EFFERENT PROJECTIONS OF THE OLIVARY PRETECTAL NUCLEUS IN THE ALBINO-RAT SUBSERVING THE PUPILLARY LIGHT REFLEX AND RELATED REFLEXES - A LIGHT-MICROSCOPIC TRACING STUDY

The olivary pretectal nucleus is a primary visual centre sensitive to luminance changes. It is involved in the pupillary light reflex, the c onsensual pupillary light reflex and related reflexes, such as the lid closure reflex whereby pupillary constriction takes place. Since the olivary pretectal nucleus is a small nucleus, previous studies using d egeneration, horseradish peroxidase and radioactive amino acid tracing were limited regarding to the exclusiveness of the projections from t he olivary pretectal nucleus. In the present study the position of the olivary pretectal nucleus in the rat was first localized by physiolog ical recording of the neurons upon luminance stimulation. Subsequently , an anterograde tracer Phaseolus vulgaris leucoagglutinin was injecte d iontophoretically. This allows a much more precise localization of t he olivary pretectal nucleus projections. Ascending and descending pat hways originating from the olivary pretectal nucleus were observed. As cending fibres project bilaterally to the intergeniculate leaflet, the ventral part of the lateral geniculate nucleus and ipsilaterally to t he anterior pretectal nucleus. In addition, contralateral projections were observed to the zona incerta and the fields of Forel. Descending fibres project bilaterally to the periaqueductal gray, the nucleus of Darkschewitsch, the interstitial nucleus of Cajal, the Edinger-Westpha l nucleus and the intermediate gray layer of the superior colliculus. Also a contralateral projection to the oculomotor nucleus and an ipsil ateral projection to the pontine nucleus and the nucleus of the optic tract were found. Furthermore, the contralateral olivary pretectal nuc leus received a small projection. Retrograde tracing experiments using two fluorescent dyes revealed that the fibres projecting to the contr alateral olivary pretectal nucleus and to the contralateral interstiti al nucleus of Cajal are collaterals. The projection from the olivary p retectal nucleus to the facial nucleus which has been described to rec eive an input in cats could not be confirmed for the rat. The fact tha t the Edinger-Westphal nucleus, the interstitial nucleus of Cajal and the superior colliculus receive an input from the olivary prerectal nu cleus suggests that this primary visual centre is not only involved in the pupillary light reflex, but also in controlling eye and head posi tion and saccadic eye movements. Although visual acuity largely depend s on receptive field sizes of retinal ganglion cells and their central connections, the stronger sympathetic influence during the pupillary light reflex in animals with frontally placed eyes compared to animals with laterally placed eyes may also contribute to the higher visual a cuity in animals with frontally placed eyes.