Interhippocampal transfer in passive avoidance task modifies metabolic activity in limbic structures

The importance of the context in the hippocampus and brain related areas throughout the performance of a fear conditioning task

The importance context has been broadly studied in the management of phobias and in the drug addiction literature. The way in which changes to a context influence behavior after the simple acquisition of a passive avoidance task remains unclear. The hippocampus has long been implicated in the contextual and spatial processing required for contextual fear, but its role in encoding the aversive component of a contextual fear memory is still inconclusive. Our work tries to elucidate whether a change in context, represented as differences in the load of the stimuli, is critical for learning about the context-shock association and whether this manipulation of the context could be linked to any change in metabolic brain activity requirements. For this purpose, we used an avoidance conditioning task. Animals were divided into three different experimental conditions. In one group, acquisition was performed in an enriched stimuli environment and retention was performed in a typically lit chamber (the PA-ACQ-CONTX group). In another group, acquisition was performed in the typically lit chamber and retention was undertaken in the highly enriched chamber (the PA-RET-CONTX group). Finally, for the control group, PA-CN-CONTX, acquisition, and retention were performed in the enriched stimuli environment. Our results showed that the PA-ACQ-CONTX group had longer escape latencies and poorer retention than the PA-RET-CONTX and PA-CN-CONTX groups after 24 h of acquisition under contextual changes. To study metabolic brain activity, histochemical labelling of cytochrome c-oxidase (CO) was performed. CO results suggested a neural circuit including the hippocampus, amygdala, thalamus, parahippocampal cortices, and mammillary nuclei that is involved in the learning and memory processes that enable context-dependent behavior. These results highlight how dysfunction in this network may be involved in the contextualization of fear associations that underlie several forms of psychopathology, including post-traumatic stress disorder, schizophrenia, and substance abuse disorders.

Hippocampal inactivation with TTX impairs long-term spatial memory retrieval and modifies brain metabolic activity

Functional inactivation techniques enable studying the hippocampal involvement in each phase of spatial memory formation in the rat. In this study, we applied tetrodotoxin unilaterally or bilaterally into the dorsal hippocampus to evaluate the role of this brain structure in retrieval of memories acquired 28 days before in the Morris water maze. We combined hippocampal inactivation with the assessment of brain metabolism using cytochrome oxidase histochemistry. Several brain regions were considered, including the hippocampus and other related structures. Results showed that both unilateral and bilateral hippocampal inactivation impaired spatial memory retrieval. Hence, whereas subjects with bilateral hippocampal inactivation showed a circular swim pattern at the side walls of the pool, unilateral inactivation favoured swimming in the quadrants adjacent to the target one. Analysis of cytochrome oxidase activity disclosed regional differences according to the degree of hippocampal functional blockade. In comparison to control group, animals with bilateral inactivation showed increased CO activity in CA1 and CA3 areas of the hippocampus during retrieval, while the activity of the dentate gyrus substantially decreased. However, unilateral inactivated animals showed decreased CO activity in Ammon's horn and the dentate gyrus. This study demonstrated that retrieval recruits differentially the hippocampal subregions and the balance between them is altered with hippocampal functional lesions.

Spatial learning-related changes in metabolic activity of limbic structures at different posttask delays

The aim of this study was to assess the functional contribution of brain limbic system regions at different moments after the acquisition of a short-term spatial memory task performed in the Morris water maze. Adult male Wistar rats were submitted to a matching-to-sample procedure with a hidden platform. The trials were made up of two daily identical visits to the platform, sample (swim-1) and retention (swim-2). To study oxidative metabolic activity, we applied cytochrome oxidase (COx) histochemistry. Densitometric measurements were taken at 1.5, 6, 24, and 48 hr posttask. An untrained group was added to explore the COx changes not specific to the learning process. The brain regions studied showed a different pattern of metabolic activity at different time points after the spatial memory task. Specifically, a significant increase of COx was found in the septal dentate gyrus, anteromedial thalamus, medial mammillary nucleus, and entorhinal cortex at early moments after learning. The entorhinal cortex maintained an increase of COx at later stages of the posttask period. In addition, an increase of COx activity was found in the supramammillary nucleus and the retrosplenial, perirhinal, and parietal cortices a long time after learning. These findings suggest that diencephalic and cortical regions are involved in this spatial learning and contribute at different moments to process this information.

