By Christopher Chandler
The researchers were initially interested in why lesbian women living together did not synchronize their menstrual cycles the way most groups of women do. They thought that one possible explanation was the absence of semen, which somehow set off the pheromones to synchronize. So they took a look at the literature, and found that semen contained many unexpected ingredients, including anti-depressants. One researcher suggested that the presence of semen in a woman might lessen depression. Studies had shown that compounds from semen can be found in a woman’s blood an hour after sex, so it was only logical to propose that semen might act as an anti-depressant.
They designed an experiment to see if having unprotected sex might lessen depression in women. They surveyed 293 college women, and the evidence was overwhelming. Women who practiced unprotected sex scored much lower on the standard “Beck Depression Inventory.” Women who used condoms and women who abstained both showed much higher levels of depression. They published their findings in the Journal of Sexual Behavior on March 12, 2002 with the title “Does Semen Have Antidepressant Properties?”
Two of the researchers, Gordon Gallop and Rebecca Burch, went on to survey the research on other properties of semen. It’s long been known that sperm makes up only five per cent of the semen. The rest, called “seminal plasma,” has compounds that help nurture the sperm and combat resistance from the immune system. But they discovered that there are at least fifty three different compounds in semen, and that they do all different kinds of things.(See Table, from the Chapter “The Psychobiology of Human Semen,” in “Female Infidelity and Paternal Uncertainty,” Cambridge University Press, 2006)
There are many hormones, but the highest concentration by far is “lutenizing hormone,” which promotes ovulation and sex drive. There are also endorphins and neurotransmitters and female sex hormones and hormones that promote maternal care, a long list of compounds that would effect many aspects of a woman’s life.
These are new findings, and will take some time to digest. Already the president elect of the American College of Surgeons, Lazar Greenfield, had to resign when he wrote about this research for Valentine’s Day, saying “So there’s a deeper bond between men and women than St. Valentine would have suspected, and now we know there’s a better gift that day than chocolates.”
It really does seem to be a box of chocolates, providing a woman with a steady male partner with many benefits. Considering how this intricate system must have evolved, Dr. Greenfield was right again when he said that it seems “nature is trying to promote a stronger bond between men and women.”
Here are the first ten pages of the chapter “The Psychobiology of Human Semen” from the book “Female Infidelity and Paternal Uncertainty,” Cambridge University Press, 2006. They give a more detailed description of the effects of different compounds found in semen.
The psychobiology of human semen
REBECCA L. BURCH
SUNY at Oswego
AND
GORDON G. GALLUP, JR.
SUNY at Albany
Introduction
Our interest in the psychological properties of semen arose as a byproduct of an initial interest in menstrual synchrony. In reviewing that literature we discovered several articles (Trevathan. Burleson. & Gregory. 1993: Weller & Weller. I998) reporting that lesbians who live together fail to show menstrual synchrony. Since the evidence suggests that menstrual synchrony is mediated by the exchange of subtle olfactory cues among cohabitating women (1)reti Cal. 1986. Stern & McClintock. 1998) this struck us as peculiar, because lesbians would be expected to be In closer, more intimate contact with one another on a daily basis than other females who live together. What is it about heterosexual females that promotes menstrual synchrony. o COnvencly what is it about leShiant that prevents menStntal synchrony? It occurred 10 us that one feanire that distinguishes heterosexual women from lesbians is the presence or absence of semen in the rental° reproductive tract. Lesbians have semen-free sex.
Human semen is a mycomplicated MiXtUre of many different ingredients. If you extract the sperm from semen, what is left is called seminal plasma. We speculated that there may be chemicals in seminal plasma that, through vaginal absorption. altect female biology and triggers the release of phemmones that function to entrain menstrual cycles among cohabitating women. Some 01(1w components in semen pass through vagina le pit helial tissue. and within an hour Or two After intercourse heightened levels of certain seminal CheiitiCalS can be detected in the female blnodstream (Henziger & Edleson. 19213). num. among a pair of cohabitating females the one with semen in her reproductive tract may entrain/drive her roommate’s menstrual cycle. This might also explain the variance in menstrual synchrony among women, with some females who show it and some who do not as a function of whether they are sexually active and whether they are using condoms (Gallup, Burch & Platek, 2002). Other studies have linked heterosexual sexual activity with menstrual cycle length (Cutler, Garcia, & Krieger, 1979) and menstrual synchrony (Jarett, 1984; Matte°, 1987).
