MECHANISMS AND FUNCTIONS OF DENDRITIC DOPAMINE RELEASE IN THE BRAIN

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Neurons in the brain that use the chemical messenger dopamine (DA) play a key role in the control of movement, motivated behaviors and cognition. Although DA release from the neurons' nerve endings occurs through a form of regulated exocytosis, DA is also released from the cell body and dendrites of DA neurons (STD DA release) and very little is known about how this occurs and what roles it plays in brain function. This integrated research program, initiated 4 years ago, may lead to a better understanding of the roles of DA neurons. It will also characterize a mechanism that will help explain the dendritic release of other chemical messengers such as 5-HT. There is also a paucity of tools to detect the impact of the somatodendritic activity of DA neurons on the dynamics of the multiple neurotransmitters present in the ventral midbrain, where these neurons are located. During the last term of this grant, we have begun to develop new animal models to better characterize STD DA release and explore its mechanisms. In collaboration with a chemist colleague, we also established a new approach called optophysiology to examine neurotransmitter dynamics. The development of better tools and better mouse models in which STD DA release is perturbed will represent a major advance in the field and may lead to a better comprehension of the roles of functional and plasticity of DA neurons. We will address the following two aims: Aim #1: Identification of the mechanisms and roles of STD DA release. We will examine the properties of STD DA release in brain sections prepared from mice in which synaptotagmins (syts), proteins that are key calcium triggers of exocytosis, are genetically knocked out. We will examine in particular the roles of syt4 and syt7, which we have found to be present in the STD compartment of DA neurons, as well as syt1, present only in axon terminals. We hypothesize that individual constitutive KO of syt4 and syt7 will allow compensation, with only the double KO leading to impaired STD DA release. We further hypothesize that conditional ablation of syt1 from DA neurons will generate mice in which DA neurons only release DA from their STD compartment. Aim #2: Development and use of optophysiology to characterize neurotransmitter dynamics in the ventral midbrain in response to STD activity of DA neurons. We have teamed up with a chemist colleague to develop a novel approach allowing multiplex detection of neurotransmitters based on custom-designed gold nanoparticle-plated electrodes and surface-enhanced Raman spectroscopy. We propose to further develop this “optophysiology” technique and optimize it for work in brain slices and intact animals, to characterize how the firing of DA neurons and the STD release of DA affects the dynamics of multiple neurotransmitters in the ventral midbrain. We will test the hypothesis that STD DA release regulates the secretion of neurotransmitters that are released from terminals expressing DA D1 and D2 receptors.