[tackem (OP)]

With even more fresh doubt being raised among experts concerning the theory linking lack of serotonin with depression can I put forward an alternative hypothesis.
Carbon dioxide blood levels.
In days gone by we used to be more active, for example we would walk a lot more and carry out manual work. Our bodies had a natural breathing rhythm which was much deeper than our current day breathing rhythm. The deeper breathing would expel more carbon dioxide from our bodies, wheras today's shallower breathing allows a build up of carbon dioxide.
Any excess carbon dioxide is dissolved in water and enters the bloodstream as carbonic acid. This reduces the PH  and turns the blood acidic. The homeostatic PH control should kick in and release bicarbonate ions and increase the depth of breath to expel more carbon dioxide.
However, what if the amygdala has reacted quicker to the PH threat and gone into fight or flight mode?
Your kidneys have turned off, so do not release any bicarbonate ions and your alveoli have gone into maximising oxygen mode meaning more carbon dioxide is being reabsorbed into the bloodstream multiplying the amygdala response.
Fight or flight response would logically lead to a panic or anxiety attack where the body will eventually recover naturally. Thus we can link carbon dioxide to panic and anxiety attacks.
If the blood levels of carbon dioxide are constantly heightened but not enough to go into a full panic attack then the brain can be under a threat of constant low level anxiety, which can be a classic symptom of depression.
Thus my alternative hypothesis indicates poor carbon dioxide clearance sets off an amygdala response leading to a variety of mental health symptoms which may include depression. If we can sequester the carbon dioxide (or carbonic acid) we may reduce the amygdala response and resolve the depression.

[paul cotter]

Depression does not discriminate as to whom it attacks, a manual labourer or an athlete both of whom breathe deeply are just as likely to suffer from depression as say, an office worker. Blood ph is very tightly controlled and if it drifted significantly you would become very ill. I do partly agree that ascribing it to low levels of serotonin is a very simplistic and most likely an incomplete explanation. The complexity of the brain with all the various transmitter molecules, feedback loops, interconnections together with the astroglia -neuron interactions means that we are far removed from a deep understanding of depression.

[trevorjohnson32]

With even more fresh doubt being raised among experts concerning the theory linking lack of serotonin with depression can I put forward an alternative hypothesis.
Carbon dioxide blood levels.
In days gone by we used to be more active, for example we would walk a lot more and carry out manual work. Our bodies had a natural breathing rhythm which was much deeper than our current day breathing rhythm. The deeper breathing would expel more carbon dioxide from our bodies, wheras today's shallower breathing allows a build up of carbon dioxide.
Any excess carbon dioxide is dissolved in water and enters the bloodstream as carbonic acid. This reduces the PH  and turns the blood acidic. The homeostatic PH control should kick in and release bicarbonate ions and increase the depth of breath to expel more carbon dioxide.
However, what if the amygdala has reacted quicker to the PH threat and gone into fight or flight mode?
Your kidneys have turned off, so do not release any bicarbonate ions and your alveoli have gone into maximising oxygen mode meaning more carbon dioxide is being reabsorbed into the bloodstream multiplying the amygdala response.
Fight or flight response would logically lead to a panic or anxiety attack where the body will eventually recover naturally. Thus we can link carbon dioxide to panic and anxiety attacks.
If the blood levels of carbon dioxide are constantly heightened but not enough to go into a full panic attack then the brain can be under a threat of constant low level anxiety, which can be a classic symptom of depression.
Thus my alternative hypothesis indicates poor carbon dioxide clearance sets off an amygdala response leading to a variety of mental health symptoms which may include depression. If we can sequester the carbon dioxide (or carbonic acid) we may reduce the amygdala response and resolve the depression.

Nice thought. Here's some additional information to consider. Gestures like tapping your toes, or krinkling a piece of paper in you pocket, anxious like fidgeting, is a result of stifling a tailbone wag. The tailbone starts to wag when you release adrenaline. Adrenaline is preceded by changes in the blood caused by changes in breathing. Once the tailbone starts to wag, it can make you dizzy and the balance system is the center for emotion and thought processes, making it dizzy amplifies those emotions. Depression can be an unnatural tailbone wag where you stare in wonder what makes a person fidget normally until you are lost and feel isolated. Music and writing, anything that is exciting for problem solving, can keep a healthy tailbone wag.

[Kryptid]

Gestures like tapping your toes, or krinkling a piece of paper in you pocket, anxious like fidgeting, is a result of stifling a tailbone wag. The tailbone starts to wag when you release adrenaline. Adrenaline is preceded by changes in the blood caused by changes in breathing. Once the tailbone starts to wag, it can make you dizzy and the balance system is the center for emotion and thought processes, making it dizzy amplifies those emotions. Depression can be an unnatural tailbone wag where you stare in wonder what makes a person fidget normally until you are lost and feel isolated. Music and writing, anything that is exciting for problem solving, can keep a healthy tailbone wag.

You're stating these things like they are facts. Can you give citations from reputable sources?

[trevorjohnson32]

You're stating these things like they are facts. Can you give citations from reputable sources?

https://academic.oup.com/biomedgerontology/article/71/2/243/2605620

[Kryptid]

I don't see anything in that paper about tailbone wagging.

[Bored chemist]

The tailbone starts to wag when you release adrenaline.

No, It does not.

Do you have any relevant  reputable sources?