A retinohypothalamic projection in the rat

A direct projection from the retina to the hypothalamus was demonstrated in the rat. Following injection of tritiated leucine or proline into the posterior chamber of the eye, labelled protein was shown autoradiographically in the suprachiasmatic nuclei of the medial hypothalamus, both ipsilateral and contralateral to the injected eye. The labelling of the nucleus was heaviest in its ventral portion but extended throughout the nucleus. No evidence for a projection to the supraoptic nucleus or any other hypothalamic nucleus was observed. All of the known terminal nuclei of the primary and accessory optic tracts were heavily labelled. The projection to the suprachiasmatic nucleus could not be clearly confirmed in material prepared using the Fink‐Heimer method for the demonstration of degenerating axon terminals. Electron microscopic study of the suprachiasmatic nucleus following orbital enucleation showed degenerating endings making synaptic contacts with small dendrites of the suprachiasmatic nucleus cells. These first appeared at three days after operation and were nearly gone by seven days. Thus, the retinohypothalamic tract in the rat appears to arise from the ganglion cells of the retina and to terminate on the smaller dendritic branches of the neurons of the suprachiasmatic nucleus.