Meditation can cause a lot of rest and peace. But can it also change the levels of hormone and neurotransmitters? Do they increase or decrease? I tend to think that fe your adrenaline is getting down and perhaps your testosteron. So can you become less 'male' or 'female' of it?
So far I have only one first-hand report of a practitioner diagnosed with ADD due to dopamine re-uptake problems, who says meditation helps him regulate the levels of dopamine.
From buddhist theory we know that the Four Jhanas meditation is intended to produce joy, which should correlate with the functioning of the reward mechanism of the brain, which is primarily dopamine-based. So another way to see the Jhanas, esp. the first and second one, is as mastery over one's dopamine levels by taking control of the reward mechanism.
As far the levels of other hormones and neurotransmitters, I can only assume they get "better", but there is nothing I know first-hand. A nice book on the topic seems to be Zen and the Brain: Toward an Understanding of Meditation and Consciousness but I have not read it.
from my personal experience, joy from meditation has exact feeling as when I take opiate but without crash landing or feeling down after. And given history of stokes in my family, I try to get brain MRI regularly every 5 years. I have a friend who is a neuro-radiologist to help me read my film. (I did my best to meditate while i was in that loud banging tunnel of MRI scanner). I think it had effect on my brain that control breathing....i forgot the name of it. it showed slightly brighter than normal.. not much but just a notch. he said typically that part of the brain would show brighter when activity is slow such as someone who is rescued from drowning or someone is taken very small amount of cyanide or when brain activities are slowing down (no need for large amount of O2 in that moment). I do not know if it has anything to do with my meditation but there it was. But, the over all finding was "normal".
I am reading an interesting book, "The molecule of more" recommended by a life-coach. It is basically dividing major brain chemicals into 2 categories. First is the so-called "desire circuit", aka dopamine, and the second is referred to as H&Ns (here and now) - serotonin, oxytocin, vasopressin, endorphins, etc. Dopamine, which drives desire, or wanting is always present for anticipation, motivation and something called "reward prediction error" - basically you get a hit of dopamine when an unexpected (surprise) reward happens. Although it would seem that dopamine is the bad chemical from a Buddhist perspective, given its strong tie to desire, there is another key role for dopamine, the control circuit. The authors describe this as dopamine vs. dopamine. To sum it up, it allows us to add delay to our reward-seeking for a better future...maybe call it sila (control of mind). A highly-motivated Buddhist might say, "i will not get nirvana today, or tomorrow, nor in this lifetime, but after 7 more human lives I will definitely obtain the unexcelled freedom!" This is dopamine working the pre-frontal cortex. Dopamine of immediate pleasure-seeking is substituted with long-term goals, and as Andrei points out it's "mastery over one's dopamine levels..."
As per meditation (concentration) itself, it might help to strengthen the control circuit, though I would think concentration had a specific goal for Buddha, ie, entering the jhana..if one fails, dopamine would stop flowing, causing a person to feel bad. If concentration never achieved its purpose, effort would cease - kind of like me.