The relationship between stereotypy and memory improvement produced by amphetamine

mTORC2/rictor signaling disrupts dopamine-dependent behaviors via defects in striatal dopamine neurotransmission

Disrupted neuronal protein kinase B (Akt) signaling has been associated with dopamine (DA)-related neuropsychiatric disorders, including schizophrenia, a devastating mental illness. We hypothesize that proper DA neurotransmission is therefore dependent upon intact neuronal Akt function. Akt is activated by phosphorylation of two key residues: Thr308 and Ser473. Blunted Akt phosphorylation at Ser473 (pAkt-473) has been observed in lymphocytes and postmortem brains of schizophrenia patients, and psychosis-prone normal individuals. Mammalian target of rapamycin (mTOR) complex 2 (mTORC2) is a multiprotein complex that is responsible for phosphorylation of Akt at Ser473 (pAkt-473). We demonstrate that mice with disrupted mTORC2 signaling in brain exhibit altered striatal DA-dependent behaviors, such as increased basal locomotion, stereotypic counts, and exaggerated response to the psychomotor effects of amphetamine (AMPH). Combining in vivo and ex vivo pharmacological, electrophysiological, and biochemical techniques, we demonstrate that the changes in striatal DA neurotransmission and associated behaviors are caused, at least in part, by elevated D2 DA receptor (D2R) expression and upregulated ERK1/2 activation. Haloperidol, a typical antipsychotic and D2R blocker, reduced AMPH hypersensitivity and elevated pERK1/2 to the levels of control animals. By viral gene delivery, we downregulated mTORC2 solely in the dorsal striatum of adult wild-type mice, demonstrating that striatal mTORC2 regulates AMPH-stimulated behaviors. Our findings implicate mTORC2 signaling as a novel pathway regulating striatal DA tone and D2R signaling.

Effects of post-training heroin and d-amphetamine on consolidation of win-stay learning and fear conditioning

It has been proposed that the reinforcing properties of drugs of abuse are due, in part, to their ability to enhance memory consolidation. To test this hypothesis, heroin (0.03-3 mg/kg, SC) and d-amphetamine (0.5-2 mg/kg, SC) were administered to male Sprague-Dawley rats immediately or 4 h after training on win-stay and fear conditioning tasks. On the win-stay, immediate post-training administration of lower doses of heroin and d-amphetamine enhanced acquisition, and probe tests further revealed that these drugs enhanced different aspects of learning. Higher doses had no effect or impaired performance, particularly when administered repeatedly. On fear conditioning, the memory-enhancing effects of immediate post-training administration of lower heroin and d-amphetamine doses were revealed only when a single tone-shock pairing procedure was employed. Therefore, under appropriate experimental conditions, mildly stimulatory doses of heroin and d-amphetamine enhanced the acquisition of tasks thought to involve different types of learning. These results support the hypothesis that one of the ways in which drugs of abuse such as opiates and psychomotor stimulants reinforce behavior is by enhancing memory consolidation processes.

Memory enhancement produced by post-training exposure to sucrose-conditioned cues

A number of aversive and appetitive unconditioned stimuli (such as shock and food) are known to produce memory enhancement when they occur during the post-training period. Post-training exposure to conditioned aversive stimuli has also been shown to enhance memory consolidation processes. The present study shows for the first time that post-training exposure to conditioned stimuli previously paired with consumption of a sucrose solution also enhances memory consolidation. Male Long Evans rats were trained on a one-session conditioned cue preference (CCP) task on a radial arm maze. Immediately or 2 hours after training, rats consumed a sucrose solution or were exposed to cues previously paired with consumption of sucrose or cues previously paired with water. Twenty-four hours later, the rats were tested for a CCP. Immediate, but not delayed, post-training consumption of sucrose enhanced memory for the CCP. Immediate, but not delayed, post-training exposure to cues previously paired with sucrose, but not with water, also enhanced CCP memory. The possibility that rewarding and aversive conditioned stimuli affect memory by a common physiological process is discussed.

Impaired neurocognitive functions affect social learning processes in oppositional defiant disorder and conduct disorder: implications for interventions

In this review, a conceptualization of oppositional defiant (ODD) and conduct disorder (CD) is presented according to which social learning processes in these disorders are affected by neurocognitive dysfunctions. Neurobiological studies in ODD and CD suggest that the ability to make associations between behaviors and negative and positive consequences is compromised in children and adolescents with these disorders due to reduced sensitivity to punishment and to reward. As a result, both learning of appropriate behavior and learning to refrain from inappropriate behavior may be affected. Likewise, problem solving is impaired due to deficiencies in inhibition, attention, cognitive flexibility, and decision making. Consequently, children and adolescents with ODD and CD may have difficulty learning to optimize their behavior in changeable environments. This conceptualization of ODD and CD is relevant for the improvement of the effect of psychological treatments. Behavioral and cognitive-behavioral interventions that have been shown to be modestly effective in ODD and CD are based on social learning. Limited effectiveness of these interventions may be caused by difficulties in social learning in children and adolescents with ODD and CD. However, although these impairments have been observed at a group level, the deficits in reward processing, punishment processing, and cognitive control mentioned above may not be present to the same extent in each individual with ODD and CD. Therefore, the neurocognitive characteristics in children and adolescents with ODD and CD should be assessed individually. Thus, instead of delivering interventions in a standardized way, these programs may benefit from an individualized approach that depends on the weaknesses and strengths of the neurocognitive characteristics of the child and the adolescent.

