The Neurochemistry of Glutamate and MSG

Dopamine is neurotoxic. So the postsynaptic neuron has a method for dealing with it. It down regulates the receptors so there are fewer of them. Next time you need a bigger hit to get the same rush and the receptors go down. And we call that tolerance. Then when the neurons start to die, that's addiction. Now you've got a real recipe for both addiction and depression.