Long-term habituation of the gill-withdrawal reflex in aplysia requires gene transcription, calcineurin and L-type voltage-gated calcium channels

Different stressors uniquely affect the expression of endocannabinoid-metabolizing enzymes in the central ring ganglia of Lymnaea stagnalis

The endocannabinoid system (ECS) plays an important role in neuroprotection, neuroplasticity, energy balance, modulation of stress, and inflammatory responses, acting as a critical link between the brain and the body's peripheral regions, while also offering promising potential for novel therapeutic strategies. Unfortunately, in humans, pharmacological inhibitors of different ECS enzymes have led to mixed results in both preclinical and clinical studies. As the ECS has been highly conserved throughout the eukaryotic lineage, the use of invertebrate model organisms like the pond snail Lymnaea stagnalis may provide a flexible tool to unravel unexplored functions of the ECS at the cellular, synaptic, and behavioral levels. In this study, starting from the available genome and transcriptome of L. stagnalis, we first identified putative transcripts of all ECS enzymes containing an open reading frame. Each predicted protein possessed a high degree of sequence conservation to known orthologues of other invertebrate and vertebrate organisms. Sequences were confirmed by qualitative PCR and sequencing. Then, we investigated the transcriptional effects induced by different stress conditions (i.e., bacterial LPS injection, predator scent, food deprivation, and acute heat shock) on the expression levels of the enzymes of the ECS in Lymnaea's central ring ganglia. Our results suggest that in Lymnaea as in rodents, the ECS is involved in mediating inflammatory and anxiety-like responses, promoting energy balance, and responding to acute stressors. To our knowledge, this study offers the most comprehensive analysis so far of the ECS in an invertebrate model organism.

Gill histology and ultrastructure in Aplysia depilans (Mollusca, Euopisthobranchia)

The gill of Aplysia depilans consists of several wedge-shaped pinnules with a highly folded structure, differing from the typical ctenidial gills of mollusks. Light microscopy and transmission electron microscopy were used to investigate this organ in juveniles and adults. In this species, the gill epithelium comprised ciliated, unciliated, and secretory cells. The ultrastructural analysis suggests other functions for the gill besides respiration. The deep cell membrane invaginations associated with mitochondria in the basal region of epithelium point to a role in ion regulation. Endocytosis and intracellular digestion were other activities detected in epithelial cells. In juveniles, an intranuclear crystalline structure was seen in some ciliated cells. The presence of an intranuclear crystalline structure was frequently associated with chromatin decondensation, swelling of the nuclear envelope and endoplasmic reticulum cisternae, and abundance of Golgi stacks. As these intranuclear inclusions were not found in the gill of the adult specimens, their occurrence in the two juveniles seems likely to be an anomalous condition whose cause cannot be established at the moment. Mucous cells were the most abundant secretory cells in the epithelium, but a few epithelial serous cells were also found. In addition, large protein-secreting subepithelial cells had the main cell body inserted in the connective tissue and a long thin neck crossing the epithelium. Mucous cells can be considered responsible for the production of the mucus layer that protects the epithelium, but the specific functions of the epithelial and subepithelial protein-secreting cells remain elusive. Below the epithelium, a layer of connective tissue with muscle cells lined the narrow hemolymph space. The connective tissue included cells with a large amount of rough endoplasmic reticulum cisternae. Bacteria were found on the surface of the gill, and the most abundant had a thin stalk for attachment to the epithelial cells.

