I don't know if there are any really good resources on this but the two major hormones involved in BF are prolactin and oxytocin. Some studies indicate that BFing moms are less likely to develop PPD. Other studies show that the hormones help BFing moms regulate stress hormones better. Other studies show that giving up breastfeeding does not help alleviate depression.
Interesting question, let me know what you turn up.
From personal experience, if the depression is not directly related to nursing, then nursing ,and the hormones it triggers while doing it, didnt help for me. In other words, if the depression is being caused by, lets say, an unhappy relationship, then the way you feel during nursing, only lasts as long as you nurse for, so for 40mins I might feel quite good, and even after for a while, but the old feelings would still come back, so my answer would be yes it helps alleviate depression(while nursing) but in the long term no.
I hope that made sense