Hippocampal expression of the orphan nuclear receptor gene hzf-3/nurr1 during spatial discrimination learning

The immediate-early gene hzf-3, also known as nurr1, is a member of the inducible orphan nuclear receptor family and is one candidate in the search for genes associated with learning and memory processes. Here we report that acquisition of a spatial food search task is accompanied by elevated levels of hzf-3 mRNA in the hippocampus. Adult male Long-Evans rats were handled, food-restricted, and allowed to habituate to the maze prior to training. During acquisition, rats were given one training session per day for 5 days. Each training session consisted of five trials in which animals searched the maze for food located in 4 of 16 holes in the floor of the maze. Training resulted in spatial acquisition of the task. Northern blot analysis showed significant increases in hippocampal hzf-3 mRNA 3 h after training in the maze. Next, brains were obtained from Naive, Habituated, Day 1, Day 3, and Day 5 animals and processed for in situ hybridization. The results showed significant increases of hzf-3 mRNA in CA1 and CA3 subregions of the dorsal hippocampus during acquisition of the task. We conclude that expression of the hzf-3 gene in the brain is associated with long-term spatial memory processes. The present results are the first to implicate an orphan nuclear receptor in long-term information storage in the hippocampus.     

Effects of chronic cocaine administration on spatial learning and hippocampal spine density in two genetically different strains of rats

Lewis and Fischer-344 rats have been proposed as an addiction model because of their differences in addiction behaviour. It has been suggested that drug addiction is related to learning and memory processes and depends on individual genetic background. We have evaluated learning performance using the eight-arm radial maze (RAM) in Lewis and Fischer-344 adult rats undergoing a chronic treatment with cocaine. In order to study whether morphological alterations were involved in the possible changes in learning after chronic cocaine treatment, we counted the spine density in hippocampal CA1 neurons from animals after the RAM protocol. Our results showed that Fischer-344 rats significantly took more time to carry out test acquisition and made a greater number of errors than Lewis animals. Nevertheless, cocaine treatment did not induce changes in learning and memory processes in both strains of rats. These facts indicate that there are genetic differences in spatial learning and memory that are not modified by the chronic treatment with cocaine. Moreover, hippocampal spine density is cocaine-modulated in both strains of rats. In conclusion, cocaine induces similar changes in hippocampal neurons morphology that are not related to genetic differences in spatial learning in the RAM protocol used here.     

Evidence for the substantia nigra pars compacta as an essential component of a memory system independent of the hippocampal memory system

The aim of the present study was to test if the nigrostriatal pathway is an essential component for a water maze cued task learning and if it works independently of the hippocampal memory system. This hypothesis was tested using an animal model of Parkinson's disease in which male Wistar rats were lesioned in the substantia nigra pars compacta (SNc) by the intranigral infusion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), thus causing a partial depletion of striatal dopamine. SNc-lesioned and sham-operated animals were implanted bilaterally with guide cannulae above the dorsal hippocampus in order to be tested after the administration of 0.4 microl 2% lidocaine or saline into this structure. The animals were tested in a spatial or in a cued version of the water maze, memory tasks previously reported to model hippocampal-dependent spatial/relational and striatal-dependent S-R learning, respectively. Hippocampal inactivation, but not SNc lesion, impaired learning and memory in the spatial version of the water maze. An opposite situation was observed with the cued version. No significant interaction was observed between the SNc lesion and hippocampal inactivation conditions affecting scores in the spatial or in the cued version of the water maze. These results suggest that the nigrostriatal pathway is an essential part of the memory system that processes S-R learning and that it works independently of the hippocampal memory system that processes spatial/relational memories.     

Intraseptal administration of muscimol produces dose-dependent memory impairments in the rat

