Learning and Memory

In the experiments reported here we have developed a new group-training protocol for assessing long-term memory for habituation in Caenorhabditis elegans. We have replicated all of the major findings of the original single-worm protocol using the new protocol: (1) distributed training produced long-term retention of training, massed training did not; (2) distributed training at long interstimulus intervals (ISIs) produced long-term retention, short ISIs did not; and (3) long-term memory for distributed training is protein synthesis-dependent as it could be blocked by heat shock during the inter-block interval. In addition, we have shown that long-term memory for habituation is graded, depending on the number of blocks of stimuli in training. The inter-block interval must be >40 min for long-term retention of training to occur. Finally, we have tested long-term memory for habituation training in a strain of worms with a mutation in a vesicular glutamate transporter in the sensory neurons that transduce tap (eat-4). The results from these eat-4 worms indicate that glutamate release from the sensory neurons has an important role in the formation of long-term memory for habituation.

There are significant sex differences in vulnerability to develop fear-related anxiety disorders. Females exhibit twice the rate of post-traumatic stress disorder (PTSD) as males and sex differences have been observed in fear extinction learning in both humans and rodents, with a failure to inhibit fear emerging as a precipitating factor in the development of PTSD. Here we report that female mice are resistant to fear extinction, and exhibit increased DNA methylation of Bdnf exon IV and a concomitant decrease in mRNA expression within the medial prefrontal cortex. Activation of BDNF signaling by the trkB agonist 7,8-dihydroxyflavone blocks the return of fear in female mice after extinction training, and thus represents a novel approach to treating fear-related anxiety disorders that are characterized by a resistance to extinction and increased propensity for renewal.

One of the most rigorously investigated problems in modern neuroscience is to decipher the mechanisms by which experience-induced changes in the central nervous system are translated into behavioral acquisition, consolidation, retention, and subsequent recall of information. Brain-derived neurotrophic factor (BDNF) has recently emerged as one of the most potent molecular mediators of not only central synaptic plasticity, but also behavioral interactions between an organism and its environment. Recent experimental evidence indicates that BDNF modulates synaptic transmission and plasticity by acting across different spatial and temporal domains. BDNF signaling evokes both short- and long-term periods of enhanced synaptic physiology in both pre- and postsynaptic compartments of central synapses. Specifically, BDNF/TrkB signaling converges on the MAP kinase pathway to enhance excitatory synaptic transmission in vivo, as well as hippocampal-dependent learning in behaving animals. Emerging concepts of the intracellular signaling cascades involved in synaptic plasticity induced through environmental interactions resulting in behavioral learning further support the contention that BDNF/TrkB signaling plays a fundamental role in mediating enduring changes in central synaptic structure and function. Here we review recent literature showing the involvement of BDNF/TrkB signaling in hippocampal-dependent learning paradigms, as well as in the types of cellular plasticity proposed to underlie learning and memory.

The induction of long-term potentiation (LTP) and long-term depression (LTD) at excitatory synapses in the hippocampus can be strongly modulated by patterns of synaptic stimulation that otherwise have no direct effect on synaptic strength. Likewise, patterns of synaptic stimulation that induce LTP or LTD not only modify synaptic strength but can also induce lasting changes that regulate how synapses will respond to subsequent trains of stimulation. Collectively known as metaplasticity, these activity-dependent processes that regulate LTP and LTD induction allow the recent history of synaptic activity to influence the induction of activity-dependent changes in synaptic strength and may thus have an important role in information storage during memory formation. To explore the cellular and molecular mechanisms underlying metaplasticity, we investigated the role of metaplasticity in the induction of LTP by υ-frequency (5-Hz) synaptic stimulation in the hippocampal CA1 region. Our results show that brief trains of υ-frequency stimulation not only induce LTP but also activate a process that inhibits the induction of additional LTP at potentiated synapses. Unlike other forms of metaplasticity, the inhibition of LTP induction at potentiated synapses does not appear to arise from activity-dependent changes in NMDA receptor function, does not require nitric oxide signaling, and is strongly modulated by β-adrenergic receptor activation. Together with previous findings, our results indicate that mechanistically distinct forms of metaplasticity regulate LTP induction and suggest that one way modulatory transmitters may act to regulate synaptic plasticity is by modulating metaplasticity.

