Aplysia

Scientific Importance

The marine mollusk Aplysia californica is an important animal for experiments in cellular, molecular and behavioral neuroscience because of the distinctive organization of its nervous system, which makes it appropriate for cellular analysis of a variety of behaviors, learning and memory. Aplysia presents a one-of-a-kind model for developmental and neurophysiological studies.

Image and diagram of the abdominal ganglion of Aplysia californica. The identified cells are labeled L or R (left or right hemiganglion) and assigned a number (diagram from Kupfermann et al (1974) Local, reflex, and central commands controlling gill and siphon movements in Aplysia. J Neurophysiol. 1974 37(5):996-1019). Click on diagram for larger version.

Neurons isolated from the pleural ganglion of Aplysia californica and grown in cell culture. Such neurons can be used for studies of morphology, synapse formation, electrical properties (neurophysiology) and gene expression.

Aplysia has provided fundamental insights into the basic organization of neuronal functions.

For more than 50 years, the simple organism Aplysia has been the focus of study for a large segment of the neurobiology community. Aplysia's large neurons allow examination of neuronal architecture, physiology and control of instinctive and learned behaviors at the level of single characterized cells and defined signaling pathways.

The distinction of Aplysia as a neurobiological model system is due to the following:

Its nervous system has a relatively small number of neurons.
Many of these cells are large (sometimes gigantic, up to 1 mm in diameter).
As a result of the neurons' consistent size, pigmentation, and position in the nervous system, hundreds of neurons have been uniquely identified at the single cell level and have been reliably linked specific behaviors.
These neurons can be isolated and cultured in vitro and they form circuits, which can be explored at molecular and cellular detail.
The animal generates a variety of behaviors many of which can be specified in terms of their mediating circuitry.
Some of these behaviors can be modified by learning. As a result, one can have access to molecular mechanisms of basic neuronal functions and can study these mechanisms in real physiological time with single-neuron resolution. No other models today can provide similar single-neuron resolution and its coupling to molecular mechanisms.
The cells provide enough messenger RNA to conduct gene expression studies on single cells.

Significant breakthroughs in understanding of cellular and molecular mechanisms of memory formation have come from studies on Aplysia. In 2000, Dr. Eric Kandel won the Nobel Prize in Physiology or Medicine for his research in honw neurons are able to form and store memories (http://nobelprize.org/nobel_prizes/medicine/laureates/2000/). For this award-winning research, he used Aplysia from Rosenstiel School's National Resource for Aplysia along with neurons from mice. Other studies of Aplysia have led to drugs now in clinical trial aimed at reversing memory loss in patients with degenerative mental diseases.

Aplysia provide a highly tractable paradigm to understand how enormous neuronal diversity is generated in the nervous system (perhaps more than 2,000 different cell types in the mammalian brain) and how it is related to behavioral functions. Although the complexity of the brain appears to be overwhelming, many major fundamental questions can be addressed at the level of single neurons. These problems are more evident when viewed through an Aplysia's approximately 20,000 central nerve cells.

Aplysia is an emerging model for developmental biology.

Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and morphogenesis, which is the process that gives rise to tissues, organs and anatomy. The diverse life history strategies adopted by many species and the extreme body plan modifications, make mollusks an ideal group with which to investigate mechanisms of development and evolution. For example, one of the most remarkable and dramatic steps taken during the course of mollusk evolution was the advent of gastropod torsion, a unique reorganization of organ positions associated with the loss of some organs and reorganization and varying degrees of centralization of the nervous system. Genomic information from Aplysia could help explain how gene expression controls major events in spiralian development, torsion-detorsion processes, neurogenesis, network organization and maturation.

Aplysia as a model for comparative and functional genomics.

Aplysia californica are representative of Mollusks, the second largest animal phylum with more than 100,000 extant marine, freshwater and land species that trace their origin to the Cambrian period. Because of their shell and chitinous radula (a major part of the feeding apparatus) this phylum has one of the best paleontological records. More than 500 million years of their evolutionary history has been reconstructed.

For example, the origin of the opisthobranch mollusks can be traced back to the late Paleozoic era (ca. 350 Mya during the Carboniferous period). The suspected sister group to anaspideans (sea hare and Aplysia-like ancestors) dates back to the Jurassic period (ca. 200 Mya). Akera, the most primitive sea hare, dates back to ca. 165 Mya and the first Aplysia-like records only appear in the Miocene era (ca. 25 Mya). Comparative genomics studies of such model organisms are of great importance to advance our understanding of the general mechanisms of evolution. Combination of genomic, behavioral and neurophysiological approaches using Aplysia should also allow study of how changes in gene expression in specific cells affect neural function and even behavior.

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http://nobelprize.org/nobel_prizes/medicine/laureates/2000/kandel-autobio.html