Effects of haloperidol, sulpiride and SCH 23390 on passive avoidance learning in mice

Acute administration of a dopamine D2/D3 receptor agonist alters behavioral and neural parameters in adult zebrafish

The dopaminergic neurotransmitter system is implicated in several brain functions and behavioral processes. Alterations in it are associated with the pathogenesis of several human neurological disorders. Pharmacological agents that interact with the dopaminergic system allow the investigation of dopamine-mediated cellular and molecular responses and may elucidate the biological bases of such disorders. Zebrafish, a translationally relevant biomedical research organism, has been successfully employed in prior psychopharmacology studies. Here, we evaluated the effects of quinpirole (dopamine D2/D3 receptor agonist) in adult zebrafish on behavioral parameters, brain-derived neurotrophic factor (BDNF) and neurotransmitter levels. Zebrafish received intraperitoneal injections of 0.5, 1.0, or 2.0 mg/kg quinpirole or saline (control group) twice with an inter-injection interval of 48 h. All tests were performed 24 h after the second injection. After this acute quinpirole administration, zebrafish exhibited decreased locomotor activity, increased anxiety-like behaviors and memory impairment. However, quinpirole did not affect social and aggressive behavior. Quinpirole-treated fish exhibited stereotypic swimming, characterized by repetitive behavior followed by immobile episodes. Moreover, quinpirole treatment also decreased the number of BDNF-immunoreactive cells in the zebrafish brain. Analysis of neurotransmitter levels demonstrated a significant increase in glutamate and a decrease in serotonin, while no alterations were observed in dopamine. These findings demonstrate that dopaminergic signaling altered by quinpirole administration results in significant behavioral and neuroplastic changes in the central nervous system of zebrafish. Thus, we conclude that the use of quinpirole administration in adult zebrafish may be an appropriate tool for the analysis of mechanisms underlying neurological disorders related to the dopaminergic system.

Long-lasting behavioral effects of quinpirole exposure on zebrafish

The human brain matures into a complex structure, and to reach its complete development, connections must occur along exact paths. If at any stage, the processes are altered, interrupted, or inhibited, the consequences can be permanent. Dopaminergic signaling participates in the control of physiological functions and behavioral processes, and alterations in this signaling pathway are related to the pathogenesis of several neurological disorders. For this reason, the use of pharmacological agents able to interact with the dopaminergic signaling may elucidate the biological bases of such disorders. We investigated the long-lasting behavioral effects on adult zebrafish after quinpirole (a dopamine D2/D3 receptor agonist) exposure during early life stages of development (24 h exposure at 5 days post-fertilization, dpf) to better understand the mechanisms underlying neurological disorders related to the dopaminergic system. Quinpirole exposure at the early life stages of zebrafish led to late behavioral alterations. When evaluated at 120 dpf, zebrafish presented increased anxiety-like behaviors. At the open tank test, fish remained longer at the bottom of the tank, indicating anxiety-like behavior. Furthermore, quinpirole-treated fish exhibited increased absolute turn angle, likely an indication of elevated erratic movements and a sign of increased fear or anxiety. Quinpirole-treated fish also showed altered swimming patterns, characterized by stereotypic swimming. During the open tank test, exposed zebrafish swims from corner to corner in a repetitive manner at the bottom of the tank. Moreover, quinpirole exposure led to memory impairment compared to control fish. However, quinpirole administration had no effects on social and aggressive behavior. These findings demonstrate that dopaminergic signaling altered by quinpirole administration in the early life stages of development led to late alterations in behavioral parameters of adult zebrafish.

N-(3-{4-[3-(trifluoromethyl)phenyl]piperazin-1-yl}propyl)-1H-indazole-3-carboxamide (D2AAK3) as a potential antipsychotic: In vitro, in silico and in vivo evaluation of a multi-target ligand

Schizophrenia is a mental illness of not adequately understood causes that is not satisfactorily enough treated by current antipsychotics. In search for novel potential antipsychotics we performed structure-based virtual screening aimed to identify new dopamine D₂ receptor antagonists. We found compound D2AAK3 with affinity to dopamine D₂ receptor of 115 nM. D2AAK3 possesses additional nanomolar or low micromolar affinity to D₁, D₃, 5-HT_1A, 5-HT_2A and 5-HT₇ receptors, which makes it a good hit for further development as a multifunctional ligand. The compound has also some affinity to M₁ and H₁ receptors. We used homology modeling, molecular docking and molecular dynamics to study interactions of D2AAK3 with its molecular targets at the molecular level. In behavioral studies D2AAK3 decreases amphetamine-induced hyperactivity (when compared to the amphetamine-treated group) measured as spontaneous locomotor activity in mice. In addition, passive avoidance test demonstrated that D2AAK3 improves memory consolidation after acute treatment in mice. Elevated plus maze tests indicated that D2AAK3 induces anxiogenic activity 30 min after acute treatment, whereas this effect has no longer been observed 60 min after administration of the studied compound in mice.

Dopamine dependent setting of a circadian oscillator underlying the memory for time of day

Animals learn and remember the time of day that significant conditions occur, and anticipate recurrence at 24-h intervals, even after only one exposure to the condition. On several place-conditioning tasks, animals show context avoidance or preference only near the time of day of the experience. The memory for time of day is registered by a circadian oscillator that is set at the time of the training. We show that manipulations of dopamine (DA) neurotransmission can set a time memory in place preference and avoidance tasks, indicating that time of day is part of the context that is learned. Single injections of the DA agonist, d-amphetamine sulfate given without further exposure to the conditioning apparatus, can reset the timing of anticipatory behavior evoked by previously acquired place-event associations. The data support a model for time memory in which DA signaling sets the phase of a circadian oscillator, which returns to the same state at regular 24-h intervals. The data also raise the possibility that some apparent impairments of memory formation or retention could reflect post-experience resetting of the optimal retrieval time rather than impairment of memory or retrieval per se.

