Berberine
Berberine has helped some guys with PSSD
https://www.pssdforum.org/viewtopic.php?p=30015&hilit=phenibut#p30015
"Berberine is a presynaptic agonist and postsynaptic antagonist of the 5HT1A receptor. Perhaps after taking it for 2 weeks, it has downregulated the presynaptic receptors while upregulating the postsynaptic ones. It's also a 5HT2 antagonist.
https://www.sciencedirect.com/science/article/abs/pii/S0024320504006502?via%3Dihub
Berberine is also an AMPK activator, similar to Metformin, so perhaps it also upregulates TrkB receptor and BDNF expression, being neurotrophic. It's such an interesting supplement, but it's risky to try."
Berberine inhibits PDE5 mRNA expression -
https://pubmed.ncbi.nlm.nih.gov/15638014/
Effects NOS mRNA expression -
https://link.springer.com/article/10.1007/BF02873556
https://link.springer.com/article/10.1007/BF02873556
Berberine can inhibit prolyl oligopeptidase, raising neuropeptides -
https://www.researchgate.net/publication/6510674_The_Natural_Product_Berberine_is_a_Human_Prolyl_Oligopeptidase_Inhibitor