LESIONS IN THE MAGNOCELLULAR PREOPTIC NUCLEUS DECREASE OLFACTORY INVESTIGATION IN RATS

The nuclear complex of the horizontal limb of the diagonal band and th e magnocellular preoptic nucleus, components of the basal forebrain ma gnocellular system affected in Alzheimer-type dementia, supply centrif ugal innervation to the olfactory bulb. The lateral magnocellular preo ptic nucleus provides significant GABAergic input. Since its stimulati on may facilitate olfactory bulb mitral cells, we have investigated th e effect of sub-total electrolytic lesions in this nucleus on performa nce in a simple test of olfactory investigation and its habituation. T wo groups of rats were used with lesions which occupied restricted vol umes, approximately 30 and 15%, of the magnocellular preoptic nucleus. Behaviourally, there was interference with olfactory investigation, w ith increased investigation latency and decreased investigation times, in the group with larger lesions at 6 and 16 days after operation. Th ere was no significant effect of the smaller lesions. No effects on pa tterns of olfactory habituation or discrimination were seen. The impai rment of olfactory investigation could not be explained by interruptio n of medial forebrain bundle fibres traversing the nucleus. It is sugg ested that bilateral partial destruction of magnocellular preoptic neu rones may produce significant deficits in either olfactory sensitivity or olfactory motivation.