Effects of dopaminergic drugs on working and reference memory in rats

Impacts of Neonatal Methylmercury on Behavioral Flexibility and Learning in Spatial Discrimination Reversal and Visual Signal Detection Tasks

Early postnatal development in rodents is sensitive to neurotoxic effects of the environmental contaminant, methylmercury. While juvenile and adolescent exposure also produce long-term impairments in behavior, the outcome of neonatal exposure is less understood. Neural development during the neonatal period in rodents is akin to that seen in humans during the third trimester of pregnancy but methylmercury exposure occurring during the neonatal period has not been modeled, partly because breast milk is a poor source of bioavailable methylmercury. To examine this developmental period, male Long-Evans rats were exposed to 0, 80, or 350µg/kg/day methylmercuric chloride from postnatal days 1 to 10, the rodent neonatal period. As adults, behavioral flexibility, attention, memory, and expression of the dopamine transporter in these rats was assessed. Rats exhibited changes in behavioral flexibility assessed in a spatial discrimination reversal procedure. Those rats exposed to the highest dose of methylmercury displayed subtly altered patterns of perseveration compared to control animals. During acquisition of the attention/memory procedure, rats exposed to this dose also had slower acquisition, and achieved lower overall accuracy during training, compared to controls despite neither attention nor memory being affected once the task was acquired. Finally, dopamine transporter expression in the striatum, prefrontal cortex, and hippocampus was unchanged in these adult rats. The results of this study replicate the trend of findings seen with exposure during gestation or during adolescence.

The Influence of CB2-Receptor Ligands on the Memory-Related Responses in Connection with Cholinergic Pathways in Mice in the Passive Avoidance Test

Background: Dysfunction of the cholinergic system is associated with the development of Alzheimer’s disease (AD). One of the new possible strategies for the pharmacological modulation of memory-related problems typical of AD, is connected with the endocannabinoid system (ECS) and the cannabinoid (CB: CB1 and CB2) receptors. Methods: The aim of the study was to determine the influence of the selective CB2 receptor ligands: agonist (JWH 133) and antagonist (AM 630) on different stages of memory and learning in mice, in the context of their interaction with cholinergic pathways. To assess and understand the memory-related effects in mice we used the passive avoidance (PA) test. Results: We revealed that co-administration of non-effective dose of JWH 133 (0.25 mg) or AM 630 (0.25 mg/kg) with the non-effective dose of cholinergic receptor agonist - nicotine (0.05 mg/kg) enhanced cognition in the PA test in mice; however, an acute injection of JWH 133 (0.25 mg/kg) or AM 630 (0.25 mg/kg) had no influence on memory enhancement induced by the effective dose of nicotine (0.1 mg/kg). Co-administration of JWH 133 (0.25 mg) or AM 630 (0.25 mg/kg) with the effective dose of the cholinergic receptor antagonist scopolamine (1 mg/kg) attenuated the scopolamine-induced memory impairment in the PA test in mice. Conclusion: Our experiments have shown that CB2 receptors participate in the modulation of memory-related responses, especially those in which cholinergic pathways are implicated.

The Acute Pharmacological Manipulation of Dopamine Receptors Modulates Judgment Bias in Japanese Quail

We have studied the effects of dopamine antagonists and agonists on Japanese quail behavior in the spatial judgment task. Twenty-four Japanese quail hens were trained in the spatial discrimination task to approach the feeder placed in the rewarded location (Go response, feeder containing mealworms) and to not approach the punished location (No-Go response, empty feeder plus aversive sound). In a subsequent spatial judgment task, the proportion of Go responses as well as approach latencies to rewarded, punished, and three ambiguous locations (near-positive, middle, near-negative, all neither rewarded nor punished) were assessed in 20 quail hens that successfully mastered the discrimination task. In Experiment 1, each bird received five treatments (0.1 and 1.0 mg/kg of dopamine D1 receptor antagonist SCH 23390, 0.05 and 0.5 mg/kg of dopamine D2 receptor antagonist haloperidol, and saline control) in a different order, according to a Latin square design. All drugs were administered intramuscularly 15 min before the spatial judgment test, with 2 days break between the treatments. Both antagonists caused a significant dose-dependent increase in the approach latencies as well as a decrease in the proportion of Go responses. In Experiment 2, with the design analogous to Experiment 1, the hens received again five treatments (1.0 and 10.0 mg/kg of dopamine D1 receptor agonist SKF 38393, 1.0 and 10.0 mg/kg of dopamine D2 receptor agonist bromocriptine, and saline control), applied intramuscularly 2 h before the test. The agonists did not have any significant effect on approach latencies and the proportion of Go responses in the spatial judgment task, as compared to the saline control, except for 10.0 mg/kg SKF 38393, which caused a decrease in the proportion of Go responses. The approach latency and the proportion of Go responses were affected by the cue location in both experiments. Our data suggest that the dopamine D1 and D2 receptor blockade leads to a decrease in the reward expectation and the negative judgment of stimuli. The effect of dopamine receptor activation is less clear. The results reveal that dopamine receptor manipulation alters the evaluation of the reward and punishment in the spatial judgment task.

Selective dopaminergic effects on attention and memory in male mice exposed to Methylmercury during adolescence

Gestational exposure to methylmercury disrupts dopamine-mediated behavior and produces heightened sensitivity to monoamine agonists later in life. This has been reported and replicated following both pre- and post-natal exposure. Impacts of methylmercury when exposure occurs during the sensitive period of adolescence, a key period of dopaminergic development, remain underexplored. There have been variable results thus far in studies investigating links between adolescent exposure to methylmercury and alterations in executive function and altered sensitivity to monoamine agonists. The current study was designed to investigate adolescent exposure by exposing male mice to 0, 0.3, or 3 ppm methylmercury during adolescence and training them in a hybrid task to assess two executive functions, attention and memory, in adulthood. Behavior in these animals was probed with a range of doses of the dopamine agonist, d-amphetamine, and the norepinephrine agonist, desipramine. Attention and memory in these mice were sensitive to disruption by d-amphetamine and interacted with methylmercury exposure. Choice latencies were also longer in the MeHg-exposed mice. Desipramine did not affect behavior in these animals nor did it interact with methylmercury. It is concluded that methylmercury-related inhibition of behavior observed in this study were differentially sensitive to acute disruption in dopamine, but not norepinephrine, neurotransmission.

Methylmercury, attention, and memory: baseline-dependent effects of adult d-amphetamine and marginal effects of adolescent methylmercury

Methylmercury (MeHg) is an environmental neurotoxicant known to disrupt behavior related to dopamine neurotransmission in experimental models. Such disruptions are sensitive to dopamine agonists when administered acutely after exposure to MeHg has ended or when administered concurrently with MeHg exposure. Sustained attention and short-term remembering, components of attention-deficit/hyperactivity disorder (ADHD), are partially mediated by dopamine neurotransmission. In order to observe MeHg-related alterations in sustained attention and short-term memory, as well as determine sensitivity of MeHg exposed animals to dopamine agonists commonly used in the treatment of ADHD symptoms, rats were exposed to 0, 0.5, or 5 ppm MeHg throughout adolescence and trained in a hybrid sustained attention/short term memory visual signal detection task in adulthood. Behavior was then probed with acute i.p. injections of the dopamine agonist, d-amphetamine, which improves impaired attention and inhibits short-term memory in clinical syndromes like ADHD. Acute d-amphetamine dose-dependently decreased short-term memory as well as sustained attention. While MeHg alone did not impair accuracy or memory, it did interact with d-amphetamine to produce baseline-dependent inhibition of behavior. These findings further show that changes in behavior following low-level exposure to MeHg during adolescence are augmented by dopamine agonists. Observed impairments in memory following acute d-amphetamine are consistent with previous findings.