The present study examined the effects of intraseptal administration of the GABAergic agonist muscimol on performance of a radial-arm maze (RAM) task. Male Long-Evans rats were trained to perform a RAM task in which a 1-h delay was imposed between the sample and the test session. In this task rats have access to four out of eight maze arms during a predelay session. Following a 1-h delay, rats are returned to the maze and allowed to freely choose among all eight arms. Arms not blocked during the predelay session are baited, and entry into an arm chosen during the predelay session or a repeated entry into a postdelay chosen arm constitutes an error. Following acquisition, animals were implanted with a single cannula aimed at the medial septum. A within-subjects design was utilized to examine the effects of intraseptal administration of muscimol (0.0, 0.75, 1.5 or 3.0 nmol) on performance in this task. All drugs or artificial cerebrospinal fluid were administered immediately following the predelay session. Muscimol, a GABA-A agonist, produced a dose-dependent impairment in maze performance as evidenced by fewer correct choices in the first four postdelay choices and an increase in the number of errors. Intraseptal administration of muscimol did not significantly alter latency per choice on the RAM task nor did it affect locomotor activity levels. Muscimol-induced impairments were also observed when a 4-h delay was imposed between the fourth and the fifth maze selection, suggesting that the behavioral deficit represents an inability to store or retain spatial working memories rather than a general performance deficit. These data indicated that pharmacological manipulation of GABA-A receptors within the medial septum modifies working memory processes. The potential interaction of GABAergic and cholinergic mechanisms in the modulation of working memory processes is discussed.     

Knockdown of Nurr1 in the rat hippocampus: implications to spatial discrimination learning and memory

Nurr1 expression is up-regulated in the brain following associative learning experiences, but its relevance to cognitive processes remains unclear. In these studies, rats initially received bilateral hippocampal infusions of control or antisense oligodeoxynucleotides (ODNs) 1 h prior to training in a holeboard spatial discrimination task. Such pre-training infusions of nurr1 antisense ODNs caused a moderate effect in learning the task and also impaired LTM tested 7 d later. In a second experiment, ODN infusions were given immediately after the animals had received two sessions of training, during which all animals showed normal learning. Although antisense treated rats were significantly impaired during the post-infusion stages of acquisition of the task, no group differences were observed during the LTM test given 7 d later. These animals were subjected 3 d later to reversal training in the same maze in the absence of any additional treatments. Remarkably, rats previously treated with antisense ODNs displayed perseveration: The animals were fixated with the previously learned pattern of baited holes, causing them to be significantly impaired in the extinction of acquired spatial preferences and future learning. We postulate that Nurr1 function in the hippocampus is important for normal cognitive processes.     

Paradoxical sleep deprivation impairs acquisition, consolidation, and retrieval of a discriminative avoidance task in rats

The aim of the present study was to investigate the effects of paradoxical sleep deprivation (PSD) for 96 h on the learning/memory processes in rats submitted to the plus-maze discriminative avoidance task (PM-DAT), which simultaneously evaluates learning, memory, anxiety and motor function. Four experiments were performed in which rats were submitted to: (1) post-training and pre-test PSD; (2) post-training or pre-test PSD; (3) pre-training PSD or pre-training paradoxical sleep (PS) rebound (24 h) and (4) pre-test PSD rebound. Concerning Experiment I, post-training and pre-test PSD induced memory deficits, an anxiolytic-like behavior and an increase in locomotor activity. In Experiment II, both post-training PS-deprived and pre-test PS-deprived groups showed memory deficits per se. However, only the pre-test PS-deprived animals presented anxiolytic-like behavior and increased locomotor activity. In Experiment III, pre-training PS-deprived rats showed learning and memory deficits, anxiolytic-like behavior and increased locomotor activity. A 24h-sleep recovery period after the PSD abolished the learning and memory deficits but not anxiety and locomotor alterations. Finally, sleep rebound did not modify acquisition (Experiment III) and retrieval (Experiment IV). This study strengthened the critical role of paradoxical sleep (but not sleep rebound) in all the phases of learning and memory formation. In addition, it suggests that PSD effects on acquisition and consolidation do not seem to be related to other behavioral alterations induced by this procedure.     

The effects of clozapine on delayed spatial alternation deficits in rats with hippocampal damage