In reviewing the literature on semen chemistry we discovered an article by Ney (1986) who, on the basis of a depressed female patient he was seeing, speculated that human semen may have antidepressant properties. It occurred to us that one way to test this hypothesis would be to compare levels of depression in women as a function of whether they were using condoms (Gallup, Burch & Platek, 2002). Sexually active heterosexual females who use condoms and lesbians share one important feature in common, they are both having semen-free sex. Using scores on the Beck Depression Inventory to index depression, we discovered that sexually active female college students who were not using condoms were significantly less depressed than those who were. Interestingly, those who were using condoms did not differ in terms of their BDI scores from those who were not having sex. Consistent with the possibility that semen may have antidepressant properties, BDI scores among those who were not using condoms were correlated with the elapsed time since their last sexual encounter (i.e. as the time since their last exposure increased, so did their depressive symptoms). When we took into account those who were using hormonal contraceptives and those who were in committed relationships, the differences in depression among those who were using condoms and those who were did not prevail.
Semen chemistry: a brief overview
It is possible that the presence of semen in the reproductive tract of women not only affects depressive symptoms, as Ney suggested, but may trigger physiological changes in menstrual cycle length, variability, and synchrony, perimenopausal symptoms, and psychological changes in menopausal and postpartum depression, premenstrual syndrome, and female-initiated sexual activity. The following compounds have been identified as being present in human semen. Each of these compounds has also been shown to affect female sexual behavior and physiology. A list of the compounds and their concentrations are shown in Table 8.1.
First of all cholesterol, the precursor to all steroid hormones, is found in semen (Valsa, Skandhan, & Umarvanshi, 1992). Concentrations of cortisol (formed from cholesterol) in human seminal plasma, as estimated by immunoassay, were about 60% of random levels in blood serum (Brotherton, 1990a).
Abbaticchio et al. (1981) also found that levels of cortisol in the blood prove to be much greater than in the seminal plasma. The researchers found no significant differences between normozoospermic and oligo-azoospermic subjects, either in the blood, or in the seminal plasma. In other words, both fertile men and men with fertility issues have similar levels of cortisol. Transcortin (corticosteroidbinding globulin) has also been found in human seminal plasma, although transcortin concentrations were only about 10% of levels in blood serum (Brotherton, 1990a).
Corticoids are best known for their role in response to stress. The adrenal cortex releases glucocorticoids (as well as prolactin, thyroid hormones, and vasopressin) in response to virtually any stressor (Nelson, 2000). Glucocorticoids accentuate the effects of dopamine, increasing reinforcement for behaviors. Blood levels of cortisol have been shown to increase approach behaviors, including parental care and interpersonal affection. This is thought to be due to an increased arousal and reinforcement of the stimuli the person is experiencing. Although there have been examples of glucocorticoid involvement in the induction of parental care, it is possible that the changes are due to the increases in oxytocin, corticotropin-releasing hormone, and opioids which accompany glucocorticoid release (Nelson, 2000).
Testosterone is in relatively high concentration in human semen compared to other compounds, approximately 559 pg/ml (Ney, 1986). Other androgens are also present, some in higher concentrations (see Table 8.1). These compounds can be used to derive other steroids and they themselves appear to have immunological properties. Levels of testosterone in semen are also correlated with sperm motility. In fact, seminal concentrations of testosterone and dihydrotestosterone were significantly higher in subjects with sperm in their ejaculate than in vasectomized men (Asch et al., 1984). Purvis et al. (1975) found a positive correlation between testosterone and dihydrotestosterone levels of the seminal plasma of normal and azoospermic subjects, indicating that one may increase the other. Another interesting finding regarding androgens in seminal plasma indicates that the ratio between testosterone and dihydrotestosterone is different from one person to the other, due to the heterogeneity of seminal plasma which stems for the most part from the male accessory sex glands, and the prostate and seminal vesicles. This ratio may be useful in identifying the person’s semen (Doss 81 Louca, 1991).