Contribution of dopamine neurotransmission in proconvulsant effect of Toxoplasma gondii infection in male mice

Epilepsy is one of the most common neurologic disorders worldwide with no distinguishable cause in 60% of patients. One-third of world's population is infected with Toxoplasma gondii (T. gondii). This intracellular parasite has high tendency to excitable cells including neurons. We assessed seizure susceptibility and involvement of dopaminergic system in male mice with acute and chronic T. gondii infection. Mice were infected by intraperitoneal injection of T. gondii cysts. Acute and chronic stages of infection were determined by quantification of SAG1/BAG1 transcripts and level of repetitive REP-529 sequence in the brain of mice by real-time PCR. Threshold of clonic seizures was measured by tail vein infusion of pentylenetetrazole. The infected mice were pretreated with D1 and D2 dopamine receptor antagonists, and seizure threshold was measured. Moreover, seizure threshold was determined after treatment of toxoplasmosis by sulfamethoxazole and trimethoprim. SAG1 level reached the maximum at week 2 after infection and then declined. The maximum level of BAG1 was observed at the week 3 and preserved till the week 8. REP-529 was detected at first week after infection, reached maximum at the week 3 and kept at this level till the eighth week. Threshold of seizures significantly decreased in both acute and chronic phases of infection. D1 and D2 receptors antagonists inhibited proconvulsant effect of toxoplasmosis. Chemotherapy inhibited parasite growth and multiplication, and returned seizure susceptibility to the level of non-infected mice. Dopaminergic neurotransmission participates in proconvulsant effect of T. gondii. The effect of parasite is eliminated by antibiotic therapy. © 2017 Wiley Periodicals, Inc.

Antipsychotic activity of standardized Bacopa extract against ketamine-induced experimental psychosis in mice: Evidence for the involvement of dopaminergic, serotonergic, and cholinergic systems

CONTEXT

Schizophrenia is a chronic disabling psychiatric disorder affecting 1% of the population worldwide. Due to the adverse effects of available antipsychotic medications, recent investigations have focused on the search for well-tolerated, safe molecules from natural resources to control the severity and progression of schizophrenia.

OBJECTIVE

To screen the standardized extract of Bacopa monniera Linn. (Scrophulariaceae) (BM) for its antipsychotic potential in the ketamine-induced psychosis model with mice.

MATERIALS AND METHODS

Graded dose of BM (40, 80, and 120 mg/kg, p.o.) were given to the mice 1 h prior to ketamine administration and tested for positive symptoms and cognitive deficits. A chronic ketamine treatment regimen was used to study the effect of BM on negative symptoms such as immobility enhancement. Each mouse was used once for the behavioral studies.

RESULTS

BM reduced ketamine-induced hyperactivity with an EC50 value of 76.60 mg/kg. The 80 mg/kg dose was used for all other behavior analysis. Pretreatment with BM at 80 mg/kg showed two-fold increases in transfer latency time (TLT) in passive avoidance task. Chronic BM pretreatment (80 mg/kg p.o. daily × 10 d) ameliorated the ketamine-induced enhanced immobility effect by 21% in the forced swim test. BM treatment reversed ketamine-induced increase in monoamine oxidase activity in both cortex and striatum and normalized the acetylcholinesterase activity and the glutamate levels in the hippocampus.

DISCUSSION AND CONCLUSION

Overall our findings suggest that BM possesses antipsychotic properties which might be due to its modulatory action on dopamine, serotonin, and glutamate neurotransmission.

Involvement of dopamine D1/D2 receptors on harmane-induced amnesia in the step-down passive avoidance test

Ingestion of harmane and other alkaloids derived from plant Peganum harmala has been shown to elicit profound behavioural and toxic effects in humans, including hallucinations, excitation, feelings of elation, and euphoria. These alkaloids in the high doses can cause a toxic syndrome characterized by tremors and convulsions. Harmane has also been shown to act on a variety of receptor systems in the mammalian brain, including those for serotonin, dopamine and benzodiazepines. In animals, it has been reported to affect short and long term memory. In the present study, effects of dopamine D1 and D2 receptor antagonists on the harmane (HA)-induced amnesia and exploratory behaviors were examined in mice. One-trial step-down and hole-board paradigms were used for the assessment of memory retention and exploratory behaviors in adult male NMRI mice respectively. Intraperitoneal (i.p.) administration of HA (5 and 10 mg/kg) immediately after training decreased memory consolidation, while had no effect on anxiety-like behavior. Memory retrieval was not altered by 15- or 30 min pre-testing administration of the D1 (SCH23390, 0.025, 0.05 and 0.1 mg/kg) or D2 (sulpiride 12.5, 25 and 50 mg/kg) receptor antagonists, respectively. In contrast, SCH23390 (0.05 and 0.1 mg/kg) or sulpiride (25 and 50 mg/kg) pre-test administration fully reversed HA-induced impairment of memory consolidation. Finally, neither D1 nor D2 receptor blockade affected exploratory behaviors in the hole-board paradigm. Altogether, these findings strongly suggest an involvement of D1 and D2 receptors modulation in the HA-induced impairment of memory consolidation.

Anxiolytic-like property of risperidone and olanzapine as examined in multiple measures of fear in rats

Atypical antipsychotics are also used in the treatment of anxiety-related disorders. Clinical and preclinical evidence regarding their intrinsic anxiolytic efficacy has been mixed. In this study, we examined the potential anxiolytic-like effects of risperidone and olanzapine, and compared them with haloperidol, chlordiazepoxide (a prototype of sedative-anxiolytic drug) or citalopram (a selective serotonin reuptake inhibitor). We used a composite of two-way avoidance conditioning and acoustic startle reflex model and examined the effects of drug treatments during the acquisition phase (Experiment 1) or extinction phase (Experiments 2 and 3) on multiple measures of conditioned and unconditioned fear/anxiety-like responses. In Experiment 4, we further compared risperidone, olanzapine, haloperidol, citalopram and chlordiazepoxide in a standard elevated plus maze test. Results revealed three distinct anxiolytic-like profiles associated with risperidone, olanzapine and chlordiazepoxide. Risperidone, especially at 1.0mg/kg, significantly decreased the number of avoidance responses, 22kHz ultrasonic vocalization, avoidance conditioning-induced hyperthermia and startle reactivity, but did not affect defecations or time spent on the open arms. Olanzapine (2.0mg/kg, sc) significantly decreased the number of avoidance responses, 22kHz vocalization and amount of defecations, but it did not inhibit startle reactivity and time spent on the open arms. Chlordiazepoxide (10mg/kg, ip) significantly decreased the number of 22kHz vocalization, avoidance conditioning-induced hyperthermia and amount of defecations, and increased time spent on the open arms, but did not decrease avoidance responses or startle reactivity. Haloperidol and citalopram did not display any anxiolytic-like property in these tests. The results highlight the importance of using multiple measures of fear-related responses to delineate behavioral profiles of psychotherapeutic drugs.

