Modulation of long-term memory and extinction responses induced by growth hormone (GH) and growth hormone releasing hormone (GHRH) in rats.

The purpose of this work is to study the participation of growth hormone (GH) and growth hormone releasing hormone (GHRH) in the modulation of long-term memory and the extinction response of a passive avoidance task in rats. However, the effect on memory vary according to the age of the animals due to plasma levels of either hormone being modified during the aging process. Male Wistar rats were divided according to age into two experimental blocks (young rats 3 months old and aged rats 24 months old at the start of the experiment) where each block received the same treatment. Each experimental block was then divided randomly into three groups where two were experimental and the other served as control. The animals were then submitted to a one-trial passive avoidance conditioning and tested for memory retention 24 hrs after as well as twice a week until the extinction response occurred. The control group received an isotonic saline solution and the other two groups received 0.8 U.I. of GH or 4 mcg of GHRH respectively. All substances were in a 0.08 ml volume and applied 24 hrs before training as well as 24 hrs before each retention session. The results indicate that GH and GHRH modulate long-term memory as well as the extinction response and in either case the response seems to vary with age. GH and GHRH facilitates long-term memory in young rats but not in aged rats. Finally, whereas GH delays the extinction response in both groups, GHRH retards the extinction only in aged rats.