Chronic Treatment with a Promnesiant GABA-A α5-Selective Inverse Agonist Increases Immediate Early Genes Expression during Memory Processing in Mice and Rectifies Their Expression Levels in a Down Syndrome Mouse Model.

Decrease of GABAergic transmission has been proposed to improve memory functions. Indeed, inverse agonists selective for α5 GABA-A-benzodiazepine receptors (α5IA) have promnesiant activity. Interestingly, we have recently shown that α5IA can rescue cognitive deficits in Ts65Dn mice, a Down syndrome mouse model with altered GABAergic transmission. Here, we studied the impact of chronic treatment with α5IA on gene expression in the hippocampus of Ts65Dn and control euploid mice after being trained in the Morris water maze task. In euploid mice, chronic treatment with α5IA increased IEGs expression, particularly of c-Fos and Arc genes. In Ts65Dn mice, deficits of IEGs activation were completely rescued after treatment with α5IA. In addition, normalization of Sod1 overexpression in Ts65Dn mice after α5IA treatment was observed. IEG expression regulation after α5IA treatment following behavioral stimulation could be a contributing factor for both the general promnesiant activity of α5IA and its rescuing effect in Ts65Dn mice alongside signaling cascades that are critical for memory consolidation and cognition.

Fiber Tracts Anomalies in APPxPS1 Transgenic Mice Modeling Alzheimer's Disease.

Amyloid beta (Aß) peptides are known to accumulate in the brain of patients with Alzheimer's disease (AD). However, the link between brain amyloidosis and clinical symptoms has not been elucidated and could be mediated by secondary neuropathological alterations such as fiber tracts anomalies. In the present study, we have investigated the impact of Aß overproduction in APPxPS1 transgenic mice on the integrity of forebrain axonal bundles (corpus callosum and anterior commissure). We found evidence of fiber tract volume reductions in APPxPS1 mice that were associated with an accelerated age-related loss of axonal neurofilaments and a myelin breakdown. The severity of these defects was neither correlated with the density of amyloid plaques nor associated with cell neurodegeneration. Our data suggest that commissural fiber tract alterations are present in Aß-overproducing transgenic mice and that intracellular Aß accumulation preceding extracellular deposits may act as a trigger of such morphological anomalies.

Specific targeting of the GABA-A receptor α5 subtype by a selective inverse agonist restores cognitive deficits in Down syndrome mice.

An imbalance between inhibitory and excitatory neurotransmission has been proposed to contribute to altered brain function in individuals with Down syndrome (DS). Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system and accordingly treatment with GABA-A antagonists can efficiently restore cognitive functions of Ts65Dn mice, a genetic model for DS. However, GABA-A antagonists are also convulsant which preclude their use for therapeutic intervention in DS individuals. Here, we have evaluated safer strategies to release GABAergic inhibition using a GABA-A-benzodiazepine receptor inverse agonist selective for the α5-subtype (α5IA). We demonstrate that α5IA restores learning and memory functions of Ts65Dn mice in the novel-object recognition and in the Morris water maze tasks. Furthermore, we show that following behavioural stimulation, α5IA enhances learning-evoked immediate early gene products in specific brain regions involved in cognition. Importantly, acute and chronic treatments with α5IA do not induce any convulsant or anxiogenic effects that are associated with GABA-A antagonists or non-selective inverse agonists of the GABA-A-benzodiazepine receptors. Finally, chronic treatment with α5IA did not induce histological alterations in the brain, liver and kidney of mice. Our results suggest that non-convulsant α5-selective GABA-A inverse agonists could improve learning and memory deficits in DS individuals.

Altered emotional and motivational processing in the transgenic rat model for Huntington's disease.

Huntington disease (HD) is caused by an expansion of CAG repeat in the Huntingtin gene. Patients demonstrate a triad of motor, cognitive and psychiatric symptoms. A transgenic rat model (tgHD rats) carrying 51 CAG repeats demonstrate progressive striatal degeneration and polyglutamine aggregates in limbic structures. In this model, emotional function has only been investigated through anxiety studies. Our aim was to extend knowledge on emotional and motivational function in symptomatic tgHD rats. We subjected tgHD and wild-type rats to behavioral protocols testing motor, emotional, and motivational abilities. From 11 to 15 months of age, animals were tested in emotional perception of sucrose using taste reactivity, acquisition, extinction, and re-acquisition of discriminative Pavlovian fear conditioning as well as reactivity to changes in reinforcement values in a runway Pavlovian approach task. Motor tests detected the symptomatic status of tgHD animals from 11 months of age. In comparison to wild types, transgenic animals exhibited emotional blunting of hedonic perception for intermediate sucrose concentration. Moreover, we found emotional alterations with better learning and re-acquisition of discriminative fear conditioning due to a higher level of conditioned fear to aversive stimuli, and hyper-reactivity to a negative hedonic shift in reinforcement value interpreted in term of greater frustration. Neuropathological assessment in the same animals showed a selective shrinkage of the central nucleus of the amygdala. Our results showing emotional blunting and hypersensitivity to negative emotional situations in symptomatic tgHD animals extend the face validity of this model regarding neuropsychiatric symptoms as seen in manifest HD patients, and suggest that some of these symptoms may be related to amygdala dysfunction.