Importance of D1 and D2 receptors in the dorsal caudate-putamen for the locomotor activity and stereotyped behaviors of preweanling rats

Dopaminergic compounds often affect the unlearned behaviors of preweanling and adult rats differently, although the brain regions underlying these age-dependent behavioral effects have not been specified. A candidate brain region is the dorsal caudate-putamen (CPu); thus, a goal of the present study was to determine whether D1 and D2 receptors in the dorsal CPu are capable of modulating the unlearned behaviors of preweanling rats. In Experiments 1 and 2, selective and nonselective dopamine agonists were bilaterally microinjected into the dorsal CPu on postnatal day (PD) 18 and both locomotor activity and stereotypy were measured. In Experiment 3, the functional coupling of D1 and D2 receptors was assessed by microinjecting the D1 agonist SKF-82958 and the D₂/D₃ agonist quinpirole either alone or in combination. In Experiments 4 and 5, quinpirole and the D1 receptor antagonist SCH-23390, or SKF-82958 and the D2 receptor antagonist raclopride, were co-administered into the dorsal CPu to further assess whether a functional D1 or D2 receptor system is necessary for the expression of quinpirole- or SKF-82958-induced behaviors. Results showed that selective stimulation of D1 or D2 receptors in the dorsal CPu increased both the locomotor activity and stereotypy of preweanling rats. Receptor coupling was evident on PD 18 because co-administration of a subthreshold dose of SKF-82958 and quinpirole produced more locomotor activity than either agonist alone. Lastly, the dopamine antagonist experiments showed that both D1 and D2 receptor systems must be functional for SKF-82958- or quinpirole-induced locomotor activity to be fully manifested. When the present data are compared to results from non-ontogenetic studies, it appears that pharmacological manipulation of D1 and D2 receptors in the dorsal CPu affects the behavior of preweanling and adult rats in a generally similar manner, although some important age-dependent differences are apparent. For example, D1 and/or D2 agonists preferentially induce locomotor activity, and not intense stereotypy, in younger animals.

Participation of D 1-4 dopamine receptors in the pro-cognitive effects of angiotensin IV and des-Phe 6 angiotensin IV

Angiotensin IV (Ang IV) and des-Phe(6)Ang IV are naturally occurring neuroactive peptides of the renin-angiotensin system (RAS) involved in memory processing. However, the relevant mechanisms are poorly understood. In this review it is proposed that the pro-cognitive effects of these peptides are, at least partly, mediated by dopamine (DA). Recent studies demonstrated that the improvement of several memory aspects; recall of appetitively and aversively motivated behaviors and learning of spatial tasks by Ang IV and des-Phe(6)Ang IV was abolished, or significantly diminished by behaviorally inactive per se doses of the D(1) and D(2) receptor blockers SCH 23390 (R-[+]-7-chloro-8-hydroxy-3 methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) and remoxipride, respectively. The D(3) receptor inhibition with nafadotride was almost ineffective but again, the D(4) receptor blockade by L745,870 hydrochloride (3-{[4-(4-chlorophenyl)piperazin-1-yl]methyl}-1H-pyrrolo[2,3-b]pyridine hydrochloride) diminished all, except for spatial memory, improving actions of the peptides. These results suggest that Ang IV and des-Phe(6)Ang IV enhance memory in a brain region-specific manner, dependent on local DA receptor subpopulations and the memory aspects controlled by them. The data reviewed here, demonstrating DA-Ang IV and des-Phe(6)Ang IV interactions in brain, strongly suggest probability of clinically relevant effects of concomitant use of antipsychotic and RAS affecting drugs.

Coherent gamma oscillations couple the amygdala and striatum during learning

The basolateral amygdala (BLA) mediates the facilitating effects of emotions on memory. The BLA's enhancing influence extends to various types of memories, including striatal-dependent habit formation. To shed light on the underlying mechanisms, we carried out unit and local field potential (LFP) recordings in BLA, striatum, auditory cortex and intralaminar thalamus in cats trained on a stimulus-response task in which the presentation of one of two tones predicted reward delivery. The coherence of BLA, but not of cortical or thalamic, LFPs was highest with striatal gamma activity, and intra-BLA muscimol infusions selectively reduced striatal gamma power. Moreover, coupling of BLA-striatal unit activity increased when LFP gamma power was augmented. Early in training, the rewarded and unrewarded tones elicited a modest increase in coherent BLA-striatal gamma. As learning progressed, this gamma coupling selectively increased in relation to the rewarded tone. Thus, coherent gamma oscillations coordinate amygdalostriatal interactions during learning and might facilitate synaptic plasticity.

Some highlights of research on the effects of caudate nucleus lesions over the past 200 years

This review describes experiments on the effects of caudate nucleus lesions on behavior in monkeys, cats and rats. Early work on monkeys and cats focused on the relationship of the caudate to the cortex in motor control, leading to the idea that the caudate serves to inhibit behaviors initiated by the cortex. However, investigation of this hypothesis with systematic behavioral testing in all three species did not support this idea; rather, these studies provided evidence that caudate lesions affect memory functions. Two main types of memory tasks were affected. One type involved reinforced stimulus-response (S-R) associations, the other involved spatial information, response-reinforcer contingencies, or working memory. Recent evidence, mainly from rats, suggests that the dorsolateral part of the caudoputamen is central to the processing and consolidation of memory for reinforced S-R associations, and that the more medial and anterior parts of the same structure are part of a neural circuit that (in some cases) also includes the hippocampus, and mediates relational information and certain forms of working memory. The possibility that the spatial distribution of the patch and matrix compartments within the caudoputamen underlies these regional differences is discussed.