5-HT2A receptors modulate dopamine D2-mediated maternal effects

Serotonin 5-HT_2A receptors are expressed throughout the mesolimbic and mesocortical dopamine pathways, and manipulation of this receptor system has a profound impact on dopamine functions and dopamine-mediated behaviors. It is highly likely that 5-HT_2A receptors may also modulate the D₂-mediated maternal effects. The present study investigated this issue and also explored the possible behavioral mechanisms. We tested the effects of two D₂ drugs (an agonist quinpirole: 0.5, 1.0 mg/kg, and a potent D₂ antagonist haloperidol: 0.05, 0.10 mg/kg, sc) and their combinations with two 5-HT_2A drugs (a selective 5-HT_2A agonist TCB-2: 2.5 mg/kg, and 5-HT_2A antagonist MDL100907, 1.0 mg/kg, sc) on maternal behavior in Sprague-Dawley postpartum females. Individually, TCB-2 (2.5 mg/kg, sc) and quinpirole (0.5 and 1.0 mg/kg, sc) reduced pup preference and disrupted home-cage maternal behavior. In contrast, haloperidol (0.10 mg/kg, sc) only disrupted home-cage maternal behavior, but did not suppress pup preference. MDL100907 (1.0 mg/kg, sc) by itself had no effect on either pup preference or maternal behavior. When administered in combination, pretreatment of TCB-2 did not alter quinpirole's disruption of pup preference and home-cage maternal behavior (possibly due to the floor effect), however, it did enhance haloperidol's disruption of pup retrieval in the home cage. MDL100907 had no effect both quinpirole's and haloperidol's disruption of pup preference and home-cage maternal behavior. Interestingly, haloperidol attenuated TCB-2's disruptive effect on pup preference. These findings suggest that activation of 5-HT_2A receptors tends to enhance D₂-mediated maternal disruption, whereas blockade of 5-HT_2A receptors is less effective. They also suggest that 5-HT_2A receptors may have a direct effect on maternal behavior independent of their interaction with D₂ receptors. The possible behavioral and neural mechanisms by which 5-HT_2A- and D2-mediated maternal effects and their interaction are discussed.

The Dopamine Allosteric Agent, PAOPA, Demonstrates Therapeutic Potential in the Phencyclidine NMDA Pre-clinical Rat Model of Schizophrenia

PAOPA, a potent analog of prolyl-leucyl-glycinamide, has shown therapeutic potential at the preclinical stage for dopaminergic related illnesses, including animal models of schizophrenia, Parkinson’s disease and haloperidol-induced extrapyramidal movement disorders. PAOPA’s unique allosteric mechanism and dopamine D2 receptor specificity provide a unique composition of properties for the development of potential therapeutics for neuropsychiatric illnesses. We sought to investigate PAOPA’s therapeutic prospects across the spectrum of schizophrenia-like symptoms represented in the established phencyclidine-induced rat model of schizophrenia, (5 mg/kg PCP twice daily for 7 days, followed by 7 days of drug withdrawal). PAOPA was assessed for its effect on brain metabolic activity and across a battery of behavioral tests including, hyperlocomotion, social withdrawal, sensorimotor gating, and novel object recognition. PAOPA showed therapeutic efficacy in behavioral paradigms representing the negative (social withdrawal) and cognitive-like (novel object recognition) symptoms of schizophrenia. Interestingly, some behavioral indices associated with the positive symptoms of schizophrenia that were ameliorated in PAOPA’s prior examination in the amphetamine-sensitized model of schizophrenia were not ameliorated in the PCP model; suggesting that the deficits induced by amphetamine and PCP—while phenotypically similar—are mechanistically different and that PAOPA’s effects are restricted to certain mechanisms and systems. These studies provide insight on the potential use of PAOPA for the safe and effective treatment of schizophrenia.

Spatial Working Memory in Male Rats: Pre-Experience and Task Dependent Roles of Dopamine D1- and D2-Like Receptors

The dopaminergic system is known to be involved in working memory processed by several brain regions like prefrontal cortex (PFC), hippocampus, striatum. In an earlier study we could show that Levodopa but not Modafinil enhanced working memory in a T-maze only during the early phase of training (day 3), whereas the later phase remained unaffected. Rats treated with a higher dose performed better than low dose treated rats. Here we could more specifically segregate the contributions of dopamine type 1- and 2- like receptors (D1R; D2R) to the training state dependent modulation of spatial working memory by intracerebroventricular (ICV) application of a D1R-like (SKF81297) and D2R-like agonist (Sumanirole) and antagonist (SCH23390, Remoxipride) at a low and high dose through 3 days of training. The D1R-like-agonist at both doses enhanced working memory at day 1 but only in the low dose treated rats enhancement persists over training compared to control rats. Rats treated with a high dose of a D1R-like-antagonist show persistent enhancement of working memory over training, whereas in low dose treated rats no statistical difference at any time point could be determined compared to controls. The D2R-like-agonist at both doses does not show an effect at any time point when compared to control animals, whereas the D2R-like antagonist at a low dose enhanced working memory at day 2. For the most effective D1R-like agonist, we repeated the experiments in a water maze working memory task, to test for task dependent differences in working memory modulations. Treated rats at both doses did not differ as compared to controls, but the temporal behavioral performance of all groups was different compared to T-maze trained rats. The results are in line with the view that spatial working memory is optimized within a limited range of dopaminergic transmission, however suggest that these ranges vary during spatial training.

Working Memory in the Prefrontal Cortex

The prefrontal cortex participates in a variety of higher cognitive functions. The concept of working memory is now widely used to understand prefrontal functions. Neurophysiological studies have revealed that stimulus-selective delay-period activity is a neural correlate of the mechanism for temporarily maintaining information in working memory processes. The central executive, which is the master component of Baddeley's working memory model and is thought to be a function of the prefrontal cortex, controls the performance of other components by allocating a limited capacity of memory resource to each component based on its demand. Recent neurophysiological studies have attempted to reveal how prefrontal neurons achieve the functions of the central executive. For example, the neural mechanisms of memory control have been examined using the interference effect in a dual-task paradigm. It has been shown that this interference effect is caused by the competitive and overloaded recruitment of overlapping neural populations in the prefrontal cortex by two concurrent tasks and that the information-processing capacity of a single neuron is limited to a fixed level, can be flexibly allocated or reallocated between two concurrent tasks based on their needs, and enhances behavioral performance when its allocation to one task is increased. Further, a metamemory task requiring spatial information has been used to understand the neural mechanism for monitoring its own operations, and it has been shown that monitoring the quality of spatial information represented by prefrontal activity is an important factor in the subject's choice and that the strength of spatially selective delay-period activity reflects confidence in decision-making. Although further studies are needed to elucidate how the prefrontal cortex controls memory resource and supervises other systems, some important mechanisms related to the central executive have been identified.