Testosterone is absorbed by the vagina, and at higher levels than testosterone administered transdermally. Approximately 63% of the testosterone administered vaginally was absorbed (Wester, Noonan, 8r Maibach, 1980). There are suggestions in the literature that normal variations in androgens during the menstrual cycle could be responsible for reported cyclic fluctuations in sexual interest (Guay, 2001; Van Goozen et al., 1997). The adrenal cortex produces significant quantities of androgens, and adrenalectomy is detrimental to female sexual behavior (Dixson, 1987; Hepburn et al., 1996). Furthermore, since the 1940s exogenous androgen treatments have been given to human females for diverse medical purposes. Even in very low doses, androgen treatments can increase sexual motivation in some human females (Carter, 1992). Morris et al. (1987) measured testosterone levels in 43 married women. Testosterone levels correlated significantly with sexual intercourse frequency.
Human semen contains both estrone and estradiol (Asch et al., 1984), although the proportion of estrone is more than twice that of estradiol (Ney, 1986). According to Luboshitzky, Shen-Orr, and Herer (2002), estradiol was in significantly higher concentration in seminal than blood plasma in males. When estrone was administered in the vagina, it was rapidly absorbed and plasma levels increased 24-fold. These high levels were maintained for 2 h (Schiff, Tulchinsky ik Ryan, 1977). Rigg et al. (1977) administered 17-11-estradiol and observed a rapid peak concentration in the blood 110 times the basal level. This also lasted for 2 h. Research also suggests that the vaginal administration of estrogen assists in the absorption of other compounds, including progesterone (Villanueva, Casper, 81- Yen, 1981).
Estrogen, along with luteinizing hormone and follicle-stimulating hormone, work in concert to trigger ovulation in females and, along with luteinizing hormone and follicle-stimulating hormone, reaches its peak at ovulation. Peak fertility more or less coincides with the estrogen peak (Nelson, 2000). Estrogen, whether secreted by the ovarian follicle or given as replacement therapy, may have broad effects on behavior. Estrogen apparently can increase female sexual motivation and may facilitate peripheral changes such as the production of odors that make the female more attractive to the male partner. In human females, estrogen also increases vaginal lubrication and thus can influence sexual behavior indirectly (Carter, 1992).
Deficits in estrogen have been associated with depression. In one study, over 90% of the depressed women treated with estrogen significantly improved their mood (Klaiber et a/., 1979). It is important to note that high, pharmacological doses were used in this study. Administration of estrogen in physiological doses improves mood in normal women (Sherwin & Gelfand, 1985) but not in clinically depressed women (Schneider et al., 1997). A great deal of research has found that estrogen is effective in the treatment of menopausal depression (Lebowitz, Pollock & Schneider, 1997; Lebowitz et al., 1997). In a meta-analysis conducted by Zweifel and O’Brien’s (1997), the effect of exogenous estrogen and other steroidal hormones were related to the alleviation of depressed mood. Progesterone alone, and in combination with estrogen, was associated with smaller reductions in depressed mood and androgen alone or in combination with estrogen was associated with greater reductions in depressed mood.
The most interesting facet of luteinizing hormone in semen is its astounding concentration. Semen contains 220 inIU/m1 luteinizing hormone, the highest concentration of any hormone. Follicle-stimulating hormone is found in a much lesser concentration (Ney, 1986). Researchers have found that concentrations of luteinizing hormone in seminal plasma are as much as five times higher than in the blood (Mondina et al., 1976; Sheth, Shah, & Mugatwala, 1976). This is rather unique, since most other peptide hormones in semen are in equal or lower concentrations than in the blood. Luteinizing hormone in semen is also linked to higher numbers and motility of sperm, compounding the male’s fertility (Asch et al., 1984). Human seminal plasma also contains luteinizing hormone- releasing hormone (Chan & Tang, 1983).
Little research has been done to determine levels of vaginal absorption of these hormones. The few studies that do mention follicle stimulating hormone and luteinizing hormone only discuss the resultant decreases in levels after the administration of high, pharmacological doses of estrogens (Keller et al., 1981). Follicle-stimulating hormone and luteinizing hormone (both glycoproteins) are structurally similar and both stimulate steroidogenesis in the gonads, as well as the development and maturation of gametes. In other words, they both act in females to produce and release eggs, and both peak at ovulation (Nelson, 2000). The deposition of follicle-stimulating hormone and luteinizing hormone, and their subsequent absorption into the female bloodstream could act to facilitate or even induce ovulation (see below).
