PROJECTIONS FROM SUBSTANTIA-NIGRA AND ZONA INCERTA TO THE CATS NUCLEUS OF DARKSCHEWITSCH

The goal of the present experiments was to examine the relationships o f the nucleus of Darkschewitsch (ND) with the substantia nigra pars re ticulata (SNr), the zona incerta (ZI), and the oculomotor nuclei by us ing wheat germ agglutinin-horseradish peroxidase (WGA-HRP) as a retrog rade and anterograde neuronal tracer injected into various sites of th e cat's brain. To eliminate the possibility that fibres of passage fro m the motor cortex passing through the SNr and ZI were responsible for the ND label, WGA-HRP also was injected into the SNr or the ZI after a large area of the frontal cortex, including the motor area, was dest royed. Retrograde axonal transport demonstrated that many cells of the rostromedial part of the ZI project to the ND, with the ipsilateral p rojections being dominant. Some cells of the caudomedial part of the S Nr project to the ND, again, with the ipsilateral projection being dom inant. A few small cells in the ND project bilaterally to the oculomot or nucleus. Anterograde tracer demonstrated that the SNr-ND terminal f ields are less dense than the ZI-ND terminal fields. A few fine termin al fibres were observed bilaterally in the oculomotor, trochlear, and abducens nuclei. Electron microscopic examination demonstrated that th ese fine, labelled terminals contain pleomorphic vesicles and have sym metrical synaptic contacts with dendrites. These results indicate that the ND, a structure that is known to be important for the control of axial muscles (i.e., eye, head, and body muscles), is the target of pr ojections from restricted areas of the SNr and ZI: areas that, during saccadic eye movement, may lead to disinhibition of the ND-oculomotor projection. Accordingly, the ND may function to inhibit the activity o f extraocular muscles during saccades. (C) 1998 Wiley-Liss, Inc.