Now let's say that a man loses his job, becomes depressed, and wants a prescription for Prozac. What made him depressed isn't the imbalance of serotonin in his brain but the loss of his job. Yet science continues to offer this kind of wrong explanation all the time. It mistakes agency for cause. The brain is serving as the agent of the mind, it isn't causing mind.Tristero rightly points out that this is dualistic nonsense, since it's unclear what kind of non-brain mental causation (or "agency") he might be talking about other than some kind of spooky causation by supernatural entities. (God or souls/ghosts.)
I would add to Tristero's complaint that it's particularly ridiculous to describe the brain as the "agent" of the mind. Agency, which means to me the ability to perform purposeful actions, is something that people possess, not something that their individual body parts possess. Just as it's a bit odd to say that my hand is my agent, since my hand has no will of its own, it's odd to say that the brain is any kind of agent. I assume Chopra is operating on some other sense of the word "agency", but I really can't tell what this might be. He seems to be saying that the mind is (or can be) a cause of events but the brain is just an agent; on my reading, this is at best the opposite of the truth and at worst totally incoherent. People really need to be careful about their definitions when they talk about these things.
Apart from my objections, however, I'd like to point out that there isn't literally nothing to what Chopra is saying.
It's true that physical, chemical causes play a very important role in many forms of depression, and that pharmaceutical interventions can be entirely appropriate and are sometimes absolutely necessary. What is not true, however, is that depression ought to be defined as a particular kind of chemical imbalance or other simple, "merely" physical state.
A state of mind or way of living counts as a form of depression only if it conforms to a certain set of societal expectations. If someone functions perfectly well, self-reports a reasonable degree of happiness and displays none of the commonly accepted signs of depression in her interactions with people, she will not and should not be considered depressed, regardless of her brain chemistry. As far as what counts as depression goes, your head might as well be full of straw; what matters is how you live your life.
Now, of course, it's true that the correlation between chemical imbalance and serious psychiatric depression is quite strong. Some serotonin/dopamine imbalances are (at least) almost always present when depressive behavior and self-reporting are present. Depressing life events can trigger these imbalances, just as they trigger depression itself, and if such imbalances were artificially induced--as they can be by frequent use of drugs like ecstasy--they (at least) almost always lead to major depression.
This very strong form of correlation is not enough to identify depression itself with a particular kind of brain state. Correlation, even 100% correlation, is not the same as identification. In this case, the identification doesn't work because it doesn't actually make sense. It makes no sense to say (except, perhaps, as a sort of metaphor) that your brain is depressed, any more than it would make sense to say that a rock is depressed. People get depressed, not brains, because people can do things, can feel emotions and perform actions that reveal their emotions. People have life histories, relationships with others, careers, etc. Brains do not. (It is nonsense, for instance, to say that my brain teaches philosophy at a community college, even though it is obviously necessary for me to have a brain to do so--I am the one doing the teaching, not my brain.)
In short: a brain is a part of someone, if an absolutely critical part; it is not what defines them as a person. What defines us as people has as much to do with things outside our body--our society, our historical era, our personal history--as it has to do with whatever the current chemical state of our brain may be. (Note that none of these factors are spooky and supernatural: "society" is a bunch of human bodies moving around, making noises and altering the surrounding environment in various ways--a very, very complicated physical process, but a physical process nonetheless.)
None of this excuses Chopra, of course. He is still wrong to say that the brain doesn't cause depression--it can and does. But this doesn't mean that these brain states literally are the same thing as depression. And this is, I would hope, a good starting point for a more legitimate critique of psychiatric practice: to an unfortunate extent, psychiatrists have come to identify brain states and depression, and thus (I think) to value pharmaceutical treatments more than counselling. We don't have to be dualists to find this problematic.