Delayed-non-match-to-sample performance in the radial arm maze: effects of dopaminergic and gabaergic agents

Diazepam induces retrograde facilitation of object recognition and object location memory in male mice

Benzodiazepines are widely prescribed for patients suffering from anxiety and insomnia. Although amnesic effects of benzodiazepines are commonly known as side effects, it has also been reported that these drugs improve memory for information learned before drug intake, a phenomenon called retrograde facilitation. However, the retrograde effects of benzodiazepines on cognitive performances in rodents remain controversial. It should be considered that studies on diazepam-induced retrograde facilitation in humans have been conducted using a recall paradigm focused on short-term memory, whereas these studies in rodents have been conducted using memory tasks that mainly target long-term memory and/or require negative or positive reinforcers. In the current study, we investigated whether diazepam, a benzodiazepine, induces retrograde facilitation for object recognition memory and spatial memory in mice, using a novel object recognition test and an object location test, respectively. These tests are available for short-term memory and do not require any reinforcer. The mice treated with diazepam retained object recognition memory for at least 180 min and spatial memory for at least 150 min. In contrast, vehicle-treated control mice retained object recognition memory for 120 min but not 150 min or longer, and spatial memory for 90 min but not 120 min or longer. These data clearly demonstrated diazepam-induced retrograde facilitation for both object recognition and spatial memories in mice. The present study is expected to contribute to the elucidation of the neural basis of retrograde facilitation.

The novel dehydroepiandrosterone (DHEA) derivative BNN27 counteracts cognitive deficits induced by the D1/D2 dopaminergic receptor agonist apomorphine in rats

RATIONALE

Schizophrenia is a devastating mental disease that affects nearly 1% of the population worldwide. It is well documented that the dopaminergic (DAergic) system is compromised in schizophrenia. It is of note that the mixed dopamine (DA) D₁/D₂ receptor agonist apomorphine induces schizophrenia-like symptoms in rodents, including disruption of memory abilities. Neuroactive steroids, comprising dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulphate (DHEAS), were shown to affect brain DAergic system and to be involved in schizophrenia. BNN27 is a novel DHEA derivative, which is devoid of steroidogenic activity. It has recently been reported that BNN27 counteracted schizophrenia-like behavioural deficits produced by glutamate hypofunction in rats.

OBJECTIVES

The aim of the present study was to investigate the ability of BNN27 to attenuate non-spatial, spatial recognition and discrete memory deficits induced by apomorphine in rats.

METHODS

To this end, the object recognition task (ORT), the object location task (OLT) and the step-through passive avoidance test (STPAT) were used.

RESULTS

BNN27 (3 and 6 mg/kg, i.p.) attenuated apomorphine (0.5 mg/kg, i.p.)-induced non-spatial, spatial recognition and discrete memory deficits. Interestingly, the effects of compounds on memory cannot be ascribed to changes in locomotor activity.

CONCLUSIONS

Our findings suggest that BNN27 is effective to DA dysfunction caused by apomorphine, attenuating cognitive impairments induced by this D1/D2 receptor agonist in rats. Additionally, our findings illustrate a functional interaction between BNN27 and the DAergic system that may be of relevance for schizophrenia-like behavioural symptoms.

Midazolam impairs acquisition and retrieval, but not consolidation of reference memory in the Morris water maze

Amnesia is one of the most discussed properties of the benzodiazepine class of drugs. The effects of benzodiazepines on human memory are usually anterograde, while changes in retrograde memory functions were seldom reported. Such inconsistent findings have prompted numerous animal studies investigating the influences of these positive modulators of inhibitory neurotransmission on different stages of memory. Among the benzodiazepines, memory effects of midazolam are of special interest due to its many and varied clinical applications. The present Morris water maze study in adult male Wistar rats was performed in three experiments in which midazolam was administered at doses of 0.5, 1 and 2 mg/kg intraperitoneally, before or immediately after each of five daily learning sessions, with two trials in a session, as well as before the probe test. Midazolam impaired acquisition and subsequent retention of spatial learning of the position of the hidden platform even at a pre-training dose of 0.5 mg/kg. This low dose was not associated with impairment of the procedural component of learning, manifested by increased time spent in the periphery of the pool. The lack of midazolam effect on consolidation has not been confounded by the observed below-chance performance of the control group since our additional experiment using diazepam also administered immediately after each of five learning sessions has revealed a similar pattern of results. Finally, midazolam administered before the probe test impaired retrieval of reference memory at all tested doses. Hence, induction of retrograde, besides anterograde amnesia should be kept in mind as a possibility when midazolam is used in clinical settings.

