Saleem M. Nicola - US grants

The proposed research seeks to determine the contribution of dopamine to the firing of neurons in the rat nucleus accumbens while the aimal self-administers cocaine. Dopamine. Psychostimulants, dopaine agonists and dopamine antagonists will be iontoporetically applied to recorded accumbens cell during the performance of a lever-pressing task that results in the delivery of a cocaine reward to the animal. Locailized iontophoretic application of drugs allows for their effects on individual neurons to be observed while the behavior of the the animals continues unchanges by the drug. The effects of these drugs on the firing of these cells during the lever-press and between repsonse will be analyzed to determine what role dopamine plays in the emergence, timing and shing of patterns of firing that are specifically associated with the lever-press. The results of these studies will indicate how dopamine modifies information processing in the accumbers, and will asuggest mechanisms by hopaminergic modulation of accumberns cell firing leads to the motivation to obtain the drug.

Project Summary When addicts encounter conditioned stimuli (CSs) associated with drugs, they often find themselves engaging in drug-seeking behavior, even after a period of abstinence. Understanding how CSs become able to promote drug- and reward-seeking behavior is thus of primary importance to understanding addiction and relapse. In this proposal, we focus on how conditioned approach behavior ? the conditioned locomotor response to the CS that often brings the subject closer to the predicted reward ? is learned. We will test three related hypotheses. First, previous studies show that NMDA receptor activation in the nucleus accumbens (NAc) is required for ac- quisition of the conditioned approach response, suggesting that synaptic plasticity within the NAc is responsible for this learning. Studies from our laboratory have shown that in animals that have already learned the response, many NAc neurons are excited by CSs, that the excitations precede approach movement initiation and predict its latency, and that they are causal to approach. Therefore, we hypothesize that synaptic plasticity within the NAc gives rise to CS-evoked excitations during task acquisition, and that this mechanism is causal to learning. Second, the NAc receives a prominent excitatory projection from the basolateral amygdala (BLA), which, in trained animals, is required for both conditioned approach behavior and for the CS-evoked excitations of NAc neurons. Previous studies suggest that many BLA neurons encode sensory salience in that they fire in response to prominent stimuli even before the animal has learned their predictive value. NAc neurons, on the other hand, do not begin to fire in response to CSs until the subject begins to learn the approach response to the CS. Therefore, we hypothesize that synaptic plasticity at the BLA-NAc synapse is required for emergence of the CS- evoked excitations of NAc neurons, and that this mechanism is causal to learning. Finally, NAc CS-evoked excitations also require dopamine, which is provided by the projection from the ventral tegmental area (VTA). Stimulation of dopamine neurons is strongly reinforcing, possibly because it facilitates the formation and maintenance of associations between stimuli and positive outcomes. We hypothesize that NAc cue-evoked excitations are a product of dopamine neuron-mediated plasticity, and that therefore stimulation of dopamine neurons is sufficient to maintain cue-evoked excitations (and hence cue-evoked approach behavior) on subsequent encounters with the cue. We will test these hypotheses with a unique and powerful combination of cutting-edge techniques. We will record the unit firing activity of neurons in the NAc and BLA throughout task acquisition learning, while simultaneously injecting an NMDA antagonist into the same structure from which we record. We will also use this method in combination with chemogenetic silencing of BLA terminals in the NAc by local microinjection of a DREADD agonist into the NAc. Finally, we will use optogenetic control over VTA dopamine neurons to examine how the VTA-NAc dopamine projection impacts the NAc neuronal activity that underlies learning.

DESCRIPTION (provided by applicant): Bulimia nervosa and binge eating disorder (binge eating disorders) are serious public health problems, in part because people with these disorders tend also to suffer other medical complications, such as obesity and depression. Recently, animal models of binge eating have been developed, in which rats are provided intermittent access to sweet and/or high fat food. As consumption of this food escalates over several weeks of access, neurochemical changes in the nucleus accumbens (NAc) are observed, including increases in expression of opioid receptors. Pilot experiments show that injection of a broad-spectrum opioid receptor antagonist into the NAc has more pronounced effects on sweet/fat liquid consumption in binge eating than in control rats. These results suggest that binge eating could be due, at least in part, to upregulation of opioidergic neurotransmission in the NAc. To test this hypothesis, we will first determine which of three opioid receptors (mu, delta and kappa) in the NAc contributes to consumption of palatable liquid, and whether these contributions are different in binge eating and control rats. We will also determine whether these contributions are specific to the core or shell regions of the NAc. In addition, we will determine whether encoding of palatability by NAc neurons differs from controls in binge eating rats. Finally, we will test the hypotheses that endogenous opioids contribute to palatability encoding by NAc neurons, and that this contribution is different in binge eating vs control animals. Our goal is to elucidate the neural mechanisms that underlie binge eating, so that pharmaceutical treatments for binge eating disorders can be developed that specifically target these mechanisms. Because the same neural circuits are involved in drug addiction, our studies will also contribute towards understanding the neural mechanisms of addiction, and of reward-seeking behavior in general. PUBLIC HEALTH RELEVANCE: Approximately 5% of the American population suffers, during at least part of adult life, from binge eating disorder or bulimia nervosa (binge eating disorders); these are serious public health problems because of the associated medical complications such as depression and obesity. The research proposed here will use an animal model of binge eating to help us understand how brain circuits that are critical for reward-seeking and feeding control these behaviors, and how circuit activity changes to produce binge eating. Identifying specific neural mechanisms responsible for binge eating will potentially stimulate the development of pharmacological interventions that specifically target these mechanisms to treat binge eating disorders.

