Age-diminished motor neuronal function of central neuron L7 in Aplysia

An age-related decline in the cholinergic synaptic response may cause the firing pattern in the jaw-closing motor neurons, which resembles the aversive taste response in the feeding behavior of old Aplysia kurodai

Anorexia due to aging is recognized as a syndrome of animal feeding behavior. Age-related functional disorders of the brain often cause behavioral changes. We used Aplysia kurodai to study this neural mechanism, following our previous study on food preference behaviors. The age of each wild animal was defined by a previously described method, and a significant age-related decline in food intake was observed. In this study, we explored the effects of aging on a specific inhibitory synaptic response in jaw-closing (JC) motor neurons produced by cholinergic multiaction (MA) neurons, the size of which determines the delay between MA and JC firings and this delay is reduced during aversive taste responses; in our analyses, we found a significant age-related decline in the synaptic response. Thereafter, we further explored whether such functional decline affects the JC firing pattern during the normal feeding response. During the feeding-like rhythmic responses induced by electrical nerve stimulation, the firing of the JC motor neurons advanced toward that of the MA burst, which typically happens during aversive taste responses. These results suggest that the age-related decline in the cholinergic synaptic response may partly cause the JC firing patterns that resemble the aversive taste response in old animals.

Whole-transcriptome changes in gene expression accompany aging of sensory neurons in Aplysia californica

Background

Large-scale molecular changes occur during aging and have many downstream consequences on whole-organism function, such as motor function, learning, and memory. The marine mollusk Aplysia californica can be used to study transcriptional changes that occur with age in identified neurons of the brain, because its simplified nervous system allows for more direct correlations between molecular changes, physiological changes, and their phenotypic outcomes. Behavioral deficits in the tail-withdrawal reflex of aged animals have been correlated with reduced excitation in sensory neurons that control the reflex. RNASeq was used to investigate whole-transcriptome changes in tail-withdrawal sensory neurons of sexually mature and aged Aplysia to correlate transcriptional changes with reduced behavioral and physiological responses.

Results

Paired-end sequencing resulted in 210 million reads used for differential expression analysis. Aging significantly altered expression of 1202 transcripts in sensory neurons underlying the tail-withdrawal reflex, with an approximately equal number of these genes up- and down regulated with age. Despite overall bidirectionality of expression changes, > 80% of ion channel genes that were differentially expressed had decreased expression with age. In particular, several voltage-gated K⁺ and Ca²⁺ channels were down regulated. This marked decrease in ion channel expression may play an important role in previously observed declines in aged sensory neuron excitability. We also observed decreased expression of genes and pathways involved in learning and memory. Genes involved in the stress response showed increased expression in aged Aplysia neurons.

Conclusions

Significantly altered expression of many genes between sexually mature and aged Aplysia suggests large molecular changes that may impact neuronal function. Decreased ion channel mRNA observed could mean fewer receptors present in aged neurons, resulting in reduced excitability of PVC sensory neurons, ultimately leading to reduced tail-withdrawal reflex observed in aged Aplysia. Significant changes in other genes and pathways, such as stress response and learning and memory, have previously been shown to occur with age in many vertebrate organisms. This suggests that some effects of aging are common across many animal phyla.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4909-1) contains supplementary material, which is available to authorized users.

Effects of aging on the food intake in the feeding behavior of Aplysia kurodai

In wild Aplysia, the birthdate of animals can typically not be determined. Therefore, we sought a reliable index of old age by taking into consideration the distinguished Japanese seasons. Large amounts of eggs and dead bodies were present on the coast during and after the second half of May (MayS). Body mass decreased after May. We roughly classified animals collected before and after the MayS as mature and old animals. Plots of internalized shell length (S) against body mass (B) gave distinct best-fit curves for mature and old animals. The B/S significantly decreased in the second half of June, suggesting that body mass decreases with age but shell length is maintained in each animal. Therefore, the collected animals were classified into mature and old animals using the best-fit curves for animals classified by the collection period. We examined the amount of food intake every 2 h up to 8 h after providing food. The amounts increased linearly, and the rate was significantly lower in old animals than in mature animals. The amount of 1-day food intake was also significantly lower in old animals. These results suggest that food intake may decline with age and this may cause mass loss in old animals.

