Olfactory learning is associated with increased spine density along apicaldendrites of pyramidal neurons in the rat piriform cortex

We studied the effect of olfactory learning on the dendritic spine density of pyramidal neurons in the rat piriform (olfactory) cortex. Rats were trai ned to distinguish between two pairs of odours in an olfactory discriminati on task. Three days after training completion, rats were killed and layer I I pyramidal neurons identified by Golgi impregnation were examined with a l ight microscope. Counts of visible spines were performed along the secondar y and tertiary branches of both the apical dendrites and the basal dendrite s, which are the sites of intracortical synaptic inputs. An estimate of the true spine density was obtained using Feldman and Peters' method (1979, Th e Journal of Comparative Neurology, 188, 527-542). The estimated true spine density along apical dendrites was higher in neurons from trained rats tha n those in pseudotrained and naive rats by 15%. As length of spiny dendrite s did not change significantly after learning, the learning-related increas e in spine density in neurons from trained rats may indicate on an increase d number of excitatory synapses interconnecting pyramidal neurons in the pi riform cortex, following olfactory learning.