DESCRIPTION (provided by applicant): Impulsivity is a tendency to respond immediately and rashly to reward-associated stimuli and, as a consequence, forego potentially greater reward that may be available after consideration of alternatives. People with impulsive personalities are at greater risk for developing problems with drug abuse, and impulsive behavior in addicts may contribute both to drug-seeking behavior and to poor financial, personal and other life choices. Understanding the neural mechanisms that drive impulsive behavior is therefore highly relevant to the goal of understanding addiction at a neural level, and to developing interventions that help addicts resist responding to drug-related stimuli and make better choices. Unfortunately, different forms of impulsivity are the result of dif- ferent behavioral and neural processes, and n single animal behavioral model captures all facets of impul- sivity. Therefore, unique forms of impulsivity must be studied individually to discover their neural mechanisms. Although impulsive action (premature responses in tasks that require waiting before responding) and heightened delay discounting (choosing a smaller immediate reward rather than a larger delayed reward) are well-studied, one form of impulsivity that has received little attention is spatial discounting choosing a smaller more proximate reward at the expense of larger reward available at greater physical distance. Recent studies (e.g., Howe et al, Nature 500:575, 2013) indicate that the mesostriatal and mesolimbic dopamine signals increase with proximity to a movement target associated with reward, and neurons in the nucleus accumbens are more strongly activated by cues that predict reward when the animal is in close proximity to the reward- associated movement target than when the animal is farther away (McGinty et al, Neuron 78:910, 2013). These results suggest that the classically-defined reward system (including the nucleus accumbens and its dopamine input) contribute to spatial discounting; however, the nature of this contribution remains unknown. The proposed experiments use a novel decision-making task for rats to investigate the neural mechanisms underlying the form of impulsivity defined by steep spatial discounting. In this task, the subject's proximity to a reward-associated lever varies across trials. Pilot data show that the likelihood of an impulsive choice of a lever that delivers suboptimal outcome (small reward) increases with greater proximity to the lever. Moreover, in this task neurons in the nucleus accumbens encode proximity independently of expected reward magnitude, suggesting that their firing promotes proximity-driven impulsivity by a mechanism independent of expected outcome evaluation. The proposed experiments explore this mechanism in more detail. They take advantage of a unique approach - recording the unit activity of accumbens neurons in behaving animals while infusing dopamine antagonists into the same structure - to assess whether the observed neural encoding is causal to proximity-driven impulsivity. The results will describe, in unprecedented detail, an important contribution of dopamine and the nucleus accumbens to decision-making and impulsivity.

? DESCRIPTION (provided by applicant): Electrophysiological recording in awake animals allows neuroscientists to measure the neural circuit activity related to sensory or motor events. The information obtained from this approach is essential for analyzing how neural circuits produce behaviors related to drug addiction, including drug- and reward-seeking. However, with conventional electrophysiological methods, the data obtained in this way is purely correlative. In addition, merely recording neural activity is insufficient to determine how information encoded by neurons in one part of the circuit influences the representation of information by neurons in a downstream node - an important goal of neural circuit analysis. Here, we propose a method for manipulating neural transmission in a temporally and pharmacologically specific fashion, allowing the experimenter to establish both how upstream neurons contribute to recorded neural activity and how the activity causes a particular behavioral event. The proposed method will allow us to apply caged neurotransmitter agonists and antagonists to neurons whose activity we record in awake, behaving animals. Caged compounds are biologically active molecules that are rendered ineffective by a covalently attached chromophore. Application of light of the appropriate wavelength causes the bond to be broken, releasing the active compound. This method is widely used in electrophysiological studies in vitro, but no study to date has reported its application in vivo in awake behaving mammals. To apply this method in awake rats, we will take advantage of our expertise with application of drugs to neurons being recorded in awake animals. We will modify this approach by including light delivery into the brain via fiber optics, allowing us to uncage compounds introduced to the tissue via local perfusion. The proposed experiments will develop and test apparatus capable of such experiments, and demonstrate the effectiveness of the technique by determining how a glutamate receptor agonist influences behaviorally- relevant firing in the nucleus accumbens.