Potential-dependent Ca channels of neurons in the mollusc Lymnaea stagnalis in aging: effect of norepinephrine

The Great Pond Snail (Lymnaea stagnalis) as a Model of Aging and Age-Related Memory Impairment: An Overview

With the increase of life span, normal aging and age-related memory decline are affecting an increasing number of people; however, many aspects of these processes are still not fully understood. Although vertebrate models have provided considerable insights into the molecular and electrophysiological changes associated with brain aging, invertebrates, including the widely recognized molluscan model organism, the great pond snail (Lymnaea stagnalis), have proven to be extremely useful for studying mechanisms of aging at the level of identified individual neurons and well-defined circuits. Its numerically simpler nervous system, well-characterized life cycle, and relatively long life span make it an ideal organism to study age-related changes in the nervous system. Here, we provide an overview of age-related studies on L. stagnalis and showcase this species as a contemporary choice for modeling the molecular, cellular, circuit, and behavioral mechanisms of aging and age-related memory impairment.

Phospholipase A2 - nexus of aging, oxidative stress, neuronal excitability, and functional decline of the aging nervous system? Insights from a snail model system of neuronal aging and age-associated memory impairment

The aging brain undergoes a range of changes varying from subtle structural and physiological changes causing only minor functional decline under healthy normal aging conditions, to severe cognitive or neurological impairment associated with extensive loss of neurons and circuits due to age-associated neurodegenerative disease conditions. Understanding how biological aging processes affect the brain and how they contribute to the onset and progress of age-associated neurodegenerative diseases is a core research goal in contemporary neuroscience. This review focuses on the idea that changes in intrinsic neuronal electrical excitability associated with (per)oxidation of membrane lipids and activation of phospholipase A₂ (PLA₂) enzymes are an important mechanism of learning and memory failure under normal aging conditions. Specifically, in the context of this special issue on the biology of cognitive aging we portray the opportunities offered by the identifiable neurons and behaviorally characterized neural circuits of the freshwater snail Lymnaea stagnalis in neuronal aging research and recapitulate recent insights indicating a key role of lipid peroxidation-induced PLA₂ as instruments of aging, oxidative stress and inflammation in age-associated neuronal and memory impairment in this model system. The findings are discussed in view of accumulating evidence suggesting involvement of analogous mechanisms in the etiology of age-associated dysfunction and disease of the human and mammalian brain.

Insights into CNS ageing from animal models of senescence

In recent years, novel model systems have made significant contributions to our understanding of the processes that control the ageing of whole organisms. However, there are limited data to show that the mechanisms that gerontologists have identified as having a role in organismal ageing contribute significantly to the ageing of the central nervous system. Two recent discoveries illustrate this particularly well. The first is the consistent failure of researchers to demonstrate a simple relationship between organismal ageing and oxidative stress--a mechanism often assumed to have a primary role in brain ageing. The second is the demonstration that senescent cells play a causal part in organismal ageing but remain essentially unstudied in a CNS context. We argue that the animal models now available (including rodents, flies, molluscs and worms), if properly applied, will allow a paradigm shift in our current understanding of the normal processes of brain ageing.

Age-related changes in climbing behavior and neural circuit physiology in Drosophila

Identifying the cellular and molecular basis for functional decline remains key to understanding aging. To this end, we have characterized age-dependent changes in climbing and the electrophysiology of the giant fiber circuitry in wild type (Wt) and mutant flies with altered lifespan (methuselah and fragile-X). Our data demonstrate a gradual decline in climbing in Wt and methuselah flies aged 5-45 days. In contrast, fragile-X flies climbed poorly even at 5 days and failed completely at 45 days. We then examined whether synaptic transmission to indirect flight muscles along the giant fiber circuit was altered with aging. At 5 days, the dorsal longitudinal muscle (DLM) in Wt flies followed high frequency stimulation well (at 130 Hz or above). At 35 and 45 days, these flies only followed 60-80 Hz. Methuselah flies did not follow stimuli as well as the Wt flies did at 5 and 25 days, but they were similar to Wt flies at older ages. Fragile-X flies responded poorly even at 5 days (40 Hz) and worsened at 35 days (30 Hz). Unlike DLMs, the tergotrochanteral muscle followed high frequency stimuli relatively well in all genotypes, suggesting that the peripheral interneuron along the DLM pathway or the DLM muscular synapse is prone to age-dependent functional decline. These studies reveal subcellular structures as potential targets of aging, indicating that the giant fiber pathway can be used as a model circuit for quantitative studies of aging in flies as well as fly models of age-related human neurological disorders.

Calcium homeostasis in aged neurones

Mechanisms of cytoplasmic calcium homeostasis were investigated in peripheral and central neurones isolated from neonatal, adult and old Wistar rats and in granule neurones in acutely prepared cerebellar slices of adult and old CBA mice. The cytoplasmic calcium concentration ([Ca2+]i) was measured by either indo-1-or fura-2-based microfluorimetry. The resting [Ca2+]i was significantly higher in senile neurones. The depolarization-induced [Ca2+]i transients were markedly altered in old neurones when compared with adult ones: the age-associated changes in stimulus-evoked [Ca2+]i signalling comprised of (i) significant decrease of the amplitudes of [Ca2+]i transients; (ii) prolongation of the rising phase and (iii) prominent deceleration of the recovery of the [Ca2+]i elevation towards the resting level after the end of depolarization. The amplitudes of calcium release from caffeine/Ca(2+)-sensitive endoplasmic reticulum calcium stores became significantly smaller in old central neurones, whereas they remained unaffected in peripheral neurones. Based on our observations we can conclude that ageing of the nervous system is associated with significant changes in mechanisms of [Ca2+]i homeostasis in individual neurones. These changes lead to a stable increase in the resting [Ca2+]i and to a substantial prolongation of stimulus-evoked [Ca2+]i signals. We could suggest also that the ability of the old neurones to handle Ca2+ loads is diminished, which may determine higher vulnerability of aged neurones to excess of calcium ions.

