Long-term sensitization training produces spike narrowing in Aplysia sensory neurons

Transcriptional changes before and after forgetting of a long-term sensitization memory in Aplysia californica

Most long-term memories are forgotten, becoming progressively less likely to be recalled. Still, some memory fragments may persist, as savings memory (easier relearning) can be detected long after recall has become impossible. What happens to a memory trace during forgetting that makes it inaccessible for recall and yet still effective to spark easier re-learning? We are addressing this question by tracking the transcriptional changes that accompany learning and then forgetting of a long-term sensitization memory in the tail-elicited siphon withdrawal reflex of Aplysia californica. First, we tracked savings memory. We found that even though recall of sensitization fades completely within 1 week of training, savings memory is still detectable at 2 weeks post training. Next, we tracked the time-course of regulation of 11 transcripts we previously identified as potentially being regulated after recall has become impossible. Remarkably, 3 transcripts still show strong regulation 2 weeks after training and an additional 4 are regulated for at least 1 week. These long-lasting changes in gene expression always begin early in the memory process, within 1 day of training. We present a synthesis of our results tracking gene expression changes accompanying sensitization and provide a testable model of how sensitization memory is forgotten.

Transcriptional correlates of memory maintenance following long-term sensitization of Aplysia californica

We characterized the transcriptional response accompanying maintenance of long-term sensitization (LTS) memory in the pleural ganglia of Aplysia californica using microarray (N = 8) and qPCR (N = 11 additional samples). We found that 24 h after memory induction there is strong regulation of 1198 transcripts (748 up and 450 down) in a pattern that is almost completely distinct from what is observed during memory encoding (1 h after training). There is widespread up-regulation of transcripts related to all levels of protein production, from transcription (e.g., subunits of transcription initiation factors) to translation (e.g., subunits of eIF1, eIF2, eIF3, eIF4, eIF5, and eIF2B) to activation of components of the unfolded protein response (e.g., CREB3/Luman, BiP, AATF). In addition, there are widespread changes in transcripts related to cytoskeleton function, synaptic targeting, synaptic function, neurotransmitter regulation, and neuronal signaling. Many of the transcripts identified have previously been linked to memory and plasticity (e.g., Egr, menin, TOB1, IGF2 mRNA binding protein 1/ZBP-1), though the majority are novel and/or uncharacterized. Interestingly, there is regulation that could contribute to metaplasticity potentially opposing or even eroding LTS memory (down-regulation of adenylate cyclase and a putative serotonin receptor, up-regulation of FMRFa and a FMRFa receptor). This study reveals that maintenance of a "simple" nonassociative memory is accompanied by an astonishingly complex transcriptional response.

Nonassociative learning in invertebrates

The simplicity and tractability of the neural circuits mediating behaviors in invertebrates have facilitated the cellular/molecular dissection of neural mechanisms underlying learning. The review has a particular focus on the general principles that have emerged from analyses of an example of nonassociative learning, sensitization in the marine mollusk Aplysia. Learning and memory rely on multiple mechanisms of plasticity at multiple sites of the neuronal circuits, with the relative contribution to memory of the different sites varying as a function of the extent of training and time after training. The same intracellular signaling cascades that induce short-term modifications in synaptic transmission can also be used to induce long-term changes. Although short-term memory relies on covalent modifications of preexisting proteins, long-term memory also requires regulated gene transcription and translation. Maintenance of long-term cellular memory involves both intracellular and extracellular feedback loops, which sustain the regulation of gene expression and the modification of targeted molecules.

Immediate and persistent transcriptional correlates of long-term sensitization training at different CNS loci in Aplysia californica

Repeated noxious stimulation produces long-term sensitization of defensive withdrawal reflexes in Aplysia californica, a form of long-term memory that requires changes in both transcription and translation. Previous work has identified 10 transcripts which are rapidly up-regulated after long-term sensitization training in the pleural ganglia. Here we use quantitative PCR to begin examining how these transcriptional changes are expressed in different CNS loci related to defensive withdrawal reflexes at 1 and 24 hours after long-term sensitization training. Specifically, we sample from a) the sensory wedge of the pleural ganglia, which exclusively contains the VC nociceptor cell bodies that help mediate input to defensive withdrawal circuits, b) the remaining pleural ganglia, which contain withdrawal interneurons, and c) the pedal ganglia, which contain many motor neurons. Results from the VC cluster show different temporal patterns of regulation: 1) rapid but transient up-regulation of Aplysia homologs of C/EBP, C/EBPγ, and CREB1, 2) delayed but sustained up-regulation of BiP, Tolloid/BMP-1, and sensorin, 3) rapid and sustained up-regulation of Egr, GlyT2, VPS36, and an uncharacterized protein (LOC101862095), and 4) an unexpected lack of regulation of Aplysia homologs of calmodulin (CaM) and reductase-related protein (RRP). Changes in the remaining pleural ganglia mirror those found in the VC cluster at 1 hour but with an attenuated level of regulation. Because these samples had almost no expression of the VC-specific transcript sensorin, our data suggests that sensitization training likely induces transcriptional changes in either defensive withdrawal interneurons or neurons unrelated to defensive withdrawal. In the pedal ganglia, we observed only a rapid but transient increase in Egr expression, indicating that long-term sensitization training is likely to induce transcriptional changes in motor neurons but raising the possibility of different transcriptional endpoints in this cell type.

