Quote: Then why don't they simply modify the NTs chemically so that they pass through the BB barrier? Wouldn't this be much easier? Then you could simply take whatever NT you need instead of taking reuptake inhibitors.

Some drugs are actually like this. Drugs like DMT and psilocybin are serotonin mimics, but unlike serotonin, they have affinity for only a few serotonin receptors. The issue has to due withis modified on the molecule.The problem with modifying structures is that each NT has a unique structure-acitivity relationship. For example, ring-constrained molecules (i.e Bromo-Dragonfly) have full-agonist ability on the 5ht2a receptor because they are able to fit into the binding pocket very well. Mu-opioid agonists work by interaction with negatively charged residues on one side and positively charged residues on another. Modification of dopamine by removing the two phenolic hydroxyl groups and attachment of the alpha-methyl carbon gives amphetamine. Addition of the methyl group on the amine gives methamphetamine, which has dopamine receptor agonist properties as well as increased BBB penetration properties.I believe there are also some "GABA" prodrugs that metabolize to GABA. Heroin metabolizes to 6-MAM in the brain and acts as a prodrug for morphine as well.The reason they think SSRIs promote brain cell growth is an indirect increase in the amount of brain-derived neurotrophic factor produced (BDNF). This causes branching of neurons, particularly in the hippocampus.Interestingly enough, cannabinoids (synthetic cannabinoids according to the Xia Zhang publication; neuroprotective effects demonstrated with marijuana and its constituents) have been shown to do the exact same thing.With that said, we share remarkable homology with many mammals -- in fact, out genetic material is quite similar even to worms. The thing is that our brains are designed in a completely different fashion than most mammals (barring primates, our closest relatives). It's not what receptors are present (most of our receptors are conserved in our ancestors), but