells of the nucleus basalis magnocellularis, are differentially involved in working and reference memory. To evaluate this hypothesis the effects on memory of scopolamine (0, 0.3, 0.6 mg/kg) or unilateral kainic acid (4.7 nmoles in 1 microliter) lesions of the basal forebrain of rats were tested. Working memory, the recall of recent events of transient importance that is vulnerable to interference, was tested using a T-maze alternation task; reference memory, information stored over the long term that is relatively resistant to interference, was evaluated using a spatial discrimination task in the T-maze. The differential sensitivity of the two tasks to interference effects was confirmed by the finding that the insertion of a 30-sec delay between trials significantly reduced performance in the alternation but not the spatial discrimination task. Furthermore, scopolamine or the lesions significantly impaired alternation but not spatial discrimination performance. Biochemical assays of the kainate-injected brains confirmed that the cortical cholinergic marker, choline acetyltransferase, was significantly reduced. These results support the hypothesis that working and reference memory may be differentially controlled by cholinergic systems.     

Comparison of δ9-THC,LSD-25 and scopolamine on non-spatial single alternation performance in the runway

Rats were trained in a straight runway on a non-spatial single alternation (NSSA) which involved presentation of reward (R) and nonreward (N) in a fixed repeating sequence (i.e. R-N-R-N ...). Patterned running results since rats learned to run fast on R trials and slow on N trials. This task can be regarded as an animal analog of the goal directed serial alternation task employed with humans. After patterned running was well established the effects of graded doses of ⁹-THC, LSD and scopolamine were delineated. Although all drugs modified alternation performance, each agent produced distinctly specific effects on the different components of NSSA. ⁹-THC disrupted alternation by decreasing running speed on R trials and increasing running speed on N trials. Lower doses of LSD increased running speed on N trials while leaving R trial speeds unchanged. At the highest dose, LSD decreased running speed on R trials while leaving N trial speeds only slightly elevated from baseline. Scopolamine disrupted alternation solely by decreasing speed on R trials. The results were discussed with reference to the effects of these drugs on internal inhibition, registration and recall of internal cues and timing behavior.This research was supported by U.S.P.H.S. Grant No. MH 18392-02.     

Effects of δ9-THC,LSD-25 and scopolamine on continuous,spontaneous alternation in the Y-maze

The effects of ⁹-THC, LSD-25 and scopolamine on continuous, spontaneous alternation of the hooded rat in the Y-maze were determined. LSD and scopolamine decreased the number of arm entries (responsivity) while concomitantly reducing percent alternation. THC however, reduced responsivity and percent alternation more at the lower dose (1 mg/kg) when compared to control than at the higher dose (3 mg/kg). Only scopolamine induced a significant increase in stimulus perseveration. The results are discussed in relation to the advantages afforded by the continuous spontaneous alternation procedure.     

Effects of pesticides and drugs on working memory in rats: continuous delayed response

Effects of four pesticides (carbaryl, propoxur, chlordimeform, and deltamethrin) and four reference drugs (physostigmine, scopolamine, methscopolamine, and chlordiazepoxide) were measured in two delayed response, working memory procedures: go-no go alternation in which rats initiated their own trials, and spatial reversals. Four of these compounds (carbaryl, propoxur, physostigmine, and scopolamine) were also tested in a go-no go alternation procedure in which animals did not initiate their trials. The pesticides and physostigmine did not selectively affect working memory in any of the procedures: low doses only moderately decreased response accuracy, whereas higher doses suppressed responding indiscriminately. The pesticides and physostigmine had similar effects on go-no go alternation (i.e., working memory) and analogous go-no go discrimination performance. Effects on go-no go alternation performance did not depend on whether the animals initiated their own trials. Scopolamine, in contrast, appeared to disrupt working memory. It profoundly disrupted accuracy at doses that only moderately decreased over-all responding and impaired go-no go alternation accuracy much more than discrimination accuracy.     

