Aging affects the ocular circadian pacemaker of Aplysia californica

Synchrony and desynchrony in circadian clocks: impacts on learning and memory

Circadian clocks evolved under conditions of environmental variation, primarily alternating light dark cycles, to enable organisms to anticipate daily environmental events and coordinate metabolic, physiological, and behavioral activities. However, modern lifestyle and advances in technology have increased the percentage of individuals working in phases misaligned with natural circadian activity rhythms. Endogenous circadian oscillators modulate alertness, the acquisition of learning, memory formation, and the recall of memory with examples of circadian modulation of memory observed across phyla from invertebrates to humans. Cognitive performance and memory are significantly diminished when occurring out of phase with natural circadian rhythms. Disruptions in circadian regulation can lead to impairment in the formation of memories and manifestation of other cognitive deficits. This review explores the types of interactions through which the circadian clock modulates cognition, highlights recent progress in identifying mechanistic interactions between the circadian system and the processes involved in memory formation, and outlines methods used to remediate circadian perturbations and reinforce circadian adaptation.

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.

Effects of age-related dopaminergic neuron loss in the substantia nigra on the circadian rhythms of locomotor activity in mice

Elderly people often develop sleep and autonomic dysfunctions, which are regulated by circadian rhythm. Recently, we reported on the degradation of neural output from the central circadian clock in the suprachiasmatic nucleus (SCN) with aging. However, it is likely that many other factors contribute to the age-related decline in the functioning of the circadian system. In this study, we examined the effects of dopaminergic neuronal loss in the substantia nigra (SN) on circadian rhythms of mice to assess whether age-related degeneration of the dopamine system influences circadian rhythm. Young male C57BL/6J mice were administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a compound that selectively destroys dopaminergic neurons in the SN, and their wheel-running activities were recorded. We observed that MPTP-treated mice lost 43% of their dopaminergic neurons in the SN (on average) and demonstrated longer period of wheel-running activity rhythm in constant darkness compared with control mice. However, all the remaining circadian parameters in the MPTP-treated mice remained constant. Our findings suggest that in addition to SCN output dysfunction, age-related degeneration in the dopamine system of the brain leads to circadian rhythm irregularities.

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).

Resetting of central and peripheral circadian oscillators in aged rats

The mammalian circadian timing system is affected by aging. Analysis of the suprachiasmatic nucleus (SCN) and of other circadian oscillators reveals age-related changes which are most profound in extra-SCN tissues. Some extra-SCN oscillators appear to stop oscillating in vivo or display altered phase relationships. To determine whether the dynamic behavior of circadian oscillators is also affected by aging we studied the resetting behavior of the Period1 transcriptional rhythm of peripheral and central oscillators in response to a 6h advance or delay in the light schedule. We employed a transgenic rat with a luciferase reporter to allow for real-time measurements of transcriptional rhythmicity. While phase resetting in the SCN following an advance or a delay of the light cycle appears nearly normal in 2-year-old rats, resynchronization of the liver was seriously disrupted. In addition, the arcuate nucleus and pineal gland exhibited faster resetting in aged rats relative to 4-8-month-old controls. The consequences of these deficits are unknown, but may contribute to organ and brain diseases in the aged as well as the health problems that are common in older shift-workers.

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.