Clozapine is an atypical antipsychotic drug that has been shown to improve spatial memory in some animal models; however its efficacy in reversing spatial memory impairment in rats with hippocampal lesions is unknown. To address this issue, we tested the effects of clozapine on delayed spatial alternation deficits in rats with hippocampal damage in three separate experiments. In each experiment, adult male rats received sham surgery or direct stereotaxic infusions of the excitotoxin, NMDA, into the hippocampus. In the first study, seven days after surgery, the sham control animals received daily saline injections while the lesioned animals were split into two groups that received daily saline or clozapine (2.0 mg/kg, sc) injections. During the fifth week of injections, all animals were tested in a food-motivated delayed spatial alternation task. Saline-treated rats with excitotoxic hippocampal damage displayed significant deficits in delayed spatial alternation. Daily clozapine injections completely reversed this deficit. In a second experiment, it was found that clozapine treatment limited to the testing days only did not improve alternation performance in lesioned rats. Finally, in a third experiment, chronic clozapine treatment did not improve alternation performance in lesioned rats that were pre-trained in the alternation task prior to surgery. These results suggest that chronic, but not acute, clozapine treatment enables rats with hippocampal damage to develop new spatial learning, but can not rescue old spatial learning established prior to damage. These results may have implications for the treatment of cognitive deficits caused by hippocampal dysfunction in disorders such as schizophrenia, Alzheimer's disease, and others.     

Testosterone fails to reverse spatial memory decline in aged rats and impairs retention in young and middle-aged animals

Recently, the vasopressin (AVP) innervation in the rat brain was shown to be restored in senescent rats following long-term testosterone administration. In order to investigate whether this restoration is accompanied by an improvement in learning and memory, both sham- and testosterone-treated young (4.5 months), middle-aged (20 months), and aged (31 months) male Brown-Norway rats were tested in a Morris water maze. All animals learned to localize a cued platform equally well, indicating that the ability to learn this task was not affected by sensory, motoric, or motivational changes with aging or testosterone treatment. There were no significant differences in retention following cue training. Subsequent training with a hidden platform in the opposite quadrant of the pool (place training) revealed impaired spatial learning in middle-aged and aged animals. Retention following place training was significantly impaired in the sham-treated aged rats as compared with sham-treated young rats. Testosterone treatment did not improve spatial learning nor retention of spatial information, but, on the contrary, impaired retention in young and middle-aged animals. The present results confirm earlier reports on an impairment of spatial learning and memory in senescent rats but fail to support a role of decreased plasma testosterone levels and central AVP innervation in this respect.     

Forced treadmill exercise prevents oxidative stress and memory deficits following chronic cerebral hypoperfusion in the rat

Physical activity impacts functional recovery following stroke in humans, however its effects in experimental animals submitted to chronic cerebral hypoperfusion have not been investigated. The aim of this study was to evaluate the therapeutic potential of exercise, as assessed by cognitive activity in the Morris water maze and the brain oxidative status, through measurement of macromolecules damage, TBARS levels and total cellular thiols, as well as antioxidant enzymes in hippocampus, striatum and cerebral cortex. Adult male Wistar rats were submitted to the modified permanent bilateral occlusion of the common carotid arteries (2VO) method, with right common carotid artery being first occluded, and tested 3 months after the ischemic event. The effects of three different exercise protocols were examined: pre-ischemia, post-ischemia and pre+post-ischemia. Physical exercise consisted of sessions of 20-min, 3 times per week during 12 weeks (moderate intensity). Rats were submitted to cognitive assessment, in both reference and working spatial memory and after the last testing session were sacrificed to have oxidative stress parameters determined. Hypoperfusion caused a significant cognitive deficit in both spatial water maze tasks and this effect was reversed in rats receiving exercise protocol post and pre+post the ischemic event. Moreover, forced regular treadmill exercise regulated oxidative damage and antioxidant enzyme activity in the hippocampus. These results suggest that physical exercise protects against cognitive and biochemical impairments caused by chronic cerebral hypoperfusion.     

Blocking in the spatial domain with arrays of discrete landmarks.

A characteristic feature of associative conditioning is that learning a predictive relationship between two events can block later learning about an added event. It is not yet well established whether blocking occurs in the spatial domain or the circumstances in which it does. We now report, using rats trained to search for hidden food near landmarks in an open field arena, that blocking can occur in spatial learning. The animals noticed the added landmark at the start of the blocking phase and explored it, but either failed to incorporate it into their spatial map or developed a representation in which only some landmarks actually control behavior. Additionally, performance at asymptote was controlled by the shape of the landmark array rather than the individual landmarks comprising it, indicating that blocking in the spatial domain may represent a failure to alter the encoded geometry of a learned array.     