Little research has also been conducted on the vaginal absorption of prolactin. However, the absorption and subsequent rise in estrogen levels triggers an increase in prolactin as well (Keller et al., 1981; Yamazaki, 1984). Human semen contains approximately 86 ng/ml prolactin (Ney, 1986). Seminal plasma prolactin concentrations are related directly to sperm concentrations and motilities and are much lower in men suffering from infertility (Aiman, McAsey, & Harms, 1988). However, other researchers (Asch et al., 1984) found no differences between fertile and vasectomized men in seminal concentrations of prolactin.
Prolactin is a hormone common throughout vertebrate evolution, and can have hundreds of unique physiological functions including effects during pregnancy and lactation (Nelson, 2000). Prolactin has been reported to influence numerous brain functions, including maternal behavior, feeding and appetite, oxytocin secretion, and ACTH (corticotropin) secretion in response to stress. Hence, prolactin may be a key player in the coordination of neuroendocrine and behavioral adaptations of the maternal brain (Grattan, 2001).
Prolactin, as studied in rodents, aids in the formation of the corpora lutea and also seems to potentiate the release of progesterone from the corpus luteum, which facilitates pregnancy. Deficiencies in prolactin release involve the mono- amine neurotransmitter systems that have been implicated in depression (Golden et al., 2002; Nelson, 2000). Deficiencies in prolactin secretion are also implicated in postpartum depression (Hendrick, Altshuler & Sun, 1998) and premenstrual syndrome (Derzko, 1990).
Although Homberg and Samuelsson (1966) isolated the presence of 13 different pros taglandins in human semen, little research has focused on these substances. However, some prostaglandins have been shown to be absorbed rapidly through the vagina, namely E1, E2, and F2a (Sandberg et al., 1968). Increased frequency of uterine contraction has resulted from prostaglandin El administration. Certain prostaglandins, Fla and F2a, are active in the dissolution of the corpora lutea and in ovulation (Nelson, 2000) and these compounds cause powerful contractions of the uterus (Mackenzie, Bradley, & Mitchell, 1980). Based on this information, it is possible that prostaglandins in semen are used to assist in ovulation. Ney (1986) also hypothesized that prostaglandins assisted in alleviating depression, citing Abdullah and Hamadah (1975) who found that drug-free patients with depression possessed significantly less prostaglandin E1, while manic patients possessed significantly more.
Other studies have shown prostaglandins to have inununosuppressive capabilities (Kelly, 1995). Skibinski et al. (1992) found that prostaglandins E1 and E2 exerted a greater immunosuppressive effect than 19-0H prostaglandin E, but considerably higher levels of 19-0H prostaglandin E in semen might contribute to the majority of immunosuppressive activity in vivo. Thus, with the presence of prostaglandins, immunological reactions would be inhibited for a period of time after intercourse.
Other suppressive agents are present in semen and may exert specific effects (Kelly, 1995). Maegawa et al. (2002) reported a repertoire of cytokines in seminal plasma (see Table 8.1). Each of these inununosuppressants may act in the female to dull her immunological reaction to the foreign bodies (sperm) invading her vagina and cervix. Thus the chemistry of human semen would appear to include components that increase the likelihood of impregnation.
Human semen contains large amounts of opioid peptides (such as 0-endorphin) and cytokines. Zalata et al. (1995) concluded that 13-endorphin in seminal plasma may play an immune-suppressive role. Singer et al. (1989) suggested that 0-endorphin and calcitonin may affect sperm motility. Mungan et al. (2001) also studied levels of calcitonin in seminal fluid and its effects on sperm motility and found that seminal calcitonin levels were significantly correlated with sperm motility. While these endorphins may assist in sperm motility, it is important to investigate the role that these chemicals play in the female immunological response and any possible analgesic properties the compounds may have when absorbed through the vagina.
Enkephalins are one of the opioids present in human semen and to date their function in this context remains unknown (Fernandez et al., 2002). Sastry, Janson, and Owens (1991) found high levels of leucine enkephalin in human seminal plasma as well as lower levels of substance P. While substance P increased sperm motility, leucine enkephalin depressed it. It was concluded that substance P-like tachykinins may play a role in sperm maturation, in expulsion of fluid from the epididymis, and in initiation of motility, whereas leucine enkephalin-like peptides may contribute to the orgasmic experience and detumescence. It is not currently known whether these peptides would affect the orgasmic experience or postcoital experiences of the female. Endorphins and enkephalins have numerous physiological effects, but primarily they decrease anxiety, induce analgesia and drowsiness, and assist in immune function and reinforcing effects (Stahl, 2001). Absorption rates and levels in the female bloodstream during and after coitus still need to be examined.
