'Happy Hormone' and Human Depression?

Serotonin is actually a neurotransmitter and not a hormone. The chemical has been dubbed the 'happy hormone' (aliteration) because some people have repeatedly suggested that low concentrations of this factor can be linked to depression. Indeed, many of the psychoactive drugs used to treat this neurosis are linked to serotonin receptors. In spite of this, a major review has concluded that there is little evidence depression is a consequence of low brain serotonin levels. A recent study in Biological Psychiatry seems to counter the review's 'firm' conclusion (https://www.theguardian.com/society/2022/nov/05/study-finds-first-evidence-of-link-between-low-serotonin-levels-and-depression). The new study contrasted 17 patients with a major depressive disorder or depression linked to Parkinson's disease (a movement condition linked to low brain levels of another neurotransmitter, dopamine) with 20 healthy volunteers. All were scanned using Positron Emission Tomography (PET). This scan used a radioactive tracer, to determine how much serotonin was bound to designated brain receptors. All subjects were then given amphetamine ('speed'), a psychoactive drug that stimulates serotonin release and rescanned. Only the depressed patients, showed a reduced serotonin response. This was taken as evidence that serotonin does play an important role in depression. The Biological Psychiatry study is, however, small and doubts have been raised about the statistical analysis used. Perhaps the following points are worth noting? Firstly, there has long been antagonism between folk who advocate treatment of depression with psychoactive drugs and those who favour other therapies (e.g. talking therapies). Secondly, 'depression' is certainly not a specific 'disease', like, for example, a simple infection. Depression covers a range of conditions, of varying intensity (and causation?). Thirdly a simple link between concentrations of a neurotransmitter in specific brain regions and a neurosis or psychosis seems inheritently unlikely. The kinetics of neurotransmitter actions appear complex. There are sub-types of many transmitters and all may exhibit binding, release from the binding site, reuptake etc. It's also very evident that different parts of the brain 'talk to eachother', in complex ways. As they say, 'This one will run and run'!