Reduced cortical neurotransmitter receptor complex levels in fetal Down syndrome brain

In this study, cortical receptor complex levels were determined in fetal Down syndrome (DS, trisomy 21) brain. Frontal cortices were obtained from individuals with DS (19th-22nd week of gestation) and controls. Membrane proteins were extracted, assayed on blue native gels and immunoblotted with brain receptor antibodies. Levels of a D1R-containing complex were markedly decreased in male and female cortices of DS individuals. Females with DS had significant reductions of nicotinic acetylcholine receptors α4 and α7, NMDA receptor GluN1 and AMPA receptor GluA1- and GluA3-containing receptor complexes. Levels of other brain receptor complexes (5-hydroxytryptamine 1A, GluA2 and GluR4 receptor-containing complexes) were comparable between the groups of females. Levels of GluA2- and GluA3-containing complexes were significantly increased in males. Decreased levels of D1R complexes in both sexes, along with the significant reduction of α4, α7-containing receptor complexes observed in females, may explain the brain deficits and impaired cognition observed in DS.

Dopamine in socioecological and evolutionary perspectives: implications for psychiatric disorders

Dopamine (DA) transmission in brain areas such as the prefrontal cortex (PFC) and nucleus accumbens (NAcc) plays important roles in cognitive and affective function. As such, DA deficits have been implicated in a number of psychiatric disorders such as schizophrenia and attention deficit/hyperactivity disorder (ADHD). Accumulating evidence suggests that DA is also involved in social behavior of animals and humans. Although most animals organize and live in social groups, how the DA system functions in such social groups of animals, and its dysfunction causes compromises in the groups has remained less understood. Here we propose that alterations of DA signaling and associated genetic variants and behavioral phenotypes, which have been normally considered as “deficits” in investigation at an individual level, may not necessarily yield disadvantages, but even work advantageously, depending on social contexts in groups. This hypothesis could provide a novel insight into our understanding of the biological mechanisms of psychiatric disorders, and a potential explanation that disadvantageous phenotypes associated with DA deficits in psychiatric disorders have remained in humans through evolution.

The dopamine D2/D3 receptor agonist quinpirole increases checking-like behaviour in an operant observing response task with uncertain reinforcement: a novel possible model of OCD

Excessive checking is a common, debilitating symptom of obsessive-compulsive disorder (OCD). In an established rodent model of OCD checking behaviour, quinpirole (dopamine D2/3-receptor agonist) increased checking in open-field tests, indicating dopaminergic modulation of checking-like behaviours. We designed a novel operant paradigm for rats (observing response task (ORT)) to further examine cognitive processes underpinning checking behaviour and clarify how and why checking develops. We investigated i) how quinpirole increases checking, ii) dependence of these effects on D2/3 receptor function (following treatment with D2/3 receptor antagonist sulpiride) and iii) effects of reward uncertainty. In the ORT, rats pressed an 'observing' lever for information about the location of an 'active' lever that provided food reinforcement. High- and low-checkers (defined from baseline observing) received quinpirole (0.5mg/kg, 10 treatments) or vehicle. Parametric task manipulations assessed observing/checking under increasing task demands relating to reinforcement uncertainty (variable response requirement and active-lever location switching). Treatment with sulpiride further probed the pharmacological basis of long-term behavioural changes. Quinpirole selectively increased checking, both functional observing lever presses (OLPs) and non-functional extra OLPs (EOLPs). The increase in OLPs and EOLPs was long-lasting, without further quinpirole administration. Quinpirole did not affect the immediate ability to use information from checking. Vehicle and quinpirole-treated rats (VEH and QNP respectively) were selectively sensitive to different forms of uncertainty. Sulpiride reduced non-functional EOLPs in QNP rats but had no effect on functional OLPs. These data have implications for treatment of compulsive checking in OCD, particularly for serotonin-reuptake-inhibitor treatment-refractory cases, where supplementation with dopamine receptor antagonists may be beneficial.

Attenuation of the disruptive effects of (+/-)3,4-methylenedioxymethamphetamine and cocaine on delayed matching-to-sample performance with D1 versus D2 antagonists

Evidence suggests that acute exposure to (+/-)3,4-methylenedioxymethamphetamine (MDMA) produces qualitatively similar effects on recognition task performance as other stimulant-type drugs. The current study examined whether there was a similar neurochemical basis to these memory effects by examining the effects of a D1 receptor antagonist (SCH23390) and D2 antagonist (eticlopride) on MDMA- or cocaine-induced impairments in delayed matching-to-sample performance in rats. At low doses it was shown that eticlopride was ineffective in antagonizing either MDMA or cocaine's effects, and at higher doses exacerbated their effects. In contrast, the D1 receptor antagonist SCH23390 was only able to significantly attenuate the disruption caused by MDMA, but not cocaine's effects. Therefore, although present evidence suggests that the effect of acute MDMA on memory-task performance may be related to its effects at D1 receptor sites, there may be differences between MDMA and cocaine in the precise neurochemical pathways involved despite their having similar cognitive effects.

Effect of dopamine and serotonin receptor antagonists on fencamfamine-induced abolition of latent inhibition

The purpose of this investigation was to verify the role of dopamine and serotonin receptors in the effect of fencamfamine (FCF) on latent inhibition. FCF is a psychomotor stimulant with an indirect dopaminergic action. Latent inhibition is a model of attention. Latent inhibition is blocked by dopaminergic agents and facilitated by dopamine receptor agonists. FCF has been shown to abolish latent inhibition. The serotonergic system may also participate in the neurochemical mediation of latent inhibition. The selective dopamine D(1) receptor antagonist SCH 23390 (7-chloro-3-methyl-1-phenyl-1,2,4,5-tetrahydro-3-benzazepin-8-ol), D(2) receptor antagonists pimozide (PIM) and methoclopramide (METH), and serotonin 5-HT(2A/C) receptor antagonist ritanserin (RIT) were used in the present study. Latent inhibition was evaluated using a conditioned emotional response procedure. Male Wistar rats that were water-restricted were subjected to a three-phase procedure: preexposure to a tone, tone-shock conditioning, and a test of the effect of the tone on licking frequency. All of the drugs were administered before the preexposure and conditioning phases. The results showed that FCF abolished latent inhibition, and this effect was clearly antagonized by PIM and METH and moderately attenuated by SCH 23390. At the doses used in the present study, RIT pretreatment did not affect latent inhibition and did not eliminate the effect of FCF, suggesting that the FCF-induced abolition of latent inhibition is not mediated by serotonin 5-HT(2A/C) receptors. These results suggest that the effect of FCF on latent inhibition is predominantly related to dopamine D(2) receptors and that dopamine D(2) receptors participate in attention processes.

Differential effects of dopamine receptor D1-type and D2-type antagonists and phase of the estrous cycle on social learning of food preferences, feeding, and social interactions in mice

The neurobiological bases of social learning, by which an animal can 'exploit the expertise of others' and avoid the disadvantages of individual learning, are only partially understood. We examined the involvement of the dopaminergic system in social learning by administering a dopamine D1-type receptor antagonist, SCH23390 (0.01, 0.05, and 0.1 mg/kg), or a D2-type receptor antagonist, raclopride (0.1, 0.3, and 0.6 mg/kg), to adult female mice prior to socially learning a food preference. We found that while SCH23390 dose-dependently inhibited social learning without affecting feeding behavior or the ability of mice to discriminate between differently flavored diets, raclopride had the opposite effects, inhibiting feeding but leaving social learning unaffected. We showed that food odor, alone or in a social context, was insufficient to induce a food preference, proving the specifically social nature of this paradigm. The estrous cycle also affected social learning, with mice in proestrus expressing the socially acquired food preference longer than estrous and diestrous mice. This suggests gonadal hormone involvement, which is consistent with known estrogenic regulation of female social behavior and estrogen receptor involvement in social learning. Furthermore, a detailed ethological analysis of the social interactions during which social learning occurs showed raclopride- and estrous phase-induced changes in agonistic behavior, which were not directly related to effects on social learning. Overall, these results suggest a differential involvement of the D1-type and D2-type receptors in the regulation of social learning, feeding, and agonistic behaviors that are likely mediated by different underlying states.

