I think what is important here is NMDA/glutamate. Some PSSD types may be related to dysfunction of the hypothalmus. This would explain HPA axis issues we see in some people, like myself with low cortisol. A hypoactive hypothalmus reduces glutamate firing in general.
What we should try to do is improve glutamate firing either by improving the HPA axis or via other receptors. Glutamate is heavily involved in hedonistic and emotional processing. Glutamate firing is likely inhibited by high serotonin. It is also inhibited by its autoreceptor, mGluR2. Downregulating mGluR2 will increase glutamate levels.
Psychedelics like psilocybin activate the 5HT2A-mGluR2 complex. This is a receptor complex formed by a combination of 5HT2A and mGluR2 receptors. Only some 5HT2A agonists like psychedelics activate the 5HT2A-mGluR2 complex. By activating it acutely, mGluR2 can be downregulated afterwards. This could explain why some people improve after psychedelics like mushrooms, because it heavily activates 5HT2A-mGluR2, downregulating mGluR2. Then after the drug leaves the system you are left with a downregulated mGluR2, allowing glutamate levels to rise. This effect lasts anywhere from days to months, but it is not clear if it can be permanent.
Direct acute mGluR2 agonists could have a similar effect, but there are not really any readily available safe options. Allosteric modulators of mGluR2 could also work, but these are also not readily available.
Now onto the HPA axis:
As has been previously suggested, downregulating GR (cortisol receptor) on the pituitary gland could help people with low cortisol from overly sensitive GR. Simply increasing cortisol is probably not enough to fix things, as the
HPA axis is complicated (mathematical model). Activation of GR actually upregulates GR on the pituitary gland, so more cortisol = more sensitive GR = even more cortisol!
This is one of the ways PTSD/CSF could happen. High stress leads to high cortisol which upregulates GR. Then the patient will be stuck with overly sensitive GR. This leads to being stuck in a state of low cortisol, sensitive GR, and underactive hypothalmus, leading to less glutamate firing = less pleasure/emotions.
This study uses the HPA mathematical modelto suggest that external stressors applied for the right duration at the right time can REVERSE the low cortisol, oversensitive GR state! So stress yourself out just a little bit, and you might be able to paradoxically reverse HPA axis dysfunction! But figuring out what counts as enough "stress" and for how long is very difficult.
Another study analyzes the model and suggests GR antagonists can return one from this state to normal. Unfortunately RU-486 probably won't work for this because it doesn't enter the brain, and instead actually raises cortisol in the brain by blocking cortisol from crossing the blood-brain barrier. One would need to antagonize GR on the pituitary gland in the brain or somehow lower cortisol levels.
I should probably make a new thread on this to organize it better. But basically, we want to increase glutamate in the brain by fixing the HPA axis, if that is indeed a problem for you. Additionally, high serotonin or upregulated 5HT2A receptors could be a problem inhibiting glutamate.