Impaired neurogenesis, neuronal loss, and brain functional deficits in the APPxPS1-Ki mouse model of Alzheimer's disease.

Amyloid-ß peptide species accumulating in the brain of patients with Alzheimer's disease are assumed to have a neurotoxic action and hence to be key actors in the physiopathology of this neurodegenerative disease. We have studied a new mouse mutant (APPxPS1-Ki) line developing both early-onset brain amyloid-ß deposition and, in contrast to most of transgenic models, subsequent neuronal loss. In 6-month-old mice, we observed cell layer atrophies in the hippocampus, together with a dramatic decrease in neurogenesis and a reduced brain blood perfusion as measured in vivo by magnetic resonance imaging. In these mice, neurological impairments and spatial hippocampal dependent memory deficits were also substantiated and worsened with aging. We described here a phenotype of APPxPS1-Ki mice that summarizes several neuroanatomical alterations and functional deficits evocative of the human pathology. Such a transgenic model that displays strong face validity might be highly beneficial to future research on AD physiopathogeny and therapeutics.

[Morphologic and molecular neuropathology of Alzheimer's disease]. / Neuropathologie morphologique et moléculaire de la maladie d'Alzheimer.

Alzheimer disease lesions include the abnormal accumulation of two proteins normally present in neurons: tau protein and Abeta peptide. Tau protein aggregates into fibrils in the cell body of neurons (neurofibrillary tangles), in dendrites (neuropil threads) and in degenerating axons that constitute the corona of the senile plaque. Tau pathology progresses in the brain areas in a stereotyped manner and in parallel with the clinical symptoms. Abeta extracellular deposits may be diffuse or focal. The Abeta focal deposit constitutes the core of the senile plaque. Progression of the Abeta lesions, which initially affect the isocortex, then the hippocampus, basal ganglia, various brainstem nuclei and cerebellum, is not directly correlated with symptoms. Mutations involving the genes implicated in Abeta peptide metabolism are responsible for familial Alzheimer disease. Mutations of the tau gene are not associated with Alzheimer disease but with frontotemporal dementia. The link between altered Abeta peptide metabolism and tau pathology has not been fully elucidated. Animal models mimic several aspects of the disease and have contributed to a better understanding of the mechanisms of the lesions.

One-year longitudinal evaluation of sensorimotor functions in APP751SL transgenic mice.

Intracerebral amyloid-beta (Abeta) peptide deposition is considered to play a key role in Alzheimer's disease and is designated as a principal therapeutic target. The relationship between brain Abeta levels and clinical deficits remains, however, unclear, both in human patients and in animal models of the disease. The purpose of the present study was to investigate, in a transgenic mouse model of brain amyloidosis, the consequences of Abeta deposition on basic neurological functions using a longitudinal approach. Animals were phenotyped at different ages corresponding to graded neuropathological stages (from no extracellular Abeta deposition to high amyloid loads). Sensory functions were evaluated by assessing visual and olfactory abilities and did not show any effects of the amyloid precursor protein (APP) transgene. Motor functions were assessed using multiple experimental paradigms. Results showed that motor strength was considerably reduced in APP transgenic mice compared with control animals. No deficit was noted in a motor coordination test although APP transgenic mice displayed decreased locomotion on a stationary beam. Hypolocomotion was also observed in the standard open-field test. Measures of anxiety obtained in the elevated plus-maze show some evidence of hyperanxiety in 15-month-old transgenic mice. Some of the neurological impairments showed by APP mice had an early onset and worsened with progressive aging, in parallel to gradual accumulation of Abeta in brain parenchyma. Relationships between neuropathologically assessed amyloid loads and behavioral deficits were further explored, and it was observed that motor strength deficits were correlated with cortical amyloid burden.

The role of the rat prelimbic/infralimbic cortex in working memory: not involved in the short-term maintenance but in monitoring and processing functions.