Serotonergic and dopaminergic modulation of gambling behavior as assessed using a novel rat gambling task

Pathological gambling (PG) is characterized by persistent, maladaptive gambling behavior, which disrupts personal and professional life. Animal models of gambling behavior could make a significant contribution to improving our understanding of the neural and neurochemical basis of gambling, and the treatment of PG. When gambling, failing to win critically results in the loss of resources wagered as well as the absence of additional gain. Here, we have incorporated these concepts into a novel rat gambling task (rGT), based, in part, on the 'Iowa' gambling task (IGT) commonly used clinically to measure gambling-like behavior. Rats choose among four different options to earn as many sugar pellets as possible within 30 min. Each option is associated with the delivery of a different amount of reward, but also with a different probability and duration of punishing time-out periods during which reward cannot be earned. The schedules are designed such that persistent choice of options linked with larger rewards result in fewer pellets earned per unit time. Rats learn to avoid these risky options to maximize their earnings, comparable with the optimal strategy in the IGT. Both d-amphetamine and the 5-HT(1A) receptor agonist, 8-OH-DPAT, impaired task performance. In contrast, the dopamine D(2) receptor antagonist, eticlopride, improved performance, whereas the D(1) receptor antagonist, SCH23390, had no effect. These data suggest that both serotonergic and dopaminergic agents can impair and improve gambling performance, and indicate that the rGT will be a useful tool to study the biological basis of gambling.

Estrogen prevents norepinephrine alpha-2a receptor reversal of stress-induced working memory impairment

Understanding effects of estrogen on the medial prefrontal cortex (PFC) may help to elucidate the increased prevalence of depression and post-traumatic stress disorder in women of ovarian cycling age. Estrogen replacement in ovariectomized (OVX) young rats amplifies the detrimental effects of stress on working memory (a PFC-mediated task), but the mechanisms by which this occurs have yet to be identified. In male rats, stimulation of norepinephrine alpha-2 adrenoceptors protects working memory from stress-induced impairments. However, this effect has not been studied in females, and has not been examined for sensitivity to estrogen. The current study asked whether OVX females with estrogen replacement (OVX+Est) and without replacement (OVX+Veh) responded differently to stimulation of alpha-2 adrenoceptors after administration of the benzodiazepine inverse agonist FG7142, a pharmacological stressor. The alpha-2 agonist, guanfacine, protected working memory from the impairing effects of FG7142 in OVX+Veh, but not in OVX+Est rats. Western Blot analysis for alpha-2 receptors was performed on PFC tissue from each group, but no changes in expression were found, indicating that the behavioral effects observed were likely not due to changes in receptor expression. These findings point to possible mechanisms by which estrogen may enhance the stress response, and hold implications for the gender discrepancy in the prevalence of stress-related mental illness.

The effects of psychotomimetic and putative cognitive-enhancing drugs on the performance of a n-back working memory task in rats

RATIONALE

Working memory impairment is a core symptom of schizophrenia, but no existing treatment remediates this deficit. Inconsistent conceptualizations and few reliable translational measures are major hindrances to understanding the neurobiology of this aspect of cognition. Using comparable task designs may help bridge clinical and preclinical research efforts.

OBJECTIVE

A novel rodent procedure was designed to translate the n-back working memory task used in schizophrenic patients.

MATERIALS AND METHODS

Rats were trained in five-lever operant chambers to recall either the last (one-back) or penultimate (two-back) lever from random sequences of lever presentations of variable lengths. Psychotomimetic doses of amphetamine, dizocilpine maleate (MK801), and (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) were tested for disruption of accuracy, and cognitive-enhancing doses of amphetamine, nicotine, and (+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride (SKF38393 hydrochloride) were examined for improvements in performance.

RESULTS

High doses of amphetamine (0.8 and 1.6 mg/kg) significantly reduced accuracy while increasing total trials; 0.1 mg/kg MK801 and 2.0 mg/kg DOI also reduced accuracy, but the latter concurrently impaired responding. At the lowest dose (0.2 mg/kg), amphetamine increased total trials and rewards without affecting accuracy; 1.0 mg/kg nicotine reduced accuracy without affecting total trials, whereas 10.0 mg/kg SKF38393 had the opposite effect.

DISCUSSION

Although the possibility for mediating behaviors may exist, the rodent n-back task provides a clinically relevant model of working memory. Amphetamine and MK801 produced selective impairments without disrupting responding. The cognitive enhancers did not improve working memory, but low doses of amphetamine improved response efficiency. This novel procedure may be useful for examining cognitive deficits and their potential reversal in animal models of schizophrenia.

Selective blockade of dopamine D(3) versus D(2) receptors enhances frontocortical cholinergic transmission and social memory in rats: a parallel neurochemical and behavioural analysis

Though dopaminergic mechanisms modulate cholinergic transmission and cognitive function, the significance of specific receptor subtypes remains uncertain. Here, we examined the roles of dopamine D(3) versus D(2) receptors. By analogy with tacrine (0.16-2.5 mg/kg, s.c.), the selective D(3) receptor antagonists, S33084 (0.01-0.63) and SB277,011 (0.63-40.0), elicited dose-dependent, pronounced and sustained elevations in dialysis levels of acetylcholine (ACh) in the frontal cortex, but not the hippocampus, of freely-moving rats. The actions of these antagonists were stereospecifically mimicked by (+)S14297 (1.25), whereas its inactive distomer, (-)S17777, was ineffective. The preferential D(2) receptor antagonist, L741,626 (10.0), failed to modify levels of ACh. S33084 (0.01-0.63) and SB277,011 (0.16-2.5) also mimicked tacrine (0.04-0.63) by dose-dependently attenuating the deleterious influence of scopolamine (1.25) upon social memory (recognition by an adult rat of a juvenile conspecific). Further, (+)S14297 (1.25) versus (-)S17777 stereospecifically blocked the action of scopolamine. Using an intersession interval of 120 min (spontaneous loss of recognition), S33084 (0.04-0.63), SB277,011 (0.16-10.0) and (+)S14297 (0.63-10.0) likewise mimicked tacrine (0.16-2.5) in enhancing social memory. In contrast, L741,626 (0.16-10.0) displayed amnesic properties. In conclusion, selective blockade of D(3) receptors facilitates frontocortical cholinergic transmission and improves social memory in rats. These data support the pertinence of D(3) receptors as a target for treatment of disorders in which cognitive function is compromised.