Modulatory influence of dopamine receptors on consolidation of object recognition memory

The role of dopamine receptors in regulating the formation of recognition memory remains poorly understood. Here we show the effects of systemic administration of dopamine receptor agonists and antagonists on the formation of memory for novel object recognition in rats. In Experiment I, rats received an intraperitoneal (i.p.) injection of vehicle, the selective D1 receptor agonist SKF38393 (1.0 and 5.0mg/kg), or the D2 receptor agonist quinpirole (1.0 and 5.0mg/kg) immediately after training. In Experiment II, rats received an injection of vehicle, the dopamine receptor antagonist SCH23390 (0.1 and 0.05 mg/kg), or the D2 receptor antagonist raclopride (0.5 and 0.1mg/kg) before training, followed by an injection of vehicle or the nonselective dopamine receptor agonist apomorphine (0.05 mg/kg) immediately after training. SKF38393 at 5mg/kg produced an enhancement of novel object recognition memory measured at both 24 and 72 h after training, whereas the dose of 10mg/kg impaired 24-h retention. Posttraining administration of quinpirole did not affect 24-h retention. Apomorphine enhanced memory in rats given pretraining raclopride, suggesting that the effect was mediated by selective activation of D1 receptors. The results indicate that activation of D1 receptors can enhance recognition memory consolidation. Importantly, pharmacological activation of D1 receptors enhanced novel object recognition memory even under conditions in which control rats showed significant retention.

Differential effects of scopolamine and lorazepam on working memory maintenance versus manipulation processes

Between-study comparisons of benzodiazepine and anticholinergic drugs on working memory suggest that anticholinergics may produce greater impairment in maintenance processes, whereas benzodiazepines may produce greater impairment in manipulation processes. This study directly compared acute effects ofthe benzodiazepine lorazepam (1.0 and 2.0 mg/70 kg, orally administered) and the anticholinergic scopolamine (0.25 and 0.50 mg/70 kg, subcutaneously administered) on working memory maintenance (storage and rehearsal) and manipulation processes in a placebo-controlled, double-dummy, double-blind, crossover design in 20 healthy volunteers. Using a modified Sternberg paradigm, storage, rehearsal, and manipulation processes were parametrically manipulated by varying memory load, delay between stimulus presentation and test, and number of operations performed on the letter strings, respectively, while controlling for drug effects on nonmemory processes. As predicted, the results suggested greater impairment in maintenance processes (rehearsal) with scopolamine than with lorazepam and greater impairment in manipulation processes with lorazepam than with scopolamine. In addition, the results suggested greater overall slowing of working memory processes with lorazepam.

Contrasting effects of bromocriptine on learning of a partially baited radial arm maze task in the presence and absence of restraint stress

RATIONALE

Severe, traumatic stress or repeated exposure to stress can result in long-term deleterious effects, including hippocampal cell atrophy and death, which, in turn, result in memory impairments and behavioural abnormalities. The dopaminergic D(2) receptor agonist, bromocriptine, has been shown to modulate learning, and chronic stress is associated with dopaminergic dysfunction.

OBJECTIVES

In the present study, we evaluated the effects of bromocriptine in the presence or absence of restraint stress.

MATERIALS AND METHODS

Adult male Wistar rats were subjected to restraint stress for 21 days (6 h/day) followed by bromocriptine treatment, and learning was assessed in the partially baited radial arm maze task. In a separate group of animals, the effects of bromocriptine per se was evaluated. Dopamine levels were estimated by high-performance liquid chromatography with electrochemical detection.

RESULTS

Stressed rats showed impairment in both acquisition and retention of the radial arm maze task, and bromocriptine treatment after stress showed a reversal of stress-induced impairment. Interestingly, in the absence of stress, bromocriptine exhibited dose-dependent differential effects on learning. While rats treated with bromocriptine 5 mg/kg, i.p., demonstrated impairment in learning, the bromocriptine 10 mg/kg and vehicle-treated groups did not differ from normal controls. To understand the neurochemical basis for the effects of bromocriptine, dopamine levels were estimated. The stress-induced decrease in dopamine levels in the hippocampus and frontal cortex were restored by bromocriptine treatment. In contrast, bromocriptine alone (5 mg/kg, i.p.) decreased dopamine levels in the frontal cortex and striatum.

