Extinction-induced "despair" in aged and adult rats: links to neurotrophins in frontal cortex and hippocampus

In the search for animal models of human geriatric depression, we found that operant extinction of escape from water results in the expression of immobility in different age groups, indicative of behavioral "despair", which was also associated with the resistance-to-extinction (RTE) expressed by these animals. With respect to the neurotrophin hypothesis of depression, nerve-growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) protein levels in frontal cortex (FC) and hippocampus (HP) were examined and related to behavioral immobility and RTE in the water maze in aged and adult Wistar rats. Age-related increases in levels of NGF were found in HP and of NT-3 in FC. Indices of immobility showed relationships in the aged with NGF and, in adults, with BDNF, pointing to a dissociation of neurotrophic involvement in extinction trial-induced "despair" in aged and adult rats. The present results support the hypothesis, that extinction-induced immobility in the water maze reflects a state akin to behavioral despair and point to age-related differences of neurotrophic involvement in depressive-like symptoms. The concept of extinction-induced behavioral "despair" in the aged subsumes several aspects of human geriatric depression, such as co-morbidity of learning impairment and anxiety, and, thus could represent a useful paradigm to examine the neuronal mechanisms underlying depression, especially in aged rodents.     

Corticosterone effects on BDNF mRNA expression in the rat hippocampus during morris water maze training

Corticosterone and Brain-Derived Neurotrophic Factor (BDNF) have both been shown to be involved in spatial memory formation in rats. In the present study we have investigated the effect of corticosterone on hippocampal BDNF mRNA expression after training in the Morris water maze in young adult Wistar rats. Therefore, we first studied BDNF mRNA levels in the hippocampus in relation to corticosterone levels at several time points after 4 training trials in the Morris water maze. Corticosterone levels were significantly increased after this procedure, and hippocampal BDNF mRNA levels only displayed a minor change: an increase in CA1 at 1 hr after training. However, in a previous study we observed dramatically decreased hippocampal BDNF mRNA levels in dentate gyrus and CA1 at 3 hr after injection of corticosterone. In order to analyze this discrepancy, we subsequently investigated if hippocampal BDNF mRNA expression is affected by corticosterone at 3 hr after water maze training. Therefore, we incorporated ADX animals and ADX animals which were injected with corticosterone in our study. ADX animals which were subjected to water maze training displayed similar hippocampal BDNF mRNA levels 3 hr after training compared to control ADX animals. Furthermore, ADX animals which were injected with corticosterone showed decreased BDNF mRNA levels in all hippocampal regions compared to control ADX animals. Water maze training did not alter this effect. Thus, the increased corticosterone levels during water maze training do not affect hippocampal BDNF mRNA expression, although exogenous corticosterone is effective under these conditions. Hence, our results suggest that in this situation BDNF is resistant to regulation by endogenous corticosterone, which may be important for learning and memory processes.     

Emotional and learning behaviour in mice overexpressing heat shock protein 70

The effects of inducible heat shock protein 70 (HSP70) on emotional and learning behaviour as well as hippocampal long-term potentiation was investigated in transgenic HSP70 overexpressing mice. In active two-way avoidance learning (shuttle box) as well as spatial 8-arm radial maze learning, the HSP70 overexpressing mice showed diminished learning performance. In several tests there was no indication of differences in anxiety behaviour between transgenic mice and wild-type mice. This suggests that impairment in learning behaviour is unrelated to the learning task and motivational aspects of behaviour. To investigate the neurophysiological correlate of learning, long-term potentiation experiments were performed. In transversal hippocampal slices, an enhanced amplitude of the population spike was found in HSP70 overexpressing mice. It was hypothesised that enhanced potentiation in conjunction with potentiation effects due to learning led to learning impairment.     

