A triphasic curve characterizes the retention of lever-pressing behavior rewarded by lateral hypothalamic stimulation during the immediate-post-trial period in rats: implications for a transient-intermediate stage between short- and long-term memory.
We investigated the retention of lever-pressing behavior in rats rewarded by lateral hypothalamic (LHT) stimulation. Rats with electrodes implanted into the lateral hypothalamus were trained to press a lever to receive LHT stimulation. More than 90% of rats which had self-stimulated for longer than 30 min retained this behavior when tested 1 or 7 days later, indicating that intracranial self-stimulation (ICSS) produced a stable long-term memory lasting for at least 7 days. Subsequently, we examined retention at shorter ICSS-test intervals. Retention rates were measured 15, 30, 60, 90, 120, 180 or 360 min after the end of a 90-min period of ICSS, using a different group of 15 rats for each interval. Unexpectedly, retention rates in shorter interval tests were lower than those observed after 1 or 7 days. We observed a characteristic fluctuation in retention rates with lower rates at 30 and 180 min, giving a triphasic form to the retention curve with peaks at 15, 90 and 360 min. When a group of rats that had been previously stimulated and shown to have retained the lever-pressing behavior was allowed to re-stimulate a second time, no fluctuations were observed in short-term interval tests. This indicates that the fluctuations in the retention curve immediately after initial ICSS are closely related to the initial acquisition of the memory. Our results support a three-phase model of memory formation that includes a transient-intermediate stage between short- and long-term memory.
Mitsubishi Kasei Institute of Life Sciences, Machida, Tokyo, Japan. ysaitoh@libra.1s.m-kagaku.co.jp
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D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D004558:Electric Stimulation; D007026:Hypothalamic Area, Lateral / Q000502:physiology*; D008297:Male; D008568:Memory / Q000502:physiology; D009043:Motor Activity / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D012153:Retention, Psychology / Q000502:physiology*; D012201:Reward; D012653:Self Stimulation; D013997:Time Factors