Synaptic targets of the intrinsic axon collaterals of supragranular pyramidal neurons in monkey prefrontal cortex

The principal axons of supragranular pyramidal neurons in the cerebral cortex travel through the white matter and terminate in other cortical areas, whereas their intrinsic axon collaterals course through the gray matter and form both local and long-distance connections within a cortical region. In the monkey prefrontal cortex (PFC), horizontally oriented, intrinsic axon collaterals from supragranular pyramidal neurons form a series of stripe-like clusters of axon terminals (Levitt et al. [1993] J Comp Neurol 338:360-376; Pucak et al. [1996] J Comp Neurol 376:614-630). The present study examined the synaptic targets of the intrinsic axon collaterals arising from supragranular pyramidal neurons within the same stripe (local projections). Approximately 50% of the within-stripe axon terminals in monkey PFC area 9 targeted dendritic spines. In contrast, for both the intrinsic axon collaterals that travel between stripes (long-range projections), and the axon terminals that project to other PFC areas (associational projections), over 92% of the postsynaptic structures were dendritic spines (Melchitzky et al. [1998] J Comp Neurol 390:211-224). The other 50% of the within-stripe terminals synapsed with dendritic shafts. Dual-labeling studies confirmed that these within-stripe terminals contacted gamma-aminobutyric acid-immunoreactive dendritic shafts, including the subpopulation that contains the calcium-binding protein parvalbumin. The functional significance of the differences in synaptic targets between local and long-range intrinsic axon collaterals was supported by whole-cell, patch clamp recordings in an in vitro slice preparation of monkey PFC. Specifically, the small amplitude responses observed in layer 3 pyramidal neurons during long-range, low-intensity stimulation were exclusively excitatory, whereas local stimulation also evoked di/polysynaptic inhibitory responses. These anatomic and electrophysiological findings suggest that intrinsic connections of the PFC differ from other cortical regions and that within the PFC, feedback (within-stripe) inhibition plays a greater role in regulating the activity of supragranular pyramidal neurons than does feedforward inhibition either between stripes or across regions.