Involvement of BDNF receptor TrkB in spatial memory formation

The N-methyl-D-aspartate (NMDA) receptors are involved in long-term potentiation (LTP), and are phosphorylated by several tyrosine kinases including a Src-family tyrosine kinase Fyn. Brain-derived neurotrophic factor (BDNF) is a neurotrophin, which also enhances hippocampal synaptic transmission and efficacy by increasing NMDA receptor activity. Here, we show that Fyn is a key molecule linking the BDNF receptor TrkB with NMDA receptors, which play an important role in spatial memory formation in a radial arm maze. Spatial learning induced phosphorylation of TrkB, Fyn, and NR2B, but not NR2A, in the hippocampus. Fyn was coimmunoprecipitated with TrkB and NR2B, and this association was increased in well-trained rats compared with control animals. Continuous intracerebroventricular infusion of PP2, a tyrosine kinase inhibitor, in rats delayed memory acquisition in the radial arm maze, but PP2-treated animals reached the same level of learning as the controls. The phosphorylation of Fyn and NR2B, but not TrkB, was diminished by PP2 treatment. Our findings suggest the importance of interaction between BDNF/TrkB signaling and NMDA receptors for spatial memory in the hippocampus.     

Neuronal NT-3 Is not Required For Synaptic Transmission or Long-Term Potentiation in Area CA1 of the Adult Rat Hippocampus

Neurotrophic factors, including BDNF and NT-3, have been implicated in the regulation of synaptic transmission and plasticity. Previous attempts to analyze synaptic transmission and plasticity in mice lacking the NT-3 gene have been hampered by the early death of the NT-3 homozygous knockout animals. We have bypassed this problem by examining synaptic transmission in mice in which the NT-3 gene is deleted in neurons later in development, by crossing animals expressing the CRE recombinase driven by the synapsin I promoter to animals in which the NT-3 gene is floxed. We conducted blind field potential recordings at the Schaffer collateral–CA1 synapse in hippocampal slices from homozygous knockout and wild-type mice. We examined the following indices of synaptic transmission: (1) input-output relationship; (2) paired-pulse facilitation; (3) post-tetanic potentiation; and (4) long-term potentiation: induced by two different protocols: (a) two trains of 100-Hz stimulation and (b) theta burst stimulation. We found no difference between the knockout and wild-type mice in any of the above measurements. These results suggest that neuronal NT-3 does not play an essential role in normal synaptic transmission and some forms of plasticity in the mouse hippocampus.     

Preserved fronto-striatal plasticity and enhanced procedural learning in a transgenic mouse model of Alzheimer's disease overexpressing mutant hAPPswe

Mutations in the amyloid precursor protein (APP) gene inducing abnormal processing and deposition of beta-amyloid protein in the brain have been implicated in the pathogenesis of Alzheimer's disease (AD). Although Tg2576 mice with the Swedish mutation (hAPPswe) exhibit age-related Abeta-plaque formation in brain regions like the hippocampus, the amygdala, and the cortex, these mice show a rather specific deficit in hippocampal-dependent learning and memory tasks. In view of recent findings showing that neural systems subserving different forms of learning are not simply independent but that depressing or enhancing one system affects learning in another system, we decided to investigate fronto-striatal synaptic plasticity and related procedural learning in these mutants. Fronto-striatal long-term depression (LTD) induced by tetanic stimulation of the cortico-striatal input was similar in Tg2576 and wild-type control mice. Behavioral data, however, pointed to an enhancement of procedural learning in the mutants that showed robust motor-based learning in the cross maze and higher active avoidance scores. Thus, in this mouse model of AD, an intact striatal function associated with an impaired hippocampal function seems to provide neural conditions favorable to procedural learning. Our results suggest that focusing on preserved or enhanced forms of learning in AD patients might be of interest to describe the functional reorganization of the brain when one memory system is selectively compromised by neurological disease.     

Microinjections of leupeptin in the frontal cortex or dorsal hippocampus block spatial learning in the rat

Adult male Long-Evans hooded rats were given bilateral microinjections of 18 mM leupeptin or saline through cannulae implanted with tips aimed at the frontal cortex or CA1 or CA3 hippocampal cell fields. Five minutes following injections animals were allowed to complete an eight-arm radial maze. The acquisition criterion required that the animal make 7 correct choices of the first 8, and 8 correct choices of the first 10, for five consecutive sessions. Leupeptin slowed acquisition of the eight-arm radial maze task when injected into the CA1 and CA3 hippocampal fields or the frontal cortex, but did not influence spontaneous activity. These results suggest that earlier reports of the amnestic effect of leupeptin when administered into the lateral cerebral ventricle may have been due to effects within the cortex and hippocampus. The present experiment represents the first attempt to identify behaviorally those brain areas in which leupeptin acts to alter learning.     