CONCLUSIONS

Our study shows that amelioration of stress-induced learning impairment correlates with restoration of dopamine levels by bromocriptine treatment.

Morris water maze learning in Long-Evans rats is differentially affected by blockade of D1-like and D2-like dopamine receptors

Dopaminergic neurotransmission is involved in several brain functions including spatial cognition. In the present study we examine the effects of systemic administration of D1-like receptor antagonist SCH23390 and D2-like receptor antagonist sulpiride on the acquisition of the Morris water maze task. We used visible versus hidden platform versions of the MWM in order to distinguish between the effects of the drugs on the procedural versus cognitive aspects of the task. SCH23390 was found to prolong escape latencies to the visible platform at a higher dose (0.05mg/kg), whilst the lower dose (0.02mg/kg) left both procedural and cognitive functions almost unchanged. SCH23390 was also found to reduce swimming speed. Sulpiride did not affect the visible platform learning at any of three doses studied (30, 60 and 100mg/kg); the highest dose of sulpiride (100mg/kg) impaired place navigation to the hidden platform, without affecting the swim speed. The results of the present study show a difference in the involvement of D1-like and D2-like receptors in the MWM acquisition.

Chronic treatment with sulbutiamine improves memory in an object recognition task and reduces some amnesic effects of dizocilpine in a spatial delayed-non-match-to-sample task

The effect of a sulbutiamine chronic treatment on memory was studied in rats with a spatial delayed-non-match-to-sample (DNMTS) task in a radial maze and a two trial object recognition task. After completion of training in the DNMTS task, animals were subjected for 9 weeks to daily injections of either saline or sulbutiamine (12.5 or 25 mg/kg). Sulbutiamine did not modify memory in the DNMTS task but improved it in the object recognition task. Dizocilpine, impaired both acquisition and retention of the DNMTS task in the saline-treated group, but not in the two sulbutiamine-treated groups, suggesting that sulbutiamine may counteract the amnesia induced by a blockade of the N-methyl-D-aspartate glutamate receptors. Taken together, these results are in favor of a beneficial effect of sulbutiamine on working and episodic memory.

Within-subject memory decline in middle-aged rats: effects of intraseptal tacrine

A longitudinal design was used to examine spatial working memory performance in aging Long-Evans rats on a 12-arm, delayed-non-match-to-sample radial maze task. Compared to performance at 12-13 months of age, the same rats exhibited a significant performance deficit at 15-16 months of age across all retention intervals (1.5-10h). All rats exhibited some degree of decline, and no rat performed as well as they had 3 months earlier. This early onset deficit may relate to the degree of difficulty required to perform accurately in a task that maximizes both spatial information processing and flexible working memory representations. Following our observation, rats were implanted with a chronic cannula aimed at the medial septal nucleus. Acute intraseptal tacrine treatments (0.0-25 micrograms/0.5 microl) did not significantly affect any index of performance. Rats exhibited further memory decline over the course of testing (up to 20 months of age). Detection of early onset dysfunction could allow for experimental analysis of underlying mechanisms and therapeutic strategies early in the course of age-related changes.

Intraseptal infusion of oxotremorine impairs memory in a delayed-non-match-to-sample radial maze task

The medial septal nucleus is part of the forebrain circuitry that supports memory. This nucleus is rich in cholinergic receptors and is a putative target for the development of cholinomimetic cognitive-enhancing drugs. Septal neurons, primarily cholinergic and GABAergic, innervate the entire hippocampal formation and regulate hippocampal formation physiology and emergent function. Direct intraseptal drug infusions can produce amnestic or promnestic effects depending upon the type of drug administered. However, intraseptal infusion of the cholinomimetic oxotremorine has been reported to produce both promnestic and amnestic effects when administered prior to task performance. The present study examined whether post-acquisition intraseptal infusion of oxotremorine would be promnestic or amnestic in a delayed-non-match-to-sample radial maze task. In this task rats must remember information about spatial locations visited during a daily sample session and maintain that information over extended retention intervals (hours) in order to perform accurately on the daily test session. Treatments may then be administered during the retention interval. Alterations in maze performance during the test session an hour or more after treatment evidences effects on memory. In the present study, intraseptal infusion of oxotremorine (1.0-10.0 microg) produced a linear dose-related impairment of memory performance. Importantly, we also observed disrupted performance on the day after treatment. This persistent deficit was related only to memory over the retention interval and did not affect indices of short-term memory (ability to avoid repetitive or proactive errors during both the pre- and post-delay sessions). The persistent deficit contrasts with the acute amnestic effects of other intraseptally administered drugs including the cholinomimetics carbachol and tacrine. Thus, intraseptal oxotremorine produced a preferential disruption of memory consolidation as well as a persistent alteration of medial septal circuits. These findings are discussed with regards to multi-stage models of hippocampal-dependent memory formation and the further development of therapeutic strategies in the treatment of mild cognitive impairment as well as age-related decline and Alzheimer's dementia.

