Functional connections between medial prefrontal cortex and caudate-putamen in brain-stimulation reward of rats

Rats were trained to self-stimulate for trains of cathodal pulses delivered via electrodes placed in the medial prefrontal cortex (MPFC) and caudate-putamen (CPu). When the pulses were delivered via 9 ipsilateral MPFC and CPu sites alternately, summation varied from 13% to 40%. However, the overall summation for 2 contralateral MPFC-CPu pairs was 5%, thus indicating a greater integration of ipsilateral than contralateral reward signals. When the interval between alternate pulses decreased in 4 of the 9 ipsilateral pairs, the summation also decreased, an outcome consistent with collision of action potentials passing between the MPFC and CPu sites. The size of the collision effect ranged from 15% to 33%. Estimates of conduction velocity varied between 0.4 to 5.4 m/s, with a 1.7 m/s average. According to these values, the neurons connecting the MPFC and CPu self-stimulation sites appear to be slower than the ones that have been shown to link reward fibers that course between posterior brain regions.