A study of the reciprocal connections between the septum and the entorhinal area using anterograde and retrograde axonal transport methods in the rat brain

The reciprocal connections between the septum and the entorhinal area (EA) was studied in the rat brain using antero‐ and retrograde axonal transport methods. After injections of large volumes (2 × 100 nl) of horseradish peroxidase (HRP) conjugated to wheat‐germ agglutinin (WGA) into the medial septum (MS) and the diagonal band of Broca (dbB), anterogradely transported HRP‐WGA was found primarily in layers II and IV of the medial and lateral EA. Injections of HRP‐WGA (50–100 nl) or fluorescent dyes (50–100 nl) into different parts of the retrohippocampal region resulted in labeling, by retrograde axonal transport, of cells in the MS and dbB, both ipsi‐ and contralateral to the injected hemisphere. The labeled cells were either small (long axis of soma: 10–15 μm), round, and oval, or medium (15–25 μm) to large (25–35 μm) of fusiform or multipolar shape. By using the method of retrograde fluorescent double labeling, the septal afferents to the EA were found to give off collaterals to other parts of the hippocampal region. A much smaller number of septal cells appeared to send bilateral projections to the EA of both hemispheres. Studies employing retrograde transport of HRP in combination with acetylcholinesterase (AChE) histochemistry on the same tissue section showed that, while a large number of cells projecting to the EA contain AChE, many projecting cells are devoid of AChE reaction products. These findings suggest that the septo‐entorhinal projection consists of a cholinergic as well as a noncholinergic component.