The effect of pergolide on cognitive performance of young and middle-aged rats

In this study, we investigated the effect of pergolide, a dopaminergic agonist, on cognitive ability in young and middle-aged rats using the Morris Water Maze (MWM). Pergolide 0.5/mg/day i.p. was administered to young and middle-aged rats, whereas only vehicle was given to their age-matched controls. During the acquisition period of 6 days, young rats showed normal escape latency pattern, which was not affected by pergolide. Middle-aged rats, however, showed poor escape latency pattern, and this poor pattern was also not affected by pergolide. On the 7th day, pergolide decreased retention time of young rats compared to control values. Middle-aged rats also showed reduced retention time. In contrast to the findings of young rats, retention time was not affected by pergolide in middle-aged rats. We concluded that pergolide does not alter escape latency at any age. It has a negative effect on retention time of young rats, whereas it has no effect on middle-aged ones.

Effects of diazepam and beta-CCM on working memory in mice: relationships with emotional reactivity

This study was aimed at determining the effects of systemic administration of diazepam and methyl beta-carboline-3-carboxylate (beta-CCM) both on spatial working memory and on emotional reactivity in mice. Results showed that diazepam and beta-CCM induced opposite effects in both memory and emotional reactivity tests. Indeed, as a function of dose, diazepam reduced anxiogenic-like reactions but increased vulnerability to interference in the memory task at a 30-s but not at a 5-s delay interval. As a function of dose, beta-CCM reduced vulnerability to interference and increased emotional reactivity, these effects being antagonised by concurrent administration of flumazenil (RO 15-1788). Thus, our study showed the bidirectional effects of these two drugs on a spatial working memory task involving a spontaneous processing of information and suggested a direct link between the emotional effects of the drugs and memory performance.

Hormones, genes and the structure of sexual arousal

Despite the inherent difficulty of connecting individual genes with integrated mammalian behaviors, it has been determined that a series of genes are turned on by estrogenic hormones acting in forebrain. Their products are, in turn, facilitatory for female reproductive behaviors such as lordosis. The causal routes by which two genes contribute to the control of lordosis behavior, the classical estrogen receptor gene (ER-alpha) and a thyroid hormone (TH) receptor gene (TR-beta), have been delineated. Beyond the mechanisms underlying the expression of concrete, specific natural behaviors, lies the question of sexual motivation. Required as an intervening variable to explain fluctuations in natural behaviors in the face of constant stimuli, motivational states have both general and specific features. Most theoretical and experimental approaches toward the general aspects of motivation have depended heavily on concepts of 'arousal.' Sexual arousal is likely to depend both on very general, broadly distributed neuronal influences and on specific affiliative and sexual tendencies. Is 'general arousal' a monolithic, undifferentiated process? In no way can a review at this time settle such issues, but the reasons behind six new experimental approaches to these questions are described.

Behavioural assessment of mice lacking D1A dopamine receptors

Dopamine D1A receptor-deficient mice were assessed in a wide variety of tasks chosen to reflect the diverse roles of this receptor subtype in behavioural regulation. The protocol included examination of exploration and locomotor activity in an open field, a test of sensorimotor orienting, both place and cue learning in the Morris water maze, and assessment of simple associative learning in an olfactory discrimination task. Homozygous mice showed broad-based impairments that were characterized by deficiencies in initiating movement and/or reactivity to external stimuli. Data obtained from flash evoked potentials indicated that these deficits did not reflect gross visual impairments. The partial reduction in D1A receptors in the heterozygous mice did not affect performance in most tasks, although circumscribed deficits in some tasks were observed (e.g., failure to develop a reliable spatial bias in the water maze). These findings extend previous behavioural studies of null mutant mice lacking D1A receptors and provide additional support for the idea that the D1A receptor participates in a wide variety of behavioural functions. The selective impairments of heterozygous mice in a spatial learning task suggest that the hippocampal/cortical dopaminergic system may be uniquely vulnerable to the partial loss of the D1A receptor.