Decline in the Recovery from Synaptic Depression in Heavier Aplysia Results from Decreased Serotonin-Induced Novel PKC Activation

The defensive withdrawal reflexes of Aplysia are important behaviors for protecting the animal from predation. Habituation and dishabituation allow for experience-dependent tuning of these reflexes and the mechanisms underlying these forms of behavioral plasticity involve changes in transmitter release from the sensory to motor neuron synapses through homosynaptic depression and the serotonin-mediated recovery from depression, respectively. Interestingly, dishabituation is reduced in older animals with no corresponding change in habituation. Here we show that the cultured sensory neurons of heavier animals (greater than 120g) that form synaptic connections with motor neurons have both reduced recovery from depression and reduced novel PKC Apl II activation with 5HT. The decrease in the recovery from depression correlated better with the size of the animal than the age of the animal. Much of this change in PKC activation and synaptic facilitation following depression can be rescued by direct activation of PKC Apl II with phorbol dibutyrate, suggesting a change in the signal transduction pathway upstream of PKC Apl II activation in the sensory neurons of larger animals.

Behavioral aging is associated with reduced sensory neuron excitability in Aplysia californica

Invertebrate models have advantages for understanding the basis of behavioral aging due to their simple nervous systems and short lifespans. The potential usefulness of Aplysia californica in aging research is apparent from its long history of neurobiological research, but it has been underexploited in this model use. Aging of simple reflexes at both single sensory neuron and neural circuit levels was studied to connect behavioral aging to neurophysiological aging. The tail withdrawal reflex (TWR), righting reflex, and biting response were measured throughout sexual maturity in 3 cohorts of hatchery-reared animals of known age. Reflex times increased and reflex amplitudes decreased significantly during aging. Aging in sensory neurons of animals with deficits in measures of the TWR and biting response resulted in significantly reduced excitability in old animals compared to their younger siblings. The threshold for firing increased while the number of action potentials in response to depolarizing current injection decreased during aging in sensory neurons, but not in tail motoneurons. Glutamate receptor-activated responses in sensory neurons also decreased with aging. In old tail motoneurons, the amplitude of evoked EPSPs following tail shock decreased, presumably due to reduced sensory neuron excitability during aging. The results were used to develop stages of aging relevant to both hatchery-reared and wild-caught Aplysia. Aplysia is a viable aging model in which the contributions of differential aging of components of neural circuits may be assessed.

Decreased response to acetylcholine during aging of aplysia neuron R15

How aging affects the communication between neurons is poorly understood. To address this question, we have studied the electrophysiological properties of identified neuron R15 of the marine mollusk Aplysia californica. R15 is a bursting neuron in the abdominal ganglia of the central nervous system and is implicated in reproduction, water balance, and heart function. Exposure to acetylcholine (ACh) causes an increase in R15 burst firing. Whole-cell recordings of R15 in the intact ganglia dissected from mature and old Aplysia showed specific changes in burst firing and properties of action potentials induced by ACh. We found that while there were no significant changes in resting membrane potential and latency in response to ACh, the burst number and burst duration is altered during aging. The action potential waveform analysis showed that unlike mature neurons, the duration of depolarization and the repolarization amplitude and duration did not change in old neurons in response to ACh. Furthermore, single neuron quantitative analysis of acetylcholine receptors (AChRs) suggested alteration of expression of specific AChRs in R15 neurons during aging. These results suggest a defect in cholinergic transmission during aging of the R15 neuron.

Age-associated bidirectional modulation of gene expression in single identified R15 neuron of Aplysia

Background

Despite the advances in our understanding of aging-associated behavioral decline, relatively little is known about how aging affects neural circuits that regulate specific behaviors, particularly the expression of genes in specific neural circuits during aging. We have addressed this by exploring a peptidergic neuron R15, an identified neuron of the marine snail Aplysia californica. R15 is implicated in reproduction and osmoregulation and responds to neurotransmitters such as acetylcholine, serotonin and glutamate and is characterized by its action potential bursts.