Characterization of the rapid transcriptional response to long-term sensitization training in Aplysia californica

We used a custom-designed microarray and quantitative PCR to characterize the rapid transcriptional response to long-term sensitization training in the marine mollusk Aplysia californica. Aplysia were exposed to repeated noxious shocks to one side of the body, a procedure known to induce a long-lasting, transcription-dependent increase in reflex responsiveness that is restricted to the side of training. One hour after training, pleural ganglia from the trained and untrained sides of the body were harvested; these ganglia contain the sensory nociceptors which help mediate the expression of long-term sensitization memory. Microarray analysis from 8 biological replicates suggests that long-term sensitization training rapidly regulates at least 81 transcripts. We used qPCR to test a subset of these transcripts and found that 83% were confirmed in the same samples, and 86% of these were again confirmed in an independent sample. Thus, our new microarray design shows strong convergent and predictive validity for analyzing the transcriptional correlates of memory in Aplysia. Fully validated transcripts include some previously identified as regulated in this paradigm (ApC/EBP and ApEgr) but also include novel findings. Specifically, we show that long-term sensitization training rapidly up-regulates the expression of transcripts which may encode Aplysia homologs of a C/EBPγ transcription factor, a glycine transporter (GlyT2), and a vacuolar-protein-sorting-associated protein (VPS36).

Transcriptional changes following long-term sensitization training and in vivo serotonin exposure in Aplysia californica

We used Aplysia californica to compare the transcriptional changes evoked by long-term sensitization training and by a treatment meant to mimic this training, in vivo exposure to serotonin. We focused on 5 candidate plasticity genes which are rapidly up-regulated in the Aplysia genus by in vivo serotonin treatment, but which have not yet been tested for regulation during sensitization: CREB1, matrilin, antistasin, eIF3e, and BAT1 homolog. CREB1 was rapidly up-regulated by both treatments, but the regulation following training was transient, falling back to control levels 24 hours after training. This suggests some caution in interpreting the proposed role of CREB1 in consolidating long-term sensitization memory. Both matrilin and eIF3e were up-regulated by in vivo serotonin but not by long-term sensitization training. This suggests that in vivo serotonin may produce generalized transcriptional effects that are not specific to long-term sensitization learning. Finally, neither treatment produced regulation of antistasin or BAT1 homolog, transcripts regulated by in vivo serotonin in the closely related Aplysia kurodai. This suggests either that these transcripts are not regulated by experience, or that transcriptional mechanisms of memory may vary within the Aplysia genus.

Changes in wingstroke kinematics associated with a change in swimming speed in a pteropod mollusk, Clione limacina

In pteropod mollusks, the gastropod foot has evolved into two broad, wing-like structures that are rhythmically waved through the water for propulsion. The flexibility of the wings lends a tremendous range of motion, an advantage that could be exploited when changing locomotory speed. Here, we investigated the kinematic changes that take place during an increase in swimming speed in the pteropod mollusk Clione limacina. Clione demonstrates two distinct swim speeds: a nearly constant slow swimming behavior and a fast swimming behavior used for escape and hunting. The neural control of Clione's swimming is well documented, as are the neuromuscular changes that bring about Clione's fast swimming. This study examined the kinematics of this swimming behavior at the two speeds. High speed filming was used to obtain 3D data from individuals during both slow and fast swimming. Clione's swimming operates at a low Reynolds number, typically under 200. Within a given swimming speed, we found that wing kinematics are highly consistent from wingbeat to wingbeat, but differ between speeds. The transition to fast swimming sees a significant increase in wing velocity and angle of attack, and range of motion increases as the wings bend more during fast swimming. Clione likely uses a combination of drag-based and unsteady mechanisms for force production at both speeds. The neuromuscular control of Clione's speed change points to a two-gaited swimming behavior, and we consider the kinematic evidence for Clione's swim speeds being discrete gaits.

Serotonin- and training-induced dynamic regulation of CREB2 in Aplysia

Long-term memory and plasticity, including long-term synaptic facilitation (LTF) of the Aplysia sensorimotor synapse, depend on the activation of transcription factors that regulate genes necessary for synaptic plasticity. In the present study we found that treatment with 5-HT and behavioral training produce biphasic changes in the expression of CREB2, a transcriptional repressor. An immediate increase in CREB2 protein was followed by a subsequent decrease. The effects of these treatments persist for at least 24 h and are observed in isolated sensory neurons. This study suggests that the dynamics of CREB2 expression could contribute to the consolidation of memory.

More than synaptic plasticity: role of nonsynaptic plasticity in learning and memory

Decades of research on the cellular mechanisms of memory have led to the widely held view that memories are stored as modifications of synaptic strength. These changes involve presynaptic processes, such as direct modulation of the release machinery, or postsynaptic processes, such as modulation of receptor properties. Parallel studies have revealed that memories might also be stored by nonsynaptic processes, such as modulation of voltage-dependent membrane conductances, which are expressed as changes in neuronal excitability. Although in some cases nonsynaptic changes can function as part of the engram itself, they might also serve as mechanisms through which a neural circuit is set to a permissive state to facilitate synaptic modifications that are necessary for memory storage.