A study published in the Nutrition Journal done demonstrated that 1000mg Vitamin C used alongside fluoxetine (an antidepressant aka Prozac) reduced symptoms of depression. They suggest that this may be the result of the anti-oxidant properties of vitamin C (since there may be oxidative side effects from fluoxetine). Your brain loves Vitamin C. It was really interesting reading their discussion, they cite many papers that link vitamin C to a reduction in all sorts of psychological issues. Promising area of research. Vitamin C is one of my favourite vitamins. This is the sort of research I love!
I will also add that vitamin C is also needed for dopamine production. Dopamine is a neurotransmitter that is often forgotten about, it is vital for mental clarity and motivation. I like to explain dopamine as being the controller of the filing system in your brain. It ensures you are able to focus, absorb and store the right information.
Best sources of vitamin C: Kale (and pretty much all greens), Broccoli, Peppers, Berries.