Retrograde enhancement of memory in goldfish by a time dependent temperature shift

Goldfish (N = 1528) at 25°C were trained to suppress their spontaneous upstream swimming into a quiet well in a single trial punished by brief electric shock. After graded delays of 4–1024 min the trained fish were cooled abruptly to 10°C, held at 10°C for graded durations of 0.07–960 min, then rewarmed abruptly to 25°C and held at 25°C. Brain temperature followed the abrupt water temperature shift within 60 sec. Retention of the learned suppression, tested 1, 2, or 4 days later, was enhanced compared to retention by trained controls held at 25°C throughout. The enhanced retention was an inverted-U function of the delay of cooling and of the duration at 10°C. Enhancement occurred only when the rewarming step fell within a sensitive period of 12–96 min after training, with a maximum at approximately 36 min, regardless of the delay to cooling or duration at 10°C. Cooling per se was not essential for enhancement of retention; the critical factor was the interval between training and rewarming. The time-dependent enhancement may result from one or more temperature-sensitive memory processes that were suppressed at 10°C, then stimulated above normal by return to 25°C.