I remember some years ago looking up the definition of major depressive disorder (MDD).  The book described major depression as a "psychiatric disorder" characterized by a set of typical symptoms, which it went on to describe in some detail.

But to my mind this was no definition at all, because it contained the undefined word 'psychiatric' within its supposedly defining text.  To my mind, a normal mind is one which is & remains in exact sync with reality, both in space & in time. On that basis, depressive symptoms are perfectly normal if & when they are appropriate to present, perhaps dire circumstances.  Depression should only be pathologized (considered an illness) if it takes on a life of its own, where the mind becomes mired in a place of negativity even after one's problems have been resolved or if its degree of severity is disproportionate to the severity of one's actual circumstances, to the point for example, of experiencing suicidal ideations.

Mood

Compounding our difficulties in coming to grips with mental illnesses, not only does the field of psychiatry in particular but the broader realm of modern medicine is general have the bad habit of oversimplifying the way they look at disease or disorder.  First of all they are dualistic, viewing their patients as being either completely normal or floridly diseased, a black or white approach to a gray world.  They by & large ignore the idea of pre-diseases or as they are termed, prodromes existing as preludes to or precursors of disease.  Thus in the field of diabetes we appreciate the existence of pre-diabetic states which have the capacity of progressing into floridly diabetic states.  On the other hand thanks to the use of preventive medicines such as metformin or through alterations in diet or exercise, pre-diabetics may be prevented from converting to states of full-blown diabetes. Applying this kind of thinking to the field of mental health, recent reports from the North American Prodrome Longitudinal Study indicate that cognitive dysfunction predates the development of psychosis, pointing towards the need for preventive measures in this group of patients, such as avoiding the use of cannabis, while finally focusing on concepts such as pre-depression or pre-psychosis as useful diagnostic & therapeutic ploys. 

Oversimplification is also a problem for many patients who manifest more than one mental "disorder", such as attention deficit, hyperactivity, OCD, &/or anxiety, yet who are pigeonholed with a single, reductionistic diagnosis that their insurance companies insist upon receiving.  And finally there is the tendency to focus exclusively on mental symptoms when dealing with psychiatric patients.  For example when physical symptoms are observed in depressed patients we describe them as the somatic manifestations of depression, as if these symptoms were not an intrinsic component of the disorder itself, but a separate secondary manifestation.  It would therefore be preferable to consider depressive disorder as a mind/body disease rather than an exclusively 'mental' disorder harboring secondary physical complications. 

Major Depressive Disorder (MDD).

Simplistically speaking, Major depressive disorder is a disorder of mood involving emotive, cognitive & physical, including autonomic symptoms that occurs in people who have inherited a depression-prone set of brain circuits or who have acquired them earlier in life as a result of exposure to trauma(s).  Trauma, if severe enough & reiterated sufficiently, leaves its semi-permanent or permanent mark upon the victim’s systems by etching its influence on to the patient’s stress response machinery, located in the hypothalamus.  Unlike a mechanical seismograph, which because it is  inorganic doesn’t retain a memory of previous catastrophic experiences, the mammalian stress machinery retains a memory of previous traumas, which programs it to react more & more easily & more robustly to subsequent traumas.  This process is called ‘kindling' & explains why depressive illnesses become more frequent & severe with advancing age.

Mood & the stress response.

The stress response machinery is comprised of several collections of transducer cells, cells that are nerve-like at their upper ends, allowing them to communicate nervously with the emotion-supporting or so-called limbic brain modules sitting above them, such as the hippocampus & the amygdala.  At their lower ends these cells behave in an endocrine manner, secreting tiny amounts of peptide hormones that then circulate to the nearby pituitary gland. Two populations of stress-related transducer cells exist, one group secreting the tiny peptide hormone CRF & the other secreting AVP.  Once carried to the anterior pituitary, these neuro-hormones promote the release of ACTH & beta endorphin, after which ACTH travels to the adrenal glands & in turn instructs them to produce & release the steroid hormone known as cortisol. Cortisol is the defining hormone when it comes to the human responses to stress, whereas in rats the equivalent hormone is called cortisone. 

So one can see that disturbances in cortisol secretion are a core feature of the mental illnesses, including the mood disorders & PTSD, indeed even being disrupted in patients with psychosis.  But psychosis is a condition characterized by nerve cell  loss, showing up on imaging studies as a loss of brain cell volume along with expansion of the fluid-filled ventricular spaces within the brain.  And tying the story together, surges in cortisol have the power to damage, even to kill nerve cells in the brain, most importantly in a tiny though vital area known as the hippocampus. One of our greatest errors however in modern medicine has been the tendency to ignore the role of the sexual hormones in this stress-related brain/body arrangement, focusing exclusively instead on the adrenal hormone cortisol.  The sex hormones actually play an important role in the regulation of the stress response, so it should not come as a surprise to us that sexual hormone disturbances play a major part in the development of mental illnesses.  But if that is true, it follows that the sex hormones may also carry the potential to aide such patients in their recoveries.