Re: How are mood disorders caused?Are they due only to biochemical disturbances

     “If a drug such as lithium carbonate alleviates symptoms of mood 
disorders, then it must be that mood disorders are caused ONLY by 
biochemical disturbances”.  The answer to your question seems to be “yes” 
and “no”. Yes, some mood disorders do seem to be biological in nature, but 
environmental factors can cause mood disorders as well.  Everyone 
experiences depression to some extent after having a bad day or after a 
romantic breakup.  Clinical depression can also occur after a traumatic 
experience or after the death of a loved one. I think part of your dilemma 
is the mind-body “curse” that psychology and related fields have been 
struggling with for hundreds of years.  For example, I suppose in some 
sense everything we do can be reduced to a biochemical process, but I don’t 
think this really helps us as a science.  I prefer the assumption that the 
mind and body cannot really be divided or studied separately.
     Getting back to the first part of your question, it is clear that mood 
disorders can be brought on by biological factors. For example, we know 
that depression (at least clinical depression) is related to reduced levels 
or reduced utilization of several neurotransmitters.  These include 
serotonin, norepinephrine and probably dopamine.  For example, if you give 
a group of “normal” people Reserpine (which is a drug that reduces  
serotonin and norepinephrine in the brain) some persons (but not everyone) 
will become depressed.  We also know that many anti-depressant drugs, such 
as selective serotonin reuptake inhibitors (SSRI) or the tricyclics (TCA) 
work by blocking reuptake of these neurotransmitters. The result is that 
there is an increased amount of neurotransmitter left in the synaptic cleft 
and more of it can reach the postsynaptic neuron (actually this is only 
part of the reason why antidepressants work, but the other factor is much 
more complicated).  Monoamine oxidase inhibitors work by decreasing the 
enzyme monoamine oxidase, which deactivates serotonin, norepinephrine and 
dopamine.  By decreasing monoamine oxidase, the level of these 
neurotransmitters also increases.
     You mention bipolar disorder in your question. This  is an interesting 
disorder that is possibly caused by too much norepinephrine or perhaps an 
imbalance of serotonin, norepinephrine and dopamine. It is true that 
lithium (which is a salt) decreases symptoms in some individuals.  Lithium 
is not really an anti-depressant. It is a mood stabilizer.  However, the 
exact mechanism of action is not really well understood (by the way, 
recently lithium has been replaced as the first drug of choice for bipolar 
disorder by anti-seizure medications such as Valproic acid). 
      In addition to treating mood disorders with drugs, psychological 
treatments have also been found to be effective. For example, some studies 
show that cognitive therapy is as effective as medication in treating 
certain types of major depression.  In fact, this type of treatment may be 
the best over the long run because when medication is stopped, the 
depression often returns. This is less likely to occur with cognitive 
therapy.  In addition, brain imaging studies done with these patients show 
actual brain changes after psychological treatment.  So you see that even 
though biological factors can cause psychological disorders, psychological 
factors can also change biological processes. This gets back to my original 
idea that the mind and body should not be viewed as being separate.  I hope 
this helps. A college level text that I recommend that you read is 
“Abnormal Psychology” by Gerald C. Davison and John M. Neale.