In humans exposure to body odorants can produce sex-specific neuroendocrine responses, permit the discrimination of scents based on differences in major histocompatibility alleles, and sex-dependently activate regions of the hypothalamus involved in reproduction. However, failure to identify the neural pathways through which such signals access reproductive circuits has impeded our understanding of how human "pheromones" may affect behavior. Moreover, despite evidence in animals that such biologically relevant odors may act as natural rewards, the routes through which odors are conveyed to the mesolimbic dopamine system that mediates reinforcement have yet to be identified. Using mice, we have identified a direct projection of the main olfactory bulb (MOB) to the medial amygdala (Me), which projects to areas in the hypothalamus that control reproduction. Preservation of this pathway in humans could explain how odors influence sex partner preference and mating behavior. As chemosignals can be detected by both the main and accessory olfactory systems in the mouse, we will compare the involvement of each system in regulating responses to olfactory cues that control sexual motivation and reward. First, we will determine whether the direct pathway from the ventral MOB to the Me is differentially activated by male and female urinary volatiles in the two sexes and whether adult exposure to testosterone modulates the intensity of such responses. Next we will determine whether a subset of olfactory sensory neurons that express the M5 subtype of a TRP cation channel project to MOB glomeruli that are innervated by Me projecting mitral cells and selectively respond to opposite-sex urinary volatiles. Because we have observed that many of the ventral MOB projections to the Me also send branches to the olfactory cortex, we will map the targets of these projections to determine if specific sites in the olfactory cortex are preferentially innervated and differ in males and females. Choi et al (2005) suggested that odors of reproductive significance in mice activate a circuit of neurons that express the transcription factor, Lhx6, in the Me and bed nucleus of the stria terminalis (BNST) and then terminate in the ventromedial hypothalamic nucleus (VMH). We will determine the extent to which main vs accessory olfactory inputs selectively induce Fos (a marker of neuronal activity) in Lhx6- expressing neurons in the Me, BNST and VMH of male mice, and determine whether these neurons send axons to the ventral tegmental area, the location of dopaminergic cell bodies of the mesolimbic system. Finally, we will use a conditioned place preference paradigm to compare the incentive qualities of body odorants detected by the main vs accessory systems in male mice. Our results should provide insight into the olfactory and limbic circuits that process reproductively salient odor cues, and may also help elucidate the mechanism whereby non-olfactory (visual, auditory, or purely cognitive) inputs control sex partner preference and sexual arousal in humans.
We have identified a new direct connection from odor sensors in the nose of mice to parts of the brain important for mating. Preservation of this pathway in humans may help explain other results showing that body odor components can activate similar brain areas in people. We propose to study this pathway in the mouse to determine how odors may be involved in regulating mate choice, sexual motivation and reward.
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