Memory and the septo-hippocampal cholinergic system in the rat

This study examined the effects of intrahippocampal injections of scopolamine (a muscarinic antagonist drug) on performance of a working-memory task (contingently) reinforced T-maze alternation) and a reference-memory task (visual discrimination) by the same rats in the same maze. Rats in the first shipment were trained in delayed alternation, received bilateral implantation of cannulae aimed at the CA3 field of the dorsal hippocampus, and were tested for retention with 1 l microinjections of scopolamine (35 g) and saline on alternate days. These rats were then trained on visual discrimination and tested alternately under scopolamine or saline as described above. It was found that scopolamine impaired performance of delayed alternation to a greater extent than performance of visual discrimination. Data from rats in the second shipment replicated this finding, with the order of the tasks reversed, and, additionally, showed that delayed alternation, but not visual discrimination, was impaired at a dose of 12 g/l. A dose of 4 g/l had no effect on either task. It is concluded that performance of a workingmemory task is significantly more sensitive to disruption of cholinergic mechanisms in the hippocampus than performance of a reference-memory task.Supported by PHS Training Grant MH-14577. Now at the Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Stanford, CA 94305, USA     

Retarded acquisition of a temporal discrimination following destruction of noradrenergic neurones by systemic treatment with DSP4

This experiment examined the effect of destroying central noradrenergic neurones, using the selective neurotoxin DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) on the acquisition and performance of discrimination between two time intervals. Rats that had received systemic treatment with DSP4 and vehicle-treated control rats were trained in a series of discrete trials to press lever A following a 2-s presentation of a light stimulus and lever B following an 8-s presentation of the same stimulus. Both groups acquired the discrimination (>90% correct choices) within 15 sessions; however, the DSP4-treated group showed significantly slower acquisition than the control group. When stable performance had been attained, probe trials were introduced in which the light was presented for intermediate durations. Both groups showed sigmoid functions relating percent choice of lever B to log stimulus duration. Neither the bisection point (duration corresponding to 50% choice of lever B) nor the Weber fraction differed significantly between the DSP4-treated and control groups. The levels of noradrenaline were markedly reduced in the neocortex and hippocampus of the DSP4-treated group, but the levels of dopamine and 5-hydroxytryptamine were not altered. The results indicate that noradrenaline depletion induced by DSP4 retarded the acquisition of temporal discrimination, but did not impair steady-state discriminative precision.     

Assessment of working memory in rats using spatial alternation behavior with variable retention intervals: effects of fixed-ratio size and scopolamine

The effects of fixed-ratio (FR) size, scopolamine, and the interactions between FR size and scopolamine were investigated in male F344 rats on working memory as assessed by spatial alternation behavior maintained under FR schedules of food presentation where the interval between trials was varied among values of 2, 4, 8, 16, and 32 s within each session. The magnitude of the FR size on the correct and incorrect levers was varied systematically from 1 response to 2, 4, 8, or 16 responses in order to determine whether the FR size influenced either the percentage of correct responding, rates of responding, or both. Under the primary baseline condition, that is when the FR size on both the correct and incorrect levers was one response (designated FR1 FR1), the percentage of correct responses decreased with increasing retention interval duration. Increasing the FR size on the correct lever produced FR-dependent increases in the percentage of correct responding as well as in rates of responding. Increasing the FR size on the incorrect lever produced FR-dependent decreases in correct responding, but had little effect on rates of responding. Dose-effect curves for scopolamine were determined on performance maintained under FR values on the correct and incorrect levers, respectively, of FR1 FR1, FR1 FR10, FR10 FR1, and FR10 FR10. In general, scopolamine produced dose-related decreases in the percentage of correct responding, although the magnitude of the effects of scopolamine varied not only with dose, but also with the length of the retention interval and with changes in FR size. Rates of responding during trials were dose-dependently decreased by scopolamine under all schedule parameters. The present results are consistent with the interpretation that scopolamine can selectively impair time-dependent memory processes such as working memory, but also can impair time-independent variables which affect performance, dependent on dose and schedule maintaining the behavior.     