Contrary to human and primate, working memory in the rodent is usually considered as a simple short term memory buffer and mainly investigated using delayed response paradigms. The aim of the present study was to further investigate the role of the rat prelimbic/infralimbic cortex in different spatial delayed tasks in order to dissociate its involvement in temporary storage from other information processes, such as behavioral flexibility and attention. In experiment 1 rats were trained in a standard elimination win-shift task in a radial-arm maze after which a 1-min delay was inserted mid trial. Prelimbic/infralimbic lesions induced only a transient disruption of performance following introduction of the delay. In experiment 2, rats were trained directly in a win-shift task with a 5-min delay that was subsequently extended to 30 min. Prelimbic/infralimbic lesions did not significantly affect behavior. Nevertheless, transient disruptions of performance (correlated with lesion extent) were noted repeatedly in lesioned rats when sets of interfering events were presented. The present findings indicate that prelimbic/infralimbic cortex is not directly involved in the short term maintenance of specific information but is implicated when changes, such as sudden introduction of a delay or exposure to unexpected interfering events, alter the initial situation. It appears that working memory in rodents should be considered, as in humans and primates, to encompass both storage and monitoring functions. The present results along with previous ones strongly suggest that prelimbic/infralimbic cortex is not involved in the temporary on-line storage but rather in the control of information required to prospectively organize the ongoing action.

Alzheimer pathology disorganizes cortico-cortical circuitry: direct evidence from a transgenic animal model.

It has been proposed that Alzheimer disease (AD) is associated with a "disconnection syndrome" due to the gradual loss of morphological and functional integrity of cortico-cortical pathways. This hypothesis derives from indirect neuropathological observations, but definitive evidence that AD primarily targets cortico-cortical networks is still lacking. By means of neuroanatomical anterograde tracing methods, we have investigated, in a murine transgenic model of AD, the impact of the amyloid burden on axonal terminals in different neural systems. Axonal tracings revealed, in accordance with the "disconnection syndrome" hypothesis, that cortico-cortical fibers are significantly disorganized. Terminal fields in local and distant cortical areas contained numerous swollen dystrophic neurites often grouped in grape-like clusters at the plaque periphery. In contrary to fibers of cortical origin, those originating from subcortical brain structures only showed limited signs of degeneration upon reaching their cortical targets. These observations suggest a selective disruption of cortico-cortical connections induced by AD brain pathology.

Projections from the parahippocampal region to the prefrontal cortex in the rat: evidence of multiple pathways.

The purpose of the present study was to investigate, by means of anterograde tracing methods, the detailed organization of the parahippocampal-prefrontal projections in the rat brain. Efferents from the perirhinal cortex were found to terminate principally in both the ventromedial (prelimbic and infralimbic cortices) and lateral (agranular insular cortex) regions of the prefrontal cortex. Terminal fields were observed mainly in the superficial layers of the prefrontal cortex. Projections arising from the dorsolateral entorhinal cortex, which borders the perirhinal cortex along its ventral extent, were similarly directed to the ventromedial and lateral prefrontal cortices but also encompassed other frontal areas (dorsomedial and orbital prefrontal regions). Terminal fields of entorhinal projections were also found in the superficial layers of the prefrontal cortex. A third pathway, taking its source in the post-rhinal cortex, presented striking topographical differences with the two other output systems. Hence, post-rhinal efferences terminated only in the ventrolateral orbital area. The results indicate that two main routes originate from the parahippocampal region to reach the prefrontal cortex. One pathway involves the rostral and lateral portions of the parahippocampal region (perirhinal and dorsolateral entorhinal cortices), and the other relies on its most caudal region, the post-rhinal cortex. The presence of such different multiple parahippocampal-prefrontal pathways may have functional relevance for learning and memory processes.

Involvement of the dorsal anterior cingulate cortex in temporal behavioral sequencing: subregional analysis of the medial prefrontal cortex in rat.

We recently demonstrated that rats with dorsal anterior cingulate (ACd) cortical lesions were disrupted in the acquisition of a temporal Go/No-Go alternation task, suggesting either an involvement of this medial prefrontal subregion in sequencing temporally ordered behaviors or alternatively in more general response selection processes 'Gisquet-Verrier et al., Psychobiology 28 (2000) 248'. In the present study, the effects of ACd lesions were investigated in two different training situations. Both tasks involved response selection but only one of the two tasks required behavioral sequencing. In experiment 1, rats with ACd lesions were normally able to select Go and No-Go responses, when selection processes were based on a tone/light conditional rule that does not require temporal patterning. In experiment 2, ACd-lesioned rats were disrupted during the acquisition of a spatial alternation task that relies on temporal patterning. These results provide further evidence of ACd involvement in behavioral sequencing, regardless of the nature (spatial or non-spatial) of chained responses. A comparison of ACd lesions-induced effects with those obtained with lesions to the adjacent prelimbic-infralimbic area 'Delatour & Gisquet-Verrier, Behav. Neurosci. 110 (1996) 1282; Delatour & Gisquet-Verrier, Behav. Neurosci. 113 (1999) 941' reveals a double dissociation of these two regions that reflects a functional regionalization of the medial prefrontal cortex in the rat.