The explicit learning of new names for known objects is improved by dexamphetamine

A randomised, double-blind, placebo-controlled, between subjects study design (N=37) was used to investigate the effects of dexamphetamine on explicit new name learning. Participants ingested 10mg of dexamphetamine or placebo daily over 5 consecutive mornings before learning new names for 50 familiar objects plus fillers. The dexamphetamine group recognised and recalled the new names more accurately than the placebo group over the 5 days and 1 month later. Word learning success was not associated with baseline neuropsychological performance, mood, cardiovascular arousal, or sustained attention. These results may have implications for the pharmacological treatment of acquired naming difficulties.

Dexamphetamine enhances explicit new word learning for novel objects

Past research suggests that dexamphetamine (Dex) can facilitate learning and memory in healthy individuals and after a neurological lesion. This study investigated the effects of Dex on the learning of names for new objects in young healthy adults (n=37) within an explicit learning paradigm by using a double-blind, placebo-controlled between- subjects design. Participants received 10 mg Dex or a placebo each morning over five consecutive days before viewing 100 novel objects with non-word names plus matched fillers. Compared to the placebo, Dex enhanced both the rate of learning and the retention of the words 1 wk and 1 month later. The improved word learning correlated with baseline attention and memory scores for participants in the Dex group only. No correlations were observed between word-learning success and sustained attention, mood or cardiovascular arousal. It was concluded that the improved explicit word learning may have reflected dexamphetamine-induced changes in short-term memory and/or memory consolidation.

The value of spontaneous alternation behavior (SAB) as a test of retention in pharmacological investigations of memory

Because of its reliance on memory, the tendency for rats, mice and other animals to alternate successive choices of T- or Y-maze arms has assumed considerable popularity in pharmacological studies of spatial memory as a quick and simple measure of retention that avoids the need for extensive training and the use of conventional reinforcers. Two forms of this tendency have been utilized, namely two-trial and continuous spontaneous alternation behavior (SAB). However, as the behavior can also reflect drug-related changes in sensory/attentional, motivational and performance processes, SAB should not be unquestionably accepted as a measure of memory alone. While assessments of post-acquisition drug effects on longer term memory may be possible through the appropriate timing of drug administration, this is more problematic if SAB is used as a measure of shorter term memory. Even though SAB can be a useful index of responsiveness to novelty, its value as a measure of retention is less certain. In this latter respect, a possible alternative to SAB testing might be the recently developed form of the related procedure, responsiveness to change.

Mnemonic functions of dorsal striatum and hippocampus in aversive conditioning

These experiments examined the mnemonic functions of hippocampus and dorsal striatum in Pavlovian aversive conditioning. Rats were trained in a single session by presenting three paired tones and footshocks. Immediately after training, they were given microinjections of D-amphetamine or vehicle into either dorsal hippocampus or dorsal striatum. Twenty-four hours later conditioned freezing (measured as cessation of movement by infrared motion detectors) to the experimental context and to the tone were measured separately. Compared to vehicle injections, amphetamine injections into dorsal hippocampus significantly increased conditioned freezing to the context but not to the tone. Injections into dorsal striatum increased conditioned freezing to both the context and the tone. These results converge with findings from lesion experiments suggesting that hippocampus is involved in aversive conditioning with contextual CSs only, and that dorsal striatum is involved in aversive conditioning with both contextual and discrete cue CSs. The functions of the these two structures in relation to that of the amygdala in the mediation of conditioned freezing are discussed.

Learning and memory functions of the Basal Ganglia

Although the mammalian basal ganglia have long been implicated in motor behavior, it is generally recognized that the behavioral functions of this subcortical group of structures are not exclusively motoric in nature. Extensive evidence now indicates a role for the basal ganglia, in particular the dorsal striatum, in learning and memory. One prominent hypothesis is that this brain region mediates a form of learning in which stimulus-response (S-R) associations or habits are incrementally acquired. Support for this hypothesis is provided by numerous neurobehavioral studies in different mammalian species, including rats, monkeys, and humans. In rats and monkeys, localized brain lesion and pharmacological approaches have been used to examine the role of the basal ganglia in S-R learning. In humans, study of patients with neurodegenerative diseases that compromise the basal ganglia, as well as research using brain neuroimaging techniques, also provide evidence of a role for the basal ganglia in habit learning. Several of these studies have dissociated the role of the basal ganglia in S-R learning from those of a cognitive or declarative medial temporal lobe memory system that includes the hippocampus as a primary component. Evidence suggests that during learning, basal ganglia and medial temporal lobe memory systems are activated simultaneously and that in some learning situations competitive interference exists between these two systems.

Cocaine enhances retention of avoidance conditioning in rats

The effects of post-training cocaine administration were tested on retention of a one-way active avoidance task in rats. A 5.0 mg/kg IP dose of cocaine enhanced retention of the avoidance task, in three separate experiments, as indicated by an increase in the number of avoidances made when animals were tested 24 h after training, while both a lower (2.5 mg/kg) and a higher (7.5 mg/kg) cocaine dose had no effect. Lidocaine (4-8 mg/kg) administered post-training did not reliably affect retention in the same task. Cocaine's ability to enhance retention depended on the interval between training and drug injection such that only cocaine administered directly after training enhanced retention the following day. The results show that post-training cocaine administration enhances retention of an active avoidance task in rats, and that this effect is probably independent of the anesthetic properties of the drug.

