Prefrontal Cortex of the Mouse Defined as Cortical Projection Area of the Thalamic Mediodorsal Nucleus

In the mouse small amounts of horseradish peroxidase were injected iontophoretically into different portions of the frontal lobe. Out of a large number of mice, 18 with typical injections which together covered most of the anterior half of the cortex were selected, and their retrogradely labeled thalamic cells were described and illustrated in detail. Special emphasis was laid on the cortical projections of the mediodorsal nucleus which were considered to define locus and extent of the prefrontal cortex. The results reveal that topographically the extent of the mouse, prefrontal cortex closely resembles that of the rat, another species of the rodent order. It includes a small area within and dorsal to the rhinal sulcus and a comparatively larger region within the medial half of the anterior cortex. Furthermore, the dorsal tip of the frontal pole is reached by a considerable number of afferents from the mediodorsal nucleus. Whether this last projection also exists in the rat is still disputed. As in the rat, only the sulcal prefrontal cortex seems to be reached by mediodorsal cells alone, whereas the rest of the prefrontal cortical fields have overlapping projections from the anteromedial nucleus. Though a considerable area within the frontal lobe of the mouse is reached by mediodorsal afferents, the significance and usefulness of this definition of the prefrontal cortex is questioned.