Results

We examined changes in gene expression in R15 neurons during aging by microarray analyses of RNAs from two different age groups, mature and old animals. Specifically we find that 1083 ESTs are differentially regulated in mature and old R15 neurons. Bioinformatics analyses of these genes have identified specific biological pathways that are up or downregulated in mature and old neurons. Comparison with human signaling networks using pathway analyses have identified three major networks [(1) cell signaling, cell morphology, and skeletal muscular system development (2) cell death and survival, cellular function maintenance and embryonic development and (3) neurological diseases, developmental and hereditary disorders] altered in old R15 neurons. Furthermore, qPCR analysis of single R15 neurons to quantify expression levels of candidate regulators involved in transcription (CREB1) and translation (S6K) showed that aging is associated with a decrease in expression of these regulators, and similar analysis in three other neurons (L7, L11 and R2) showed that gene expression change during aging could be bidirectional.

Conclusions

We find that aging is associated with bidirectional changes in gene expression. Detailed bioinformatics analyses and human homolog searches have identified specific biological processes and human-relevant signaling pathways in R15 that are affected during aging. Evaluation of gene expression changes in different neurons suggests specific transcriptomic signature of single neurons during aging.

Do different neurons age differently? Direct genome-wide analysis of aging in single identified cholinergic neurons

Aplysia californica is a powerful experimental system to study the entire scope of genomic and epigenomic regulation at the resolution of single functionally characterized neurons and is an emerging model in the neurobiology of aging. First, we have identified and cloned a number of evolutionarily conserved genes that are age-related, including components of apoptosis and chromatin remodeling. Second, we performed gene expression profiling of different identified cholinergic neurons between young and aged animals. Our initial analysis indicates that two cholinergic neurons (R2 and LPl1) revealed highly differential genome-wide changes following aging suggesting that on the molecular scale different neurons indeed age differently. Each of the neurons tested has a unique subset of genes differentially expressed in older animals, and the majority of differently expressed genes (including those related to apoptosis and Alzheimer's disease) are found in aging neurons of one but not another type. The performed analysis allows us to implicate (i) cell specific changes in histones, (ii) DNA methylation and (iii) regional relocation of RNAs as key processes underlying age-related changes in neuronal functions and synaptic plasticity. These mechanisms can fine-tune the dynamics of long-term chromatin remodeling, or control weakening and the loss of synaptic connections in aging. At the same time our genomic tests revealed evolutionarily conserved gene clusters associated with aging (e.g., apoptosis-, telomere- and redox-dependent processes, insulin and estrogen signaling and water channels).

Parallel processing in an identified neural circuit: the Aplysia californica gill-withdrawal response model system

The response of the gill of Aplysia calfornica Cooper to weak to moderate tactile stimulation of the siphon, the gill-withdrawal response or GWR, has been an important model system for work aimed at understanding the relationship between neural plasticity and simple forms of non-associative and associative learning. Interest in the GWR has been based largely on the hypothesis that the response could be explained adequately by parallel monosynaptic reflex arcs between six parietovisceral ganglion (PVG) gill motor neurons (GMNs) and a cluster of sensory neurons termed the LE cluster. This hypothesis, the Kupfermann-Kandel model, made clear, falsifiable predictions that have stimulated experimental work for many years. Here, we review tests of three predictions of the Kupfermann-Kandel model: (1) that the GWR is a simple, reflexive behaviour graded with stimulus intensity; (2) that central nervous system (CNS) pathways are necessary and sufficient for the GWR; and (3) that activity in six identified GMNs is sufficient to account for the GWR. The available data suggest that (1) a variety of action patterns occur in the context of the GWR; (2) the PVG is not necessary and the diffuse peripheral nervous system (PNS) is sufficient to mediate these action patterns; and (3) the role of any individual GMN in the behaviour varies. Both the control of gill-withdrawal responses, and plasticity in these responses, are broadly distributed across both PNS and CNS pathways. The Kupfermann-Kandel model is inconsistent with the available data and therefore stands rejected. There is, no known causal connection or correlation between the observed plasticity at the identified synapses in this system and behavioural changes during non-associative and associative learning paradigms. Critical examination of these well-studied central pathways suggests that they represent a 'wetware' neural network, architecturally similar to the neural network models of the widely used 'Perceptron' and/or 'Back-propagation' type. Such models may offer a more biologically realistic representation of nervous system organisation than has been thought. In this model, the six parallel GMNs of the CNS correspond to a hidden layer within one module of the gill-control system. That is, the gill-control system appears to be organised as a distributed system with several parallel modules, some of which are neural networks in their own right. A new model is presented here which predicts that the six GMNs serve as components of a 'push-pull' gain control system, along with known but largely unidentified inhibitory motor neurons from the PVG. This 'push-pull' gain control system sets the responsiveness of the peripheral gill motor system. Neither causal nor correlational links between specific forms of neural plasticity and behavioural plasticity have been demonstrated in the GWR model system. However, the GWR model system does provide an opportunity to observe and describe directly the physiological and biochemical mechanisms of distributed representation and parallel processing in a largely identifiable 'wetware' neural network.