Role of dopamine in the behavioural actions of nicotine related to addiction

Experimental impairment of dopamine function by 6-hydroxydopamine lesions or by dopamine receptor antagonists shows that dopamine is involved in nicotine's discriminative stimulus properties, nicotine-induced facilitation of intracranial self-stimulation, intravenous nicotine self-administration, nicotine conditioned place-preference and nicotine-induced disruption of latent inhibition. Therefore, nicotine depends on dopamine for those behavioural effects that are most relevant for its reinforcing properties and are likely to be the basis of the abuse liability of tobacco smoke. On the other hand, in vivo monitoring studies show that nicotine stimulates dopamine transmission in specific brain areas and in particular, in the shell of the nucleus accumbens and in areas of the extended amygdala. These effects of nicotine resemble those of a reward like food except that nicotine-induced release of dopamine does not undergo single-trial, long-lasting habituation. It is speculated that repeated non-habituating stimulation of dopamine release by nicotine in the nucleus accumbens shell abnormally facilitates associative stimulus-reward learning. Acute effects of nicotine on dopamine transmission undergo acute and chronic tolerance; with repeated, discontinuous exposure, sensitization of nicotine-induced stimulation of dopamine release in the nucleus accumbens core takes place while the response in the shell is reduced. It is speculated that these adaptive changes are the substrate of a switch from abnormal incentive responding controlled by consequences (action-outcome responding) into abnormal habit responding, triggered by conditional stimuli and automatically driven by action schemata relatively independent from nicotine reward. These two modalities might coexist, being utilized alternatively in relation to the availability of tobacco. Unavailability of tobacco disrupts the automatic, implicit modality of abnormal habit responding switching responding into the explicit, conscious modality of incentive drug-seeking and craving.

In mice tonic estrogen replacement therapy improves non-spatial and spatial memory in a water maze task

We investigated the effects of estrogen replacement therapy on water maze non-spatial and spatial navigation in mice. Three groups of mice were ovariectomized and two of these groups being implanted with s.c. pellets that produce blood levels of estrogen close to those found in estrous (estrogen low, 75-100 pg/ml blood) or proestrous (estrogen high, 300-400 pg/ml). The behavioral assessment was initiated 7 days after pellet implantation. Non-spatial navigation to a clearly visible platform was stimulated by low and high levels of estrogen. However, spatial navigation to a hidden platform was improved by low estrogen levels. We found that estrogen improves two different types of memory processes that depend on striatal (non-spatial navigation) and hippocampal (spatial) memory systems.

Posttraining estrogen and memory modulation

The present paper provides a review of recent research carried out in this laboratory investigating the effects of posttraining peripheral and intrahippocampal injection of estradiol on memory in rats, and estradiol-acetylcholine interactions in memory modulation. Ovariectomized rats received an eight-trial training session in a hippocampal-dependent hidden platform water maze task. Immediately following training, rats received a posttraining peripheral or intrahippocampal injection of estradiol-cyclodextrin complex or vehicle. Twenty-four hours later rats were returned to the maze for a retention test session, and latency to escape was used as a measure of memory for the previous day's training. Peripheral posttraining injection of estradiol enhances memory relative to vehicle-treated rats. Injections of estradiol given 2 h posttraining has no effect on retention, indicating a time-dependent effect of estradiol on memory storage processes. A time-dependent memory enhancing effect of posttraining intrahippocampal injections of estradiol has also been observed in both male and ovariectomized female rats. The memory enhancing effect of peripheral posttraining injection of estradiol in ovariectomized rats is blocked by a subeffective dose of the acetylcholine muscarinic receptor antagonist scopolamine, suggesting that estradiol interacts with cholinergic systems in memory modulation. Concurrent peripheral posttraining injection of a subeffective dose of estradiol and a subeffective dose of the cholinergic agonist oxotremorine produces a synergistic memory enhancing effect. The findings suggest that: (1) estradiol selectively influences memory storage independent of an effect on nonmnemonic processes, (2) the hippocampus is a potential neuroanatomical site of action mediating estrogenic effects on memory, and (3) estradiol interacts with cholinergic systems in memory modulation.

Infusion of the dopamine D1 receptor antagonist SCH 23390 into the amygdala blocks fear expression in a potentiated startle paradigm

Dopamine (DA) D1 receptors are distributed in the nucleus accumbens and the amygdala, two regions of the mesocorticolimbic DA system known to be activated by aversive environmental stimuli. The objective of the present study was to determine the contribution of D1 receptors in these brain regions to the expression of a fear-motivated behavior, notably, potentiated startle in rats. Injection of the DA D1 receptor antagonist SCH 23390 into the amygdala blocked the ability of a conditioned light stimulus previously paired with footshock to enhance acoustic startle amplitudes. Bilateral intracerebral administration of SCH 23390 into the nucleus accumbens had no effect on fear-potentiated startle. The observed opposing effects of amygdaloid DA D1 receptor antagonism on fear expression, along with earlier research demonstrating the involvement of ventral tegmental area (VTA) DA neurons on fear-potentiated startle, suggest a role for mesoamygdaloid activity in conditioned excitatory fear reactions.