Age-associated changes of cytoplasmic calcium homeostasis in cerebellar granule neurons in situ: investigation on thin cerebellar slices

Mechanisms of cytoplasmic calcium homeostasis were investigated in adult and old CBA mice. The cytoplasmic calcium concentration ([Ca2+]i) was measured on fura-2/AM loaded granule neurons in acutely isolated cerebellar slices. The resting [Ca2+]i was significantly higher in senile cerebellar granule neurons, being on average 60 +/- 15 nM (n = 163) in adult and 107 +/- 12 nM (n = 129) in old neurons. The depolarization-induced [Ca2+]i transients were markedly altered in old neurons as compared with adult ones: their amplitude was smaller by about five times, the rate of rise was prolonged about two times, and the complete recovery to the resting level after the end of depolarization was about five times longer. The amplitude of calcium release from caffeine/Ca(2+)-sensitive endoplasmic reticulum calcium stores also become significantly smaller in old neurons (the amplitudes of [Ca2+]i transients evoked by 30 mM caffeine were 75 +/- 27 nM (n = 29) in adult and 25 +/- 10 nM (n = 23) in old neurons). We conclude that neuronal aging is associated with prominent changes in the mechanisms responsible for [Ca2+]i regulation. These changes presumably include lowering of voltage-gated plasmalemmal Ca2+ influx and slowing down of Ca2+ extrusion from the cytoplasm.

Aging of neurons in the mollusc Lymnaea stagnalis small parietal ganglion: a morpho-functional comparison in the same neuron

The aim of this study was to compare the functional and structural changes in similarly identified neurons of the small parietal ganglion in 56 molluscs (Lymnaea stagnalis) of two age groups: adult (10-12 months) and old (20-22 months). No age changes were found in the values of membrane potential, resistance of the neuronal membrane, amplitude, duration, or rate of increase of the anterior action potential front. With aging, the thresholds of direct stimulation were significantly increased, the rate of action potential repolarization decreased, and the amplitude of trace hyperpolarization decreased. The most marked age-dependent changes were observed in the frequency of neuronal spontaneous activity. A clear relationship was established between the frequency of action potentials of the neuron and its structure in adult and old individuals alike. In the molluscs of both age groups, the neurons with a high frequency of action potential displayed ultrastructural features of high activity in the organelles involved in protein biosynthesis. The cytoplasm of these neurons was filled with numerous ribosomes and had a well-developed rough endoplasmic reticulum. The structure of cells with low spontaneous activity in old molluscs differed considerably from that of the corresponding neurons of the adult individuals. The former had significantly marked morphological signs of reduction of the protein-synthesizing processes, as well as of destructive and dystrophic changes. A decrease in the lability of neurons may be an important mechanism of aging.

Calcium currents in aged rat dorsal root ganglion neurones

1. The whole-cell voltage clamp technique was used to record calcium currents in the somatic membrane of rat cultured dorsal root ganglion neurones. 2. Neurones were enzymatically isolated from animals of three age groups (neonatal, 2-7 days; adult, 7 months; and old, 30 months) and maintained in primary culture 3-14 days. 3. The neurones isolated from neonatal and old rats showed two distinct types of Ca2+ currents, a low-threshold transient current and a high-threshold sustained current, whereas neurones from old rats showed only a high-threshold calcium current. 4. The density of the high-threshold calcium current was 28.4 +/- 6.3 pA/pF (mean +/- S.E.M., n = 54) in neonatal, 39.1 +/- 7.2 pA/pF (n = 62) in adult and 11.0 +/- 4.6 pA/pF (n = 64) in old dorsal root ganglion neurones. 5. We found no difference in elementary high-threshold Ca2+ current characteristics in neurones from different age groups. The single-channel conductance was (with 60 mM Ca2+ in the recording pipette) 16.0 +/- 2.7 pS (mean +/- S.E.M., n = 9) in neonatal, 16.2 +/- 1.7 pS (n = 11) in adult and 16.4 +/- 1.2 pS (n = 12) in old neurones. 6. Current-voltage relations and kinetics of high-threshold calcium currents showed no detectable age-dependent difference. 7. The run-down of high-threshold calcium currents in dorsal root ganglion neurones from old rats was practically insensitive to intracellular administration of cyclic AMP and ATP. The same intervention caused a significant deceleration of Ca2+ current run-down in the majority of neonatal and in some adult cells. 8. We suggest that the disappearance of the low-threshold calcium current and reduction of high-threshold calcium current with ageing is due to a depression of calcium channel expression during late ontogenesis. The decrease of sensitivity of high-threshold calcium channels to phosphorylation by cyclic AMP-dependent protein kinase in aged neurones could also be a reason for altered turnover between silent and functional pools of calcium channels, which may underlie the age-dependent decline in the density of high-threshold calcium channels.