Serotonin and acetylcholine release response in the rat hippocampus duringa spatial memory task

By using in vivo microdialysis we monitored the extracellular levels of ace tylcholine and serotonin in the hippocampus of rats performing a spatial me mory task. After rats were trained for 10 consecutive days to master a food -reinforced radial-arm maze task, they were implanted with a microdialysis probe in the dorsal hippocampus. On day 12, rats were tested in the maze an d acetylcholine and serotonin outputs were monitored before the test, durin g the wailing phase and while performing the trials. In trained, food-rewar ded rats, hippocampal acetylcholine levers increased during the waiting per iod (181 +/- 9% of baseline) and further increased during the radial-maze p erformance to 236 +/- 13% of baseline values, while serotonin levels did no t change during the waiting period but increased to 142+/-3% during the maz e performance. To discriminate whether the increase of acetylcholine and se rotonin levels during the testing was associated with memory performance or with food consumption, Ne monitored hippocampal acetylcholine and serotoni n release in rats that were trained, but not food rewarded, or in rats that were not trained, but rewarded only on the test day. In the trained, non-r ewarded group, acetylcholine release increased during the waiting phase to 168 +/- 6%, but did not increase further during the task performance. In co ntrast, no change in serotonin release was observed in this group in any ph ase of the test. In rats which were not trained, but food rewarded, acetylc holine increased only during the maze period (150+/-5%). Serotonin increase d gradually and become significant at the end of the trials. (130+/-3%). Wh ile both neurotransmitters could be implicated in feeding behaviour, only a ctivation of cholinergic neurotransmission appears to be associated with me mory function. Our results support the following hypotheses: (i) hippocampal acetylcholine could be involved in attentional and cognitive functions underlying motiva tional processes; (ii) serotonin could be implicated in non-cognitive proce sses (i.e. in the control of motor and feeding behaviour). Since serotonin and acetylcholine neurotransmission is simultaneously activated during the spatial memory task, this suggests that these neurotransmitter systems regu late behavioural and cognitive functions. (C) 1999 IBRO. Published by Elsev ier Science Ltd.