Behavioral functions of nucleus accumbens dopamine: empirical and conceptual problems with the anhedonia hypothesis

Nucleus accumbens (DA) has been implicated in a number of different behavioral functions, but most commonly it is said to be involved in "reward" or "reinforcement". In the present article, the putative reinforcement functions of accumbens DA are summarized in a manner described as the "General Anhedonia Model". According to this model, the DA innervation of the nucleus accumbens is conceived of as a crucial link in the "reward system", which evolved to mediate the reinforcing effects of natural stimuli such as food. The reward system is said to be activated by natural reinforcing stimuli, and this activation mediates the reinforcing effects of these natural stimuli. According to this view, other stimuli such as brain stimulation and drugs can activate this system, which leads to these stimuli being reinforcing as well. Interference with DA systems is said to blunt the reinforcing effects of these rewarding stimuli, leading to "extinction". This general model of the behavioral functions of accumbens DA is utilized widely as a theoretical framework for integrating research findings. Nevertheless, there are several difficulties with the General Anhedonia Model. Several studies have observed substantial differences between the effects of extinction and the effects of DA antagonism or accumbens DA depletions. Studies involving aversive conditions indicate that DA antagonists and accumbens DA depletions can interfere with avoidance behavior, and also have demonstrated that accumbens DA release is increased by stressful or aversive stimuli. Although accumbens DA is important for drug abuse phenomena, particularly stimulant self-administration, studies that involve other reinforcers are more problematic. A large body of evidence indicates that low doses of dopamine antagonists, or depletions of accumbens DA, do not impair fundamental aspects of food motivation such as chow consumption and simple instrumental responses for food. This is particularly important, in view of the fact that many behavioral researchers consider the regulation of food motivation to be a fundamental aspect of food reinforcement. Finally, studies employing cost/benefit analyses are reviewed, and in these studies considerable evidence indicates that accumbens DA is involved in the allocation of responses in relation to various reinforcers. Nucleus accumbens DA participates in the function of enabling organisms to overcome response costs, or obstacles, in order to obtain access to stimuli such as food. In summary, nucleus accumbens DA is not seen as directly mediating food reinforcement, but instead is seen as a higher order sensorimotor integrator that is involved in modulating response output in relation to motivational factors and response constraints. Interfering with accumbens DA appears to partially dissociate the process of primary reinforcement from processes regulating instrumental response initiation, maintenance and selection.

Amphetamine enhances memory retention and facilitates norepinephrine release from the hippocampus in rats

The present study investigated the effects of intrahippocampal amphetamine on memory retention and the role of hippocampal norepinephrine (NE) in memory consolidation in rats. One-way inhibitory avoidance learning paradigm was adopted. Animals were trained to avoid the foot shock. The latency to step into the shock compartment was recorded as the retention measure. The ceiling score (full retention) was 600 s. Results indicated that intra-hippocampal injections of amphetamine produced a dose-dependent enhancement of memory retention with doses at 0.6 micrograms and 1.6 micrograms reaching a significant effect. The beta-adrenergic blocker propranolol, at a dose which did not affect retention alone (80 ng), antagonized the memory-enhancing effect of amphetamine. Along with this memory-enhancing effect, amphetamine also elevated the level of NE release, and this effect was significant in animals not showing a full retention score (nonresponders) than in animals showing a full retention score (responders), as assayed by in vivo microdialysis. Within the control group, the responders also had a higher level of NE than the nonresponders. All these results are probably due to the fact that responders have a higher level of NE release than nonresponders. The effect of amphetamine on NE release is, therefore, not as obvious in responders. These results together support our hypothesis that NE plays a facilitatory role in the memory process and amphetamine enhances retention performance, at least in part, through facilitation of hippocampal NE release.

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.

Ginsenoside Rg1 inhibits the brain cAMP phosphodiesterase activity in young and aged rats

1. The in vitro effect of ginsenoside Rg1 from Panax ginseng on the low- and high-KM cyclic AMP phosphodiesterase (cAMP PDE) activity in the frontal cortex, striatum, hypothalamus and hippocampus of young (4-5-month old) and aged (22-month old) rats has been studied. 2. Administered in increasing concentrations (from 5 x 10(-5) M up to 5 x 10(-4) M), ginsenoside Rg1 exerted a pronounced inhibitory effect on the low- and high-KM enzyme activity in all brain structures studied in rats of both age groups. 3. Ginsenoside Rg1 exhibited inhibitory potency similar to that of theophylline. 4. The present results provide evidence for the CNS effects of ginsenoside Rg1 through inhibition of the intracellular level of cAMP.

Memory enhancement by post-training peripheral administration of low doses of dopamine agonists: possible autoreceptor effect

These experiments examined the effect of post-training injections of low doses of dopamine (DA) agonists on the acquisition of two 8-arm radial maze tasks. On a winstay simultaneous discrimination task, a light cue signaled the location of food in four randomly selected arms on each trial, and animals were required to visit each of the lit arms twice within a trial. Animals received one food trial per day and were injected immediately after training on Day 5. The direct DA receptor agonist, apomorphine (0.05 mg/kg), and the direct D2-DA receptor agonists, LY 177555 (quinpirole: 0.05, 0.1 mg/kg) and B-HT 920 (0.05 mg/kg), all improved acquisition of winstay radial maze behavior relative to saline-injected controls. On a win-shift task, rats were allowed to obtain food from four randomly selected maze arms, followed by a delay period in which they were removed from the maze. Animals were returned to the maze for a retention test in which only those arms that had not been visited prior to the delay contained food. After training on shorter delays, a delay of 18 h was imposed between the first four and second four choices, and DA agonists were injected immediately after the first four choices. Apomorphine, LY 171555, and B-HT 920 (all at 0.05 mg/kg), all improved win-shift retention relative to saline-injected controls. On both tasks, delaying the injections for 2 h post-training eliminated the memory-improving effects of all drugs. The results indicate that post-training administration of DA agonists at doses that may preferentially stimulate autoreceptors improves memory.