Circadian regulation of a transcription factor, ApC/EBP, in the eye of Aplysia californica

The transcription factor, ApC/EBP (Aplysia CCAAT enhancer-binding protein) is an immediate early gene that is rapidly induced by serotonin and the cAMP signaling pathway. ApC/EBP acts as an important link following the activation of protein kinase A (PKA) in the consolidation of long-term memory in Aplysia californica. In this study, we report that levels of ApC/EBP mRNA in the eye of Aplysia are modulated by serotonin or light. These responses of ApC/EBP to serotonin and light are mimicked by analogs of cAMP and cGMP. Expression of ApC/EBP in the eye is also under the control of the circadian oscillator with circadian rhythms of ApC/EBP mRNA present under constant dark conditions. Therefore, ApC/EBP is a candidate gene for a circadian transcription factor to mediate circadian responses activated by the cAMP and cGMP second messenger signaling pathways.

Dopamine receptor subtype density as a function of age in Aplysia californica

The age-associated changes in dopamine subtype receptors were examined in Aplysia californica. The density of the subtype receptors D1, D2, D3 and D4 was examined in the ganglia from 4.5-, 6-, 8-, 9- and 12-month animals. Receptor analysis was performed by examining the binding of radiolabeled ligands to the individual subtypes. [3H]SCH23390 and [3H]Clozapine were used to analyze D1 and D4 specific binding. [3H]Quinpirole was used for determining D2 and D3 specific binding. Specific binding was found to be present for all four receptor subtypes. All receptor subtypes showed an increase in density from 4.5 to 6 months. From 6 to 8 months D2 and D3 decreased, while D1 and D4 increased. D4 showed the strongest increase. All four subtypes examined showed decreases from 8 to 12 months. ANOVA results indicated age was a significant factor in the subtype receptor density for all receptor types.

Multiple synaptic connections of a single neuron change differentially with age

The efficacy of chemical synaptic connections of a single identified interneuron with different types of follower neurons was studied throughout the adult life of the pond snail Lymnaea stagnalis. Simultaneous intracellular recordings were made from the interneuron RPeD1 and its follower neurons in isolated CNS preparations from animals of different age groups (3-18 months of age). The presence of postsynaptic responses to RPeD1 action potentials was tested. With increasing age, the number of A-group neurons that was found with a response to evoked RPeD1 action potentials decreased, yet the number of HIJK-group neurons responding to RPeD1 input increased. The number of G-group neurons and the number of individual neurons VD2/3 and VD4 with RPeD1 input did not differ significantly between age groups. However, there was variability in the presence of responses in these individual neurons. Thus, synaptic connections of the single interneuron RPeD1 change differentially throughout the adult life of L. stagnalis. Within the A-group we found indications that changes in RPeD1 input apply to the entire A-group. In the A-group neurons changes in several electrical properties could not account for the observed age-related changes in the number of neurons responding to RPeD1 action potentials.

