Collateral innervation of the medial and lateral prefrontal cortex by amygdaloid, thalamic, and brain-stem neurons.

The distribution of the afferents to the rat's prefrontal cortex originating in the thalamic mediodorsal nucleus and the amygdala was investigated with two fluorescent tracers. Special emphasis was laid on detecting the loci of neurons which project via axonal collaterals into both lateral and medial portions of the prefrontal cortex. It was found that a high number of neurons of the anterior portion of the basolateral amygdaloid nucleus terminate via collaterals in both the medial and lateral subfields of the prefrontal cortex. On the other hand, only a small number of mediodorsal thalamic cells were found to project to both sides of the prefrontal hemisphere via bifurcating axonal collaterals. These cells were situated exclusively in the lateral part of the medial segment of the mediodorsal nucleus. The majority of both thalamic and amygdaloid neurons with bifurcating axons originate from subregions whose cells innervate primarily the medial prefrontal cortex. In brain-stem, neurons of the nucleus raphé dorsalis also project via collaterals to the medial and lateral prefrontal regions. Furthermore, neurons of the dorsal and ventral premamillary nuclei, the lateral mamillary nucleus, the ventral tegmental area of Tsai, and the ventral tegmental nucleus of Gudden were found to project to the medial prefrontal cortex. Our results indicate a differential collateral organization of thalamic and amygdaloid afferents to prefrontal cortical fields. The anterior basolateral amygdala (which innervates via collaterals both the medial and lateral prefrontal subfields) may add a common input to either subfield, such as information on the significance of incoming stimuli to the animal's behavior, while the mediodorsal nucleus (whose segments are principally connected to only one prefrontal subfield) may add segment-specific information, for example, of a spatial-cognitive nature for the lateral segment and of an emotional nature for the central and medial segments. The existence of a basolateral limbic circuit, composed of the amygdala, the thalamic mediodorsal nucleus, and the prefrontal cortex, is confirmed and knowledge on its interconnectivity is extended. From an anatomical point of view these data provide arguments for both unitary and diverging functions of the prefrontal cortex.