Possible novel features of synaptic regulation during long-term facilitation in Aplysia

Most studies of molecular mechanisms of synaptic plasticity have focused on the sequence of changes either at individual synapses or in the cell nucleus. However, studies of long-term facilitation at Aplysia sensory neuron–motor neuron synapses in isolated cell culture suggest two additional features of facilitation. First, that there is also regulation of the number of synaptic contacts between two neurons, which may occur at the level of cell pair-specific branch points in the neuronal arbor. Branch points contain many molecules that are involved in protein synthesis-dependent long-term facilitation including neurotrophins and the RNA binding protein CPEB. Second, the regulation involves homeostatic feedback and tends to keep the total number of contacts between two neurons at a fairly constant level both at rest and following facilitation. That raises the question of how facilitation and homeostasis can coexist. A possible answer is suggested by the findings that they both involve spontaneous transmission and postsynaptic Ca²⁺, which can have bidirectional effects similar to LTP and LTD in hippocampus. In addition, long-term facilitation can involve a change in the set point of homeostasis, which could be encoded by plasticity molecules such as CPEB and/or PKM. A computational model based on these ideas can qualitatively simulate the basic features of both facilitation and homeostasis of the number of contacts.

Phylogenetic analysis of ionotropic L-glutamate receptor genes in the Bilateria, with special notes on Aplysia californica

Background

The neurotransmitter L-Glutamate (L-Glu) acting at ionotropic L-Glu receptors (iGluR) conveys fast excitatory signal transmission in the nervous systems of all animals. iGluR-dependent neurotransmission is a key component of the synaptic plasticity that underlies learning and memory. During learning, two subtypes of iGluR, α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR) and N-methyl-D-aspartate receptors (NMDAR), are dynamically regulated postsynaptically in vertebrates. Invertebrate organisms such as Aplysia californica (Aplysia) are well-studied models for iGluR-mediated function, yet no studies to date have analyzed the evolutionary relationships between iGluR genes in these species and those in vertebrates, to identify genes that may mediate plasticity. We conducted a thorough phylogenetic analysis spanning Bilateria to elucidate these relationships. The expression status of iGluR genes in the Aplysia nervous system was also examined.

Results

Our analysis shows that ancestral genes for both NMDAR and AMPAR subtypes were present in the common bilaterian ancestor. NMDAR genes show very high conservation in motifs responsible for forming the conductance pore of the ion channel. The number of NMDAR subunits is greater in vertebrates due to an increased number of splice variants and an increased number of genes, likely due to gene duplication events. AMPAR subunits form an orthologous group, and there is high variability in the number of AMPAR genes in each species due to extensive taxon specific gene gain and loss. qPCR results show that all 12 Aplysia iGluR subunits are expressed in all nervous system ganglia.

Conclusions

Orthologous NMDAR subunits in all species studied suggests conserved function across Bilateria, and potentially a conserved mechanism of neuroplasticity and learning. Vertebrates display an increased number of NMDAR genes and splice variants, which may play a role in their greater diversity of physiological responses. Extensive gene gain and loss of AMPAR genes may result in different physiological properties that are taxon specific. Our results suggest a significant role for L-Glu mediated responses throughout the Aplysia nervous system, consistent with L-Glu’s role as the primary excitatory neurotransmitter.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0871-1) contains supplementary material, which is available to authorized users.

Acute Sleep Deprivation Blocks Short- and Long-Term Operant Memory in Aplysia

STUDY OBJECTIVES

Insufficient sleep in individuals appears increasingly common due to the demands of modern work schedules and technology use. Consequently, there is a growing need to understand the interactions between sleep deprivation and memory. The current study determined the effects of acute sleep deprivation on short and long-term associative memory using the marine mollusk Aplysia californica, a relatively simple model system well known for studies of learning and memory.

METHODS

Aplysia were sleep deprived for 9 hours using context changes and tactile stimulation either prior to or after training for the operant learning paradigm, learning that food is inedible (LFI). The effects of sleep deprivation on short-term (STM) and long-term memory (LTM) were assessed.

RESULTS

Acute sleep deprivation prior to LFI training impaired the induction of STM and LTM with persistent effects lasting at least 24 h. Sleep deprivation immediately after training blocked the consolidation of LTM. However, sleep deprivation following the period of molecular consolidation did not affect memory recall. Memory impairments were independent of handling-induced stress, as daytime handled control animals demonstrated no memory deficits. Additional training immediately after sleep deprivation failed to rescue the induction of memory, but additional training alleviated the persistent impairment in memory induction when training occurred 24 h following sleep deprivation.