Increased age affects properties characterizing behavioral plasticity in freely behaving Aplysia

In the marine mollusc Aplysia, in vitro studies showed that the gill withdrawal reflex (GWR) and its neuronal substrates were altered by age. In contrast, age minimally affected the gill respiratory pumping movements (GPM) and its neuronal substrates. Based on the respective properties of the GWR- and GPM-pathways in vitro, we proposed that the more pronounced the effect of age, the greater the expression of plasticity in a pathway. This conclusion may hold for in vitro preparations, but it remained to be demonstrated in intact animals. Based on this conclusion, the GWR should exhibit greater plasticity than the GPM in intact animals. Using freely behaving Aplysia, we tested for plasticity of the GWR and the GPM in three age groups (young, mature, and old). The tests for behavioral plasticity were: Graded responses to varying stimulus strength, response decrement (or habituation) to repetitive stimulation, enhanced response to dishabituating stimuli, and the effect of the GWR stimulus on the GPM and the GPM stimulus on the GWR. The GWR in mature animals exhibited all four properties, but in old animals, graded responses and habituation were significantly altered and in young animals habituation and dishabituation were absent. The GPM exhibited fewer of the properties than the GWR, only graded responses and response decrement, both of which were generally the same in the three groups. We found that behavioral plasticity and age-induced plasticity are related in freely behaving animals and are consistent with in vitro findings. The effect of age on properties characterizing plasticity at both the behavioral and pathway levels is discussed.

Differences in aging in two neural pathways: proposed explanations from the nervous system of Aplysia

A basic question in studies of the neurobiology of aging is to what extent age-related changes are genetically preprogrammed or epigenetically mediated. Our approach to this question is to compare the effect of age on two neural pathways in the marine mollusc, Aplysia. The advantage of Aplysia as a model of neural aging is that age-sensitive properties in the pathways can be studied at the behavioral, physiological, and morphological levels. The two pathways we are investigating respond differently to aging; a comparison of the pathways' properties provides a means of distinguishing the effect of age from other variables in the same animal. Age effects are expressed in the gill withdrawal reflex pathway at the three levels but are minimal in the gill respiratory pathway. The behavioral and physiological expressions of the reflex pathway are weakened in old animals (250 days of age and older) when compared to those in mature ones (ca. 160 days of age). The major differences between the two pathways are: (1) the reflex pathway appears to exhibit more plasticity than the respiratory pathway, and (2) the level of use of the respiratory pathway is more regular and frequent than that of the reflex pathway. The greater plasticity intrinsic to the reflex pathway and its level of use may well be the characteristics upon which age-related changes depend. An interplay between genetic and epigenetic factors is suggested to help explain the differential aging in the two pathways.

Age-dependent alterations of synaptic performance and plasticity in crustacean motor systems

Age-related changes in synaptic performance and plasticity are surveyed in crustacean neuromuscular systems. These systems are functionally differentiated into phasic and tonic types, with different attributes of synaptic function and plasticity. Conversion of phasic neuromuscular junctions to a more phasic phenotype can be brought about by altering the activity of selected neurons. This type of plasticity disappears in older animals in some motor neurons, but is retained in others. Developmental programs set constraints on the age-dependent modifications of plasticity. Crustacean motor neurons are often characterized by great longevity, with progressive addition of new branches and synapses to keep up with growth of innervated muscle cells. Certain age-related compensatory mechanisms found in neuromuscular junctions of other species may not be required in crustaceans.

Age-related decrease in electrical coupling of two identified neurons in the mollusc Lymnaea stagnalis

Electrophysiological characteristics of two identified giant electrotonically coupled neurosecretory cells in the central nervous system of the mollusc Lymnaea stagnalis were studied in mature animals of different age. The coupling coefficient of the neurons decreased considerably with age. The possibility that the decrease is due to an increase in the junctional resistance between the cells is discussed.

