BTW/FWIW there's a connection between serotonin and BDNF (brain derived neurotrophic factor) that promotes neuron growth, new neurons from stem cells - and it might do the same (no data either way) for astrocytes. There's also a connection between serotonin and depression. SSRIs (serotonin selective reuptake inhibitors) increase the level of serotonin - and BDNF. There's more to depression than low serotonin (SSRIs only help about half the people with depression); people with depression are often prescribed SSRIs. The study is observational, but a difference between therapists who are allowed to prescribe (typically psychiatrists) and those who are not might be used as a proxy for SSRI use.
There's no evidence that low serotonin leads to low BDNF, but it might be going on. (ran across this while looking at the interlocked systems that make up the 'BBB'). Again, on a mobile, no links - sorry.