FE65 in Alzheimer's disease: neuronal distribution and association with neurofibrillary tangles.

FE65, a protein expressed in the nervous system, has the ability to bind the C-terminal domain of the amyloid precursor protein. This suggests a role for FE65 in the pathogenesis of Alzheimer's disease (AD). The present study was conducted to find out if the distribution of FE65 immunoreactivity was affected during the course of AD, and to determine the degree of co-localization of FE65 with other proteins known to be involved in AD. Single immunoperoxidase-labeling experiments, conducted on six sporadic AD patients and six nondemented age-matched controls, showed that the proportion of volume occupied by FE65 immunoreactivity was not modified in the isocortex of AD patients. However, in hippocampal area CA4, increased FE65 immunoreactivity seemed to be associated with the severity of the disease. Double-immunofluorescent labeling did not show any clear co-localization of FE65 with the amyloid precursor protein. FE65 immunoreactivity was also absent from focal and diffuse deposits of the beta-amyloid peptide. Unexpectedly double labeling experiments showed a co-localization of FE65 and tau proteins in intracellular tangles. Ultrastructural observations confirmed that FE65 was associated with paired helical filaments.

Alleviation of overtraining reversal effect by transient inactivation of the dorsal striatum.

In this study, we investigated the role of the dorsal striatum in the acquisition and the use (retrieval) of a specific learning developed during overtraining. The paradigm was such that rats had to respond differentially to two signals in order to obtain food or to avoid an electrical footshock. Overtraining was aimed at eliciting a facilitative effect on discrimination reversal as compared to simply trained rats. In this way, transient inactivation of the dorsal striatum by lidocaine enabled us to investigate, separately, the role of this structure during overtraining and reversal. The data show that inactivating the dorsal striatum before each reversal session prevented the overtraining reversal effect observed in control rats. Moreover, inactivation of the dorsal striatum during overtraining had no effect on the level of discriminative performance just as it did not affect the subsequent facilitative effect on reversal. These results show that even though the striatum might normally be part of a routine automatic system, clearly its contribution is not essential. Indeed, despite inactivation of the striatum in overtrained rats, their ability to develop an efficient selection process that can be used during reversal was observed. However, the integrity of the striatum became essential in order to mediate the modification of behaviour when this behavioural routine formed during overtraining had to be modified during reversal.

Functional role of rat prelimbic-infralimbic cortices in spatial memory: evidence for their involvement in attention and behavioural flexibility.

The involvement of the medial prefrontal cortex (mPFC), and more particularly the prelimbic and infralimbic cortices (PL-IL area), in spatial memory remains controversial. The present study investigates the effects of neurotoxic lesions restricted to the PL-IL area of the mPFC in rats trained in two different spatial tasks. In experiment 1, PL-IL lesioned rats showed normal acquisition of a delayed non-matching to position task. They were also able to plan their responses for a prospective strategy but were transiently disrupted when the initial delay was extended. In experiment 2, rats were trained to locate one baited box among 13 identical boxes distributed on a circular arena. Lesioned rats performed normally when trained from a single start position but were severely disrupted when four start positions were used. A probe trial showed this deficit was not due to failure to learn the goal location. The addition of a proximal cue signalling the goal box helped lesioned rats to directly open the goal box, but did not compensate for greater distances that they travelled to reach it. Results from both experiments indicate that the PL-IL area is directly involved neither in allocentric spatial representations nor prospective memory and is not specifically involved in working memory. This area seems more likely to be involved in both attentional processes and behavioural flexibility that may be important for processing information for working memory as well as for spatial memory.

Lesions of the prelimbic-infralimbic cortices in rats do not disrupt response selection processes but induce delay-dependent deficits: evidence for a role in working memory?

