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 hormo ne (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 a ge into two experimental blocks (young rats 3 months old and aged rats 24 months old at the start of the experiment) where each block receiv ed the same treatment. Each experimental block was then divided random ly into three groups where two were experimental and the other served as control. The animals were then submitted to a one-trial passive avo idance conditioning and tested for memory retention 24 hrs after as we ll as twice a week until the extinction response occurred. The control group received an isotonic saline solution and the other two groups r eceived 0.8 U.I. of GH or 4 meg of GHRH respectively. All substances w ere 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 longterm memory as well as the extinction response and in either case the response seems to vary with age. GH and GHRH facili tates long-term memory in young rats but not in aged rats. Finally, wh ereas GH delays the extinction response in both groups, GHRH retards t he extinction only in aged rats.