Behavioural, Pharmacological, and Immunohistochemical Investigation of 50 kHz USVs as an Expression of Positive Emotional Arousal in the Long Evans Rat
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The emission of ultrasonic vocalizations (USVs) in Rattus norvegicus is thought to effectively represent an underlying emotional state within the organism manifested at the behavioural level. The main goal of my thesis was to characterize, at multiple levels of analysis, the 50 kHz USVs of the adult rat as an overtly expressed form of positive emotional arousal. In chapter 2, I found evidence of individual differences in 50 kHz emission possibly reflective of a trait that does not merely overlap with approach motivation. The predisposition to emit 50 kHz USVs was found to provide additional information about the USV response to psychostimulant administration beyond approach motivation alone. In chapter 3, I found evidence that various social and non-social behavioural contexts appeared to exert influence on the frequency-modulation characteristics of 50 kHz USVs. My findings suggest that the highest rates of calling and frequency modulation inducible by non-pharmacological stimuli may be observed following exposure of a male rat to a naturally cycling female. Moreover, my research in chapter 3 established that despite context-specific modulation of 50 kHz USVs all such calling could be blocked by antagonism of dopamine receptors. In chapter 4, I utilized microinjections of dopamine into the shell of the nucleus accumbens to establish that dopamine is sufficient to induce 50 kHz USVs. Additionally, my findings from chapter 4 supported the observed association between frequency-modulated 50 kHz USVs and call rate typically induced by psychostimulants. In chapter 5, I used a minimal sensitization protocol with amphetamine to establish that 50 kHz USVs and measures of general ergometric activity could be dissociated. Additionally, in chapter 5 I attempted to find brain region activation patterns associated with calling. My chapter 5 findings failed to find any direct relations between immunostained brain regions and behavioural expression. However, exploratory analyses suggest possible associations between prefrontal and striatal regions may be involved in the USV behavioural response to amphetamine. In aggregate, my empirical findings are consistent with the existence of a putative subcomponent of the ascending mesolimbic dopamine system responsible for positive emotional arousal reflected by emission of 50 kHz USVs in the rat.