Chronic glucocorticoids increase hippocampal vulnerability to neurotoxicity under conditions that produce CA3 dendritic retraction but fail to impair spatial recognition memory

Abstract

We previously found that chronic stress conditions producing CA3 dendritic retraction and spatial memory deficits make the hippocampus vulnerable to the neurotoxin ibotenic acid (IBO). The purpose of this study was to determine whether exposure to chronic corticosterone (CORT) under conditions that produce CA3 dendritic retraction would enhance CA3 susceptibility to IBO. Male Sprague Dawley rats were chronically treated for 21 d with CORT in drinking water (400 microg/ml), and half were given daily injections of phenytoin (40 mg/kg), an antiepileptic drug that prevents CA3 dendritic retraction. Three days after treatments stopped, IBO was infused into the CA3 region. Conditions producing CA3 dendritic retraction (CORT and vehicle) exacerbated IBO-induced CA3 damage compared with conditions in which CA3 dendritic retraction was not observed (vehicle and vehicle, vehicle and phenytoin, CORT and phenytoin). Additionally, spatial recognition memory was assessed using the Y-maze, revealing that conditions producing CA3 dendritic retraction failed to impair spatial recognition memory. Furthermore, CORT levels in response to a potentially mild stressor (injection and Y-maze exposure) stayed at basal levels and failed to differ among key groups (vehicle and vehicle, CORT and vehicle, CORT and phenytoin), supporting the interpretations that CORT levels were unlikely to have been elevated during IBO infusion and that the neuroprotective actions of phenytoin were not through CORT alterations. These data are the first to show that conditions with prolonged glucocorticoid elevations leading to structural changes in hippocampal dendritic arbors can make the hippocampus vulnerable to neurotoxic challenges. These findings have significance for many disorders with elevated glucocorticoids that include depression, schizophrenia, Alzheimer's disease, and Cushing's disease.

Figure 1.

Experimental timeline. Experiment 1 investigated the effects of chronic glucocorticoid (CORT) and phenytoin on Y-maze performance and susceptibility to IBO. Experiment 2 investigated the effects of chronic glucocorticoid and chronic phenytoin on CA3 dendritic complexity.

Figure 2.

Percentage of CA3 hippocampal damage after IBO infusion. Rats chronically treated with CORT drinking water and daily injections of vehicle (CV) showed more IBO-induced CA3 damage compared with control groups (VV, VP). In contrast, rats treated with CORT drinking water and daily phenytoin injections (CP) showed statistically similar levels of damage as the controls (VV, VP), indicating that phenytoin prevented the negative consequences of CORT on CA3 susceptibility to IBO-induced damage. *p < 0.05 compared with CP. n = 6 per group. Error bars indicate means ± SEM.

Figure 3.

Influence of chronic CORT and phenytoin on Y-maze performance. A, All groups showed a significantly higher percentage of entries into the novel arm than the other arm, indicating that recognition of the novel arm was intact. *p < 0.05 novel versus other arm using Wilcoxon analysis. B, Animals given injections of phenytoin showed higher positive difference scores than groups given injections of vehicle, which shows that phenytoin improves novel arm recognition in the Y-maze above the standard performance and that novel arm recognition is unaltered by a history of chronic CORT. C, Rats given CORT in their drinking water made fewer total arm entries than did rats given the vehicle drinking solution. Error bars indicate means ± SEM.

Figure 4.

Quantification of CA3 apical and basal dendritic morphology. Treating rats with chronic CORT in the drinking water and daily vehicle injections (CV) reduced apical CA3 dendritic branch points but did not significantly alter apical CA3 dendritic length. Basal CA3 dendritic branch points and length were unaffected by chronic CORT and daily phenytoin treatment (CP). *p < 0.05 compared with VV. n = 6–9 per group. Error bars indicate means ± SEM.

Figure 5.

Representations of Golgi-stained CA3 neurons. Tracings were made using a camera lucida drawing tube (320×). Note the reduced apical CA3 dendritic arbors of the CV condition relative to the other treatments.