My laboratory is interested in understanding the cellular and molecular mechanisms that underlie synaptic changes during learning. A major goal is to elucidate molecular mechanisms of associative integration of behavioral stimuli that initiate produce of synaptic connections during associative learning. We study this issue in a simple model system: classical conditioning of the defensive withdrawal reflex of the marine snail Aplysia. One mechanism that contributes to associative increases in synaptic strength is associative activation of a dually-regulated enzyme: the Ca²⁺ calmodulin-sensitive adenylyl cyclase. Working with Eric Kandel, I found that during conditioning, Ca²⁺calmodulin-sensitive adenylyl cyclase provides a site of associative stimulus convergence for two cellular representations of behavioral events: Ca²⁺ influx and modulatory neurotransmitter. The resulting increase in cAMP levels, which results from convergent activation of adenylyl cyclase, initiates short-term and long-term facilitation of the synapses from the sensory neurons that are activated by the conditioned stimulus. In biochemical studies, we have asked whether the activation properties of this enzyme can account for some of the key features of conditioning. For example in most forms of conditioning, animals learn the predictive relationship between the conditioned and unconditioned stimuli only if the conditioned stimulus is presented shortly before the unconditioned stimulus during training; pairing in the backward direction is relatively ineffective. Previously, my laboratory demonstrated that neural adenylyl cyclase displays a sequence preference that parallels that of the behavior: activation is more effective if Ca²⁺ (the signal from the conditioned stimulus) precedes modulatory neurotransmitter (the signal from the unconditioned stimulus). We recently found that this mechanism involves a temporal shift in the response to a transient Ca²⁺ stimulus so that the enzyme is activated with a delay, with most of the activation occurring after the stimulus has ended; this delay causes the Ca²⁺ response to coincide with the arrival of modulatory transmitter released by the unconditioned stimulus. We are using recombinant mammalian adenylyl cyclase to investigate the kinetic basis for this delayed activation by Ca²⁺CaM. By altering expression of specific isoforms of adenylyl cyclase, we are determining what role the dually-regulated adenylyl cyclase plays in associative learning.

Another interest is the mechanism of synaptic plasticity that contributes to behavioral habituation. When these sensory neuron synapses are activated with single action potentials (as would occur with a recurring weak behavioral stimulus that produces habituation), these synapses rapidly undergo profound depression. We have found through parallel computer modeling and cellular experiments that this synaptic depression involves the abrupt switching off, or silencing, of release sites. Our current studies are devoted to identifying the cellular signaling cascade that mediates this switching off. We recently identified a novel mechanism of synaptic plasticity in which the pattern of presynaptic firing determines whether release sites are switched on or off. Whereas single action potentials in the sensory neurons result in rapid synaptic depression, the activation of these synapses with brief bursts of action potentials protects these synapses from depression. This burst-dependent protection against synaptic depression is mediated by protein kinase C, which is activated by the Ca²⁺ influx during the brief burst of activity. Burst-dependent protection may function as a sensitive switch to maintain responsiveness when sensory afferents are repeatedly activated by moderate intensity, behaviorally significant, stimuli that would otherwise produce habituation.
-- transient Ca²⁺ and Ca²⁺ calmodulin-K⁺ currents are modulated in the presynaptic sensory neurons; using a combined electrophysiological and computer simulation approach,