Interrogation of the neural mechanisms by which male and female rats assess aversive and mixed valence stimuli in learning and performing a task
The appropriate evaluation of reward and punishment is critical to mammalian survival, and its disruption contributes to numerous psychiatric illnesses. Few animal models capture this tension and rely on learned fear rather than assessment of affective valence. In contrast, conditioned taste aversion (CTA) is a behavioral paradigm in which a palatable substance is paired with an aversive visceral experience, generally an injection of a nausea-inducing agent such as lithium chloride (LiCl), to produce aversion to the substance at subsequent exposure. CTA utilizes a critical decision-making mechanism for assessing the relative danger or safety of a stimulus. This process has broad implications in human psychology and mental health disorders, including anorexia nervosa, anxiety, and post-traumatic stress disorder. Although these disorders all show female predominance, understanding the neural circuits activated in CTA is fundamentally unstudied in females.The purpose of the present study was to deepen our understanding of both the behavioral outputs and neural mechanisms underlying reinforcement versus aversion in male and female rats. We used ultrasonic vocalizations (USVs) as a means of assessing affective valence. We found striking sex differences in this behavior. Males produced 55 kHz USVs, associated with a positive emotional valence, when anticipating a reward, and inhibited these calls when the reward was devalued with LiCl. Females produced 55 kHz based on their estrous cycle, but were more likely to make 22 kHz vocalizations, associated with a negative emotional valence, when anticipating the devalued stimulus. We measures c-fos response to each of the acute and conditioned stimuli during CTA. C-fos responses were similar in males and females after boost or LiCl. Females engaged the gustatory cortex more than males during the reinforcing task and males engaged the amygdala more than females in both the reinforcing and devalued tasks. We used a new technology, RNAScope, to identify a population of dopamine receptor-expressing neuronal subtypes that are activated in the process of balancing reward and aversion during CTA. We found that the central nucleus of the amygdala (CeA) responds to hedonic and aversive stimuli in a cell-specific manner. The basolateral amygdala (BLA), in contrast, did not show receptor-type cell specificity. The BLA showed a strong response in the females regardless of treatment group. In contrast, males engaged the BLA when treated with LiCl and during expression of the CTA tasks. The BLA processes affective valence and current studies suggest it is necessary for supporting motivated behavior. The present study suggests that males and females differentially process the affective valence of a stimulus to produce the same goal-directed behavior. Further studies on the mechanisms underlying these differences will broaden our understanding of how the amygdala differentially processes reward and aversion in males and females, which could result in new therapeutic targets for the treatment of a variety of neurobiological and psychiatric disorders. We also used a separate but complementary behavioral conditioning paradigm, conditioned nausea, as a model of chemotherapy-induced anticipatory nausea. Twenty-five to fifty % of chemotherapy patients will develop anticipatory nausea and vomiting (ANV), in which symptoms occur in anticipation of treatment. ANV is triggered by environmental cues and shows little response to traditional antiemetic therapy, suggesting unique neural pathways mediate this response. We used a rat model of ANV that pairs a novel context (test cage) with a high dose of the emetic drug lithium chloride (LiCl) to produced conditioned nausea behaviors in the LiCl-paired environment even in the absence of the toxin. Male and female rats were treated on days 0, 2, 4, and 6. On day 8, they were placed in the context and we measured gaping, an analogue of human vomiting. To identify brain regions associated with acute LiCl and ANV, we measured c-fos activation by immunohistochemical staining on days 0 and 8. We found both male and female rats exhibited gaping at equal rates but showed remarkable sex differences in their c-fos response. Acute LiCl activated brain regions responsive to aversive visceral stimuli, including the central nucleus of the amygdala (CeA), nucleus of the solitary tract (NTS), and area postrema (AP) that were not active during ANV. ANV activated c-fos expression in the frontal cortex and insula of males but not females. These data suggest that therapies such as ondansetron which target the AP are not effective in ANV because it is not a site activated during the ANV response. Further studies aimed at characterizing the cell types activated in the conditioned nausea response will help identify novel therapeutic targets for the treatment of this condition, improving both quality of life and outcomes for patients undergoing chemotherapy.
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