While several DA systems exist in the brain, the mesolimbic DA system appears to be the most important in this respect. Both D1 and D2 receptors though partially functional antagonists are both significantly expressed in the NA. Other DA receptors (D3-5) are also linked to the Espanjan morsiamet laillinen limbic system and are substantially present in the amygdala and the hippocampus. Their functions include reward and motivation and appear to share common morphologic evolutionary and molecular roots. Endogenous opioids may also be involved in this process.

Early studies that involved functional MRI, which used the partner's photograph as a visual stimuli confirm the involvement of the right ventral segmental area (VTA) which is a central region of the brain's reward system associated with pleasure, general arousal, focused attention, and motivation to pursue and acquire rewards. The VTA projects into several regions including the caudate nucleus which plays a role in reward detection, expectation, representation of goals, and integration of sensory inputs to prepare for action. These appear to be true of both early intense (7.4 months) and a little later and not so intense love (28.8 months).

Clearly, sexual activity is an important component of the reinforcement of the reward system, and this appears to reinforce attachment. Increasing levels of testosterone and oestrogen promote DA release. Similarly, elevated activity of dopaminergic pathways appear to increase the release of testosterone and oestrogen. The sympathetic nervous system also appears to contribute.

Behavioral data support the complementary but distinct pathways for love and sex drive: (a) While sexual drive is often expressed toward a range of individuals while love is focused on one particular individual (b) the sex drive can be quelled when satiated; love does not decrease with coitus and persists unabated for months or years. Sex drive enables individuals to initiate courtship and mating with a range of partners; love focuses mating energy to specific individuals conserving time and metabolic energy (6).

The relationship between elevated central DA sex steroids, sexual arousal, and performance appears to be conserved in humans (7)

Pleasure and reward activate behavioral patterns that get memorized for the goal of repetition and faster and better recognition later. There is clear evidence to support a connection between attachment behaviors and pleasure pathways that involve hippocampal mechanisms.

Complex interactions between gonadal reward and sympathetic systems demonstrate there are distinct but overlapping neural networks involved in love and sex the latter contributing to the reinforcement of the former

The VP is a major target of the NA. The interaction of OT, AVP, and the DA systems within the reward circuitry appears to be the foundation of monogamy. In promiscuous species, the male feels the attraction but does not associate the pleasurable feeling with a specific partner and so does not initiate long-term attachment (23). The relationship with the DA reward systems also appears weaker in these species. Their interdependence also distinguishes romantic love from more platonic attachments including friendship and maternal love (vide above).

It is interesting to extrapolate this to humans. Humans engage in sexual activity throughout the cycle which may serve to strengthen the pair bond. Interestingly and in contrast to other species human females have enlarged mammary tissue independent of lactation. Breast and nipple stimulation are integral to human sexual activity. Nipple stimulation during lactation is one of the most potent stimulus for OT release. This part of sexual activity may reinforce pair bonding in humans. OT levels are elevated in women during orgasm and AVP in men increase during sexual arousal adding validity to the notion that sexual activity indeed reinforces the bond. In the postpartum state when sexual activity and desire decreases probably as a trade off in reproductive interests. This appears to be mediated by OT through activation of reward centers in the VTA.