A model of working memory capacity in the radial-arm maze task

The radial arm maze is one of the most commonly used tests for assessing working memory in laboratory animals. However, to date, there exists no quantitative method of estimating working memory capacity from performance on this task. Here, we present a mathematical model of performance on the radial arm maze from which we can derive estimates of capacity. We derive explicit results for the two most commonly used measures of performance as functions of number of arms in the maze and memory capacity, assuming a uniform random search. We simulate random non-uniform search strategies. Comparing our model to previous experiments, we show that our model predicts a working memory capacity in the range of 3-9 at the level of performance observed in these experiments. This estimate is within the typical estimate of human working memory capacity. Performance of rats on large mazes (e.g. 48 arms) has been used as evidence that the working memory capacity of rats may be significantly larger than that of humans. We report that memory capacity in the range 3-9 is sufficient to explain the performance of rats in very large radial mazes. Furthermore, when we simulate non-uniform random search strategies observed in the experiments, the resulting estimates do not differ significantly from those assuming a uniform random search. We conclude that a list-based model of working memory with modest capacity is more powerful than previously expected.     

Exploration of virtual mazes by rhesus monkeys (<Emphasis Type="Italic">Macaca mulatta</Emphasis>)

A chasm divides the huge corpus of maze studies found in the literature, with animals tested in mazes on the one side and humans tested with mazes on the other. Advances in technology and software have made possible the production and use of virtual mazes, which allow humans to navigate computerized environments and thus for humans and nonhuman animals to be tested in comparable spatial domains. In the present experiment, this comparability is extended even further by examining whether rhesus monkeys (Macaca mulatta) can learn to explore virtual mazes. Four male macaques were trained to manipulate a joystick so as to move through a virtual environment and to locate a computer-generated target. The animals succeeded in learning this task, and located the target even when it was located in novel alleys. The search pattern within the maze for these animals resembled the pattern of maze navigation observed for monkeys that were tested on more traditional two-dimensional computerized mazes.     

Experimental evidence for spatial learning in cuttlefish (Sepia officinalis)

Laboratory mazes were used to study spatial-learning capabilities in cuttlefish (Sepia offcinalis), using escape for reinforcement. In preliminary observations, cuttlefish in an artificial pond moved actively around the environment and appeared to learn about features of their environment. In laboratory experiments, cuttlefish exited a simple alley maze more quickly with experience and retained the learned information. Similar improvement was not found in open-field mazes or T mazes, perhaps because of motor problems. Cuttlefish learned to exit a maze that required them to find openings in a vertical wall. The wall maze was modified to an arena, and simultaneous discrimination learning and reversal learning were demonstrated. These experiments indicate that cuttlefish improve performance over serial reversals of a simultaneous, visual-spatial discrimination problem.     

Algorithmic responding on the radial maze in rats does not always imply absence of spatial encoding

Two experiments were conducted to determine whether consistent algorithmic response patterning on 8- and 10-arm versions of the radial maze is independent of spatial encoding. On the 8-arm version well-trained hooded rats were tested in darkness, after maze rotation that rendered room cues ambiguous with respect to arm positions, or with room cues unsystematically relocated. Ambiguous maze rotation was also used with well-trained subjects on the 10-arm version. If algorithmic patterning is a learned, non-spatial strategy, animals using it consistently ought not to have been affected by changes in the spatial layout of the test environment, and the type of pattern used by each subject would have remained constant. On the 8-arm radial maze, responses were most often made to arms 2 or 3 from that just visited. In many animals patterns were interchangeable, switching occurring between preferred angles of turn from day to day. Performance fell when animals were tested in darkness and upon ambiguous maze rotation early (but not later) in training. Testing in darkness increased the angle through which animals turned when responding, perhaps due to the disturbance of intramaze cue use. On the 10-arm maze the “consecutive arm” pattern was used persistently by several animals and appeared to protect their performance from disruption by ambiguous maze rotation. Animals not using rigid patterning were adversely affected. However, on both mazes animals using patterning correctly identified maze arms that had been omitted from otherwise patterned choice sequences. Animals adopted continuous patterning only when spatial encoding had been established. Response patterning appears to serve a mnemonic function and in rats complements rather than replaces the use of a spatial representation of the environment. It was concluded that a complex, flexible relationship exists between spatial functioning and its expression via motor responses.     

