Journal of Neuroscience, Vol 12, 2417-2432, Copyright © 1992 by Society for Neuroscience
Functional influence of interlaminar connections in the hamster's superior colliculus
RD Mooney, X Huang and RW Rhoades
Department of Anatomy, Medical College of Ohio, Toledo 43699.
The aim of this study was to determine the functional importance of
intrinsic connections within the hamster's superior colliculus (SC) in the
development of the visual responses of neurons in the deep layers of this
nucleus. We used localized injections of lidocaine to block action
potentials of fibers that passed through the superficial layers and
injections of CoCl2 to block synaptic transmission in these layers. We also
determined the morphology of a number of the deep layer cells recorded in
these experiments by intracellular injection of HRP. Injection of lidocaine
into the superficial layers completely abolished the visual- and/or optic
chiasm-evoked responses of all 40 deep layer cells tested. Thus, fibers
that either pass through or synapse in the superficial layers are necessary
for the visual responses of deep layer neurons. Injections of CoCl2
restricted to the superficial layers significantly reduced the visual
responsivity of 86% of 92 deep layer neurons tested and abolished the
visual responses of 68% of these cells. Superficial layer injections of
CoCl2 were equally effective in reducing the responses of neurons with
dendrites that ascended into the superficial layers (all seven cells tested
and recovered) and those of cells with dendrites restricted to the deep
layers (six of seven cells tested and recovered). Injections of CoCl2 into
the deep layers, in the region of the cell being recorded, significantly
reduced the visual responses of 59% of 37 cells and abolished the visual
responses of 40% of the neurons tested. Deep CoCl2 injections abolished the
visual responses of three of four cells with dendrites restricted to the
deep layers and only one of four cells with dendrites that ascended into
the superficial layers. These results indicate that descending interlaminar
axons from visually responsive superficial layer cells may be responsible
for the visual responsivity of most neurons in the deep SC layers of
hamster. Also, some deep layer neurons in this species may receive
effective visual input through their dendrites that ascend into the
superficial layers, where they are likely to be contacted by retinal axons
or axon collaterals of superficial layer cells.
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