Distinctive patterns of connectivity between the zona incerta and the red nucleus of rats.

In an effort to understand better the function of the zona incerta of the thalamus, this study has examined the organisation of its connections with the red nucleus of the midbrain. Injections of the tracers biotinylated dextran (BD) or cholera toxin subunit B (CTb) were made into either the zona incerta or red nucleus of Sprague Dawley rats and the resultant labelling was examined. The results indicate that there are strong connections between the zona incerta and red nucleus; further, that these connections make distinct patterns in each nucleus. After incertal injections, labelling is found mainly within the smaller parvocellular lamina of the red nucleus, leaving the larger magnocellular lamina barren of labelling. After rubral injections, labelling is seen across all of the four distinct sectors of the zona incerta, but with a concentration within the medial region of each. Overall, the results indicate that the zona incerta is in a position to influence the motor related functions of the parvocellular lamina of the red nucleus. It is suggested that these results, when taken together with recent reports of a large interpositoincertal pathway, render the zona incerta as a synaptic interface between cerebellar and rubral circuits.