Working memory deficits in transgenic rats overexpressing human adenosine A2A receptors in the brain

Adenosine receptors in the central nervous system have been implicated in the modulation of different behavioural patterns and cognitive functions although the specific role of A(2A) receptor (A(2A)R) subtype in learning and memory is still unclear. In the present work we establish a novel transgenic rat strain, TGR(NSEhA2A), overexpressing adenosine A(2A)Rs mainly in the cerebral cortex, the hippocampal formation, and the cerebellum. Thereafter, we explore the relevance of this A(2A)Rs overexpression for learning and memory function. Animals were behaviourally assessed in several learning and memory tasks (6-arms radial tunnel maze, T-maze, object recognition, and several Morris water maze paradigms) and other tests for spontaneous motor activity (open field, hexagonal tunnel maze) and anxiety (plus maze) as modification of these behaviours may interfere with the assessment of cognitive function. Neither motor performance and emotional/anxious-like behaviours were altered by overexpression of A(2A)Rs. TGR(NSEhA2A) showed normal hippocampal-dependent learning of spatial reference memory. However, they presented working memory deficits as detected by performance of constant errors in the blind arms of the 6 arm radial tunnel maze, reduced recognition of a novel object and a lack of learning improvement over four trials on the same day which was not observed over consecutive days in a repeated acquisition paradigm in the Morris water maze. Given the interdependence between adenosinic and dopaminergic function, the present results render the novel TGR(NSEhA2A) as a putative animal model for the working memory deficits and cognitive disruptions related to overstimulation of cortical A(2A)Rs or to dopaminergic prefrontal dysfunction as seen in schizophrenic or Parkinson's disease patients.     

Individual differences in anxiety trait are related to spatial learning abilities and hippocampal expression of mineralocorticoid receptors

Although high levels of anxiety might be expected to negatively influence learning and memory, it remains to be shown whether individual differences in anxiety may influence spatial learning and memory in outbred rat populations. We have studied this possibility in male Wistar rats whose levels of anxiety were first characterized as either high (HA) or low (LA) according to their behavior in the elevated plus maze or in the open field test. Subsequently, their performance in the Morris water maze was studied, a task dependent on hippocampal activity. Interestingly, LA rats showed a faster acquisition and better memory in the water maze when compared to HA rats. Indeed, this difference in performance could mainly be attributed to the increase in thigmotactic behavior (swimming in circles close to the maze walls) displayed by HA rats during spatial navigation. Glucocorticoids are known to affect the state of anxiety and the hippocampus is the main target of glucocorticoids in the brain. Hence, we investigated whether the hippocampal expression of the two classical corticosteroid receptors, mineralocorticoid (MR) and glucocorticoid (GR) differed in the two groups of rats. We found that LA rats displayed higher hippocampal expression of MR but not GR than HA rats. Indeed, the expression levels for these receptors were positively correlated with the amount of time spent by the animals in the open arms of the elevated plus maze. Moreover, we present evidence that the levels of anxiety quantified in the first stages of our study constitute a trait rather than a state. Taken together, this study has generated evidence of a close interaction between the anxiety trait, hippocampal MR expression and the learning abilities of individuals in stressful spatial orientation tasks.     

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.     