Studies on the time course and the effect of cholinergic and adrenergic receptor blockers on the stimulus effect of nicotine

This study investigated the stimulus property of nicotine in the rat. The primary objectives of the study were 1. to determine the time course of the nicotine stimulus and its relationship to brain levels of the drug and 2. to determine whether the nicotine stimulus is dependent upon the integrity of specific neurotransmitter systems. A lever choice discrimination was used. After injection of nicotine, depression of one lever in an operant test chamber resulted in food reinforcement according to a variable interval schedule of 15 sec. When saline was administered, the opposite lever was reinforced. A high degree of discriminated responding was observed when either 400 g/kg or 200 g/kg of nicotine was used as a discriminative stimulus. The degree of discrimination decreased as the length of the time period between the injection of nicotine and the test of discrimination was increased. This decline in discrimination was similar to the decline in brain levels of nicotine suggesting that nicotine discrimination is directly related to the concentration of nicotine in the brain. Atropine, mecamylamine, dibenamine, propranolol and -methyl-para-tyrosine (AMPT) were all tested, in a range of doses, for effects upon nicotine discrimination. Of these, only mecamylamine antagonized the nicotine stimulus. These results indicate that the stimulus effect of nicotine is mediated specifically through nicotinic-cholinergic receptors and not muscarinic-cholinergic or adrenergic receptors.A preliminary report of this investigation appeared in the Pharmacologist 15, 452 (1973).     

Facilitated acquisition of a temporal discrimination following destruction of the ascending 5-hydroxytryptaminergic pathways

This experiment examined the effect of destroying the 5-hydroxytryptaminergic (5HTergic) pathways on the acquisition and performance of discrimination between two brief time intervals. Rats that had received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei, and sham-lesioned control rats were trained in a series of discrete trials to press lever A following a 200-ms presentation of a light stimulus and lever B following an 800-ms presentation of the same stimulus. Both groups gradually acquired accurate performance, attaining 80%–85% accuracy by the end of 40 sessions. The lesioned group learnt the task significantly faster than the control group. When stable performance had been attained, probe trials were introduced in which the light was presented for intermediate durations. Both groups showed sigmoid functions relating percent choice of lever B to log stimulus duration. The bisection point (duration corresponding to 50% choice of lever B) did not differ significantly between the two groups; however, the Weber fraction was significantly smaller in the lesioned group than in the control group. The levels of 5HT and 5-hydroxy-indole-acetic acid were markedly reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered. The results indicate that destruction of the 5HTergic pathways facilitates acquisition of a temporal discrimination. The lack of an effect of the lesion on the bisection point contrasts with our previous finding using longer stimulus durations; it is suggested that different behavioural processes may underlie millisecond-range and second-range temporal discrimination, and that these may be differently affected by 5HT depletion.     

Attenuation of scopolamine-induced impairment of spontaneous alternation behaviour by antagonist but not inverse agonist and agonist β-carbolines

Mice were tested in a simple automated Y-maze. Total number of arm entries and alternation behaviour were measured. The latter is thought to reflect working memory capacity at a rudimentary level. During an 8-min session, vehicle-treated mice performed 32.4±7.4 arm entries, 51.0±12.4% of which were organized in alternations (triplets). The two variables showed a negative correlation. Scopolamine (1.0 mg/kg) significantly enhanced activity, reduced alternation behaviour and diminished the correlation between the two variables. The effects of benzodiazepine receptor inverse agonist, antagonist and agonist -carbolines on this spontaneous behaviour and on the effects of scopolamine were examined. The effects of inverse agonists and agonists on locomotor activity were complex in interaction with both vehicle and scopolamine. The scopolamine-induced reduction of alternation behaviour was significantly reversed by the antagonist ZK 93426 but not by inverse agonists; furthermore, partial agonists and agonists showed no effects. It is hypothesized that the interaction of antagonist -carbolines with scopolamine is based on a direct GABA-ergic control of cholinergic neurotransmission, and suggests an ability of antagonist -carbolines to antagonize amnestic properties of scopolamine.     

