Horizontal synaptic connections in monkey prefrontal cortex: An in vitro electrophysiological study

In monkey dorsolateral prefrontal cortex (PFC), long-distance, horizontally oriented intrinsic axon collaterals interconnect clusters of pyramidal neu rons in the supragranular layers. In order to study the electrophysiologica l responses mediated by these long-distance projections, an in vitro slice preparation of monkey PFC was used to obtain whole-cell patch clamp recordi ngs from layer 3 pyramidal neurons. Using in vivo tracer injections, we fou nd that long-distance projections were well preserved in PFC slices cut in the coronal plane. Postsynaptic currents were evoked by low-intensity elect rical extracellular stimulation applied successively to 20-30 discrete site s located up to 2200 mu m lateral to the recorded cell. Several criteria we re applied to discriminate between mono- and polysynaptic responses. Long-d istance monosynaptic connections were mediated by fibers with relatively sl ow conduction velocity (0.14 m/s). Excitatory postsynaptic currents (EPSCs) evoked by stimulation of short- or long-distance horizontal connections di d not differ in kinetic properties. The majority (77%) of the 35 layer 3 PF C neurons studied were monosynaptic targets of long-distance connections. E PSCs mediated by long-distance connections had amplitudes that were similar or even larger than short-distance EPSCs, suggesting that excitatory input provided by the former was relatively robust. For most neurons (87.5%) in which a full complement of monosynaptic EPSCs was evoked by multisite stimu lation, the EPSC amplitude as a function of stimulation distance from the r ecorded cells exhibited statistically significant peaks. The spacing betwee n peaks was similar to the spacing between interconnected clusters of neuro ns observed in previous anatomical studies. The results show that long-dist ance excitatory connections constitute a significant intrinsic pathway of s ynaptic communication in layer 3 of monkey PFC.