Transneuronal retinal input to the primate Edinger-Westphal nucleus.

The Edinger-Westphal nucleus of the oculomotor nuclear complex provides preganglionic parasympathetic innervation to the pupil. We labelled its retinal input by transneuronal autoradiography after an eye injection of [3H]proline in the macaque monkey. The primary retinal projection to the pretectum terminated in the ipsilateral and contralateral olivary nuclei. These nuclei were intensely labelled and sharply delimited, with a mean diameter of 590 microm and a rostrocaudal length of 2.52 mm. The caudal half of the olivary nucleus on each side broke into multiple clumps of label. Fragments of label also surrounded each olivary nucleus. The exact pattern of pretectal labelling varied considerably among animals and even from side to side in the same animal. In 5 of 6 monkeys, label from the olivary nucleus reached the Edinger-Westphal nucleus transneuronally. In transverse sections, the Edinger-Westphal label appeared as a circular patch located on either side of the midbrain ventral to the cerebral aqueduct in the central gray matter. It averaged 230 microm in diameter and 610 microm in length. In Nissl-stained sections, the autoradiographic label corresponded to a distinct nucleus comprised of neurons that were smaller than neurons in nearby somatic subdivisions of the oculomotor complex. The mean area of Edinger-Westphal neurons was 295 microm2. Transneuronal retinal input to the Edinger-Westphal nucleus mediating pupillary constriction terminates in a single, well-defined cell group in the midbrain.