Clozapine and olanzapine exhibit an intrinsic anxiolytic property in two conditioned fear paradigms: contrast with haloperidol and chlordiazepoxide

Psychotic fear and anxiety disturbances are seen at a relatively high frequency in patients with schizophrenia. Atypical anti-psychotics are believed to show superior efficacy in reducing these symptoms. However, clinical and preclinical evidence regarding their anxiolytic efficacy has been mixed. In this study, we evaluated the possible anxiolytic property of two atypicals clozapine and olanzapine and compared them with typical haloperidol and chlordiazepoxide (a prototype of sedative-anxiolytic drug) in two preclinical models of fear. In Experiment 1, we used a fear-induced passive avoidance and conditioned place aversion paradigm and examined the effects of clozapine (20 mg/kg, sc), haloperidol (0.05 mg/kg, sc) and chlordiazepoxide (10 mg/ kg, ip). In Experiments 2 and 3, we used a two-way active avoidance conditioning paradigm and further compared the effects of clozapine (20 mg/kg, sc), haloperidol (0.05 mg/kg, sc), chlordiazepoxide (10 mg/kg, ip) and three doses of olanzapine (0.5, 1.0, and 2.0 mg/kg, sc). Results show that clozapine and chlordiazepoxide, but not haloperidol, significantly attenuated the shock conditioning-induced place aversion, decreased the amount of defecations and the number of the 22-kHz vocalizations. Clozapine also reduced the shock conditioning-induced hyperthermia. Similar to clozapine, olanzapine also significantly decreased the amount of defecations and reduced the shock conditioning-induced hyperthermia, but it did not inhibit the 22-kHz vocalizations. This study demonstrates that clozapine and olanzapine possess an intrinsic anxiolytic property, which is not attributable to its superior anti-"psychotic" effect or its favorable effects on motor functions or learning and memory processes. These findings also suggest that the combined use of passive avoidance and active avoidance conditioning models can be useful in better differentiating typical and atypical anti-psychotics as well as anxiolytics.

Involvement of dopamine D2 receptors in the induction of long-term potentiation in the basolateral amygdala-dentate gyrus pathway of anesthetized rats

We have previously found that synaptic pathway from the basolateral amygdala (BLA) to the dentate gyrus (DG) displays N-methyl-D-aspartate (NMDA) receptor-independent form of long-term potentiation (LTP), which should be a valuable model for elucidating neural mechanisms linking emotion and memory. To explore its cellular mechanisms, we investigated possible involvement of the beta-adrenergic, muscarinic cholinergic and dopaminergic systems on LTP in this pathway of anesthetized rats. The induction of BLA-DG LTP was not affected by administration of the beta-adrenoceptor antagonist propranolol (50-150nmol, i.c.v.), the muscarinic receptor antagonist scopolamine (2-6mg/kg, i.p.), the cholinesterase inhibitor physostigmine (50 nmol, i.c.v.) or the dopamine D(1) receptor antagonist SCH23390 (100nmol, i.c.v.), but significantly inhibited by the dopamine D2 receptor antagonists, chlorpromazine (15nmol, i.c.v.) and haloperidol (0.15-0.5mg/kg, i.p.), and significantly promoted by the dopamine D2 receptor agonist quinpirole (78nmol, i.c.v.). Furthermore, lesioning with 6-hydroxydopamine of the ventral tegmental area (VTA), the origin of mesolimbic dopaminergic neurons, resulted in attenuated BLA-DG LTP. These results suggest that the D2-dopaminergic system, but not the beta-adrenergic, muscarinic or D1-dopaminergic system, is involved in the induction of BLA-DG LTP. In addition, inhibition of BLA-DG LTP by haloperidol or VTA lesion was abolished by blockade of GABAergic inhibition with picrotoxin. It is probable that the D2-dopaminergic system promotes the induction of BLA-DG LTP by suppressing GABAergic inhibition.

Lurasidone (SM-13496), a novel atypical antipsychotic drug, reverses MK-801-induced impairment of learning and memory in the rat passive-avoidance test

Lurasidone (SM-13496) is a novel atypical antipsychotic with high affinities to dopamine D2, serotonin 5-HT7, 5-HT2A, 5-HT1A receptors and alpha2C adrenoceptor. In this study, the effects of lurasidone on the rat passive-avoidance response and its impairment by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine) were evaluated and compared with those of other antipsychotics. The passive-avoidance response was examined by measuring the step-through latency, 1 day after the animals received foot-shock training. When given before the training session, lurasidone did not affect the passive-avoidance response at any dose tested (1-30 mg/kg, p.o.). All the other atypical antipsychotics examined (i.e., risperidone, olanzapine, quetiapine, clozapine and aripiprazole), however, significantly reduced the step-through latency at relatively high doses. A pre-training administration of lurasidone significantly and dose-dependently reversed the MK-801-induced impairment of the passive-avoidance response. At doses lower than those that affected the passive-avoidance response, risperidone, quetiapine, and clozapine partially reduced the MK-801-induced impairment, whereas haloperidol, olanzapine, and aripiprazole were inactive. In addition, the post-training administration of lurasidone was as effective in countering the MK-801 effect as the pre-training administration, suggesting that lurasidone worked, at least in part, by restoring the memory consolidation process disrupted by MK-801. These results suggest that lurasidone is superior to other antipsychotics in improving the MK-801-induced memory impairment and may be clinically useful for treating cognitive impairments in schizophrenia.

