Effect of cardiac arrest on cognitive impairment and hippocampal plasticity in middle-aged rats

Abstract

Cardiopulmonary arrest is a leading cause of death and disability in the United States that usually occurs in the aged population. Cardiac arrest (CA) induces global ischemia, disrupting global cerebral circulation that results in ischemic brain injury and leads to cognitive impairments in survivors. Ischemia-induced neuronal damage in the hippocampus following CA can result in the impairment of cognitive function including spatial memory. In the present study, we used a model of asphyxial CA (ACA) in nine month old male Fischer 344 rats to investigate cognitive and synaptic deficits following mild global cerebral ischemia. These experiments were performed with the goals of 1) establishing a model of CA in nine month old middle-aged rats; and 2) to test the hypothesis that learning and memory deficits develop following mild global cerebral ischemia in middle-aged rats. To test this hypothesis, spatial memory assays (Barnes circular platform maze and contextual fear conditioning) and field recordings (long-term potentiation and paired-pulse facilitation) were performed. We show that following ACA in nine month old middle-aged rats, there is significant impairment in spatial memory formation, paired-pulse facilitation n dysfunction, and a reduction in the number of non-compromised hippocampal Cornu Ammonis 1 and subiculum neurons. In conclusion, nine month old animals undergoing cardiac arrest have impaired survival, deficits in spatial memory formation, and synaptic dysfunction.

Fig 1. Survival rate following Sham or ACA procedure.

Kaplan-Meier curve of animal survival for seven days following Sham or ACA procedures. At day seven, the survival of Sham animals was 95% and 38% for ACA rats (Sham n = 22 and ACA n = 45).

Fig 2. Histopathology of the hippocampus seven days following Sham or ACA.

A) Representative histological images of hippocampal coronal slices of Sham and ACA animals with 40X images of the i) CA1 and ii) subiculum region. There was a visible increase in small pyknotic and eosinophilic cells (arrows) following ACA in the CA1 and subiculum regions. B) Bar graph depicting the number of non-compromised neurons per field to left (LH) or right (RH) hemisphere to their respective groups, where there was a significant decrease in the number non-compromised neurons following ACA in the CA1 region and RH of the subiculum (*p<0.05; Sham n = 6 and ACA n = 5).

Fig 3. Hippocampal acute slice electrophysiology seven days following Sham or ACA procedure.

A) Mean (± S.E.M.) input/output curve (I/O) of fEPSP amplitudes across stimulation intensities (4–15 V) in control and ACA animals (not normalized). B) Example tracings of sham fEPSP recorded in the CA1 stratum radiatum region of hippocampal slices, before (black) and after (gray) LTP-induction. C) Average slope of fEPSP (averaged to the 30 min pre-tetanus values) before and after the induction of LTP. LTP was induced by 100-Hz tetanic stimulation for 1 s and is indicated by the arrow. D) Example tracings of sham paired-pulse response. E) Average fold increase of paired-pulse response; Stimulation 2 (S₂)/Stimulation 1 (S₁) amplitude following sham and ACA procedures (*p<0.05, n = 6).

Fig 4. Barnes maze cognitive measurements following Sham or ACA procedure.

Average distance traveled A) per day [F (1, 3) = 6.68, *p<0.05, group effect] or B) across all trials (**p<0.01) on Barnes circular platform maze (testing occurred 3, 4, 5, and 6 days following sham or ACA procedures). The average latency to enter escape tunnel C) per day (*p<0.05, group effect) or D) across all trials (***p<0.005, group effect) on Barnes circular platform maze. The percentage of each animal using a search strategy (spatial, serial, or random) was also quantified on the Barnes maze. E) Percentage of trials where an animal used a spatial versus a non-spatial (serial + random) search strategy (*p<0.05). F) Percentage of trials where an animal used a systematic (spatial + serial) versus a random search strategy (***p<0.005) (Sham n = 10 and ACA n = 8).

Fig 5. Fear conditioning spatial memory deficits following Sham or ACA procedure.

Average increase in percent time frozen from baseline following fear conditioning was significantly higher in sham animals (51.44 ± 5.21%) compared to the ACA animals (33.97 ± 2.9%, *p<0.05) (Sham n = 9 and ACA n = 6).