Development of plastic mechanisms related to learning at identified chemical synaptic connections in Aplysia.

In the marine snail Aplysia californica learned changes in behavior have been traced to alterations in synaptic efficacy. With the ability to raise the animals in the laboratory, we have explored the development of four types of plastic mechanisms at identified synapses: post-tetanic potentiation and pre-synaptic inhibition, which do not as yet have known behavioral functions, and homosynaptic depression, the cellular mechanism of short-term habituation, and heterosynaptic facilitation, the basis of short-term sensitization. Homosynaptic depression and pre-synaptic inhibition are present early in juvenile life. In contrast, post-tetanic potentiation and heterosynaptic facilitation appear only later, after a discrete interval. The step-wise ontogeny of synaptic plastic mechanisms in Aplysia parallels the gradual emergence of behavior in successive developmental stages. Interestingly, senescence reverses the developmental sequence for habituation and sensitization mechanisms. To pursue further an understanding of the relationship between synapse formation and plasticity underlying learning it will be necessary to extend these studies to dissociated cell culture where mechanisms can be explored on the molecular level. Cell culture may also permit examination of the development of cellular mechanisms underlying classical and operant conditioning which may clarify differences between associative and non-associative mechanisms.