Involvement of the histaminergic system on appetitive learning and its interaction with haloperidol in goldfish

This study investigated the actions of the histaminergic system on appetitive learning and memory, and its interaction with the dopaminergic system in goldfish. It consisted of nine sessions, in which fish were tested in a four-arm tank. On day 1, the animals were habituated for 10 min. On day 2, they were placed in one arm and had to find food at the left or the right arm. Time to begin feeding was recorded, and the procedure repeated for more 3 days (training phase). On training day 4, seven groups were injected with saline, seven with haloperidol (2.0 mg/kg) and one with DMSO solution before training and after feeding, three groups received saline, six chlorpheniramine (CPA) (1.0, 4.0 and 8.0 mg/kg), and six l-histidine (LH) (25, 50 and 100 mg/kg). Saline groups were considered as control of CPA and LH treated groups and DMSO as control of haloperidol. A non-injected group was also included. Testing occurred after 24 h. A reversal procedure was conducted 24h after testing and repeated for 3 days. The groups receiving CPA at 1.0 and 8.0 mg/kg and LH at 25, 50 and 100 mg/kg differed between Test and Reversal day 1. Pre-treatment with haloperidol plus 8.0 mg/kg of CPA and 25 and 50 mg/kg of LH reverted the treatment effect. However, in the groups treated with 1.0 mg/kg of CPA and 100 mg/kg of LH, the difference remained. This study confirmed the interaction between the histaminergic and the dopaminergic systems on memory process in goldfish.

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.

Differential effects of haloperidol, clozapine and olanzapine on learning and memory functions in mice

Many schizophrenic patients exhibit impairments in neurocognitive functions. Typical antipsychotic drugs such as haloperidol, have limited or even detrimental influence on cognitive functions. In contrast, atypical antipsychotic drugs, such as clozapine and olanzapine, may improve memory function in schizophrenics. However, only a few studies have been conducted to directly compare the effects of olanzapine, clozapine and haloperidol on memory functions in animal models. Thus, their effects on this issue were investigated in the present studies by using one-way step-through passive avoidance task and Morris water maze as models of learning and memory. The results showed that olanzapine did not affect acquisition, consolidation or retrieval process in step-through test. Moreover, it improved spatial learning function in mice in Morris water maze task. Clozapine and haloperidol appeared to impair acquisition process and consolidation process, respectively, in step-through test. Both drugs impaired spatial learning function in mice in Morris water maze task. The results suggested a positive implication for the clinical medication of olanzapine in schizophrenic treatment.

Effects of Clozapine and Sulpiride on Morphine State-Dependent Memory in the Step-Down Passive Avoidance Test

The effects of antipsychotic drugs sulpiride and clozapine on morphine state-dependent memory of passive avoidance task were examined in mice. Post-training administration of morphine (5 mg/kg) led to state-dependent learning with impaired memory retrieval on the test day which was reversed by pre-test administration of the same dose of the opioid (5 mg/kg). In animals where memory was impaired by post-training morphine, the administration of either sulpiride or clozapine before pre-test morphine reduced the improvement of memory produced by the opioid. Co-administration of sulpiride with clozapine did not potentiate their antagonistic response. In conclusion, the inhibition of improvement of memory retrieval by morphine treatment on the test day by the two dopamine receptor antagonists seems to be induced through two different receptor mechanisms.

Differently lasting effects of prenatal and postnatal chronic clozapine/haloperidol on activity and memory in mouse offspring

We evaluated the behavioral effects of chronic haloperidol (HAL) and clozapine (CLO) during gestation and CNS development, compared with transient treatments that stopped 1-3 weeks before the test.

RESULTS

1) Chronic HAL (6 mg/l in drinking water) but not HAL-withdrawal caused hypo-activity in open-field test on postnatal days (PNDs) 35, 42 and 56. However, hyper-activity was found in both CLO (90 mg/l) and CLO-withdrawal pups. 2) In the step-through test, retention performance was significantly higher in HAL-treated mice than in the controls on PND 42, as well as in withdrawal mice on PNDs 35 and 42. However, both chronic CLO (90 mg/l) exposure and CLO-withdrawal tended to improve the acquisition of memory. Furthermore, chronic CLO (180 mg/l) ameliorated scopolamine (3 mg/kg)-induced impairment of memory on PND 56. 3) In the water-maze test, both chronic HAL and HAL-withdrawal treatments significantly impaired performance on the 4th training day at PND 35, but not PNDs 42 and 56. Neither chronic CLO exposure nor CLO-withdrawal affected spatial memory. 4) Body weight following HAL/CLO administration decreased when compared with the controls during PND 21-35, but approached control levels at PND 40.

CONCLUSION

HAL doesn't elicit permanent behavioral changes in offspring. By contrast, CLO had longer-lasting effects than HAL. The pups at age before PND 35 seem more sensitive to HAL/CLO than elder pups.

Inhibiting effect of D1, but not D2 antagonist administered to the striatum on retention of passive avoidance in the chick

The avian lobus parolfactorius, equivalent to the medial striatum (caudate-putamen) of mammals, has been shown to be of crucial importance in passive avoidance training in day-old domestic chicks, where the aversive stimulus is the bitter tasting substance methylanthranilate. Here we report that the specific D1 antagonist SCH23390, injected into the lobus parolfactorius of day-old chicks (Gallus domesticus) prior to training, impaired performance on testing 30min post-training at low doses (0.5 and 25nmol). Sulpiride, a D2 antagonist, had no significant effect on performance in comparable doses. The early D1 activation may signify an essential mechanism leading to storage itself or to the canalisation of the relevant association to a permanent store.

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.

Facilitation and disruption of memory for the passive avoidance task in the day-old chick using dopamine D1 receptor compounds

This series of studies provides a behavioural account of dopamine D1-receptor-dependent facilitation and disruption of memory for the single-trial passive avoidance task in the day-old chick. The D1 antagonist, SCH23390, induced memory disruption in a dose-dependent manner from 60 min after training with a strong (100% methyl anthranilate) aversant experience. The D1 agonist, SKF38393, was found to facilitate memory in chicks given a weak (20% vol/vol methyl anthranilate) training experience. The D2 antagonist, sulpiride, and the D2 agonist, quinpirole, showed no memory effects. The research indicates an important role for dopamine D1-dependent mechanisms in memory formation in the chick.

