Dopamine and oxytocin are both needed for bonding. In this research, the link between the two is made clearer than ever. Dopamine induces the release of oxytocin in the reward circuit, and reinforces sexual activity (stimulates erections in rats).
But what does this mean? On the surface it logically suggests that hot foreplay might increase your emotional bond with your lover - since oxytocin released in the brain is associated with bonding. However, we think the more important implication is that when dopamine drops, oxytocin levels can't rise, weakening emotional bonds.
Dopamine naturally drops after orgasm, and prolactin surges (which keeps dopamine down). Not only that, androgen receptors (that is, receptors for sex hormones, such as testosterone) decrease for up to a week in male rats. (Androgens stimulate the production of dopamine, too.) The fewer the receptors, the weaker the stimulus.
In short, researchers need to look at the bigger picture if they would understand the full implications of the link between dopamine and oxytocin. Lovers often think that they can make up for any sexual hangover by engaging in lots of affectionate or sexually-stimulating behavior, but this research suggests that once dopamine drops, it may be hard to raise oxytocin. In short, the natural low dopamine phase of the orgasm cycle may be interfering with lovers' desire to bond emotionally.
As we project these neurochemical changes onto our lover, we may too easily conclude that we have fallen out of love, and it is time to find a more stimulting partner with whom to repeat the cycle. This appears to be Mother Nature's objective.
Here's the link to the abstract of the study:
Stimulation of dopamine receptors in the paraventricular nucleus of the hypothalamus of male rats induces penile erection and increases extra-cellular dopamine in the nucleus accumbens: Involvement of central oxytocin