Trace fear conditioning is reduced in the aging rat

Auditory trace fear conditioning is a hippocampus-dependent learning task that requires animals to associate an auditory conditioned stimulus (CS) and a fear-producing shock-unconditioned stimulus (US) that are separated by an empty 20-s trace interval. Previous studies have shown that aging impairs learning performance on hippocampus-dependent tasks. This study measured heart rate (HR) and freezing fear responses to determine if aging impairs hippocampus-dependent auditory trace fear conditioning in freely moving rats. Aging and Young rats received one long-trace fear conditioning session (10 trials). Each trial consisted of a tone-CS (5 s) and a shock-US separated by an empty 20-s trace interval. The next day rats received CS-alone retention trials. Young rats showed significantly larger HR and freezing responses on the initial CS-alone retention trials compared to the Aging rats. A control group of aging rats received fear conditioning trials with a short 1-s trace interval separating the CS and US. The Aging Short-Trace Group showed HR and freezing responses on the initial CS alone retention trials that were similar to the Young Long-Trace Group, but greater than the Aging Long-Trace Group. A second aging control group received unpaired CSs and USs, and showed no HR or freezing responses on CS-alone retention trials. These data show that HR and freezing are effective measures for detecting aging-related deficits in trace fear conditioning.     

Effects of excitotoxic brain lesions on taste-mediated odor learning in the rat

The association learning between taste and odor is important in ingestive behavior. For a better understanding of this learning, we have developed a convenient and useful paradigm to assess the taste-mediated odor learning. In the training session, Wistar male rats drank water from two bottles in their home cages and from eight small glass dishes. In the learning session they were exposed in their home cages and also in a circular open-field apparatus to 0.005 M Na-saccharin and 0.02 M quinine hydrochloride which contained either banana or almond odors. One learning trial consisted of this pair of exposures. The preceding behavioral experiment has shown that these two odors are not aversive and are differentially perceived by rats. In the test session, the animals were put in the open-field apparatus equipped with eight dishes: four contained water with banana, and another four, with almond. Normal control rats preferred to drink water with the odor previously associated with saccharin. Stronger and more persistent preference was attained after two or three learning trials. To elucidate the brain sites responsible for this taste-mediated odor learning, the same procedure was assessed on brain-lesioned rats. Rats with lesions in the amygdala showed rapid extinction of preference to the saccharin-associated odor, whereas control rats did not. However, rats with lesions in the insular cortex showed retention of learning similar to that of the control rats. Rats with lesions in the sulcal prefrontal or cingulate cortices showed moderate disruptive effects on preference to the saccharin-associated odor. In conclusion, the odor learning established in our experimental paradigm is based on the association between the quality of odor and hedonics of taste. The amygdala may play a role in the formation, at least in the retention process, of this taste-odor association learning.     

AMPA/kainate, NMDA, and dopamine D1 receptor function in the nucleus accumbens core: a context-limited role in the encoding and consolidation of instrumental memory

Neural integration of glutamate- and dopamine-coded signals within the nucleus accumbens (NAc) is a fundamental process governing cellular plasticity underlying reward-related learning. Intra-NAc core blockade of NMDA or D1 receptors in rats impairs instrumental learning (lever-pressing for sugar pellets), but it is not known during which phase of learning (acquisition or consolidation) these receptors are recruited, nor is it known what role AMPA/kainate receptors have in these processes. Here we show that pre-trial intra-NAc core administration of the NMDA, AMPA/KA, and D1 receptor antagonists AP-5 (1 microg/0.5 microL), LY293558 (0.01 or 0.1 microg/0.5 microL), and SCH23390 (1 microg/0.5 microL), respectively, impaired acquisition of a lever-pressing response, whereas post-trial administration left memory consolidation unaffected. An analysis of the microstructure of behavior while rats were under the influence of these drugs revealed that glutamatergic and dopaminergic signals contribute differentially to critical aspects of the initial, randomly emitted behaviors that enable reinforcement learning. Thus, glutamate and dopamine receptors are activated in a time-limited fashion-only being required while the animals are actively engaged in the learning context.     