Zuclopenthixol facilitates memory retrieval in rats: possible involvement of noradrenergic and serotonergic mechanisms

Although disturbed memory function often coexists with psychosis, the cognitive effects of antipsychotic medications with diverse pharmacodynamic properties are rarely investigated. The neurocognitive profile of zuclopenthixol, a thioxanthene dopaminergic antagonist and a conventional neuroleptic agent, has yet to be investigated despite the effect of the drug on a variety of neurotransmitter systems involved in mediation of learning and memory processes. In this study, the effect of zuclopenthixol was tested on memory retrieval 24 h after training using an inhibitory avoidance task in rats. Acute administration of zuclopenthixol (0.7 and 1.4 mg/kg i.p.) before retrieval testing increased step-through latency during the test session. The same doses of zuclopenthixol did not affect the ambulatory activity of rats in the openfield test and therefore the facilitatory effect of the drug on memory function could not be confounded with any motoric properties. This study also investigated the effect of zuclopenthixol on cortical and hippocampal monoaminergic neurotransmitters' levels together with acetylcholinesterase enzyme (AChE) activity, both of which are known to be important in control of cognitive function. Administration of zuclopenthixol (0.7 and 1.4 mg/kg i.p.) neither affected dopamine (DA) level nor AChE activity in rat cortex and hippocampus. On the other hand, the lower dose of zuclopenthixol elevated cortical norepinephrine (NE) level, while the higher dose elevated both cortical and hippocampal NE level together with hippocampal serotonin (5-HT) level. These results may suggest the involvement of adrenergic and serotonergic mechanisms in the facilitatory effect of zuclopenthixol on retrieval memory. Zuclopenthixol may therefore be a better alternative than other commonly used antipsychotic medications reported to impair cognitive function of schizophrenic patients.

Facilitatory effect of the dopamine D4 receptor agonist PD168,077 on memory consolidation of an inhibitory avoidance learned response in C57BL/6J mice

The still unknown contribution of the D4 receptors to memory consolidation was studied examining the memory effects of the dopamine D4 agonist PD168,077, the putative dopamine D4 antagonist L745,870, their mutual combination, and the combination of the D4 agonist with representative compounds acting as agonist or antagonist on the D1, D2 and the D3 receptors. Memory consolidation was assessed in C57BL/6J mice using the one-trial step-through inhibitory avoidance task, the compounds being injected immediately after training (foot-shock) and performance measured 24h later. PD168,077 (0.5-10mg/kg) dose-dependently improved memory performance and L745,870 (0.05-5mg/kg) at doses lower than 1mg/kg increased and at doses higher than 1mg/kg impaired memory performance. PD168,077 did not affect the paradoxical promnesic effect of low doses (0.1-0.5mg/kg) of L745,870, but antagonised the memory-impairing effect induced by 5mg/kg L745,870. The D1 antagonist SCH23390 (0.025-0.05 mg/kg) and the D2 antagonist eticlopride (0.01-0.05 mg/kg) antagonised the promnesic effects of PD168,077, which attenuated the decreasing effect on memory consolidation of both D1 and D2 antagonists. Accordingly, the D1 agonist SKF38393 (5-20mg/kg) and the D2 agonist quinelorane (0.1-1 mg/kg) both synergistically magnified the memory-improving effects of the D4 agonist. The dopamine D3 antagonist U99194A (2.5-10mg/kg) did not affect the promnesic effects induced by the D4 agonist, which nevertheless abolished the U99194A-induced promnesic effects. Additionally, the amnesic effects produced by the D3 agonist 7-OH-DPAT (0.01-1 microg/kg) was attenuated by PD168,077. These results suggest a potential role of dopamine D4 receptors in memory consolidation, which would be similar to that of the D1 and D2 receptors and probably opposite to that of the D3 receptors.

The muscarinic receptor agonist BuTAC, a novel potential antipsychotic, does not impair learning and memory in mouse passive avoidance

(5R,6R)-6-(3-butylthio-1,2,5-thiadiazol-4-yl)-1-azabicyclo[3.2.1]octane) (BuTAC) is a novel, selective muscarinic receptor ligand with partial agonist mode of action at muscarinic M2 and M4 and antagonist mode of action at M1, M3 and M5 receptor subtypes in cloned cell lines. BuTAC exhibits functional dopamine receptor antagonism despite its lack of affinity for dopamine receptors, and parasympathomimetic effects in mice are produced only at doses well beyond the doses exhibiting the antipsychotic-like effects. In the present study we investigated the effects of BuTAC and the antipsychotic compounds clozapine, sertindole and olanzapine using one trial passive avoidance with mice as a model of learning and memory. Pharmacologically relevant doses of BuTAC and reference antipsychotics were identified, based on inhibition of apomorphine-induced climbing in mice as an assay measuring antidopaminergic potency. When ratios between the minimum effective dose (MED) for impairment of retention in passive avoidance and the MED for inhibition of apomorphine-induced climbing were calculated, BuTAC displayed a high ratio of >10, compared with clozapine (0.3), sertindole (3) and olanzapine (3). These data suggest that BuTAC is a potential novel antipsychotic which may have favourable effects on aspects of learning and memory.

Opposite effects of dopamine D2 and D3 receptors on learning and memory in the rat

Mesolimbocortical dopamine plays a role in learning and memory. The specific receptor subtypes mediating the effects of dopamine, however, are still unknown. Dopamine D2, D3 and D4 receptors are expressed in the hippocampus and dopamine D3 receptors are present in the septal area, suggesting that these receptor subtypes can contribute to the behavioral effects of dopamine D2-like receptor agonists. We now investigated the role of dopamine D2 and D3 receptors in learning and memory by using the transient amnesia induced by scopolamine in the passive avoidance test as experimental model. The data strongly suggest that both dopamine D2 and D3 receptors mediate the effects of dopamine on the integrative function of learning and memory. In particular, we show that the non-selective dopamine agonist apomorphine prevents the scopolamine-induced disruption of consolidation of the previously acquired passive avoidance behavior. This effect is mediated by receptors belonging to the dopamine D2 family since it was antagonized by (-)-sulpiride and mimicked by quinpirole. Nafadotride, a relatively selective antagonist for dopamine D3 receptors, antagonized scopolamine-induced memory disruption and potentiated the facilitatory effect of quinpirole. Taken together, these results suggest that the effects of dopamine on memory consolidation are the result of a balance between dopamine D2 receptor-mediated facilitation and dopamine D3 receptor-mediated inhibition, and that dopamine D2 and D3 receptors play opposite roles in the control of the mechanisms leading to memory consolidation.

