Donald B. Katz - US grants

The proposed research represents an initial attempt to characterize interactions between the cerebellar cortex (CTX) and deep interpositus nucleus (INP) during rabbit eyeblink conditioning. Studies 1 and 2 will investigate whether CTX activity can influence the ability to make and learn conditioned responses. Rabbits will receive brief intracortical pulses of stimulation at different time points within paired conditioning trials, and the impact of these pulses will be analyzed. Study 1 will involve a within-subjects design, in which timing of stimulation will vary randomly between trials; analyses will reveal any stimulation timings that interfere with CR expression. Study 2 will use a between- subjects design, in which one group of rabbits will consistently receive stimulation shown to interfere with CR expression; analyses will reveal if particularly-timed CTX stimulation interferes with learning. Study 3 will examine whether or not INP-independent learning-related neural activity is seen in CTX. Trained rabbits will receive either neurotoxic INP lesions or sham lesions, after which single-unit activity from CTX cells will be recorded during additional training. Analysis will reveal whether CTX can sustain learning-related changes without the presence of the INP (and, thus, without CRs). Overall, these studies should add to our understanding of how parts of the cerebellar learning system interact to produce well-timed CRs.

As a rat feeds, information about the food on the tongue both its taste and its texture is transmitted to populations of multimodal neurons in the gustatory cortex (GC), amygdala (AM), and elsewhere. Little is known about how this information is processed and interpreted, but recent findings suggest that somatosensory, chemosensory, and hedonic sources of GC activity can be pulled out of single-neuron responses via examination of the responses' temporal and spectral properties. The proposed research will extend this basic understanding of the mechanisms of gustatory response in two ways: 1) we will analyze when and how AM activity, thought to be involved in the processing of palatability, influences GC responses; and 2) we will examine how the various influences on GC activity change with associative learning. Chronic recordings will be taken from small ensembles of GC and AM neurons in rats trained to accept controlled doses of tastes ranging from bitter to sweet. It is expected that certain facets of GC activity, identified via time-series analysis, will be attributable to input from the ipsilateral AM; these facets will characterized with regard to their relationship to the palatability of the administered taste. Furthermore, the relationship between AM and GC activity is expected to change with induction of an aversion to a formerly preferred taste. The sources and behavioral relevance of these patterns will be examined by temporarily inactivating AM during stimulus exposure, which should inhibit both learning and any learning-related patterns expected during post-training trials. This project will expand our understanding of gustatory (and general perceptual) processing in new directions.

DESCRIPTION (provided by applicant): Little is known about what happens as animals learn to associate meaning with taste, although it is known that gustatory cortex (GC) and amygdala (AMG) are involved in the process. The goal of these experiments is to record simultaneously from these areas as rats learn to associate illness with a taste that they formerly thought palatable (this kind of learning is called conditioned taste aversion, or CTA). Ensembles of GC and AMG single neurons will be recorded through the entire learning process, as the rats progress from being naive to trained. The specific aims of the project are: 1) to determine the aspects of temporal codes in both brain areas that change during learning, as well as how interactions between the areas change; 2) to examine whether GC and AMG are specifically involved in learned reflexes that are produced as the rats try to expel the newly noxious taste; 3) to test whether the inactivation of AMG during training keeps these changes from appearing during testing; and 4) to test whether the development of plasticity in AMG is a specific part of these changes in temporal coding. Overall, this project will shed light on the neural mechanisms underlying an experiential phenomenon that affects (sometimes adversely) the lives of all mammals, including humans.

DESCRIPTION (provided by applicant): The neural system underlying taste is, like so many other neural systems, highly complex: the two parallel pathways that carry taste information to the cortex, one of which ascends via thalamus and one via amygdala, cross and feedback at several levels. It is reasonable to hypothesize, therefore, that taste perception and learning may rely on such neural interactions-interactions that, if real, should cause time- varying taste responses that are coupled between regions and covariant with taste acceptance and rejection. In fact, our recent work has revealed just these sorts of taste response dynamics linking basolateral amygdala (BLA) and gustatory cortex (GC), reciprocally connected parts of the forebrain taste circuit. The experiments proposed here will provide the first direct tests of whether such dynamics are a vital part of normal perception. We will use site-specific pharmacology, manipulations of learning, and electrical brain stimulation to perturb BLA-GC functional connectivity, and test whether each manipulation affects the putative temporal codes appropriately, and in lock-step with perception. We will first test whether the presence of hedonic information in the time-course of GC taste responses depends upon amygdala-cortical connectivity by knocking out BLA (using infusions of the GABA agonist muscimol) in awake rats as they (the rats) respond to naturally palatable and aversive tastes. We will then use similar infusions to test whether BLA is important for temporally-specific changes to these GC taste response properties wrought by conditioned taste aversion (CTA), a paradigm whereby rats learn that a taste once thought palatable is in fact noxious. Next, we will examine whether plasticity at amygdala- cortical synapses are the source of learning-related changes in taste dynamics, by infusing a compound that inhibits synaptic plasticity (H-7) just after training sessions. Finally, we will test whether the results of the first 3 experiments occur in variants of the CTA paradigm that do not require BLA, in order to test not only the existence of amygdala-cortical cooperation but its relevance to the specifics of what is learned. Together, these experiments will directly test a uniquely systems-level view of perception and learning, in the process revealing the neural mechanisms of an experiential phenomenon that affects (sometimes adversely) all mammals including humans. PUBLIC HEALTH RELEVANCE: This project investigates the nature of naive and learned emotional responses to tastes-disgust responses generated by a neural system similar to the one underlying fear responses. While most research focuses on describing the role on each individual part (in this case, cortex and amygdala) of such systems, the experiments proposed here will bring a battery of cutting-edge technology to bear on explaining how the interconnected networks of neurons work together to make perception and learning happen. As such, it provides an unprecedentedly sophisticated view of how mammals learn that a stimulus should be thought of as aversive;the knowledge generated by this work bears on a variety of human aversions to food, including those induced by cancer medications, as well as on more general processes relating to normal and abnormal disgust responses.

PROJECT SUMMARY The investigators will combine experimental studies with computational simulations to uncover the neural circuit mechanisms underlying taste choice behavior. A fundamental question to be addressed is how the neural activity induced by a given taste is impacted by the recent sampling of another taste and/or by the expectation that another taste is forthcoming. We will analyse data produced from ensemble recordings of neural activity and make use of optogenetic techniques that alter neural activity, to further our work on how specific features of such activity impact behavior during a decision making task. These data will provide fuel to our modeling efforts, with which we will produce a framework that can generalize to other sensory modalities, whenever decisions are based on a consideration of one stimulus at a time (even if two stimuli are present) rather than on a parallel processing of two or more stimuli simultaneously?that is, this novel framework will allow us to understand the behavior of animals when faced with a choice between any two separate stimuli. Thus, a transformative aspect of the research will be the development of a new framework for the analysis of decision-making when the choice to be made is whether to stick with the current stimulus or to switch to another. In such decision making tasks, which are ubiquitous in natural settings, the stimulus itself is chosen by the subject rather than being controled by the experimenter. Our recordings of the animal's behavior simultaneously with its neural activity allow us to tightly constrain these first dynamic models of such a process. Taste is an important, albeit underused, modality for addressing issues of sensory processing, in that it has a strong connection to behavior?indeed it is well-nigh impossible for an animal to sample a taste without a behavioral response. Moreover, given that tastants can be intrinisically hedonic or aversive, animals are internally driven to make responses and will do so without the months of training?which inevitably rewires the brain?required for animal training in behavioral tasks based on other sensory modalities. Therefore taste is ideally suited to the study of naturalistic decision making.