Olfactory discrimination learning deficit in heterozygous reeler mice.

Adult mice heterozygous for the 'reeler' mutation (HR mice) have been found not to have a reeler phenotype but to express a number of abnormal traits including reduced sensorimotor gating, lower density of dendritic spines in frontoparietal cortex and hippocampus, and selectively decreased expression of one form of glutamic acid decarboxylase (GAD67). Since reelin is expressed in olfactory areas of the adult mouse brain, we have tested for olfactory functions in HR mice that express only half of the reelin found in wild-type (WT) mice. HR and WT mice were trained to criterion performance of 90% correct in a block of 20 trials on eight distinct simultaneous-cue, two-odor discriminations. HR mice required significantly more training sessions and made more errors than did WT mice in acquiring the first olfactory discrimination. Subsequent discriminations were learned equally rapidly by both HR and WT mice. Memory retention for the final discrimination was tested 1 week after training and was equally good for both groups. Both HR and WT mice showed equivalent sensitivity in discriminating low concentrations of either ethyl acetate or butanol from non-odorized air. Whether or not the olfactory learning deficit observed in HR mice is related to the low dendritic spine density or GAD67 reduction observed in hippocampus and cortex are currently under investigation.