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.

Nefiracetam (DM-9384) reverses apomorphine-induced amnesia of a passive avoidance response: delayed emergence of the memory retention effects

Nefiracetam is a novel pyrrolidone derivative which attenuates scopolamine-induced learning and post-training consolidation deficits. Given that apomorphine inhibits passive avoidance retention when given during training or in a defined 10-12h post-training period, we evaluated the ability of nefiracetam to attenuate amnesia induced by dopaminergic agonism. A step-down passive avoidance paradigm was employed and nefiracetam (3 mg/kg) and apomorphine (0.5 mg/kg) were given alone or in combination during training and at the 10-12h post-training period of consolidation. Co-administration of nefiracetam and apomorphine during training or 10h thereafter produced no significant anti-amnesic effect. However, administration of nefiracetam during training completely reversed the amnesia induced by apomorphine at the 10h post-training time and the converse was also true. These effects were not mediated by a dopaminergic mechanism as nefiracetam, at millimolar concentrations, failed to displace either [3H]SCH 23390 or [3H]spiperone binding from D1 or D2 dopamine receptor subtypes, respectively. It is suggested that nefiracetam augments molecular processes in the early stages of events which ultimately lead to consolidation of memory.

Effects of nicotine on memory retrieval in mice

The effect of nicotine was tested on retrieval 24 h after training on a passive avoidance task. Intraperitoneal (i.p.) injection of nicotine (0.25-1.5 mg/kg) increased the step-down latency in mice dose dependently. Pretreatment with the nicotinic receptor antagonist mecamylamine (0.5-1 mg/kg) decreased, whereas pretreatment with the dopamine D1 receptor antagonist SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol maleate) (0.01, 0.05 and 0.1 mg/kg) and the beta-adrenoreceptor antagonist propranolol (10 mg/kg) increased the nicotine response. The dopamine receptor D2 receptor antagonist sulpiride (5-10 mg/kg), the anti-muscarinic agent atropine (2.5-10 mg/kg), the peripheral nicotinic receptor antagonist hexamethonium (0.01-0.5 mg/kg), the alpha-adrenoceptor antagonist phenoxybenzamine (1 and 10 mg/kg) and the peripheral dopamine D2 receptor antagonist domperidone (5 and 10 mg/kg) did not change the response induced by nicotine. Single administration of the antagonists did not cause response; however, a high dose of domperidone (10 mg/kg) and propranolol alone increased the step-down latencies. It may be concluded that a nicotinic receptor mechanism is involved in the nicotine-induced improvement of memory retrieval.

The attenuation of suppression of motility by triazolam in the conditioned fear stress task is exacerbated by ethanol in mice

We investigated whether triazolam attenuated the suppression of motility in the conditioned fear stress task in mice and whether ethanol modified the effects of triazolam. When mice were placed 24 hours later (retention test) in the same environment in which they had previously been exposed to an electric foot shock (training), they exhibited a marked suppression of motility (conditioned fear stress). Triazolam (0.01-0.1 mg/kg, s.c.), administered before training, attenuated the suppression of motility in the conditioned fear stress task in a dose-dependent manner, without affecting the sensitivity to an electric foot shock. The doses of triazolam that attenuated the suppression of motility were much lower that those of chlordiazepoxide (5-10 mg/kg, s.c.). Neither drug, administered before the retention test, attenuated the suppression of motility in the conditioned fear stress task. These results suggest that both benzodiazepines may inhibit the process of acquisition, but not the process of recall, of memory. Ethanol (1 g/kg, p.o.), which, by itself, did not affect either the suppression of motility or the sensitivity to an electric foot shock, exacerbated the attenuation of the suppression of motility in the conditioned fear stress task induced by both triazolam (0.01 mg/kg) and chlordiazepoxide (5 mg/kg). These results suggest that ethanol exacerbates the effects of benzodiazepines.

Ameliorating effects of sigma receptor ligands on the impairment of passive avoidance tasks in mice: involvement in the central acetylcholinergic system

Three sigma receptor ligands were examined for their ameliorating effects on p-chloroamphetamine-induced amnesia in mice. p-Chloroamphetamine was administered intraperitoneally 30 min before the training session of the passive avoidance response. Each sigma receptor ligand was administered 60 min before or immediately after the training session, or 60 min before the retention test. (+)-N-Allylnormetazocine ((+)-SKF-10,047), a prototype benzomorphan sigma receptor ligand, significantly reduced the p-chloroamphetamine-induced amnesia in these three administration schedules, as do acetylcholinesterase inhibitors. On the contrary, the significant anti-amnesic effects elicited by non-benzomorphan sigma receptor ligands, 1,3-di-(2-tolyl)guanidine (DTG) or (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperizine ((+)-3-PPP), were observed depending upon the timing of their administration. In addition, the ameliorating effect of (+)-SKF-10,047 against the p-chloroamphetamine-induced amnesia was superior to that of (-)-SKF-10,047. The (+)-SKF-10,047-induced anti-amnesic effect was significantly antagonized by the concurrent administration of either scopolamine, a muscarinic receptor antagonist, or hemicholinium-3, an inhibitor of the Na(+)-dependent high-affinity choline uptake site. These findings indicated that sigma receptor ligands had anti-amnesic effects against drug-induced memory impairment. In addition, the anti-amnesic effect of (+)-SKF-10,047 was superior to those of other sigma receptor ligands, and was mediated by both the sigma receptor and the central acetylcholinergic system.

