Neptune was an important source of extracted deuterium and He-3 in the Interplanetary age; first reached by a Brazilian mission in 250 AT which colonized its moons Proteus and Triton, but the mission was subverted at long range by the Augmentation Activist faction. The AAs continued to hold Proteus even after the arrival of other explorers, including a secondary colonization mission from Jupiter.
The first bubblehab was established in Neptune's atmosphere in the 290's, and although distance from the other parts of Solsys slowed immigration and development a great variety of groups, some eccentric and isolationist, established themselves there in the following centuries. As with Saturn and Neptune, but to a lesser degree, the Great Expulsion saw an influx of refugees from Old Earth, some independent and some sponsored by agents of GAIA. After a long period of conflict during Solsys' Dark Ages these groups came an accommodation with each other and with the orbital polities, one that was eventually so peaceful and prosperous that Neptune was a late and cautious member of the First Federation, with many individual holdout polities.
Currently Neptune and its many atmospheric and orbital settlements are integrated to one degree or another with the Solar Organization.