The late phase of long-term potentiation (L-LTP) is correlated with some types of long-term memory, but the mechanisms by which L-LTP is modulated by prior synaptic activity are undefined. Activation of protein phosphatases by low-frequency stimulation (LFS) given before induction of L-LTP may significantly modify L-LTP. Using cellular electrophysiological recording methods in mouse hippocampal slices, we show that LFS given before induction of L-LTP inhibited L-LTP in an activity-dependent manner without affecting either basal synaptic strength or the early phase of LTP (E-LTP). This anterograde inhibitory effect of LFS was persistent, required N-methyl-D-aspartate (NMDA) receptor activation, and was blocked by inhibitors of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A). These data indicate that certain patterns of LFS can activate PP1 and/or PP2A, and that long-lasting activation of these phosphatases by prior LFS can suppress the subsequent expression of L-LTP without affecting E-LTP. Because this inhibition of L-LTP is caused by prior synaptic activity that, alone, produced no net effect on synaptic efficacy, we suggest that this is a “silent” form of metaplasticity that may influence long-term information storage by modulating the capacity of synapses to express L-LTP after repeated bouts of activity.

The perineuronal net (PNN) surrounds neurons in the central nervous system and is thought to regulate developmental plasticity. A few studies have shown an involvement of the PNN in hippocampal plasticity and memory storage in adult animals. In addition to the hippocampus, plasticity in the medial prefrontal cortex (mPFC) has been demonstrated to be critical for the storage of long-term memory, particularly memories for temporally separated events. In the present study, we examined the role of PNN in the acquisition and retention of memories for trace (in which the conditioned and unconditioned stimuli are temporally separated) and delayed (in which the conditioned and unconditioned stimuli overlap) fear conditioning in both the hippocampus and the mPFC. Consistent with a role for the hippocampus in memory storage in both delayed and trace fear conditioning, removal of hippocampal PNN disrupted contextual and trace fear memory. Disruption of the PNN in the mPFC impaired long-term trace and conditioned stimulus (CS)-elicited fear memory in the trace fear conditioning task. Interestingly, CS-elicited fear memory was also impaired when a delayed fear conditioning paradigm was used. These findings further support a role for the PNN in neural plasticity and implicate PNN-regulated plasticity in neocortical memory storage.

Regulated expression of a constitutively active form of cAMP response element-binding protein (CREB), VP16-CREB, lowers the threshold for the late phase of long-term potentiation in the Schaffer collateral pathway in a de novo gene expression-independent manner, and increases the excitability and reduces afterhyperpolarization of neurons at the amygdala and the hippocampus. We explore the consequences of these changes on the consolidation of fear conditioning and find that the expression of VP16-CREB can bypass the requirement for de novo gene expression associated with long-term memory formation, suggesting that CREB-dependent gene expression is sufficient for fear memory consolidation.

In CA1 stratum oriens of hippocampal slices, a robust long-term potentiation (LTP) induced by tetanic stimulation (20 pulses at 100 Hz, 10 trains delivered at 0.1 Hz) was accompanied by a slowly developing potentiation in a second, untetanized pathway. N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (D-APV, 50 or 100 microM) reduced the homosynaptic LTP by 80% but failed to affect heterosynaptic LTP. The metabotropic receptor antagonist DL-2-amino-3-phosphonopropionic acid DL-AP3, 300 microM) or (+)-alpha-methyl-4-carboxyphenylglycine (MCPG, 500 microM), applied with DL-APV, further reduced homosynaptic LTP and blocked heterosynaptic LTP. The inhibitor of Ca(2+)-induced Ca2+ release dantrolene (20 microM), also applied with DL-APV, blocked both components of LTP. Importantly, when low-frequency test stimulation (0.017 Hz) to the untetanized, heterosynaptic pathway was interrupted for 30 min after homosynaptic tetanization, heterosynaptic LTP did not develop. These results demonstrate homosynaptic and heterosynaptic LTP inductions in stratum oriens of the area CA1 and suggest that the heterosynaptic LTP is induced by NMDA-independent, novel associative processes between tetanized and untetanized pathways.

A common cellular alteration, reduced post-burst afterhyperpolarization (AHP) in CA1 neurons, is associated with acquisition of the hippocampus-dependent tasks trace eyeblink conditioning and the Morris water maze. As a similar increase in excitability is correlated with these two learning paradigms, we sought to determine the interactive behavioral effects of training animals on both tasks by using either a consecutive or simultaneous training design. In the consecutive design, animals were trained first on either the trace eyeblink conditioning task for six sessions, followed by training on the water maze task for six sessions, or vice versa. The simultaneous design consisted of six or 11 training days; animals received one session/day of both trace eyeblink conditioning and water maze training. Separate groups were used for consecutive and simultaneous training. Animals trained on both tasks simultaneously were significantly facilitated in their ability to acquire the trace eyeblink conditioning task; no effect of simultaneous training was seen on the water maze task. No effect was seen on acquisition for either task when using the consecutive training design. Taken together, these findings provide insight into how the hippocampus processes information when animals learn multiple hippocampus-dependent tasks.