Properties of muscle cells and remodeling of neuromuscular junctions as related to age in Aplysia

Age-related morphologic properties of muscle cells and of their innervation in the gill were sought to explain reduced contractility in old Aplysia. As a consequence of previous physiological findings, properties of two muscle groups were examined: one group, MPn, innervated by motor neuron, L7, whose ability to elicit muscle contraction was reduced during aging, and the other group, LEV, innervated by motor neuron, LDG1, whose ability was not. In neither group did muscle properties, such as cell diameter and density and thick filament diameter and density, and resting potential, change with age. In contrast, age-related remodeling of nmjs did occur. The results show that remodeling is expressed differently in the two types of junctions: In L7-nmjs contact between terminals and muscle cells significantly increased with age; in LDG1-nmjs the terminal perimeter was enlarged significantly and not the contact between terminals and muscle cells. With increased age, the proportion of the terminal perimeter in contact with muscle in L7-nmjs increased significantly, and in LDG1-nmjs it remained essentially the same. Accompanying the remodeling of LDG1 terminals was a significant increase of vesicles in them; no significant change in vesicle number was measured in L7 terminals. The effect on transmission of the age-related presynaptic changes in the two types of junctions is discussed.

Survival and aging of a small laboratory population of a marine mollusc, Aplysia californica

In an investigation of the postmetamorphic survival of a population of 112 Aplysia californica, five animals died before 100 days of age and five after 200 days. The number of survivors among the 102 animals which died between 100 and 220 days declined approximately linearly with age. The median age at death was 155 days. The animals studied were those that died of natural causes within a laboratory population that was established to provide Aplysia for sacrifice in an experimental program. Actuarial separation of the former group from the latter was justified by theoretical consideration. Age-specific mortality rates were calculated from the survival data. Statistical fluctuation arising from the small size of the population was reduced by grouping the data in bins of unequal age duration. The durations were specified such that each bin contained approximately the same number of data points. An algorithm for choosing the number of data bins was based on the requirement that the precision with which the age of a group is determined should equal the precision with which the number of deaths in the groups is known. The Gompertz and power laws of mortality were fitted to the age-specific mortality-rate data with equally good results. The positive values of slope associated with the mortality-rate functions as well as the linear shape of the curve of survival provide actuarial evidence that Aplysia age. Since Aplysia grow linearly without approaching a limiting size, the existence of senescence indicates especially clearly the falsity of Bidder's hypothesis that aging is a by-product of the cessation of growth.

Modulation of the Aplysia gill withdrawal reflex by dopamine

The ability of dopamine to modulate gill contractions was tested in Aplysia. When dopamine was perfused through the gill vasculature, gill contractions caused by siphon stimulation (gill withdrawal reflex) and by depolarization of the gill motor neuron L7 were increased in amplitude, as compared with those evoked during seawater perfusion. Habituation of gill movements, brought about by repetitive stimulation of the siphon or of L7, was prevented by dopamine. Despite the absence of reflex habituation, the number of action potentials in central gill motor neurons, evoked by siphon stimulation, showed normal decrement. Dopamine's effects were blocked when the ctenidial nerve was cut or when L7 hyperpolarized. These data suggest that dopamine acts peripherally to increase the efficacy of L7's synaptic transmission onto gill muscle or elements of the gill neural plexus.

Dopamine produces muscle contractions and modulates motoneuron-induced contractions in Aplysia gill

1. Dopamine has been reported to exist in unusually large quantities in Aplysia gill. The physiological role of this neurotransmitter in this organ was examined. 2. The addition of dopamine to a gill perfusate results in the contractions of the lateral and medial external pinnule muscles, the circular and longitudinal muscles of the afferent vessel, and the circular muscles of the efferent vessel. 3. Dopamine-induced contractions persist after chemical synaptic transmission is eliminated in the gill. This suggests that excitatory dopamine receptors are present on gill smooth muscle fibers themselves. 4. Dopamine also potentiates the gill response to action potentials in single identified gill motoneurons. Evidence presented suggests that muscle contractions and modulation of motoneuron contractions are independent phenomena. 5. While modulation may in part be mediated by increases in excitatory junction potential (EJP) amplitude, in many cases large increases in muscle contractions occur while the enhancement of EJPs is disproportionately small. 6. Dopamine's ability to produce muscle contractions suggests that there may be dopaminergic motoneuron innervation of the gill. We suggest that dopamine's modulatory actions may be mediated via modification of excitation-contraction coupling in smooth muscle fibers.