Strain-dependent effects of D2 dopaminergic and muscarinic-cholinergic agonists and antagonists on memory consolidation processes in mice

Spatial learning and memory deficit of low level polybrominated diphenyl ethers-47 in male adult rat is modulated by intracellular glutamate receptors

Polybrominated diphenyl ethers (PBDEs), a class of widely used flame retardants, are extensively diffused in the environment. Of particular concern are the reported highly sensitivity of PBDEs in children or developmental animals, however, almost no information is available on their potential effects on adults and the mechanisms are still unknown. In the present study, we investigated the neurotoxic effects of sub-chronic PBDE-47 exposure on adult male Sprague-Dawley rats. Thus, PBDE-47, 0.1, 0.5 and 1 mg/kg per day was administered to rats by gavage for 30 days. The learning and memory function was tested by Morris water maze. Further, in order to explore the potential mechanism, the expression of NMDA-receptors was evaluated by using both immunohistochemistry (IHC) and RT-PCR. Our results showed that sub-chronic exposure to PBDE-47 produced learning and memory deficits in male adult rats. Also, significant decrease in the CA1, CA3 and dentate gyrus areas of hippocampus affected by all three doses of PBDE-47 on the expression of NR(1), NR(2)B and Glu were found by IHC. In addition, the evaluation of expression of the NR(1), NR(2)B and NR(2)C showed statistically significant decrease in mRNA expression in rats exposed to PBDE-47. These findings showed that sub-chronic exposure to PBDE-47 could also induce behavioral alterations and the neurotoxic effects might due to the down-regulation expression of NMDA receptors. Our data indicated that the possibility of exposure of adults to PBDE-47 warranted further studies to characterize their potential neurotoxicity.

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.

Effects of nicotine vary across two auditory evoked potentials in the mouse

BACKGROUND

Schizophrenia patients display sensory processing deficits, reduced alpha7-nicotine receptor expression, and increased incidence of smoking, prompting investigation of nicotine receptor agonists as possible treatments. We evaluated the effects of acute and chronic nicotine, using an animal model that incorporates genetic variation for sensory processing and nicotine sensitivity.

METHODS

C57BL/6J and DBA/2Hsd mice received 2 weeks of 4.2 mg/kg chronic nicotine or saline. Auditory evoked potentials were recorded before and after acute nicotine injection of 1.05 mg/kg on day 14, with a paired-click paradigm (S1/S2). Amplitude and gating of the P20 and N40 were compared between conditions.

RESULTS

Acute nicotine increased the amplitude and gating of the P20 and decreased the amplitude and gating of the N40 across all groups, primarily by acting on S1. Chronic nicotine attenuated the effects of acute nicotine on the N40.

CONCLUSIONS

Our data support the notion that the mouse P20 shares pharmacological response properties with the human P50. In addition, findings suggest that nicotine might increase the initial sensory response (S1), with a resulting improvement in gating of some components.

The involvement of cholinergic and noradrenergic systems in behavioral recovery following oxotremorine treatment to chronically stressed rats

Chronic stress in rats has been shown to impair learning and memory, and precipitate several affective disorders like depression and anxiety. The mechanisms involved in these stress-induced disorders and the possible reversal are poorly understood, thus limiting the number of drugs available for their treatment. Our earlier studies suggest cholinergic dysfunction as the underlying cause in the behavioral deficits following stress. Muscarinic cholinergic agonist, oxotremorine is demonstrated to have a beneficial effect in reversing brain injury-induced behavioral dysfunction. In this study, we have evaluated the effect of oxotremorine treatment on chronic restraint stress-induced cognitive deficits. Rats were subjected to restraint stress (6 h/day) for 21 days followed by oxotremorine treatment for 10 days. Spatial learning and memory was assessed in a partially baited eight-arm radial maze task. Stressed rats exhibited impairment in performance, with decreased percentage of correct choices and an increase in the number of reference memory errors (RMEs). Oxotremorine treatment (0.1 or 0.2 mg/kg, i.p.) to stressed rats resulted in a significant increase in the percent correct choices and a decrease in the number of RMEs compared with stress as well as the stress+vehicle-treated groups. In the retention test, oxotremorine treated rats committed less RMEs compared with the stress group. Chronic restraint stress decreased acetylcholinesterase (AChE) activity in the hippocampus, frontal cortex and septum, which was reversed by both the doses of oxotremorine. Further, oxotremorine treatment also restored the norepinephrine levels in the hippocampus and frontal cortex. Thus, this study demonstrates the potential of cholinergic muscarinic agonists and the involvement of both cholinergic and noradrenergic systems in the reversal of stress-induced learning and memory deficits.

