Confocal mapping of cortical inputs onto identified pyramidal neurons

Using a confocal microscopy protocol, we carried out a microcircuitry investigation of cortical connections in monkey temporal cortex. Inputs were labeled by BDA injections in posterior area TE, and potential postsynaptic pyramidal neuron targets were labeled with EGFP, by injection of retrogradely transported adenovirus. We scored the number and distribution of putative contacts onto dendritic compartments of neurons in different layers. Initial results show that about 50% of apical dendrites of layer (L.) 6 neurons receive contacts, as they ascend through L.4 (n= 1 brain), but only 30-35% of those from L.5 neurons (n= 2). Basal dendrites of L. 3 neurons also receive few contacts in L.4. This supports the role of layer 4 as an interlaminar relay in association cortex. In addition, our results indicate spatial heterogeneity in the occurrence and number of contacts, possibly due to subtype specificity in target preference. The maximum number of contacts, for a L. 2 neuron projecting from anterior to posterior TE, was 29. This approach seems a useful alternative or complement to electron microscopic analyses of long distance connectivity.