Postmortem Changes in Rat Brain: Studies on Membrane‐Bound Enzymes and Receptors

Publisher Website
Google Scholar
Abstract
The relationship between the stability of potential neurochemical markers and autolysis time was studied at 4°C and 25°C using postmortem brain samples from two rat strains. In general, qualitatively similar results were obtained with either N/Nih or Sprague‐Dawley rats; however, quantitative differences were often observed, particularly in regard to benzodiazepine receptor changes. For every enzyme activity or binding property examined, no significant change was found when brains were kept at 4°C for up to 72 h prior to freezing at ‐70°C. Na,K‐ATPase and low‐affinity Ca‐ATPase activities were also stable in brains kept at 25°C for up to 72 h. Mg‐ATPase activity was reduced in brains kept at 25°C for 24 and 48 h. [3H]Guanidinoethylmercapto‐succinic acid ([3H]GEMSA) binding to enkephalin conver‐tase in the cytosol was not significantly changed in brains kept at 25°C; however, a small increase was seen for [3H]GEMSA binding to the membrane fraction at 24, but not 48 and 72 h postmortem. [3H]Quinuclidinyl benzilate ([3H]QNB) binding to muscarinic cholinergic receptors decreased in brains kept at 25°C for 72 h. Opioid receptor binding also decreased in brains kept at 25°C. Using [3H]2‐D‐alanine‐5‐D‐leucine enkephalin to label δ opioid receptors, a statistically significant decrease in binding was observed as early as 6 h postmortem, and was completely abolished after 72 h at 25°C. In contrast, [3H]naloxone binding was unchanged after 24 h at 25°C, but was decreased after 48 and 72 h. A statistically significant increase in [3H]diazepam binding was observed in Sprague‐Dawley rat brains kept at 25°C for 24–72 h; however, the increase was not significant when N/Nih rat brains were used. The increase in [3H]diaz‐epam binding was due to an increase in both receptor affinity and apparent number. The mechanism(s) underlying these changes are unclear. The results of this study demonstrate that some, but not all, membrane‐bound enzymes and receptors are subject to postmortem changes when brains are kept at 25°C for varying periods of time. However, the time course and direction of the changes appear unique for each system examined. Extrapolating to humans, the results indicate that postmortem delay time is a potentially important variable that must be considered when utilizing human brain autopsy samples.