Hello,
Thanks for another interesting article. Let me try to answer your question. Of course there are others people in this forum who know much more than I do regarding this subject.

Intranasal is one of the best ways send a drug to brain, but the problem is with dopamine. It has be coated with something or formulated (as in the article you posted) so that it doesnt react or irritate the nasal mucosal layers or the lungs. Second degree of difficulty is it is a small molecule and has both activating and inhibitory properties depending on which receptor it binds to. So any coating substance used has to preserve its native structure to certain extent for Dopamine to work the way it should.
Insulin which is what the article talks about, is a protein and I guess is relatively easier to formulate than Dopamine. Dopamine is a very small molecule and possibly loses its configuration and function when formulated. If the formulation is good, Dopamine inhalation should work.

This is my theory! Any other thoughts?

Girija