Adult-born neurons inhibit developmentally-born neurons during spatial learning

Ongoing neurogenesis in the dentate gyrus (DG) subregion of the hippocampus results in a heterogenous population of neurons. Immature adult-born neurons (ABNs) have physiological and anatomical properties that may give them a unique role in learning. For example, compared to older granule neurons, they have greater somatic excitability, which could facilitate their recruitment into memory traces. However, recruitment is also likely to depend on interactions with other DG neurons through processes such as lateral inhibition. Immature ABNs target inhibitory interneurons and, compared to older neurons, they receive less GABAergic inhibition. Thus, they may induce lateral inhibition of mature DG neurons while being less susceptible to inhibition themselves. To test this we used a chemogenetic approach to silence immature ABNs as rats learned a spatial water maze task, and measured activity (Fos expression) in ABNs and developmentally-born neurons (DBNs). A retrovirus expressing the inhibitory DREADD receptor, hM4Di, was injected into the dorsal DG of male rats at 6w to infect neurons born in adulthood. Animals were also injected with BrdU to label DBNs or ABNs. DBNs were significantly more active than immature 4-week-old ABNs. Silencing 4-week-old ABNs did not alter learning but it increased activity in DBNs. However, silencing ABNs did not affect activation in other ABNs within the DG. Silencing ABNs also did not alter Fos expression in parvalbumin- and somatostatin-expressing interneurons. Collectively, these results suggest that ABNs may directly inhibit DBN activity during hippocampal-dependent learning, which may be relevant for maintaining sparse hippocampal representations of experienced events.

Electroconvulsive Shock, but Not Transcranial Magnetic Stimulation, Transiently Elevates Cell Proliferation in the Adult Mouse Hippocampus

Hippocampal plasticity is hypothesized to play a role in the etiopathogenesis of depression and the antidepressant effect of medications. One form of plasticity that is unique to the hippocampus and is involved in depression-related behaviors in animal models is adult neurogenesis. While chronic electroconvulsive shock (ECS) strongly promotes neurogenesis, less is known about its acute effects and little is known about the neurogenic effects of other forms of stimulation therapy, such as repetitive transcranial magnetic stimulation (rTMS). Here, we investigated the time course of acute ECS and rTMS effects on markers of cell proliferation and neurogenesis in the adult hippocampus. Mice were subjected to a single session of ECS, 10 Hz rTMS (10-rTMS), or intermittent theta burst stimulation (iTBS). Mice in both TMS groups were injected with BrdU 2 days before stimulation to label immature cells. One, 3, or 7 days later, hippocampi were collected and immunostained for BrdU + cells, actively proliferating PCNA + cells, and immature DCX + neurons. Following ECS, mice displayed a transient increase in cell proliferation at 3 days post-stimulation. At 7 days post-stimulation there was an elevation in the number of proliferating neuronal precursor cells (PCNA + DCX +), specifically in the ventral hippocampus. iTBS and rTMS did not alter the number of BrdU + cells, proliferating cells, or immature neurons at any of the post-stimulation time points. Our results suggest that neurostimulation treatments exert different effects on hippocampal neurogenesis, where ECS may have greater neurogenic potential than iTBS and 10-rTMS.