The effect of amygdala kindling on hippocampal neurogenesis coincides with decreased reelin and DISC1 expression in the adult dentate gyrus

Abstract

Temporal lobe seizures can induce the proliferation and abnormal migration of newly generated dentate granule cells, but little is known about the molecular mechanisms that govern these pathological events. Reelin and DISC1 (disrupted-in-schizophrenia 1) are proteins that play a regulatory role in the maturation and integration of new neurons in the developing and adult brain. In this study, we examined whether amygdala kindling results in aberrant neurogenesis and altered expression of reelin and DISC1 in the adult dentate gyrus. Using doublecortin immunohistochemistry, we found that short-term kindling (i.e., 30 electrical stimulations) significantly increased the number of immature neurons in the dentate subgranular zone (SGZ), whereas long-term kindling (i.e., 99 electrical stimulations) did not. However, doublecortin-labeled neurons in long-term kindled rats showed greater dendritic complexity than they did in short-term kindled or control rats. We also found that long-term kindling decreased the number of reelin-positive cells and decreased DISC1 expression in the dentate granule cell layer and subgranular zone. Interestingly, kindling-induced changes in reelin and DISC1 expression coincided with the appearance of ectopically located Prox1-labeled granule cells in the hilus. These effects occurred independently of alterations in granule cell layer length, dentate volume, or the number of hilar neurons. Taken together, these findings suggest a novel role for DISC1 in the pathophysiology of temporal lobe epilepsy and further suggest that changes in reelin and DISC1 expression may contribute to aberrant neurogenesis in the kindling model.

Aberrant hippocampal neurogenesis after limbic kindling: Relationship to BDNF and hippocampal-dependent memory

“…Interestingly, limbic-kindled rats also had significantly more DCX-ir cells containing basal dendrites. These findings are consistent with previous reports that seizures facilitate dendritic outgrowth and the presence of hilar basal dendrites [16,17,54,55]. What mechanisms may account for this hypertrophic dendritic growth?…”

“…Previous reports have indicated that limbic seizures downregulate the expression of migratory proteins, such as the extracellular matrix protein reelin and the susceptibility gene disrupted in schizophrenia 1 (DISC1), in the SGZ or GCL [16,52,53]. This likely contributes to aberrant migration of adult generated neurons after limbic kindling.…”

“…For all counts, we employed unbiased stereology using a modified optical fractionator method that excludes cells in focus at the uppermost focal plane to reduce oversampling [46]. The total number of BrdU-ir and DCX-ir cells was estimated using the following formula: N total = ∑ Q − × 1 / ssf × A(x, y step) / a(frame) × t / h. ∑Q − represents the number of counted cells, ssf is the section sampling fraction (1 in 12), A(x, y step) is the area associated with each x, y movement (10,000 μm 2 ), a(frame) is the area of the counting frame (3600 μm 2 ), t is the weighted average section thickness, and h is the height of the dissector (12 μm) [16,45]. To avoid counting sectioning artifacts, we used a guard zone of 2 μm.…”

“…Category five and six cells represented the postmitotic stage and included cells with one major process extending into the molecular layer (category five), or a defined dendritic tree with delicate branching in the granule cell layer (category six). As seizures can increase the number of adult generated neurons containing hilar basal dendrites [16,17], we also recorded the number of randomly sampled cells containing processes extending into the hilus. The data are presented as the percentage of DCX-ir cells in each of the six categories and the percentage of sampled cells containing hilar basal dendrites.…”

“…For example, seizures are known to increase hippocampal cell proliferation and survival, and some of these newborn neurons migrate ectopically into the hilus and develop basal dendrites [14,[16][17][18][19]. Many other newborn neurons do migrate normally into the GCL, but they mature more rapidly than normal.…”