Epinephrine Effects on Memory Are Not Dependent on Hepatic Glucose Release

Epinephrine released or administered soon after a given training task modulates memory processes. Since epinephrine does not readily cross the blood–brain barrier, studies have suggested that some of the central effects of epinephrine might be mediated by peripheral release of glucose. These experiments examined the involvement of blood glucose levels in the posttraining effects of peripherally administered epinephrine. The effects of the administration of epinephrine (25 and 625 mg/kg) on memory of an inhibitory avoidance task were evaluated in fed and fasted rats (depleted glycogen stores in liver). Blood glucose levels after the task in each group were also measured. Female Wistar rats were divided in two groups. Fed and 48-h-fasted animals were submitted to the inhibitory avoidance task and received ip epinephrine or saline immediately after training. The test session was carried out 48 h after training. Epinephrine (25 or 625 mg/kg) caused an increased glycemia in fed rats, but no effect was observed in fasted animals. Administration of epinephrine 25 mg/kg induced a facilitation of memory, while epinephrine 625 mg/kg impaired retention (either in fasted or in fed animals). There was no relation between increased glycemia induced by epinephrine and its effects on memory, since this drug presented its classical effects independently of the previous state of the animal (fed or fasted). The results of the present study suggest that the effects of systemic released or administered epinephrine on memory processes are not dependent on hepatic glucose release.     

The effects of p-chloroamphetamine, a depletor of brain serotonin, on the performance of rats in two types of positively reinforced complex spatial discrimination tasks

Learning in male Sprague-Dawley rats was assessed in two types of positively reinforced complex spatial discrimination tasks (Stone 14-unit T-maze and eight-arm radial-arm maze) following cytotoxic lesions of central serotonergic terminal projection fields with p-chloroamphetamine (PCA). Learning, as expressed as mean number of errors per day and mean number of trails required to reach criterion, was significantly enhanced in the PCA-lesioned animals trained in the Stone maze. On the other hand, the performance of the PCA-lesioned animals trained in the eight-arm radial-arm maze was not found to differ significantly from that of saline-injected animals. The improved acquisition of the PCA-lesioned rats trained in the Stone maze was completely abolished following pretreatment with the selective serotonergic reuptake inhibitor norzimeldine. Neurochemical analyses of the brains of representative animals revealed that the levels of serotonin and its major metabolite, 5-hydroxy-3-indoleacetic acid, were both significantly reduced by PCA in all regions examined. While it is clear from these and other studies that the serotonergic nervous system plays an important role in the processes underlying learning and memory, these results further underscore the selective role of this neurotransmitter system in the way information is processed by the brain.     

Intracranial self-stimulation improves memory consolidation in rats with little training

Post-training intracranial electrical self-stimulation can improve learning and memory consolidation in rats. However, the molecular mechanisms involved are not known yet. Since previous paradigms of this kind of facilitation are relatively unsuitable to try a molecular approach, here we develop a single and short model of learning and memory facilitation by post-training self-stimulation that could make easier the research of its neural and molecular basis. Thus, three consecutive experiments were carried out to ascertain whether post-training self-stimulation is able to facilitate memory when learning consists of only a brief (5 trials) two-way active avoidance conditioning session. The results of Experiment 1 showed that it is actually possible, and that 48 h after the acquisition session is a very good time to observe the memory improvement. As a way to probe the retroactive effect of self-stimulation, in Experiment 2 we observed that the same self-stimulation treatment given to the subjects not post-training but 48 h before a single two-way active avoidance session does not improve the acquisition of conditioning. In Experiment 3, we showed that the SS facilitative effect observed 48 h after the acquisition session in Experiment 1 was still maintained one week later. We concluded that post-training intracranial self-stimulation can consistently improve memory consolidation even when little acquisition training is given to the animals in a single training session.     

Functional networks involved in spatial learning strategies in middle-aged rats

Our aim was to assess the way that middle-aged rats solve spatial learning tasks that can be performed using different strategies. We assessed the brain networks involved in these spatial learning processes using Principal Component Analysis. Two tasks were performed in a complex context, a four-arm radial maze, in which each group must use either an allocentric or an egocentric strategy. Another task was performed in a simple T-maze in which rats must use an egocentric strategy. Brain metabolic activity was quantified to evaluate neural changes related to spatial learning in the described tasks. Our findings revealed that two functional networks are involved in spatial learning in aged rats. One of the networks, spatial processing, is composed of brain regions involved in the integration of sensory and motivational information. The other network, context-dependent processing, mainly involves the dorsal hippocampus and is related to the processing of contextual information from the environment. Both networks work together to solve spatial tasks in a complex spatial environment.