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Endorphin theory of love
|Structure of morphine (C17H19NO3 ), an endorphin-like chemical substance.|
The etymology of the word endorphin is a combination of the prefix endo- and the sufix -orphin; or a shorted formation of the words endogenous and morphine, intended to mean "a morphine-like substance originating from within the body." 
In 1975, endorphin receptors were discovered by two independent teams of researchers, John Hughes and Hans Kosterlitz in Scottland and Rabi Samantov and Solomon Snyder in the U.S. In 1976, American neuroscientist Candace Pert, who had played a role in the discovery of the opiate receptor while completing her PhD in pharmacology in the laboratory of Synder, along with American researcher Nancy Ostrowski, together built a small sex guillotine. With this device, they encouraged mice to have sex and then decapitated them while they were in the act. They then quickly then blenderized the brains and found that the brain cells had been pouring out endorphins. 
In particular, Ostrowski, who was a relative expert on brain mechanisms of animal sex, would inject the animals with a radioactive opiate before copulation, and then, at various points in the cycle, decapitate them and remove their brains. Using autoradiographic visualization of the animal’s brains, the two of them were able to see where the endorphins were released during orgasm. The found that blood endorphin levels increased by about 200 percent from the beginning to the end of the act. 
|Loretta Breuning’s 2012 Meet Your Happy Chemicals: Dopamine, Endorphin, Oxytocin, Serotonin (Ѻ), talks about endorphin and love, in overview.|
They then tested how exercise affected the release of endorphins by recruiting twelve young psychiatrists who were serious runners and taking blood samples before and after their daily runs.  The results showed that levels increased significantly after a run. A later paper, written by Pert and exercise physiologist Peter Farell, popularized and substantiated the physiological validation of the phenomenon now known as “runner’s high”.
Later, under the guidance of Pert, a number of data collections were made of saliva at various moments during human sexual intercourse. These results, although suggestive and presented at some early neuroscience meets, however, were never published as they “lacked sufficient clarity to be written up and accepted by a medical journal.” 
Chemistry of love
Into the late 1970s, the understanding of the function of endorphins in the body was improving. In the 1983 book The Chemistry of Love American psychiatrist Michael Liebowitz broke the mold when he outlined the relationship between endorphins and love. Prior to this year, no one had ever serious investigated the underlying biochemical or neurochemical basis of love. Liebowitz, for instance, states that, prior to the writing of his book, when “looking into the matter more closely I began to realize that no one had ever seriously tried to examine the biochemical basis for our romantic drives”. 
In the 1996 book The Alchemy of Love, by American physician Theresa Crenshaw, a former pupil of famed human-sexuality researchers William Masters and Virginia Johnson, arrived at the same point of view. Crenshaw stated that "when it was discovered in the 1980s that nerve cells have specific receptor sites for chemicals such as endorphins, a flood of research followed ... we soon learned that the vast tributaries of the brain are awash in hormones (such as testosterone and estrogen)."  This research soon led to the "hormone theory of love".
In regards to endorphin, however, Liebowitz noted that endorphins are more effective than morphine in reducing separation-induced crying in guinea pigs and chicks.  Subsequently, Liebowitz reasons, animals and likely humans are genetically programmed to secrete endorphins in situations of social comfort (such as sex), which would help them to feel both less anxious and also give them a sense of well-being. Moreover, Liebowitz notes that when people feel socially isolated or cut off from familiar figures, the brain shuts off endorphin production. Thus, according to Liebowitz, “as most adults feel some anxiety when separated from important figures in their lives, and some sense of increased security when their closest relationships seem stable”, attachment, as in the form of a human chemical bond, is mediated internally by the release of endorphins in the brain.
See main: FearIn 2001, American neurobiologist Donald Pfaff and associates showed that genetically modified mice made to be incapable of producing endogenous opiates (lacking endorphins), behaved such that the smallest fright made the animals freeze.  In large rooms, they stayed close to walls and showed other typical signs of fear. 
1. (a) Goldstein, A. &. Lowery, P.J. "Effect of the opiate antagonist naloxone on body temperature in rats." Life Science. 1975 Sep 15;17(6):927-31.
(b) Role of endorphins discovered (history) - PBS.com
2. Bodanis, David. (2005). Electric Universe – How Electricity Switched on the Modern World, (pg. 216-18). New York: Three Rivers Press.
3. Pert, Candace B. (1997). Molecules of Emotion – the Science of Mind-Body Medicine, (pg. 102-04). New York: Simon & Schuster.
4. Liebowitz, Michael R. (1983). The Chemistry of Love, (pgs. 5, 107-09). Boston: Little, Brown and Co.
5. (a) Crenshaw, Theresa L. (1996). The Alchemy of Love and Lust - Discovering our Sex Hormones and How they Determine who we Love, when we Love, and how often we Love, (pg. xx). New York: G. P. Putman's Sons.
(b) Teresa Crenshaw (1942-2001) - Stanford Magazine, Class Notes.
6. Frobose, Gabriele, and Frobose, Rolf. (2006). Lust and Love: Is it More than Chemistry? (pg. 107). RSC Publishing.
7. Ragnauth, A., Schuller, A., Morgan, M., Chan, J., Ogawa, S., Pintar, J., Bodnar, R.J. and Pfaff, D.W. (2001). “Female preproenkephalin-knockout mice display altered emotional responses.” Proc. Natl. Acad. Sci. USA, 98: 1958-63.
● Endorphin – Wikipedia.
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