Age-dependent decline in learning and memory performances of WAG/Rij rat model of absence epilepsy

Recent clinical studies revealed emotional and cognitive impairments associated with absence epilepsy. Preclinical research with genetic models of absence epilepsy however have primarily focused on dysfunctional emotional processes and paid relatively less attention to cognitive impairment. In order to bridge this gap, we investigated age-dependent changes in learning and memory performance, anxiety-like behavior, and locomotor activity of WAG/Rij rats (a valid model of generalized absence epilepsy) using passive avoidance, Morris water maze, elevated plus maze, and locomotor activity cage. We tested 5 month-old and 13 month-old WAG/Rij rats and compared their performance to age-matched Wistar rats. Results revealed a decline in emotional and spatial memory of WAG/Rij rats compared to age-matched Wistar rats only at 13 months of age. Importantly, there were no significant differences between WAG/Rij and Wistar rats in terms of anxiety-like behavior and locomotor activity at either age. Results pointed at age-dependent learning and memory deficits in the WAG/Rij rat model of absence epilepsy.

A role for dorsal and ventral hippocampus in response learning

The hippocampus and the striatum have been traditionally considered as part of different and independent memory systems despite growing evidence supporting that both brain regions may even compete for behavioral control in particular learning tasks. In this regard, it has been reported that the hippocampus could be necessary for the use of idiothetic cues in several types of spatial learning tasks. Accordingly, the ventral striatum receives strong anatomical projections from the hippocampus, suggesting a participation of both regions in goal-directed behavior. Our work examined the role of the dorsal and ventral hippocampus on a response learning task. Cytochrome c oxidase (C.O.) quantitative histochemistry was used as an index of brain oxidative metabolism. In addition, determination of C.O. subunit I levels in the hippocampus by western blot analysis was performed to assess the contribution of this subunit to overall C.O. activity. Increased brain oxidative metabolism was found in most of the studied hippocampal subregions when experimental group was compared with a swim control group. However, no differences were found in the amount of C.O. subunit I expressed in the hippocampus by western blot analysis. Our results support that both the dorsal and ventral hippocampus are associated with the use of response strategies during response learning.

Memantine partly rescues behavioral and cognitive deficits in an animal model of neurodegeneration

Memantine, a non-competitive NMDA receptor antagonist, is used for the treatment of Alzheimer's disease (AD) and off-label as an anti-depressant. Here we investigated possible anti-depressant, cognitive enhancing and neuroprotective effects of memantine in the olfactory bulbectomized (OBX) rat. OBX is used as a screening model for antidepressants and shows cognitive disturbances. In Experiment I, memantine treatment started 14 days after OBX surgery (this setup is similar to what we use for screening of potential antidepressants) and 2 days before surgery in experiment II. In both experiments, memantine (20 mg/kg, p.o) was administered once daily for 28 days. Animals were tested in the open field (locomotor activity), passive avoidance (fear learning and memory), and holeboard (spatial acquisition and memory) before and after the bulbectomy. Memantine, when administered before surgery, prevented OBX-induced hyperactivity and partly fear memory loss. These behavioral effects were present for at least 3 weeks after cessation of treatment. Memantine, however did not improve spatial memory. When administered 2 weeks after OBX surgery, memantine was ineffective in normalizing open field hyperactivity and improving cognitive deficits. Interestingly, after the animals were retrained in passive avoidance, memantine- treated OBX rats (both in experiment I and II) showed improved fear learning and memory. Our findings suggest that memantine has both neuroprotective and cognitive enhancing effects without antidepressant-like properties in the OBX rat. Based on our results, we propose that memantine may be more beneficial to AD patients when administered early in the disease process.