Scopolamine impairs delayed matching in an olfactory task in rats.

The action of the cerebral cholinergic system seems to be important for remembering events over short time intervals. We decided to test this hypothesis in the rat by developing an original model of short term memory based on the olfactory sensory modality which is a major determinant in the animal behaviour. The principle of the experiment was a "delayed match-to-sample" test performed in a classical T maze divided into two compartments. In the first compartment, rats received an odorant stimulation, then, in the second, they had to discriminate between the two arms odorized differently. To receive a food reinforcement, the animals had to enter the arm signaled by the odor presented in the first part of the maze. The test was performed with (Short-term memory condition) or without (Immediate memory condition) a variable delay between the first odor sampling and the discrimination task. Both tests were performed with control and scopolamine-treated animals (0.5, 0.125 and 0.0625 mg/kg IP). An injection of scopolamine (0.5 mg/kg) impaired performances, even when no retention of the odor was required. However, lower doses (0.125, 0.0625 mg/kg) selectively altered performances in the short term memory condition. These results suggest that intact muscarinic transmission is required for an olfactory cue to be used over a short time after its presentation.