Effects of pimozide on cognition in children with Tourette syndrome: interaction with comorbid attention deficit hyperactivity disorder

Pimozide, a dopamine receptor antagonist, dopamine uptake inhibitor and L-type calcium channel blocker, is used in children suffering from Tourette syndrome (TS), but its effect on frequently comorbid attention deficit hyperactivity disorder (ADHD) is unknown. We have characterized the cognitive effects of pimozide and haloperidol in TS in relation to comorbid ADHD. Sixty-six consecutive outpatients with TS (DSM-III-R), ages 7-16, were randomly assigned to pimozide, haloperidol or medication-free clinical treatment and evaluated cognitively after 6 weeks. Continuous performance task (CPT) and memory search efficiency were compared across treatments. Treatment, comorbid ADHD and their interaction were significant on the CPT task. Pimozide treatment was superior to haloperidol and improved memory search efficiently over the no-drug condition.

Characterization of the effects of scopolamine on the habituation of exploratory activity: differential effects of oxotremorine and physostigmine

1. Habituation was regarded as a difference between exploratory activity measured first (session 1) and that measured second (session 2) in a novel environment. 2. Scopolamine (1.0 mg/kg) significantly increased the horizontal activity in sessions 1 and 2 when administered prior to session 1, resulting in the impairment of habituation. 3. Haloperidol (0.2 mg/kg) inhibited scopolamine-induced hypermotility in session 1, but it did not inhibit the scopolamine-induced impairment of habituation in session 2. 4. The direct cholinergic agonist oxotremorine (0.03 mg/kg), unlike the cholinesterase inhibitor physostigmine, significantly inhibited the scopolamine-induced impairment of habituation in the horizontal and vertical activities. 5. These results suggest that the direct stimulation of cholinergic receptors is more effective for scopolamine-induced amnesia than the indirect stimulation of cholinergic receptors by cholinesterase inhibitors in the habituation task.

The involvement of nucleus accumbens dopamine in appetitive and aversive motivation

In recent years, considerable emphasis has been placed upon the putative role of nucleus accumbens dopamine systems in appetitive motivation and positive reinforcement. However, considerable evidence indicates that brain dopamine in general, and nucleus accumbens dopamine in particular, is involved in aspects of aversive motivation. Administration of dopamine antagonists or localized interference with nucleus accumbens dopamine systems has been shown to disrupt active avoidance behavior. In addition, accumbens dopamine release and metabolism is activated by a wide variety of stressful conditions. A review of the literature indicates that there are substantial similarities between the characteristics of dopaminergic involvement in appetitive and aversive motivation. There is conflicting evidence about the role of dopamine in emotion, and little evidence to suggest that the profound and consistent changes in instrumental behavior produced by interference with DA systems are due to direct dopaminergic mediation of positive affective responses such as hedonia. It is suggested that nucleus accumbens dopamine is involved in aspects of sensorimotor functions that are involved in both appetitive and aversive motivation.

Effects of several amnesic drugs on the habituation of exploratory activity in mice as indexed by horizontal and vertical activities

1. The effects of several drugs on the habituation of exploratory activity in mice were investigated by using animal movement analyzing systems which could detect horizontal and vertical activities. 2. Habituation was regarded as the difference between exploratory activity measured first (session 1) and that measured second (session 2). 3. Scopolamine (1, 3 and/or 6 mg/kg), pirenzepine (20 and 40 micrograms) injected immediately after session 1, cycloheximide (30 mg/kg), diazepam (1, 2 and/or 4 mg/kg) and dizocilpine (0.05, 0.1 and/or 0.2 mg/kg), unlike butylscopolamine (1, 3 and 6 mg/kg) or haloperidol (0.2 and 0.4 mg/kg), significantly increased horizontal activity but not always vertical activity in session 2. 4. From the results obtained, it appears that the increase in horizontal activity in session 2 is more appropriate as an indicator of the impairments of memory processes than that in vertical activity.

Dopaminergic involvement in the improving effects of dynorphin A-(1-13) on scopolamine-induced impairment of alternation performance

The present study was designed to clarify whether dopamine systems are involved in the effect of dynorphin A-(1-13), an endogenous kappa-opioid receptor agonist, on the scopolamine-induced impairment of spontaneous alternation performance related to working memory in mice. Sulpiride (10 and/or 30 mg/kg), a dopamine D2-selective antagonist, markedly improved the impairment of alternation performance and significantly reduced the increase in total arm entries (i.e. locomotor activity) induced by scopolamine (1 mg/kg). In contrast, SCH 23390 (0.01, 0.03 and 0.1 mg/kg), a dopamine D1-selective antagonist, did not influence the impairment of alternation performance, whereas it dose dependently reduced total arm entries in scopolamine-treated mice RU 24213 (1 mg/kg), a dopamine D2-selective agonist, almost completely reversed the improving effect of dynorphin A-(1-13) (3 micrograms) on the scopolamine-induced impairment of alternation performance and reduced total arm entries, although it was without effects on behavioral responses in normal mice. However, SKF 38393 (3 and 10 mg/kg), a dopamine D1-selective agonist, failed to change alternation performance or total arm entries in normal or scopolamine-treated mice. These findings suggest that the impairment of spontaneous alternation performance induced by scopolamine is improved by the blockade of dopamine D2 receptors. Furthermore, the improving effect of dynorphin A-(1-13) on the scopolamine-induced impairment of spontaneous alternation performance may be based upon the inhibition of dopaminergic activity through the mediation of kappa-opioid receptors.

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.

