Summary The dentate gyrus is hypothesized to function as a “gate”, limiting the flow of excitation through the hippocampus. During epileptogenesis, adult-generated granule cells (DGC) form aberrant neuronal connections with neighboring DGC, disrupting the dentate gate. Hyperactivation of the mTOR signaling pathway is implicated in driving this aberrant circuit formation. While the presence of abnormal DGC in epilepsy has been known for decades, direct evidence linking abnormal DGC to seizures has been lacking. Here, we isolate the effects of abnormal DGC using a transgenic mouse model to selectively delete PTEN from postnatally-generated DGC. PTEN deletion led to hyperactivation of the mTOR pathway, producing abnormal DGC morphologically similar to those in epilepsy. Strikingly, animals in which PTEN was deleted from ≥9% of the DGC population developed spontaneous seizures in about four weeks, confirming that abnormal DGC – which are present in both animals and humans with epilepsy – are capable of causing the disease.

Aberrantly interconnected granule cells are characteristic of temporal lobe epilepsy. By reducing network stability, these abnormal neurons may contribute directly to disease development. Only subsets of granule cells, however, exhibit abnormalities. Why this is the case is not known. Ongoing neurogenesis in the adult hippocampus may provide an explanation. Newly generated granule cells may be uniquely vulnerable to environmental disruptions relative to their mature neighbors. Here, we determine whether there is a critical period after neuronal birth during which neuronal integration can be disrupted by an epileptogenic insult. By bromodeoxyuridine birthdating cells in green fluorescent protein-expressing transgenic mice, we were able to noninvasively label granule cells born 8 weeks before (mature), 1 week before (immature), or 3 weeks after (newborn) pilocarpineepileptogenesis. Neuronal morphology was examined 4 and 8 weeks after pilocarpine treatment. Strikingly, almost 50% of immature granule cells exposed to pilocarpine-epileptogenesis exhibited aberrant hilar basal dendrites. In contrast, only 9% of mature granule cells exposed to the identical insult possessed basal dendrites. Moreover, newborn cells were even more severely impacted than immature cells, with 40% exhibiting basal dendrites and an additional 20% exhibiting migration defects. In comparison, Ͻ5% of neurons from normal animals exhibited either abnormality, regardless of age. Together, these data demonstrate the existence of a critical period after the birth of adult-generated neurons during which they are vulnerable to being recruited into epileptogenic neuronal circuits. Pathological brain states therefore may pose a significant hurdle for the appropriate integration of newly born endogenous, and exogenous, neurons.

Prolonged isoflurane exposure in neonatal mice led to increased immediate brain cell degeneration, however, no significant reductions in adult neuronal density or deficits in spontaneous locomotion, spatial learning, or memory function were observed.

In neonatal mice, equipotent doses of the three commonly used inhaled anesthetics demonstrated similar neurotoxic profiles, suggesting that developmental neurotoxicity is a common feature of all three drugs and cannot be avoided by switching to newer agents.