Insights into the Role of Dopamine Receptor Systems in Learning and Memory

It is well established that learning and memory are complex processes involving and recruiting different brain modulatory neurotransmitter systems. Considerable evidence points to the involvement of dopamine in various aspects of cognition, and interest has been focused on investigating the clinical relevance of dopamine systems to age-related cognitive decline and manifestations of cognitive impairment in schizophrenia, Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. In the past decade or so, in spite of the molecular cloning of the five dopamine receptor subtypes, their specific roles in brain function remained inconclusive due to the lack of completely selective ligands that could distinguish between the members of the D1-like and D2-like dopamine receptor families. One of the most important advances in the field of dopamine research has been the generation of mutant mouse models permitting evaluation of the dopaminergic system using gene targeting technologies. These mouse models represent an important approach to explore the functional roles of closely related receptor subtypes. In this review, we present and discuss evidence on the role of dopamine receptors in different aspects of learning and memory at the cellular, molecular and behavioral levels. We compare evidence using conventional pharmacological, lesion or electrophysiological studies with results from mice with targeted deletions of different subtypes of dopamine receptor genes. We particularly focus on dopamine D1 and D2 receptors in an effort to delineate their specific roles in various aspects of cognitive function. We provide strong evidence, from our own recent work as well as others, that dopamine is part of the network that plays a very important role in cognitive function, and that although multiple dopamine receptor subtypes contribute to different aspects of learning and memory, the D1 receptor seems to play a more prominent role in mediating plasticity and specific aspects of cognitive function, including spatial learning and memory processes, reversal learning, extinction learning, and incentive learning.

The effects of adulthood olanzapine treatment on cognitive performance and neurotrophic factor content in male and female rats neonatally treated with quinpirole

Male and female Sprague-Dawley rats were administered quinpirole (1 mg/kg, i.p.) or saline once daily from postnatal day (P)1 to P21. This drug treatment has been shown to produce long-term priming of the D2 receptor. Beginning on P62, rats were administered the atypical antipsychotic olanzapine (2.5 mg/kg) or saline twice daily (i.p.) for 28 days. One day after olanzapine treatment ceased, rats were tested on the place and match-to-place versions of the Morris water maze (MWM) for seven consecutive days. Dopamine D2 receptor priming was verified through a yawning behavioural test, a D2 receptor-mediated event, before olanzapine was administered as well as after olanzapine treatment and behavioural testing were complete. Results showed that neonatal quinpirole treatment induced D2 priming that was eliminated by olanzapine treatment. On the MWM place version, D2-primed rats demonstrated a significant impairment that was eliminated by olanzapine treatment, but olanzapine treatment to animals neonatally treated with saline produced a significant deficit on the place version of the MWM. There were no significant deficits on the match-to-place version. Brain tissue analyses revealed that neonatal quinpirole treatment produced a significant decrease in hippocampal NGF, BDNF and ChAT that was eliminated by olanzapine treatment. Neonatal quinpirole treatment produced a significant decrease in BDNF and ChAT in the frontal cortex that was unaffected by olanzapine treatment. These results show that olanzapine eliminates D2 receptor priming and cognitive impairment and also alleviates decreases in neurotrophins and acetylcholinergic markers produced by D2 priming in the hippocampus.

Reinforcing effects of nicotine microinjections into the ventral tegmental area of mice: Dependence on cholinergic nicotinic and dopaminergic D1 receptors

We used an intracranial self-administration (ICSA) procedure to assess the involvement of the ventral tegmental area (VTA) nicotinic receptors in the rewarding effects of nicotine. We then challenged intra-VTA nicotine self-administration via systemic or local injections of dopamine (DA)-D1 and nicotinic receptor antagonists. C57BL/6J mice were stereotaxically implanted unilaterally with a guide cannula above the VTA. After 1 week of recovery, mice were allowed to discriminate between two arms of a Y-maze over seven daily sessions, one arm being reinforced by intracranial nicotine microinjection. Mice exhibited nicotine self-administration at both doses tested, i.e. 10 ng (21.6 pmol) and 100 ng (216 pmol)/50-nl injection. In contrast, mice receiving a 216-pmol nicotine dose 0.8 mm above VTA performed at chance level. Once the ICSA response was acquired, systemic pretreatment with the DA-D1 receptor antagonist SCH 23390 (25 microg/kg i.p.) or co-infusion of the nAChR antagonist DHbetaE with nicotine disrupted ICSA. Replacement of SCH 23390 by vehicle, or withdrawal of DHbetaE from nicotine/DHbetaE mixed solutions led to recovery of intra-VTA nicotine self-administration. We conclude that nicotinic receptors in the VTA, presumably alpha4beta2 nAChRs are critically to mediate the rewarding effects of nicotine and that DA-D1 receptors are also directly implicated.

Dopamine agonists and antagonists can produce an attenuation of response bias in a temporal discrimination task depending on discriminability of target duration

The current study examined the effects of the D2 agonist (quinpirole) and D2 antagonist (eticlopride) on temporal discrimination performance in a conditional discrimination task (Experiment I) and a delayed conditional discrimination task (Experiment II). In both experiments rats discriminated between a scheduled stimulus duration of 3 s versus 9 s. Consistent with previous reports, overall discrimination performance decreased in a dose-dependent manner with both drugs. Changes in response bias (the tendency to choose-short or choose-long irrespective of actual stimulus duration), however, were best characterized in terms of both drugs tending to attenuate the bias effects normally observed during baseline drug-free performance. Specifically, the 'choose-short' bias observed in Experiment I and at a relatively short, 0.1 s, delay in Experiment II became less extreme with increasing doses. In addition, the 'choose-long' bias observed at a relatively long, 6 s, delay in Experiment II also became less extreme with increasing doses. Thus, whether there was an apparent shift from a short response bias to long, or vice versa, was the product of the delay interval between stimulus presentation and choice rather than whether the drug in question was a D2 agonist or antagonist. Such an attenuation of bias may have arisen because of subjects confounding the delay interval with the actual discriminative stimulus duration.

Context-dependent modulation by D₁ receptors: Differential effects in hippocampus and striatum

Place-specific firing by hippocampal and striatal neurons was recorded simultaneously following injection of a D(1) receptor antagonist (SCH23390) and during spatial working memory task performance. SCH23390-induced changes in unit responses were observed during light and dark test conditions. Although hippocampal place field locations were altered by the contextual change, the reliability and specificity of place fields was disrupted only by combining D(1) antagonism and a change in context. Striatal place field locations were reorganized after either contextual change or D(1) antagonism, without altering place field reliability and specificity. Disrupted velocity encoding by place cells in both regions was induced by darkness, whereas greater stability in acceleration encoding followed removal of D(1) receptor activity. Dopamine may differentially regulate hippocampal context learning and striatum-based predictive codes.

Co-administration of fluoxetine and WAY100635 improves short-term memory function

The aim of this study was to determine whether the action of the antidepressant fluoxetine or the anxiolytic buspirone could be modified by specific 5-hydroxytriptamine (5-HT(1A)) receptor blockade in a short-term memory paradigm. Male Wistar rats were trained to perform the putative short-term memory task, delayed non-matching to position. WAY100635, a selective 5-HT(1A) receptor antagonist (0.15 mg/kg), was administered 15 min before either the selective serotonin reuptake inhibitor fluoxetine (3 mg/kg), or the partial 5-HT(1A) receptor agonist and dopamine D2 receptor antagonist, buspirone (0.3 mg/kg). 8-Hydroxy-di-n-propylamino tetralin (8-OH-DPAT), a full 5-HT(1A) receptor agonist (0.3 mg/kg), was also included in the study as a positive control. WAY100635 alone had no effect on any behavioural parameter measured (response accuracy, delay lever press activity and trial completion). 8-OH-DPAT impaired response accuracy in a delay-dependent manner, an effect reversed by WAY100635. Fluoxetine also impaired response accuracy delay-dependently. WAY100635 pretreatment not only reversed this deficit but improved response accuracy, in the presence of a significant deficit in trial completion. At the dose used, buspirone showed no significant differences compared to the control group. The data suggest that fluoxetine impairs short-term memory function by the indirect activation of 5-HT(1A) receptors, but that its co-administration with WAY100635 improves short-term memory function.