Facilitation of memory for extinction of drug-induced conditioned reward: role of amygdala and acetylcholine

These experiments examined the effects of posttrial peripheral and intra-amygdala injections of the cholinergic muscarinic receptor agonist oxotremorine on memory consolidation underlying extinction of amphetamine conditioned place preference (CPP) behavior. Male Long-Evans rats were initially trained and tested for an amphetamine (2 mg/kg) CPP. Rats were subsequently given limited extinction training, followed by immediate posttrial peripheral or intrabasolateral amygdala injections of oxotremorine. A second CPP test was then administered, and the amount of time spent in the previously amphetamine-paired and saline-paired apparatus compartments was recorded. Peripheral (0.07 or 0.01 mg/kg) or intra-amygdala (10 etag/0.5 microL) postextinction trial injections of oxotremorine facilitated CPP extinction. Oxotremorine injections that were delayed 2 h posttrial training did not enhance CPP extinction, indicating a time-dependent effect of the drug on memory consolidation processes. The findings indicate that memory consolidation for extinction of approach behavior to environmental stimuli previously paired with drug reward can be facilitated by posttrial peripheral or intrabasolateral amygdala administration of a cholinergic agonist.

Dopamine D2 receptors mediate two-odor discrimination and reversal learning in C57BL/6 mice

Background

Dopamine modulation of neuronal signaling in the frontal cortex, midbrain, and striatum is essential for processing and integrating diverse external sensory stimuli and attaching salience to environmental cues that signal causal relationships, thereby guiding goal-directed, adaptable behaviors. At the cellular level, dopamine signaling is mediated through D₁-like or D₂-like receptors. Although a role for D₁-like receptors in a variety of goal-directed behaviors has been identified, an explicit involvement of D₂ receptors has not been clearly established. To determine whether dopamine D₂ receptor-mediated signaling contributes to associative and reversal learning, we compared C57Bl/6J mice that completely lack functional dopamine D₂ receptors to wild-type mice with respect to their ability to attach appropriate salience to external stimuli (stimulus discrimination) and disengage from inappropriate behavioral strategies when reinforcement contingencies change (e.g. reversal learning).

Results

Mildly food-deprived female wild-type and dopamine D₂ receptor deficient mice rapidly learned to retrieve and consume visible food reinforcers from a small plastic dish. Furthermore, both genotypes readily learned to dig through the same dish filled with sterile sand in order to locate a buried food pellet. However, the dopamine D₂ receptor deficient mice required significantly more trials than wild-type mice to discriminate between two dishes, each filled with a different scented sand, and to associate one of the two odors with the presence of a reinforcer (food). In addition, the dopamine D₂ receptor deficient mice repeatedly fail to alter their response patterns during reversal trials where the reinforcement rules were inverted.

Conclusions

Inbred C57Bl/6J mice that develop in the complete absence of functional dopamine D₂ receptors are capable of olfaction but display an impaired ability to acquire odor-driven reinforcement contingencies. Furthermore, the ability of dopamine D₂ receptor deficient mice to adjust their responding to a previously reinforced stimulus when unexpected outcomes are encountered is significantly impaired. These findings suggest that signaling mediated by the dopamine D₂ receptor is important for regulating associative and reversal learning and may have implications for the treatment of human attention disorders.

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.

Decline in motor functions in aging is related to the loss of NMDA receptors

The aim of the study was to assess the contribution of central dopaminergic and glutamatergic systems to the age-dependent loss of motor functions in rats. Rats of three age groups were compared: young (3-5-month-old), middle-aged (20-21-month-old) and old (29-31-month-old). The obtained results showed an age-dependent decline in the electromyographic (EMG) resting and reflex activities in the gastrocnemius and tibialis anterior muscles, as well as in the T-maze performance. Although these disturbances were accompanied with significant age-dependent decreases in the binding to NMDA, AMPA and dopamine D2 receptors, and a decline in the number of nigral dopamine neurons, they were significantly correlated with the loss of the binding to NMDA receptors only. The reduction in T-maze performance with aging was additionally correlated with a decrease in motor functions (EMG activity). The study suggests a crucial role of the loss of NMDA receptors in age-dependent motor disabilities, as well as in disturbances measured in the T-maze.

D2 dopamine receptor blockade immediately post-training enhances retention in hidden and visible platform versions of the water maze

Considerable evidence shows that post-training administration of dopamine agonists can enhance memory through actions on consolidation processes, but relatively little is known regarding the effects of dopamine antagonists on consolidation. These experiments investigated the effects of post-training systemic administration of the D2 receptor antagonist sulpiride on consolidation of memory for two versions of the Morris water maze task. Rats trained in either the hidden (spatial) or visible (cued) platform version received a subcutaneous injection of sulpiride or vehicle immediately following training. Retention testing 48 hr later revealed that relative to vehicle controls, sulpiride reduced platform latencies in both task versions, suggesting that like dopamine agonists, sulpiride can also have memory-enhancing effects.

