T HE R OLE OF S EXUAL A CTIVITY
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 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.