224 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”(see Sec. 10.8 below with respect to vertebrates), resulting in the activa-
tion of the post-synaptic receptors and the growth of the dendritic spines.
The net outcome is facilitation of transmission across the synapse.^12
A newly proposed mechanism of Aplysia learning involves prion
proteins. Prions are a type of proteins that exists in two conformations,
the active and the inactive, the former being able to activate the latter
in a self-sustained manner. A neuronal protein of the CPEB family that
facilitates protein synthesis has been found to have prion-like proper-
ties. The conversion of CPEB in the synapses to the active form follow-
ing learning is believed to contribute to memory storage by maintaining
long-term synaptic changes. Once CPEB in a given synapse is activated,
new proteins continue to be made and stored in a synapse-specific man-
ner, providing a mechanism for “tagging” of memory trace to those syn-
apses that have experienced learning.^13 Prion-like proteins have also
been associated with long-term human memory.^14
Another invertebrate model from which useful data on memory
can be obtained is the fruit fly Drosophila melanogaster. In this animal,
associative learning between odor and shock persists for 24 hours.^15 The
process of memory consolidation can be tracked down to protein synthe-
sis that occurs in two specific neurons.^16
The roundworm Caenorhabditis elegans serves as another use-
ful model for invertebrate memory, as it is capable of non-associative
and associative learning. It undergoes habituation to mechanical and
chemical stimuli, and can learn from smell, taste, temperature, and
oxygen-level cues to the benefit of its survival. Specific genes underlying
cellular and molecular mechanisms of learning have been identified.^17
10.8 Cellular and Molecular Basis of Memory: Lessons
From Vertebrates
In the vertebrates, much work has been carried out on the hippo-
campus of the rodent brain, both in live animals and in brain slices.
One of the most remarkable observations that correlate with memory is