Spatio-temporal organization of a branched tecto-spinal/tecto-diencephalic neuronal system.

The aim of the present study was to identify in the rat the diencephalic nuclei addressed by ascending collaterals of tecto-spinal neurons. For this purpose we made use of anterograde axonal transport method to determine the pattern of diencephalic projections arising from the lateral portion of the superior colliculus where most of tecto-spinal neurons are lying. Next, we used the antidromic activation method to analyse whether some of these colliculo-diencephalic projections were provided through collaterals of tecto-spinal neurons. Following injections of wheatgerm agglutinin, conjugated with horseradish peroxidase, in the lateral part of the superior colliculus, anterograde labelling was observed in: the contralateral superior colliculus, the ipsilateral pretectal nuclei, subthalamic area (zona incerta and Forel field) and thalamic structures namely: dorsal and ventral lateral geniculate, parafascicular, posterior nuclear group, reuniens nuclei and lateral portion of medio-dorsal nucleus. Moreover, bilateral projections were revealed in the following thalamic nuclei: lateral posterior, ventro-medial, paracentral and central-lateral. In the electrophysiological study we established that the thalamic nuclei medio-dorsal/central-lateral, paracentral, ventral-medial and the zona incerta receive ascending collaterals of the tecto-spinal neurons. In addition, an axonal branch to the contralateral superior colliculus was also revealed. The various ascending and descending collaterals of each branched neuron exhibited a similar conduction time for action potentials. It is thus likely that the tecto-spinal/tecto-diencephalic neurons provide a synchronized influence on their targets. The functional implication of such a branched collicular efferent pathway is discussed. Considering that tecto-spinal neurons are one of the collicular neuronal populations on which the substantia nigra exerts its influence, new perspectives in the analysis of basal ganglia collicular relationships are given.