Let's see if I can help you out with an answers rather then just say...everyone reacts differently...that wouldnt be satisfactory to a man of your intellect and knowledge...so here goes.
Dopamine is one of the brains most abundant neurotransmitters. Depending on the location of upregulation will depend on its effect. There are 5 kinds of receptors d1-d5 throughout the brain. Depending on the area will depend on the concentration of each receptor.
The area of upregulation you where previously concerned with would be the substantia nigra. This is the area associated with parkinsons. Rest assured the only studies i see that take this into account state that supplementation with L-Dopa extracted from mucuna is neuro protective for this area of the brain. Here is one for you to chew through and spit out look up more and then become more knowledgeable then I lol (
http://onlinelibrary.wiley.com/doi/10.1002/ptr.1514/abstract) Neuroprotective effects of the antiparkinson drug Mucuna pruriens
Bala V. Manyam1,*, Muralikrishnan Dhanasekaran1, Theodore A. Hare2
(
http://onlinelibrary.wiley.com/doi/10.1002/ptr.1407/abstract)
Effect of antiparkinson drug HP-200 (Mucuna pruriens) on the central monoaminergic neurotransmitters
Bala V. Manyam1,*, Muralikrishnan Dhanasekaran1, Theodore A. Hare2
This still doesnt answer your question...is it a nuerostimulant? Well not like caffeine or geranium but it is one of the major contributors to out overall sense of well being and motivation. I will use a quote from Affective Neuroscience: The Foundations of Human and Animal Emotions (1998), Jaak Panksepp dopamine circuits "appear to be major contributors to our feelings of engagement and excitement as we seek the material resources needed for bodily survival, and also when we pursue the cognitive interests that bring positive existential meanings into our lives." He explains that without dopamine, "human aspirations remain frozen…." He goes on to say that when dopamine is abundant in synapses, "a person feels as if he or she can do anything." (this excerpt can be found at
http://mybrainnotes.com/brains2.html#dopamine).
So why take it before bed? Well obviously for its effects on GH,which can also be beneficial pre-lift or post-lift. But GABA being a sleep aid should help relax the body to a state where it is more susceptible to fall asleep and then the synergy between the mucuna and the GABA will both raise GH release while asleep.
But again...as you well know...this isnt the cause for everyone b/c some individuals can take GABA and not relax...rather it can help stimulate them and they take it before a lift. The best way is to literally find out what it does for you.