CONCLUSIONS

Acute sleep deprivation inhibited the induction and consolidation, but not the recall of memory. These behavioral studies establish Aplysia as an effective model system for studying the interactions between sleep and memory formation.

Transcriptional analysis of a whole-body form of long-term habituation in Aplysia californica

Habituation is the simplest form of learning, but we know little about the transcriptional mechanisms that encode long-term habituation memory. A key obstacle is that habituation is relatively stimulus-specific and is thus encoded in small sets of neurons, providing poor signal/noise ratios for transcriptional analysis. To overcome this obstacle, we have developed a protocol for producing whole-body long-term habituation of the siphon-withdrawal reflex (SWR) of Aplysia californica. Specifically, we constructed a computer-controlled brushing apparatus to apply low-intensity tactile stimulation over the entire dorsal surface of Aplysia at regular intervals. We found that 3 d of training (10 rounds of stimulation/day; each round = 15 min brushing at a 10-sec ISI; 15-min rest between rounds) produces habituation with several characteristics favorable for mechanistic investigation. First, habituation is widespread, with SWR durations reduced whether the reflex is evoked by tactile stimulation to the head, tail, or the siphon. Second, long-term habituation is sensitive to the pattern of training, occurring only when brushing sessions are spaced out over 3 d rather than massed into a single session. Using a custom-designed microarray and quantitative PCR, we show that long-term habituation produces long-term up-regulation of an apparent Aplysia homolog of cornichon, a protein important for glutamate receptor trafficking. Our training paradigm provides a promising starting point for characterizing the transcriptional mechanisms of long-term habituation memory.

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).

Habituation of Arctic ground squirrels (Urocitellus parryii) to handling and movement during torpor to prevent artificial arousal

Hibernation is a unique physiological adaptation characterized by periods of torpor that consist of repeated, reversible, and dramatic reductions of body temperature, metabolism, and blood flow. External and internal triggers can induce arousal from torpor in the hibernator. Studies of hibernating animals often require that animals be handled or moved prior to sampling or euthanasia but this movement can induce changes in the hibernation status of the animal. In fact, it has been demonstrated that movement of animals while they are hibernating is sufficient to induce an artificial arousal, which can detrimentally alter experimental findings obtained from animals assumed to be torpid. Therefore, we assessed a method to induce habituation of torpid hibernators to handling and movement to reduce inadvertent arousals. A platform rocker was used to mimic motion experienced during transfer of an animal and changes in respiratory rate (RR) were used to assess responsiveness of torpid Arctic ground squirrels (AGS, Urocitellus parryii). We found that movement alone did not induce a change in RR, however, exposure to handling induced an increase in RR in almost all AGS. This change in RR was markedly reduced with increased exposures, and all AGS exhibited a change in RR ≤ 1 by the end of the study. AGS habituated faster mid-season compared to early in the season, which mirrors other assessments of seasonal variation of torpor depth. However, AGS regained responsiveness when they were not exposed to daily handling. While AGS continued to undergo natural arousals during the study, occurrence of a full arousal was neither necessary for becoming habituated nor detrimental to the time required for habituation. These data suggest that even when torpid, AGS are able to undergo mechanosensory habituation, one of the simplest forms of learning, and provides a reliable way to reduce the sensitivity of torpid animals to handling.