Effect of lesions of the ascending 5-hydroxytryptaminergic pathways on timing behaviour investigated with an interval bisection task

Seventeen rats received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei: 12 rats received sham injections. The rats were trained in a series of discrete trials to press lever A following a 2-s presentation of a light stimulus and to press lever B following an 8-s presentation of the same stimulus. Both groups learnt the task rapidly and maintained >90% accuracy throughout the experiment. When stable performance had been attained, probe trials were introduced in which the light was presented for intermediate durations. Both groups showed sigmoid functions relating percent choice of lever B to log stimulus duration. The bisection point (duration corresponding to 50% choice of lever B) was significantly shorter in the lesioned group than in the control group. There was no significant difference between the slopes of the psychophysical functions or the Weber fractions derived for the two groups. The levels of 5-hydroxytryptamine (5HT) and 5-hydroxyindoleacetic acid in the parietal cortex, hippocampus, amygdala, nucleus accumbens and hypothalamus were markedly reduced in the lesioned group, but the levels of noradrenaline and dopamine were not significantly affected by the lesion. The results confirm the involvement of 5HTergic function in timing behaviour, but suggest that destruction of these pathways does not disrupt the capacity for temporal discrimination.     

Effects of scopolamine,d-amphetamine and other drugs affecting catecholamines on spontaneous alternation and locomotor activity in mice

Locomotor activity and Y-maze spontaneous alternation were examined in three strains of inbred mice (A/J, DBA/2J and C57BL/6J) following various drug treatments. Although the strains exhibited different levels of locomotor activity, the level of spontaneous alternation was comparable among the strains. Scopolamine produced dose dependent increases in locomotor activity in the A and DBA/2 strains, but produced a transient inhibitory effect upon locomotor activity in C57BL/6. Nevertheless, spontaneous alternation was eliminated equally, regardless of strain. d-Amphetamine increased locomotor activity and reduced alternation significantly below chance levels (perseveration). -Methyl-p-tyrosine (-MpT) and FLA-63 reduced the locomotor stimulating effects of d-amphetamine; however, the effectiveness of these agents was found to be strain dependent. Neither -MpT nor FLA-63 reduced the perseverative behavior. A subsequent study employing Swiss-Webster mice revealed that with pretreatment of reserpine, both -MpT and FLA-63 eliminated the amphetamine-induced perseverative behavior. Results were interpreted in terms of (a) the role of cholinergic and catecholaminergic systems in modulating alternation behavior, (b) qualitative differences in the behavioral effects elicited by scopolamine and d-amphetamine, (c) strain-specificity regarding pharmacological effects, and (d) role of newly synthesized norepinephrine and dopamine in subserving amphetamine-induced locomotor activity and perseveration.     

Destruction of central noradrenergic neurones with DSP4 impairs the acquisition of temporal discrimination but does not affect memory for duration in a delayed conditional discrimination task

This experiment examined the effect of destroying central noradrenergic neurones using the selective neurotoxin N-(2-chloroethyl)-n-ethyl-2-bromobenzylamine (DSP4) on the acquisition of a temporal discrimination and on memory for duration, using a delayed conditional discrimination task. In phase I, rats that had received systemic treatment with DSP4 and vehicle-treated control rats were trained in a series of discrete trials to press lever A following a 2-s presentation of a light stimulus, and lever B following an 8-s presentation of the same stimulus. Following stimulus offset, a response on a panel placed midway between the two levers was required to initiate lever presentation; a single response on either lever resulted in withdrawal of both levers and, in the case of a “correct” response, reinforcer delivery. Both groups acquired accurate discrimination, achieving 90% correct choices within 50 sessions; the DSP4-treated group acquired accurate performance more slowly than the control group. In phase II, delays were interposed between stimulus offset and lever presentation in 50% of the trials. In the absence of a delay, discriminative accuracy was lower in the DSP4-treated group than in the control group. Accuracy declined as a function of post-stimulus delay in both groups; both groups showed a delay-dependent bias towards responding on lever A (“choose-short” bias). Neither of these effects differed significantly between the two groups. The concentrations of noradrenaline in the parietal cortex and hippocampus were reduced by 90% and 89% in the DSP4-treated group, compared to the levels in the control group, but the levels of dopamine, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid did not differ significantly between the groups. The results confirm the deleterious effect of DSP4 on the acquisition of temporal discrimination, but do not provide evidence for a role of the noradrenergic innervation of the hippocampus and neocortex in temporal working memory. Received: 8 July 1996/Final version: 2 October 1996     

Atropine,scopolamine and hippocampal lesion effects on alternation performance of rats

