Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Neuroscience

Supervisor

MacDougall-Shackleton, Scott A

Abstract

Breeding-typical levels of estradiol in songbirds has been shown to lead to selective auditory processing and induce a release of serotonin in auditory regions of the forebrain. These findings triggered the question of whether auditory discrimination is driven by estradiol directly, or by the associated release of serotonin. I treated non-breeding female white-throated sparrows (Zonotrichia albicollis) with either blank implants, 17β-estradiol, serotonin enhancer and blank implants, or serotonin antagonist and 17β-estradiol implants. Birds then heard male conspecific songs or control tones, and ZENK-immunoreactivity was quantified in the caudomedial nidopallium (NCM) and caudomedial mesopallium (CMM). While I did not find that breeding-typical levels of estradiol drives auditory selectivity in the brain, I did find a significant interaction between treatment and stimulus in the serotonin-enhancer group in the CMM, and clear trends in the same direction in the NCM. This suggests that the previously found effects of estradiol may be mediated by serotonin.

Summary for Lay Audience

The “Cocktail Party Effect” is a psychological phenomenon, which refers to the ability of humans to selectively attend to a relevant sound, such as someone’s voice, while simultaneously ignoring irrelevant background noise. However, this is not a uniquely human trait, and female songbirds during the breeding season have demonstrated to have a selective auditory processing ability of their own. Research has shown that a facilitator of this auditory selectivity is estradiol, a sex hormone. Sex hormones, such as estradiol, fluctuate drastically over the year, with elevated levels in the spring (breeding season). Previous research has shown that females with breeding-typical levels of estradiol pay attention to males of their own species singing significantly more than other auditory stimuli. Breeding-typical levels of estradiol also induce a release of serotonin, a brain chemical that is involved in various biological functions, including cognition, mood, and reward learning behaviours. This finding has led to the question of whether songbirds' ability to selectively attend to song during the breeding season is driven by estradiol directly, or by the associated release of serotonin. To address this question, I surgically implanted birds with either blank capsules, or with capsules packed with estradiol. I also made use of serotonin drugs that either facilitate (enhancer) or block the effects of serotonin (antagonist). I treated female white-throated sparrows with either i) blank implants, ii) 17β-estradiol implants, iii) serotonin enhancer and blank implants, or iv) serotonin antagonist and 17β-estradiol implants. Birds then heard either males of their own species singing or control tones. I then euthanized birds and labelled brain cells that were active during the sound exposure. I quantified cells in auditory areas of the brain and found that there was a significant interaction between treatment and sound stimulus occurring in the birds treated with the serotonin enhancer in the absence of estradiol, in some auditory regions. In the regions that a significant interaction was not obtained, I observed clear trends in the same direction with the serotonin enhancer group. Taken together, these findings suggest that serotonin may be mediating the previously found effects of estradiol on auditory selectivity in the brain.

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