Increased conditioned immobility and weight loss in rats following mechanical impacts to the skull that do not produce loss of consciousness

Rats either received a single vertical impact (15 km/h) of mechanical energy to their right dorsal skulls over the parietal region or served as handled controls. About 50% of the rats appeared normal after the impact. Thirty days later there were conspicuous areas containing neurons with shrunken and darkly stained somas within the cortices beneath the impact site and within the amygdala and entorhinal cortices. These neurons, occupying an average total area that ranged from 0.50 mm² to 5 mm², were evident even in rats that showed no stunning following the impact. These neurons were not seen in control rats. Subsequent decreases in body weight for rats that received the impact (even with no obvious stunning) were attenuated by oral access to 10% glucose but not by treatment with acetaminophen or ketamine. The rats that sustained the impact also displayed increased immobility within settings with which an aversive stimulus had been associated. Post-impact injection with ketamine did not normalize this behaviour. These results show that quantitative changes in some neuronal soma remain weeks after a single impact of mechanical energy that is not associated with immediate changes in behaviour. Concomitant with these neuronal alterations was increased emotional responsiveness to contexts associated with a single aversive episode and transient decreases in body weights.