Interhemispheric and subcortical collaterals of medial prefrontal cortical neurons in the rat

Efferent neurons of rat medial prefrontal cortex, projecting to subcortical structures and contralateral homotypical areas, were analyzed using anatomical and electrophysiological methods. Anterograde labelling with radioactive amino acids demonstrated the pathways of these efferents in the rostral part of the brain; terminal fields in contralateral cortical areas and localization of fibers projecting subcortically were particularly examined. The laminar location of cells projecting to the contralateral prefrontal cortex or through the striatum was investigated by means of the retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injected in these two structures. Cells innervating the contralateral prefrontal cortex were distributed in layers II-III and V, whereas injection of WGA-HRP in the striatum labelled cells in layer V. The antidromic activation technique was used to identify the cortical neurons which innervate the ipsilateral and contralateral subcortical structures as well as contralateral homotypical cortical areas. Among 743 recorded neurons, 282 neurons were antidromically driven from at least one of the stimulated sites (e.g. right striatum (RS), left striatum (LS), and contralateral prefrontal cortex (L-Cx]. The mean conduction velocities were 0.6 m/s and 0.8 m/s for subcortical and cortical efferents, respectively. The reciprocal collision test provided evidence for the existence of branched axons for 35% of the antidromically activated cells. All the possible branching patterns were found. The results of this study thus demonstrate the existence of single neocortical neurons that send axon collaterals to contralateral cortex and subcortical structures.