Diencephalic and mesencephalic efferents of the medial prefrontal cortex in the rat: electrophysiological evidence for the existence of branched axons.

Efferents originating from the rat medial prefrontal cortex and projecting to diencephalic and mesencephalic structures were analyzed electrophysiologically. The antidromic activation technique was used in order to identify cortical neurons which innervate the medio-dorsal nucleus of the thalamus (MD), lateral habenula, ventro-medial mesencephalic tegmentum (VMT), substantia nigra (SN), superior colliculus (SC) and central gray (CG). Corresponding cortical neurons, found mainly in layers V and VI, were distributed in the rostro-caudal extension of the medial prefrontal cortex. Their conduction velocities were relatively slow and ranged between 0.4 and 5.5 m/s. The reciprocal collision test allowed us to determine whether a given cortical neuron innervated only one structure or whether it sent axon collaterals to other subcortical areas. Twelve patterns of axonal branching were established: MD-VMT, MD-SN, MD-SC, VMT-SN, VMT-SC, VMT-CG, SN-SC, habenula-VMT, habenula-CG, MD-SN-SC, VMT-SN-SC and VMT-habenula-CG. Finally, it was estimated that a large proportion of the prefrontal cells which project to a given subcortical structure also innervate one or two other subcortical areas. Results are discussed in the light of anatomical data and their functional significance.