Thanks for pointing out the likely importance of the habenula for stress related neuropsychiatric disorders. I was not aware that these nuclei are involved in many functions like stress responses, pain processing, reproductive behavior, sleep-wake cycles, and even learning. Not so sure how the mast cells link to the habenulae?

Not sure there's any mast cells in the habenula, but you might be interested in this paper linking lateral habenula astrocytic glutamate uptake dysfunction to depressive-like behaviors 

Glial Dysfunction in the Mouse Habenula Causes Depressive-

Like Behaviors and Sleep Disturbance

The Journal of Neuroscience, December 3, 2014 • 34(49):16273–16285 • 16273

It's porbably very good hint to look at he above paper by W. cui et al. 

http://www.jneurosci.org/cgi/pmidlookup?view=long&pmid=25471567

because Mast cells regulate glutamine-co-transporters.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4405831/pdf/12876_2015_Article_275.pdf