The effects of eticlopride on Morris water task performance in male and female rats neonatally treated with quinpirole

RATIONALE

Previous studies have shown that neonatal quinpirole treatment which results in long-term dopamine D2 receptor supersensitization (D2 receptor priming) produces cognitive deficits in preweanling and adult rats behaviorally tested on the Morris water task (MWT).

OBJECTIVE

This study was designed to analyze whether pretraining administration of the D2 antagonist eticlopride alleviates cognitive deficits produced by neonatal quinpirole treatment.

METHODS

Both male and female Sprague-Dawley rats were treated with quinpirole HCl (1 mg/kg) or saline from postnatal days 1 to 21. From P22 to P24, rats were tested on the place version of the MWT in which a hidden platform remains stationary throughout training. From P25 to P28, rats were tested on the match-to-place version of the MWT, and rats are given a pair of daily training trials to locate the hidden platform that was moved to a new location each day. Fifteen minutes before each training session, rats were intraperitoneally administered with eticlopride (0.01 or 0.02 mg/kg) or saline.

RESULTS

Pretraining eticlopride treatment alleviated cognitive deficits produced by neonatal quinpirole treatment in both male and female rats on the place version of the MWT, as well as in males tested on the match-to-place version of the MWT. However, there were no significant deficits produced by neonatal quinpirole treatment in females tested on the match-to-place version of the MWT, and control males demonstrated superiority over control females on this version of the task.

CONCLUSIONS

Pretraining administration of the dopamine D2 antagonist eticlopride alleviated cognitive deficits produced by neonatal quinpirole treatment. However, it appears that the dopamine D2 receptor may have a more important influence on cognitive performance in males than in females, which may be related to increased sensitivity of the D2 receptor in males.

The BDNF Val66Met polymorphism has a gender specific influence on planning ability in Parkinson's disease

Parkinson's disease (PD) patients show a range of cognitive deficits,which may relate to abnormalities in dopaminergic transmission in fronto-striatal circuitry. In this study, we have investigated the impact of brainderived neurotrophic factor (BDNF) val66met polymorphisms on performance of the Tower of London (TOL) test of planning by PD patients. This polymorphism significantly influences BDNF secretion in the CNS, and BDNF is known to influence dopaminergic neurons and cognitive processes. Patients with PD totalling 291 who had undergone detailed motor and cognitive assessments as part of a population-based study of PD were genotyped for the BDNF val66met polymorphism. The impact of this polymorphism on cognitive ability was determined using multivariate analysis to adjust for possible confounding variables. Patients with low rates of BDNF secretion (met alleles) performed significantly better at the TOL task than those with high rates of secretion (val alleles). Furthermore, subgroup analyses revealed that the effect is most apparent in women and among patients with prior dopaminergic exposure. We speculate that BDNF may interact with dopaminergic transmission and dopamine receptor stimulation in the frontostriatal circuitry, with subsequent consequences on cognition in Parkinson's disease.

Captopril and enalapril improve cognition and depressed mood in hypertensive patients

In this study, we evaluate the effects of two angiotensin converting enzyme inhibitors (ACEIs), captopril and enalapril given chronically as antihypertensive treatment, on certain cognitive and emotional processes in humans. Thirty-nine subjects with mild to moderate hypertension and fifteen normotensive controls were divided into four groups consisting of normotensive and hypertensive subjects taking captopril, enalapril, or no medication at all. The Rey Auditory Verbal Learning Test and the Wechsler Memory Scale were used to evaluate their cognitive functioning. Mood changes in all subjects were assessed using the Beck Depression Inventory and the Hopkins Symptom Check- list (HSC).

RESULTS

Untreated hypertensive patients scored lower than normotensive controls in cognitive tests and significantly worse in cumulative recall (P < 0.05) and paired words association (P < 0.01). When compared with normotensive subjects, untreated hypertensive patients also scored significantly higher on the depression with anxiety subscale in HSC (P < 0.05). No significant influence of hypertension was found in any other examined aspect of cognition and mood. In most cases captopril improved and enalapril reversed the adverse memory effects of hypertension. High arterial blood pressure is significantly associated with an impairment of cognition and the occurrence of depression with anxiety in humans. Enalapril and, to a lesser extent, captopril reversed these deficits.

(±)3,4-Methylenedioxymethamphetamine, d-Amphetamine, and Cocaine Impair Delayed Matching-to-Sample Performance by an Increase in Susceptibility to Proactive Interference

This study compared the effects of (+/-)3,4-methylenedioxymethamphetamine, d-amphetamine, and cocaine on performance of rats in a delayed matching-to-sample procedure using a variety of indices of performance to determine the mechanism by which working memory task impairments arise. All 3 drugs produced an overall delay-independent decrease in accuracy rather than a delay-dependent increase in the rate of forgetting. This impairment arose as a result of current-trial choice responses being progressively more affected by responses made in the immediately preceding trial as drug dose increased. Therefore, all 3 drugs produced qualitatively similar disruptions in memory task performance best characterized as an impairment arising from proactive sources of interference.

Regulation of working memory by dopamine D4 receptor in rats

Working memory is regulated by neurotransmitters in prefrontal cortex (PFC), including dopamine and norepinephrine. Previous studies of dopamine function in working memory have focused on the D1 and D2 receptors, with most evidence suggesting a dominant role for the D1 receptor. Since the dopamine D4 receptor is highly expressed in PFC, we hypothesize that it may also contribute to working memory. To test this hypothesis, we examined behavioral effects of L-745,870, a highly selective, centrally active, D4 antagonist, using a delayed alternation task in rats. Task performance was dose-dependently affected by the D4 antagonist, depending on individual baseline functional status of working memory. In rats with good baseline performance, the D4 antagonist had no effects at low doses, whereas high doses disrupted working memory. In rats with poor baseline working memory, the D4 antagonist significantly improved working memory at low doses, and higher doses were not distinguishable from vehicle controls. Effects of the D4 antagonist among poor performers were most robust when task demand for working memory was high, with lesser effects at lower demand level, suggesting that such effects were selective for working memory. The present findings indicate a significant role of the D4 receptor in working memory, and suggest innovative, D4-based, treatment of cognitive deficits associated with neuropsychiatric disorders.

