Role of the CaV2 channel in regulating transmitter release in Aplysia sensory neurons

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Neurons communicate through synaptic connections and these synapses underlie most brain function. At synapses, the presynaptic cell releases neurotransmitter though calcium-dependent release of synaptic vesicles that contain the neurotransmitters. The post-synaptic cell then responds through neurotransmitter-gated ion channels. Importantly, many synaptic connections are specialized for specific brain functions and control transmitter release in unique ways. The synaptic connections between the sensory neuron and the motor neuron that underlie the simple defensive withdrawal reflexes of the marine mollusc Aplysia californica have long been used to study the mechanisms underlying the control of synaptic transmission. The reflexes habituate with repeated activation through reducing release of the neurotransmitter-containing synaptic vesicles at the sensory-to-motor neuron synapses, termed homosynaptic depression. The sensory neurons must have specializations that mediate this depression, since most synapses do not decrease their release of synaptic vesicles under these conditions. A noxious stimulus to the animal results in the recovery from depression and behavioral dishabituation of the reflexes. The reduction in transmitter release during homosynaptic depression is known to involve changes in the efficacy of calcium to trigger the release of neurotransmitter: a change in calcium-secretion coupling. However, the molecular mechanisms of both homosynaptic depression and how it is reversed remain unknown. Evidence from a wide variety of synapses suggests calcium-secretion coupling is regulated through interactions between the specific calcium channel, CaV2, the source of calcium that triggers neurotransmitter release, and the synaptic vesicle. In the present application we will examine whether different sensory neuron-specific modifications of CaV2 or of the proteins that link CaV2 to the synaptic vesicles underlie homosynaptic depression and its reversal. We will measure depression and its reversal at sensory-motor neuron synapses where the sensory neurons express a modified CaV2 that either lacks sensory-neuron specific splicing, or specific binding sites for proteins important for interacting with synaptic vesicles. We will also examine the effects of expressing proteins that link CaV2 to synaptic vesicles that are not normally expressed in sensory neurons. These experiments will identify why sensory neurons depress and how the reversal of depression is mediated. These results will determine how transmitter release is regulated at this specialized synapse and provide general answers to the question of how calcium-secretion coupling functions and how transmitter release can be specialized at specific synapses in order to optimize brain function. **