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The Journal of Neurophysiology Vol. 81 No. 2 February 1999, pp. 954-958
Copyright ©1999 by the American Physiological Society
RAPID COMMUNICATION
1Department of Cell Biology and Anatomy and 2Department of Ophthalmology and Neuroscience Center of Excellence, Louisiana State University, Medical Center, New Orleans, Louisiana 70112
Retinal input induces three firing patterns in neurons of the
superficial superior colliculus of neonatal rats. By using an in vitro isolated brain stem preparation, we recorded extracellular responses to electrical stimulation of the optic tract (OT) from 71 neurons in the superficial superior colliculus (SC) of neonatal rats
(P1-13). At postnatal day 1 (P1), all tested neurons
(n = 10) already received excitatory input from the
retina. Sixty-nine (97%) superficial SC neurons of neonatal rats
showed three response patterns to OT stimulation, which depended on
stimulus intensity. A weak stimulus evoked only one spike that was
caused by activation of
non-N-methyl-D-aspartate (NMDA) glutamate
receptors. A moderate stimulus elicited a short train (<250 ms) of
spikes, which was induced by activation of both NMDA and non-NMDA
receptors. A strong stimulus gave rise to a long train (>300 ms) of
spikes, which was associated with additional activation of L-type
high-threshold calcium channels. The long train firing pattern could
also be induced either by temporal summation of retinal inputs or by
blocking 
-aminobutyric acid-A receptors. Because retinal ganglion
cells show synchronous bursting activity before eye opening at P14, the
retinotectal inputs appear to be sufficient to activate L-type calcium
channels in the absence of pattern vision. Therefore activation of
L-type calcium channels is likely to be an important source for calcium
influx into SC neurons in neonatal rats.
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