The anatomical identification of saccadic omnipause neurons in the rat brainstem

Omnipause neurons (OPNs) represent a crucial component for the generation of saccadic eye movements. They inhibit saccadic premotor neurons in the paramedian pontine reticular formation (PPRF) as well as in the rostral interstitial nucleus of the medial longitudinal fascicle (RIMLF) during the intersaccadic interval. In turn, inhibition of OPNs is a prerequisite in order to generate saccadic eye movements. Although the anatomy of the saccadic system including the OPNs has been extensively studied in primates and cats, no detailed anatomical description of these neurons in rats has been performed so far. The aim of the present study was the identification of putative OPNs in the rat brainstem based on their projection target, localization, and histochemical characteristics. Stereotactic tract-tracer injections into the rostral mesencephalon including the RIMLF in rat resulted in back-labeling of a neuron group adjacent to the midline at the level of traversing fibers of the abducens nerve, which are considered as OPNs lying in the nucleus raphe interpositus. Combined immunohistochemical staining for various markers revealed in these neurons the expression of parvalbumin, chondroitin sulfate proteoglycan, and glycine, but a lack of serotonin. The results of our study demonstrate the striking similarity between individual elements of the premotor saccadic network in rats and primates. The exact knowledge of their location in rats provides a basis for in vitro studies of the OPNs in rat brainstem slices.