Cocaine and amphetamine facilitate retention of jump-up responding in rats

The effects of cocaine and d-amphetamine administration on the acquisition of an automated jump-up active avoidance task were examined in two separate experiments. On days 1 and 2, male Sprague-Dawley rats received one escape-only training trial, followed immediately by the intraperitoneal injection of cocaine, amphetamine, or saline. On day 3, subjects received eight escape/avoidance trials. The posttraining administration of cocaine (2.75 and 5.55 mg/kg) and amphetamine (0.3 and 1.0 mg/kg) on days 1 and 2 facilitated jump-up avoidance performance on day 3. Importantly, both cocaine and amphetamine enhanced learning and memory under experimental conditions that allowed for drug-free training and testing.

D-1 receptor involvement in reward-related learning

A comparison of the effects of apomorphine, amphetamine and dopamine (DA) receptor subtype-specific agonists in responding for conditioned reward, self-administration and place conditioning paradigms provides insights into the possible involvement of D-1 and D-2 receptors in reward-related learning. Amphetamine and the D-2 agonists bromocriptine and quinpirole enhanced responding for conditioned reward, were self-administered and produced place preferences. Apomorphine impaired responding for conditioned reward by enhancing responding on two levers, was self-administered and produced a place preference. The D-1 agonist SKF 38393 impaired responding for condition reward, did not support self-administration and produced a place preference. The failure of SKF 38393 to support self-administration may have been related to effects of this drug, for example, peripheral aversive effects or a slow onset of action, unrelated to its action at the D-1 receptor. It was suggested that a D-1 agonist might be expected to be self-administered from the point of view of the hypothesis that it is the action at D-1 receptors of DA released in association with reward that produces reward-related learning. This hypothesis was supported by the remaining data. Thus, apomorphine and SKF 38393 may have masked the DA signal associated with reward in the conditioned reward paradigm leading to a loss of control of responding by the conditioned rewarding stimulus. In self-administration, apomorphine would have its onset of action after the performance of the response which is followed immediately by a conditioned reward. The conditioned reward may effectively maintain control of behaviour by the lever and related stimuli while the drug may maintain the effectiveness of the conditioned reward. In place conditioning, there is no specific environmental stimulus that must come to control responding; therefore, apomorphine and SKF 38393 may have been seen to produce place preferences in spite of their relatively tonic action at D-1 receptors. Finally, the finding that the D-1 antagonists SCH 23390 or SCH 39166 blocked the effects of reward in these paradigms was taken as further evidence that the D-1 receptor may be critically involved in the learning produced by rewarding stimuli.

Localized intracaudate dopamine D2 receptor activation during the post-training period improves memory for visual or olfactory conditioned emotional responses in rats

Rats with cannulas aimed at the posteroventral (PV) or ventrolateral (VL) areas of the caudate nucleus were trained on a conditioned emotional response (CER) task. Post-training microinjections of the indirect catecholamine agonist, d-amphetamine (5 micrograms), or of the dopamine D2 receptor agonist, LY171555 (1 microgram), into the PV area improved retention of a CER with a visual CS, but had no effect on a CER with an olfactory CS. Post-training injections of the same two drugs into the VL area improved retention of a CER with an olfactory CS, but had no effect on a CER with a visual CS. Post-training injections of the dopamine D1 receptor agonist, SKF38393 (0.5, 1.0, 2.0 micrograms), into either site had no effects on either CER. These findings suggest that different areas of the caudate nucleus mediate acquisition of CERs with different CSs, possibly implicating the topographically organized corticostriatal innervation in the acquisition of certain types of memories in the caudate nucleus. The findings also suggest that dopamine D2 receptors in the caudate nucleus are involved in the acquisition of these CERs.

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.

Acute effects of d-amphetamine in a monkey operant behavioral test battery

The acute effects of d-amphetamine were assessed using a battery of complex food-reinforced operant tasks that included responding in delayed matching-to-sample (DMTS, n = 6), conditioned position responding (CPR, n = 7), progressive ratio (PR, n = 8), temporal response differentiation (TRD, n = 4), and incremental repeated acquisition (IRA, n = 9) tasks. Performance in these tasks is thought to depend upon specific brain functions such as short-term memory and attention (DMTS), color and position discrimination (CPR), motivation to work for food (PR), time perception (TRD), and learning (IRA). d-Amphetamine sulfate (0.01-1.0 mg/kg IV), given 15-min pression produced significant dose-dependent decreases in the number of reinforcers obtained in each task. Response accuracy was significantly decreased at doses of 0.3 and 1.0 mg/kg for TRD and at 1.0 mg/kg for CPR when compared to saline injections. Accuracy was not consistently affected in the DMTS or IRA tasks. Response rates decreased or response latencies increased significantly at doses of 0.3 and 1.0 mg/kg in the PR and DMTS tasks. A dose of 0.1 mg/kg for the IRA and TRD tasks, 0.3 mg/kg for DMTS and 1.0 mg/kg for the CPR tasks significantly decreased percent task completed. Thus, the relative sensitivities of these tasks for detecting d-amphetamine behavioral effects were IRA = TRD greater than PR = DMTS greater than CPR.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1 and alpha 2-adrenergic receptor blockade influences angiotensin II facilitation of avoidance behavior and stereotypy in rats

