Learning-induced plasticity of N-methyl-D-aspartate receptors is task and region specific

Fear conditioning performance and NMDA receptor subtypes: NR2A differential expression in the striatum

While considerable evidence implicates NMDA receptors in the hippocampus in contextual fear conditioning, the role of other brain regions is less well understood. To further investigate this issue, rats were subjected to a contextual fear conditioning task and then classified as high or low responders according to performance. Density of NMDA receptors was evaluated using [3H]MK-801 autoradiography in 52 brain areas and expression of NR2A and NR2B subunits was studied with in situ hybridization in the same brains. Results revealed no differences between high- and low-performance rats in NMDA receptor binding in any of the brain areas studied. Similarly, NR2B subunit expression was also not different between groups. However, NR2A expression was significantly higher in the caudate-putamen of low-performance rats. These results suggest that NMDA receptors in the caudate-putamen may also be involved in contextual fear conditioning performance.

Impact of enriched-environment housing on brain-derived neurotrophic factor and on cognitive performance after a transient global ischemia

Environmental enrichment promotes structural and functional changes in the brain, including enhanced learning and memory performance in rodents. Transient global cerebral ischemia (15 min) causes specific damage to dorsal hippocampal area CA1 pyramidal cells of the rat concomitantly with cognitive deficits. Thus, we investigated if environmental enrichment can protect rats against the cognitive and neurological consequences of transient ischemia. We evaluated the impairment of learning and memory with three tasks: odour discrimination, object exploration and spatial learning. Contrary to expectation, we found that the enriched environment improved performances for both ischemic and sham rats in odour discrimination and object exploration tasks compared with standard condition housed rats. After exposure to an enriched environment, ischemic rats performed better in the water maze than those in the standard housing conditions. However, exposure to an enriched environment does not protect against actual loss of CA1 pyramidal cells. Brain-derived neurotrophic factor (BDNF) levels were increased in environmental enrichment animals compared to those housed in standard conditions. We conclude that environmental enrichment has positive effects that are independent of the effects of ischemic brain lesions.

Consolidation of memory for odour-reward association requires transient polysialylation of the neural cell adhesion molecule in the rat hippocampal dentate gyrus

Cell adhesion molecule function is involved in hippocampal synaptic plasticity and is associated with memory consolidation. At the infragranular zone of the dentate gyrus, neurons expressing the polysialylated form of the neural cell adhesion molecule (NCAM PSA) transiently increase their frequency at the 12-hr posttraining time in behaviours elicited by stressful stimuli, such as those associated with conditioned avoidance, water maze, and fear conditioning paradigms. To determine whether learning-induced modulation of NCAM polysialylation is limited to stressful paradigms, we employed a reward-based odour discrimination task. Animals show a rapid acquisition and recall of this task in terms of latency to identify the food-associated odour and the number of choice errors. Immunohistochemical procedures were employed to determine the change in NCAM PSA expression following task acquisition. NCAM PSA immunoreactivity in the hippocampal formation was most intense on the granule-like neurons in the infragranular zone of the dentate gyrus, and their frequency transiently increased in the 12-hr posttraining period. The nature of the transient increase in NCAM PSA-immunoreactive neurons was indistinguishable from that observed following avoidance conditioning or spatial learning, in that it occurred at the same time. The transient increase in NCAM PSA expression is suggested to facilitate dendritic elaboration in response to the acquisition of novel behavioural repertoires.

Blockade of NMDA receptors in prelimbic cortex induces an enduring amnesia for odor-reward associative learning

The competitive antagonist 2-amino-5-phosphonoeptanoic acid (APV) was injected intracerebroventricularly to determine the involvement of NMDA receptors in different stages of memory consolidation. Subsequent experiments used local injections to determine possible sites of drug action. Rats were trained in a rapidly learned olfactory task to find palatable food in a hole in a sponge impregnated with the target odor in the presence of two other sponges with nonrewarded odors. APV injections were made intracerebroventricularly 5 min or 2 hr after the end of the training, and a retention test was given 48 hr later. The results showed that blockade of NMDA receptors immediately after training induces a profound and enduring amnesia with no effect when the treatment is delayed at 2 hr after training. To address the question of the effective sites of action of the intracerebroventricular treatment, APV injections into the hippocampus and into the prelimblic region of the frontal cortex (PLC) were made. Blockade of NMDA receptors into the PLC but not into the hippocampus impaired memory formation of the odor-reward association. The amnesia is not transient, because the retention tests were made 48 hr after training. These results underlie the role of NMDA receptors in the early stage of consolidation of a simple odor-reward associative memory and confirm the role of the PLC in the consolidation of long-term memory.

Testosterone decreases CA1 plasticity in vivo in gonadectomized male rats

Estrogen has been reported to enhance CA1 functional plasticity in adult rats, as measured by the induction of long-term potentiation (LTP). In the present study, the effects of androgens on CA1 LTP were assessed in adult male Sprague-Dawley rats in vivo following peripubertal castration. Castrated rats with cholesterol implants showed significantly greater plasticity, both in degree and duration of potentiation, than castrated rats given testosterone or dihydrotestosterone implants. An LTP paradigm reported to produce decremental LTP in vitro produced nondecremental LTP in androgen-deprived rats, but decremental LTP in androgen-treated rats. These results suggest that androgens, unlike estrogens, act to reduce CA1 plasticity in the adult rat.

Consolidation of Memory for Odor–Reward Association: β-Adrenergic Receptor Involvement in the Late Phase

Experimentally naive rats can learn rapidly to discriminate among three odors to obtain food reinforcement. After three massed trials, they show almost errorless performance. This task has proved to be useful in studying time-dependent postacquisition intracellular processes necessary for long-term memory. The present experiments evaluated the temporal dynamics of the role of β-noradrenergic receptors in long-term consolidation. Rats were implanted with intracerebroventricular cannulae and trained in a single session to find reinforcement in a hole in a sponge impregnated with a particular odor. Injections of the β-receptor antagonist timolol were made at 5 min, 1, 2, or 5 hr after training. Memory and relearning ability were evaluated 48 hr later. Rats treated with timolol 2 hr after training showed a memory deficit at the retention test, but were able to relearn the task normally. Injections at the earlier or later time points were ineffective. The results reinforce previous observations with systemic injections that β-noradrenergic receptors are involved in the late phase of memory consolidation and suggest a critical time window during which they are necessary. The time window is compatible with the current view that long-term memory depends on late involvement of the cAMP cascade leading to new protein synthesis necessary for synaptic reorganization.