Steroid hormones exert profound effects on social behavior, but those effects are typically slow because steroids activate molecules that turn genes on and off, and changes in gene activity take a long time to produce effects on behavior. However, it has recently been shown that steroids can also affect behavior through more rapid mechanisms that do not depend on interactions with genes. Thompson will test the hypothesis that two sex steroids, testosterone and estradiol, affect reproductive behavior in male goldfish by rapidly changing how goldfish see the world, particularly female sexual stimuli. The proposal outlines a series of behavioral, electrophysiological, and neuroanatomical experiments that will also determine the kinds of molecules in the brain that these steroid hormones act upon to produce behavioral effects. Additionally, the project will identify where within the brain hormones produce those effects. Together, these experiments will increase our understanding of the fundamental cellular mechanisms through which steroid hormones affect behavior, particularly those related to reproduction. The principal investigator will perform many experiments related to this grant in an upper level laboratory course at Bowdoin College, providing meaningful research experiences to undergraduate students. Dr. Thompson will also teach a summer neuroscience outreach program at Bowdoin College for teams of high school teachers and their students. This will be followed by his students setting up presentations in the classes of teachers who have participated.
The scientific goal of this project was to determine how steroid hormones rapidly affect tissue in ways that can immediately alter an individual's physiology and/or behavior. Until recently, we thought that steroids like testosterone exclusively affect tissue by activating receptors inside of cells that turn genes on and off, which ultimately affects how much protein a cell makes. This "genomic" mechanism is slow, often taking hours, if not days, to produce enough changes in protein levels to alter an individual's physiology and/or behavior. Steroids have therefore been traditionally thought of as molecules that prepare organisms for future events, not immediate environmental challenges. However, we now know that steroid hormones can rapidly (within seconds to minutes) influence the physiology of cells through actions on recently discovered receptors that sit on the surfaces (membranes) of cells. Steroids thus have the potential to produce immediate effects on physiology and behavior. To better understand how they may do so, we took advantage of an organism in which testosterone rapidly affects physiological and behavioral responses to social stimuli, goldfish. The testosterone produced by goldfish is exactly the same molecule as that produced in all other species, including humans, and the peripheral tissues (gonads) and brain regions upon which we think it acts in this species are even remarkably similar across vertebrate animals. Thus, learning how testosterone works in this system will help us better understand how this hormone may rapidly influence physiology and behavior in a range of vertebrates, including humans. We found that not only does testosterone influence social approach responses in male goldfish too rapidly to be associated with the slow, genomic mechanism discussed above, but that estradiol, and estrogen, does also. In fact, we found that testosterone’s rapid behavioral effects actually depend upon its biochemical conversion within the brain into estradiol. Thus, the brain is quickly converting testosterone, an androgen, into estradiol, an estrogen, which then rapidly activates estrogen receptors on cells to influence masculine patterns of behavior. We found similar effects on peripheral, physiological processes (the production of sperm), and determined that those effects are also dependent on testosterone's conversion into estradiol and the subsequent activation of membrane versions of estrogen receptors. This is significant because estrogen receptors typically reside inside of cells, not on their surfaces, and they usually mediate the slow, genomic effects of estrogens, not their rapid influences. Learning more about the biochemical and molecular mechanisms through which testosterone and estradiol work will ultimately help us understand how steroids promote behavioral and physiological flexibility in changing social environments. In the brain, we were able to visualize a particular estrogen receptor, ERbeta, in glial cells. These glial cells are the precursors to neurons that are born in adulthood in fish, and estrogens increase the production of new neurons in adulthood in several fish species. Thus, our findings suggest that the ability of estrogens to stimulate neuron birth (neurogenesis) in adulthood, a mechanism that promotes adaptability to changing environmental conditions, may depend upon influences on this receptor. Because hormone regulatory mechanisms are remarkably similar across vertebrates, what we have learned about these mechanisms in goldfish will also help us better understand how estrogens influence behavioral flexibility in other organisms, and maybe even how they exert neuroprotective effects in human brains. We also found ERbeta in the retina, suggesting that estrogens may rapidly affect the way these animals sense (see) the world, thus altering an individual's "moment-to-moment" perception of the environment, including its social environment. Indeed, further experiments showed that estradiol rapidly increases retinal sensitivity to light, indicating that estrogens could affect behavior, in some cases, by rapidly affecting sensory abilities and thus altering perceptions of the social environment. These results thus demonstrate a novel way that sex steroid hormones may affect behavior in vertebrates. As an RUI award (Research at Undergraduate Institutions), a second major goal of the proposal, and one associated with the Broader Impact directive of the NSF, was to expand undergraduate opportunities in research and thus provide a more "hands-on" educational opportunity for our future scientists and medical specialists. Many of the experiments associated with this protocol were first conducted by students enrolled in a laboratory course that I teach, thus exposing them to modern methods that can be used to understand the brain while refining the experimental protocols that ultimately succeeded in the laboratory. Furthermore, many of the students who took that class went on to do independent study and senior honors projects in my laboratory. Four have already been co-authors on peer-reviewed publications that resulted from this work, and 9 have presented their findings at national academic meetings. These experiences have been vital to their academic development and thus to their future successes in science and medicine.
