We want to examine the relationship between the cellular and molecular mechanisms underlying short-term (minutes to hours) and long-term changes (days or weeks) in synaptic efficacy underlying simple forms of learning. In our studies, we will use the gill and siphon withdrawal reflex of the marine mollusk Aplysia. This reflex can be facilitated (sensitized) or depressed (habituated) for short or long periods of time according to the training protocols. Many of the neurons which mediate it have been identified and some critical sites of physiological changes are known. Finally, this reflex can be studied in a variety of experimental preparations, from intact animals to dissociated cell cultures; it is possible in this system to carry out electrophysiological, pharmacological, biochemical and molecular studies at the level of single identified neurons. In this proposal, we will focus on two areas: 1-Studies of short-term physiological changes at the polysynaptic component of the reflex. We will examine the role of recurrent inhibition and its cholinergic component, the multiple sites of change in the neuronal network of the gill and siphon withdrawal reflex during sensitization and the role excitatory amino acids play in the neurotransmission of the network. 2-Studies of the long-term physiological changes at the polysynaptic component of the reflex. We will focus our investigation on the contribution of excitatory and inhibitory interneurons and the modulatory interneurons. Many previous studies, including our own, have focused on the monosynaptic component of the reflex. The unique contribution of our program is to pay special attention to the polysynaptic component of the reflex, the parallel processing and the many sites of modulation this model system has. In addition, our recent studies have opened up two novel directions of research of great theoretical interest: the existence of branch specific heterosynaptic facilitation and the role of acetylcholine and recurrent inhibition during learning. The advantages of this invertebrate model should help us understand general mechanisms mediating short and longer lasting alterations evoked by environmental changes and learning or how memories are encoded, stored and retrieved. In addition, this research program will permit us to examine how many sites in a neuronal circuit can change in parallel and are involved in higher forms of learning. This could also help us understand how neuromodulators of learning and memory are acting in more complex nervous systems and what may happen during various types of memory impairments.
