Diet Restriction Increases Enkephalin- and Dynorphin-like Immunoreactivity in Rat Brain and Attenuates Long-term Retention of Passive Avoidance

The present study examines effects of diet restriction (DR) on behavior and on the opioid peptides enkephalin (Enk) and dynorphin (Dyn). Female rats were assigned to ad libitum food intake (AL), DR 60% (DR60) or 40% (DR40) of AL. After 4 weeks, DR reduced fearful behavior in the elevated plus maze. DR rats displayed good retention of passive avoidance at 24 h, but DR40 rats had reduced retention, at 5 and 11 days post training. Changes in Enk- and Dyn-like immunoreactivity (LI) in the hippocampal mossy fibers (MF), hypothalamus, septum, central nucleus of amygdala (CeAm) and thalamus depended on the severity of DR. In DR60, Enk-LI and Dyn-LI were not changed except for reduction in CeAm. In DR40, Dyn-LI increased significantly above AL levels in MF, CeAm and hypothalamus, whereas Enk-LI increased significantly above AL levels in the CA3 subregion of the MF system and in thalamus. Serum glucose was tightly correlated with Enk-LI reaching highest values in the MF (r=-0.82). Increased opioid-LI in CeAm and MF was associated with reduced fearfulness in the elevated plus maze. Thus, hippocampal and amygdala opioid subsystems are uniquely sensitive to DR and may be relevant to psychophysiological problems in human starvation including anorexia.