Mother rats who took really good care of their pups produced more oxytocin and showed more "reward" in their dopamine system. If oxytocin was blocked, dopamine reward didn't happen. Same thing might be going on with bonding behaviors between lovers. As oxytocin response gets stronger, so does dopamine reward.??
[Endocrinology. 2010 May;151(5):2276-86. Epub 2010 Mar 12.
Shahrokh DK, Zhang TY, Diorio J, Gratton A, Meaney MJ.
Sackler Program for Epigenetics and Psychobiology at McGill University, Douglas Mental Health University Institute, McGill University, Montréal, Canada.
Comment in: Endocrinology. 2010 May;151(5):1978-80. ]
Variations in maternal behavior among lactating rats associate with differences in estrogen-oxytocin interactions in the medial preoptic area (mPOA) and in dopamine levels in the nucleus accumbens (nAcc). Thus, stable, individual differences in pup licking/grooming (LG) are abolished by oxytocin receptor blockade or treatments that eliminate differences in the nAcc dopamine signal. We provide novel evidence for a direct effect of oxytocin at the level of the ventral tegmental area (VTA) in the regulation of nAcc dopamine levels. Mothers that exhibit consistently increased pup LG (i.e. high LG mothers) by comparison with low LG mothers show increased oxytocin expression in the mPOA and the paraventricular nucleus of the hypothalamus and increased projections of oxytocin-positive cells from both mPOA and paraventricular nucleus of the hypothalamus to the VTA. Direct infusion of oxytocin into the VTA increased the dopamine signal in the nAcc. Finally, high compared with low LG mothers show greater increases in dopamine signal in the nAcc during bouts of pup LG, and this difference is abolished with infusions of an oxytocin receptor antagonist directly into the VTA. These studies reveal a direct effect of oxytocin on dopamine release within the mesocorticolimbic dopamine system and are consistent with previous reports of oxytocin-dopamine interactions in the establishment and maintenance of social bonds.