The connections of the frog interpeduncular nucleus (ITP) demonstrated by horseradish peroxidase (HRP)

Summary

HRP was injected into the ITP of the frogRana esculenta. Golgi-like labelled cells were consistently demonstrated bilaterally in the dorsal habenular nuclei and in a dorsolateral tegmental nucleus of the mesencephalic central grey. A few labelled cells of this type were also observed in the lateral hypothalamus, in the epiphysis and medial to the nucleus isthmi. Granular deposits of HRP were observed in cells of the bed nucleus of the hippocampal commissure, in the dorsal hypothalamus and in the dorsal midline of the midbrain and isthmic tegmentum. Accumulations of granules were also apparent in suprachoroidal cells. A quantity of labelled fibres were observed bilaterally in the fasciculus retroflexus (FR) and a few in the periventricular grey, the pineal recess and the ependymal layer of the third ventricle.

The frog dorsal habenulae, homologous to the mammalian medial habenulae, are morphologically asymmetric. HRP labelling occurred in the right habenula as well as in both subdivisions of the left habenula. The labelling pattern remained unchanged in those specimens in which the habenular commissure was cut immediately prior to HRP injection in the ITP. In no case were the ventral habenular nuclei, homologous to the lateral habenulae in mammals, found labelled.

Our results demonstrate that the habenulo-inter-peduncular afferents originate solely from the dorsal habenular nuclei and that labelling of the asymmetric portion of the left dorsal habenula is effected through the FR, which seems to be of the same size on both sides of the brain. In addition, the possibility is raised that there exists a connection between ITP, the pineal and the third ventricle choroid plexus in the frog.