Visual corticopontine projections in the guinea pig: an autoradiographic study

The goal of this study is to characterize the anatomical organization of the visual cortical output to the basal pontine nuclei in the guinea pig. Data from the literature show that guinea pigs exhibit different optokinetic oculomotor behaviors with respect to rats and rabbits. Namely, they present a fast rise in eye movement velocity at stimulus onset and a better performance in monocular horizontal stimulation. Possible differences in the visual corticopontocerebellar pathway might explain these peculiarities. The pontine projections from the primary visual cortex were studied with the method of the anterograde axonal transport of [3H]leucine. The terminal labeling forms prominent patches, ipsilaterally to the cortical injection, throughout the rostrocaudal extent of the pontine nuclei, predominantly in the dorsolateral region. At the intermediate rostrocaudal level, some foci of labeling are found ventrolaterally as well. Sparse fields are present also in the medial pontine nuclei and in the nucleus reticularis tegmenti pontis, but only when the injection site extends to secondary visual areas, either lateral or medial. The present description of the corticopontine projections in guinea pigs is in substantial agreement with the projections previously described in rats, with a few differences, namely: (1) the recipient area extends more caudally; (2) secondary visual areas project to the nucleus reticularis tegmenti pontis.