The gustatory system processes tastants according to specific sensory and hedonic categories. In nave animals, gustatory stimuli are represented for the taste quality they evoke ? i.e., sweet, salty, umami, bitter, sour ? and for their hedonic value (palatable or aversive). However, gustatory circuits are highly plastic and representations can be modified by learning. The best example of gustatory plasticity comes from studies on conditioned taste aversion (CTA) - a learning paradigm in which an innately palatable tastant becomes aversive once paired with gastric illness. Electrophysiological recordings and imaging experiments demonstrate that CTA leads to a persistent remapping of cortical representations of taste. While these studies have been instrumental in demonstrating that the gustatory cortex (GC) can reshape how it represents taste, they do not clarify whether persistent remapping is specific to aversive learning and to the hedonic dimension, or if it can occur also for other forms of taste experience. The experiments in this grant will address this unanswered question by investigating whether reward-driven, taste-action associations can modify and remap sensory representations. To this purpose, we modified a classic two-alternative choice procedure. In this task, mice sample one out of four tastants (two sweets: sucrose [S1] and maltose [S2]; two bitters: quinine [B1] and cycloheximide [B2]) from a central licking spout and respond by licking one of two lateral spouts. In the main version of this task, mice will produce the same response to incongruent pairs of tastants (i.e., sucrose or quinine -> lick lateral spout 1 vs maltose or cycloheximide -> lick lateral spout 2). This version of the paradigm requires mice to ignore innate taste similarities and produce i) similar responses for tastants with different qualities and opposite hedonics; ii) different responses for tastants with similar qualities and hedonics. A number of additional behavioral paradigms will be used as controls. The experiments in this grant will test the overarching hypothesis that GC allows mice to form taste-action associations and that this behavior is associated with task-related activity and plasticity of taste-evoked responses. The proposed research focuses on the following aims:
Aim #1 will develop a well-controlled behavioral paradigm for training mice to form new taste-action associations. In addition, the experiments will rely on chemogenetic and optogenetic inactivation of GC to determine its involvement in the performance of a taste- action association task.
Aim #2 will use electrophysiological methods to unveil plastic changes of single unit spiking activity in GC of alert mice learning and performing a taste-action association task. Waveform analyses will allow us to separate putative excitatory and inhibitory neurons and to follow their activity across days. Finally, Aim #3 will rely on 2-photon calcium imaging in Gad2-T2a-NLS-mCherry mice to track how associative learning changes the spatial patterns of activity in large ensembles of excitatory and GABAergic neurons across days. These experiments will allow us to investigate the possible role of inhibition in GC plasticity. Furthermore, comparing patterns of activity in alert and anesthetized mice will inform us on the state-dependency of remapping. Altogether, the proposed experiments will open a new alley of research on the function of GC and establish an experimental pipeline to investigate reward-based taste learning and GC plasticity. If successful, these studies will demonstrate that GC representation of taste accounts for taste-action associations. Proving this principle will have important consequences for our understanding of the role of GC in ingestive behaviors and decisions.
Gustatory stimuli are typically perceived according to their sensory quality (i.e., sweet, savory, salty, sour, bitter) and their hedonic value (i.e., palatable or aversive). Neural responses to stimuli sharing the same gustatory quality are more similar than responses to stimuli evoking different qualities. The same principle applies to hedonic value, with palatable solutions having a common neural signature that is distinct from that of aversive stimuli. However, the representation of these variables is not immutable throughout the life of an animal. The high levels of plasticity seen in the gustatory system, and in the gustatory cortex in particular, suggest that learning could reshape how gustatory stimuli are represented. The goal of this proposal is to understand how learning to associate different tastants with different actions changes the cortical representation of taste. The results of this research will provide important information on how feeding behaviors change the gustatory system, helping us understand the sensory impact of overeating and eating habits in general. This research will greatly contribute to our understanding of the role of the gustatory cortex in eating disorders.
