Most of the activty of the dentate gyrus birthing new neurons is in during early development and when a baby. During these times there's a stream neurons from the sub-granular zone (beneath the granular cells in the dentate gyrus in the hippocampus). The perceptions of the external environment and the internal physiological state modulate the rate of survival of these new neurons.
It's kind of open in the sub-granular zone and the stem cells are able to move freely and assume multiple intermediate cell architectures before finally becoming dentate granule neurons that migrate to and integrate with the existing hippocampal circuitry along the other parts of the two nesting C shapes. They're even somewhat glia-like during part of this intermediate development. It's a complex process that's easy to interrupt. Most of the cells die and do not become integrated into the hippocampus.
So, to get to and restate the question: how do neuroactive chemicals and things like ECT potentially alter the rate of new sub-granular zone stem cells? If the current development state of the cell has receptors or the glia around it are effected by chemicals that bind to those receptors it's easy to see how things could be thrown off.
It is not necessarily a good thing to increase the rate of cell survival of new neurons in the adult. The rate of new neurons has important effects on the ability of the neuronal circuits in the rest of the hippocampus (involving CA1, CA3, etc) to do the types of memory encoding tasks researchers call pattern separation and pattern completion. So while there are indeed chemical and electrophysiological ways to increase the the survival rate of new neurons there should be reasons to believe the balance needed to be adjusted up.
Just as a throwaway, THC is one of the chemical compounds found to increase new dentate granual neuron survival and integration.