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.

A role for D2, but not D1, dopamine receptors in the response-reinstating effects of food reinforcement

Although the reinforcing properties of food are reduced in the presence of dopamine antagonist drugs, controversy exists about the relative roles of D1 vs D2 receptor subtypes in the actions of these drugs. The current experiment compared the effects of raclopride (a selective D2 receptor antagonist) and SCH 39166 (a selective D1 receptor antagonist) in the response-reinstating effects of food reinforcement. Hungry rats were trained to run a straight-alley for food reinforcement during single daily trials. The operant was then extinguished during consecutive daily non-reinforced trials. Subjects were then injected with one of four doses of raclopride (0.0, 1.0, 0.5, and 0.25 mg/kg, i.p.) or SCH 39166 (0.0, 1.0, 0.5, and 0.1 mg/kg i.p.) 30 min prior to a single reinforced treatment trial. Twenty-four h later, a test trial was conducted in an unbaited runway. The single reinforced trial in the midst of extinction was observed to reinstate operant runway performance. Raclopride, but not SCH 39166, dose-dependently attenuated this reinstatement. Motor control groups ruled out the possibility that these results were due to differential residual motor effects of the drugs. Results suggest that D2, but not D1, dopamine receptors, are involved in the response-reinstating properties of food reinforcement.

A synergistic interaction between dopamine D1 and D2 receptor subtypes in the memory impairments induced by concussive brain injury (CBI) in mice

Profound latent learning and memory deficits with increased monoamine levels in the brain following concussive brain injury (CBI) have been documented in our previous work. The purpose of the present study was to determine the role of dopamine (DA) receptor subtypes in the memory deficits associated with CBI. Profound latent learning and memory impairments were observed in the vehicle-treated CBI mice. SCH-23390 administered 15 min post-injury had no significant effects on the impairments of latent learning and memory in the CBI mice. Sulpiride significantly improved the impairments of latent learning and memory in a dose-dependent manner, indicating that activation of dopaminergic neuronal function is involved in the CBI-induced amnesia. Interestingly, co-administration of sulpiride and SCH-23390, at doses which alone has no significant effect, significantly ameliorated the impairments of latent learning and memory. These results strongly suggested that D1 and D2 receptor subtypes are synergistically involved in the dysfunction of learning and memory associated with CBI.

Spatial working memory over long retention intervals: dependence on sustained cholinergic activation in the septohippocampal or nucleus basalis magnocellularis-cortical pathways?

Previous direct neurochemical studies of the temporal dynamics of cholinergic activation in the septohippocampal and nucleus basalis magnocellularis-cortical pathways at various stages during repeated testing of mice with selective spatial reference or working memory protocols [Durkin and Toumane (1992), Behav. Brain Res. 50, 43-52] showed that the post-test durations of cholinergic activation in each pathway varied as a function of the type of memory tested and the level of task mastery. Since (i) the hippocampal formation is considered to constitute a critical component of a temporary memory buffer, and (ii) working memory items are not thought to be submitted to consolidation and permanent storage, we postulated that the duration of testing-induced cholinergic activation in the septohippocampal pathway may govern the maintenance of the working memory trace over the retention interval. In order to test directly this hypothesis C57 B1/6 mice were extensively trained (one trial/day, 25-30 days) on an identical selective working memory task to attain high levels of retention (> 80% correct), but using either 5 min (Group 1), or 60 min (Group 2) retention intervals. At various times (30 s-75 min) following the initial acquisition phase of the test, cholinergic activity in the hippocampus and frontal cortex was quantified using measures of high-affinity choline uptake. Whereas cholinergic activation was observed in both pathways at 30 s post-acquisition and throughout the 5 min retention interval in Group 1, the situation in Group 2 is different, activation of the septohippocampal pathway being maintained for only 15 min, while activation in the nucleus basalis magnocellularis-cortical pathway is maintained for the totality of the 1 h retention interval. The nucleus basalis magnocellularis-cortical cholinergic pathway, in addition to its role in long-term reference memory storage processes may, thus, via an intervention in the temporal encoding of information, also subsume a complementary intermediate-term buffer storage role in working memory situations requiring retention intervals in excess of 15 min in mice. This secondary, "backup", function of the nucleus basalis magnocellularis-cortical pathway would thus liberate the septohippocampal complex from its primary active role in the temporary maintenance and/or accessibility of the working memory trace in these particular cases requiring long retention intervals.

