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1.  Aging in Sensory and Motor Neurons Results in Learning Failure in Aplysia californica 
PLoS ONE  2015;10(5):e0127056.
The physiological and molecular mechanisms of age-related memory loss are complicated by the complexity of vertebrate nervous systems. This study takes advantage of a simple neural model to investigate nervous system aging, focusing on changes in learning and memory in the form of behavioral sensitization in vivo and synaptic facilitation in vitro. The effect of aging on the tail withdrawal reflex (TWR) was studied in Aplysia californica at maturity and late in the annual lifecycle. We found that short-term sensitization in TWR was absent in aged Aplysia. This implied that the neuronal machinery governing nonassociative learning was compromised during aging. Synaptic plasticity in the form of short-term facilitation between tail sensory and motor neurons decreased during aging whether the sensitizing stimulus was tail shock or the heterosynaptic modulator serotonin (5-HT). Together, these results suggest that the cellular mechanisms governing behavioral sensitization are compromised during aging, thereby nearly eliminating sensitization in aged Aplysia.
PMCID: PMC4430239  PMID: 25970633
2.  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.
PMCID: PMC4023074  PMID: 24847260
L-glutamate; NMDA; D-aspartate; pleural ganglion; buccal ganglion
3.  Pharmacological evidence that D-aspartate activates a current distinct from ionotropic glutamate receptor currents in Aplysia californica 
Brain and Behavior  2012;2(4):391-401.
D-Aspartate (D-Asp) activates a nonspecific cation current of unknown identity independent of L-glutamate (L-Glu) in neurons of Aplysia californica. Whole-cell voltage clamp studies were conducted using primary cultures of Aplysia buccal S cluster (BSC) neurons to characterize these receptor channels pharmacologically. The N-methyl-D-aspartate receptor (NMDAR) coagonist glycine potentiated D-Asp currents only at −30 mV, while D-serine did not potentiate D-Asp currents at any amplitude. Portions of D-Asp currents were blocked by the L-Glu antagonists kynurenate, DL-2-amino-5-phosphonopentanoic acid (APV), (2S,3R)-1-(phenanthren-2-carbonyl)piperazine-2,3-dicarboxylic acid (PPDA), and 1,3-dihydro-5-[3-[4-(phenylmethyl)-1–2H-benzimidazol-2-one (TCS46b), suggesting that L-Glu channels, particularly NMDAR-like channels, may partially contribute to D-Asp whole-cell currents. In contrast, L-Glu currents were unaffected by APV, and showed greater block by kynurenate, suggesting that D-Asp and L-Glu act, in part, at different sites. The excitatory amino acid transport blocker DL-threo-b-Benzyloxyaspartic acid (TBOA) blocked a fraction of D-Asp currents, suggesting that currents associated with these transporters also contribute. Non-NMDA L-GluR antagonists that preferentially block alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate receptors significantly increased D-Asp currents, suggesting a possible allosteric potentiating effect of these antagonists on D-Asp receptors. L-Glu-induced currents were significantly reduced in the presence of bath-applied D-Asp, whereas bath-applied L-Glu had no effect on D-Asp-induced currents. The mixed effects of these agents on D-Asp-induced currents in Aplysia illustrate that the underlying channels are not uniformly characteristic of any known agonist associated channel type.
PMCID: PMC3432962  PMID: 22950043
APV; buccal ganglion; coagonist; electrophysiology; mollusk; NMDA

Results 1-3 (3)