Central gene expression changes associated with enhanced neuroendocrine and autonomic response habituation to repeated noise stress after voluntary wheel running in rats

Accumulating evidence indicates that regular physical exercise benefits health in part by counteracting some of the negative physiological impacts of stress. While some studies identified reductions in some measures of acute stress responses with prior exercise, limited data were available concerning effects on cardiovascular function, and reported effects on hypothalamic-pituitary-adrenocortical (HPA) axis responses were largely inconsistent. Given that exposure to repeated or prolonged stress is strongly implicated in the precipitation and exacerbation of illness, we proposed the novel hypothesis that physical exercise might facilitate adaptation to repeated stress, and subsequently demonstrated significant enhancement of both HPA axis (glucocorticoid) and cardiovascular (tachycardia) response habituation to repeated noise stress in rats with long-term access to running wheels compared to sedentary controls. Stress habituation has been attributed to modifications of brain circuits, but the specific sites of adaptation and the molecular changes driving its expression remain unclear. Here, in situ hybridization histochemistry was used to examine regulation of select stress-associated signaling systems in brain regions representing likely candidates to underlie exercise-enhanced stress habituation. Analyzed brains were collected from active (6 weeks of wheel running) and sedentary rats following control, acute, or repeated noise exposures that induced a significantly faster rate of glucocorticoid response habituation in active animals but preserved acute noise responsiveness. Nearly identical experimental manipulations also induce a faster rate of cardiovascular response habituation in exercised, repeatedly stressed rats. The observed regulation of the corticotropin-releasing factor and brain-derived neurotrophic factor systems across several brain regions suggests widespread effects of voluntary exercise on central functions and related adaptations to stress across multiple response modalities.

Learning and memory in zebrafish larvae

Larval zebrafish possess several experimental advantages for investigating the molecular and neural bases of learning and memory. Despite this, neuroscientists have only recently begun to use these animals to study memory. However, in a relatively short period of time a number of forms of learning have been described in zebrafish larvae, and significant progress has been made toward their understanding. Here we provide a comprehensive review of this progress; we also describe several promising new experimental technologies currently being used in larval zebrafish that are likely to contribute major insights into the processes that underlie learning and memory.

Protein phosphatase-dependent circadian regulation of intermediate-term associative memory

The endogenous circadian clock is a principal factor modulating memory across species. Determining the processes through which the circadian clock modulates memory formation is a key issue in understanding and identifying mechanisms to improve memory. We used the marine mollusk Aplysia californica to investigate circadian modulation of intermediate-term memory (ITM) and the mechanisms through which the circadian clock phase specifically suppresses memory using the operant learning paradigm, learning that food is inedible. We found that ITM, a temporally and mechanistically distinct form of memory, is rhythmically expressed under light-dark and constant conditions when induced by either massed or spaced training. Strong circadian regulation of ITM occurs with memory exhibited only by animals trained during the early subjective day; no apparent memory is expressed when training occurs during the late subjective day or night. Given the necessity of multiple persistent kinase cascades for ITM, we investigated whether protein phosphatase activity affected circadian modulation. Inhibition of protein phosphatases 1 and 2A blocked ITM when animals were trained during the early (subjective) day while resulting in phase-specific memory rescue when animals were trained late in the subjective day and early night. In contrast, inhibition of calcineurin did not block ITM when animals were trained during the early day and permitted ITM when animals were trained during the late subjective day, early evening, and throughout the night. These results demonstrate that levels of protein phosphatase activity are critical regulators of ITM and one mechanism through which the circadian clock regulates memory formation.

Endocannabinoid-dependent long-term depression in a nociceptive synapse requires coordinated presynaptic and postsynaptic transcription and translation

