You can send it to Nikon, but since it's fungus there may be no fix for it other than to replace some of the elements (after a thorough cleaning, of course). Last time I called them, their minimum repair charge was about $180 but I'd bet the bill would be higher, per lens of course. These are AI series lens and I'm sure you could find decent used replacements that were fungus free for less than the cost of the repair.

A few years ago (after the latest Nikons weren't working with MF lenses), I sold my 300 f4.5 and it was the ED version. It had seen a lot of use but was optically excellent and I think I only got about $250 for it, maybe less. BTW, it was one of the best lenses I've ever owned but I sold it because of the compatibility problems.