Effects of glucose and fructose on recently reactivated and recently acquired memories.

1. The effects of glucose and fructose on memory reactivation were investigated. 2. Rats were trained originally on a brightness discrimination passive avoidance task. 3. Memory reactivation treatment consisted of re-exposing the rats 24 hr later to the footshock unconditioned stimulus in the experimental room. Glucose or fructose (32, 100, 320, 1000, or 2000 mg/kg) was administered immediately after reactivation. 4. Twenty-four hr after reactivation (48 hr after training) the rats were tested for their ability to acquire an active avoidance (reversal) task. 5. The dose-response functions for the effects of both glucose and fructose on the reactivated memory followed identical cubic trends. However, a combined dose of glucose and fructose was significantly less effective at modulating memory than was an equimolar dose of either sugar alone. 6. We compared analytically the effects of combined glucose and fructose treatment on new versus old memories. The dose-response functions for both types of memories follow cubic trends, suggesting that similar multiple interacting mechanisms operate when memories are originally stored and when they are later re-encoded.

Time-dependent effects of fructose on the modulation of a reactivated memory.

1. A passive-avoidance-to-active-avoidance negative transfer paradigm was used to investigate systematically the time-dependent effects of fructose on reactivated memories in rats. 2. Memory reactivation consisted of re-exposing the rats 24 hr after passive-avoidance conditioning to environmental and learning cues present during training; post-reactivation injections of fructose (100 mg/kg, sc) or saline were followed 24 hr later by active-avoidance (discrimination reversal) conditioning. Fructose or saline was administered in the experimental room 0, 2, 5, or 30 min, or in the colony room 60 min, after reactivation. 3. The results showed a time-dependent decrease in the ability of fructose to modulate a reactivated memory. 4. These results suggest that the time-dependent effects for the modulation of a reactivated memory by fructose (a hexose that does not readily pass the blood-brain barrier) and glucose (a hexose that readily passes the blood-brain barrier) follow similar trends.

Comparable dose-response functions for the effects of glucose and fructose on memory.

A passive avoidance-to-active avoidance negative transfer paradigm was used to investigate in rats the effects of glucose and fructose on recently acquired memories. Immediate post-passive avoidance conditioning injections of glucose, fructose, or saline were followed 48 h later by active avoidance conditioning. Equimolar 10, 32, 100, and 2000 mg/kg sc doses of the two sugars significantly impaired acquisition of the reversal task, whereas 3.2 mg/kg doses of both sugars were without significant effect on subsequent performance and 320 mg/kg doses of both sugars significantly enhanced subsequent performance. The cubic trends for both dose-response functions were statistically significant and did not differ from each other. This is the first demonstration that glucose and fructose affect recently acquired memories in accord with comparable cubic dose-response functions, and that there are doses of both sugars that can enhance memory (as indicated by an increase in the number of trials required to reach criterion on the reversal task) and doses of both sugars that can impair memory (as indicated by a decrease in the number of trials required to reach criterion on the reversal task), compared to saline treatment. The similar cubic dose-response functions for glucose and fructose suggest that their mechanisms of action when they are injected peripherally are similar. In addition, because fructose does not readily pass the blood-brain barrier, the results suggest that these two monosaccharides may act through a common peripheral pathway.