Endocannabinoids (eCBs) play an important role in long-term regulation of synaptic signaling in both vertebrates and invertebrates. In this study, the role of transcription- and translation-dependent processes in presynaptic versus postsynaptic neurons was examined during eCB-mediated synaptic plasticity in the CNS of the leech. Low-frequency stimulation (LFS) of non-nociceptive afferents elicits eCB-dependent long-term depression (eCB-LTD) heterosynaptically in nociceptive synapses that lasts at least 2 h. Bath application of emetine, a protein synthesis inhibitor, blocked eCB-LTD after afferent LFS or exogenous eCB application, indicating that this depression was translation dependent. Bath application of actinomycin D, an irreversible RNA synthesis inhibitor, or 5,6-dichlorobenzimidazole 1-β-d-ribofurandoside (DRB), a reversible RNA synthesis inhibitor, also prevented eCB-LTD. Selective injection of DRB or emetine into the presynaptic or postsynaptic neuron before LFS indicated that eCB-LTD required transcription and translation in the postsynaptic neuron but only translation in the presynaptic cell. Depression observed immediately after LFS was also blocked when these transcription- and translation-dependent processes were inhibited. It is proposed that induction of eCB-LTD in this nociceptive synapse requires the coordination of presynaptic protein synthesis and postsynaptic mRNA and protein synthesis. These findings provide significant insights into both eCB-based synaptic plasticity and understanding how activity in non-nociceptive afferents modulates nociceptive pathways.

NMDA receptor blockade by memantine does not prevent adaptation to recurrent hypoglycaemia in healthy men

AIMS

Recurrent hypoglycaemia leads to an attenuation of hypoglycaemic symptoms and hormonal counterregulatory responses. This phenomenon poses a severe problem in the treatment of patients with diabetes mellitus, but the underlying neuroendocrine mechanisms are unclear. On the basis of animal experimental findings, we hypothesized that counterregulatory attenuation represents a basic adaptive learning process relying on synaptic long-term potentiation or depression. If so, attenuation should be prevented by blocking glutamatergic N-methyl-D-aspartate (NMDA) receptors.

METHODS

Sixteen healthy young men participated in two conditions, separated by 4 weeks. Participants received the NMDA antagonist memantine over 5 days (15 mg/day) in one condition and placebo in the other one. After 3 days of drug administration, participants underwent two hypoglycaemic clamps on day 4 and another one on day 5. We assessed blood concentrations of counterregulatory hormones (cortisol, ACTH, epinephrine, norepinephrine, growth hormone and glucagon) as well as subjective symptoms of hypoglycaemia and word-list recall as an indicator of short-term memory.

RESULTS

Counterregulatory responses of all hormones as well as neuroglycopenic and autonomic symptom ratings showed robust attenuation following the third as compared to the first hypoglycaemia (p < 0.05). NMDA receptor antagonization by memantine impaired memory function but did not alter any neuroendocrine measure of counterregulatory attenuation (p > 0.17).

CONCLUSIONS

Attenuation of the endocrine as well as symptomatic counterregulatory response to recurrent hypoglycaemia is not prevented by the NMDA receptor blocker memantine. Our results do not support the view that adaptation to repeated hypoglycaemia relies on NMDA receptor-mediated plastic processes involving long-term potentiation or depression.

Proteomic characterization of the abdominal ganglion of Aplysia californica-a protein resource for neuroscience

Aplysia californica (AC) is a widely used model for testing learning and memory. Although ESTs have been generated, proteomics studies on AC proteins are limited. Studies at the protein level, however, are mandatory, not only due to the fact that studies at the nucleic acid level are not allowing conclusions about PTMs. A gel-based proteomics method was therefore applied to carry out protein profiling in abdominal ganglia from AC. Abdominal ganglia were extirpated, proteins extracted and run on 2DE with subsequent in-gel digestion with trypsin, chymotrypsin, and partially by subtilisin. Peptides were identified using a nano-LC-ESI-LTQ-FT-mass spectrometer. MS/MS data were analyzed by searching the NCBI nonredundant public AC EST database and the NCBI nonredundant public AC protein database. A total of 477 different proteins represented by 363 protein spots were detected and were assigned to different protein pathways as for instance signaling (receptors, protein kinases, and phosphatases), metabolism, protein synthesis, handling and degradation, cytoskeleton and structural, oxido-redox, heat shock and chaperone, hypothetical, predicted and unnamed proteins. The generation of a protein map of soluble proteins shows the existence of so far hypothetical and predicted proteins and is allowing and challenging further work at the protein level, in particular in the field of neuroscience.