Effects of neurotoxic lesions of the prelimbic-infralimbic cortex (PL-IL) were examined in rats performing 2 conditional tasks. PL-IL-lesioned rats showed normal acquisition of a visuospatial conditional discrimination in a Y maze as well as a tone-light conditional discrimination in an operant chamber, indicating that the PL-IL is not necessary for response selection processes. When the working memory load was subsequently increased in the tone-light conditional discrimination, rats with PL-IL lesions showed a delay-dependent disruption of performance. This suggests a role of the PL-IL in some working memory processes. However, the present results, considered along with previous studies, suggest that the PL-IL does not seem to be directly involved in the processes necessary to maintain specific items over a delay period but rather in the planning of forthcoming behavioral responses on the basis of previously acquired information.

Rule-based learning impairment in rats with lesions to the dorsal striatum.

The present study examined the effects of lesions to the dorsal striatum (DS) in Sprague-Dawley rats, when tested on the acquisition and successive shifts in the position of a goal arm in an eight-arm radial maze. In the procedure we used, rats had to retrieve the location of one baited arm among the eight arms of the maze after it had just been presented as a sample during a forced trial. After attainment of a fixed learning criterion, rats were submitted to five successive shifts in the goal location. Results showed that DS rats were able to learn the position of the goal arm during the acquisition phase as efficiently as sham-operated rats. In contrast, when the position of the goal arm was shifted, although DS rats were able to learn its new position, they made significantly more errors and required more sessions to reach criterion than sham-operated rats. These results suggested that both groups did not solve the task using the same behavioral strategy. The analysis of responses made suggested that sham-operated rats solved the task using the pairing rule between the forced and the free run (matching-to-sample rule), while DS rats solved the task using only visuo-spatial processing. These data therefore suggest that the dorsal striatum plays an important role in rule-learning ability.

[Alzheimer's disease: lesions and their progression]. / Maladie d'Alzheimer: les lésions et leur progression.

Alzheimer disease appears to be a stereotyped mode of reaction of the central nervous system to various types of aggression such as different mutations involving various proteins, trisomy 21 or repeated head trauma as in dementia pugilistica. Rather than a disease, it appears to be a clinicopathological syndrome due to various causes. Lesions may be considered under 3 headings: neurofibrillary pathology, A beta peptide deposits and loss (neuronal and synaptic). Neurofibrillary pathology includes the neurofibrillary tangle, the crown of the senile plaque and the neuropil threads. All those lesions are characterized by the same ultrastructure--i.e. the accumulation of paired helical filaments--and the same immunohistochemistry: they are labelled by antibodies directed against the tau proteins. The amyloid deposits, present in the core of the senile plaque and in the vascular walls, are made of a 40 to 42 amino-acids long peptide, named A beta, derived from the amyloid precursor protein (APP). Antibodies directed against the A beta peptide also label diffuse deposits that are devoid of the tinctorial affinities and of the biochemical properties of amyloid substances. Those diffuse deposits are insufficient to cause dementia since they may be observed in high density in aged people without intellectual deterioration. Neuronal loss occurs after neurofibrillary pathology. The role of the synaptic pathology remains discussed. Besides tau proteins, A beta peptide and APP, several other proteins may play an important role: apolipoprotein E which could act as a chaperone protein, inducing or facilitating the formation of amyloid, presenilins 1 and 2, mutated in some cases of familial Alzheimer disease, alpha-synuclein which is present in the Lewy bodies found in Parkinson disease and in dementia with Lewy bodies. The A beta deposits are diffusely distributed in the cerebral cortex; the neurofibrillary changes have a hierarchical distribution. The progression of the neurofibrillary pathology in the various cortical areas follow a stereotyped sequence that may help to grade the severity of the disease. Progression may take decades. The relations between aging and Alzheimer disease are still poorly understood. Frequency of Alzheimer type lesions in old people could suggest that they are the inevitable burden of age, but this has been discussed.

Prelimbic cortex specific lesions disrupt delayed-variable response tasks in the rat.

The role of the prelimbic cortex (PL) in rats was investigated with excitotoxic lesions. PL lesions altered the alternation scores in spontaneous and reinforced spatial delayed-alternation tasks. PL lesions induced a delay in conditioning under a temporal go/no-go alternation schedule but not under a continuous food-reinforcement schedule in a runway. PL lesions had no effect on the acquisition of a standard radial-arm-maze task nor on a fixed-goal location task but disrupted the acquisition of a variable-goal location task in a radial-arm maze. The present results indicate that PL lesions replicated most of the behavioral deficits obtained with larger prefrontal lesions. PL lesions disrupted the acquisition of delayed-variable response tasks while leaving unaffected fixed-response tasks. These results are discussed in relation with a working-memory, a response-selection, and an attentional hypothesis.