Effects of hippocampal electrical stimulation on longterm memory and on cholinergic mechanisms in three inbred strains of mice.
Two sets of experiments have been carried out in an attempt to determine the role of hippocampal cholinergic mechanisms in a long-term memory storage. Three inbred strains of mice were presented with two different learning tasks in order to estimate their long-term retention abilities as well as changes in this ability after a post-trial hippocampal stimulation. In parallel experiments the enzymes involved in acetylcholine metabolism were studied under different experimental conditions. Our results indicate: (a) The capacity for long-term memory of the BALB/c line is much greater than that of either the C57BL/6 or C57BR strain. (b) Hippocampal post-trial electrical stimulation leads to an improvement of this capacity in the BALB/c strain. This phenomenon is less pronounced in C57BL/6 and non-existent in C57BR mice. (c) Choline acetyltransferase activity in the hippocampus is significantly higher in BALB/c than in the other two strains. In BALB/c this enzyme activity is greatly changed by the post-trial stimulation whereas in the C57BL/L strain only a slight variation of enzyme activity is observed. No modification occurs in C57BR. The results suggest that the more active acetylcholine synthesizing enzyme in the hippocampus of BALB/c may be related to a greater acetylcholine availability, thus favoring the establishment of a long-term memory, perhaps by releasing greater amounts of acetylcholine in the hippocampus immmediately after the learning session. The electrical stimulation of the hippocampus acts to magnify or accelerate this phenomenon. It is suggested that the efficiency of the stimulation would be related to the genetically determined higher cholinergic activity of the hippocampus.