Sorry to chime in, but I have definitely read studies showing a causal relationship with D activating/regulating Tryptophan Hydroxylase. Theoretically, activation would convert more substrate toward Serotonin. The links show this increase in the CNS, but inhibition outside of the BBB. However, Vitamin D also activates Tyrosine Hydroxlyase at similar doses.
Yes, see my response to Joeyd above.