Functional antagonism of the mesolimbic dopaminergic system on mesolimbic cholinergic system in the vocal expression of an emotional state
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The overarching goal of this thesis was to determine if the initiation of a positive emotional state could antagonize the expression of a negative emotional state in rats. The hypothesis of the thesis argued that the initiation of a positive emotional state would ameliorate the vocal expression of a negative emotional state. The subjective emotional state of the rat was indexed by the quantity and type of pharmacologically induced ultrasonic vocalizations (USVs). Adolescent and adult rats can emit vocalizations above the upper threshold of human hearing (>20 kHz) termed ultrasonic vocalizations (USVs). These USVs are broadly divided into 50-kHz, reflective of a positive emotional state, and 22-kHz USVs, reflective of a negative emotional state. Pharmacologically, injection of dopamine agonists into the nucleus accumbens shell is sufficient for the initiation of 50-kHz USVs, while injection of cholinergic agonists into the anterior hypothalamic-medial preoptic area (AH-MPO) or the lateral septum (LS) can initiate 22-kHz USVs. In chapter two of the thesis, I demonstrated that microinjection of the dopamine agonist, apomorphine, into the medial shell of the nucleus accumbens attenuated the extent of carbachol-induced 22-kHz USVs from the AH-MPO. I also demonstrated that this effect was dependent upon the microinjection of apomorphine into the central region of the nucleus accumbens shell. In chapter three, I demonstrated that apomorphine could also decrease the extent of carbachol-induced 22-kHz USVs from the LS providing evidence that the effect reported in chapter two was not isolated to the AH-MPO, but rather extending along the medial cholinoceptive vocalization strip. In the third chapter. I also demonstrated that the magnitude of the reduction in the number of 22-kHz USVs was correlated to the number of emitted frequency-modulated (FM) 50-kHz USVs induced by apomorphine. In the fourth chapter, I investigated whether blocking dopamine receptors, either systemically using the typical D2-antipsychotic agent, haloperidol, or microinjection of the D2 antagonist, raclopride, into the nucleus accumbens shell could increase the emission of carbachol-induced 22-kHz USVs from the LS. The results showed that antagonism of dopamine receptors, either systemically or intracerebrally, did not increase the number of 22-kHz USVs. Interestingly, it was also observed that after the prolonged recording of carbachol-induced 22-kHz USVs, some 50-kHz USVs spontaneously appeared after roughly 300 s into the recording. I argued that these 50-kHz USVs, which I defined as “rebound 50-kHz USVs” are not initiated by carbachol since they occurred when the carbachol-response weaned. It was also demonstrated these rebound 50-kHz USVs were dependent upon dopamine release within the nucleus accumbens since both systemic, and intracerebral application of dopamine antagonists into the central division of the nucleus accumbens shell blocked the occurrence of rebound 50-kHz USVs. Altogether, the data supports the thesis that activation of a positive emotional state decreases the expression of the negative emotional state in rats when measured using ultrasonic vocalizations.