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The most important ingredients in love chemistry are hormones. They are behind all emotions and also regulate our feelings of love and attachment. Oxytocin and vasopressin are the most prominent hormones implicated in pair-bonding and love, not just between partners, but also between friends, or mother and child [1]. Production of oxytocin, nicknamed the "cuddle and trust" hormone, is initiated by caressing and cuddling and is responsible for the pleasant feeling and comfort this gives us [2]. Principally, oxytocin triggers the muscular contractions required for birth and milk release during lactation, thereby also creating unconditional maternal love. Mothers who have had a caesarian section have, especially in the beginning, a weaker instinct for the cry of their child compared to mums who gave birth the natural way, initiated by oxytocin [3] (read more on maternal love in this issue's 'of Mothers, Methylation and Modernity' on pp. 8). Vasopressin plays its principal role in cardiovascular function and maintains blood pressure [4]. It is however also known as the attachment hormone. It is more important in males; while their oxytocin production is lower than in females, they use this hormone for pair-bonding instead [2,5].
Pair-Bonding
Both hormones are produced by the paraventricular and supraoptic nuclei of the hypothalamus and released into circulation by the pituitary gland, where they will then look for their respective receptors [6]. There are three different types of vasopressin receptors and one type of oxytocin receptor discovered thus far. The vasopressin receptor 1a (V1a) as well as the oxytocin receptor are present in many brain regions associated with love, including the dopamine reward system [7]. This means that at least part of the effect of oxytocin and vasopressin is dopamine-dependent. Research in prairie voles, small monogamous mammals that are used as the animal model for attachment and love, shows that the distribution and density of these receptors play an essential role in pair-bonding [8]. There are also some differences in the effect of oxytocin and vasopressin found in prairie voles. Oxytocin has anxiolytic and stress-reducing effects, and induces partner-bonding in females. Vasopressin on the other hand increases fear and stress responses, and induces partner-bonding in males [2]. This is also true in humans, and it has been suggested that it originates in prehistoric parent-child bonding, where mothers cared and fathers protected against danger [5] (see also "Mono or Poly? Which is our Nature?" on pp. 9).
Dopamine
When binding to the dopamine reward system, oxytocin and vasopressin induce dopamine release, making love a rewarding experience [9]. Dopamine production as well as expression of dopamine receptor 1 (D1) or 2 (D2) in the nucleus accumbens determines the exclusiveness of pair-bonding. Stimulation of D1 induces neuroplasticity and reward-related learning and memory, and blocks the formation of pair-bonds in prairie voles. D2 expressing neurons on the other hand project to the ventral pallidum, which integrates information from the D2-positive neurons with information from the vasopressinergic system to activate neuronal networks that aid in the formation of pair bonds [10]. In prairie voles, D1 expression is up-regulated after the first bonding has taken place, preventing promiscuous behavior [11].
First Phase Hormones
Decreases in serotonin levels are also related to love, especially the rather manic and obsessive behavior during the first phase of romantic love [1]. After this first phase in the relationship serotonin levels recover to normal levels again (see also 'The Course of True Love Never Did Run Smooth' on pp. 5). Norepinephrine [12], cortisol and testosterone [2] levels are also especially important in this first phase.
All in all, many different hormones play an important role in love and how we experience our relationships. However, their effects are not just dependent on hormone levels, but also on number and distribution of receptors. This ensures that the chemistry behind love cannot be summarized and generalized into one protocol, but is an individual mixture.
So, unfortunately, chemistry lessons will stay rather static and not so exciting; love potion brewing is not yet ready to be taught in the classroom.
[1] Zeki, FEBS lett, 2007
[2] De Boer, Neuroscience, 2012
[3] Weisman, Arch Wom Ment Health, 2010
[4] Earley, Ann N Y Acad Sci, 1966
[5] Mieras, Liefde, 2010
[6] Debiec, FEBS lett, 2007
[7] Bartels, Neuroimage, 2004
[8] Insel, Proc Natl Acad Sci U S A, 1992
[9] Young, Nat Neurosci, 2004
[10] Edwards, Nat Neurosci, 2006
[11] Aragona, ILAR J, 2003
[12] uberdionysus.livejournal.com
by Judith Houtman
This article originally appeared June 2014 volume 7, issue 02, The Neuroscience of Love
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