-Mark Zhang
mzhang59@gmail.com
Wednesday, March 21, 2012
The Evidence - Hormonal Regulation
In citing an example of the complex gene-environment complex at play, Shenk refers to steroid hormones. Specifically, "testosterone levels change as a function of sexual experience, and hormones like testosterone are known to be able to diffuse across membranes where - once they have been bound by specific receptors - they can bind with DNA to regulate gene expression" (184). This matches almost perfectly with Campbell's description of cellular response pathways for hormones, noting how "Binding of hormones to receptors induces changes in cytoplasmic molecules and sometimes alters gene transcription (Campbell 978)." Therefore, it seems fairly clear that hormones certainly do play a key role in the GXE model. But aside from testosterone and estradiol, what other hormones are out there that are both released as a result of some external environmental factor and help regulate gene expression, particularly ones that contribute to skill or intelligence. Try to find an example with as much detail as possible, such as where the hormone is produced, what the gene expression results in, and what kind of selective advantage that hormonal response provides the human organism as a whole. As you research this idea and post a response, be sure to keep in mind the theme of continuity and change; how are the release of hormones supposed to combat, address, or adapt to a specific environmental stressor?
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Melatonin is a hormone all teenagers should identify with. A hormone that induces sleep, melatonin is produced by the pineal glands, a small mass of tissue near the center of the brain. Melatonin is sensitive to light or darkness and to seasons marked by changes in day length. Darkness is the environment stimulus that triggers the release of melatonin. For example, in the winter where days are short and nights are long, more melatonin is secreted. Melatonin targets the suprachiasmatic nucleus (SCN—a group of neurons) that functions as the biological clock (Campbell 994). David Shenk mentions, “genes are constantly activated and deactivated by environmental stimuli, nutrition, hormones, nerve impulses, and other genes” (22). Thus, hormones play a role in gene expression. In this case, melatonin plays a crucial role in the regulation of rhythmic clock gene expression. The rhythmic clock is also known as the circadian rhythm, which is a daily cycle of rest and activity normally synchronized with the light and dark cycles of the environment (Campbell 1122).
ReplyDeleteDr. von Gall and her team (of Anatomisches Institut II in Germany) tested, in mice, the effects of the absence of melatonin receptors on the genes related to the biological clock: mPer, mCry, Clock, and Bmal1. The results showed a significant reduction of the expression of those genes (mPer, mCry, Clock, and Bmal1) without melatonin present (http://onlinelibrary.wiley.com/doi/10.1196/annals.1327.105/abstract). Thus, melatonin is a hormone that stimulates the expression of important genes related to the circadian cycle. mPer, for example, is a gene whose product regulates the circadian clock (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Circadian.html). If this gene is not expressed or not enough expressed due to the lack of melatonin, there could be consequences such as sleep disturbances, jet lag, insomnia, depression, etc.
In addition to those possible consequences, the circadian rhythm does not only apply to sleep—it applies to pulse rate, blood pressure, temperature, rate of cell division, blood cell count, metabolic rate, and so on (Campbell 838). Thus, it would definitely be a selective advantage to maintain normal melatonin levels and a steady circadian rhythm since it is so universally applicable. Furthermore, melatonin can indirectly contribute to intelligence. During sleep, the brain is involved in learning and memory—and regions of the brain activated during a learning task can be active again during sleep (Campbell 1071). In a sense, the brain, during sleep, is processing and learning the information that the conscious brain had pondered about earlier. In a study carried out by the University of California, young adults who napped for 90 minutes performed better in rigorous learning tasks than those who did not take a nap. In fact, those who napped actually increased their ability to learn (http://www.newsmaxhealth.com/headline_health/nap_make_smarter/2010/02/23/313146.html ). Thus, as melatonin expresses genes essential for the circadian rhythm, it is also contributing to intelligence by ensuring sleep.
Melatonin is also relevant to a theme—continuity and change. A specific environment stressor such as jetlag disturbs the circadian rhythm since the body has not adjusted to the day-night cycle of the destination. Melatonin is often taken as a medicine to decrease of the effects of jetlag. Melatonin, being a player in regulating bodily rhythm, will re-align the body with the new place. For example, it might be released when the nighttime comes in the new location—forcing the body to sleep and thus adapt to the new setting. And since the circadian rhythm affects various other areas of the body, melatonin would have indirectly also helped other areas to adjust to the new setting.
Linda Xu (lindaxu22@hotmail.com)
(Melatonin—a sleep inducing hormone. An ironic topic, I think, to teenagers nationwide)
Thyroxine is also an essential hormone in the human body which contributes to one’s skill and intelligence and further enhances Shenk’s GxE model. Thyroxine is mostly involved maintaining homeostasis in the body, primarily through regulating the rate of metabolic processes. However, thyroxine also helps to “maintain normal blood pressure, heart rate, and muscle tone” and assists in the regulating the digestion and reproduction in humans, (Campbell 990). By controlling different factors of the circulatory system as well as muscular development, thyroxine is particularly beneficial to athletes who need to continually regulate their bodies during and outside of their sports. Deviating from a homeostatic set point with either hyperthyroidism or hypothyroidism, having too much or too little thyroid hormone in the blood, respectively, can often cause severe metabolic disorders. Thyroxine is secreted by the thyroid gland and targets nearly every cell in the human body. The secretion of thyroxine requires the presence of iodine and is stimulated by thyroid-stimulating hormone, or TSH. Thyroxine is able to diffuse passively through a cell’s plasma membrane, and within a cell nucleus thyroxine often binds to the CRYM gene to code for the protein NADP-regulated thyroid-hormone-binding protein (THBP), which plays a role in the regulatory and developmental processes as mentioned above.
ReplyDeleteIn the context of evolution, thyroxine proves to be a selective advantage in that it provides a mechanism of controlling one’s metabolism. Additionally, because of its regulation of muscle tone and heart rate, among other factors, thyroxine also seems to contribute to athletic genius. During competition with an increase in the demand on an athlete’s body, it would be a selective advantage for an athlete to be able to increase heart rate in order to distribute more oxygen to muscle cells for cellular respiration. Furthermore, thyroxine’s significant “protein biosynthesis” and the fact that it “stimulates the rate of amino acid incorporation” and production would assist during an athlete’s training, contributing to muscular development (Sokoloff http://www.ncbi.nlm.nih.gov/pmc/articles/PMC300337/pdf/pnas00183-0096.pdf) Thus, athletic environmental stressors and “genes constantly respond[ing] to” thyroxine can result in the increased effect of thyroxine on the body and potentially improve and contribute to athletic genius (22).
Linda’s above example of melatonin is another such hormone that significantly affects the human body and contributes to overall genius. Because of its everyday involvement in regulating sleep, it seems to more directly affect one’s potential for increasing skill and intelligence. Therefore, the proper secretion of melatonin would help to maintain one’s circadian rhythm as Linda addressed and to induce sleep which allows for necessary physical and recuperation. An imbalance of melatonin would pose significant health risks that would often times inhibit one’s full potential for skill and intelligence.
David Ribot (ribotdavid@gmail.com)