Axosomatic input to subpopulations of cortically projecting pyramidal neurons in primate prefrontal cortex

Pyramidal cells, the major class of cortical excitatory neurons, can be divided into different subpopulations based upon the target region of their principal axon projection. The activity of pyramidal neurons is regulated in part through inhibitory synaptic inputs to the soma from local circuit neurons. However, little is known about how the density of these axosomatic inputs differs among subpopulations of pyramidal neurons in the prefrontal cortex of primates. In this study, retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was used to identify pyramidal neurons in monkey prefrontal cortex (areas 9 and 46), which were labeled via either associational (ipsilateral hemisphere) or callosal (contralateral hemisphere) principal axon projections. Ultrastructural analysis revealed that the relative number of terminals apposed to the somatic membrane did not differ between associational and callosal neurons. However, neurons in the supragranular layers were apposed by a significantly greater number of axon terminals than were neurons in the infragranular layers. These findings suggest that the laminar environment of a neuron may play a more important role than principal axon projection in determining the amount of axosomatic inhibitory input it receives.