Strain-dependent interactions between MK-801 and cocaine on retention of C57BL/6 and DBA/2 mice tested in a one-trial inhibitory avoidance task: involvement of dopaminergic mechanisms

Two sets of experiments were carried out with C57BL/6 (C57) and DBA/2 (DBA) mice tested in a one-trial inhibitory avoidance task. In the first set C57 and DBA mice were injected posttraining with saline or with the D1 DA receptor antagonist SCH 23390 and then with saline, cocaine (5 mg/kg), MK-801 (0.1 mg/kg), or with a combination of these two drugs. Cocaine enhanced retention in the C57 strain and impaired it in the DBA strain, and MK-801 potentiated the effects of cocaine in both strains. Furthermore, pretreatment with SCH 23390 completely antagonized the potentiation of the effects of cocaine exerted by MK-801. In the second set of experiments mice belonging to these same two strains were injected posttraining with vehicle or with the D2 DA receptor antagonist (-)-sulpiride and then with saline, cocaine (5 mg/kg), MK-801 (0.1 mg/kg), or with a combination of these two drugs. Pretreatment with the D2 DA receptor antagonist completely antagonized in both strains the potentiation of the effect of cocaine exerted by MK-801. The results of the present research show that the noncompetitive NMDA receptor antagonist MK-801 enhances the effect of cocaine on retention performance in C57 and DBA mice and that dopaminergic mechanisms are involved in this potentiation.

Changes in binding to muscarinic and nicotinic cholinergic receptors in the chick telencephalon, following passive avoidance learning

Changes in nicotinic and muscarinic cholinergic receptors 30 min after one-trial passive avoidance training were studied in day-old chicks (Gallus domesticus), by quantitative receptor autoradiography. [3H]-alpha-bungarotoxin (alpha-BgT) and [3H]-quinuclidinyl benzilate (QNB) were used to monitor changes in 15 forebrain regions for nicotinic and muscarinic receptors, respectively. A significant increase occurred bilaterally in the quantity of bound alpha-BgT in the lobus parolfactorius, while the amount of bound QNB decreased significantly, and bilaterally, in the hippocampus, hyperstriatum ventrale, lobus parolfactorius and posterolateral telencephalon, pars dorsalis. The data support an involvement of cholinergic receptor types in the neural mechanisms underlying passive avoidance learning.

Contribution of cholinergic and gabaergic functions to memory processes in BALB/cANnCrlBR mice

Several lines of evidence indicate that glucose influences on memory depend on interactions between glucose, glucoregulation and hippocampal cholinergic function. We previously demonstrated that glucose and scopolamine differentially affected memory consolidation for an operant bar pressing task in two closely-related BALB/c mouse strains. Whereas glucose normally improves memory in several animal strains, memory consolidation was not effected by systemic glucose injections in BALB/cANnCrlBR mice. Moreover, these mice were relatively insensitive to the normally observed amnestic effects of scopolamine. We therefore sought to determine whether cholinergic mechanisms in the dorsal hippocampus were involved in such atypical drug effects on memory processing in that strain of mice. In Experiment 1, we examined whether post-training oxotremorine would also atypically influence memory consolidation for an appetitively reinforced operant bar pressing task following microinjection in the dorsal hippocampus. In Experiment 2, we examined the effects of intrahippocampal GABAA drugs on memory consolidation. The non-selective muscarinic agonist, oxotremorine, dose-dependently impaired memory and the GABAA antagonist, bicuculline, improved retention in BALB/cANnCrlBR mice. It was concluded that GABA-mediated influences on hippocampal pyramidal output in BALB/cANnCrlBR mice and other strains are similar; but the amnestic effects of oxotremorine from the dorsal hippocampus were opposite to facilitating effects normally observed in other animal strains. Results are discussed relative to possible altered septo-hippocampal cholinergic neurotransmission in BALB/cANnCrlBR mice.

Strain-dependent involvement of D1 and D2 dopamine receptors in muscarinic cholinergic influences on memory storage

These experiments examined the interaction of muscarinic and dopaminergic systems in influencing memory for one-trial inhibitory avoidance training in mice of the C57BL/6 and DBA/2 strains. In both strains, immediate post-training systemic administration of the muscarinic cholinergic agonist oxotremorine enhanced retention and the cholinergic antagonist atropine impaired retention. No effects were seen with injections 2 h post-training. Furthermore, the drugs did not affect retention performance of animals that received no footshock on the training trial. These results confirm previous findings indicating that muscarinic cholinergic drugs affect memory by influencing memory consolidation. In C57 mice, pretreatment with selective D1 or D2 dopamine (DA) receptor agonists (SKF 38393 or LY 171555, respectively) in otherwise non-effective doses (5 and 0.25 mg/kg, respectively) potentiated the effects of oxotremorine (0.04 mg/kg). Furthermore, in C57 mice pretreatment with selective D1 or D2 receptor antagonists (SCH 23390 or (-)-sulpiride) in otherwise non-effective doses (0.025 and 6 mg/kg, respectively) blocked the memory enhancing effects of oxotremorine. The memory impairing effects of atropine (3 mg/kg) were blocked by the D1 and D2 selective agonists and potentiated by the selective D1 or D2 antagonists. In contrast, in DBA mice, the D1 and D2 selective agonists antagonised the memory enhancing effects of oxotremorine (0.02 mg/kg) and potentiated the effects of atropine (2 mg/kg). Furthermore, the D1 and D2 antagonists potentiated the effects of oxotremorine and antagonised those of atropine. These findings indicate that although muscarinic cholinergic influences on memory storage are comparable in mice of these two strains, the cholinergic-dopaminergic interactions are opposite in the two strains. These results have implications for hypotheses of cholinergic and dopaminergic regulation of memory storage.