Involvement of D1 dopamine receptors in the cognitive effects of angiotensin IV and des-Phe6 angiotensin IV

An important role for angiotensin IV (Ang IV) in the processes of learning and memory has now been well established. We have previously found that intracerebroventricular (ICV) administration of Ang IV as well as des-Phe6-Ang IV enhances learning of conditioned avoidance responses (CARs), facilitates recall of a passive avoidance (PA) task, and improves object recognition (OR) in rats. Since the dopaminergic system is crucial for the cognitive processes, in this study our aim was to determine the dopaminergic D1 mediation of these effects using SCH 23390 as a selective D1 receptor antagonist. Male Wistar rats (180-200 g), pretreated with SCH 23390 (R-[+]-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) 0.05 mg/kg intraperitoneally (IP), were given Ang IV or des-Phe6-Ang IV (1 nmol ICV) 1 h later and then tested in the above cognitive paradigms, as well as in the open field and an elevated 'plus' maze to control for the unspecific, respectively, motor and emotional, effects of our treatments. Both, Ang IV and des-Phe6-Ang IV effectively enhanced learning of CARs (P < 0.05), recall of PA (P < 0.001), and improved OR (P < 0.001). Pretreatment with SCH 23390 abolished the cognitive effects of both peptides. SCH 23390, Ang IV, and des-Phe6-Ang IV, given at the same doses and routes as in the cognitive tests, did not significantly influence crossings, rearings and bar approaches in the open field, nor the parameters measured in the elevated 'plus' maze, thus making a major contribution of the unspecific effects of our treatments to the results of the memory tests improbable. In conclusion, these results indicate that the functional dopaminergic D1 receptors are necessary for the Ang IV and des-Phe6-Ang IV cognitive effects to occur.

Effects of methylphenidate on working memory in pigeons

To assess the effects of methylphenidate on working memory, pigeons were trained in a delayed matching-to-sample task. Delay interval duration (0.2, 1, 3, 6, or 12 sec) was varied within sessions in order to separate delay-dependent from delay-independent effects of the drug on performance. A reduction in the sample response requirement from five responses to one response effectively reduced attention to the stimulus and impaired overall accuracy. Methylphenidate was administered in doses of 0.0 (saline control), 0.25, 2.5, and 10 mg/kg. Relative to performance with saline, accuracy was significantly reduced with 10 mg/kg methylphenidate to the same extent in both fixed ratio (FR) 1 and FR 5 conditions. The smaller doses had no effect, and there was no evidence that accuracy improved with drug administration. Intercepts and slopes of exponential functions fitted to measures of discriminability plotted as a function of delay showed that methylphenidate affected delay-independent aspects of performance (initial discriminability), but not delay-dependent aspects (rate of forgetting).

Haloperidol and chlorpheniramine interaction in inhibitory avoidance in goldfish

The purpose of this study was to investigate a possible interaction between histaminergic and dopaminergic systems in learning and memory processes, in an inhibitory avoidance test in goldfish. Haloperidol, a dopaminergic antagonist, was administrated pre-training and the chlorpheniramine (CPA), a histaminergic antagonist, post-training. The inhibitory avoidance procedure was performed in 3 days, using a rectangular aquarium divided into two compartments (black and white), with a central door. On the first day, the animals were habituated for 10 min. On the second day, they were injected with 2 mg/kg of haloperidol or dimethyl sulfoxide (DMSO) 20 min before training. Then, the animals were placed in the white compartment, the central door was opened and the time spent for crossing between compartments was recorded. After the fish crossed the line between compartments a 45 g weight was dropped. This procedure was done five times in a row. Immediately after the fifth trial, the fish were injected intraperitoneally (i.p.) with either saline or CPA (0.4, 1.0, 4.0, 8.0 or 16 mg/kg). On the next day (test) the time to cross was recorded again. On the training trials, the animals treated with DMSO or haloperidol presented a significant increase in the latencies indicating learning (Friedman P = 0.0062 and 0.0001). The latencies in the test day showed that groups pre-treated with haloperidol and treated with CPA presented a dose-dependent increase in latencies, and those treated with the 16 mg/kg CPA group showed a significant increase (ANOVA two-way followed by Student-Newman-Keuls (SNK) P < 0.01). Thus, it can be suggested that the facilitatory action occurs due to an additive interaction between both systems, in a dose-dependent way.

Pharmacological challenge reveals long-term effects of perinatal phencyclidine on delayed spatial alternation in rats

Administration of the N-methyl-D-aspartate (NMDA) antagonist phencyclidine (PCP) may produce alterations in behavior that resemble those that have been observed in animal models of schizophrenia. This study was designed to examine the effects of early postnatal injection of PCP on later acquisition and performance of a delayed spatial alternation task, a procedure that is sensitive to manipulations of the prefrontal cortex. At the beginning of the study, we injected cross-fostered female rat pups subcutaneously with either saline or 10 mg/kg PCP on postnatal (PN) days 7, 9, and 11. On PN34, the rats began training in a delayed spatial alternation task consisting of 10 daily trials with a 10-s intertrial interval. Although accuracy improved significantly faster in the saline-treated group than in the PCP-treated group, by PN70, both groups had acquired the task with approximately equal accuracies. Pharmacological challenges with the NMDA antagonists, PCP and ketamine, and with the dopamine modulator, amphetamine, decreased accuracy to a similar extent in both groups of rats when intertrial delays were held constant at 10 s; however, nicotine did not decrease accuracy in either group. In contrast, dizocilpine (a high-affinity NMDA open-channel blocker) produced a more pronounced decrease in accuracy in the PCP-treated rats than in the saline-treated rats. When delays were lengthened to 30 s, PCP also decreased accuracy in the PCP-treated rats to a greater extent than in saline-treated rats. These results suggest that perinatal administration of PCP may produce long-term alterations in cognition that are revealed by pharmacological challenge and manipulation of task difficulty.

Novel drug interactions at D(2) dopamine receptors: modulation of [3H]quinpirole binding by monoamine oxidase inhibitors

D(2) dopamine receptors are the principal target of drugs used to treat schizophrenia and Parkinson's disease. Recent findings suggest novel drug interactions at D(2) receptors, specifically interactions of monoamine oxidase inhibitors (MAOIs) at a novel binding site that modulates the binding of [3H]quinpirole to the D(2) receptor. That MAOIs inhibit [3H]quinpirole binding challenges the traditional understanding of ligand interactions at dopamine receptors and may shed light on the mechanism of behavioral sensitization to psychostimulants and the pharmacology and toxicity of MAOIs.

The effects of local application of D2 selective dopaminergic drugs into the medial prefrontal cortex of rats in a delayed spatial choice task

The study examined the effects of modulation of dopamine D2 receptors-mediated neurotransmission in the rat's prefrontal cortex (PFC) on storage and executive components of working memory. Rats were trained on delayed (delay interval, 3 s) and non-delayed choice in a U-maze. The prominence of proactive interference was evaluated by sorting errors in a current trial on the basis of animal reactions in a preceding trial. The erroneous runs to the same arm of the maze as in the previous trial were identified as the repetitions (RE) and the erroneous runs to the other arm in comparison with the previous trial were classified as alternations (AE). The bilateral microinfusion of D2 agonists PPHT (0.004 microg, 0.04 microg, 0.4 microg/1 microl) into medial wall of the PFC produced a dose-dependent increase in the error rate of the delayed-response task and did not influence non-delayed choice. In delay condition PPHT enhanced the perseverative tendencies (the rate of RE was significantly higher than the rate of AE), in non-delayed choice the erroneous performance was mainly represented by AE. In contrast, the infusion of D2-receptor antagonist sulpiride (0.03 microg, 0.3 microg, 3 microg/1 microl) increased the accuracy of delayed choice and changed the mode of intertrial dependence-rats made significantly more AE than RE. The results are discussed in terms of the involvement of D2 receptor dependent transmission of the PFC in different cognitive processes related to the delayed performance in U-maze (within-trial short-term storage of information versus dynamic control of between-trials working memory processing).