Interactions between effects of hippocampal lesions and cholinergic blocking agents were examined using two behavioral tasks, delayed spatial alternation and go-no go temporal alternation. Atrophine and scopolamine were administered to control rats and to rats with lesions in three areas of the hippocampal formation. Large hippocampal lesions disrupted performance on both tasks as did administration of atrophine or scopolamine to control rats and to rats with lesions in restricted areas of the hippocampus. However, the effects of atropine and scopolamine on performance of rats with large lesions involving all regions of the hippocampal formation depended upon the alternation task used. Atropine and scopolamine disrupted delayed spatial alternation performance of all groups tested, but atropine failed to significantly disrupt go-no go alternation performance of the group with large unrestricted hippocampal lesions. Both the size of the lesion and the behavioral task were found to be important determinants of the effects of cholinergic blockers on the performance of rats with hippocampal lesions.     

Effects of scopolamine on components of delayed response performance in the rat

The effects of scopolamine and methyl scopolamine on working memory processes were investigated in a paired trial, go-no go delayed response procedure in which rats initiated their own trials. Drug effects were examined concurrently on performance at three delays--no, 0, and 2.5 sec. Scopolamine disrupted no-delay (discrimination) performance in a dose-related manner. Scopolamine also progressively reduced performance at 0-delay and 2.5 sec delay more than at no-delay, but only at the highest administered dose (0.5 mg/kg). Scopolamine affected sensitivity, but not response bias. Although both scopolamine and methyl scopolamine reduced the probability of trial initiation, only scopolamine disrupted accuracy of performance on the initiated trials.     

Scopolamine effects on delayed spatial alternation in the rat.

Rats were trained to press two levers in alternation on discrete trials spaced 10 sec apart. During the final sessions of alternation training, error responses per opportunity on the trials that followed reinforced trials (initial trials) did not differ from error responses per opportunity on repetitive (correction) trials (Experiment 1). Scopolamine did not increase the rats' tendency to perseverate: drug treatment did not cause the error responses per opportunity to increase over runs of consecutive error responses (Experiment 2). Scopolamine did not impair performance when alternation was controlled by visual stimuli present in the external environment at the time of the response (Experiment 3). The disruption in delayed alternation performance produced by scopolamine was attributed to effects on stimulus discrimination, resulting in impairment of control of responding by stimuli not present in the environment at the time of the response.     

A reappraisal of scopolamine effects on inhibition

A series of related experiments was conducted to examine the effects of scopolamine on discrimination performance in the presence of a stimulus signalling non-reinforcement. In Experiment 1, rats trained to respond on 1 of two levers in the presence of a 1000-Hz tone and on the other lever in the presence of a 3000-Hz tone were not reinforced when white noise was added to 1 of the tones. Pairing white noise with the other tone during an extinction session demonstrated that the white noise had become a conditioned inhibitory stimulus. In Experiment 2, scopolamine decreased responding and discrimination accuracy on the excitatory (reinforced) trials, and increased responding on the inhibitory (non-reinforced) trials. The magnitude of the drug's effect was similar on excitatory and inhibitory trials. Using combinations of visual and auditory discriminative stimuli, Experiment 3 confirmed the results of experiment 2. These experiments show that scopolamine disrupts animals' ability to discriminate, and that scopolamine-induced increases in non-rewarded responses cannot be attributed solely to a disinhibitory effect of the drug as Carlton (1969) and others have claimed.     

Scopolamine effects on two-trial delayed-response performance in the rat

A two-trial procedure examined the importance of stimulus type, presence or absence of the stimuli at the time of choice, and time since presentation of the stimuli in determining the effects of scopolamine. Some rats were required to “switch” response levers on trial 2 to the incorrect trial-1 lever, whereas other rats were required to operate the same lever for both trials. Two sets of trial-1 stimuli were employed for different rats. Scopolamine reduced choice accuracy more in the presence of one set of trial-1 stimuli than for the other set. The reduction of accuracy under scopolamine was greater when the rats were required to “switch” on trial 2 than for the “stay” condition. Presence vs. absence of the stimuli had an inconsistent influence on drug effect and there was no interaction between “length of absence” of the stimuli and scopolamine-induced decreases in accuracy, thus precluding an interpretation that scopolamine exerted its effects through a disruption of memory processes.