No, the minor depressions can be solved with psychological treatment. Antidepressants
are used for the serious depressions with the patients that don't show their symptomatology in the time and way they were expected to after some time.
An effective psychotherapy can modify the brain's biology, because the mind has an influence on it.
Experiments can be used to alter the physic but in other cases the time has been too long and it has changed the neuronal processes and then the antidepressant medicines try to balance
the neurotransmission levels (See What is the depression/psychofarmacs)