The crossed nigrostriatal projection decussates in the ventral tegmental decussation.

Horseradish peroxidase (HRP) tract-tracing techniques were used in 44 rats in order to establish the site of decussation of the crossed nigrostriatal projection. Somata in both the ipsilateral and the contralateral ventromedial mesencephalon were labelled after injection of HRP into the caudate nucleus. In agreement with previous studies, contralateral labelling constituted about 3% of the ipsilateral labelling. Midsagittal transection of the mesodiencephalic junction did not prevent the contralateral labelling. However, mid-sagittal transection of the ventral mesencephalon, or selective 6-hydroxydopamine (6-OHDA) lesions of the ventral tegmental decussation did prevent the contralateral labelling. Moreover, 6-OHDA lesions of the substantia nigra ipsilateral to the horseradish peroxidase injection also prevented contralateral labelling. We conclude that the crossed nigrostriatal projection decussates in the ventral tegmental decussation, and that this projection is susceptible to damage by standard 6-OHDA lesions located on the opposite side to the origin of the crossed pathway.

Hemispheric disconnection and rotational behaviour.

Several studies have demonstrated the existence of crossed pathways interconnecting the bilateral extrapyramidal system. The present study has evaluated the role of the thalamus in the interhemispheric control of nigrostriatal function by observing the effect of midsagittal thalamic transection on amphetamine-induced rotation in rats. The effect of thalamic transection on net rotational asymmetry did not differ from the effects of sham operations. Also, the transection did not affect the rotational asymmetry induced by subsequent lesioning of the dominant hemisphere substantia nigra. The failure to affect the rotation asymmetry by the transection suggests either that the inter-hemispheric pathway does not control extrapyramidal asymmetry or that the crossing takes place outside the thalamus. In an additional group of rats, thalamic transection did not interrupt retrograde labeling of somata in the substantia nigra and ventral tegmental area by horseradish peroxidase injected in the contralateral caudate. Thus, the crossed nigrostriatal projection does not decussate via the thalamus. It is suggested that this pathway decussates in the ventral mesencephalon.