Increases in hippocampal and frontal cortical acetylcholine release associated with presentation of sensory stimuli

In vivo microdialysis was employed to monitor acetylcholine release in the hippocampus and frontal cortex of freely behaving rats. Four stimuli were presented on separate occasions in the course of a dialysis session to rats with microdialysis probes implanted in the hippocampus or frontal cortex. Visual, auditory, olfactory and tactile stimuli elicited a number of different responses such as exploratory and consummatory behaviours. Presentation of two of the stimuli (auditory and tactile) also produced periods of alert immobility (freezing). All of the stimuli increased acetylcholine release in both the hippocampus and cortex: in the hippocampus, this increase was statistically significant with all except the olfactory stimulus, whereas in the cortex all but the visual stimulus resulted in significant increases. In the hippocampus, there were no significant differences between the increases in acetylcholine release produced by the four stimuli. In contrast, in the cortex, there was significant variation between the magnitude of acetylcholine release produced by the different stimuli: acetylcholine release elicited by tactile stimulation was greater than that produced by the other stimuli. There was no significant variation in the duration of increases in acetylcholine release produced by the stimuli in either the hippocampus or cortex. These results provide evidence that acetylcholine release is associated with a variety of behavioural responses to stimuli designed to produce arousal, and point to a role for cortical and hippocampal cholinergic mechanisms in arousal or attention. Further, the results suggest that under some circumstances cortical and hippocampal acetylcholine release may be regulated differentially.