Levemopamil injection after cerebral oligemia reduces spatial memory deficits in rats

Transient reduction of cerebral blood flow to oligemic levels as produced by bilateral clamping of carotid arteries (BCCA) in pentobarbital anesthetized Wistar rats leads to spatial orientation deficiencies in a water maze test 8-10 days after surgery. These deficiencies are more pronounced in 4-month-old than in 6-week-old animals. Levemopamil [(2S)-2-isopropyl-5-(methylphenethylamino)-2-phenylvaleronitril e hydrochloride], a Ca2+ channel blocker and 5-HT2 antagonist, prevents the deficiencies in groups of animals of both ages, even when administered 24 h after the transient vessel occlusion. Levemopamil did not influence the maze performance of sham-operated control rats. Levemopamil, therefore, appears to modulate mechanisms that are altered specifically either by, or as a consequence of, the BCCA procedure. Levemopamil did not influence the altered GABA or ACh content in different vulnerable brain structures following BCCA, showing that the substance acts via additional mechanisms affected by the BCCA procedure.

Effects of amphetamine and medial septal lesions on acquisition and retention of radial maze learning in rats

Procholinergic drugs have failed to overcome the memory deficit induced by alterations of the cholinergic system because their neurochemical target in the brain is either lacking or disorganised. However, there are many reports on a relative involvement of the dopaminergic system in learning and memory that may compensate for the cholinergic deficit because of the interaction or balance between neurotransmitters and the redundancy of the brain. The aim of our experiments is to examine the activation of the dopaminergic system on the performance of normal and medial septal lesioned rats in the radial maze test involving continuous choices. In the first experiment different groups of normal rats were treated with either 0.5, 1.0 or 2.0 mg/kg of D-amphetamine and tested in the radial maze. In the second experiment, medial septal lesioned rats which had learnt pre-op the radial maze test were retested a month later. Amphetamine had no effect on the memory measures provided by the radial maze test in normal and lesioned rats, but non-memory measures were significantly affected: amphetamine decreased the sequential choice responses and the time taken by the rats to perform the test. The present results show that the activation of the dopaminergic system does not compensate for the alteration of the cholinergic activity inducing amnesia, however, they support the recent data on the improving effect of amphetamine on locomotor activity. The interpretation of drug/lesion interaction effects is discussed in this paper in relation to the literature on the effect of promnesic drugs.

Effects of dopaminergic drugs on working and reference memory in rats

Changes in dopaminergic function have been associated with alterations in motor and cognitive function in man and in animals. This study was designed to assess the effects of dopaminergic drugs on these aspects of conditioned behavior in animals. Male Long-Evans rats were trained to perform an appetitive operant task that allowed daily quantification of working memory (accuracy of spatial delayed nonmatching-to-position), reference memory (accuracy of visual discrimination) and motor function [choice lever-press latency and nosepoke interresponse time (IRT) during delay]. The indirect dopamine agonist d-amphetamine (0.3-1.0 mg/kg) reduced nonmatching accuracy without significantly affecting discrimination accuracy, response latency, or nosepoke IRT. The D2/D3 agonist quinpirole (0.01-0.056 mg/kg) also decreased nonmatching accuracy without changing discrimination accuracy, but increased choice response latency and nosepoke IRT as well. The D1 agonist SKF 38393 (1.0-3.0 mg/kg) and the D1 antagonist SCH 23390 (0.01-0.03 mg/kg) only affected nosepoke IRT, at doses below those causing response failure. The D2 antagonist raclopride (0.056-0.177 mg/kg) exerted no significant effects at doses that did not suppress responding completely. The selective reduction of nonmatching accuracy by d-amphetamine and quinpirole indicates a mnemonic impairment specific to working memory (relative to reference memory). These results suggest further 1) that stimulation of D2/D3, but not D1, receptors may account for the d-amphetamine-induced deficit in working memory; 2) that stimulation of D2/D3 receptors alone by quinpirole may also impair spatial working memory, but only in conjunction with motor slowing; and 3) that antagonism of either receptor type (by SCH 23390 or raclopride) does not significantly affect memory at doses causing motor slowing and response failure.