Neurobiology of learning and memory
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Neurobiol Learn Mem · May 2010
US habituation, like CS extinction, produces a decrement in conditioned fear responding that is NMDA dependent and subject to renewal and reinstatement.
Just as fear can be learned, it can also be inhibited. The most common way of reducing learned fear is through extinction, where the conditioned stimulus (CS) previously paired with an aversive unconditioned stimulus (US) is repeatedly presented on its own. Another, much less commonly studied, way to inhibit learned fear is by habituating, or devaluing, the US. ⋯ We then showed that the reduction in freezing resulting from either US habituation or CS extinction was context-specific (i.e., a change in context led to a renewal of the learned fear response; Experiment 2) and, furthermore, was attenuated when a pre-test shock was given (i.e., reinstatement of fear was observed in both cases; Experiment 3). Finally, Experiment 4 demonstrated that an injection of the NMDA antagonist MK-801 prior to US habituation impaired long-term retention of the learning that takes place during this procedure. Together, these results suggest that the decrement in conditioned fear responses produced by US habituation and CS extinction could rely on overlapping processes.
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Neurobiol Learn Mem · Mar 2010
Late expression of brain-derived neurotrophic factor in the amygdala is required for persistence of fear memory.
In many instances, increase in neuronal activity can induce biphasic secretion of a modulator. The initial release of the modulator triggers the induction of synaptic plasticity, whereas the second-phase release reinforces the efficacy of synaptic transmission and growth of dendrites and axons. In this study, we showed that fear conditioning not only induced the first but also a second peak of brain-derived neurotrophic factor (BDNF) expression. ⋯ The level of growth-associated protein 43 (GAP-43), a marker of newly formed synapses, in the amygdala increased 7 days after fear conditioning. Moreover, conditioned rats had higher AMPA/NMDA ratio than unpaired rats. These results suggest that consolidated memory could be continuously modulated by previous molecular changes produced during memory acquisition.
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Neurobiol Learn Mem · Feb 2010
Human frontal midline theta and its synchronization to gamma during a verbal delayed match to sample task.
The involvement of oscillatory activity, especially at theta and gamma frequency, in human working memory has been reported frequently. A salient pattern during working memory is electroencephalographic frontal midline theta activity which has been suggested to reflect monitoring functions in order to deal with a task. In general, theta activity has been credited with integrative functions of distributed activity. ⋯ Cross-frequency phase synchronization between frontal midline theta activity and distributed gamma activity, on the other hand, was predominant during the first half of the delay period and was stronger for manipulation compared to retention. We suggest that coupling of frontal midline theta to gamma activity reflects monitoring functions on the temporal segregation of memory items, whereas higher frontal midline theta power in the second half of the delay period might be associated with rehearsal processes. Rehearsal processes in the manipulation condition are likely more pronounced, because rehearsal of a new letter string in a limited time window requires higher mental effort compared to pure retention where rehearsal processes may already start at the beginning of the delay period.
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Neurobiol Learn Mem · Oct 2009
Association learning-dependent increases in acetylcholine release in the rat auditory cortex during auditory classical conditioning.
The cholinergic system has been implicated in sensory cortical plasticity, learning and memory. This experiment determined the relationship between the acquisition of a Pavlovian conditioned approach response (CR) to an auditory conditioned stimulus (CS) and the release of acetylcholine (ACh) in the primary auditory cortex in rats. Samples of ACh were collected via microdialysis during behavioral training in either an auditory classical conditioning task or in a non-associative control task. ⋯ Training was conducted on three consecutive days, with microdialysis samples being collected on Days 1 and 3 in separate sub-groups. The level of ACh released in the auditory cortex during conditioning trials increased from the first to the third day of training in the conditioning group as rats acquired the CR, but did not change in the control group, which did not acquire a CR. These data provide direct evidence for the hypothesis that ACh release increases in the primary auditory cortex during natural memory formation, where cholinergic activation is known to contribute to the formation of specific associative representational plasticity in conjunction with specific memory formation.
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Neurobiol Learn Mem · Oct 2009
Effects of cannabinoids infused into the dorsal hippocampus upon memory formation in 3-days apomorphine-treated rats.
In the present study, the effects of intra-dorsal hippocampus (intra-CA1) injection of cannabinoid receptor agents on memory formation have been investigated in 3-days apomorphine-treated rats. Step-through inhibitory avoidance task of memory has been used to examine retrieval of memory formation, 24h after training. Apomorphine was injected subcutaneously (S. ⋯ The D1 receptor antagonist, SCH 23390 (0.01, 0.02, 0.07 and 0.1 mg/kg/day x 3-days, S. C.), was ineffective in this respect. The results suggest that subchronic apomorphine treatment may induce dopamine D2 receptor sensitization, which in turn prevented amnesia induced by WIN55,212-2.