Pretreatment of rats with prazosin (PRA), an alpha 1-adrenergic receptor blocker, abolished the increased rate of learning of conditioned avoidance responses stimulated by intracerebroventricular angiotensin II (AII) administration. Yohimbine (YOH), an alpha 2-receptor blocker, reversed the effect of AII. PRA did not affect, and YOH abolished, the improvement of recall of a passive avoidance behavior caused by AII. The stereotypies produced by apomorphine (APO) and amphetamine (AMP) were enhanced by AII. PRA changed neither stereotypy, but it abolished the AII effect in both cases. YOH did not alter APO stereotypy and abolished the enhancement of that behavior caused by AII. YOH increased AMP stereotypy and had an additive effect with AII. No significant changes of exploratory motor activity were caused by PRA, YOH, or their combination, with AII. These findings indicate that functioning alpha 1- and alpha 2-adrenergic receptors are necessary for the facilitation of learning by AII, while only alpha 2-receptors appear to be involved in AII improvement of recall. The central dopaminergic system may in part be responsible for the modulation by PRA and YOH of the effects of AII on learning and recall.

Memory facilitation produced by dopamine agonists: role of receptor subtype and mnemonic requirements

The role of dopamine (DA) receptor subtypes in the acquisition of two memory tasks in the 8-arm radial maze was examined. The receptors were manipulated with posttraining, subcutaneous injections of an indirect DA receptor agonist (D-amphetamine), a selective D2 receptor agonist (LY171555), and a selective D1 receptor agonist (SKF-38393). On a win-stay task (sensitive to caudate nucleus lesions) a light cue signalled the location of food in 4 randomly selected arms on each trial. Rats were given one trial per day and injected after training on day 5. D-Amphetamine (2.0 mg/kg) and LY171555 (2.0 mg/kg) improved performance relative to controls; however SKF-38393 (1-4 mg/kg) had no effect on the acquisition of win-stay behavior. On a win-shift task (sensitive to fornix/hippocampal lesions) a delay of 18 hr was imposed between the first 4 and second 4 choices; drugs were injected after the first 4 choices. D-Amphetamine (1.0 mg/kg) and LY171555 (2.0 mg/kg) significantly improved retention relative to controls. SKF-38393 (1-4 mg/kg) had no effect on win-shift retention. These results suggest that the memory-improving properties of DA agonists on tasks sensitive to both hippocampal and caudate lesions are mediated by the D2 receptor.

Reward or reinforcement: what's the difference?

The histories of the terms "reward" and "reinforcement" are reviewed to show the difference in their origins. Reward refers to the fact that certain environmental stimuli have the property of eliciting approach responses. Evidence suggests that the ventral striatum (nucleus accumbens area) is central to the mediation of this behavior. Reinforcement refers to the tendency of certain stimuli to strengthen learned stimulus-response tendencies. The dorsolateral striatum appears to be central to the mediation of this behavior. Neuroanatomical and neurochemical data are adduced suggesting that reward may be mediated by a neural circuit including the neostriatal patch system, together with the hippocampus, limbic system (amygdala, prefrontal cortex) and ventral pallidum. The evidence also suggests that reinforcement, in the form of dopamine release in the striatal matrix, acts to promote the consolidation of sensori-motor associations. Thus, the matrix may mediate stimulus-response memory as part of a circuit including the cerebral cortex, substantia nigra pars reticulata and its projections to thalamic and brainstem motor areas.

Dissociation of visual and olfactory conditioning in the neostriatum of rats

In an experiment designed to demonstrate a double dissociation, the effects of bilateral electrolytic lesions of either the posteroventral or the ventrolateral regions of the neostriatum on the conditioned emotional response (CER) were examined. Posteroventral lesions impaired acquisition of the CER with a visual CS but not with an olfactory CS. Sham-operated posteroventral and ventrolateral lesioned animals acquired the visual CER normally. Ventrolateral lesions impaired acquisition of the CER with the olfactory CS but not with the visual CS. Sham-operated ventrolateral and posteroventral lesioned animals acquired the olfactory CER normally. In a second experiment the effect of post-training unilateral intrastriatal microinjections of (+)-amphetamine on acquisition of the visual and olfactory CERs was studied. Posteroventral injections improved retention of the visual, but not the olfactory CER. Ventrolateral injections improved retention of the olfactory, but not the visual CER. Saline and delayed (+)-amphetamine injection controls demonstrated that the improvement of retention in each case was a retroactive improvement of memory for the recently acquired CERs by (+)-amphetamine. These findings are consistent with previous reports of post-training memory facilitation mediated by dopaminergic function in the neostriatum. The results of both experiments are consistent with a regional functional heterogeneity hypothesis: the idea that anatomically linked areas of cortex and neostriatum process memories involving different stimuli in similar ways and that the integrity of these structures and their connections is necessary to establish and consolidate associative memory.

A functional hypothesis concerning the striatal matrix and patches: mediation of S-R memory and reward

Recent neurochemical and anatomical findings delineating two compartments, the patches and the matrix, in the mammalian striatum, are described. Anatomical and neurochemical aspects of manipulations affecting behaviors related to S-R memory and reward correspond in certain respects to the features of these striatal compartments. These coincidences lead to an hypothesis concerning the function of the striatal compartments and their participation in neural systems controlling both immediate and learned behaviors.

