BRANCHED CONNECTIONS TO THE SEPTUM AND TO THE ENTORHINAL CORTEX FROM THE HIPPOCAMPUS, AMYGDALA, AND DIENCEPHALON IN THE RAT

Neuronal cell populations giving origin to bifurcating projections to the septum and the entorhinal cortex were studied in the rat by means of double retrograde labeling using the fluorescent tracers Fast Blue and Diamidino Yellow, Double labeled pyramidal neurons were consistent ly detected in the temporal level of the CA1 area and subiculum of the hippocampal formation, where they represented at least 50% of the cel ls retrogradely labeled from the entorhinal injections, Double labeled neurons were also detected in the amygdala, where they prevailed in t he basal complex, Scattered double labeled neurons were observed in a number of hypothalamic nuclei, with a slight predominance in the preop tic region. Finally, a few double labeled cells were detected in the m idline thalamus, and especially in the thalamic paraventricular nucleu s, In all these structures, double labeled neurons were located ispila terally to the injection sites. The present data indicate that the sep tum and entorhinal cortex are tightly interconnected by axonal bifurca tions deriving from a variety of telencephalic and diencephalic source s.