Dendritic organization of neurons of the superior colliculus in animals with different visual capability

The aim of the study was to compare several morphological characteristics o f neurons in the superficial layers of the superior colliculus in diurnal a nd nocturnal mammals with different visual specialization. Thus, we investi gated the rat (Rattus norvegicus), a nocturnal animal; the tree shrew (Tupa ia glis), a diurnal animal, and the Mongolian rodents, Microtus brandti (no cturnal) and Alticola barakshin (diurnal), The investigation was focused on the study of the organization and extent of dendrites of Golgi-impregnated projection neurons, which were divided in two classes: narrow-field and wi de-field cells. We determined that the ratios between the volumes of dendri tic fields of the investigated neuronal types and the total volume of the s uperior colliculus differed to a great extent between the different species . The tree shrew had the largest superior colliculus and the smallest wide- field neurons, while the rat had the largest wide-field neurons. As for the Mongolian rodents, we provided the first description of superior colliculu s neurons. The day-active animal Alticola barakshin was found to have a 50% larger volume of the superior colliculus than that of the night-active ani mal Microtus brandti, and the size of the dendritic field of both wide-fiel d neurons and narrow-field neurons was smaller than that of Microtus brandt i, Electron microscopic investigation of wide-field neurons performed in th e rat revealed only a few symmetric synaptic contacts on the arborizations of distal and terminal dendrites and numerous asymmetric synapses on the de ndritic stem. Our findings support the hypothesis that whereas the narrow-f ield neurons are relay neurons in the retino-tecto-thalamic pathway of the visual system, the wide-field neurons may play additional roles in the reti no-tecto-reticulo-spinal system. (C) 2000 Elsevier Science Inc.