Cholinergic and dopaminergic agents which inhibit a passive avoidance response attenuate the paradigm-specific increases in NCAM sialylation state

The influence of cholinergic and dopaminergic agents on the acquisition of a passive avoidance response in the rat is demonstrated. Trifluoperazine (0.12 mg/kg), a dopamine antagonist, inhibited task acquisition when present during training or later, during consolidation, at the 10-12 h post-training period and at no other intervening time point. Induction of amnesia was dose-dependent and was not apparent when the dose exceeded 0.12 mg/kg. This effect appears to be due to an increase in dopamine release through presynaptic receptor antagonism as similar results could be obtained by the administration of apomorphine (0.5 mg/kg), a dopamine agonist, and this effect could be antagonized by the D1 receptor selective antagonist SCH-23390. Scopolamine (0.15 mg/kg), a muscarinic antagonist, impaired acquisition of the passive avoidance response when administered during training and, separately, at the 6 h post-training period. This could not be attributed to presynaptic antagonism as oxotremorine (0.2 mg/kg), a muscarinic agonist, had no amnesic action. Administration of apomorphine or scopolamine during training and at the appropriate post-training period prevented subsequent paradigm-specific increases of neural cell adhesion molecule sialylation state in hippocampal immunoprecipitates obtained at 24 h after task acquisition and 4 h following intraventricular infusion of the labelled sialic acid precursor - N-acetyl-D-mannosamine. Oxotremorine alone did not influence neural cell adhesion molecule sialylation state. These observations provide further evidence of a regulatory role for neural cell adhesion molecule sialylation state in information storage processes.

Strain-dependent effects of post-training dopamine receptor agonists and antagonists on memory storage in mice

Post-training administration of the selective D1 or D2 agonists SKF 38393 and LY 171555 dose dependently impairs retention of an inhibitory avoidance response in DBA/2 mice. In agreement, the selective D1 or D2 antagonists SCH 23390 and (-)-sulpiride improve retention. These effects are opposite to those observed in the C57BL/6 strain, as previously reported. Moreover, B6D2F1 hybrids present a response to SKF 38393, LY 171555, SCH 23390, and (-)-sulpiride that parallels that of the C57BL/6 strain, thus suggesting that the neural mechanisms underlying the effects of DA agonists or antagonists on memory processes may be inherited through a dominant mode of inheritance.

Effects of dopamine receptor agonists on passive avoidance learning in mice: interaction of dopamine D1 and D2 receptors

The present study examined the effects of dopamine D1 and D2 receptor agonists on the acquisition stage of passive avoidance learning and on locomotor activity in mice. The D2 agonist, RU 24213 (1-10 mg/kg s.c.), and the non-selective agonist, apomorphine (0.3-3 mg/kg s.c.), but not the D1 agonist, SKF 38393 (1-10 mg/kg s.c.), impaired learning and activated locomotion. RU 24213 (1 mg/kg s.c.) was more effective in impairing learning than in activating locomotion. The concurrent administration of SKF 38393 (10 mg/kg i.p.) and RU 24213 (1 and 3 mg/kg s.c.) produced a synergistic effect in both behavioral situations. The D1 antagonist, SCH 23390 (0.025 mg/kg i.p.), slightly inhibited the effects of apomorphine and of the combination of SKF 38393 and RU 24213 on learning but not on locomotion. The D2 antagonist, (-)-sulpiride (40 mg/kg i.p.), completely blocked these effects in both situations. These results suggest that dopamine receptor agonists impair passive avoidance learning through the D2 receptor, and that D1 and D2 receptors act synergistically in this impairment, as they do in their effects on locomotion. The involvement of D1 and D2 receptors is qualitatively similar in each of these behaviors, although some small differences may exist.

Effect of D-1 or D-2 receptor stimulation on memory retrieval in mice

Mice were trained in one-way active avoidance procedure and retention was tested at 4, 8, 16 and 24 h after training of animals and compared with non-shocked or untrained animals. The effect of drugs was tested on retrieval 24 h after training in other groups of mice. High doses of apomorphine or bromocriptine impaired, while low doses of the drugs improved, retrieval of avoidance. High doses of sulpiride reversed the impairment induced by high doses of these dopamine agonists. Low doses of sulpiride antagonized the improvement of retrieval induced by low doses of apomorphine. SKF 38393 treatment of animals also improved the retrieval. The retrieval impairment induced by higher doses of apomorphine or the improvement induced by different doses of SKF 38393 was antagonized by SCH 23390 pre-treatment. Single administration of SCH 23390 or low doses of sulpiride also impaired retrieval. It is concluded that stimulating post-synaptic D-2 dopamine receptors impairs retrieval whilst activation of pre-synaptic D-2 or post-synaptic D-1 receptors improves memory retrieval.

Post-training dopamine receptor agonists and antagonists affect memory storage in mice irrespective of their selectivity for D1 or D2 receptors

Post-training administration of the selective D1 and D2 agonists SKF 38393 and LY 171555 dose-dependently facilitated retention of an inhibitory avoidance response in mice, while the selective D1 or D2 antagonists SCH 23390 and (-)sulpiride produced an impairment of retention. These effects are not to be ascribed to a nonspecific action of the drugs on retention performance, as the latencies during the retention test of those mice that had not received a footshock during the training were not increased by the post-training drug administration. The effects on retention performance induced by DA agonists and antagonists seem to be due to an effect on memory consolidation, since they have been observed when drugs were given at short, but not at long, periods of time after training. These results showing a similar role of D1 and D2 receptor types on memory storage appear not to be consistent with a body of neuropharmacological, neurophysiological, and behavioral evidence pointing to a different functional role of these types of DA receptors. This discrepancy is discussed in terms of possible involvement of different brain systems, peripheral systems, or possible second messenger processes activated by the two receptor types and leading to similar effects on memory processes.

Effects of specific dopaminergic agonists and antagonists in the open-field test

It has been found that dopaminergic transmission could be involved in some aspects of anxiety. The present study aims to explore this hypothesis further, using specific DA1 (SKF 38393) and DA2 (bromocriptine) agonists or DA1 (SCH 23390), and DA2 (zetidoline) antagonists in the open-field test. The results confirm previous studies indicating that DA1 and DA2 agonists predominantly increase locomotor activity, while DA1 and DA2 antagonists predominantly decrease it. However, at low doses, the four drugs increase the peripheral ambulation score significantly and, with the exception of zetidoline, also increase the central ambulation score. The observations made with zetidoline confirm the hypothesis that a specific presynaptic DA2 antagonism could be determinant for the disinhibitory effects of low doses of neuroleptics. A collateral action on 5HT transmission is also suggested to explain an hypothetic anxiolytic action of DA agonists and SCH 23390 at lower doses.