Disturbances in exploratory behavior and functional recovery in the Y and radial mazes following dopamine depletion of the lateral septum

The effects of 6-hydroxydopamine (6-OHDA) injected into the lateral septum in rats were investigated for spontaneous alternation behavior in a Y maze and for spatially oriented behavior in an 8-arm radial maze. The performance of the animals in these tests was assessed under two physiological states, food-satiated and food-deprived. The selective depletion of septal dopaminergic concentrations leads to behavioral disturbances in both the Y and the radial mazes. These deficits disappeared when the animals with 6-OHDA lesions were food-deprived. These results confirm other studies from our laboratory and support two conclusions. First, lesions of dopaminergic neurons lead to behavioral impairments which resemble those found after the total lesion of the structure they innervate. Second, these behavioral impairments are responsive to therapeutic treatments, manipulations of the internal or external environment, or the level of arousal, since under certain conditions a recovery of function can occur in the absence of dopaminergic neurons. These two points provide additional support for a nonspecific role for the dopaminergic neurons originating in the ventral tegmental area. These neurons could have a permissive role in the functioning of the forebrain structures they innervate.     

Accelerated cognitive aging in diabetic rats is prevented by lowering corticosterone levels

Diabetes and normal aging are both characterized by increases in levels of glucocorticoids. Because long-term exposure to elevated glucocorticoids can be detrimental to hippocampal function, we evaluated the performance of young diabetic rats in the 14-unit T-maze, a task that is sensitive to hippocampal deficits. To assess the contribution of diabetes-induced elevations in corticosterone levels, we examined maze learning in diabetic rats that had levels of corticosterone 'clamped' through adrenalectomy and low-dose corticosterone replacement. For comparison, we also tested a separate group of young and aged rats in the maze. Adrenally intact diabetic rats learned poorly in the 14-unit T-maze. Preventing the increases in corticosterone levels that accompanies the onset of experimental diabetes also prevented deficits in complex maze learning. The pattern of errors made by adrenally intact diabetic rats was similar to the pattern of errors made by aged rats, suggesting that the cognitive profiles of diabetic and aged rats share common features.     

Ciproxifan, an H3 receptor antagonist, alleviates hyperactivity and cognitive deficits in the APP Tg2576 mouse model of Alzheimer's disease

Previous research has indicated that the blockade of H(3)-type histamine receptors may improve attention and memory in normal rodents. The purpose of this study was to determine if ciproxifan, an H(3) receptor antagonist, could alleviate the hyperactivity and cognitive deficits observed in a transgenic mouse model (APP(Tg2576)) of Alzheimer's disease. APP(Tg2576) mice displayed significantly greater locomotor activity than wild-type mice, but APP(Tg2576) mice provided with daily ciproxifan treatment showed activity levels that did not differ from wild-type mice. In the swim maze, APP(Tg2576) mice exhibited significantly longer escape latencies, but the APP(Tg2576) mice treated daily with ciproxifan had latencies that were indistinguishable from controls. In probe trials conducted one hour after the last training trial, ciproxifan-treated APP(Tg2576) mice spent more time near the previous platform location and made more crossings of this area than did saline-treated APP(Tg2576) mice. APP(Tg2576) mice also demonstrated a significant impairment in the object recognition task that was reversed by acute treatment with ciproxifan (3.0mg/kg). These data support the idea that modulation of H(3) receptors represents a novel and viable therapeutic strategy in the treatment of Alzheimer's disease.     

Octopuses (Octopus bimaculoides) and cuttlefishes (Sepia pharaonis, S. officinalis) can conditionally discriminate

In complex navigation using landmarks, an animal must discriminate between potential cues and show context (condition) sensitivity. Such conditional discrimination is considered a form of complex learning and has been associated primarily with vertebrates. We tested the hypothesis that octopuses and cuttlefish are capable of conditional discrimination. Subjects were trained in two maze configurations (the conditions) in which they were required to select one of two particular escape routes within each maze (the discrimination). Conditional discrimination could be demonstrated by selecting the correct escape route in each maze. Six of ten mud-flat octopuses (Octopus bimaculoides), 6 of 13 pharaoh cuttlefish (Sepia pharaonis), and one of four common cuttlefish (S. officinalis) demonstrated conditional discrimination by successfully solving both mazes. These experiments demonstrate that cephalopods are capable of conditional discrimination and extend the limits of invertebrate complex learning.