Rafiq Huda - US grants

DESCRIPTION (provided by applicant): Midbrain dopamine neurons display phasic responses to rewards and cues that predict rewards. During associative learning, when a sensory stimulus (such as a light flash or auditory tone) is paired with a reward, with training th sensory stimulus comes to predict the reward. This raises an interesting question. Since an animal receives a vast amount of sensory data at any given moment, what distinguishes the sensory information associated with a reward from other incoming information? One possibility is that neuromodulatory reward signals encoded by dopamine alter the representation of reward-predicting stimuli in sensory cortices (for instance, by preferentially enhancing the gain of neuronal responses elicited by reward-predicting stimuli), so that they can be differentiated from other sensory inputs. The cellular and circuit properties of the primary visual cortex (V1) have been extensively characterized; hence, it is an ideal model system to test the hypothesis that dopaminergic signaling modulates the gain of sensory information. In Aim 1, channelrhodopsin-2 (ChR2) will be selectively expressed in dopamine neurons of the ventral tegmental area (VTA), which provides dopaminergic projections to the cortex. Fast-scan cyclic voltammetry (FSCV) will be used to establish the laminar profile of light-evoked dopamine release in V1. To test if dopamine signaling modulates the response properties of visually evoked activity, in-vivo single-unit and two-photon targeted cell-attached recordings from specific cell-types will be made. Neuronal responses to visual stimuli consisting of gratings at specific orientations will be recorded. In each animal, a randomly chosen orientation will be conditioned by pairing with optogenetic activation of dopaminergic fibers in V1 or of the ChR2- expressing cell bodies of dopamine neurons in the VTA. This experiment will establish whether pairing sensory input with local (optogenetic stimulation of V1) or global (VTA stimulation) dopaminergic signaling can modify the representation of visual stimuli, and how specific cell-types contribute to this modulation. After establishing the role of dopaminergic signaling in-vivo in Aim 2 mechanistic studies will be performed in-vitro. Different cell-types, especially molecularly-defined subtypes of inhibitory cells, make specific contributions to sensory processing. Hence, dopamine may preferentially recruit specific cell-types to exert its effects. In order to understand the cellular mechanisms of dopamine action, how dopamine impacts the intrinsic and synaptic properties of excitatory and subtypes of inhibitory cells will be determined Together, these studies will test the hypothesis that dopaminergic signals modify the encoding of sensory stimuli in V1 and will provide a solid groundwork for future work to investigate the role of dopamine signaling in the visual cortex during associative learning.

My long term career goal is to establish an independent research program aimed at leveraging cutting-edge optical technologies available in mice to study neural circuits underlying cognitive functions. To facilitate this goal, I have received training in a variety of techniques including cellular neurophysiology, functional two-photon microscopy, and viral-based circuit tracing. During the mentored phase of this award, I will work under the supervision of Professor Mriganka Sur, a pioneer in optical methods for interrogation of neural circuits, with advice from a mentoring team consisting of Professors Ann Graybiel, Kay Tye, and Wasim Malik. Additional training in behavioral task design, projection-specific optogenetic manipulations, and computational methods for data analysis from these mentors will equip me with the tools necessary to probe neural underpinnings of cognitive functions and springboard me to an independent career. This work will be done at the Brain and Cognitive Sciences department at MIT, which offers both expansive infrastructural resources and a vibrant intellectual community necessary for making fundamental discoveries on the circuit-basis of behavior. During my NRSA-sponsored postdoctoral training, I identified a crucial role for visual cortical inputs to the anterior cingulate subdivision of the prefrontal cortex (PFC) in visual decision making. My immediate goal is to ascertain exactly which cognitive functions are supported by this area and the underlying circuit-level mechanisms. While the PFC has been widely implicated in guiding attention and motor planning, it is unclear if the same or distinct neural substrates underlie these functions. According to the ?pre-motor theory of attention?, attention is an emergent property of networks that implement actions and, hence, the same set of neurons contribute to both attention and motor planning. However, recent evidence suggests that these functions are served by distinct cell-types. In this K99 application, I will test the hypothesis that distinct PFC cell-types target either the visual cortex or superior colliculus, a midbrain structure that coordinates motor behavior, to guide attentional modulation of sensory processing or motor planning, respectively. In Aim 1, I will use optogenetic inactivation to test the contribution of visual cortex, superior colliculus, or PFC to performance on a novel two-choice visual task with specific temporal epochs for attentional engagement or motor planning. (Aim 2a) Next, I will use projection-specific optogenetic inactivation to test the hypothesis that PFC cell- types that project to visual cortex contribute to attentional processing of visual stimuli, whereas cells projecting to superior colliculus contribute to motor planning. (Aim 2b) Using two-photon microscopy, I will measure the neural signatures of attentional engagement and motor planning in these two cell-types. (Aim 3) In the independent phase of the award, I will use a viral-based disynaptic tracing strategy to identify the sources of inputs onto specific projection cell-types in the PFC. Using dual-color two-photon microscopy and optogenetic inhibition, I will determine how long-range input axons contribute to neural coding of task variables by specific PFC projection cell-types Together, these studies will clarify circuit-level mechanisms of PFC contributions to attention and motor planning.