An hypothesis on the role of glucose in the mechanism of action of cognitive enhancers

This review presents evidence that some cognition enhancing drugs produce their beneficial effects on learning and memory by increasing the availability of glucose for uptake and utilization into the brain. The hypothesis further suggests that many cognition enhancing drugs act through a peripheral mechanism rather than directly on the brain. The general hypothesis is supported by four independent and converging pieces of evidence: 1) Some cognition enhancing drugs may not cross the blood-brain barrier, but can still facilitate memory; 2) Some cognition enhancing drugs are effective only when injected peripherally, but not when injected directly into the brain; 3) Many cognition enhancing drugs are not effective after adrenalectomy; 4) Cognitive function is correlated with glucose regulation in aged animals and humans. These four lines of research have implications for the role of glucose in the action of specific cognitive enhancers.

Effect of angiotensin II and saralasin on motor activity and the passive avoidance behavior of rats

One nmole of angiotensin II (ANG II) or saralasin, given intracerebroventricularly, failed to alter the motor activity of rats in open field. A combined injection of both peptides caused a significant decrease of the number of crossings and rearings. In the electromagnetic motimeter horizontal activity of animals was changed by neither of the peptides while the vertical activity was increased by ANG II. Again, a combined injection of saralasin and ANG II inhibited both horizontal and vertical activity. Stereotypies evoked by both apomorphine (2 mg/kg) and amphetamine (6.5 mg/kg), given intraperitoneally, were markedly intensified by ANG II and saralasin. A five-fold increase of the re-entry latencies in the passive avoidance situation was observed after pre-test administration of ANG II or saralasin but not the two in combination. These results suggest that ANG II and saralasin may improve processes related to learning and memory through an unspecific mechanism involving central dopamine systems.

Effect of nigrostriatal dopamine depletion on the post-training, memory-improving action of amphetamine

Systemic, post-training injections of d-amphetamine improve memory in a variety of learning situations. Evidence from self-stimulation and intra-striatal injection studies suggests that this effect may be a result of the amphetamine-promoted release of endogenous dopamine from nigro-striatal neurons. In the present study this hypothesis was supported by the demonstration that destruction of these neurons with intra-nigral injections of 6-hydroxydopamine eliminate the memory-improving effect of systemic, post-training injections of amphetamine.

Behavioral effects of angiotensin II and angiotensin II-(4-8)-pentapeptide in rats

One nM of angiotensin II (AII) or angiotensin II-(4-8)-pentapeptide [AII(4-8)] given intracerebroventricularly did not affect locomotor and exploratory behavior of rats in open field. AII significantly increased and AII(4-8) did not affect vertical activity of animals in electromagnetic motimeter. Neither of the peptides influenced horizontal activity in the motimeter. Both peptides intensified stereotypy produced by apomorphine and amphetamine. AII significantly improved, while AII(4-8) did not affect, consolidation of memory of the correct way to food in T-maze. Similarly, AII increased and AII(4-8) did not change the rate of acquisition of conditioned avoidance responses in a shuttle-box. Of the two examined peptides only AII significantly improved retrieval of memory of the passive avoidance behavior. The results show that AII(4-8) influences central dopaminergic system but, unlike its parent peptide AII, has no apparent effect on memory.

Effects of systemic and intracranial amphetamine injections on behavior in the open field: a detailed analysis

Systemic injections of amphetamine result in profound changes in the behavior of animals in an open field. There is an increase in activity, certain species-typical behaviors are produced, and there is a tendency for any elicited behavior to be repeated in a stereotyped way. The present study examined the contributions of dopamine terminal regions to these effects in rats by microinjecting amphetamine directly into one of six discrete sites (medial frontal cortex, nucleus accumbens, anteromedial caudate nucleus, ventrolateral caudate nucleus, amygdala, or the region surrounding the area postrema) and making detailed behavioral observations. This data was compared with the behavior of systemically injected rats that were also observed in the open field. An observer recorded the occurrence of twelve categories of behavior and recorded photocell beam interruptions during five post-injection observation periods. The results confirmed and extended previous accounts of the behavior of systemically injected rats, adding increased snout contact with the environment as an additional effect of amphetamine. Intracranial injections produced changes in activity level from several of the injection sites but there was no increase in the species-typical behaviors associated with stereotypy. Changes in the occurrence of some recorded behaviors were produced by injections into most of the sites and these data are presented in detail.

The conditioned place preference is affected by two independent reinforcement processes

The conditioned place preference method for measuring the affective properties of reinforcing events was studied using treatments of known affective value. The size of the place aversion observed increased with dose when the reinforcer was injections of lithium chloride. The size of the place preference observed increased with concentration when the reinforcer was drinking sucrose solutions. However, when the reinforcer was solutions of saccharin (that were consumed in the same amounts as the sucrose solutions) no place preferences were observed. This finding was explained in terms of the dual reinforcement hypothesis [20] which postulates that although sucrose and saccharin both have positive affective properties (based on their tastes) only sucrose has memory improving properties (based on its post-ingestive action). It was therefore proposed that conditioned place preferences depend on the activation of both affective and memory improving processes. This hypothesis was confirmed by the observation of place preferences with a saccharin solution as the reinforcer when the pairing trials were followed by non-contingent, post-pairing injections of glucose or amphetamine (both of which are known to improve memory). Therefore, behavior in the place preference method depends upon both the affective and the memory improving properties of the reinforcers under test.