Contribution of the accessory olfactory system to the expression of proceptive and receptive behaviors in female mice
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The integration of chemosensory cues regulates the expression of courtship behaviors in estrous female mice. Previous investigators showed that surgical destruction of different segments of the accessory olfactory system (AOS) (including the vomeronasal organ (VNO), accessory olfactory bulb (AOB) and medial amygdala (Me)) decreased females’ motivation to investigate male chemosensory cues and display lordosis, a reflexive arched-back posture, shown in response to male mounts. This thesis further examined the role of the AOS in female courtship behavior by reversibly disrupting AOS function using optogenetic and pharmacogenetic methods, and by experimentally stimulating the AOS using male chemosignals. Sexually naïve estrous female mice initially show low levels of lordosis; however, their performance improves with repeated testing. In Experiment 1, repeated pre-exposure of estrous female mice to male pheromones known to activate the AOS failed to augment the expression of lordosis but did increase the time females spent holding the lordosis posture. Pre-test exposure to male pheromones also increased the time that estrous females spent investigating the male. In Experiment 2, optogenetic silencing of the AOB neurons projecting to the Me reduced the expression of lordosis in sexually experienced estrous females, indicating that continuous AOS signaling is required for full expression of this behavior. In Experiment 3, a pharmacogenetic method (DREADD) was used to reversibly silence Me neurons, first in sexually naive and then in sexually experienced estrous females. DREADD-induced silencing of Me neurons reduced females’ preference to investigate male urinary chemosensory cues and greatly attenuated the progressive increase in lordosis otherwise observed in control females after repeated testing with a male. DREADD-induced silencing of the Me failed to significantly disrupt lordosis after females had received mating experience; however, the withdrawal of this experimental inhibition of Me signaling did significantly augment females’ investigation of the male’s body. My results provide a new insight into the contribution of the AOS chemosensory signaling pathway by establishing its essential, ongoing role in the preference of estrous females to seek out male pheromones and to display receptive lordosis behavior.