Neuroanatomical identification of the frog habenular connections using peroxidase (HRP).

A study of the habenular nuclei connections by means of horseradish peroxidase (HRP) has never been carried out in amphibia. In the present paper we have investigated the afferent projections of the left and right habenular nuclei of the frog Rana esculenta using this technique. Cells, labelled by HRP, were either in a Golgi-like pattern or in a granular pattern. It appears that the habenular nuclei on the two sides of the epithalamus do not show different connections even though they are morphologically asymmetric. In fact, each habenula is connected bilaterally with the septal area and the bed nucleus of the hippocampal commissure, and ipsilaterally with the hypothalamic areas, the entopeduncular nucleus, the periventricular gray of the third ventricle and the interpeduncular nucleus. However, the habenular commissure has typical commissural fibres which apparently do not involve the medial portion of the left habenula. The habenular connections in the frog are generally similar to those reported in the literature for mammals. In addition, our results show the possibility that HRP is transported both retrograde and anterograde.