The thalamic reticular nucleus does not send commissural projection to the contralateral parafascicular nucleus in the rat

The reticular nucleus of the thalamus (NRT) projects to virtually all thalamic nuclei ipsilaterally. In addition, recent studies suggest that NRT sends contralateral projections through an intrathalamic commissural fiber system to several thalamic nuclei, including the NRT itself. In the present study we used retrograde cell labeling, multi-unit anterograde labeling and immunohistochemical methods to study both ipsi- and contralateral NRT projection to the parafascicular nucleus (Pf) in the rat. Injections of the fluorescent tracers true blue or fluorogold in Pf led to massive retrograde cell labeling in rostral and dorsal portions of the ipsilateral NRT, whereas the same sectors of the contralateral NRT were devoid of labeling. Some retrogradely labeled cells were nevertheless present on the contralateral side in the borderline region between NRT and the zona incerta (ZI). Retrograde cell labeling experiments with cholera toxin B subunit (CTb) combined to immunohistochemistry for parvalbumin (PV) and calbindin D-28k (CB) indicated that the few retrogradely labeled cells encountered at the border between NRT and ZI displayed immunoreactivity for CB but not for PV. Since PV and CB label neurons belonging to NRT and ZI, respectively, it is concluded that these contralateral retrogradely labeled cells belong to ZI and not to NRT. Multi-unit cell anterograde labeling experiments with biocytin showed that NRT cells that project to Pf arborize extensively only on the ipsilateral side. The same approach, however, has revealed NRT cells projecting to both ipsi- and contralateral ventromedial thalamic nuclei. The axon of these NRT neurons arborizes more profusely ipsilaterally than contralaterally. These results reveal that the NRT projection to Pf in rodents is strictly unilateral. These findings are at variance with the emerging concept that NRT exerts a prominent bilateral influence upon most thalamic nuclei.