Oxytocin is found in the seminal plasma of normal men and is slightly lower in patients with poor semen quality and vasectornized patients. No statistically significant relationships have been found between the oxytocin levels and sperm characteristics (Goverde et al., 1998). Oxytocin has been shown to have wide-ranging effects in humans (Nelson, 2000). Although it is well known for its effects during parturition and lactation, oxytocin also increases production of other hormones, such as prostaglandins and testosterone, and assists in the stimulation of ovulation and blastocyst development. Oxytocin has been thought of as an “affiliation hormone” because research on non-human mammals has demonstrated that it plays a key role in the initiation of maternal behavior and the formation of adult pair bonds. Oxytocin is implicated in male penile erection and female orgasm as well as the social aspects of romantic relationships (Turner et al., 1999). It is possible that oxytocin, transferred to the female during coitus and absorbed, can strengthen the pair bond and make the sexual activity more rewarding.
Vasopressin, a peptide hormone similar in structure to oxytocin, has also been identified in semen. In the animal literature, vasopressin has been associated with behaviors that might be classified broadly as “defensive,” including enhanced arousal, attention, or vigilance, increased aggressive behavior, and a general increase in sympathetic functions (Carter 82• Altemus, 1997; Nelson, 2000). Virtually nothing is known regarding the effects in humans of centrally administered vasopressin because it does not readily pass through the blood-brain barrier (Carter & Altemus, 1997). In animals, vasopressin has been implicated in the central mediation of complex social behaviors, including affiliation, parental care, and territorial aggression. Intense aggression towards strangers for defense of territory, nest, and mate has long been an identifying feature of monogamy in these species (Winslow et al., 1993). It is possible that vasopressin, like oxytocin, can act to strengthen pair bonds, and may induce parental behaviors and sexual jealousy.
A variety of placental proteins, including human chorionic gonadotropin (Asch etal., 1984; de Medeiros et al., 1992), human placental lactogen, pregnancy-specific 01-glycoprotein, placental protein 5 (Seppala et al., 1985), and ferritin (Brotherton, 1990b), have been found in human seminal plasma. In many cases their concentrations in follicular fluid and seminal plasma greatly exceeded those in the serum of nonpregnant women or men, and sometimes they even exceeded pregnancy levels. Concentrations of 0-human chorionic gonadotropin and luteinizing hormone were highly correlated with the numbers and motility of sperm in the ejaculate (de Medeiros etal., 1992). It is possible that these levels of human chorionic gonadotropin can increase the probability of conception and pregnancy maintenance in women. Seppala et al. (1985) also found a number of pregnancy-associated proteins (see Table 8.1) in follicular fluid and seminal plasma. The levels of placental protein 5 in seminal plasma showed an association with sperm motility, suggesting that placental protein 5 may have a significant biological function in the maintenance of sperm motility (Lee et al., 1983).
Relaxin is a polypeptide hormone produced in the human female by the corpus luteum and deciduas during pregnancy. In the male it is produced in the prostate and is present in human semen (MacLennan, 1991). According to Stewart et al. (1990), relaxin is significantly elevated 9-10 days following ovulation in women. Relaxin also increases 1-2 days prior to the first detectable increase in plasma human chorionic gonadotropin during pregnancy. Relaxin may have significant roles in sperm motility, fertilization, implantation, uterine growth and accommodation, the control of myometrial activity to prevent preterm labor, cervical ripening, and the facilitation of labor (MacLennan, 1991; Weiss, 1995). Its role in pregnancy maintenance has yet to be elucidated fully, but relaxin in seminal plasma may act to manipulate the female reproductive cycle or facilitate pregnancy. It is interesting that compounds defined by their pregnancy-maintaining effects are present in semen, implying that repeated insemination after conception may play a role in producing positive pregnancy outcomes (see Chapter 10 in this volume).
