I have read that alpha-melanocyte stimulating hormone (alpha-MSH)
inhibits appetite in humans. I understand that this hormone is
released from the pituitary by MRH from the hypothalamus, and forms
part of the melano-cortisol system. Can daily exposure to very bright
light result in higher blood levels of alpha-MSH (so that someone
living in tropical or subtropical areas would have higher alpha-MSH
levels)? Could very bright light stimulate the melano- cortisol system
generally?

a-MSH neurons in the Arcuate Nucleus of the Hypothalamus are typically
stimulated by leptin levels. aMSH neurons proeject to the lateral
hypothalamic area, where they control the inhibition of feeding
behavior. They also project to the brainstem and to preganglionic
sympathetic neurons as well as to the Paraventricular Nucleus of the
Hypothalamus. This input to the PVN then stimulates the release of
ACTH and thyrotropin from the anterior pituaitary. To my knowledge MRH
isn't involved, and light probably isn't involved, although that isn't
fact. Its a good hypothesis though. Keep in mind, nuclei of the
hypothalamus are differentially specific for light sensativity and
other functions - when thinking light, think the Suprachiasmatic
nucleus.

With regards to ACHT, I don't know if you know the pathway, but ACTH
released from the anterior pituitary results in increased Cortisol,
resulting in increased blood glucose.