Expression of cocaine sensitization: regulation by the medial prefrontal cortex

Extracellular levels of dopamine are increased in response to systemic administration of cocaine in several brain areas including the nucleus accumbens and medial prefrontal cortex. While the cocaine-induced increase in extracellular dopamine levels in the nucleus accumbens is augmented after repeated daily cocaine, the response of extracellular dopamine levels in the medial prefrontal cortex is attenuated. Since dopamine in the medial prefrontal cortex has an inhibitory effect on nucleus accumbens dopamine levels and locomotor activity, the role of medial prefrontal cortex dopamine tolerance in the expression of sensitized locomotor behavior was further examined by injection of D-amphetamine sulfate into the prelimbic portion of the medial prefrontal cortex just prior to cocaine challenge in cocaine-sensitized rats. Male Sprague-Dawley rats were non-handled (naive) or injected with either saline (1 ml/kg, i.p.) or cocaine (15 mg/kg, i.p.) for five consecutive days. After a seven to 12 day withdrawal period, rats were microinjected with either saline or various doses of amphetamine into primarily the prelimbic region of the medial prefrontal cortex followed by systemic injection of saline or cocaine. In naive rats, intramedial prefrontal cortex amphetamine produced a trend toward decreased locomotor responding to cocaine challenge while no effect of amphetamine was evident in daily saline pretreated rats. Daily cocaine pretreated rats that received saline in the medial prefrontal cortex demonstrated a sensitized locomotor response compared to their daily saline pretreated counterparts. This sensitization was blocked by a low dose of amphetamine (0.175 microg/side) in the medial prefrontal cortex, an effect which disappeared in animals administered higher amphetamine doses. The results suggest that in rats sensitized to cocaine, decreased medial prefrontal cortex dopamine levels in response to cocaine challenge may contribute to behavioral sensitization. Furthermore, the data indicate the possibility that there is an optimal range at which medial prefrontal cortex amphetamine exerts maximal behavioral inhibition. These findings implicate a role for decreased cortical control in producing sensitized behavioral responding to cocaine.

The specific dopamine uptake inhibitor GBR 12783 improves learning of inhibitory avoidance and increases hippocampal acetylcholine release

The specific dopamine uptake inhibitor, GBR 12783 was tested on the retention performance of a one-trial passive avoidance test. For a moderate electric shock intensity, GBR 12783 (10 mg/kg), injected before acquisition session, improved retention performance. Scopolamine (0.125-0.5 mg/kg) completely blocked the promnesic effect of GBR 12783. Moreover, GBR 12783 increased hippocampal acetylcholine release in vivo. These data suggest that the promnesic effect of GBR 12783 is mediated by an increase in the septo-hippocampal cholinergic transmission.

D1 receptor modulation of hippocampal-prefrontal cortical circuits integrating spatial memory with executive functions in the rat

Dopamine (DA) within the prefrontal cortex (PFC) plays an important role in modulating the short-term retention of information during working memory tasks. In contrast, little is known about the role of DA in modulating other executive aspects of working memory such as the use of short-term memory to guide action. The present study examined the effects of D1 and D2 receptor blockade in the PFC on foraging by rats on a radial arm maze under two task conditions: (1) a delayed task in which spatial information acquired during a training phase was used 30 min later to guide prospective responses, and (2) a nondelayed task that was identical to the test phase of the delayed task but lacked a training phase, thereby depriving rats of previous information about the location of food on the maze. In experiment 1, microinjections of the D1 antagonist SCH-23390 (0.05, 0.5, or 5 microg/&microl), but not the D2 antagonist sulpiride (0.05, 0.5, or 5 microg/microl), into the prelimbic region of the PFC before the test phase disrupted performance of the delayed task without affecting response latencies. In contrast, neither drug affected performance of the nondelayed task. In the present study, we also investigated the role of D1 receptors in modulating activity in hippocampal-PFC circuits during delayed responding. Unilateral injections of SCH-23390 into the PFC in the hemisphere contralateral to a microinjection of lidocaine into the hippocampus severely disrupted performance of the delayed task. Thus, the ability to use previously acquired spatial information to guide responding 30 min later on a radial arm maze requires D1 receptor activation in the PFC and D1 receptor modulation of hippocampal inputs to the PFC. These data suggest that D1 receptors in the PFC are involved in working memory processes other than just the short-term active retention of information and also provide direct evidence for DA modulation of limbic-PFC circuits during behavior.

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.

Involvement of activation of dopaminergic neuronal system in learning and memory deficits associated with experimental mild traumatic brain injury

Much evidence has indicated that a disturbance in dopamine neurotransmission following mild to moderate traumatic brain injury is involved in the development of post traumatic memory deficits. In the present study we examined the effects of a dopamine receptor agonist and some antagonists on latent learning and memory deficits associated with a concussive traumatic brain injury in mice. Anaesthetized animals were subjected to mild traumatic brain injury by dropping a weight onto the head, and a single-dose injection of apomorphine (0.3-3.0 mg/kg) or haloperidol (0.3-3.0 mg/kg) was made i.p. 15 min after the trauma. One week later, a water-finding task consisting of an acquisition trial, a retention test and a retest was employed to assess learning and memory functions. Mice that had received a traumatic brain injury were impaired in task performance, with prolonged latencies for finding and drinking in the retention test and retest. Administration of haloperidol but not of apomorphine significantly shortened the prolonged latency in both of the tests, indicating that antagonism of dopamine receptors is beneficial for the recovery of post traumatic memory deficits. In order to evaluate which receptor subtype plays the major role in this model, we examined the effects of SCH-23390 (0.03-0.3 mg/kg), a D1 receptor antagonist, and sulpiride (3.0-30 mg/kg), a D2 receptor antagonist, in the same experimental paradigm. The results showed that administration of sulpiride but not of SCH-23390 significantly improved the deficits in task performance, indicating that D2 receptors are the major site of action. However, combined treatment with SCH-23390 (0.03-0.3 mg/kg) and sulpiride (3.0 mg/kg) at doses that had no effect when the antagonists were given alone exerted a significant additive effect in improving these deficits, indicating that interaction between D1 and D2 receptors is involved in these processes. The present results suggest that a dopaminergic mechanism contributes to the memory dysfunction associated with traumatic brain injury.

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.

Effects of selective catechol-O-methyltransferase inhibitors on single-trial passive avoidance retention in male rats

The effects of new selective catechol-O-methyltransferase (COMT) inhibitors entacapone (mainly peripheral effect) and tolcapone (acting also in the brain) on normal and impaired cognitive functions were studied in aversively motivated inhibitory avoidance using a single-trial passive avoidance paradigm in young adult rats. Passive avoidance retention latency was shortened by either scopolamine (1.0 mg/kg) or bilateral lesions to nucleus basalis magnocellularis (NBM) caused by infusions of ethylcholine aziridinium (AF64A). Entacapone (30 mg/kg) administered once before training or before the retention test, 24 h after training, prevented the effect of scopolamine but did not alter extinction in these rats. However, entacapone (30 mg/kg) prolonged lag time when given during the extinction process to intact rats after training. Tolcapone administered once before training (10 mg/kg) counteracted the effect of scopolamine. It prolonged retention latency of the intact rats when given after training (10 mg/kg). Tolcapone (3 mg/kg) also prolonged lag time when given during extinction to rats bearing NBM lesions. The effect of scopolamine on extinction and retrieval was not prevented by tolcapone. Only entacapone improved memory storage. Collectively, the present results indicate that COMT inhibitors prolong retention latencies in a single-trial passive avoidance test assessed at several memory phases.

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.