Pyramidal neuron local axon terminals in monkey prefrontal cortex: differential targeting of subclasses of GABA neurons.

In monkey prefrontal cortex (PFC), approximately 50% of the local axon terminals of pyramidal neurons form synapses with the dendritic shafts of GABA neurons. Subclasses of GABA neurons can be distinguished by the presence of different calcium-binding proteins. For example, in monkey PFC, parvalbumin (PV)-containing cells comprise approximately 25% of GABA neurons and are predominantly located in layers 3b-4, whereas calretinin (CR)-containing cells, which are present in greatest density in layers 2-3a, constitute 50% of GABA neurons. Consequently, in order to determine the cell class and laminar specificity of the dendritic targets of pyramidal neuron local axon collaterals in monkey PFC area 9, we conducted ultrastructural analyses of local axon terminals labeled with the anterograde tracer, biotinylated dextran amine, and dendrites immunoreactive (IR) for PV or CR. In layer 3b, the majority of the local axon terminals targeted PV-IR dendritic shafts, whereas CR-IR dendritic shafts were targeted infrequently. This differential targeting was also present in layers 2-3a, although it was less pronounced. In addition, PV-IR dendrites had a significantly greater density of excitatory inputs than did CR-IR dendrites. These findings indicate that PV-containing interneurons, which have a potent inhibitory effect on pyramidal neurons, are selectively targeted by the excitatory local axon terminals of supragranular pyramidal neurons in monkey PFC. These connections may provide the anatomical substrate for the coordinated activity of pyramidal neurons and fast-spiking GABA neurons during working memory.