Pekary, Hershman, and Friedman (1983) reported the presence of high levels of thyrotropin-releasing hormone and a thyrotropin-releasing-hormonehomologous peptide, in the human prostate. Others have reported a thyrotropin-releasing-hoimone-like peptide in human semen (Gkonos et al., 1994; Khan et al., 1992; Khan Sz Smyth, 1993; Pekary et al., 1990). Gkonos et al. (1994) suggest that this thyrotropin-releasing-hormone-like peptide may play a role in human reproductive physiology. In addition, thyroid hormones, and specifically thyrotropin-releasing hormone, have been utilized in the investigation and treatment of depression. Administration of thyrotropin-releasing hormone stimulates the release of thyroid-stimulating hormone from the anterior pituitary gland and subsequent hormone production by the thyroid gland. In fact, administration of thyrotropin-releasing hormone has also been shown to successfully treat some sufferers of premenstrual syndrome (Roy- Byrne et al., 1984). Depressed patients have a significantly lower thyroid response to thyroid-stimulating hormone and show high levels of antibodies against thyrotropin-releasing hormone. Depressive symptoms are also treated with thyrotropin-releasing hormone (Nelson, 2000).
Serotonin has been found in human seminal plasma, and increases sperm motility, but extremely high levels have been correlated with certain types of infertility (Gonzales et al., 1989; Gonzales 8z Garcia-Hjarles, 1990). However, seminal volume, pH, sperm morphology, fructose, citric acid, and serum testoterone values were similar between groups of patients with different levels of blood serotonin. This suggests an optimal level of serotonin in semen for male reproductive physiology.
Although the role of serotonin in depression, and the treatment of depression, is well documented (Stahl, 2001), little to no infolination is available on activity of seminal serotonin in the female body, absorption levels, metabolism, or physiological reactions. Even though serotonin does not pass through the blood-brain barrier, it may affect peripheral sites and act indirectly to alter emotions and behavior. Since depression can be characterized as a serotonindeficiency syndrome (Stahl, 2001), the absorption of seminal serotonin could have an effect, at the very least by contributing the metabolized building blocks of serotonin and increasing serotonin synthesis.
Serotonin metabolite melatonin is also found in semen (Yie et al., 1991). Luboshitzky, Shen-Orr, and Herer (2002) found that melatonin levels in the semen of normal men averaged 0.6-5.0 pg/ml, significantly lower than blood levels. High levels of melatonin have been found in men with oligospermia and azoospermia, which suggests that melatonin may have an effect upon both spelin production and motility, but its effect in normal males remains unclear (Yie et al., 1991).
Several studies have linked melatonin to reproductive behaviors in humans (Nelson, 2000), possibly reflecting seasonal fluctuations in sexual behaviors and other hormonal levels (Cassone et al., 1993). It is also important to the development of the reproductive system (Davis, 1997). It appears that melatonin inhibits the negative-feedback loop of steroid hormones, increasing their levels and effects (Davis, 1997). ln fact, melatonin has been found to both stimulate and inhibit reproductive function, depending on how it is released (Weaver, 1997). Because of its relationship to serotonin, melatonin has also been linked to mood; in particular, it is negatively correlated with incidence of suicide (Souetre et al., 1987) and deficiencies implicated in Seasonal Affective Disorder. Other research shows that bipolar patients have a more vulnerable and deficient melatonin system (Lewy et al., 1984). There is a strong correlation between clinical symptoms of depressed mood, reality disturbance, and low levels of melatonin (Brown et al., 1987). By and large, the effects of melatonin are to induce sleepiness and fatigue (Weaver, 1997). It is possible that the addition of seminal melatonin could raise blood levels and function to induce sleep in the female after copula-tion. This is consistent with evidence showing that the resumption of an upright posture following insemination endangers sperm retention (see Chapter 7 in this volume). Melatonin is lipid-soluble and may be able to act fairly quickly. Fait et al. (2001) found that in addition to the precursor tyrosine, epinephrine, norepinephrine, 3,4-dihydroxyphenylalanine (DOPA), and 3,4-dihydroxyphenyl acetic acid (DOPAC) were found in the semen of healthy volunteers. Norepinepluine and DOPA were present in all specimens. These concentrations are respectively 19 times and twice as high as the normal concentration in plasma. No correlation was found between the concentration of any of the catecholamines and semen characteristics. These neurotransmitters, along with their precursor tyrosine, have a huge impact on human behavior. A deficiency in such neurotransmitters is the major hypothesis behind depressive disorders. Tyrosine, once absorbed by the body, can easily be metabolized into dopamine, the neurotransmitter released in all rewarding behaviors and responsible for reward and addiction, and norepin-ephrine, which is involved in greater concentration, attention, and arousal (Stahl, 2001).
by Chris Chandler; c.chandler_2004@sbcglobal.net
North Avenue Magazine 
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