Research reportRestriction of environmental space attenuates locomotor activity and hippocampal acetylcholine release in male rats
Introduction
The hippocampus has been suggested to play a role in the spatial learning and memory function 3, 19. Cholinergic neurons within the basal forebrain which provide major projections to the hippocampus [26]are responsible for spatial learning and memory ability 6, 21. Loss of the cholinergic projection caused by damage to the bilateral fimbria–fornix [30]or the selective lesioning of the basal forebrain cholinergic system [10]impairs learning and memory tasks requiring spatially defined responses.
It is known that hippocampal acetylcholine (ACh) release is closely associated with spontaneous locomotor activity. An increase in locomotor activity was accompanied by increased release of ACh [4], suggesting the implication of locomotor activity in the stimulation of ACh release. In our study, ACh release in the hippocampus was strongly related to spontaneous locomotor activity in rats 16, 17. Further, forced running in a treadmill resulted in the instantaneous appearance of theta waves in the hippocampal electroencephalogram and the appearance of theta waves was blocked by a muscarinic receptor antagonist, scopolamine [28]. Selective damage of septohippocampal cholinergic neurons resulted in a dramatic loss of the theta waves [11].
The question arises, however, whether the spontaneous locomotor activity but not forced activity induces the hippocampal ACh release. In the present study, therefore, we attempted to suppress spontaneous locomotor activity by restriction of the environmental space, and an in vivo microdialysis study was performed to examine whether the hippocampal ACh release changed.
Section snippets
Animals and surgery
Young male rats (3–4 months old) of Wistar–Imamichi strain (Animal Reproduction Research, Oomiya, Japan) were used in this study. The rats were housed individually at a constant temperature of 23±1°C under a light/dark cycle (lights on: 0500–1900 h). Food and water were available ad libitum in all experimental periods. All the animal housing and surgical procedures were in accordance with the guidelines of the Institutional Animal Care and Use Committee of Animal Research Center, Yokohama City
Results
Effects of the restriction of environmental space on the spontaneous locomotor activity are shown in Fig. 1. Restriction of the environmental space was started at the time around 1900 on day 0, which is shown by arrows in Fig. 1. After rats were moved into the small cage, mean locomotor activity counts decreased, whereas they did not change in rats in the large cage. ANOVA indicated the significant interaction between cage and day (p<0.01, F=10.849 in the dark phase, p<0.05, F=2.705 in the
Discussion
First, we have checked whether the restriction of the environmental space attenuates spontaneous locomotor activity. Even in the small cage, circadian fluctuation of locomotor activity was sustained throughout the experimental period. But, the mean locomotor activity count after the cage change to the small cage was significantly less than that in the large cage. The result shows that the restriction of the environmental space attenuates spontaneous locomotor activity during both the light and
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